Biology - Transport in Plants

Transpiration and water loss are an unavoidable consequence of photosynthesis. Although stomata are needed for the exchange of gases during photosynthesis. Although stomata are needed for the exchange of gases during photosynthesis, they also water molecules to pass out of the leaf. But, the leaf is adapted to be able to reduce water loss:

• The number, position, size and distribution of stomata vary between plants, depending on their environment (which affects the amount of water they need).
• The turgidity of guard cells changes in relation to the light intensity and availability of water, in order to alter the size of the stomata openings.

The xylem and phloem from a continuous system of tubes from roots to leaves, called vascular bundles.

• Xylem transports water and soluble mineral salts from the roots to the leaves (transpiration).
• Phloem allows the movement of food substances (sugars) around the plant  (translocation), up and down the stems to growing tissues and storage tissues.

Xylem vessels are made from dead plant plant cells. They have a hollow lumen. The cellulose cell walls are thickened with a waterproof substance. Phloem cells are long columns of living cells.

Root hairs have an enormous surface area for absorbing water and so increase the plants ability to take up water.

Transpiration is the diffusion and evaporation of water from inside a leaf. It causes water to be moved up xylem vessels and provides plants with water for cooling, photosynthesis and support, and brings minerals to the plant.

The  transpiration stream is powered by the evaporation of water from the leaf:

• Water evaporates from internal leaf cells through the stomata.
• Water passes by osmosis from the xylem vessels to leaf cells, which pull the thread of water in that vessel upwards by a very small amount.
• Water entres the xylem from the root tissues, to replace water which has moved upwards.
• Water enters root hair cells by osmosis to replace to replace water that has enters the xylem.

The rate of transpiration can be affected by:

• Light - more light increases the rate of photosynthesis and transpiration.
• Air movement (wind) - as the movement of the air increases, transpiration increases.
• Temperature - heat increases the temperature and the rate of photosynthesis and transpiration.
• Humidity - low humidity increases the rate of transpiration.

A leafy shoot's rate of transpiration can be measured using a mass potometer.

1. The plants roots are submerged in a sealed bag of water and placed in a beaker.

2. The beaker is placed on  digital balance.

3. Readings are then taken to see how much water lost by the plant during transpiration.

4. The conditions, e.g. light, temperature, can be changed to see how this affects water       loss.

Healthy plants need to balance the amount of water they take in and lose:

1. Water is absorbed by thr plant by the root hair cells, which have a larger surface area to take in water.

2. The water then diffuses though the plant up to the leaves.

3. When it reaches the leaves it can be lost through transpiration (evapouration).

Two adaptations reduce the rate at which water is lost by leaves:

• A way cuticle on the surface of the leaf.
• Having the majority of the stomata on the lower surface of the leaf.

Transpiration and water loss are an unavoidable consequence of photosynthesis. Although stomata are needed for the exchange of gases during photosynthesis. Although stomata are needed for the exchange of gases during photosynthesis, they also water molecules to pass out of the leaf. But, the leaf is adapted to be able to reduce water loss:

• The number, position, size and distribution of stomata vary between plants, depending on their environment (which affects the amount of water they need).
• The turgidity of guard cells changes in relation to the light intensity and availability of water, in order to alter the size of the stomata openings.

During photosynthesis, the guard cells become turgid and the stomata full open. But, if there is a lack of water the guard cells become flaccid and the stomata close to prevent unnecessary water loss and photosynthesis. Transpiration rate is affected by:

• High light intensity which causes the stomata to open, this increases the rate of water evaporation.
• High temperature which increases the movement of the water molecules - this speeds up transpiration.
• Increased air movement which blows the water molecules away from the stomata - this increases transpiration.
• High humidity which decreases the concentration gradient - this slows down transpiration.