J. M. Chesworth

Regulation of Plant Growth and Development

Plants must regulate and integrate the activities of all their parts. They must also respond to changes in their environment and do so in a bewildering variety of ways. To achieve this they have complex mechanisms for perception of external events and for controlling their growth and development. Some aspects of these control mechanisms, including the role of plant hormones, are considered in this chapter.

Muscle and Meat

Much of animal agriculture is devoted to the production of meat for which skeletal muscle provides the raw material. Although meat is derived from muscle there are important biochemical differences between the physiologically active tissue and the commercial commodity, and changes to the carcass after slaughter have to be carefully controlled to ensure the provision of a hygienic product of high eating quality.

Fatty Acids and Lipids

The term lipids is used to describe a chemically heterogeneous group of organic compounds which have in common the general property that they are insoluble in water but are soluble in organic solvents such as hydrocarbons (e.g. hexane and toluene), chloroform and alcohols. In its widest sense the term encompasses natural products such as the fat-soluble vitamins, carotenoids, steroids, terpenes, bile salts, fatty acids and their ester and amine derivatives. Commonly it is used more narrowly to include only fatty acids and their derivatives, waxes, steroids and steroid esters.

Seeds and Germination

The seed is the means by which a plant reproduces and multiplies, and which allows it to persist through adverse conditions. Inside a protective seed coat, it consists of an embryo together with nutrient reserves, and which support the growth of the embryo until the seedling is able to carry out photosynthesis and obtain nutrients from the soil via the root system.

Purines, Pyrimidines and Nucleic Acids

The properties of all proteins are determined by the sequence of the 20 different amino acids which they contain. Any errors in these sequences usually prevent them from performing their normal biological functions. The order in which the amino acids are assembled is determined by the structure of the DNA (deoxyribonucleic acid) molecules in the cells. DNA therefore acts as the ’blueprint’ which determines the nature and function of proteins. DNA also has the vital function of transmitting this information from one generation to the next, as the information in the DNA can be copied and handed on to daughter cells.

The Pentose Phosphate Pathway

The pentose phosphate pathway (also known as the phosphogluconate pathway or the hexose monophosphate shunt) provides a route by which glucose can be oxidized to carbon dioxide. As in the case of glycolysis, the substrate for the pathway is glucose-6-phosphate. An outline of the pathway is shown in Figure 15.1. The pathway can be considered in two parts: oxidative reactions and rearrangement reactions.

Lactation and Its Manipulation

The majority of the world’s supply of milk comes from cows, with water buffalo, goats, sheep and camels producing much smaller amounts. The mammary gland of a high-yielding animal has an extraordinary rate of metabolic activity, which may be greater than the rest of the whole of the animal’s metabolism put together. This metabolic activity is initiated very rapidly: the cow is able to make the transition from a non-lactating state to a high milk yield within a very few days. High-yielding dairy cows are capable of producing well over 10 tonnes of milk within a period of 10 months. During this time the rate of production is not constant but builds up to a peak over the first few months and then declines steadily to less than a half of maximum. In early lactation, the cow’s food intake is insufficient to meet the requirements for milk production.

Compartments, Membranes and Regulation

The major structural features of cells are described in Chapter 1. It is clear that as organisms have evolved, their architecture has become more complex, as reflected in both the overall shape and size of cells and the diversity of intracellular structures. Prokaryotes have no discernible intracellular membranes and can thus be considered as having a single compartment. Eukaryotic cells contain many different types of membrane-bounded structures. The plasma membrane surrounds the cell, and acts as a barrier between the extracellular environment which may undergo marked changes in composition, and the cell contents which have a relatively constant composition. Within cells are many subcellular organelles. Many of these are surrounded by membranes, e.g. the mitochondria, chloroplasts and nucleus, and have their own composition which may be markedly different from the cytosol.

Vegetative Growth of Plants

Once the root system has developed and photosynthesis is well established, seedlings are no longer dependent on nutrients supplied by the seed. However the functions of the root and the shoot are closely integrated and each is dependent on the other for its survival.

The Tricarboxylic Acid Cycle

Like all organic compounds, glucose will burn in air to produce carbon dioxide and heat. This oxidation process has been harnessed by living organisms not to produce heat, but to trap chemical energy in the form of ATP which can be used for a variety of cellular functions.