Digestion and Nutrition
A Consuming Cornucopia
Sir Walter Raleigh observed that the difference between a rich man and a poor man is that the former eats when he pleases while the latter eats when he can get it. In today’s crowded world, with nearly 80 million people added each year to the world’s population of 6 billion, the separation between the well-fed affluent and the hungry and malnourished poor reminds us that time has not diminished the shrewdness of Sir Walter’s remark. Unlike the affluent for whom food acquisition requires only the selection of prepackaged foods at a well-stocked supermarket, the world’s poor can appreciate that for them, as for the rest of the animal kingdom, procuring food is fundamental to survival. For most animals, eating is the main business of living.
Potential food is everywhere and little remains unexploited. Animals bite, chew, nibble, crush, graze, browse, shred, rasp, filter, engulf, enmesh, suck, and soak up foods of incredible variety. What an animal eats and how it eats profoundly affect an animal’s feeding specialization, its behavior, its physiology, and its internal and external anatomy—in short, both its body form and its role in the web of life. The endless evolutionary jostling between predator and prey has provided compromise adaptations for eating and adaptations for avoiding being eaten. By whatever means food may be secured, there is far less variation among animals in the subsequent digestive simplification of foods. Vertebrates and invertebrates alike use similar digestive enzymes. Even more uniform are the final biochemical pathways for nutrient use and energy transformation. The nourishment of animals is like a cornucopia in which the food flows in rather than out. A great diversity of foods procured by countless feeding adaptations streams into the mouth of the horn, is simplified, and finally applied to the common purpose of survival and reproduction.
All organisms require energy to maintain their highly ordered and complex structure. This energy is chemical bond energy that is released by transforming complex compounds acquired from the organism’s environment into simpler ones.
The ultimate source of energy for life on earth is the sun. Sunlight is captured by chlorophyll molecules in green plants, which transform a portion of this energy into chemical bond energy (food energy). Green plants are autotrophic organisms; they require only inorganic compounds absorbed from their surroundings to provide the raw material for synthesis and growth. Most autotrophic organisms are the chlorophyll-bearing phototrophs, although some, the chemosynthetic bacteria, are chemotrophs; they gain energy from inorganic chemical reactions.
Almost all animals are heterotrophic organisms that depend on already synthesized organic compounds of plants and other animals to obtain the materials they will use for growth, maintenance, and reproduction of their kind. Since the food of animals, normally the complex tissues of other organisms, is usually too bulky to be absorbed directly by cells, it must be broken down, or digested, into soluble molecules that are small enough to be used.
Animals may be divided into a number of categories on the basis of dietary habits. Herbivorous animals feed mainly on plant life. Carnivorous animals feed mainly on herbivores and other carnivores. Omnivorous forms eat both plants and animals. Saprophagous animals feed on decaying organic matter.
The ingestion of foods and their simplification by digestion are only initial steps in nutrition. Foods reduced by digestion to soluble, molecular form are absorbed into the circulatory system and transported to the body’s tissues. There they are assimilated into the structure of cells. Oxygen is also transported by blood to the tissues, where food products are oxidized, or burned to yield energy and heat. Food not immediately used is stored for future use. Wastes produced by oxidation must be excreted. Food products unsuitable for digestion are egested in the form of feces.
In this section we first examine the feeding adaptations of animals. Next we discuss digestion and absorption of food. We close with a consideration of nutritional requirements of animals.
Sir Walter Raleigh observed that the difference between a rich man and a poor man is that the former eats when he pleases while the latter eats when he can get it. In today’s crowded world, with nearly 80 million people added each year to the world’s population of 6 billion, the separation between the well-fed affluent and the hungry and malnourished poor reminds us that time has not diminished the shrewdness of Sir Walter’s remark. Unlike the affluent for whom food acquisition requires only the selection of prepackaged foods at a well-stocked supermarket, the world’s poor can appreciate that for them, as for the rest of the animal kingdom, procuring food is fundamental to survival. For most animals, eating is the main business of living.
Potential food is everywhere and little remains unexploited. Animals bite, chew, nibble, crush, graze, browse, shred, rasp, filter, engulf, enmesh, suck, and soak up foods of incredible variety. What an animal eats and how it eats profoundly affect an animal’s feeding specialization, its behavior, its physiology, and its internal and external anatomy—in short, both its body form and its role in the web of life. The endless evolutionary jostling between predator and prey has provided compromise adaptations for eating and adaptations for avoiding being eaten. By whatever means food may be secured, there is far less variation among animals in the subsequent digestive simplification of foods. Vertebrates and invertebrates alike use similar digestive enzymes. Even more uniform are the final biochemical pathways for nutrient use and energy transformation. The nourishment of animals is like a cornucopia in which the food flows in rather than out. A great diversity of foods procured by countless feeding adaptations streams into the mouth of the horn, is simplified, and finally applied to the common purpose of survival and reproduction.
All organisms require energy to maintain their highly ordered and complex structure. This energy is chemical bond energy that is released by transforming complex compounds acquired from the organism’s environment into simpler ones.
The ultimate source of energy for life on earth is the sun. Sunlight is captured by chlorophyll molecules in green plants, which transform a portion of this energy into chemical bond energy (food energy). Green plants are autotrophic organisms; they require only inorganic compounds absorbed from their surroundings to provide the raw material for synthesis and growth. Most autotrophic organisms are the chlorophyll-bearing phototrophs, although some, the chemosynthetic bacteria, are chemotrophs; they gain energy from inorganic chemical reactions.
Almost all animals are heterotrophic organisms that depend on already synthesized organic compounds of plants and other animals to obtain the materials they will use for growth, maintenance, and reproduction of their kind. Since the food of animals, normally the complex tissues of other organisms, is usually too bulky to be absorbed directly by cells, it must be broken down, or digested, into soluble molecules that are small enough to be used.
Animals may be divided into a number of categories on the basis of dietary habits. Herbivorous animals feed mainly on plant life. Carnivorous animals feed mainly on herbivores and other carnivores. Omnivorous forms eat both plants and animals. Saprophagous animals feed on decaying organic matter.
The ingestion of foods and their simplification by digestion are only initial steps in nutrition. Foods reduced by digestion to soluble, molecular form are absorbed into the circulatory system and transported to the body’s tissues. There they are assimilated into the structure of cells. Oxygen is also transported by blood to the tissues, where food products are oxidized, or burned to yield energy and heat. Food not immediately used is stored for future use. Wastes produced by oxidation must be excreted. Food products unsuitable for digestion are egested in the form of feces.
In this section we first examine the feeding adaptations of animals. Next we discuss digestion and absorption of food. We close with a consideration of nutritional requirements of animals.
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