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LevetinMcMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
49
4Plant Physiology
CHAPTER OUTLINE
Plant Transport Systems 50Transpiration 51Absorption of Water from the Soil 51
A CLOSER LOOK 4.1 MineralNutrition and the Green Clean 52
Water Movement in Plants 53Translocation of Sugar 53
Metabolism 54
A CLOSER LOOK 4.2 Sugar andSlavery 55
Energy 57Redox Reactions 57Phosphorylation 57Enzymes 58
Photosynthesis 58Energy from the Sun 58Light-Absorbing Pigments 58Overview 58The Light Reactions 59The Calvin Cycle 62Variation to Carbon Fixation 63
Cellular Respiration 64Glycolysis 65The Krebs Cycle 65The Electron Transport System 68Aerobic vs. Anaerobic Respiration 69
Chapter Summary 70
Review Questions 70Further Reading 70
KEY CONCEPTS1. The movement of water in xylem is a
passive phenomenon dependent on thepull of transpiration and the cohesionof water molecules whereas thetranslocation of sugars in the phloemis best described by the Pressure FlowHypothesis.
2. Plants are dynamic metabolic systemswith hundreds of biochemical reactionsoccurring each second, which enableplants to live, grow, and respond totheir environment.
3. Life on Earth is dependent on theflow of energy from the sun, andphotosynthesis is the process duringwhich plants convert carbon dioxideand water into sugars using this solarenergy with oxygen as a by-product.
4. In cellular respiration, the chemical-bond energy in sugars is converted intoan energy-rich compound, ATP, whichcan then be used for other metabolicreactions.
C H A P T E R
Products of photosynthesis are translocated in the phloem and stored
in various plant organs. These pumpkins are excellent examples
of how energy from the sun is transformed into food by these processes.
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LevetinMcMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
50 U N I T I I Introduction to Plant Life: Botanical Principles
Although plants lack mobility and appear static to thecasual observer, they are nonetheless active organ-
isms with many dynamic processes occurring within
each part of the plant. Materials are transported through
specialized conducting systems; energy is harnessed from
the sun; storage products are manufactured; stored foods are
broken down to yield chemical energy; and a multitude of
products are synthesized. Put simply, plants are bustling with
activity. This chapter will consider some of the major trans-
port and metabolic pathways in higher plants.
PLANT TRANSPORT SYSTEMSAs described in Chapter 3, there are two conducting, or
vascular, tissues in higher plants, the xylem and the phloem,each with component cell types. Water and mineral transport
in the xylem will be described first. Tracheids and vesselelements, which consist of only cell walls after the cyto-
plasm degenerates, are the actual conducting components in
xylem.
The source of water for land plants is the soil. Even
when the soil appears dry, there is often abundant soil mois-
ture below the surface. Roots of plants have ready access to
this soil water; leaves, however, are far removed from this
water source and are normally surrounded by the relatively
drier air. The basic challenge is moving water from the soil
up to the leaves across tremendous distances, sometimes up
to 100 meters (300 feet). This challenge is, in fact, met when
water moves through the xylem. There are three components
to this movement: transpiration from the leaves, the uptake
of water from the soil, and the conduction in the xylem(fig. 4.1).
Figure 4.1 Transpiration-Cohesion Theory of xylem transport. (a) As transpiration occurs in the leaf, it creates a cohesive pull on thewhole water column downward to the roots, where water is absorbed from the soil. (b) Vessel elements join to form a long vessel thatmay reach from the roots to the stem tip.
Xylem
Phloem
The tension createdby transpiration pullswater up into leaves.
Xylem
H2O
Phloem
Xylem
(a) Xylem transport
Water is absorbedby the roots.
Water is cohesive andforms a continuouscolumn in xylem.
H2O
Vesselelement
(b)
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LevetinMcMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
C H A P T E R 4 Plant Physiology 51
TranspirationTranspiration, the loss of water vapor from leaves, is the
force behind the movement of water in xylem. This evapora-
tive water loss occurs mainly through the stomata (90%) and
to a lesser extent through the cuticle (10%). When stomata
are open, gas exchange occurs freely between the leaf and the
atmosphere. Water vapor and oxygen (from photosynthesis)
diffuse out of the leaf while carbon dioxide diffuses into the
leaf (fig. 4.2a). The amount of water vapor that is transpired
is astounding, with estimates of 2 liters (0.5 gallons) of water
per day for a single corn plant, 5 liters (1.3 gallons) for a sun-
flower, 200 liters (52 gallons) for a large maple tree, and 450
liters (117 gallons) for a date palm. Imagine the quantities of
water lost each day from the acres of corn and wheat planted
in the farm belt of the United States! Clearly, transpiration by
plants is a major force in the global cycling of water.
It is the action of the guard cells that regulates the rate
of water lost through transpiration and, at the same time,
regulates the rate of photosynthesis by controlling the CO2
uptake. Each stoma is surrounded by a pair of guard cells,
which have unevenly thickened walls. The walls of the
guard cells that border the stoma are thicker than the outer
walls. When guard cells become turgid they can only expandoutward owing to the radial orientation of cellulose fibrils;
this outward expansion of the guard cells opens the stomata.
Stomata are generally open during daylight and closed at
night. As long as the stomata are open, both transpiration and
photosynthesis occur, but when water loss exceeds uptake,
the guard cells lose turgor and close the stomata (fig. 4.2b).
(See A Closer Look 6.1The Influence of Hormones on
Plant Reproductive Cycles.) On hot, dry, windy days the
high rate of transpiration frequently causes the stomata to
close early, resulting in a near shutdown of photosynthesis
as well as transpiration. A fine balance must be struck in this
photosynthesis-transpiration dilemma to allow enough CO2
for photosynthesis while at the same time preventing exces-
sive water loss. Some plants have evolved an alternate path-way for CO
2uptake at night when rates of transpiration are
lower (see CAM Pathway later in this chapter). Other plants
have morphological or anatomical adaptations that reduce
rates of transpiration while keeping the stomata open. These
physiological and anatomical adaptations are most common
in xerophytes, plants occurring in arid environments.
The basis of transpiration is the diffusion of water mol-
ecules from an area of high concentration within the leaf to
an area of lower concentration in the atmosphere. Unless the
atmospheric relative humidity is 100%, the air is relatively
dry compared with the interior of a leaf, where the intercellu-
lar spaces are saturated with water vapor. As long as stomata
are open, a continuous stream of water vapor transpires from
the leaf, creating a pull on the water column that extends fromthe leaf through the plant to the soil.
Concept Quiz
Xerophytes are plants that are able to grow in arid environments.
Explain how the following adaptations of xerophytes would
reduce transpiration rates and enhance these plants survival in
arid regions:
Thick cuticle
Sunken stomata (stomata are found in cavities)
Leaf surface covered with dense mat of trichomes (hairs)(Hint: See Chapter 3.)
Absorption of Water from the SoilWater and dissolved minerals enter a plant through the root
hairs and can follow two paths, via either the symplastor the
apoplast. Water molecules can diffuse through the plasma
membrane into the cytoplasm of a root hair cell and continue
on this intracellular movement through the cytoplasm of cells
in the cortex. Recall that molecules will move from an area
of high concentration to one of low concentration. The water
molecules move from the dilute soil solution and enter the
Figure 4.2 Transpiration is the basic driving force behindwater movement in the xylem. (a) When stomata are open, bothtranspiration and photosynthesis occur as H
2O molecules diffuse
out of the leaves and CO2
molecules diffuse in. (b) When guardcells are turgid, stoma are open, and when guard cells are flaccid,stoma are closed.
Open Stoma
Closed Stoma
Chloroplast
Guard cells
Nucleus
(a)
(b)
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LevetinMcMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
52
A CLOSER LOOK 4.1
Research has shown that certain minerals are requiredby plants for normal growth and development. These areincluded in the essential elements listed in Table 1.3. Thesoil is the source of these minerals, which are absorbed bythe plant with the soil water. Even nitrogen, which is a gas inits elemental state, is normally absorbed from the soil in theform of nitrate ions (NO
3). Some soils are notoriously defi-
cient in micronutrients and are therefore unable to supportmost plant life. Serpentine soils, for example, are deficient incalcium, and only plants able to tolerate the low levels of thismineral can survive. In modern agriculture, mineral depletionof soils is a major concern, since harvesting crops interruptsthe natural recycling of nutrients back to the soil.
