Biology F221 transport in plants

Biology F221

Transport in Plants

STRUCTURE AND FUNCTION OF XYLEM VESSELS, SIEVE TUBE ELEMENTS AND COMPANION CELLS;

Xylem vessels

Long, thick walls of dead cells strengthened and waterproofed by lignin.

The contents break down leaving a long empty column.

The lignin forms patters which prevents them becoming too rigid and gives flexibility.

Pits are left which allow water to leave the vessel.

Sieve Tube elements(Phloem) 

Contain very little cytoplasm and no nucleus.

They are lined up end to end to form a tube in which sugars (usually sucrose) are transported.

There are sieve plates which are perforated.

Sieve tubes have very thin walls and are five or six sided.

Companion cells(Phloem)

These are connected to the sieve tubes.

They have a dense cytoplasm, a large nucleus and many mitochondria to produce ATP for active processes.

They use ATP as a source of energy to load sucrose into the phloem.

There are plasmodesmata which are gaps in the cell walls allowing communication and flow of minerals between the cells.

DEFINE THE TERM TRANSPIRATION;

The loss of water vapour from the aerial parts of a plant due to evaporation.

EXPLAIN WHY TRANSPIRATION IS A CONSEQUENCE OF GASEOUS EXCHANGE; 

For the exchange of gases to occur in a leaf, the stomata of plants must be open. This is an easy route by which water can be lost.

To reduce this, plants have many structural and behavioural adaptations

A waxy cuticle waterproofs the leaf preventing water loss through the epidermis.

The stomata are often on the underside of leaves, to reduce evaporation due to direct heating.

Most stomata close at night- there is no light, so no photosynthesis can occur, so no need for gaseous exchange.

Deciduous plants lose their leaves in winter when temperatures are too low for photosynthesis, and the ground may be frozen, so less water is available, meaning that plants have to conserve water.

FACTORS THAT AFFECT TRANSPIRATION RATE;

·         Number of leaves

More leaves = large surface area which water can be lost from

·         Number, size and position of stomata

If leaves have many, large stomata water vapour is lost more quickly

If the stomata are on the lower surface, water loss is slower

·         Presence of cuticle

A waxy cuticle prevents water loss from the leaf surface

·         Light

In light, the stomata open to allow gaseous exchange for photosynthesis

·         Temperature

       Higher temperature will increase the rate of water loss

       Increase the rate of evaporation

       Increase the rate of diffusion as the water molecules due to more kinetic energy

       Decrease the relative water vapour potential in the air, causing the rapid diffusion of  nnnmolecules out of the leaf

·         Relative humidity

       Higher relative humidity in the air will decrease the rate of water loss.

       There will be a smaller water potential gradient between the air spaces in the leaf…