Plant biology

This section contains information on the movement of water through plants and gas exchange in leaves.

Plants require water as a reactant for photosynthesis, to maintain cell turgidity (for support), and also to transport dissolved minerals into the roots. Root systems are adapted for absorption of substances by having root hair cells in the epidermis to increase the surface area available. Once water molecules have passed into the root cells, there are two pathways which they may follow to the transport system: the apoplast or symplast pathway (detail of the vacuolar pathway is not needed).

The apoplast pathway

Most water entering the roots of a plant moves along this pathway. It follows the cell walls and the small spaces between cells before reaching the Casparian strip - a layer that surrounds the cells of the endodermis and forces water to pass through the cytoplasm of the cells.

The symplast pathway

A much smaller proportion of water that enters the roots moves along this pathway.It moves through the cytoplasm of the root cells. The cell membranes can be used to regulate the movement of water molecules and dissolved mineral ions.

Transpiration

The process of transpiration is simply the evaporation of water from any part of the plant exposed to the air. The majority of water lost by evaporation is lost from the underside of the leaves through small pores called stomata, although some is also lost through the stem. The pathway taken by water through the plant is called the transpiration stream. It is necessary for the transport of water and minerals, and also to help to prevent the plant from overheating.

There are four factors which affect the rate of transpiration. The first three all affect the water potential gradient (the concentration gradient of water) between the inside and outside of the leaf. The larger the difference in water potential, the faster the loss of water.

Humidity: the more humid the air outside the leaf, the less water will be lost
Temperature: higher temperatures increase the rate of evaporation, and also increase the amount of water vapour that the air can hold
Wind speed: air movement carries water vapour away from the stomata, increasing the water potential gradient
Light: the higher the light intensity the more stomata are open for gas exchange, increasing the rate of water loss

Cohesion tension theory of transpiration

Water moves through the xylem vessels of plants. These are made up of dead cells which are waterproof and hollow. There are three forces that act together to pull the water up through the xylem: cohesion, tension and adhesion. When water evaporates from the inside of the leaf, it is replaced by water from the xylem vessels by osmosis. Cohesion is a force that sticks water molecules together by hydrogen bonds, and so pulls the whole column of water up. Evaporation drives the upwards pull on the water column while gravity is exerting a downwards force on it - this results in tension. Adhesion is hydrogen bonding between the walls of the xylem vessel and the water molecules, and prevents the column of water breaking.

The role of phloem

The phloem vessels are the other transport system in plants. They move the sugars produced by photosynthesis and other inorganic compounds by a process called translocation. Transport in the phloem vessels can be in both directions at once. Phloem tissue contains sieve tubes, which are another group of hollow cells. Unlike xylem vessels, these are living tissue and have cells next to them known as companion cells. These actively transport sugars into the phloem vessels, which causes water molecules to move into the tubes by osmosis.

Gas exchange in the leaves

The main function of the stomata is to allow gas exchange to take place. Oxygen is needed for respiration, and carbon dioxide is needed for photosynthesis. These cannot diffuse into the leaf because the cuticle (a waxy layer that limits water loss) is impermeable to gases.

Most plants will open their stomata when the light intensity is high enough for the rate of photosynthesis to be high. The problem with this is that water vapour diffuses out through these pores.

Quiz

Google Sites

Report abuse