Question ?

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

Levels of Plant Transport

1. Cellular

2. Short Distance

3. Long Distance

Cellular Transport

The transport of solutes and water across cell membranes.

Types of transport:

1. Passive Transport

2. Active Transport

3. Water Transport

1. Passive Transport

Diffusion and Osmosis. Requires no cellular energy. Materials diffuse down

concentration gradients.

Problems

Usually very slow. How can diffusion be

assisted? Transport Proteins

Ex. K+ channel

Potassium Channel

Found in most plant cell membranes.

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

environmental stimuli (see cell signaling)

2. Active Transport

Requires cell energy. Moves solutes against a

concentration gradient. Ex: Proton Pumps

Proton Pump

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

Allow anions to move by co-transport.

Ex: NO3

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

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.

Water Potential

Has two components: Pressure potential:

Solute potential:

Comment

See the Ts lab handout for more on water potential.

Bulk Flow

The movement of water between two locations due to pressure or tension.

Bulk Flow

Much faster than osmosis. Tension (negative pressure)

pulls water from place to place.

May cause bulk flow against the diffusion gradient.

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.

Flaccid Turgid

Aquaporins

Water specific facilitated diffusion transport channels.

Help water move more rapidly through lipid bilayers.

Aquaporins with GFP

Short Distance Transport

1. Transmembrane route

2. Symplast route

3. 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.

Long Distance Transport

Problem: diffusion is too slow for long distances.

Answer: tension and bulk flow methods.

Root Hairs

Main site of absorption of water and minerals.

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.

Endodermis

Casparian Strip

Result

Plant can now control movement of materials into the stele.

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 Methods

1. Root Pressure

2. 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.

Ts Summary

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

Factors that Affect Transpiration Rate

1. Environmental

2. Plant Structures

Multiple Layer Epidermis

Stomatal Crypt

Homework

Read – Chapter 36, 39 Chapter 36 – Mon. 4/16 Test 2 – next week – Chapters

29, 30, 35, 36. A few questions may come from 37, 38 and possible 39.

Environmental Factors

1. Humidity

2. Temperature

3. Light

4. Soil Water Content

5. Wind

Plant Structure Factors

1. 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.

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.

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.