Biology Presentation

Okan Atilan

Water Uptake in Plants

Root Hair Cells

They cover the part of the root that absorb water.

They increases the surface area and hence, speeds up water uptake.

When water is absorbed, it has to through the cortex and the endodermis, to reach the xylem.

Water Potential

Water moves from areas of higher water potential to areas of lower water potential.

The soil around roots has a high water potential because the amount of water in soil is high

Leaves have a low water potential because of constant evaporation of water by transpiration

Water Potential Gradient

The difference between water potentials create a water potential gradient

Water moves from roots to leaves, down the water potential gradient

Mineral ions are transported into the root hair cell by active transport

This lowers the water potential so more water moves in by osmosis.

Apoplast and Symplast Pathway

Water can travel from roots to xylem in two different paths

Apoplast pathway- goes through non-living cell walls. Water simply diffuse through them.

Symplast Pathway- goes through living cytoplasm of the cells. Plasmodesmata connects the cytoplasm of the neighboring cells.

Apoplast and Symplast Pathway

Mainly apoplast pathway is used because of its low resistance

Water can only travel up to endodermis cells by apoplast pathway, because Casparian strip blocks this pathway in endodermis

Water has to travel in symplast pathway after it reaches to Casparian strip

The advantage is, water pass through a cell membrane so the substances in it can be controlled

After passing endodermis, water moves into the xylem.

Transport of Water and Xylem

There are 3 ways in which water moves in xylem against the force of gravity

1. Cohesion-tension theory

Water evaporates from the leaves by transpiration. This creates a tension which pulls more water into leaf. Due to hydrogen bonds, water molecules stick together (cohesion). So when the water molecules at the top are pulled, the bottom ones follow and the whole column of water moves upward

The mineral ions are forced by endodermis into xylem. They transfer to xylem by active transport. A water potential gradient develops so more water is pushed into the xylem. This creates a pressure at the bottom end of the xylem and the molecules at the top are pushed. This pressure is weak and it is not strong enough to move water to the top of big plants by itself. But, in young plants, root pressure has a great impact on water transportation from roots to leaves.

2. Root Pressure:

It occurs when water moves up in a thin tube. In plants it occur because of the molecular attraction (adhesion) between water and the inside of the xylem tube.

All three methods are used to move water up through the xylem. None of them provide sufficient power to move water on their own.

3. Capillarity

Xylem

Xylem support the plant. In stems, xylem is found near the outside and prevents bending of the stem.

Water can travel through the cells is the xylem freely because these cells do not have end plates.

Transpiration

It is the loss of water as the form of water vapour from the leaves through stomata to the atmosphere.

Transpiration occurs because of the stomata found on the leaves which allow gas exchange between cells in the leaves and atmosphere in respiration and photosynthesis.

Transpiration

Water evaporates from cell walls and accumulates in the spaces found between the cells in the leaf.

When stomata are open, water vapour diffuses out of the stomata.

This is because of the diffusion gradient found between the inside of the and the atmosphere. There is much more water vapour in the atmosphere.

Potometer

Potometer

It is used to measure the rate of transpiration.

It measures the amount of water taken up by the plant.

It is assumed that the uptake of water is directly related to water loss by transpiration.

Factors Affecting the Rate of Transpiration There are four main factors which

affect the rate of transpiration;1. Light intensity- Light itself does not

affect evaporation, but in daylight the stomata are open. This allows the water vapour to diffuse out. In the dark, transpiration is reduced because the stomata are closed.

2. Temperature- When the temperature increases, the molecules gain more energy and start to diffuse out of the leaves faster.

3. Humidity- If the air is humid, it can accept little more from the plants. The diffusion gradient between the air and leaf reduced and hence the rate of transpiration.

4. Wind- In moving air, the water vapour will be swept away from the leaf as fast as it diffuses out. A diffusion gradient would be maintained and the rate of transpiration increases.

Xerophytic Plants

Plants that are adapted to live in dry conditions are called xerophytes.

They are very efficient at water uptake and they can store water.

They have special adaptations to reduce transpiration.

Cacti

They are adapted to desert conditions. Their leaf adaptation include, presence of a thick

waxy cuticle, few stomata, stomata that are opened at night and closed by day, leaf surface covered with hair.

They have a very extensive root system. Water is stored in leaves or stems. They have spines which protect the plant from

predators.