Respiration

In order that growth can occur, the food must be broken down in a controlled manner to release energy for the production of useful structural substances such as cellulose, the main constituent of plant cell walls, and proteins for enzymes. This energy is used also to fuel cell division and the many chemical reactions that occur in the cell.

The energy requirement within the plant varies, and reproductive organs can respire at twice the rate of the leaves. Also, in apical meristems, the processes of cell division and cell differentiation require high inputs of energy. In order that the breakdown is complete, oxygen is required in the process of aerobic respiration. A summary of the process is given in Table 8.2.

Table 8.2 A summary of the process of aerobic respiration a. Written in a conventional way, the process can be expressed in the following way:

glucose plus oxygen gives rise to carbon dioxide plus water plus energy in the mitochondria of the cell.

b. Written in the form of a chemical equation, which represents molecular happenings at the sub-microscopic level, the above sentence becomes:

1 C6H12O6 molecule plus 6 O2 molecules give rise to 6 CO2 molecules plus 6 H2O molecules plus energy in the mitochondria of the cell.

It would appear at first sight that respiration is the reverse of photosynthesis (see p111). This supposition is correct in the sense that photosynthesis creates glucose as an energy-saving strategy, and respiration breaks down glucose as an energy releasing mechanism. It is also correct in the sense that the simple equations representing the two processes are mirror images of each other.

It should, however, be emphasized that the two processes have two notable differences. The first is that respiration in plants (as in animals) occurs in all living cells of all tissues at all times in leaves, stems, flowers, roots and fruits. Photosynthesis occurs predominantly in the

Respiration is the process by which sugars and related substances are broken down to yield energy, the end-products being carbon dioxide and water.

palisade mesophyll tissue of leaves. Secondly, respiration takes place in the torpedo-shaped organelles of the cell called mitochondria. Photosynthesis occurs in the oval-shaped chloroplasts. Details of biochemistry, beyond the scope of this book, would reveal how different these processes are, in spite of their superficial similarities. In the absence of oxygen, inefficient anaerobic respiration takes place and incomplete breakdown of the carbohydrates produces alcohol as a waste product, with energy still trapped in the molecule. If a plant or plant organ such as a root is supplied with low oxygen concentrations in a waterlogged or compacted soil, the consequent alcohol production may prove toxic enough to cause root death. Over-watering, especially of pot plants, leads to this damage and encourages damping-off fungi.

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