Chapter 36:

Transport in Plants

Plants

Leaves roots may be 100m apart.

Question ?

How do plants move materials from one organ to the other ?

Levels of Plant Transport1. Cellular

2. Short Distance

3. Long Distance

3 Levels of Plant Transport A) Cellular Transport

The transport of solutes and water across cell membranes.

Types of transport:1. Passive Transport Diffusion and Osmosis. Requires no cellular

energy. Materials diffuse down

concentration gradients.

Problem: very slow Mechanisms

Transport Proteins Ex: Carrier Proteins

Selective Channels Potassium Channel

Found in most plant cell membranes.

Allow K+ but not Na+ to pass. Often “gated” to respond to

environmental stimuli.

B. Active TransportC. Water Transport

2. Active Transport Requires cell energy. Moves solutes against a concentration gradient.

Ex: Proton Pump (another example of Chemiosmosis) Uses ATP to move H+ out of cells. H+ creates a membrane potential. H+ allows cotransport.

Membrane Potentials Allow cations to moved into the cell. Ex: Ca+2, Mg+2

Cotransport Couples H+ with anions to move both into cell. Ex: NO3

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Summary

3. Water Transport Osmosis - water moves from high concentration to low

concentration. Water Potential

The potential energy of water to move from one location to another.

Abbreviated as Has two components:

Pressure potential:

Solute potential:

Problem Cell wall creates a pressure in the cells. Water potential must account for this

pressure. Pressure counteracts the tendency for water to

move into plant cells.

Bulk Flow The movement of water between two

locations due to pressure. Much faster than osmosis. Tension (negative pressure). May cause bulk flow against the diffusion

gradient.

Tension Is a very important force to "pull" water from

one location to another. Plant Vacuoles

Create Turgor Pressure against the cell wall. Affect water potential by controlling water

concentrations inside cells.

Tonoplast Name for the vacuole membrane. Has proton pumps. Comment – genetic modification of these

pumps gives plants salt tolerance.

Proton Pumps

Drives solutes inside the vacuole.

Lowers water potential ()inside the

vacuole. Result

Water moves into the vacuole. Vacuole swells. Turgor pressure increases.

Turgor Pressure Important for non-woody plant support. Wilting:

Loss of turgor pressure. Loss of water from cells.

FlaccidTurgid

Aquaporins Water specific facilitated diffusion transport

channels. Help water move more rapidly through lipid

bilayers. Short Distance Transport

1. Transmembrane route2. Symplast route3. Apoplast route

1. Transmembrane

Materials cross from cell to cell by crossing each cell's membranes and cell walls.

2. Symplast

The continuum of cytoplasm by plasmodesmata bridges between cells.

3. Apoplast Extracellular pathway

around and between cell walls.

Point Movement of materials can take place by all 3

routes.

Long Distance Transport Problem: diffusion is too slow for long

distances. Answer: tension and bulk flow methods.

Start - Roots Absorb water. Take up minerals.

Root Hairs Main site of absorption. Comment - older roots

have cork and are not very permeable to water.

Root Cortex

Very spongy. Apoplast route very

common. Problem

Can't control uptake of materials if the apoplast route is used.

Solution Endodermis with its

Casparian Strip.

Casparian Strip Waxy layer of suberin. Creates a barrier between the cortex and the

stele. Forces materials from apoplast into

endodermis symplast. Result

Plant can now control movement of materials into the stele.

Endodermis

Casparian Strip

Mycorrhizae

Symbiotic association of fungi with roots of plants.

Help with water and mineral absorption (replaces root hairs in some plants).

May also prevent toxins from entering the plant.

Mycorrhizae

Xylem Sap Solution of water and minerals loaded into the

xylem by the endodermis. Endodermis - also prevents back flow of

water and minerals out of the stele.

Xylem Sap Transport Methods1. Root Pressure2. Transpiration (Ts) Root Pressure

Root cells load minerals into xylem. Water potential () is lowered. Water flows into xylem.

