WASSCE SHS
Xylem carries water and mineral salts up the plant. Phloem carries sugars from where they are made to where they are used or stored.
Xylem vessels are dead tubes, thickened with lignin, running from the roots through the stem into the leaves. Water enters a root hair by osmosis, because the cell sap is more concentrated than the soil solution. Mineral ions are taken up mainly by active transport, which needs energy, because they are often moved against a concentration gradient. Inside the leaf, water evaporates from the moist mesophyll cells into the air spaces and diffuses out through the stomata. That loss is transpiration.
The loss pulls on the column of water in the xylem. Water molecules stick to each other, which is cohesion, and the column is dragged upward. This transpiration pull is the main reason water reaches the top of a tall tree. Phloem is different. Its cells are living. It moves sucrose and other made food from a source, such as a leaf, to a sink, such as a root or a growing bud. That movement is translocation, and it can be up or down. Do not give the xylem that job.
Direction and contents are the marks. Xylem: water and mineral salts, up only. Phloem: manufactured food, up or down. Transpiration is evaporation from the mesophyll followed by diffusion out of the stomata, not "the plant sweating" with no site. Osmosis is water. Active transport is the ions. If you swap them you lose both. Cohesion is what keeps the water column intact under the pull.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
4 marks
(a) Name the tissue that transports water, and the tissue that transports sugars. (2 marks)
(b) State one structural difference between the two tissues. (2 marks)
(a) Xylem transports water. Phloem transports sugars.
(b) Xylem vessels are dead and lignified. Phloem cells are living.
4 marks
Explain how water from the soil reaches the top of a tall tree. (4 marks)
Water enters root hairs by osmosis and moves into the xylem. In the leaf, evaporation from mesophyll cells and diffusion through stomata pull on the water column. Cohesion between water molecules lets that pull draw the whole column upward.
3 marks
State two factors that increase the rate of transpiration, and say why one of them does so. (3 marks)
Higher temperature and wind both increase the rate. Wind removes humid air from around the leaf, so the diffusion gradient for water vapour stays steep.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Biology freeXylem carries water and mineral salts upward in dead, lignified tubes. Phloem carries sugars and other made food in living cells, from a source such as a leaf to a sink such as a root. Phloem transport can be up or down. Xylem water transport is up.
Transpiration from the leaves pulls on a continuous column of water in the xylem. Cohesion between water molecules keeps the column from breaking. Root pressure may help a little. It is not what lifts water to the top of a tall tree. All WASSCE Biology past questions