AP Biology 2006-2007

Transport in Plants

AP Biology

§ Vascular tissue– Transports nutrients throughout a plant;

such transport may occur over long distances

Figure 36.1

AP Biology

Transport Overview§ Physical forces drive the transport of

materials in plants over a range of distances

§ Transport in vascular plants occurs on three scalesu Transport of water and solutes by individual

cells, such as root hairsu Short-distance transport of substances from

cell to cell at the levels of tissues and organsu Long-distance transport within xylem and

phloem at the level of the whole plant

AP Biology

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 other

parts 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

Figure 36.2

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

AP Biology

Transport in plants§ H2O & minerals

u transport in xylemu transpiration

§ evaporation, adhesion & cohesion§ negative pressure

§ Sugarsu transport in phloemu bulk flow

§ Calvin cycle in leaves loads sucrose into phloem§ positive pressure

§ Gas exchangeu photosynthesis

§ CO2 in; O2 out§ stomates

u respiration§ O2 in; CO2 out§ roots exchange gases within air spaces in soil

Why doesover-wateringkill a plant?

AP Biology

Ascent of xylem fluidTranspiration pull generated by leaf

AP Biology

Water & mineral absorption§ Water absorption from soil

u osmosis u aquaporins

§ Mineral absorptionu active transportu proton pumps

§ active transport of H+

H2O

root hair

aquaporin

proton pumps

AP Biology

Mineral absorption§ Proton pumps

u active transport of H+ ions out of cell§ chemiosmosis§ H+ gradient

u creates membranepotential§ difference in charge§ drives cation uptake

u creates gradient§ cotransport of other

solutes against theirgradient

AP Biology

Water flow through root§ Porous cell wall

u water can flow through cell wall route & not enter cells

u plant needs to force water into cellsCasparian strip

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Controlling the route of water in root§ Endodermis

u cell layer surrounding vascular cylinder of rootu lined with impermeable Casparian stripu forces fluid through selective cell membrane

§ filtered & forced into xylem cells

Aaaah…Structure–Function

yet again!

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Endodermis & Casparian strip

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Root anatomy

dicot monocot

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Mycorrhizae increase absorption§ Symbiotic relationship between fungi & plant

u symbiotic fungi greatly increases surface area for absorption of water & minerals

u increases volume of soil reached by plantu increases transport to host plant

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Mycorrhizae

AP Biology

Control of transpiration§ Balancing stomate function

u always a compromise between photosynthesis & transpiration§ leaf may transpire more than its weight in

water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

AP Biology

Rate of Transpiration§ High humidity slows down

transpiration, low humidity speeds it up§ Wind can reduce humidity near the

stomates and therefore increases transpiration

§ Increased light, inc. photosynthesis, inc water vapor, inc transpiration

§ Closing stomata stops transpiration

AP Biology

ChloroplastsEpidermal cell

NucleusGuard cell

Thickened innercell wall (rigid)

Stoma open Stoma closed

H2O

water moves into guard cells

H2O H2O H2O

H2O H2O

H2O

H2O

H2O H2O H2O H2O

Control of Stomates

K+

K+

K+

K+

K+ K+

K+ K+

K+ K+K+K+

water moves out of guard cells

§ Uptake of K+ ions by guard cellsu proton pumpsu water enters by

osmosisu guard cells

become turgid§ Loss of K+ ions

by guard cellsu water leaves by

osmosisu guard cells

become flaccid

AP Biology

Transport of sugars in phloem§ Loading of sucrose into phloem

u flow through cells via plasmodesmatau proton pumps

§ cotransport of sucrose into cells down proton gradient

AP BiologyOn a plant…

What’s a source…What’s a sink?

can flow 1m/hr

Pressure flow in phloem§ Mass flow hypothesis

u “source to sink” flow§ direction of transport in phloem is

dependent on plant’s needsu phloem loading

§ active transport of sucrose into phloem

§ increased sucrose concentration decreases H2O potential

u water flows in from xylem cells§ increase in pressure due to

increase in H2O causes flow