Ch. 35 Plant Structure, Growth, and Development

&Ch. 36 Resource Acquisition and

Transport in Vascular Plants

Objectives:

LO 4.17 The student is able to analyze data to identify how molecular interactions affect structure and function.LO 4.18 The student is able to use representations and models to analyze how cooperative interactions within organisms promote efficiency in the use of energy and matter.

Plant Structure

• Roots – anchors plants, absorbs minerals/water, and stores carbohydrates

• Stems – raises/separates leaves

• Leaves – main photosynthetic organ

Plant Structure

• Leaf anatomy:– Stomata: pores for gas

exchange– Guard cells: flank

stomata to open/close it– Mesophyll:

photosynthesizing area– Epidermis: protective

layer

Nonvascular vs Vascular Plants

• Nonvascular plants have no way to transport food or water (Ex: mosses, algae)

• Vascular plants have a transportation system– xylem (moves water and

dissolved minerals upward from roots into the shoots)

– phloem (transports organic nutrients from where they are made to where they are needed)

http://thomson.fosterscience.com/Biology/Unit-ProtistsFungiPlants/XylemPhloem.jpg

36.2 Different Mechanisms for Transport over short/long distances

• Transport in vascular plants occurs on three scales:– Transport of water and solutes by individual cells, such as

root hairs– Short-distance transport of substances from cell to cell at

the levels of tissues and organs– Long-distance transport within xylem

and phloem at the level of the whole plant

Minerals

H2O

H2O

CO2 O2

Sugar

Light

CO2

O2

Transport at Cellular Level

Relies on selective permeability of membranes• Transport proteins

– Facilitated diffusion– Selective channels (K+ channels)

• Aquaporins—water-specific protein channels that facilitate water diffusion across plasma membrane

Transport at Cellular Level• Proton pumps

– create a hydrogen ion gradient that is a form of potential energy

– contribute to a voltage known as a membrane potential

CYTOPLASM

ATP

EXTRACELLULAR FLUID

Proton pumpgenerates mem-brane potentialand gradient.

Transport at Cellular Level• Plant cells use energy stored in the proton

gradient and membrane potential to drive the transport of many different solutes

CYTOPLASM EXTRACELLULAR FLUID

Cations ( , forexample) aredriven into the cellby the membranepotential.

Transport protein

Membrane potential and cation uptake

Transport at Cellular Level• In the mechanism called cotransport, a transport

protein couples the passage of one solute to the passage of another

Cell accumulatesanions ( , for example) by coupling their transport to; theinward diffusionof through a cotransporter.

Cotransport of anions

Water Potential - Review• (Ψ) Direction water will move due to differing factors. (Ψ = Ψs + Ψp)

– Movement from high Ψ → low Ψ– Measured in megapascal (Mpa)

• Ψs = solute potential. Adding solutes decreases potential; always negative.

• Ψp = pressure potential. Can be positive or negativeSolutes have a negative effect on by bindingwater molecules.

Pure water at equilibrium

H2O

Adding solutes to theright arm makes lowerthere, resulting in netmovement of water tothe right arm:

H2O

Pure water

Membrane Solutes

Positive pressure has a positive effect on by pushing water.

Pure water at equilibrium

H2O

H2O

Positivepressure

Applying positivepressure to the right armmakes higher there,resulting in net movementof water to the left arm:

Solutes and positivepressure have opposingeffects on watermovement.

Pure water at equilibrium

H2O

H2O

Positivepressure

Solutes

In this example, the effectof adding solutes isoffset by positivepressure, resulting in nonet movement of water:

Negative pressure(tension) has a negativeeffect on by pullingwater.

Pure water at equilibrium

H2O

H2O

Negativepressure

Applying negativepressure to the right armmakes lower there,resulting in net movementof water to the right arm:

Differences in Water Potential

Drive Water Transport in Plant Cells

= P + S

36.3 Transpiration

• Root tips absorb water and minerals via diffusion and active transport.

• These diffuse into the xylem.– This creates pressure potential in the roots, but

this is not greater than gravity.• The rest gets pulled up due to transpiration;

loss of water vapor from leaves.

Transportation of Xylem Sap (Water): Transpiration-Cohesion Theory

• Water evaporates from leaves through stomata—creates a low pressure at top of water column

• Water replaced by water from xylem—water in areas of high pressure move to areas of low pressure

Strong cohesion of water with the pressure difference helps to pull the entire water column up from roots to rest of plant

Transpiration

• Water evaporates from leaves through stomata, leaving a pocket of air in cells.

• Due to cohesive and adhesive properties, nearby water molecules in xylem move in and take up the air space.

• Entire xylem column of water moves up due to hydrogen bonding of water molecules.

© 2011 Pearson Education, Inc.

Animation: Transport in Roots Right-click slide / select “Play”

© 2011 Pearson Education, Inc.

Animation: Water TransportRight-click slide / select “Play”

© 2011 Pearson Education, Inc.

Animation: TranspirationRight-click slide / select “Play”

36.4 The Rate of Transpiration is Regulated by Stomata

• Stomata allow the transport of CO2 in and O2 out for photosynthesis.– However, water is also lost due to transpiration

• Stomata are flanked with guard cells which can open/close the pore.

• During the day, K+ moves into guard cells causing water to move in by osmosis.– This builds turgor and opens

the stomata.

• At night, K+ moves out causing water to move out.– This makes the cells flaccid

and opens stomata.

Radially orientedcellulose microfibrils

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal openingand closing (surface view)

(b) Role of potassium in stomatal opening and closing

K

H2OH2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Stimuli for Stomatal Opening and Closing

• Generally, stomata open during the day and close at night to minimize water loss

• Stomatal opening at dawn is triggered by– Light– CO2 depletion– An internal “clock” in guard cells

• All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

• Drought, high temperature, and wind can cause stomata to close during the daytime

• The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata

Adaptations That Reduce Evaporative Water Loss

36.5 Sugars Are Transported From Sources to Sinks Via Phloem (Translocation)

• Phloem sap is an aqueous solution that is high in sucrose

• A sugar source is an organ that is a net producer of sugar, such as mature leaves

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

Sugars are actively pumped via an H+ co-transporter into a sieve-tube

Source• Sugars are made in photosynthetic

cells and pumped by active transport into sieve tubes.

• Concentration of dissolved substances increases in the sieve tube and water flows in by osmosis

• Pressure builds up at the source end of the sieve tube

Sink • Sugars are pumped out • Water leaves the sieve tube by

osmosis• Pressure drops at the sink end of

the sieve tube

Difference in pressure causes sugars to move from source to sink

Loading of sugar

Uptake of water

Unloading of sugar

Water recycled

Source cell(leaf)Vessel

(xylem)

Sieve tube

(phloem)

SucroseH2O

H2O

H2OSucrose

Sink cell(storageroot)

Bu

lk f

low

by

neg

ativ

e p

ress

ure

Bu

lk f

low

by

po

siti

ve p

ress

ure

2

1

3

4

2

1

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