Uptake Of Water In A Plant Flashcards
When root hairs absorb water we’re does it pass through
Roots hairs absorb water by osmosis which then passes through the cortex, endodermis and pericycle to enter the root xylem.
Movement of water up a plant is due to 3 forces
Movement of water up a plant is due to transpiration pull, root pressure and capillarity.
Explain absorption of water by root hairs
Absorption of water by the root hairs
Soil water has a high water potential. The cell sap, because of its high concentration of solutes, has a low water potential. This causes a water potential gradient resulting in water entering the cytosol of the root hair, through the cell membrane by osmosis. It then enters the vacuole of the root hair cell. The root hair, having absorbed water, now has a higher water potential than the inner cortical cells of the root. This water potential gradient extends along the inner tissues to the xylem tissue.
Movement of water to the stele of the root is form two types
Major pathway
Minor pathway
Explain major pathway
Major pathway
Most of the water passes by diffusion, from the root hair cell along cell walls of the cortex to neighbouring intercellular air spaces, not passing through any cortical cells. The intercellular spaces closer to the stele have a lower water potential than those closer to the root hair. Water therefore moves along this water potential gradient, until it reaches the impermeable endodermis with its Casparian strip. Some of the cells of the endodermis are thin-walled (called passage cells) and allow water to pass through into the stele, by osmosis. Water then enters the root xylem.
Explain minor pathway
Minor pathway
Some water moves from one cortical cell to the next by osmosis. When water enters the root hair cell, the cell’s water potential is raised. The neighbouring cells have a lower water potential than the root hair cell: thus water passes by osmosis to neighbouring cells on its way to the stele. It encounters the endodermis with its Casparian strip. Some of the cells of the endodermis are thin-walled (passage cells) and allow water to pass through into the stele, by osmosis. Water then enters the xylem of the root.
Three forces are responsible for the upward movement of water from the root to leaves
Transpirational pull
Root pressure
Capillarity
Explain root pressure
The cell sap of a root hair has a higher concentration of solutes than the water surrounding soil particles. The cell sap therefore always has a lower water potential than the soil water as a result, water continually enters the root hair by osmosis. Water moves into and across the root tissues, into the xylem of the root and up the xylem of the stem. Root pressure is not a very strong force and is only responsible for moving water a short distance up the stem.
What do guttation
When humidity is high, liquid water may be lost through tiny pores called hydathodes, located on leaf margins at the end of some of the veins. The water appears as drops on the edges of leaves.
What causes guttation
Root pressure
In guttation what are the tiny pores called
Hydathodes
What are the conditions for guttation
High humidity
High soil water content
Very low transpiration rate
Explain capillarity
Capillarity
The ability of water to spontaneously rise up very narrow tubes is termed capillarity. The narrower the bore of a tube, the greater the height achieved by a rising column of water. Capillarity is the result of the adhesive forces between the tube and water molecules, and the cohesive forces between water molecules. Xylem vessels and tracheids have narrow lumen (bores), and are therefore similar to capillary tubes, enabling capillarity to play a minor role in the upward movement of water.
What is transpiration pull
Transpiration pull
When transpiration occurs water vapour escapes from the leaves via the stomata. Water is drawn from the neighbouring mesophyll cells, which in turn, receive water from the xylem tissue. A diffusion pressure gradient exists between the xylem tissue and the air outside the stomata. The water column in the xylem is continuous, extending down to the roots. This continuous water column is held together and pulled up by a combination of cohesive forces (attractive forces between water molecules), and adhesive forces (attractive forces between water and xylem tissue). As water vapour escapes from the stomata, pressure gradients cause it to be replaced by water drawn ultimately from the root system of the plant. This upward movement of water against gravity is known as transpiration pull. It is a powerful force and is the main force that enables water to travel from the roots to the leaves of the tallest trees.
