Plant Structure and Function

Chapters 35-39

Evolution of PlantsAll Plants…• multicellular, eukaryotic, autotrophic, alternation of generations

Alternation of GenerationsSporophyte (diploid)• produces haploid spores via meiosis

Gametophyte (haploid)• produce haploidgametes via mitosis

Fertilization• joins two gametes toform a zygote

Angiosperms

Monocots vs. Dicots• named for the numberof cotyledons present on the embryo of the plant

+ monocots- orchids, corn, lilies, grasses

+ dicots- roses, beans, sunflowers, oaks

Plant MorphologyMorphology (body form)• shoot and root systems + inhabit two environments

- shoot (aerial) + stems, leaves, flowers- root (subterranean) + taproot, lateral roots

• vascular tissues + transport materials between roots and shoots

- xylem/phloem

Plant Anatomy Anatomy (internal structure)• division of labor + cells differing in structure and function

- parenchyma, collenchyma, sclerenchyma (below)- water- and food-conducting cells (next slide)

ParenchymaSt: “typical” plant cellsFu: perform most metabolic functions

CollenchymaSt: unevenly thickened primary wallsFu: provide support but allow growthin young parts of plants

SclerenchymaSt: hardened secondary walls (LIGNIN)Fu: specialized for support; dead

Plant cell types

Parenchyma cells Collenchyma cells

Cell wall

Sclerenchyma cells

Plant cell types• Xylem • Phloem

WATER-CONDUCTING CELLS OF THE XYLEM

Vessel Tracheids

Tracheids and vessels

Vesselelement

Pits

Tracheids

SUGAR-CONDUCTING CELLS OF THE PHLOEM

Companion cell

Sieve-tubemember

Sieve-tube members:longitudinal view

Sieveplate

Nucleus

CytoplasmCompanioncell

Water- and Food-conducting CellsXylem (water)

• dead at functional maturity• tracheids- tapered with pits• vessel elements- regular tubes

Phloem (food)• alive at functional maturity• sieve-tube members-

arranged end to end with sieve plates &Companion cells

Plant TissuesThree Tissue Systems• dermal tissue + epidermis (skin)

- single layer of cells that covers entire body- waxy cuticle/root hairs

• vascular tissue + xylem and phloem

- transport and support• ground tissue + mostly parenchyma

- occupies the space b/n dermal/vascular tissue- photosynthesis, storage, support

Plant GrowthMeristems• perpetually embryonic tissues located at regions of growth + divide to generate additional cells (initials and derivatives)

- apical meristems (primary growth- length) + located at tips of roots and shoots- lateral meristems (secondary growth- girth)

Roots• A root

– Is an organ that anchors the vascular plant– Absorbs minerals and water– Often stores organic nutrients– Taproots found in dicots and gymnosperms– Lateral roots (Branch roots off of the taproot)– Fibrous root system in monocots (e.g. grass)

Figure 35.3

Modified Roots• Many plants have modified roots

(a) Prop roots (b) Storage roots(c) “Strangling” aerial

roots

(d) Buttress roots (e) Pneumatophores

(a) Prop roots (b) Storage roots

Primary Growth of RootsPrimary Growth of Roots• apical meristem + root cap + three overlapping zones

- cell division- elongation- maturation

Stems• A stem is an organ consisting of

– Nodes (could be opposite or alternate)– Internodes

Modified Stems

Rhizomes(d)

Tubers (c)Bulbs

Stolons

(a)

Storage leaves

Stem

Root Node

Rhizome

Root

Buds• An axillary bud

– Is a structure that has the potential to form a lateral shoot, or branch

• A terminal bud– Is located near the shoot tip and causes elongation of a young

shoot

Gardening tip:Removing the terminal bud stimulates growth of axillary buds

Primary Growth in ShootsPrimary Growth in Shoots• apical meristem (1, 7) + cell division occurs + produces primary meristems

- protoderm (4, 8)- procambium (3, 10)- ground meristem (5, 9)

• axillary bud meristems + located at base of leaf primordia • leaf primordium (2, 6) + gives rise to leaves

The leafIs the main photosynthetic organ of most vascular plants

Leaves generally consist ofBlade StalkPetiole

Leaf Morphology• In classifying angiosperms

– Taxonomists may use leaf morphology as a criterion

Petiole

(a) Simple leaf

(b) Compound leaf.

(c) Doubly compound leaf.

