Water Movement Within a Plant AP Biology Unit 5

Review: Properties of Water Water is a polar molecule Water molecules can form a “column” –due to hydrogen bonds between them Cohesion = Water is attracted to other water molecules Adhesion = Water is attracted to other polar molecules   Hydrogen bond

Question… Why do plants wilt? When there is enough water  it flows into plant cells  cells are plump with water (turgid) If there isn’t enough water  it flows out of the cell  cells are limp (flaccid)

Movement Into Roots Water is absorbed into the roots through osmosis Osmosis = diffusion of water Osmosis is a passive process– no energy required Water potential (ψ) determines the direction that osmosis will occur

Water Potential Pure H 2 O has ψ = 0 Water potential is a combination of the pressure from solutes (ψ s )and the physical pressure (ψ p ). The more solutes there are in a solution, the more negative the water potential will be.

Water Potential Water always moves from areas of higher water potential (less solutes) to lower water potential (more solutes). Moves from less negative ψ to more negative ψ. No net water movement

Pathways of Water Apoplast Pathway = water travels between cells (outside of them) Symplast Pathway = water travels through cells (inside them to get from cell to cell)

Water Movement in Roots Water and ions are able to freely diffuse up to a point in the root cells To get into the stele (where the vascular tissue is located), the water and ions must pass through the cells of the endodermis (symplast pathway).

Casparian Strip Blocks the water from crossing the endodermis through apoplast pathway control what enters the vascular tissue Prevents water and solutes from “leaking” out into the soil

Water Transport in Plant Water is transported through the plant in the xylem Cells that make up the xylem: –Tracheids– found in all plants –Vessel Elements – found in some plants

Formation of xylem 1.Cell dies. 2.Cell contents disintegrate. 3.Water can move through these hollow cells with little resistance.

Capillary Action movement of H 2 O up a very narrow tube Does account for a little water movement up the xylem, but not much (about 40 cm) Image obtained without permission from

Transpiration Pull Accounts for most of the water movement up the xylem. As H 2 O evaporates from the stomata (=transpiration), water from the xylem moves into the leaf to take its place. Because of the hydrogen bonds between water molecules, water is “pulled” up the xylem.

Tension-Cohesion-Evaporation Model

Question… When you receive a bouquet of flowers, why is it important to cut them under water? Transpiration is still continuing, so if you cut it under water, it will ensure that water (not air) gets into the xylem. Air bubble in xylem would disrupt the column of water.

Transpiration Rate Factors that affect transpiration rate include: light, humidity, temperature. The plant might also close its stomata to limit transpiration We’ll look at this in more detail in the upcoming dry lab.

Stomata and Guard Cells Stoma is Greek for "mouth" (plural = stomata) Function of Stoma = to allow gas exchange with the outside environment (CO 2 and O 2 ) Guard cells regulate the opening of the stomata

Stomatal Opening and Closing When guard cells are full of water (plump)  stretch away from each other  stoma is open When guard cells don’t have much water in them (limp)  don’t pull away from each other  stoma is closed

Role of K + K + ions (potassium) control the opening of stoma by changing the water content in the guard cells. –K + in : H 2 0 follows by osmosis  stoma open –K + out: H 2 0 follows by osmosis  stoma closed

Question… What conditions favor stomatal opening? –Plentiful water –Moderate temperatures

Hormonal Control of Stomata Abscissic Acid (plant hormone) acts on guard cells to close stomata. –“stress” hormone in plants –Farmers can use it as an anti-transpirant (reduces water loss and the need for irrigation). –There are genetically engineered plants that have a mutant era gene—this makes them highly sensitive to abscissic acid.