Chapter 4.2 - Regulation of Water Balance in Vascular Plants

transpiration

when a plant looses water through the leaves - stomata

water loss in plants

water is lost by evaporation from the leaves and stems of a plant. This loss is mostly through stomata and is called transpiration

regulatory mechanisms

are active responses by plants that are induced by changes in water balance

structural adaptations

heritable physical traits that have evolved in plant populations and equip plants to survive and reproduce under their particular environmental conditions

lateral roots

can have multiple levels of brancing and are usually restricted to the shallow levels of soil

fibrous roots

systems provide a very large surface area for water absorption

tap root systems

can penerate the soil to reach water at deeper levels

root hairs

The entry point of water in plants is through the root hairs. Root hairs greatly increase the surface area available for absorption of soil water.

stomata

pores in a leaf, mostly on the underside of a leaf through which oxygen and carbon dioxide can move

water in

Absorption of water by root hairs mainly occurs passively, through osmosis and facilitated diffusion, with neither process requiring an input of energy.

Osmosis and facilitated diffusion — can only occur in the presence of a water potential gradient between the soil water (higher potential) and the root cell water (lower potential).

water out

at leaves - through transpiration

transpiration stream

continous flow of water through the plant

how do plants regulate water losss

by specialised gaurd cells each side of the stomata, which open or close the pore

stomata open

gas exchange and transpiration rate increase

stomata closed

gas exchange and transpiration rates decrease

Processes that assits the transport of water from the roots of the plant upward

1. Transpiration pull

2. Cohesion - tension

3. Root pressure

Transpiration pull

when water evaporates from the leaves of a plant, water is pulled up to replace what was lost.

Cohesion tension

water molecules cling together as they are pulled through the plant and adhere to the walls of the xylem. the upward pull on the cohesion sap creates a tension

Root pressure

A weak 'push' effect for the water's upward movement through the plant

Structure of stomata

Stomata is bounded by two gaurd cells, which together regulate the entry and exit of gases (including water vapour). They allow the plant to prevent excessive water loss from its leaves.

How do gases move in and out of the plant

the gases move in and out of the plant and air spaces through diffusion

what gases pass through open stomata

water, carbon dixoide and oxygen

when is the stomata opened

when conditions favour having the stomata open (high carbon dioxide levels, high sunlight or high water levels), ions move into the gaurd cells.

water then follows these ions by osmosis into the gaurd cells. this results in the stomata opening.

when is the stomata closed

when water levels are low to prevent loss through transpiration.

water and ions move out of the gaurd cells, causing the gaurd cells to become flaccid. This results in the stomata closing.

explain water through plant diagram

1. water enters the root hairs by osmosis

2. water passes acorss the root, from cell to cell by osmosis. It also seeps between the cells.

3. Water is dawn up the xylem vessels because transpiration is constantly removing water from the top of them

stomata open

gaurd cells - turgid

daylight - light

co2 - high

soil water - high

stomata closed

gaurd cells - turgrid

daylight - dark

co2 - low

soil water - low

How do plants in the desert get water

Desert plants have sturctural adapatation both above ground and below ground which allows them to suvive in the warm weather.

They store water in special tubers or bulbs

They don't have leaves to prevent water loss through transpiration

They have prickles and spikes on their stems to protect against thirsty animals, like cactuses.

Some have long vertrical roots growing deep into the soil to reach the water table, while some have roots growing horizontally in the top layers of the soil to collect surface water.

Some have a waxy layer covering fleshy leaves to help store water, like aloe vera plants.