Available water is soil water between field capacity and the permanent

wilting point. Water molecules having very slight positive charges at one

end and very slight negative charges at the other end. Such molecules are

said to be polar.

Molecules and ions are in constant random motion and tend to

distribute themselves evenly in the space available to them. They move

from a region of higher concentration to a region of lower concentration

by simple diffusion along a diffusion gradient. Evenly distributed

molecules are in a state of equilibrium.

Osmosis is the diffusion of water through a semipermeable membrane.

It takes place in response to concentration differences of dissolved

substances.

Osmotic pressure or potential is the pressure required to prevent

osmosis from taking place. The pressure that develops in a cell as a result

of water entering it is called turgor. Water moves from a region of higher

water potential (osmotic potential and pressure potential combined) to a

region of lower water potential when osmosis is occurring.

Simple diffusion. A. A barrier separates two kinds of molecules. B. When the

barrier is removed, random movement of individual molecules results in both

kinds moving from a region of higher concentration to a region of lower

concentration. C. Eventually, equilibrium (even distribution) is reached. The rate

of diffusion gradually slows down as equilibrium is approached.

A. A turgid cell. Water has entered the cell by osmosis, and turgor pressure is

pushing the cell contents against the cell walls. B. Water has left the cell, and

turgor pressure has dropped, leaving the cell flaccid. The vacuole has disappeared.

×200.

A simple osmometer, made by tying a differentially permeable membrane over the

mouth of a thistle tube.

Plasmolysis is the shrinkage of the cytoplasm away from the cell

wall as a result of osmosis taking place when the water potential inside

the cell is greater than outside.

Imbibition is the attraction and adhesion of water molecules to the

internal surfaces of materials; it results in swelling and is the initial

step in the germination of seeds.

Active transport is the expenditure of energy by a cell that results

in molecules or ions entering or leaving the cell against a diffusion

gradient. Recent evidence suggests that this process involves an

enzyme complex and what has been referred to as a proton pump. The

pump involves the plasma membrane of plant and sodium ions.

The cohesion-tension theory postulates that water rises through

plants because of the adhesion of water molecules to the walls of the

capillary-conducting elements of the xylem, cohesion of the water

molecules, and tension on the water columns created by the pull

developed by transpiration.

Water that enters a plant passes through xylem and mostly

transpires into the atmosphere via stomata. Water retained by the

plant is used in photosynthesis and other metabolic activities.

A portion of a leaf of the water weed Elodea. A. Normal cells. B.

Plasmolyzed cells. ×100.

Black-eyed pea seeds before and after imbibition of water.

Pathway of water through a plant.

Capillarity in narrow tubes. The smaller the diameter of the tube, the greater the

rise of the fluid.

The translocation of food substances takes place in a water

solution, and according to the pressure-flow hypothesis, such

substances flow along concentration gradients between their sources

and sinks.

At present, the most widely accepted theory for movement of

substances in the phloem is called the pressure-flow (bulk or mass-

flow) hypothesis. According to this theory, food substances in

solution (organic solutes) flow from a source, where water enters by

osmosis (e.g., a food-storage tissue, such as the cortex of a root or

rhizome, or a food-producing tissue, such as the mesophyll tissue of a

leaf). The water exits at a sink, which is a place where food is

utilized, such as the growing tip of a stem or root. Food substances in

solution (organic solutes) are moved along concentration gradients

between sources and sinks.

One of the most important functions of water in the plant involves

the translocation (transportation) of food substances in solution by

the phloem, a process that has only recently come to be better

understood.

The pressure-flow hypothesis.

An aphid feeding on a young stem of basswood (Tilia). A droplet of “honeydew” is

emerging from the rear of the aphid. ×10.

Many of the studies that led to our present knowledge of the

subject used aphids (small, sucking insects) and organic compounds

designed as radioactive tracers. Most aphids feed on phloem by

inserting their tiny, tubelike mouthparts (stylets) through the leaf or

stem tissues until a sieve tube is reached and punctured.

Transpiration is regulated by humidity and the stomata, which

open and close through changes in turgor pressure of the guard cells.

These changes, which involve potassium ions, result from osmosis

and active transport between the guard cells and the adjacent

epidermal cells (subsidiary cells). Humidity plays an inverse but

direct role in transpiration rates: high humidity reduces

transpiration, and low humidity accelerates it.

In the absence of transpiration at night, the pressure in the xylem

elements builds to the point of forcing liquid water out of the

hydathodes in the leaves. Dew is water that is condensed from the

air. Guttation is the loss of water at night in liquid form through

hydathodes at the tips of leaf veins.

A. A small portion of the leaf epidermis of Wandering Jew (Zebrina sp.) with

several stomata interspersed among ordinary epidermal cells. Each stoma is

bordered by a pair of guard cells, and each guard cell is flanked to the outside by a

small epidermal cell called a subsidiary cell. ×100.

