Double Award - Science for GCSE

Chapter 6

Food and water for plants
Water uptake and transport in plants

1.	Water and mineral uptake by plants In order to grow, plants must take in water and minerals from the soil. Water and minerals are taken up by plants through their roots. The roots of plants have millions of root hair cells which project into the soil. These root hair cells provide a large surface area with thin cell walls to help the uptake of water and minerals.

2.	Plants need water for three important functions (jobs): photosynthesis; support of the plant; transport of solutions up and down the plant. Water rises up the stem of the plant and then to the leaves where most of it evaporates. This is called transpiration. The continuous flow of water from the roots through the plant to the leaves is called the transpiration stream.
3.	Support in plants Plant cells differ from animal cells in having strong, rigid cell walls. The cell wall is completely permeable to water and solutes, but just below the cell wall is the cell membrane which is selectively permeable. Water and dissolved minerals can pass from one cell to another through the cell membranes, from the roots to the leaves. When the plant has a good supply of water, the cells fill up and pack tightly. The cells press on one another and on the cell wall. This makes the stem and leaves firm yet flexible. The swollen plant cells are described as turgid and the support they give to the plant is called turgor. If a plant has not had enough water, it will wilt and may wither and die.
4.	Transport in plants The diagram below shows the various parts in the young stem of a plant and their different functions.

5.	Plants need certain elements so that they can grow and be healthy. The most important essential elements are oxygen, hydrogen and carbon. Plants get these elements from water in the soil and from oxygen and carbon dioxide in the air. The next three essential elements which plants need and take in from the soil as minerals are: Nitrogen in nitrates and ammonium salts. Plants need nitrogen to synthesise proteins and nucleic acids. Lack of nitrogen results in small plants with yellow leaves. Phosphorus in phosphates. Plants need phosphorus to synthesise ATP and nucleic acids. Lack of phosphorus results in a poor root system. Potassium in potassium salts. Potassium salts help in the transfer of materials across cell membranes. Lack of potassium results in yellow leaves which die back.