Axillary bud

Leaflet

Petiole

Axillary bud

Axillary bud

LeafletPetiole

Modified Leaves

Tendrils

Spines

Storage leaves

Bracts

Reproductive leaves. The leaves of some succulents produce adventitious plantlets, which fall off the leaf and take root in the soil.

Leaf AnatomyEpidermal Tissue• upper/lower epidermis• guard cells (stomata)

Ground Tissue• mesophyll +palisade/spongy parenchyma

Vascular Tissue• veins + xylem and phloem

Keyto labels

DermalGround

Vascular

Guardcells

Stomatal pore

Epidermalcell

50 µm

Surface view of a spiderwort(Tradescantia) leaf (LM)

(b)Cuticle

Sclerenchymafibers

Stoma

Upperepidermis

Palisademesophyll

Spongymesophyll

Lowerepidermis

Cuticle

Vein

Guard cells

Xylem

Phloem

Guard cells

Bundle-sheathcell

Cutaway drawing of leaf tissues(a)

Vein Air spaces Guard cells

100 µmTransverse section of a lilac(Syringa) leaf (LM)

(c)

Leaf Anatomy

Dermaltissue

Groundtissue Vascular

tissue

The Three Tissue Systems: Dermal, Vascular, and Ground

Dermal Tissue

– Protects plant from: • Physical damage• Pathogens

• H2O loss (Cuticle)

Vascular tissue– Carries out long-distance transport of materials

between roots and shoots– Consists of two tissues, xylem and phloem

Ground Tissue– Includes various cells specialized for functions such as

storage, photosynthesis, and support

– Pith = ground tissue internal to the vascular tissue– Cortex = ground tissue external to the vascular tissue

Secondary Growth

Lateral Meristems• vascular cambium + produces secondary xylem/phloem (vascular tissue)• cork cambium + produces tough, thick covering (replaces epidermis)• secondary growth + occurs in all gymnosperms; most dicot angiosperms

The Vascular Cambium and Secondary Vascular Tissue

• The vascular cambium– Is a cylinder of meristematic cells one cell thick– Develops from parenchyma cells

2° Growth

• As a tree or woody shrub ages– The older layers of secondary xylem, the

heartwood, no longer transport water and minerals

• The outer layers, known as sapwood– Still transport materials through the xylem

Cork CambiumPeriderm• protective coat of secondary plant body + cork cambium and dead cork cells

- bark

• cork cambium producescork cells

CHAPTER 36

Plant Transport

MineralsH2O CO2

O2

CO2 O2

H2O Sugar

Light

• A variety of physical processes– Are involved in the different types of transport

Sugars are produced byphotosynthesis in the leaves.5

Sugars are transported asphloem sap to roots and otherparts of the plant.

6

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

4

Transpiration, the loss of waterfrom leaves (mostly through

stomata), creates a force withinleaves that pulls xylem sap upward.

3

Water and minerals aretransported upward from

roots to shoots as xylem sap.

2

Roots absorb waterand dissolved minerals

from the soil.

1 Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.

7

The Central Role of Proton Pumps

• Proton pumps in plant cells– Create a hydrogen ion gradient– Contribute to membrane potential

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+ H+

H+

H+

H+H+

H+

Proton pump generates membrane potentialand H+ gradient.

–

––

–

– +

+

+

+

+

• Plant cells use energy stored in the proton gradient and membrane potential– To drive the transport of many different cations

+CYTOPLASM EXTRACELLULAR FLUID

Cations ( , for example) are driven into the cell by themembrane potential.

Transport protein

K+

K+

K+

K+

K+ K+

K+

K+

–

–

– +

+

(Membrane potential and cation uptake

–

–

+

+

Figure 37.6b

(b) Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairsand also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution.

H2O + CO2 H2CO3 HCO3– +

Root hair

K+

Cu2+

Ca2+

Mg2+

K+

K+

H+

H+

Soil particle

–

–– –

– – ––

–

• Cotransport– A transport protein couples the passage of H+ to

anions

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

NO3–

NO 3 –

NO3

–

NO 3–

NO3

–

NO3 –

–

–

– +

+

+

–

–

– +

+

+

NO3–

Cotransport of anions

H+of through a

cotransporter.

Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion

H+

H+

H+

H+

H+H+

H+

H+ H+

H+

SS

S

S

SPlant cells canalso accumulate a neutral solute,such as sucrose

( ), bycotransporting

down the

steep protongradient.

