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ANSWERS TO REVIEW QUESTIONS – CHAPTER 25 1. Explain why ‘total carbohydrate’ is unhelpful when it appears in a statement of food composition. (p. 578) The term ‘total carbohydrate’ includes sugars, starches and fibre, which vary enormously in their ease of digestibility. For example, all animals easily digest sugars, starches are less so, and complex fibres, such as cellulose, only by a few specialised animal groups. Thus, the amount of available energy depends on both the relative proportion of each form of carbohydrate and the type of animal doing the digesting. In addition, dieticians advise that we reduce our sugar intake and increase our fibre intake, yet both are included under the term ‘total carbohydrate’, making comparisons between different processed foods impossible. 2. Explain the terms ‘essential amino acids’ and ‘essential fatty acids’. (pp. 578–579) The terms ‘essential amino acids’ and ‘essential fatty acids’ refer to the fact that many animal groups have lost the ability to manufacture certain molecules essential to their continued wellbeing and must obtain them from their diet. The actual amino acids and fatty acids required depend on both the animal involved and the stage of its life. For example, most animals can synthesise vitamin C (ascorbic acid), but humans, other primates and fruit bats cannot and require a dietary source. 3. What are the advantages of a one-way digestive tract? (p. 589) A one-way digestive tract with a mouth and an anus allows for regional specialisation of the gut, in turn allowing for more efficient digestion and waste elimination. The one-way tract also allows food to spend different amounts of time in different regions of the tract, optimising the time needed for various stages of digestion. Humans provide a good example, with the buccal cavity being used to crush, taste and lubricate food, initiate chemical digestion and eliminate some bacteria, the stomach involved in food storage and protein breakdown, the small intestine in enzymatic digestion and adsorption, and the large intestine chiefly involved in waste elimination. 4. Use examples to explain the differing roles of physical and chemical digestion. (p. 583) Physical digestion is the breakdown of food into small particles by grinding or chewing, e.g. teeth and jaws in vertebrates, radula in snails and the gizzard in birds. Chemical digestion is the process whereby complex molecules are broken down by hydrolytic enzymes, which are usually secreted into the gut lumen—e.g. in humans: protein digestion by pepsin in the stomach, polysaccharide digestion by pancreatic amylases in the lumen of the small intestine, fat digestion by bile salts and lipase in the lumen of the small intestine. 5. What are zymogens and why are they important? (pp. 583–584) Zymogens are inactive precursors of proteases, enzymes involved in the digestion of protein. Cells secrete zymogens, rather than the active proteases to prevent damage to themselves during the secretion process. Zymogens are converted to the active form when they reach their target site. A good example is pepsinogen, secreted by cells lining the stomach wall, and which is converted to the active form pepsin, either by acid or by existing pepsin. 6. Describe some of the advantages and disadvantages of microbial fermentation in the foregut and hindgut of herbivorous mammals. (pp. 594–596 Box 25.2) Foregut fermentation allows for the extensive degradation of cellulose, but microbes first use soluble sugars and starches. In addition, dietary protein is also first used by microbes but the host can then digest the microbes themselves, which are in turn a very rich source of amino acids. Hindgut fermentation is similar to foregut fermentation with the two exceptions that the food cannot be re-chewed and the microbes which form the basis of the fermentation system in hindgut fermenters are washed out of the system and so their protein cannot be used by the host. The second problem can be overcome by behaviour known as caecotrophy or coprophagy, in which the contents of the gut are passed prior to their conversion to faeces and re-ingested by the host, enabling the microbes to be digested. 7. Outline the dietary differences faced by aquatic herbivores and terrestrial herbivores. (pp. 581–582) The nature and availability of plant material differs dramatically between aquatic and terrestrial environments. In aquatic environments most autotrophs are algae, which produce cellulose and storage carbohydrates but have no secondary thickening due to the supporting effect of the water in which they live. This means that they are far softer to eat in the first place and easier to digest than land plants. In contrast, terrestrial plants have complex cell walls and often lignified woody tissues, which makes them difficult to both eat and digest. To get at their contents, the tough cell walls need rupturing using hard teeth or jaws that can grind against each other. 8. Why are there few birds that rely on fermentation of plant cells as their major energy source? (pp. 590–591) Birds are constrained in what they eat by their mode of locomotion. Flight requires a lot of energy and there is a high cost in having heavy organ systems. Fermenting vegetation takes both time and a large amount of space, and using a digestive system virtually prohibits active flight. As one author has succinctly stated, try flying carrying a compost heap! As such, the few truly herbivorous birds, as opposed to those consuming high-energy foods such as insects, nectar or fruit, fly poorly—for example, the pheasant, which limits itself to brief lurches into the air—or not at all, the emu and other ratites that have lost the power of flight altogether. 9. Why is obesity better interpreted as an imbalance between fat intake and fat oxidation rather than simply energy balance? (pp. 597–600) Obese people have a greater fat content than lean people. Those people that do not efficiently convert dietary fat into energy accumulate body fat. Energy intake and energy expenditure are tightly coupled. For example, even a 1% discrepancy in energy balance would result in a one- to two-fold body weight increase. 10. How does the body homeostatically regulate its mass? (pp. 599–600) Humans have a variety of homeostatic feedback mechanisms that integrate short- and long-term energy stores and lead to compensatory changes in energy expenditure and appetite. These signals are integrated in the hypothalamus. Ghrelin signals that the stomach is empty and its concentration rises before meals. Gut hormones such as PYY work with insulin to signal our immediate feeding status. Longer term signals of body fat are provided by leptin, which is produced in the fat cells and travels via the blood to the hypothalamus, where it influences appetite. Together these signals have a homeostatic effect on body weight and so our weight remains largely constant.