Transport in Plants. Transport in plants. Transport across Membranes. Water potential 10/9/2016

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Transport in Plants Transport in plants How is a plant able

Transport across Membranes Diffusion: Simple and facilitated.

Water potential Water potential: the tendency of water to

Water potential of a cell = turgor pressure + solute

Water potential symbol: Ѱ Turgor pressure: pressure on plant

1 Transport in Plants Transport in plants How is a plant able to move water and nutrients from roots to the rest of the plant body? Especially tall trees? Sequoia can be over 300 feet tall! Transport across Membranes Diffusion: Simple and facilitated. Osmosis Active Transport: pumps. Water potential Water potential: the tendency of water to leave one place in favor of another. Water potential of a cell = turgor pressure + solute concentration. Water potential symbol: Ѱ Turgor pressure: pressure on plant cells due to the intake of water. Water moves by osmosis to an area of greater solute concentration. Increasing turgor pressure will slow the movement of water into a plant cell. Water potential will affect flow of water in a plant. 1

Copyright The McGraw-Hill Companies, Inc.

Water potential: the tendency of water to leave one place in favor

Water always moves from an area of high water pressure toward an

In plants, typically there is high pressure at the roots and lower

Soil Plant Air Symporters contribute to the yw gradient that

Soil Cytosol H + Mineral ions Water enters plant through roots.

0-100 ψ w Water potential (MPa) 8 Transport in xylem vessels

What pulls this solution up the plant (or gigantic tree?

2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Water potential: the tendency of water to leave one place in favor of another. Water always moves from an area of high water pressure toward an area of lower water pressure. In plants, typically there is high pressure at the roots and lower toward the top of the stem. Soil Plant Air Symporters contribute to the yw gradient that determines the directional flow of water. Soil Cytosol H + Mineral ions Water enters plant through roots. H 2O Soil Symporter Water ψ w Water potential (MPa) 8 Transport in xylem vessels Solutes and water enter the xylem vessels from the roots. What pulls this solution up the plant (or gigantic tree?) Transport in xylem vessels Water and nutrients (inorganic) solution is called xylem sap. Some force must move xylem sap up the tree against gravity. Forces: Transpiration Root pressure Cohesion/adhesion (capillary effect) Capillary action Due to hydrogen bonding, water molecules stick to each other (cohesion) and to the walls of the vessels (adhesion). Bonds pull up the side of the vessel against gravity, drawing up water. Stronger pull in narrower tubes more adhesion! 2

Transpiration Transpiration: Loss of water from the leaves (through stomata) and stems by

The column of water moves upward as water evaporates off the top.

Rate of Transpiration Rate of water movementin a plant depends upon the rate of transpiration

movement through the plant. Some factors that increasethe rate of transpiration: Dry air.

Root pressure Root pressure: Root cells are actively pumping ions into the xylem.

the higher solute. This pushes xylem sap upwards.

in the roots. When transpiration from leaves is low or absent at night.

3 Transpiration Transpiration: Loss of water from the leaves (through stomata) and stems by evaporation. This is a pullingforce. The column of water moves upward as water evaporates off the top. Constant transpiration creates a gradient of water potential. Rate of Transpiration Rate of water movementin a plant depends upon the rate of transpiration (RT). Anything that increases the rate of transpiration will increase the rate of water movement through the plant. Some factors that increasethe rate of transpiration: Dry air. Blowing wind. Higher temperatures. Anything that causes stomata to open (ex: light). Root pressure Root pressure: Root cells are actively pumping ions into the xylem. These ions accumulate in the xylem causing water (by osmosis) to flow into the xylem towards the higher solute. This pushes xylem sap upwards. Xylem Transport: Root pressure Root pressureis caused by the continuous accumulation of ions in the roots. When transpiration from leaves is low or absent at night. Causes water to move into plant and up the xylem despite the absence of transpiration. Guttation (production of dew) is loss of water from leaves when root pressure is high. Root pressure alone, however, is insufficient to explain xylem transport. Transpirationprovides the main force. Capillary action contributes. 18 3

Transport in phloem Most carbohydrates produced in leaves are distributed through phloem to rest of plant.

