Questions S
3.11 3.12 3.13 3.14 3.15
S
transport protein
What is meant by a turgid cell? What is plasmolysis? How can plasmolysis be brought about? In Figure 3.9, what fills space X? Explain your answer. Describe the events shown in Figures 3.5 and 3.6 in terms of water potential.
glucose molecule
outside the cell
3.3 Active transport
There are many occasions when cells need to take in substances which are only present in small quantities around them. Root hair cells in plants, for example, take in nitrate ions from the soil. Very often, the concentration of nitrate ions inside the root hair cell is higher than the concentration in the soil. The diffusion gradient for the nitrate ions is out of the root hair, and into the soil. Despite this, the root hair cells are still able to take nitrate ions in. They do it by a process called active transport. Active transport is an energy-consuming process by which substances are transported against their concentration gradient. The energy is provided by respiration in the cell. In the cell membrane of the root hair cells are special transport proteins. These proteins pick up nitrate ions from outside the cell, and then change shape in such a way that they push the nitrate ions through the cell membrane and into the cytoplasm of the cell. As its name suggests, active transport uses energy. The energy is provided by respiration inside the root hair cells. (You can find out about respiration in Chapter 11.) Energy is needed to produce the shape change in the transport protein. You can think of active transport as a process in which chemical energy that has been released from glucose (by respiration) is converted into kinetic energy of molecules and ions. S Most other cells can carry out active transport. In the human small intestine, for example, glucose can be actively transported from the lumen of the intestine into the cells of the villi. In kidney tubules, glucose is actively transported out of the tubule and into the blood. Figure 3.10 shows how active transport of glucose takes place.
inside the cell
cell membrane
1 The glucose molecule enters the transport protein.
2 The transport protein changes shape. The energy needed for it to do this is provided by respiration in the cell.
3 The change of shape of the transport protein pushes the glucose molecule into the cell.
Figure 3.10 Active transport.
Key definition active transport – the movement of molecules S and ions in or out of a cell through the cell membrane against a concentration gradient, using energy from respiration
Original material Š Cambridge University Press 2014
Chapter 3:
Movement in and out of cells
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