Download ANSWERS TO REVIEW QUESTIONS – CHAPTER 25

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.