Chapter 21: Plant Structure & Function · Chapter 21: Plant Structure & Function ... for water transport in plants; based on molecular properties of water and transpiration. – Roots

Chapter 21: Plant Structure & FunctionChapter 21: Plant Structure & Function

Chapter 21: Plant Structure & FunctionChapter 21: Plant Structure & FunctionAll organisms must:All organisms must:–– Take in certain materials, e.g. OTake in certain materials, e.g. O22, food, drink, food, drink–– Eliminate other materials, Eliminate other materials,

e.g. COe.g. CO22, waste products, waste products


SingleSingle--celled organisms can take celled organisms can take in/release necessary materials by in/release necessary materials by simple simple diffusiondiffusion..


Need for transport systems in larger organisms:Need for transport systems in larger organisms:1.1. Smaller surface area/volume ratioSmaller surface area/volume ratio2.2. Surface of body is not in contact with liquidSurface of body is not in contact with liquidConsider human body (vertebrates):Consider human body (vertebrates):–– Our cells are in contact with internal liquid Our cells are in contact with internal liquid

environment.environment.–– Specialized systems maintain this environment, Specialized systems maintain this environment,

provide cells with food and oxygen, remove carbon provide cells with food and oxygen, remove carbon dioxide and eliminate wastes.dioxide and eliminate wastes.

Plants, likewise, possess transport systems.Plants, likewise, possess transport systems.Transport systems: key to maintaining internal Transport systems: key to maintaining internal balance necessary for life.balance necessary for life.

SurfaceSurface--toto--volume Ratiovolume Ratio

Demo:Demo:

Surface Area = Volume =

SA/Volume Ratio =


SA/Volume Ratio =


SA/Volume Ratio =

1 cm

Transport Systems in Plants: Adaptations for Transport Systems in Plants: Adaptations for Life on LandLife on Land

First land plants probably evolved from First land plants probably evolved from green algae 430 MYA.green algae 430 MYA.Life out of water posed new challenges: Life out of water posed new challenges: e.g. loss of moisture to air.e.g. loss of moisture to air.Early adaptations included:Early adaptations included:–– Protective structure for gametes and embryosProtective structure for gametes and embryos–– WaterWater--proof covering (waxy cuticle)proof covering (waxy cuticle)

Adaptations for Life on LandAdaptations for Life on Land

Two groups evolved:Two groups evolved:Nonvascular plantsNonvascular plants: Mosses (: Mosses (BryophytesBryophytes) ) and relatives (liverworts and hornworts)and relatives (liverworts and hornworts)–– Do not grow very large.Do not grow very large.–– Restricted to damp environments; require water for Restricted to damp environments; require water for

fertilization.fertilization.–– Waxy cuticleWaxy cuticle–– No vascular tissue. Water moves through plant by No vascular tissue. Water moves through plant by

diffusion, capillary action, and diffusion, capillary action, and cytoplasmiccytoplasmic streaming.streaming.–– No woody tissues for support.No woody tissues for support.

Nonvascular PlantsNonvascular Plants


Vascular plantsVascular plants::–– Evolved specialized Evolved specialized

vascular tissue vascular tissue (cells joined into (cells joined into tubes) for tubes) for conducting water conducting water and nutrients and nutrients throughout the throughout the body of the plant.body of the plant.

Vascular PlantsVascular Plants

Vascular TissueVascular Tissue


Other challenges to life on land for plants (Other challenges to life on land for plants (See See Fig. 7.1, p. 186Fig. 7.1, p. 186):):–– Light and COLight and CO22 must be obtained above ground.must be obtained above ground.–– Water and nutrients from soil.Water and nutrients from soil.

Evolved Evolved underground root systemunderground root system for absorbing for absorbing minerals and water. minerals and water. See Fig. 7.2, p. 187See Fig. 7.2, p. 187..–– WaterWater--absorbing sections of roots generally not absorbing sections of roots generally not

covered with cuticle.covered with cuticle.–– Root hairsRoot hairs: fine, long extensions from root cells to : fine, long extensions from root cells to

maximize absorption surface of roots.maximize absorption surface of roots.–– Water acts as transport fluid, carrying nutrients from Water acts as transport fluid, carrying nutrients from

roots to leaves.roots to leaves.