S

H+

–

–

–

+

+

+

–

–

++–

H+ H+S+

–Contransport of a neutral solute

• Cotransport– Is also responsible for the uptake of sucrose by plant

cells

• Water potential– Is a measurement that combines the effects of solute

concentration and pressure– Determines the direction of movement of water

• Water– Flows from regions of high water potential to regions

of low water potential

Quantitative Analysis of Water Potential• The addition of solutes

– Reduces water potential

0.1 Msolution

H2O

Purewater

(a)

• Negative pressure– Decreases water potential

H2O

(d)

• Application of physical pressure– Increases water potential

H2O

(b)

H2O

(c)

Aquaporin Proteins and Water Transport

• Aquaporins– Are transport proteins in the cell membrane that

allow the passage of water

– Do not affect water potential

Movement of fluid in the xylem & phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes

Fluid Movement

• Water and minerals ascend from roots to shoots through the xylem

• Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant

• The transpired water must be replaced by water transported up from the roots

Pushing Xylem Sap: Root Pressure

• At night, when transpiration is very low– Root cells continue pumping mineral ions into the

xylem of the vascular cylinder, lowering the water potential

• Water flows in from the root cortex– Generating root pressure

• Root pressure sometimes results in guttation

Transpiration produces negative pressure (tension) in the leaf

Which exerts a pulling force on water in the xylem, pulling water into the leaf

The transpirational pull on xylem sapIs transmitted all the way from the leaves to the root tips and even into the soil solutionIs facilitated by cohesion and adhesion

• The stomata of xerophytes– Are concentrated on the lower leaf surface– Are often located in depressions that shelter the

pores from the dry wind

Lower epidermaltissue

Trichomes(“hairs”)

Cuticle Upper epidermal tissue

Stomata 100 m

TranslocationIs the transport of organic nutrients in the plant

Phloem sapIs an aqueous solution that is mostly sucroseTravels from a sugar source to a sugar sink

Translocation through Phloem

A sugar sourceIs a plant organ that is a net producer of sugar, such as mature leaves

A sugar sinkIs an organ that is a net consumer or storer of sugar, such as a tuber or bulb

Sugar Source & Sink

TranspirationLab

Control of TranspirationPhotosynthesis-Transpiration Compromise• guard cells help balance plant’s

need to conserve water with its requirement for photosynthesis

Stomatal closing • 1. Potassium ions move out of the vacuole and out of the cells. • 2. Water moves out of the vacuoles, following potassium ions. • 3. The guard cells shrink in size. • 4. The stoma closes.

Stomatal opening

1. Potassium ions move into the vacuoles.

2. Water moves into the vacuoles, following potassium ions.

3. The guard cells expand.

4. The stoma opens.

Plant nutrition

• Chapter 37

Plant NutritionWhat does a plant need to survive?• 9 macronutrients, 8 micronutrients + macro- required in large quantities

- C, H, N, O, P, S, K, Ca, Mg + micro- required in small quantities

- Fe, Cl, Cu, Mn, Zn, Mo, B, Ni + usually serve as cofactors of enzymatic reactions

• The most common deficiencies– Are those of nitrogen, potassium, and phosphorus

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

Hydroponics

• Remove only one macronutrient to see effects on plant

SoilTexture and Composition• texture depends on size of particles + sand-silt-clay

- loams: equal amounts of sand, silt, clay

• composition + horizons

- living organic matter- A horizon: topsoil, living organisms, humus- B horizon: less organic, less weathering than A horizon- C Horizon: “parent” material for upper layers

• soil conservation issues + fertilizers, irrigation, erosion

Soil

• A mixture of mineral particles, decaying organic material, living organisms, air, and water, which together support the growth of plants

Aeration

Soil Bacteria and Nitrogen Availability• Nitrogen-fixing bacteria convert atmospheric N2

– plants absorb ammonium (NH4+), nitrate (NO3

-)

Atmosphere

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3

(ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

Atmosphere

Nitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria

Nutritional AdaptationsSymbiotic Relationships• symbiotic nitrogen fixation + Legume root nodules contain bacteroids (Rhizobium bacteria)

- mutualistic relationship - Crop rotation (Legumes • mycorrhizae + symbiotic associations of fungi and roots