B. Left. An open stoma. The guard cells swell when turgor pressure in

them increases and the stoma opens as the thinner outer walls stretch more

than the thicker inner walls. Right. The stoma closes when the turgor

pressure in the guard cells decreases. ×400

Hydathode structure at the tip of a leaf vein through which water is

forced by root pressures. Root pressure forces liquid water out of

hydathodes, usually at night when transpiration is not occurring.

Furthermore, root pressure seems to drop to negligible amounts in

the summer, when the greatest amounts of water are moving through

the plant.

Droplets of guttation water at the tips of leaves of young barley plants.

Elements essential as building blocks for compounds synthesized by plants.

Growth phenomena are controlled by both internal and external means

and by chemical and physical forces in balance with one another. Besides

carbon, hydrogen, and oxygen, 15 other elements are essential to most

plants. When any of the essential elements are deficient in the plant,

characteristic deficiency symptoms appear.

The mineral elements are usually put into two categories: (1)

macronutrients, which are used by plants in greater amounts and

constitute from 0.5% to 3.0% of the dry weight of the plant; and (2)

micronutrients, which are needed by the plant in very small amounts, often

constituting only a few parts per million of the dry weight. The

macronutrients are carbon, hydrogen, oxygen, nitrogen, potassium,

calcium, phosphorus, magnesium, and sulfur, with the first four usually

making up about 99% of the nutrient total. Those elements remaining, the

micronutrients, are present in amounts ranging from bare traces.

Deficiency symptoms of Nitrogen are relatively uniform loss

of color in leaves (green to yellow), occurring first on the

oldest ones. Nitrogen is part of proteins, nucleic acids and

chlorophylls.

N

METABOLISM

Anabolism = building reactions

(Photosynthesis, Citric acid cycle,etc,)

Catabolism = breaking down compounds

into simpler compounds

molecules or atoms.(Respiration,etc)

sunlight, waterCO2

Fructose(carbohydrates)

Thylakoid membrane

Chloroplast stroma

oxygen

Light reactions

Dark reactions

ATP, NADPH

Fig. 10.2a

Enzymes catalyze reactions of metabolism. Many of these include

oxidation-reduction reactions. Oxidation is loss of electrons;

reduction is gain of electrons.

Photosynthesis is an anabolic process that combines carbon

dioxide and water in the presence of light with the aid of chlorophyll;

oxygen is a by-product. All life depends on photosynthesis, which

takes place in chloroplasts.

Carbon dioxide constitutes 0.038% of the atmosphere, but the

percentage has been rising in recent years. Increased carbon dioxide

levels have potential to elevate global temperatures through the

“greenhouse effect.”

Chlorophyll b and carotenoids (accessory pigments) are antenna

pigments that direct light energy to chlorophyll a. Photosynthetic

units containing chlorophylls and accessory pigments absorb units of

light energy, become excited, and pass this energy to acceptors

during the light-dependent reactions of photosynthesis.

Visible light that is passed through a prism is broken up into individual colors with

wavelengths ranging from 390 nanometers (violet) to 780 nanometers (red).

The structure of a molecule of chlorophyll a (Essential Pigment), the most

important of the pigments involved in photosynthesis. The boxlike ring

structure on the left, with magnesium and nitrogen inside, functions in

capturing light energy. The tail, which extends into the interior of a

thylakoid membrane, is insoluble in water; all chlorophyll molecules are,

however, fat soluble.

The absorption spectra of chlorophyll a, chlorophyll b, and a carotenoid. The

maximum absorption of the chlorophylls is in the blue and red wavelengths. The

maximum absorption of the carotenoids is in the blue-green to green parts of the

visible spectrum.

Less than 1% of all the water absorbed by plants is used in

photosynthesis; most of the remainder is transpired or incorporated

into cytoplasm, vacuoles, and other materials.

During the light-dependent reactions of photosynthesis, which occur

in thylakoid membranes of chloroplasts, water molecules are split, and

oxygen gas is released. Hydrogen ions and electrons are released from

water and transferred to produce NADPH and ATP.

The two types of photosynthetic units present in most chloroplasts are

photosystems I and II (PSI &PSII). The PSI present in stroma lamella and

PSII in granum. The events that take place in photosystem II come before

those of photosystem I. Each photosystem has a reaction-center molecule of

chlorophyll a (the most abundant pigment) that boosts electrons to a

higher energy level when it is excited by light energy.

Photosystem II boosts electron excitation to a level that, when it

encounters photosystem I, has the potential to reduce NADP to NADPH

through noncyclic electron flow. Photosystem I, by itself, can cycle

electrons for generation of ATP. Electron transport while the photosystems

are operating and proton movement across thylakoid membranes are both

involved in ATP production.

A simplified summary of photosynthetic reactions.

Stroma lamella

Grana thylakoid

The light-dependent reactions of photosynthesis, which occur

in more than one way. In noncyclic photophosphorylation,

involving photosystems I and II, which convert light energy to

biochemical energy in the form of ATP and NADPH, water

molecules are split, releasing electrons, protons, and oxygen

gas. The electrons are subsequently used to produce NADPH,

whereas the protons are used, in part, to enable production of

ATP. Oxygen gas is a by-product of this noncyclic

photophosphorylation, although aerobic organisms rely upon

this gas for respiration. The ATP and NADPH are used in the

carbon-fixing reactions that convert CO2 to sugars.

