Transpiration and transport around a plant

What you need to know

Reflections and Exam tips

Transpiration

Recall:

Plants need to:

• Get Carbon Dioxide
• Water
• Remove Oxygen from photosynthesis
• Transport sugars and amino acids around

Carbon dioxide, enters through the stomata on the leaves. As plants take in Carbon Dioxide, water escapes through the stomata into the air.

The evaporation of water from the leaves into the air is called Transpiration. There is less water in the air than on the moist cell walls of spongy mesophyll cells of the leaf, so water travels from its high concentration to its low concentration by diffusion.

Factors affecting rate of transpiration

1. Humidity

Humidity is how much moisture is in the air. A high humidity means more moisture in the air so there is a smaller diffusion gradient. The rate of transpiration decreases.

2. Temperature

The higher the temperature, the more evaporation of water occurs, so transpiration increases.

3. Sunlight

Sunlight plays a role in the opening of stomata. More sunlight may mean more transpiration, but plants usually have ways of limiting stomatal opening to reduce transpiration on hot days.

 

 

 

 

4. Wind Speed

Normally, a layer of moist air hangs around the lower surface of the leaf. This reduces the transpiration/evaporation gradient, reducing transpiration. The higher the wind speed, the higher the rate of transpiration as evaporated water particles are quickly removed, increasing the diffusion gradient.

Adaptations to reduce transpiration:

• Thick, waxy cuticle
• Sunken stomata
• Hairs/spines

Water and nutrients are absorbed from the soils through the roots and root hairs.

You will recall that nutrients are absorbed from the soil by Active Transport. This increases the concentration of the cytoplasm and vacuole, water follows by Osmosis.

How does the water move up to the leaves?

Xylem- Once water enters the roots, it increases the volume of the cytoplasm and vacuole. This forces excess water up through tubes called Xylem (pronounced Zy lem). These are dead cells with no cytoplasm and they form a system of tubes from the roots to the leaves. In the leaves, xylem are found in the veins.

Transpiration stream:

Water molecules in the soil, in the roots, stem, leaves and the air are all linked. The evaporation of one molecule from the leaves draws another from the soil.

This constant pull of water molecules from the leaves to the roots because of evaporation is called the transpiration stream. It also helps plants absorb water from the soil.

Phloem

Sugars made in the leaves have to be transported around the plant as well. These are carried in another tissue called Phloem (pronounced Flo em). Phloem are also tubes, but unlike xylem, they have living cells which have mitochondria for active transport.