<P> The macronutrients (excluding fiber and water) provide structural material (amino acids from which proteins are built, and lipids from which cell membranes and some signaling molecules are built) and energy . Some of the structural material can be used to generate energy internally, and in either case it is measured in joules or calories (sometimes called "kilocalories" and on other rare occasions written with a capital C to distinguish them from little' c' calories). Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram., though the net energy from either depends on such factors as absorption and digestive effort, which vary substantially from instance to instance . Vitamins, minerals, fiber, and water do not provide energy, but are required for other reasons . A third class dietary material, fiber (i.e., non-digestible material such as cellulose), seems also to be required, for both mechanical and biochemical reasons, though the exact reasons remain unclear . </P> <P> Molecules of carbohydrates and fats consist of carbon, hydrogen, and oxygen atoms . Carbohydrates range from simple monosaccharides (glucose, fructose, galactose) to complex polysaccharides (starch). Fats are triglycerides, made of assorted fatty acid monomers bound to glycerol backbone . Some fatty acids, but not all, are essential in the diet: they cannot be synthesized in the body . Protein molecules contain nitrogen atoms in addition to carbon, oxygen, and hydrogen . The fundamental components of protein are nitrogen - containing amino acids, some of which are essential in the sense that humans cannot make them internally . Some of the amino acids are convertible (with the expenditure of energy) to glucose and can be used for energy production just as ordinary glucose . By breaking down existing protein, some glucose can be produced internally; the remaining amino acids are discarded, primarily as urea in urine . This occurs normally only during prolonged starvation . </P> <P> Other dietary substances found in plant foods (phytochemicals, polyphenols) are not identified as essential nutrients but appear to impact health in both positive and negative ways . Most foods contain a mix of some or all of the nutrient classes, together with other substances . Some nutrients can be stored internally (e.g., the fat soluble vitamins), while others are required more or less continuously . Poor health can be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient . For example, both salt provides sodium and chloride, both essential nutrients, but will cause illness or even death in too large amounts . </P> <P> Autotrophs ~ Those organisms which can make food on their own, from simple substances, are known as autotrophs . This is because green plants chlorophyll and can perform photosynthesis . In other words, green plants have autotrophic mode of nutrition . </P>

Do all animals eat the same kind of food