- mutualistic relationship

• parasitic plants + plants that supplement their nutrition from host

- mistletoe, dodder plant, Indian pipe• carnivorous plants + supplement nutrition by digesting animals

Mycorrhizae and Plant Nutrition• Mycorrhizae

– Are modified roots consisting of mutualistic associations of fungi and roots

• The fungus– Benefits from a steady supply of sugar donated

by the host plant

• In return, the fungus– Increases the surface area of water uptake and

mineral absorption and supplies water and minerals to the host plant

• Unusual nutritional adaptations in plants

Staghorn fern, an epiphyte

EPIPHYTES

PARASITIC PLANTS

CARNIVOROUS PLANTS

Mistletoe, a photosynthetic parasite Dodder, a nonphotosynthetic parasite

Host’s phloem

Haustoria

Indian pipe, a nonphotosynthetic parasite

Venus’ flytrapPitcher plants Sundews

Dodder

Phytoremediation

• Poplars remove nitrates• Mustard removes uranium

Pesticide Levels (ppb) inGround Water Before & After Phytoremediation Activities

Wetlands

Uptake of Soil SolutionSymplastic Route• continuum of cytosol basedon plasmodesmata

Apoplastic Route• continuum of cell walls andextracellular spaces

Lateral transport of soilsolution alternates betweenapoplastic and symplastic routes until it reaches theCasparian strip

Casparian StripA belt of suberin (purple) that blocks the passage of water and dissolved minerals.

PLANT REPRODUCTIONChapter 38

Plant ReproductionSporophyte (diploid)• produces haploid spores via meiosis

Gametophyte (haploid)• produce haploidgametes via mitosis

Fertilization• joins two gametes toform a zygote

• An overview of angiosperm reproductionAnther attip of stamen

Filament

AntherStamen

Pollen tube

Germinated pollen grain(n) (male gametophyte)on stigma of carpel

Ovary (base of carpel)

Ovule

Embryo sac (n)(female gametophyte)

FERTILIZATIONEgg (n)

Sperm (n)

Petal

Receptacle

Sepal

Style

Ovary

Key

Haploid (n)

Diploid (2n)

(a) An idealized flower.

(b) Simplified angiosperm life cycle.See Figure 30.10 for a more detailedversion of the life cycle, including meiosis.

Mature sporophyteplant (2n) withflowers

Seed(developsfrom ovule)

Zygote(2n)

Embryo (2n)(sporophyte)

Simple fruit(develops from ovary)

Germinatingseed

Seed

CarpelStigma

Mechanisms That Prevent Self-Fertilization

Stigma

Antherwith

pollen

Stigma

Pin flower Thrum flower

The most common anti-selfing mechanism in flowering plantsIs known as self-incompatibility, the ability of a plant to reject its own pollen

Preventative Selfing

• Some plants– Reject pollen that has an S-gene matching an

allele in the stigma cells

• Recognition of self pollen– Triggers a signal transduction pathway leading

to a block in growth of a pollen tube

Double FertilizationDouble Fertilization• pollen grain lands on stigma + pollen tube toward ovule + both sperm discharged down the tube

- egg and one of the sperm produce zygote

- 2 polar nuclei and sperm cell produce endosperm

+ ovule becomes the seed coat + ovary becomes the fruit

Seed Structure and Development

Foliage leaves

Cotyledon

Hypocotyl

Radicle

Epicotyl

Seed coat

Cotyledon

Hypocotyl Cotyledon

Hypocotyl

• The radicle– Is the first organ to emerge from the germinating seed

• In many eudicots– A hook forms in the hypocotyl, and growth pushes the

hook above ground

• Monocots– Use a different method for breaking ground when they

germinate

• The coleoptile– Pushes upward through the soil and into the air

Foliage leaves

ColeoptileColeoptile

Radicle

PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS

Chapter 39

Tropisms

• Growth toward or away from a stimulus

• Gravitropism (Gravity)• Phototropism (Light)• Thigmotropism (Touch)

Etiolation

• The stems of plants raised in the dark elongate much more rapidly than normal, a phenomenon called etiolation.

Figure 39.3

CELLWALL

CYTOPLASM

  1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane

Signal Transduction Pathway

Plant hormones help coordinate growth, development, and responses to stimuli

• Hormones– Are chemical signals that coordinate the different parts of

an organism

Photoperiod, the relative lengths of night and day+ Is the environmental stimulus plants use most often to detect the time of year