Mineral deficiencies can often be detected by specificsymptoms such as chlorosis (loss of chlorophyll resulting inyellow or white leaf tissue), necrosis (isolated dead patches),anthocyanin formation (development of deep red pigmenta-tion of leaves or stem), stunted growth, and developmentof woody tissue in an herbaceous plant. Soils are mostcommonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described.Leaves develop chlorosis; stems are short and slender; and
anthocyanin discoloration occurs on stems, petioles, andlower leaf surfaces. Phosphorus-deficient plants are often
stunted, with leaves turning a characteristic dark green, oftenwith the accumulation of anthocyanin. Typically, older leavesare affected first as the phosphorus is mobilized to younggrowing tissue. Iron deficiency is characterized by chlorosisbetween veins in young leaves.
Much of the research on nutrient deficiencies is basedon growing plants hydroponically, using soil-less nutrientsolutions. This technique allows researchers to create solu-tions that selectively omit certain nutrients and then observethe resulting effects on the plants (box fig. 4.1). Hydroponicshas applications beyond basic research since it facilitates thegrowing of greenhouse vegetables during winter. Aeroponics,a technique in which plants are suspended and the rootsmisted with a nutrient solution, is another method for grow-ing plants in soil-less culture.
While mineral deficiencies can limit the growth of plants,an overabundance of certain minerals can be toxic and canalso limit growth. Saline soils, which have high concentrationsof sodium chloride and other salts, also limit plant growth, andresearch continues to focus on developing salt-tolerant vari-eties of agricultural crops. Research has focused on the toxiceffects of heavy metals such as lead, cadmium, mercury, and
aluminum; however, even copper and zinc, which are essentialelements, can become toxic in high concentrations. Although
Mineral Nutrition and the Green Clean
more concentrated cytoplasm of the root cells. The cytoplasm
of all cells is interconnected through plasmodesmata and is
referred to as the symplast. Thus, this pathway follows the
symplast from a root hair cell into the stele (fig. 4.3).
A second path is the diffusion of water through the cell
walls and intercellular spaces from the root hair through
the cortex (fig. 4.3). The intercellular spaces and the spaces
between the cellulose fibrils in the cell walls constitute the
apoplast of a plant; thus, the water molecules move unim-
peded through the apoplast until they reach the endodermis.The innermost layer of the cortex consists of a specialized
cylinder of cells known as the endodermis. The presence of
a Casparian strip on the walls of the endodermal cells regu-
lates the movement of water and minerals into the stele. The
Casparian strip is a layer of suberin (and in some instances
lignin as well) on the radial and transverse walls (top, bottom,
and sides) that prevents the apoplastic movement of water into
the stele (see Chapter 3, fig. 3.8c). The movement of water
through the selectively permeable plasma membrane is there-
fore directed to the tangential walls of the endodermis and into
the cytoplasm of these cells and, thus, the symplast. By forc-
ing the water and minerals through the symplastic pathway,
some control over the uptake of minerals is exerted. Some
Figure 4.3 Water and minerals can follow one of twopathways across the cortex into the vascular cylinder: (a) theapoplastic pathway, in which water diffuses through the cell wallsand intercellular spaces or (b) the symplastic pathway, in whichwater diffuses into the cytoplasm of a root hair cell and continuesmoving through the cytoplasm from one cell to the next. Inboth pathways, water must move through the symplast of theendodermal cells.
EpidermisCortex
Xylem
Root hair
Endodermis andCasparian strip
Symplasticpathway
Apoplasticpathway
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LevetinMcMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
53
most plants cannot survive in these soils, certain plants havethe ability to tolerate high levels of these minerals.
Scientists have known for some time that certain plants,called hyperaccumulators, can concentrate minerals atlevels 100-fold or greater than normal. Certain minerals aremore likely to be hyperaccumulated than others. A surveyof known hyperaccumulators identified that 75% of themamassed nickel. Cobalt, copper, zinc, manganese, lead, andcadmium are other minerals of choice.
Hyperaccumulators run the gamut of the plant world.They may be herbs, shrubs, or trees. Many members of the
Brassicaceae or mustard family, Euphorbiaceae or spurge
family, Fabaceae or legume family, and Poaceae or grass familyare top hyperaccumulators. Many are found in tropical andsubtropical areas of the world where accumulation of high
concentrations of metals may afford some protection againstplant-eating insects and microbial pathogens.
Only recently have investigators considered using theseplants to clean up soil and waste sites that have beencontaminated by toxic levels of heavy metalsan environ-mentally friendly approach known as phytoremediation.A green clean scenario begins with the planting of hyperac-cumulating species in the target area, such as an abandonedmine or an irrigation pond contaminated by runoff. Toxicminerals would first be absorbed by roots but later trans-located to the stem and leaves. A harvest of the shootswould remove the toxic compounds off site to be ashed orcomposted to recover the metal for industrial uses. Afterseveral years of cultivation and harvest, the site would be
restored at a cost much lower than the price of excavationand reburial, the standard practice for remediation of con-taminated soils.
In field trials, alpine pennycress (Thlaspi caerulescens)removed zinc and cadmium from soils near the site ofa zinc smelter. Indian mustard (Brassica juncea) native toPakistan and India has been effective in reducing the level ofselenium salts by 50% in contaminated soils. Much interesthas focused on Indian mustard since it has also been shownto concentrate lead, chromium, cadmium, nickel, zinc, andcopper in the laboratory. The aquatic weed parrot feather(Myriophyllum aquaticum) shows promise in restoring con-taminated waterways. Research is ongoing as the searchcontinues for the plants best suited to purify polluted sites
quickly and cheaply.
Box Figure 4.1 The effects of mineral deficiencies are shownin these sunflower plants grown in hydroponic culture. The plants
grown in complete nutrient solution are shown on the right; thoseon the left are deficient in calcium.
minerals are prevented from entering the stele while others are
selectively absorbed by active transport. (See A Closer Look
4.1Mineral Nutrition and the Green Clean.) Once inside the
cytoplasm of the endodermal cells, water moves symplasti-
cally into the living cells of the pericycle, the outermost layer
of the stele. The water moves into the conducting cells of the
xylem, drawn by the pull of transpiration.
Water Movement in PlantsOnce water is in the xylem of the stele, its movement upward
in the plant is driven by the pull of transpiration as well as
certain properties of the water molecule itself, cohesion and
adhesion. Recall from Chapter 1 that the polarity of water
molecules creates hydrogen bonds between adjacent mole-
cules. These hydrogen bonds may form between water mole-
cules themselves (cohesion) or between water molecules and
the molecules in the walls of vessel elements and tracheids
(adhesion). The presence of these water molecules adhering
to the cell walls provides a continuous source of water that
can evaporate into the intercellular spaces of the leaf and
transpire through the stomata. The cohesive force is so strong
that any force or pull on one water molecule acts on all of
them as well, resulting in the bulk flow of water within the
plant. Bulk flow is usually defined as the movement of a fluid
because of pressure differences at two locations. In the xylem,
the pull of transpiration is the force causing the bulk flow. As
transpiration occurs in the leaf, it creates a cohesive pull on
the whole water column downward from the leaf through the
xylem to the root, where water uptake occurs to replace the
water lost through transpiration (fig. 4.1). This mechanism of
water movement in plants is known as the Transpiration-Cohesion Theory and has been used to explain rates of
water movement as high as 44 meters (145 feet) per hour in
angiosperm trees.
Translocation of SugarOrganic materials are translocated by the sieve tube members
of the phloem. In contrast to the xylem, where the conduct-
ing elements function when the cells are dead, the sieve
tube members of the phloem are living but highly special-
ized cells (see Chapter 3). While water movement in the
xylem is upward from the soil, phloem translocation moves
in the direction from source (supply area) to sink (area of
metabolism or storage). In late winter, the source may be
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LevetinMcMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
54 U N I T I I Introduction to Plant Life: Botanical Principles
cells and then move symplastically into sieve tube membersthrough plasmodesmata. This highly concentrated solution in
the sieve tube members causes water to enter by osmosis from
nearby xylem elements, resulting in a buildup of pressure.
When pressure starts to build in these cells, the solute-rich
phloem sap is pushed through the pores in the sieve plate
into the adjacent sieve tube member and so on down to the
sink. This movement of material en masse is known as mass
flow. At the sink, companion cells function in active phloem
unloading, which reduces the concentration of sugars and
allows water to diffuse out of these cells. Sugars unloaded
at the sink are taken up by nearby cells and either stored as
starch or metabolized. (See A Closer Look 4.2Sugar and
Slavery.) The loading and unloading of sugars by active
transport are energy-requiring steps.
