Blood, lymph and transport in plants

What is blood made of?

Blood is a fluid connective tissue. If blood is spun fast in a tube, it separates into two parts. About $55\%$ is a pale yellow liquid, the plasma. The rest is cells.

• Plasma is mostly water. It carries food, carbon dioxide, salts and nitrogenous wastes, dissolved in it.
• Red blood cells carry oxygen. They hold a red pigment, haemoglobin, which takes up oxygen in the lungs and gives it up in the tissues. Haemoglobin makes the blood red.
• White blood cells fight germs that enter the body.
• Platelets are tiny pieces of cells. They help the blood to clot at a cut.

Platelets and clotting

When a blood vessel is cut, blood leaks out. If nothing stopped it, we would lose blood, the pressure would fall, and the pumping would fail. Platelets go round in the blood all the time. At a cut, they gather and start a clot, which plugs the leak.

Lymph

Some plasma, with salts and a little protein, leaks out through the thin walls of the capillaries. It fills the spaces between the cells. Some of it goes back into the capillaries. The rest drains into small tubes, the lymphatic capillaries, and is now called lymph.

• Lymph is like plasma, but it is colourless and has less protein.
• It flows through larger lymph vessels and finally drains into large veins.
• It carries digested and absorbed fat from the small intestine.
• It drains extra fluid from the spaces between the cells back into the blood.

Transport in plants

Plants also move water, minerals and food around. But a plant does not move from place to place, and many of its cells are dead. It uses much less energy than an animal, so its transport can be slower.

A plant has two transport tissues.

• Xylem carries water and minerals from the roots to the leaves.
• Phloem carries food, such as sucrose and amino acids, from the leaves to the other parts.

How water climbs up

Water rises in two ways.

1. Root pressure. Root cells take in ions of minerals from the soil. Now the root holds more ions than the soil, so water moves into the root. The new water pushes the column of water up the xylem. This push is enough for small plants, and it works at night.
2. Transpiration pull. In the day, water evaporates from the leaves through the stomata. This loss of water vapour is called transpiration. As water leaves, it pulls the water below it up the xylem, like a long chain. In a tall tree, this pull is the main force during the day.

Transpiration also cools the leaves, just as sweating cools us. And it keeps water and minerals flowing up all day.

You should find that the loss is zero at night and largest at noon. Double the leaf area, and the water lost in the day doubles too.

Moving food: translocation

The food made in the leaves is carried by the phloem to the roots, fruits, seeds and growing parts. This is called translocation.

• The sugar travels as sucrose. Energy from ATP is used to load it into the tubes of the phloem.
• Now the phloem holds much sugar, so water enters it by osmosis, and the pressure inside rises.
• This pressure pushes the food to the parts that have less. So food can move up or down, as the plant needs.

In spring, for example, sugar stored in the roots or the stem goes up to the buds, which need energy to grow.

Try it in Python

The program adds up the water lost by a young tree in each hour of a sunny day.

Practise with new numbers

Common mistakes

• "Phloem moves food only downwards." It moves food wherever it is needed, up or down.
• "Xylem carries food." Xylem carries water and minerals. Phloem carries food.
• "Transpiration only wastes water." It pulls water and minerals up, and it cools the leaves.
• "Lymph is the same as blood." Lymph has no red cells, and it has less protein than plasma.
• "Platelets carry oxygen." Red blood cells carry oxygen. Platelets help the blood to clot.

Remember these

• Blood is plasma and cells. Red cells carry oxygen with haemoglobin, white cells fight germs, and platelets help clotting.
• Lymph forms from fluid that leaks out of the capillaries. It carries absorbed fat and drains into the veins.
• Xylem carries water and minerals up from the roots. Root pressure helps at night, and the transpiration pull is the main force in the day.
• Phloem carries food by translocation. It uses energy, and the food can go up or down.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What gives blood its red colour, and what job does it do?

2. Why does a small cut usually stop bleeding after a few minutes?

3. Where does lymph come from, and where does it go in the end?

4. On a hot, dry and windy day, does a plant lose water faster or slower than on a cool, damp day? Why?

5. Which of the two transport tissues of a plant uses energy from ATP to do its work?