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 Transport 1. 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: 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 Transport C. 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-

Summary

3. Water Transport y = yr + yp 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 y Has two components: Pressure potential: yr Solute potential: yp y = yr + yp

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 (yp ) 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.

Turgid Flaccid

Short Distance Transport Aquaporins Water specific facilitated diffusion transport channels. Help water move more rapidly through lipid bilayers. 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.

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.

Casparian Strip Endodermis

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 Methods 1. Root Pressure 2. Transpiration (Ts) Root Pressure Root cells load minerals into xylem. Water potential (yp) 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 Structures Multiple Layer Epidermis Stomatal Crypt

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.

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 (yp) 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.