METABOLISMMetabolism is the sum total of all chemical reactions occur-
ring in living organisms. Metabolic reactions that synthesize
compounds are referred to as anabolic reactions and are
generally endergonic, requiring an input of energy. In con-
trast, catabolic reactions, which break down compounds, are
usually exergonic reactions, which release energy. Many of
these reactions also involve the conversion of energy from
one form to another.
an underground storage organ translocating sugars to apicalmeristems (the sink) in the branches of a tree. In summer,
the source may be photosynthetic leaves sending sugars for
storage to sinks such as roots or developing fruits (fig. 4.4).
In most plants, the primary material translocated in phloem
is sucrose in a watery solution that also may include small
amounts of amino acids, minerals, and other organic com-
pounds. Translocation in the phloem is quite rapid and has
been timed at speeds averaging 1 meter (3.3 feet) per hour.
The amount of material translocated is also quite impres-
sive. In a growing pumpkin, which reaches a size of 5.5 kg
(11 lbs) in 33 days, approximately 8 g (0.3 oz) of solution are
translocated each hour. Each fall at state and county fairs all
across the United States, prize-winning pumpkins routinely
weigh well over 400 kg (880 lb). The 2003 world recordholder was a 630 kg (1,385 lb) pumpkin grown by Steven
Deletas from Oregon. An even larger pumpkin (663 kg, or
1,458 lb) was grown in New Hampshire but was disqualified
because it was damaged. In the United States in 2003, there
were 63 pumpkins that weighed over 454 kg (1,000 lb).
The hypothesis currently accepted to explain transloca-
tion in the phloem is the Pressure Flow (or Mass Flow)
Hypothesis. This is a modified version of a hypothesis
first proposed by Ernst Mnch in 1926. According to this
hypothesis, there is a bulk flow of solutes from source to
sink (fig. 4.4). At the source, phloem loading takes place as
sugar molecules are first actively transported into companion
Figure 4.4 Translocation of sugar. (a) Products of photosynthesis move from source to sink. At the source, sugar molecules are loadedinto the phloem. As the sugar concentration increases, water moves in from adjacent xylem, pressure builds up, and the sugar solutionis forced through the plant to the sink, where sugar is unloaded from the phloem. (b) A physical model can be used to demonstrate thePressure Flow Hypothesis. The concentrated sugar solution on the left is comparable to the source, and the dilute solution on the rightis comparable to the sink. In bulb 1, water moves into the concentrated sugar solution from the surroundings, pressure builds up, and thesugar solution flows through the tube to bulb 2, where the sugars begin to accumulate, and the pressure causes the water to move out.Recall the discussion of osmosis and diffusion in Chapter 2.
Xylem
(a) (b)
Source
Sink
Sieve tubemember
Companioncell Flow of solution
Concentratedsugar
solution
Dilutesugar
solutionH2O
Selectivelypermeablemembranes
1 2
H2O H2O
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II. Introduction to Plant
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Companies, 2008
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Sugar and Slavery
A CLOSER LOOK 4.2
Products of photosynthesis are typically transported togrowing fruits, storage organs, and other sinks throughoutthe plant. After being unloaded from the phloem, sugars areusually converted to starch or other storage carbohydrates.Although the disaccharide sucrose is the material translo-cated in the phloem of most plants, very few species storesignificant amounts of this sugar. Only two plants, sugarcaneand sugar beet, are commercially important sources ofsucrose, commonly known as table sugar. Sugarcane is themore important crop and, in terms of sheer tonnage, leadsthe global crop production list (see fig. 12.1).
Sugarcane is native to the islands of the South Pacific andhas been grown in India since antiquity. Small amounts ofsugarcane reached the ancient civilizations in the Near Eastand Mediterranean countries through Arab trading routes,but it was not grown in those regions until the seventh cen-tury. Even after cultivation was established, honey remainedthe principal sweetener in Europe until the fifteenth century.During the Middle Ages, sugar was an expensive luxury thatfound its greatest use in medicinal compounds to disguise thebitter taste of many herbal remedies. Early in the fifteenthcentury, sugar plantations were established on islands in the
eastern Atlantic: on the Canary Islands by Spain as well as onMadeira and the Azores by Portugal.Columbus introduced sugarcane to the Caribbean islands
on his second voyage in 1493. By 1509, sugarcane was har-vested in Santo Domingo and Hispaniola and soon spread toother islands. In fact, many Caribbean islands were eventu-ally denuded of native forests and planted with sugarcane.The Portuguese saw the opportunities in South America andstarted sugar plantations in Brazil in 1521. Although late toenter the West Indies, the British established colonies in theearly seventeenth century, and by the 1640s, sugar planta-tions were thriving on Barbados. The first sugarcane grownin the continental United States was in the French colony ofLouisiana in 1753.
The growing of sugarcane was responsible for the estab-lishment of slavery in the Americas, and early in the sixteenthcentury, sugar and the slave trade became interdependent.Decimation of the native populations led to the need fornew workers on the sugar plantations. The first suggestionto use African slaves was made in 1517. Within a short time,the importation of slaves was a reality in both the Spanishand Portuguese colonies, with the greatest number of slavesimported into Brazil. The introduction of African slaves tothese colonies was an outgrowth of the slave trade in Spainand Portugal that had begun in the 1440s. Initially, Spainexported the slaves, but by 1530 slaves were sent directlyfrom Africa to the Caribbean.
The sugar production in the Caribbean came at a timewhen supplies of honey in Europe were decreasing. TheCatholic monasteries were the traditional source of honey.Beehives were kept, principally, to produce beeswaxfor church candles. During the Protestant Reformation,Catholic monasteries were suppressed, and the sources ofhoney fell short of demand. Also, in the late seventeenthcentury, the introduction and growing popularity of coffee,tea, and cocoa in Europe accelerated the demand for sugar,since Europeans generally disliked the naturally bitter tasteof these beverages. Sugar became the most important com-modity traded in the world, and eventually England becamethe dominant force in this enterprise. The Triangular Tradewas the source of many fortunes. The first leg of the tri-angle was from England to West Africa, where trinkets,cloth, firearms, salt, and other commodities were barteredfor slaves. The second leg brought slaves to the CaribbeanIslands, where they were sold. The final leg of the journeycarried rum, molasses, and sugar back to England (boxfig. 4.2a). A second triangle became important in the mid-eighteenth century, linking the West Indies, New England,and West Africa. The use of slaves on sugar plantations
continued until the early nineteenth century, when theslave trade was abolished. It has been estimated that duringthis period 10 million to 20 million African slaves had beenbrought to the New World. Approximately 40% of theslaves brought to the New World went to the Caribbean
Box Figure 4.2a (a) The Triangular Trade.
Caribbean Sea
Atlantic
Ocean
West
Africa
West
Indies
Firearms,
etc.
South
America
North
America
Great
Britain
cloth,
salt,
Rum
,su
gar
Slaves
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II. Introduction to Plant
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Islands. During the seventeenth century, the British sugarplantations in these islands created one of the harshest sys-tems of slavery in history. It was physically crueler and moredemanding than slave conditions on the North Americancontinent. Although the life of a slave was never easy, it wasespecially arduous in the sugar plantations. Few survived thehard labor; consequently, the slave population had to beconstantly replenished.
Sugarcane, Saccharum officinarum, is a perennial memberof the grass family, Poaceae (box fig. 4.2b). Several species ofSaccharum are known to exist in the tropics, and it is believedthat S. officinarum originated as a hybrid of several species.
The species owes its importance to the sucrose stored inthe cells of the stem. Sugarcane, which uses the C4
Pathwayfor photosynthesis, is considered one of the most efficientconverters of solar energy into chemical energy. Canes areoften 5 to 6 meters (15 to 20 feet) tall, with individual stalksup to 10 cm (4 in) in diameter. Plants are grown from stemcuttings, with each segment containing three or four nodesand each node at least one bud. Segments are laid horizon-tally, bud upward, in shallow trenches. Roots soon developfrom the node. Generally 1218 months are needed beforethe canes can be harvested. On fertile land, subsequent cropsdevelop from the rhizome for 2 or 3 years before replant-ing is necessary. Sugarcane thrives in moist lowland tropicsand subtropics and today provides about 60% of the worlds
sugar supply.Canes generally contain 12% to 15% sucrose. After har-vesting, canes are crushed by heavy steel rollers to extractthe sugary juice. The fibrous residue (bagasse) can be usedto make fiberboard, paper, and other products, or used ascompost. Successive boilings concentrate the sucrose; impu-rities are usually precipitated by adding lime water (calciumoxide solution) and are removed by filtering. The solutionis then evaporated to form a syrup from which the sugaris crystallized. Centrifuges separate the thick brown liquidportion from the crystals. The liquid portion is molasses,which is used in foods or is fermented to make rum, ethylalcohol, or vinegar. The crystallized sugar (about 96%97%
pure sucrose) is further refined to free it from any additionalimpurities.
