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Ch. 36 Warm-Up

Describe the process of how H₂O gets into the plant and up to the leaves.

Compare and contrast apoplastic flow to symplastic flow.

Explain the mass flow of materials in the phloem (source to sink).

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Ch. 36 Warm-Up

What is transpiration?

What are mycorrhizae?

What is the function of the Casparian strip?

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Chapter 36

Resource Acquisition and Transport in Vascular Plants

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What you need to know:

The role of passive transport, active transport, and cotransport in plant transport.
The role of diffusion, active transport, and bulk flow in the movement of water and nutrients in plants.
How the transpiration cohesion-tension mechanism explain water movement in plants.
How pressure flow explains translocation.

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What does a plant need?

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Review:

Selectively permeable membrane: osmosis, transport proteins, selective channels
Proton pump: active transport; uses E to pump H⁺ out of cell → proton gradient
Cotransport: couple H⁺ diffusion with sucrose transport
Aquaporin: transport protein which controls H₂O uptake/loss

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Solute transport across plant cell plasma membranes

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Osmosis

**Water potential (ψ): H₂O moves from high ψ → low ψ potential, solute conc. & pressure

Water potential equation: ψ = ψ_S + ψ_P
Solute potential (ψ_S) – osmotic potential
Pressure potential (ψ_P) – physical pressure on solution
Pure water: ψ_S = 0 Mpa
Ψ is always negative!
Turgor pressure = force on cell wall

Bulk flow: move H₂O in plant from regions of high → low pressure

** Review AP Bio Investigation 4

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Flaccid: limp (wilting)
Plasmolysis: shrink, pull away from cell wall (kills most plant cells) due to H₂O loss
Turgid: firm (healthy plant)

Turgid Plant Cell

Plasmolysis

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A watered impatiens plant regains its turgor.

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Evolution of Transport in Plants

Algal ancestors of land plants absorbed water, minerals, CO₂ directly from the surrounding water

The evolution of xylem and phloem made possible the long-distance transport in land plants

Adaptations represent compromises between enhancing photosynthesis and minimizing water loss

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Vascular Tissues: conduct molecules

Xylem	Phloem
Nonliving functional	Living functional
Xylem sap = H₂O & minerals	Phloem sap = sucrose, minerals, amino acids, hormones
	Source to sink (sugar made) to (sugar consumed/stored)

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Transport of H₂O and minerals into xylem:

Root epidermis → cortex → [Casparian Strip] → vascular cylinder → xylem tissue → shoot system

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At Root Epidermis

Root hairs: increase surface area of absorption at root tips
Mycorrhizae: mutualistic relationship between fungal hyphae + roots

Increase H₂O/mineral absorption

The white mycelium of the fungus ensheathes these roots of a pine tree.

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Long distance transport

Efficient long distance transport of fluid requires bulk flow, fluid movement driven by pressure
Efficient movement is possible because mature xylem (tracheids and vessel elements) have no cytoplasm, and phloem (sieve-tube elements) have few organelles in their cytoplasm

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Transport of H₂O

Water and minerals can travel through a plant by three routes:

Transmembrane route: out of one cell, across a cell wall, and into another cell

Symplastic route: via the continuum of cytosol

Apoplastic route: via the cell walls and extracellular spaces

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Transport pathways across Cortex:

Apoplast = materials travel between cells
Symplast = materials cross cell membrane, move through cytosol & plasmodesmata

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Entry into Vascular Cylinder:

Endodermis (inner layer of cortex) sealed by Casparian strip (waxy material)

Blocks passage of H₂O and minerals
All materials absorbed from roots enter xylem through selectively permeable membrane
Symplast entry only!

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Fig. 36-12a

Casparian strip

Plasma�membrane

Apoplastic�route

Symplastic�route

Root�hair

Epidermis

Cortex

Endodermis

Vessels�(xylem)

Stele�(vascular�cylinder)

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How does material move vertically (against gravity)?

Transpiration: loss of H₂O via evaporation from leaves into air

Root pressure (least important)

Diffusion into root pushes sap up

Cohesion-tension hypothesis
Transpiration provides pull
Cohesion of H₂O transmits pull from roots→shoots

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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

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Guttation: exudation of water droplets seen in morning (not dew), caused by root pressure

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Stomata regulate rate of transpiration

Stomata – pores in epidermis of leaves/stems, allow gas exchange and transpiration
Guard cells – open/close stoma by changing shape

Take up K⁺ → lower ψ → take up H₂O → pore opens
Lose K⁺→ lose H₂O → cells less bowed → pore closes

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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, and an internal “clock” in guard cells
All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

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Cells stimulated open by: light, loss of CO₂ in leaf, circadian rhythms (internal 24 hr clock)

Stomata closure: drought, high temperature, wind

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Effects of Transpiration on Wilting and Leaf Temperature�

If the lost water is not replaced by sufficient transport of water, the plant will wilt

Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

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SUGAR TRANSPORT IN PLANTS

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Sugar Transport

Translocation: transport of sugars into phloem by pressure flow
Source → Sink

Source = produce sugar (photosynthesis)
Sink = consume/store sugar (fruit, roots)

Via sieve-tube elements
Active transport of sucrose

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Fig. 36-19

Mesophyll cell

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion�(transfer) cell

Sieve-tube�element

High H⁺ concentration

Cotransporter

Proton�pump

Low H⁺ concentration

Key

Apoplast

Symplast

Mesophyll cell

Bundle-�sheath cell

Phloem�parenchyma cell

Sucrose

ATP

H⁺

S

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Bulk flow in a sieve tube

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Fig. 36-21

Sap�droplet

25 µm

Sieve-�tube�element

Stylet

Sap droplet

Aphid feeding

Stylet in sieve-tube�element

Separated stylet�exuding sap

EXPERIMENT

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Symplast is dynamic

Plasmodesmata allows movement of RNA & proteins between cells
Phloem can carry rapid, long-distance electrical signaling

Nerve-like function
Swift communication
Changes in gene expression, respiration, photosynthesis

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You should now be able to:

Describe how proton pumps function in transport of materials across membranes
Define the following terms: osmosis, water potential, flaccid, turgor pressure, turgid
Explain how aquaporins affect the rate of water transport across membranes
Describe three routes available for short-distance transport in plants

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Relate structure to function in sieve-tube cells, vessel cells, and tracheid cells
Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder
Define and explain guttation
Explain this statement: “The ascent of xylem sap is ultimately solar powered”

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Describe the role of stomata and discuss factors that might affect their density and behavior
Trace the path of phloem sap from sugar source to sugar sink; describe sugar loading and unloading
Compare and contrast water and sugar transport in plants.