CHAPTER 36. TRANSPORT IN PLANTS. Overview of Transport in a Vascular Plant. TERMS TO BE REVIEWED. 1. passive transport 10. chemiosmosis 2. transport proteins 11. proton pump 3. active transport 12. membrane potential 4. cotransport 13. turgor pressure
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TRANSPORT IN PLANTS
1. passive transport 10. chemiosmosis
2. transport proteins 11. proton pump
3. active transport 12. membrane potential
4. cotransport 13. turgor pressure
5. water potential 14. plasmolysis
7. solute concentration
1. Most water and mineral absorption occurs in the cells at the tips of the roots (Cellular Level).
2. Root hairs are modified epidermal cells that are specialized for water absorption.
3. Soil solution flows through the hydrophilic walls of epidermal cells and travels along the cell walls and the intercellular spaces into the root cortex
4. Movement of soil solution into the cell involves: osmosis, diffusion, active transport, proton pumps, aquaporins, water potential, cotransport, transport proteins
A. Transport is also regulated by the compartmental structure of plant cells
B. The plasma membrane directly controls the traffic of molecules into and out of the protoplast
C. The third major compartment in most mature plant cells is the vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume
D. In most plant tissues, the cell wall and cytosol are continuous from cell to cell
E. Water and minerals can travel through a plant root by three routes: (Short-Distance Transport)
F. Long-Distance Transport at the Whole Plant Level
Epidermis cortex endodermis stele (xylem)
2. Mineral ions enter epidermal cells by diffusion and active transport using carrier proteins
3. Movement usually a combination of apoplasticand symplasticroutes
4. Only minerals and water using the symplastic route move directly into xylem
5. Minerals and water using apoplastic route are blocked at the endodermis by the Casparian strip and must enter an endodermal cell to move into xylem
6. Mycorrhiza hyphae absorb water and selected minerals and can enable older regions of the roots to supply water and minerals to the plants.
A. Ascent of xylem sap depends on:
1. transpiration-the loss of water vapor from leaves and other aerial parts of the plant
2. physical properties of water-cohesion and adhesion
B. Xylem sap (composed of water and minerals) flows upward against gravity
C. Xylem vessels are close to each leaf cell
D. Water must move upward to replace that lost by transpiration
E. Pushing Xylem Sap: Root Pressure
3. More water entering leaves than is transpired can result in guttation (discharge of water droplets at the leaf margin)
1. Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata by transpiration.
2. Transpiration produces negative pressure in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf
4. Cavitation—formation of water vapor pockets in xylem that breaks the chain of water molecules and the pull is stopped
5. Ascent of xylem sap is ultimately solar powered.
1. About 95% of water taken in is lost by transpiration through the stomata
2. Amount of water lost by a leaf depends on the number of stomata and the average size of the opening
3. Guard cells, by controlling the size of stomata, help conserve water
4. Benefits of transpiration:
a. Assists in mineral transfer from roots to shoots
b. Reduces leaf temperatures
5. If transpiration exceeds delivery of water by xylem, plant wilts.
6. Rate of transpiration is greatest on a sunny, warm, dry, and windy day
7. Stomata are more concentrated on bottom of leaf away from the sun to reduce evaporation
8. Waxy cuticle also prevents water loss
B. How Stomata Open and Close
1. Guard Cells control stomatal diameter by changing shape.
2. When guard cells take in water, they become turgid and the gap between cells increases.
3. When guard cells lose water, they become flaccid and the gap between cells decreases.
4. Changes in turgor pressure results primarily from the reversible uptake and loss of K+ by guard cells
5. Generally, stomata are opened during the day and closed at night.
7. Stoma close during daytime because of:
1. Xerophytes—plants adapted to arid climates
2. Modifications of xerophytes:
1. Translocation—transport of organic products of PS by phloem throughout the plant
-in angiosperms it involves sieve-tube members
2. Phloemsap—aqueous solution that is mostly sucrose
3. Sugarsource—an organ that is a net producer of sugar such as mature leaves
4. Sugarsink—an organ that is a net consumer or storer of sugar, such as a tuber or bulb
4. Can involve:
6. Water follows by osmosis.
C. Pressure Flow (Bulk Flow) of Phloem Sap in Angiosperms
1. Phloem sap moves by bulk flow driven by positive pressure (pressure flow)
2. Higher levels of sugar at the source lowers the water potential and causes water to flow into the tube
3. Xylem recycles the water from sink to source.