Sugar beet, Beta vulgaris, formerly a member of the goose-foot family, Chenopodiaceae, now in the Amaranthaceae,is not closely related to sugarcane (box fig. 4.2c). It is actu-ally the same species as red beets, which are native to theMediterranean region and have been consumed since thetime of the ancient Romans. In the mid-eighteenth century, aGerman chemist, Andreas Marggraf, discovered that the rootscontained sugar that was chemically identical to that fromcane. The rise of the sugar beet industry can be tied to theEmperor Napoleon I. When a British naval blockade cut offthe sugar imports to France, Napoleon realized the value of a
domestic source of sugar and encouraged scientific researchon the sugar beet. After 1815, sugarcane imports wererestored, halting the developing sugar beet industry. By theearly twentieth century, the sugar beet industry was revivedin both Europe and North America, and today sugar beetsprovide close to 40% of the worlds supply of table sugar.
Sugar beet is a biennial plant, but it is harvested at theend of the first year, when the sucrose content is greatest.Selective breeding gradually raised the percentage of sugarin the root from 2% to approximately 20%. After harvesting,roots are shredded, steeped in hot water, and then pressedto extract the sucrose. Further processing is similar to sugar-cane processing and produces an identical final product.
Box Figure 4.2b (b) Sugarcane harvest near Luxor, Egypt.
Box Figure 4.2c (c) USDA geneticist checks growth anddisease resistance of a sugar beet variety.
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II. Introduction to Plant
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C H A P T E R 4 Plant Physiology 57
EnergyAll life processes are driven by energy, and consequently, a
cell or an organism deprived of an energy source will soon
die. Energy is defined by physicists as the ability to do work
and is governed by certain physical principles such as the
Laws of Thermodynamics.
The First Law of Thermodynamics states that energy
can neither be created nor destroyed, but it can be con-
verted from one form to another.
Among the forms of energy are radiant (light), thermal
(heat), chemical, mechanical (motion), and electrical. One
focus of this chapter is photosynthesis, the process that con-
verts radiant energy from the sun into the chemical energy of
a sugar molecule.
The Second Law of Thermodynamics states that in
any transfer of energy there is always a loss of useful
energy to the system, usually in the form of heat.
When gasoline is burned as fuel to drive an automobile
engine, chemical energy is converted into mechanical energy,
but the conversion is not very efficient. Some of the energy is
lost as heat to the surroundings.
All forms of energy can exist as either potential energy
or kinetic energy. Potential energy is stored energy that has
the capacity to do work; kinetic energy actually is doing work
or is energy in action. For example, a boulder at the top of a
hill has a tremendous amount of potential energy. If it rolls
down the hillside, the potential energy is changed into kinetic
energy, an exergonic process (fig. 4.5). To push it back up tothe top of the hill would be an endergonic process requiring
considerable input of energy. Transformations from potential
to kinetic and vice versa occur constantly in biological sys-
tems and are part of the underlying principles of both photo-
synthesis and respiration.
Redox ReactionsMany energy transformations in cells involve the transfer of
electrons or hydrogen atoms. When a molecule gains an elec-
tron or a hydrogen atom, the molecule is said to be reduced,
and the molecule that gives up the electron is said to be oxi-
dized. A molecule that has been reduced has gained energy;
likewise the oxidized molecule has lost energy. Oxidation
and reduction reactions are usually coupled (sometimes calledredox reactions); as one molecule is oxidized, the other is
simultaneously reduced.
AH2 B A BH2
(A-reduced) (B-oxidized) (A-oxidized) (B-reduced)
In many oxidation-reduction reactions, an intermediate
is used to transport electrons from one reactant to another.
One such electron intermediate is NAD (nicotinamide
adenine dinucleotide), which can exist in both oxidized
and reduced states (NAD oxidized form and NADH
H reduced form). Similarly, NADP (nicotinamide
adenine dinucleotide phosphate) and FAD (flavin adenine
dinucleotide) also can exist as NADP/NADPHH and
FAD/FADH2, respectively. NAD and FAD are common
electron carriers in respiration; NADP serves the same func-
tion in photosynthesis.
PhosphorylationOther energy transformations involve the transfer of a phos-
phate group. When a phosphate group is added to a molecule,
the resulting product is said to be phosphorylated and has a
higher energy level than the original molecule. These phos-
phorylated compounds may also lose the high-energy phos-
phate group and thereby release energy. The energy currency
of cells, ATP (adenosine triphosphate), is constantly recycled
in this way. When a phosphate group is removed from ATP,
ADP (adenosine diphosphate) is formed, and energy is
released in this exergonic reaction.
ATP ADP PO4 energy
Figure 4.5 Potential and kinetic energy. (a) The boulder at thetop of a hill has a tremendous amount of potential energy. (b) Ifthe boulder rolls down the hill, the potential energy is convertedto kinetic energy. To push the boulder back up to the top wouldrequire a considerable input of energy.
Kinetic energy
(b)
Potential energy
(a)
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Recreating ATP requires the addition of a phosphate groupto ADP (an endergonic reaction) with the appropriate input
of energy.
ADP PO4 energy ATP
EnzymesProteins that act as catalysts for chemical reactions in living
organisms are known as enzymes. Catalysts speed up the
rate of a chemical reaction without being used up or changed
during the reaction. The majority of chemical reactions in
living organisms require enzymes in order to occur at bio-
logical temperatures. Enzymes are highly specific for certain
reactants; the compound acted upon by the enzyme is known
as the substrate.The names of enzymes most commonly endin the suffix -ase, which is sometimes appended to the name
of the substrate or the type of reaction. Some enzymes func-
tion properly only in the presence of cofactorsor coenzymes.
Cofactors are inorganic, often metallic, ions such as Mg
and Mn, while organic molecules such as NAD, NADP,
and some vitamins are coenzymes. Both cofactors and coen-
zymes are loosely associated with enzymes; however, pros-
thetic groups are nonprotein molecules that are attached to
some enzymes and are necessary for enzyme action.
PHOTOSYNTHESIS
Photosynthesis is the process that transforms the vast energyof the sun into chemical energy and is the basis for most food
chains on Earth. The overwhelming majority of life depends
on the photosynthetic ability of green plants and algae, and
without these producers, life as is known today could not
survive.
Energy from the SunThe sun is basically a thermonuclear reactor producing tre-
mendous quantities of electromagnetic radiation, which
bathes Earth. Visible light is only a small portion of the elec-
tromagnetic spectrum, which includes radio waves, micro-
waves, infrared radiation, ultraviolet radiation, X rays, and
gamma rays (fig. 4.6). This radiant energy, or light, has
a dual nature consisting of both particles and waves. Theparticles are known as photons and have a fixed quantity of
energy. It is believed that the photons travel in waves and
thus display characteristic wavelengths. Wavelengths vary
from radio waves, which may be over 1 kilometer long to
gamma rays, which are a fraction of a nanometer (nm) long.
The energy content also varies and is inversely proportional
to the wavelengththat is, the longer the wavelength, the
lower the energy.
Approximately 40% of the radiant energy reaching Earth
is in the form of visible light. If visible light passes through
a prism, the component colors become apparent; these range
from red at one end of the visible band to blue-violet at
the other end. The wavelengths of visible light range from
380 nm (violet) to 760 nm (red) and are the wavelengthsmost important to living organisms (fig. 4.6). In fact, the
wavelengths within this range are the ones absorbed by the
chlorophylls and other photosynthetic pigments in green
plants and algae.
Light-Absorbing PigmentsWhen light strikes an object, the light can pass through the
object (be transmitted), be reflected from the surface, or be
absorbed. For light to be absorbed, pigments must be present.
Pigments absorb light selectively, with different pigments
absorbing different wavelengths and reflecting others. Each
pigment has a characteristic absorption spectrum, which
depicts the absorption at each wavelength. If all visible wave-lengths are absorbed, the object appears black; however, if all
wavelengths are reflected, the object appears white. Green
leaves appear green because these wavelengths are reflected.
In higher plants, the major organ of photosynthesis is
the leaf, and the green chloroplasts within the mesophyll are
the actual sites of this process. The major photosynthetic
pigments are the green chlorophylls, which are located on
the thylakoid membranes of the chloroplasts (fig. 4.7).