Only photosystem I is involved in cyclic photophosphorylation.

In this relatively simple system, electrons boosted from a

photosystem I reaction-center molecule are shunted back into

the reaction center via the electron transport system. ATP is

produced from ADP, but no NADPH or oxygen is produced.

Since rubisco catalyzes formation of the 3-carbon compound 3PGA as the

first isolated product in these light-independent reactions, plants

demonstrating this process are called C3 plants (3-carbon pathway).

The light-independent reactions occur through a series of reactions known

as the Calvin cycle, which takes place in the stroma of chloroplasts. In the first

step, carbon dioxide is combined with RuBP through catalytic action of the

enzyme rubisco to form two molecules of the 3-carbon compound, GA3P. The

ATP and NADPH from the light-dependent reactions furnish energy to

eventually convert GA3P to 6-carbon carbohydrates. This cycle also

regenerates RuBP to enable continued carbon fixation.

In the light-independent reactions of C4 plants (tropical grasses or arid

region plants), 4-carbon oxaloacetic acid is initially produced instead of 3-

carbon PGA. In the leaf mesophyll of C4 plants (4-carbon pathway), there are

large chloroplasts, which contain rubisco in the bundle sheaths, and small

chloroplasts in the mesophyll, which contain higher concentrations of PEP

carboxylase. C4 plants that facilitate the conversion of carbon dioxide to

carbohydrate at much lower concentrations than is possible in C3 plants.

CAM photosynthesis occurs in cacti and succulent plants whose stomata

are closed and admit little CO2 during the day. Regular photosynthesis occurs

as the 4-carbon compounds that accumulate at night are converted back to

carbon dioxide during the day.

The Calvin cycle

(light-independent

reactions) of

photosynthesis. The

cycle takes place in

the stroma of

chloroplasts,

where each step is

controlled by a

different kind of

enzyme. Carbon

dioxide molecules

from the air enter

the cycle one at a

time, making six

turns of the cycle

necessary to

produce one

molecule of a 6-

carbon sugar such

as Fructose

Organization of the thylakoid membrane showing the relative positions of photosystems

and protein complexes. Some hydrogen ions (protons) from the stroma are pumped into the

thylakoid space (lumen), producing a hydrogen gradient. ATP is produced when these

hydrogen ions and those from water flow from the thylakoid space into the stroma through

the ATP synthase complex.

A portion of a cross section of a leaf of corn (Zea mays), a C4 plant with Kranz anatomy leaves.

An illustration of the C4 photosynthesis pathway. Carbon dioxide is converted to

organic acids in mesophyll cells. After the acids move into bundle sheath cells, some

carbon dioxide is released and enters the Calvin cycle, where it becomes a 3-carbon

compound that moves back to a mesophyll cell; there it is converted to PEP, which

accepts carbon dioxide from the air.

CAM Plants

Cytosol

Chloroplast

However, as indicated by its name, the enzyme RuBP

carboxylase/oxygenase has the potential to fix both CO2, through its

carboxylase activity described by the Calvin cycle, and O2, through its

oxygenase activity. The oxygenase activity of rubisco enables C3 plants to

undergo a process called photorespiration.

Photorespiration requires cooperation among chloroplasts, peroxisomes,

and mitochondria to facilitate shuttling of intermediates along the

photorespiratory pathway. The products of photorespiration are the 2-carbon

phosphoglycolic acid, which is processed to some extent in peroxisomes and

eventually released as carbon dioxide in mitochondria, and the 3-carbon

phosphoglyceric acid that can reenter the Calvin cycle. No ATP is produced by

photorespiration.

Light that is too intense may change the way in which some of a cell’s

metabolism takes place. For example, higher light intensities and temperatures

may change the ratio of carbon dioxide to oxygen in the interiors of leaves,

which, in turn, may accelerate photorespiration.

Photorespiration is typically considered to be a wasteful process that uses

oxygen and releases carbon dioxide, although it may help some plants to

survive under adverse conditions. It differs from common aerobic respiration

in its chemical pathways.

Photooxidation, which involves the destruction (“bleaching”) of chlorophyll

by light, may also occur.

Effects of light on two forms of photosynthesis. Both forms of photosynthesis,

known as C3 and C4, respectively. A. In C3, the rate of photosynthesis will not

increase beyond a certain intensity of light. In C4 plants, when additional

carbon dioxide is available, photosynthetic rates undergo up to a 30% increase

in light intensity.

Effects of temperature on two forms of photosynthesis. Both forms of

photosynthesis, known as C3 and C4, respectively. B. In C3 plants, quantum yield

of photosynthesis decreases as temperatures increase, whereas in C4 plants, the

quantum yield of photosynthesis is not significantly affected by temperature

fluctuations between 10°C and 40°C.