Why is energy needed

Like all other living things, microorganisms need to acquire energy in order to survive. Energy is required:

• to maintain the structural integrity of the cell by repairing any damage to its constituents

• to synthesise new cellular components such as nucleic acids, polysaccharides and enzymes

• to transport certain substances into the cell from its surroundings

• for the cell to grow and multiply

• for cellular movement.

Metabolism is the term used to describe all the biochemical reactions that take place inside a cell; it includes those reactions that release energy into the cell, and those that make use of that energy. Figure 6.1 summarises these processes.

As we saw in Chapter 4, most microorganisms obtain their energy from the nutrients they take into the cell; these may come from an organic or an inorganic source. Once inside the cell, these nutrients must then be biochemically processed by reactions that trap some of their chemical energy, at the same time breaking them down into smaller molecules. These then serve as building blocks for the synthesis of new cellular components. Chemical compounds contain potential energy within their molecular structure, and some of this can be

Catabolism is the term used to describe reactions that break down large molecules, usually coupled to a release of energy.

Anabolism is the term used to describe reactions involved in the synthesis of macromole-cules, usually requiring an input of energy

Figure 6.1 Microorganisms use a variety of processes to generate biochemical energy in the form of the compound adenosine triphosphate (ATP). As ATP is broken down to ADP and inorganic phosphate, the energy released is used for the maintenance, reproduction and survival of the cell

released when they are broken down. In other metabolic types, energy is obtained from the sun by means of photosynthesis; once again, however, the energy is used for synthetic purposes.

Central to the metabolic processes of any cell are enzymes. Without them, the many biochemical reactions referred to above simply wouldn't take place at a fast enough rate for living cells to maintain themselves. We shall start our consideration of metabolism by taking a look at enzymes: what they are, and how they work. In the later sections of the chapter, we shall consider in more detail those processes by which energy is acquired and spent.

An enzyme is a cellular catalyst (usually protein), specific to a particular reaction or group of reactions.

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