Thylakoids can be found in stacks, known as grana, as well
as individually in the stroma, the enzyme-rich ground sub-
stance of the chloroplast. Chloroplasts may have 50 to 80
grana, each with about 10 to 30 thylakoids. The chlorophylls
can be located on the stroma thylakoids as well as in the
grana, and it is the abundance of these pigments that makesleaves appear green.
In green plants there are two forms of chlorophyll, a and
b, which differ slightly in chemical makeup. Most chloroplasts
have three times more chlorophyll a than b. The absorption
spectra of the chlorophylls show peak absorbances in the red
and blue-violet regions, with much of the yellow and green
light reflected (fig. 4.6). Other forms of chlorophyll and other
photosynthetic pigments occur in the algae (Chapter 22).
In addition to chlorophylls, chloroplasts also contain
carotenoids. These include the orange carotenes and yel-
low xanthophylls, which absorb light in the violet, blue, and
bluegreen regions of the spectrum. Carotenoids are called
accessory pigments, and the light energy absorbed by these
pigments is transferred to chlorophyll for photosynthesis.Although present in all leaves, carotenoids are normally
masked by the chlorophylls. Recall that these carotenoids
become apparent in autumn in temperate latitudes, when
chlorophyll degrades.
OverviewPhotosynthesis consists of two major phases, the light reac-
tions and the Calvin Cycle. The light reactions constitute
the photochemical phase of photosynthesis, during which
radiant energy is converted into chemical energy. During
the light reactions, water molecules are split, releasing oxy-
gen and providing electrons for the reduction of NADP to
NADPHH. The light reactions also provide the energy
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for the synthesis of ATP. The Calvin Cycle constitutes the
biochemical phase and involves the fixation and reduction of
CO2
to form sugars using the ATP and NADPH produced in
the light reactions (fig. 4.8).
The Light ReactionsThe light reactions are composed of two cooperating photo-
systems, Photosystems I and II, and take place on the thy-
lakoid membranes within the chloroplasts. Each photosystem
is a complex of several hundred chlorophyll and carotenoid
molecules (known as light-harvesting antennae) and associ-
ated membrane proteins. Countless units of these photosys-
tems are arrayed on the thylakoid membranes throughout the
chloroplast. When light strikes a pigment molecule in either
photosystem, the energy is funneled into a reaction center,
which consists of a chlorophyll a molecule bound to a mem-
brane protein (fig. 4.9). The reaction center for Photosystem I
is known as P700
, which indicates the wavelength of maximum
light absorption in the red region of the spectrum; the reaction
center for Photosystem II is P680
, again indicating the peak
absorbance. Associated with the photosystems are various
enzymes and coenzymes that function as electron carriers andare components of the thylakoid membranes.
When a photon of light strikes a pigment molecule in
the light-harvesting antennae of Photosystem I, the energy
is funneled to P700
(fig. 4.10). When P700
absorbs this energy,
an electron is excited and ejected, leaving P700
in an oxidized
state. The ejected electron is picked up by a primary elec-
tron acceptor, which then passes the electrons on to ferre-
doxin (Fd), another electron intermediate, and eventually to
NADP, reducing it to NADPH H, one of the products of
the light reactions.
Another photon of light absorbed by a chlorophyll mol-
ecule in Photosystem II will transfer its energy to the reaction
center P680
(fig. 4.10). When P680
absorbs this energy, an excited
Figure 4.6 Energy from the sun drives the process of photosynthesis. (a) Visible light is only a small portion of the electromagneticspectrum. (b) If visible light is passed through a prism, the component colors are apparent. The chlorophyll pigments in leaves absorb theblue-violet and orange-red portions of the spectrum. (c) Leaves reflect the green and yellow portions of the spectrum.
Absorption
Reflected
Absorbed
Transmitted
ElectromagneticSpectrum
Yellow Orange Red
Absorbed
Absorbed
Violet Blue Green
Gam
ma
rays
X
rays
Ult
rav
iole
tlight
Visible
light
Infra
red
light
Micr
owaves
Radi
ow
aves
Decreasing wavelength
High energy Low energy
Chlorophyllb
Chlorophylla
Transmittedand reflected
400 nm 500 nm 700 nm600 nm
(b)
(a)
(c)
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Figure 4.7 The major organ of photosynthesis is the leaf, andthe actual site of photosynthesis is the chloroplast. (a)(d) Leafcells with chloroplasts; (e)(f) internal structure of the chloroplast.Thylakoid sacs comprise the grana, sites of the light reactions. Thestroma contains the enzymes that carry out the Calvin Cycle.
(a)
Cuticle
Epidermis
Nucleus
Vacuole
Cellwall
Outer membrane
Inner membrane
Stroma
ThylakoidGranum
Chloroplast
Palisade cell
Spongycell
Stoma
Vascularbundle
(b)
(c)
(d)
(e)
Thylakoids in grana
Stroma(f)
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Figure 4.8 Photosynthesis consists of two major phases, the light reactions and the Calvin Cycle.
Light
Electrons
Chloroplast
H2O
O2
ATP
CO2
Sugar
P
NADP
ADP
Calvin
Cycle(in stroma)
NADPH
Light
Reaction(on
thylakoidmembranes)
Figure 4.9 Photosystem. Each photosystem is composed of several hundred chlorophyll and carotenoid molecules that make up light-harvesting antennae. When light strikes a pigment molecule, the energy is transferred and funneled into a reaction center.
Pigment molecule
Energy transfer
Reactioncenter
Light-harvestingantennaecomplex
Incomingphotons(light)
Light
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electron is ejected and passed on to another primary electron
acceptor, leaving P680
in an oxidized state. The electron lost by
P680
is replaced by an electron from water, in a reaction that
is not fully understood, and catalyzed by an enzyme on thethylakoid membrane that requires manganese atoms. In this
reaction, water molecules are split into oxygen and hydrogen;
the hydrogen is a source of both electrons and protons.
The primary electron acceptor passes the electron on to
a series of thylakoid membrane-bound electron carriers that
include plastoquinone (Pq),cytochrome complex,plasto-
cyanin (Pc), and others. The electron is eventually passed
to the oxidized P700
in Photosystem I. During the transfer
of electrons, ATP is synthesized as protons are passed from
the thylakoid lumen into the stroma by an ATP synthasein
the membrane. It is actually the passage of protons through
this enzyme that drives the production of ATP; however, the
mechanism for this reaction is still not completely under-
stood. This synthesis of ATP is known as photophosphory-lation, since the energy that drives the whole process is from
sunlight.
In the process just described, the two photosystems are
joined together by the one-way transfer of electrons from
Photosystem II to Photosystem I. Water is the ultimate source
of these electrons, continually replenishing electrons lost
from P680
. The photophosphorylation that occurs during this
process is referred to as noncyclic photophosphorylation
since the electron transfer is one-way, with the reduction of
NADP as the final step.
Photosystem I is also capable of functioning indepen-
dently, transferring electrons in a cyclic fashion. The elec-
trons, instead of being passed to NADP from ferredoxin, may
be passed to the cytochrome complex and then back to P700
(fig. 4.11). ATP, but not NADPH, may be generated during
this process, which is known as cyclic photophosphoryla-
tion, since the flow of electrons begins and ends with P700
.
As just described, when water is split, oxygen is released.The oxygen eventually diffuses out of the leaves into the
atmosphere and is Earths only constant supply of this gas.
The current 20% oxygen content in the atmosphere is the
result of 3.5 billion years of photosynthesis. The atmosphere
of early Earth did not contain this gas; oxygen began to accu-
mulate only after the evolution of the first photosynthetic
organisms, the cyanobacteria. Today, the vast majority of liv-
ing organisms depend on oxygen for cellular respiration and,
therefore, the energy that maintains life.
The overall light reactions proceed with breathtaking
speed as a constant flow of electrons moves from water to
NADPH, powered by the vast energy of the sun. The ATP
and NADPH that result from the light reactions are needed to
drive the biochemical reactions in the Calvin Cycle.
The Calvin CycleThe source of carbon used in the photosynthetic manufac-
ture of sugars is carbon dioxide from the atmosphere. This
gas makes up just a tiny fraction, approximately 0.035%, of
Earths atmosphere and enters the leaf by diffusing through
the stomata. The reactions that involve the fixation and reduc-
tion of CO2
to form sugars are known as the Calvin Cycle and
are sometimes referred to as the C3Pathway. These reactions
utilize the ATP and NADPH produced in the light reactions
but do not involve the direct participation of light and are
hence sometimes referred to as light-independent reactions,
or dark reactions. The Calvin Cycle takes place in the stroma
Figure 4.10 Two cooperating photosystems work together to transfer electrons from water to NADPH. The passage of electronsfrom Photosystem II to Photosystem I also drives the formation of ATP, a process known as noncyclic photophosphorylation.