Variety of sugars, amino acids, organic acids, proteins, and ions. This solution is called phloem sap.

A sugar sourceis a plant organ that is a net producer of sugar. Leaves are the primary source.

A tuber or bulb is a storage organ. Transport in phloem: pressure-flow hypothesis. Pressure-Flow Hypothesis 1.

4 Transport in phloem Most carbohydrates produced in leaves are distributed through phloem to rest of plant. Main component: sucrose (disaccharide sugar = two glucose molecules). Also transports hormones, mrna, and other molecules. Variety of sugars, amino acids, organic acids, proteins, and ions. This solution is called phloem sap. Transport in phloem Sugars are produced by photosynthesis in leaves an then transported to the rest of the plant body. A sugar sourceis a plant organ that is a net producer of sugar. Leaves are the primary source. A sugar sinkis an organ that is a net consumer or accumulator of sugar (storage). Growing roots, buds, stems, and fruits are sugar sinks. A tuber or bulb is a storage organ. Transport in phloem: pressure-flow hypothesis. Pressure-Flow Hypothesis 1. Phloem cells at the sugar source actively transport (pump) sugars into the phloem sieve tubes. 2. This creates a higher sugar solution in the tube. 3. Water moves in through osmosis from xylem. 4. The influx of water increases pressure in the phloem tube and pushes the solution. Pressure-Flow Hypothesis 5. Plant cells at the sugar sink remove sugars from the phloem tube by active transport (pumps). 6. Lower sugar solution in the phloem tube. 7. Water leaves the phloem tube by osmosis. 8. Inside the tube pressure decreasesand movement slows. 4

26 Absorption through roots Most of the water enters the plant through the root system; especially the part of the root covered in

Root hairs increase the surface area of a root, which allows a much greater rate of absorption.

Solution must pass through the plasma membrane of a root dermal cell in order to enter the plant.

Routes types There are two basic types of routes water can take entering the root: Extracellular and Intracellular.

Symplast route and Transmembrane route. Extracellular route: outside the cells (through or between walls).

Casparian strip: continuous waxy barrier separating the ground tissue from the vascular tissue. Wraps the veins (xylem and phloem).

5 26 Absorption through roots Most of the water enters the plant through the root system; especially the part of the root covered in root hairs. Some oxygen also enters into roots. Root hairs are extensions of root epidermal cells. Root hairs increase the surface area of a root, which allows a much greater rate of absorption. Absorption in roots All nutrients enter a plant root in solution. This means the nutrients are dissolved in water. They are solutes. Solution must pass through the plasma membrane of a root dermal cell in order to enter the plant. Plasma membrane is selectively permeable. Passive and Active transport. Especially osmosis. Routes types There are two basic types of routes water can take entering the root: Extracellular and Intracellular. Intracellular routes: inside the cells. Moves through plasmodesmata channels and then enters the xylem. Symplast route and Transmembrane route. Extracellular route: outside the cells (through or between walls). Stopped by the Casparian strip and forced into the cell before reaching the xylem. Apoplast route. Casparian strip: continuous waxy barrier separating the ground tissue from the vascular tissue. Wraps the veins (xylem and phloem). SpecificTransport Routes Three specific transport routes exist through cells. 1. Apoplast route movement through the cell walls and the space between cells. (extracellular) 2. Symplast route cytoplasm continuum between cells connected by plasmodesmata. (intracellular) 3. Transmembrane route membrane transport between cells and across the membranes of vacuoles within cells. (intracellular) 5

6 Note: Solutes often will take routes that are a combinationof these! However, no one gets past the selectively permeable membrane regardless of which route or combination a solute takes. Casparian strip Regulates the passage of minerals (ions) into the xylem by blocking access via cell walls and requiring all minerals to cross a selectively permeable membrane. No sneaking past the check point! Aphids love phloem sap! 6

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AP Biology Chapter 36

AP Biology Chapter 36 Chapter 36 Chapter 36 Transport in Plants 2006-2007 Transport in plants - Overview H2O & minerals transport in xylem transpiration evaporation, adhesion & cohesion negative pressure Sugars transport in