Also evolved Also evolved aerial system of stems and aerial system of stems and leavesleaves for food production.for food production.–– LigninLignin: polymer embedded : polymer embedded

within cellulose matrix that within cellulose matrix that provides rigidity (support) provides rigidity (support) to trees and other vascular to trees and other vascular plants.plants.




PithPith is a light substance is a light substance that is found in that is found in vascular vascular plantsplants..–– Consists of soft, spongy Consists of soft, spongy

parenchymaparenchyma cells, and is cells, and is located in the center of located in the center of the the stemstem..

–– Encircled by a ring of Encircled by a ring of xylemxylem ((woodywoody tissue), tissue), and outside that, a ring and outside that, a ring of of phloemphloem ((barkbark tissue).tissue).

Pith

Transport Systems in PlantsTransport Systems in Plants

Water TransportWater Transport


Corn stem


XylemXylem:: hollow tubehollow tube--shaped cells that carry water shaped cells that carry water and minerals up from the roots (and minerals up from the roots (Fig. 7.4a, p. Fig. 7.4a, p. 189189 ).).Consists of two types of waterConsists of two types of water--conducting cells conducting cells plus strong weightplus strong weight--bearing fibers:bearing fibers:–– TracheidsTracheids: cells with pointed ends and thick walls : cells with pointed ends and thick walls

with pits that connect them to neighboring cells. with pits that connect them to neighboring cells. Water moves through the pits.Water moves through the pits.

–– Vessel elementsVessel elements: wider, shorter, thinner: wider, shorter, thinner--walled and walled and lessless--tapered than tapered than tracheidstracheids; ends are perforated or ; ends are perforated or missing altogether. Water flows freely through missing altogether. Water flows freely through openings.openings.

XylemXylem

XylemXylem


Evaporation in plants is great, e.g. a Evaporation in plants is great, e.g. a typical red maple may lose 2000 L of typical red maple may lose 2000 L of water on a humid day.water on a humid day.In trees and tall plants, water must be In trees and tall plants, water must be transported up great distances.transported up great distances.Plants donPlants don’’t have pumping systems for t have pumping systems for transporting water from roots to aerial transporting water from roots to aerial system of shoots and leaves.system of shoots and leaves.

XylemXylem


CohesionCohesion--tension hypothesistension hypothesis: explanation : explanation for water transport in plants; based on for water transport in plants; based on molecular properties of water and transpiration.molecular properties of water and transpiration.–– Roots exert pressure, but insufficient to account for Roots exert pressure, but insufficient to account for

rise of water in taller plants and trees.rise of water in taller plants and trees.

CohesionCohesion: tendency of water molecules to stick : tendency of water molecules to stick together because of weak hydrogen bonds.together because of weak hydrogen bonds.AdhesionAdhesion: water molecules are polar (slightly : water molecules are polar (slightly charged); thus, they form weak bonds with charged); thus, they form weak bonds with other charged molecules, e.g. glass.other charged molecules, e.g. glass.


Capillary actionCapillary action: the process by which water : the process by which water rises in a glass tube; brought about by cohesion rises in a glass tube; brought about by cohesion and adhesion.and adhesion.Water adheres to charged walls of glass tube; Water adheres to charged walls of glass tube; cohesion causes other water molecules to cohesion causes other water molecules to follow.follow.Walls of Walls of tracheidstracheids and vessel elements also have and vessel elements also have many charged groups which take water up by many charged groups which take water up by capillary action.capillary action.Process is not very rapid; height to which water Process is not very rapid; height to which water can rise is limited by:can rise is limited by:–– Diameter of tubeDiameter of tube–– GravityGravity


Water leaving plant by Water leaving plant by transpiration tugs on water tugs on water below it. This tugging is transmitted from one water below it. This tugging is transmitted from one water molecule to another molecule to another a long chain of water molecules a long chain of water molecules continually pulled through xylem from root to leaf.continually pulled through xylem from root to leaf.Water molecules in the xylem replaces water that leaves Water molecules in the xylem replaces water that leaves the the mesophyllmesophyll cells via the stomates.cells via the stomates.Less polar liquids would not be able to do this, as they Less polar liquids would not be able to do this, as they are less cohesive.are less cohesive.