Incoming e
e
Photosystem IPhotosystem II
1O2 2H
e e
2H2O
Reaction center Reaction center
Light Light
photons(light)
Incomingphotons(light)
Cytochromecomplex
ATP
ADP
Fd
NADPH
Pq
Pc
Primaryacceptor
Primaryacceptor
NADP
reductase
NADP H
P680 P700
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of the chloroplasts, which contains the enzymes that catalyze
the many reactions in the cycle. This pathway was worked out
by Melvin Calvin, in association with Andrew Benson and
James Bassham, during the late 1940s and early 1950s. The
pathway is named in honor of Calvin, who received a Nobel
Prize for his work in 1961.The following discussion will be limited to the main
events of the Calvin Cycle, which are depicted in Figure 4.12.
The end product of this pathway is the synthesis of a six-
carbon sugar; this requires the input of carbon dioxide. Six
turns of the cycle are needed to incorporate six molecules
of CO2
into a single molecule of a six-carbon sugar. The
initial event is the fixation or addition of CO2to ribulose-1,
5-bisphosphate (RUBP), a five-carbon sugar with two
phosphate groups. This carboxylation reaction is cata-
lyzed by the enzyme ribulose bisphosphate carboxylase
(RUBISCO). In addition to its obvious importance to
photosynthesis, RUBISCO appears to be the most abundant
protein on Earth since it constitutes 12.5% to 25% of total
leaf protein. The product of the carboxylation is an unstablesix-carbon intermediate that immediately splits into two
molecules of a three-carbon compound, with one phosphate
group called phosphoglyceric acid (PGA), or phospho-
glycerate. Six turns of the cycle would yield 12 molecules
of PGA.
The 12 PGA molecules are converted into 12 molecules
of glyceraldehyde phosphate, or phosphoglyceraldehyde
(PGAL). This step requires the input of 12 NADPH H
and 12 ATP (both generated during the light reactions), which
supply the energy for this reaction. Ten of the 12 glyceral-
dehyde phosphate molecules are used to regenerate the six
molecules of RUBP in a complex series of interconversions
that require six more ATP and allow the cycle to continue.
Two molecules of glyceraldehyde phosphate are the net gain
from six turns of the Calvin Cycle; these are converted into
one molecule of fructose-1,6-bisphosphate, which is soon
converted to glucose. The glucose produced is never stored as
such but is converted into starch, sucrose, or a variety of other
products, thus completing the conversion of solar energy into
chemical energy.The complex steps of photosynthesis can be summarized
in the following simple equation, which considers only the
raw materials and end products of the process:
CHLOROPHYLL6CO
2 12H
2O sunlight C
6H
12O
6 6O
2 6H
2O
Concept Quiz
Photosynthesis consists of two major phases: the light reac-tions, in which light energy is converted into chemical energy,and the Calvin Cycle, in which the fixation and reduction of
carbon dioxide to form sugars take place.How is each phase dependent upon the other?
Variation to Carbon FixationMany plants utilize a variation of carbon fixation that consists
of a prefixation of CO2
before the Calvin Cycle. There are two
pathways in which this prefixation occurs, the C4
Pathway
and the CAM Pathway. The C4
pathway occurs in several
thousand species of tropical and subtropical plants, including
the economically important crops of corn, sugarcane, and sor-
ghum. This pathway, occurring in mesophyll cells, consists of
incorporating CO2 into organic acids, resulting in a four-carbon
Figure 4.11 Photosystem I can also function in a cyclic fashion. Instead of reducing NADP, electrons are passed back to P700
. Thisprocess results in the generation of ATP by cyclic photophosphorylation.
Photosystem I
Incomingphotons(light)
Fd
ATP
Pq
P700
Pc
e
Light
Cytochromecomplex
Primaryacceptor
Reactioncenter
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compound and, hence, the name of the pathway. This com-
pound is soon broken down to release CO2
to the Calvin Cycle,
which occurs in cells surrounding the vascular bundle. The C4
pathway ensures a more efficient delivery of CO2
for fixation
and greater photosynthetic rates under conditions of high light
intensity, high temperature, and low CO2
concentrations.
These same steps are part of the CAM (Crassulacean
Acid Metabolism) pathway, which functions in a number
of cacti and succulents, plants of desert environments. This
pathway was initially described among members of the plant
family Crassulaceae. CAM plants are unusual in that theirstomata are closed during the daytime but open at night. Thus,
they fix CO2
during the nighttime hours, incorporating it into
four-carbon organic acids. During the daylight hours, these
compounds are broken down to release CO2
to continue on
into the Calvin Cycle. This alternate pathway allows carbon
fixation to occur at night when transpiration rates are low, an
obvious advantage in hot, dry desert environments.
CELLULAR RESPIRATIONAs discussed, photosynthesis converts solar energy into
chemical energy, stored in a variety of organic compounds.
Starch and sucrose are common storage compounds in
plants and as such are the energy reserves for the plants
themselves and the animals that feed on them. Ultimately
the survival of all organisms on Earth is dependent on the
release of this chemical energy through the catabolic pro-
cess of cellular respiration. All living organisms require
energy to maintain the processes of life. Even at the cel-
lular level, life is a highly dynamic system, requiring
continuous input of energy that is used in the processes of
growth, repair, transport, synthesis, motility, cell division,
and reproduction. Cellular respiration occurs continuously,
every hour of every day, in all living cells; the need forenergy is nonstop.
Cellular respiration is actually a step-by-step breakdown
of the chemical bonds in glucose, involving many enzymatic
reactions, and results in the release of usable energy in the
form of ATP. The overall process is the complete oxidation
of glucose, resulting in CO2
and H2O and the formation of
ATP:
C6H
12O
6 6O
2 6CO
2 6H
2O 36 ATP
This equation of cellular respiration is merely a summary of
a complex step-by-step process that has three major stages
or pathways: glycolysis, the Krebs Cycle, and the Electron
Transport System.
Figure 4.12 The Calvin Cycle. For every six molecules of CO2
that enter the cycle, one six-carbon sugar is produced. The ATP andNADPH required by this cycle are generated by the light reactions.
ADPNADPH
NADP
ADP
12
12
12
12
6
6
Carbon
Glucose and other sugars
ATP
6 CO2
6 RUBP
10 PGAL12 PGAL
12 PGA
2 PGAL
ATP
Manyintermediatereactions (andrearrangementsof carbon bonds)
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Glycolysis is a series of reactions that occur in thecytoplasm and result in the breakdown of glucose into two
molecules of a three-carbon compound. Along the way, NAD
is reduced and some ATPs are produced. The Krebs Cycle
continues the breakdown of the three-carbon compounds in
the matrix of the mitochondria and results in the release of
CO2. Additional ATP, NADH, and FADH
2are also gener-
ated during these steps. The final stage of respiration, the
Electron Transport System, occurs on the cristae, the inner
membrane of the mitochondria, and consists of a series of
redox reactions during which significant amounts of ATP
are synthesized. A comparison of photosynthesis and cellular
respiration is presented in Table 4.1.
GlycolysisThe word glycolysis means the splitting of sugar. It starts
with glucose, which arises from the breakdown of poly-
saccharides, most commonly either starch or glycogen (in
animals and fungi), or the conversion from other substances,
especially other sugars. The first few steps in glycolysis
actually add energy to the molecule in the form of phosphate
groups (fig. 4.13). These phosphorylations are at the expense
of two molecules of ATP. In addition, the glucose molecule
undergoes a rearrangement that converts it to fructose-1,
6-bisphosphate. These steps prime the molecule for the later
oxidation. The next step splits fructose-1, 6-bisphosphate
into glyceraldehyde phosphate and dihydroxyacetone
phosphate, but the latter is converted into a second mole-cule of glyceraldehyde phosphate. Both glyceraldehyde
phosphate molecules continue on in the glycolytic pathway
so that each of the remaining steps actually occurs twice.
Glyceraldehyde phosphate is phosphorylated and oxidized
in the next step, which also reduces NAD to NADH H.
The resulting organic acids, with two phosphate groups, give
up both phosphates in the remaining steps of glycolysis,
yielding two molecules of pyruvate (pyruvic acid), plus 4
ATP and 2 NADH H. Note that during steps 7 and 10
a total of 4 ATP are produced; however, two of those ATP
molecules are used during the initial steps, resulting in a netgain of only 2 ATP.
