transport

In plants and animals, method by which substances such as nutrients and water move into and out of organisms and into and out of cells in the body. Water entering and leaving cells usually does so by osmosis. In both animals and plants there is a cell membrane around every body cell and this is partially permeable so osmosis can occur. When water is taken up into the body of a plant or animal, the water has to enter a cell and must do so by osmosis. However, with plants, the presence of a strong cell wall in addition to the cell membrane means that plant cells do not take up too much water. The cells just become fully filled or turgid (see turgor). Loss of water from plants is by transpiration, which is evaporation of water from leaves. The movement of nutrients across cell membranes may involve a transport process called active transport. This can be thought of as a ‘pump’ because it requires an input of energy. Plant cells use active transport to absorb minerals from the soil. Humans will use the same process for the same reasons to absorb minerals from food in the alimentary canal.

Within plants and animals there are great differences between the ways in which fluids are transported. In mammals there is a circulatory system using blood to transport mineral salts and gases for respiration. In plants there are two systems. One uses xylem tissue and carries water and minerals from roots to leaves. The other uses phloem tissue and carries sugars in solution from leaves to all parts of the plant.