Result Volume of water in xylem increases Xylem sap is pushed up the xylem tissues creating root

pressure.

Comments Root Pressure: limited way

to move xylem sap. Most apparent at night. Excess water may leave

plant through Guttation.

Transpiration (Ts) Evaporation of water from aerial plant parts. Major force to pull xylem sap up tall trees.

TCTM Theory Transpiration Cohesion Tension Mechanism

How does TCTM work? Water evaporates from leaves, especially

from the cell walls of the spongy mesophyll. Reason: water potential of the air is usually

much less than that of the cells.

As water evaporates: Cohesion: water molecules sticking together

by H bonds. Adhesion: water molecules sticking to other

materials (cell walls etc.). Result

The loss of water from the leaves creates “tension” or negative pressure between the air and the water in the plant.

Tension causes: Xylem sap to move to replace the water lost

from the mesophyll cells.

Xylem Sap Is “pulled” by the resulting tension all the

way down the plant to the roots and soil.

Summary

Xylem sap moves along a continual chain of water potential from: air leaf stem roots soil

Comments Tension is a negative pressure which causes a

decreased in the size of xylem cells. Xylem cells would collapse without

secondary cell walls.

Factors that Affect Transpiration Rate

1. Environmental

2. Plant StructuresMultiple Layer Epidermis

Stomatal Crypt

Environmental Factors1. Humidity

2. Temperature

3. Light

4. Soil Water Content

5. Wind

Plant Structure Factors1. Cuticle

2. Stomate Number

3. Hairs

Stomates Openings in the epidermis that allow water

and gas exchange. Controlled by Guard Cells. Control rate of Ts and Ps.

Guard Cells

Turgid: Swell - open stomata. Flaccid: Shrink - close stomata. Size of the cells is a result of turgor pressure

changes.

Turgid - Open Flaccid - Closed

Turgor Pressure of Guard cells

Controlled by K+ concentrations.

To Open Stomata:1. K+ enters the guard cells.2. Water potential lowered.3. Water enters guard cells.4. Turgor pressure increases.5. Guard cells swell and Stomata opens.

To Close Stomata:1. K+ leaves guard cells.

2. Water leaves guard cells.

3. Turgor pressure decreases.

4. Guard cells shrink and Stomata close.

K+ Movement Regulated by proton pumps and K+ channels. Controlled by:

Light (Blue) CO2 concentrations

Abscisic Acid (water stress)

Comment Plant must balance loss of water by

transpiration with CO2 uptake for Ps.

Adaptations for Balance C4 Ps CAM Ps

Phloem Transport

Moves sugars (food). Transported in live cells.

Ex: Sieve & Companion Cells

Source - Sink Transport Model for movement of phloem sap from a

Source to a Sink.

Source Sugar production site Ex: Ps

Starch breakdown in a storage area.

Sink Sugar uptake site. Ex: Growing areas

Storage areas Fruits and seeds

Comment The same organ can serve as a source or a

sink depending on the season.

Result Phloem transport can go in two directions

even in the same vascular bundle.

Xylem Transport: In Contrast to Phloem

Usually unidirectional. Endodermis prevents back flow. Dead cells.

Phloem Loading at the Source:1. Diffusion

2. Transfer Cells

3. Active Transport

Phloem Loading

Transfer Cells Modified cell with ingrowths of cell wall to

provide more surface area for sugar diffusion.

Result Sugar loaded into phloem.

Water potential () decreases.

Bulk flow is created.

Bulk Flow Movement of water into phloem. Pressure forces phloem sap to move toward

the sink.

At the Sink: Sugar is removed. Water potential is raised. Water moves out of phloem over to xylem.

Phloem: summary Source - builds pressure. Sink - reduces pressure. Pressure caused by:

Sugar content changes Water potential changes

Comment

Plants move materials without "moving" parts, unlike animals.

Summary Know various ways plants use to move

materials. Know how Ts works and the factors that

affect Ts. Know how phloem transport works.