Nutrient TransportNutrient TransportFig. 7.4b, p. 189Fig. 7.4b, p. 189

PhloemPhloem: system of elongated cells arranged : system of elongated cells arranged into tubes filled with streaming cytoplasm; into tubes filled with streaming cytoplasm; movement of organic materials is accomplished movement of organic materials is accomplished here by active transport.here by active transport.–– Consists of sieve tube members, Consists of sieve tube members, companion cellscompanion cells

and fibers.and fibers.–– Sieve TubesSieve Tubes: elongated cells with perforated ends : elongated cells with perforated ends

((sieve platessieve plates), resembling strainers (thus, the name ), resembling strainers (thus, the name ““sievesieve””) through which contents of cells mix.) through which contents of cells mix.

Sugars and amino acids move through phloem cells from Sugars and amino acids move through phloem cells from leaves to other parts of the plant.leaves to other parts of the plant.Rate of movement is greater than could be accounted for by Rate of movement is greater than could be accounted for by diffusion.diffusion.

PhloemPhloem

(Plasmodesmata)

PhloemPhloem

Nutrient TransportNutrient Transport

Nutrient TransportNutrient TransportPressurePressure--flow hypothesisflow hypothesis: water and dissolved sugars : water and dissolved sugars

move from areas of high pressure (move from areas of high pressure (sourcessources) to areas of ) to areas of low pressure (low pressure (sinkssinks).).SourcesSources: areas where sugars are produced or stored; : areas where sugars are produced or stored; generally, areas where energy is provided; cotyledons generally, areas where energy is provided; cotyledons and endosperm during germination; leaves during spring and endosperm during germination; leaves during spring and summer; some storage roots during springand summer; some storage roots during spring–– CotyledonCotyledon: single (: single (monocotsmonocots) or double () or double (dicotsdicots) seed leaf of a ) seed leaf of a

flowering plant embryo (flowering plant embryo (angiospermsangiosperms).).–– EndospermEndosperm: nutrient rich structure formed by the union of a : nutrient rich structure formed by the union of a

sperm cell and a large cell having two nuclei; double fertilizatsperm cell and a large cell having two nuclei; double fertilization ion is complete when a second sperm cell fertilizes the egg; the is complete when a second sperm cell fertilizes the egg; the endosperm provides nourishment to the developing embryo in endosperm provides nourishment to the developing embryo in seeds of angiosperms.seeds of angiosperms.

Nutrient TransportNutrient TransportPressurePressure--flow hypothesisflow hypothesis: water and : water and

dissolved sugars move from areas of high dissolved sugars move from areas of high pressure (sources) to areas of low pressure (sources) to areas of low pressure (sinks).pressure (sinks).SinksSinks: areas where water and sugars are : areas where water and sugars are used or food storage areasused or food storage areas–– Growing leaf budsGrowing leaf buds–– Root tipsRoot tips–– FlowersFlowers–– FruitsFruits–– SeedsSeeds

PressurePressure--flow hypothesisflow hypothesisSucrose is produced in a leaf by photosynthesis. Sucrose is produced in a leaf by photosynthesis. Then actively transported into sieve tubes from Then actively transported into sieve tubes from mesophyllmesophyll cells.cells.Companion cells produce a Companion cells produce a protein key in the transport protein key in the transport process.process.High [sucrose] draws water High [sucrose] draws water into the phloem cells, into the phloem cells, producing higher pressure.producing higher pressure.High pressure pushes sucrose High pressure pushes sucrose toward areas of lower pressure, toward areas of lower pressure, moving sucrose through sieve moving sucrose through sieve tubes, cell to cell, from source tubes, cell to cell, from source to sink.to sink.

PressurePressure--flow hypothesisflow hypothesisAt sink, active transport removes sucrose from At sink, active transport removes sucrose from phloem for use or storage.phloem for use or storage.As this occurs, water leaves As this occurs, water leaves phloem cells by osmosis, phloem cells by osmosis, mostly returning to xylem.mostly returning to xylem.Entire process depends Entire process depends upon uptake of water and upon uptake of water and sucrose by phloem cells at sucrose by phloem cells at source areas and active source areas and active removal of same materials removal of same materials from phloem cells by sink from phloem cells by sink tissues.tissues.

Nutrient Transport in PhloemNutrient Transport in Phloem

Pressure Flow HypothesisPressure Flow Hypothesis

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Chapter 21: Plant Structure & Function · Chapter 21: Plant Structure & Function ... for water transport in plants; based on molecular properties of water and transpiration. – Roots