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EnzymesEnzymes are biological catalysts. There are optimum temperatures and pH values at which their activity is greatest. Enzymes are also proteins, and usually denatured above about 45ºC. Enzymes are important in respiration. Aerobic respiration releases energy from glucose. Enzymes are biological catalysts - catalysts are substances that increase the rate of chemical reactions without being used up. Enzymes are also proteins that are folded into complex shapes that allow smaller molecules to fit into them. The place where these substrate molecules fit is called the active site. If the shape of the enzyme changes, its active site may no longer work. We say the enzyme has been denatured. They can be denatured by high temperatures or extremes of pH. Note that it is wrong to say the enzyme has been killed. Although enzymes are made by living things, they are proteins, and not alive. As the temperature increases, so does the rate of reaction. But very high temperatures denature enzymes. The graph shows the typical change in an enzyme's activity with increasing temperature. The enzyme activity gradually increases with temperature until around 37ºC, or body temperature. Then, as the temperature continues to rise, the rate of reaction falls rapidly, as heat energy denatures the enzyme. Changes in pH alter an enzyme’s shape. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5. Enzymes in the stomach have an optimum pH of about 2. Enzymes in cells catalyse photosynthesis, protein synthesis - joining amino acids together, and aerobic respiration. Respiration is not the same thing as breathing. That is more properly called ventilation. Instead, respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygen    →    carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

  • -to allow muscles to contract in animals
  • -to maintain a constant body temperature in birds and mammals

CellsAll living things are made up of cells. The structures of different types of cells are related to their functions. Animal cells and plant cells have features in common, such as a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant and algal cells also have a cell wall, and often have chloroplasts and a permanent vacuole. Bacterial and yeast cells have different structures to animal and plant cells. Dissolved substances pass into and out of cells by diffusion.  Nucleus- contains genetic material, which controls the activities of the cell

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