The Krebs CycleThe remainder of cellular respiration occurs in the mitochon-
dria of the cell (fig. 4.14). Recall from Chapter 2 that mito-
chondria are organelles with a double membrane. Although
the outer membrane is smooth, the inner membrane is
invaginated; these folds are referred to as cristae. The area
between the outer and inner membranes is referred to as the
intermembrane space; the enzyme-rich area enclosed by
the inner membrane is known as the matrix. The enzymes in
the matrix catalyze each step in the Krebs Cycle.
Once inside the mitochondrial matrix, each of the twopyruvate molecules from glycolysis undergoes several
changes before it enters the Krebs Cycle. The molecule
is oxidized and decarboxylated, losing a CO2, with the
remaining two-carbon compound joining to coenzyme A
to form a complex known as acetyl-CoA. During this step,
NAD is also reduced to NADH H. Acetyl-CoA enters
the Krebs Cycle by combining with a four-carbon organic
acid known as oxaloacetate (oxaloacetic acid) to form a
six-carbon compound known as citrate (citric acid). (The
Krebs Cycle, named in honor of Hans Krebs, who worked
out the steps in this pathway in 1937 and later received a
Nobel Prize for this work, is alternatively known as the
Citric Acid Cycle.)
The steps in the cycle consist of a series of reactionsduring which two more decarboxylations (going from a six-
carbon to a five-carbon and then to a four-carbon compound)
and several oxidations occur (fig. 4.15). During these steps,
three more molecules of NAD are reduced to NADH H,
a molecule of FAD is reduced to FADH2, and one molecule
of ATP is formed. At the end of these steps, the four-carbon
oxaloacetate is regenerated, allowing the cycle to begin
anew. For each molecule of pyruvate that entered the mito-
chondrion, three molecules of CO2
are released and 1 ATP,
4 NADH H, and 1 FADH2
are produced. Since two
Table 4.1Comparison of Photosynthesis and Cellular Respiration
Photosynthesis Cellular Respiration
Overall reaction 6CO212H
2O sunlight C
6H
12O
66O
26H
2O C
6H
12O
66O
26CO
26H
2O 36ATP
Reactants Carbon dioxide, water, sunlight Glucose, oxygen
Products Glucose Energy
By-products Oxygen Carbon dioxide water
Cellular location Chloroplasts Cytoplasm, mitochondria
Energetics Requires energy Releases energy as ATP
Chemical pathways Light reactions and Calvin Cycle Glycolysis, Krebs Cycle, and Electron Transport System
Summary Sugar synthesized using the energy of the sun Energy released from the breakdown of sugar
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Figure 4.13 Glycolysis.
1.
Glucose
Glucose6phosphate
Fructose6phosphate
Fructose1,6diphosphate
Glyceraldehyde3phosphate (G3P)
2.
3.
4. 5.
Dihydroxyacetone phosphate
13diphosphoglycerate (DPG)
6.
Occurs
twice
Carbon
3PGA
7.
2PGA
8.
PEP
9.
Pyruvate
10.
ADP
1. Phosphorylation of glucose by ATP
2-3. Rearrangement to fructose, followed by a second
ATP phosphorylation
4-5. The six-carbon molecule is split into 2 three-carbon molecules of G3P
6. Oxidation followed by phosphorylation produces
2 NADH molecules and gives 2 molecules ofDPG, each with one high-energy phosphate bond
10. Removal of high-energy phosphate by 2 ADPproduces 2 ATP molecules and gives 2 pyruvate
molecules
ADP
ADP
ATP
ADP
NADH
NAD
H2O
P
P
P
P P
P
P
P
P
P
P
P
ATP
ATP
ATP
7. Removal of high-energy phosphate by 2 ADPmolecules produces 2 ATP molecules and gives
2 molecules of 3 phosphoglyceric acid (PGA)
8-9. Phosphate is moved and removal of water gives2 phosphoenol pyruvate (PEP) molecules each
with a high-energy phosphate bond
ADP
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C H A P T E R 4 Plant Physiology 67
Figure 4.15 Krebs Cycle. Before the cycle begins, pyruvate is converted to acetyl-CoA with the loss of CO2. The two-carbon
acetyl group combines with oxaloacetate to form the six-carbon citrate. Two decarboxylations and several redox reactions regenerateoxaloacetate. For every pyruvate that enters the mitochondrion, 4 NADH, 1 FADH
2, and 1 ATP are produced, and 3 CO
2are released.
CoA
CoA
Carbon
CO2leaves cycle
Citrate
CO2leaves cycle
KrebsCycle
Pyruvate
(from glycolysis)
CO2removed
- Ketoglutarate
Succinate
Malate
Oxaloacetate
NADH
NAD
ATP ADP
NADH NAD
NADH
NAD
NADH
NAD
Acetyl-CoA
2 carbonsenter cycle
FADH2
FAD
Figure 4.14 Mitochondrial structure. The inner mitochondrial membrane has numerous infoldings known as cristae. The enzymes andcoenzymes of the Electron Transport System occur on these membranes. The matrix contains the enzymes that carry out the Krebs Cycle,(a) 85,000, TEM. (b) Cut-away diagram of mitochondrion to show internal organization.
Matrix
Cristae
(a)
Cristae
Matrix
Intermembranespace
Outer membrane
Inner membrane
(b)
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4. Plant Physiology The McGrawHill
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68 U N I T I I Introduction to Plant Life: Botanical Principles
pyruvate molecules are formed from each glucose molecule,the cycle turns twice, resulting in 6 CO
2released and yield-
ing 2 ATP, 8 NADH H, and 2 FADH2
as energy-rich
products. At this point, the entire glucose molecule has been
totally degraded; a portion of its energy has been harvested
in these Krebs Cycle products as well as the 2 ATP and 2
NADH H produced in glycolysis.
The Electron Transport SystemThe third and final stage of cellular respiration, the Electron
Transport System, occurs on the inner membranes of the
mitochondria and involves a series of enzymes and coen-
zymes, including several iron-containing cytochromes that
are embedded in this layer and function as electron carri-ers. (This series of electron carriers is similar to the ones
described in the light reactions of photosynthesis.) During
this stage, electrons and hydrogen ions are passed from the
NADH H and FADH2
molecules formed in glycolysis
and the Krebs Cycle down a series of redox reactions and
are finally accepted by oxygen-forming water in the process
(fig. 4.16).
The Electron Transport System is a highly exergonic
process and is coupled to the formation of ATP. This method
of ATP synthesis is referred to as oxidative phosphoryla-
tion. When the electron flow begins from NADH produced
within the mitochondria, enough energy is available to pro-
duce three molecules of ATP from each NADH for a total of
24 ATP from the 8 NADH. Two molecules of ATP are alsosynthesized during the flow of electrons from each FADH
2
produced in the Krebs Cycle (4 ATP) and each NADH from
glycolysis (4 ATP). During the Electron Transport System
then, a total of 32 ATP are generated. This number is added
to the net yield of 2 ATP from glycolysis and the 2 ATP
produced in the Krebs Cycle, for a grand total of 36 ATP
for each glucose molecule that completes cellular respira-
tion (table 4.2). These ATP molecules are then transported
out of the mitochondria and are available for use within
the cell. It should be noted, however, that this production
of 36 ATP harnesses only a fraction, 39%, of the original
chemical energy of the glucose molecule; the remainder is
lost as heat. (Although this 39% efficiency seems low, it is
actually much higher than energy conversions in mechanicalsystems.)
The formation of ATP during the transport of electrons
is believed to occur by the same mechanism described for
photophosphorylation during photosynthesis. During the
transfer of electrons, protons pass from the intermembrane
space into the matrix through an ATPase in the membrane
(fig. 4.17). It is actually the passage of protons through this
ATP synthase enzyme that somehow drives the production
of ATP. This model for ATP synthesis is known as chemi-
osmosis and was first proposed by Peter Mitchell during
the early 1960s. Mitchell received a Nobel Prize for this
Figure 4.16 Electron Transport System. Electrons fromNADH and FADH
2are passed along electron-carrier molecules
(number 1 to 5), including several cytochromes, and finally areaccepted by oxygen. This process drives the formation of ATP bychemiosmosis.
NADHNAD
O212
H2O
FADH2
1
2
3
4
5
FAD
2H
theory, which applies to ATP synthesis in both respiration
and photosynthesis.
Concept Quiz
All living organisms carry out cellular respiration. In plantsthe sugars that are broken down during cellular respirationwere produced by the plant during photosynthesis.
What is the immediate source of compounds broken down
during cellular respiration in humans? What is the ultimate
source of these compounds?
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Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
C H A P T E R 4 Plant Physiology 69
Aerobic vs. Anaerobic RespirationThe complete oxidation of glucose requires the presence of
oxygen and is therefore known as aerobic respiration. As
indicated, it is the last step of respiration and involves the
direct participation of oxygen as the final electron acceptor.
Without oxygen, this last step cannot occur since no othercompound can serve as the ultimate electron acceptor. In fact,
both the Electron Transport System and the Krebs Cycle are
dependent on the availability of oxygen and cannot operate
in its absence.
Some organisms, however, have metabolic pathways
that allow respiration to proceed in the absence of oxygen.
This type of respiration is known as anaerobic respiration,
or fermentation, and is found in some yeast (a unicellular
fungus), in bacteria, and even in muscle tissue. The most
familiar example is alcoholic fermentation found in cer-
tain types of yeast and utilized in the production of beer
and wine (see Chapter 24). When oxygen is not available,
the yeast cells can switch to a pathway that can convert
pyruvic acid to ethanol and CO2. In the process, NADH isoxidized back to NAD, allowing this coenzyme to recycle
back to glycolysis. Recycling of the coenzyme allows gly-
colysis to continue and thus supply the energy needs of
the yeast, at least in a limited way. The only energy yield
from this alcoholic fermentation is the 2 ATP produced
during glycolysis (compared with 36 ATP during aerobic
respiration). The still energy-rich alcohol is merely a by-
product of the oxidation of NADH. If oxygen becomes
available, yeast can switch back to aerobic respiration
with its higher energy yield. Other anaerobic pathways
also exist in bacteria and muscle cells, in which the by-
products are different from alcohol but the yield of NAD
and ATP is similar.
Figure 4.17 Chemiosmosis in mitochondria and chloroplasts. In the mitochondria, protons (H) are translocated to the intermembranespace during the transfer of electrons down the Electron Transport System. The proton gradient drives ATP synthesis as protons movethrough the ATP synthase complex back to the matrix. In the chloroplast, protons are translocated into the thylakoid compartment. Asprotons move through the ATP synthase complex back to the stroma, ATP is synthesized.
Electrontransportchain
Mitochondrion Chloroplast
Membrane
ADP P
H
ATPsynthase
H
H
H
H HH
H
ATP
High H
concentration
Low H
concentration
Intermembranespace
Thylakoidcompartment
Matrix Stroma
Table 4.2 Tally of ATP Producedfrom the Breakdown of Glucose
During Cellular Respiration
Pathway Net ATP Yield*
Glycolysis
2 ATP 2 ATP
2 NADH 4 ATP
Acetyl-CoA formation (2 turns)
1 NADH 2 6 ATP
Krebs Cycle (2 turns)
1 ATP 2 2 ATP
3 NADH 2 18 ATP
1 FADH22 4 ATP
TOTAL 36 ATP
*Each NADH produced in the mitochondrion yields 3 ATP while each NADH
produced during glycolysis has a net yield of 2 ATP. Each FADH 2 also yields 2 ATP.
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and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
70 U N I T I I Introduction to Plant Life: Botanical Principles
CHAPTER SUMMARY1. Plants obtain water from the soil, moving it up within the
xylem through the entire plant. This movement of water is
a passive phenomenon dependent on the pull of transpira-
tion and the cohesion of water molecules. Minerals are
also obtained from the soil and transported in the xylem.
The translocation of sugars occurs in the phloem, moving
from source (photosynthetic leaves or storage organs)
to sink (growing organs or developing storage tissue)
through mass flow within sieve tube members.
2. Sucrose is the usual sugar transported in the phloem;
however, very few plants actually store this economically
valuable carbohydrate. Sugarcane and sugar beet are the
major sucrose-supplying crops. The early developmentof sugarcane plantations in North America greatly influ-
enced the course of history by introducing the slave trade
to the continent.
3. Plants are dynamic metabolic systems with hundreds of
reactions occurring each second to enable plants to live,
grow, and respond to their environment. All life processes
are driven by energy, with some metabolic reactions being
endergonic and others exergonic. Energy transformations
occur constantly in biological systems and are part of the
underlying principles of both photosynthesis and cellular
respiration.
4. Photosynthesis takes place in chloroplasts of green plants
and algae and results in the conversion of radiant energyinto chemical energy (linking the energy of the sun with
life on Earth). Using the raw materials carbon dioxide and
water, along with chlorophyll and sunlight, plants are able
to manufacture sugars. In the light reactions of photosyn-
thesis, energy from the sun is harnessed, forming mol-
ecules of ATP and NADPH. During this process, water
molecules are split, releasing oxygen to the atmosphere
as a by-product. During the Calvin Cycle, carbon dioxide
molecules are fixed and reduced to form sugars, using the
energy provided by the ATP and NADPH from the light
reactions.
5. Cellular respiration is the means by which stored energy
is made available for the energy requirements of the
cell. Through respiration, the energy of carbohydrates istransferred to ATP molecules, which are then available
for the energy needs of the cell. During aerobic respira-
tion, each molecule of glucose is completely oxidized
during the many reactions of glycolysis, the Krebs Cycle,
and the Electron Transport System, resulting in the for-
mation of 36 ATP molecules. In anaerobic respiration,
only two molecules of ATP are formed from each glu-
cose molecule.
REVIEW QUESTIONS1. Explain how water enters a root.
2. What is transpiration, and how does it affect water move-
ment in plants?
3. How are the properties of water important to the theory of
water movement in plants?
4. What are the advantages and disadvantages to having
stomata open during the daylight hours?
5. Explain how the Pressure Flow Hypothesis accounts for
translocation in the phloem.
6. How is light harnessed during the light reactions of pho-
tosynthesis, and what pigments are involved?
7. What is carbon fixation? How is carbon fixed during the
Calvin Cycle?
8. Why is glycolysis important to living organisms, and
where does it occur?
9. Describe mitochondria and the respiratory events that
occur in them.
10. Few plants can survive the saline soils of deserts or
coastal areas, where mineral salts such as sodium chloride
accumulate in extremely high concentrations. Why?
11. If a 500 kg (1,100 lb) pumpkin develops during a 5-month
growing season, determine how much photosynthate
(products of photosynthesis) is transported into the grow-
ing fruit each hour.
12. How are the processes of transpiration and photosynthesis
interrelated?
13. In what way is life on Earth dependent on the energy of
the sun?
FURTHER READING
Beardsley, Tim. 1998. Catching the Rays. Scientific American
278(3): 2526.
Brown, Kathryn S. 1995. The Green Clean.BioScience 45(9):
579582.
Caldwell, Mark. 1995. The Amazing All-Natural Light
Machine.Discover16(12): 8896.
Demmig-Adams, Barbara, and William W. Adams III. 2002.
Antioxidants in Photosynthesis and Human Nutrition.
Science 298: 21492153.
Dunn, Richard S. 1972. Sugar and Slaves. W.W. Norton,
New York, NY.
Govindjee, and William J. Coleman. 1990. How Plants Make
Oxygen. Scientific American 262(2): 5056.
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5/28/2018 4. Plant Physiology
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LevetinMcMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
4. Plant Physiology The McGrawHill
Companies, 2008
C H A P T E R 4 Plant Physiology 71
Hopkins, William G. and Norman P. A. Hner. 2004.Introduction to Plant Physiology, 3rd Edition. John Wiley
& Sons, New York.
Lee, David W. and Kevin S. Gould. 2002. Why Leaves Turn
Red.American Scientist90(6): 524531.
Mintz, Sidney W. 1991. Pleasure, Profit, and Satiation.
Pages 112129 in Seeds of Change: A Quincentennial
Commemoration, ed. Herman J. Viola and Carolyn
Margolis. Smithsonian Institution Press,Washington, DC.
Moffat, Anne Simon. 1995. Plants Providing Their Worth in
Toxic Metal Cleanup. Science 269: 302303.
Moore, Randy,W. Dennis Clark, and Darrell Vodopich.
1998. Botany: Plant Form and Function, 2nd Edition.
WCB/McGraw-Hill, Dubuque, IA.Seymour, Roger S. 1997. Plants That Warm Themselves.
Scientific American 276(3): 104109.
Taiz, Lincoln, and Eduardo Zeiger. 2006. Plant Physiology,4th Edition. Sinauer Associates, Sunderland, MA.
Turgeon, Robert. 2006. Phloem Loading: How Leaves Gain
their Independence.BioScience 56(1): 1524.
Williams, John B. 2002. Phytoremediation in Wetland
Ecosystems: Progress, Problems, and Potential. Critical
Reviews in Plant Sciences 21(6): 607635.
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