Living organisms are dynamic systems that are always in need of energy and nutrients. Both are indispensable to multiple vital processes such as growth, reproduction, maintenance and repair of body structures, and protection and detoxification of cells. Nutrients are found in foods and are classified as carbohydrates, lipids (fats), and proteins. The main sources of carbohydrates in the human diet are sugars (such as sucrose [table sugar] and lactose [from dairy products]) and starches (from grains and their derivates). Lipids are found in many foods, though they are especially concentrated in butter, cheese, margarine, whole milk, eggs, nuts, olive oil, meat, and fish. In the United States, animal products (milk, eggs, and meat) are the main source of protein in many people's diets, although there are significant quantities of protein in grains, legumes (especially soybeans), nuts, and some vegetables (including spinach, broccoli, and potatoes). In addition to these major nutrients, small quantities of micronutrients--such as essential vitamins and minerals--are also indispensable to many body processes, including structural repair and metabolic functions in cells, tissues, and organs. The best sources of vitamins are whole grains, raw vegetables and fruits, and whole milk. Physiological amounts of trace minerals are present in practically every kind of food.
Nutrition starts with the intake of foods and the process of digestion, which reduces them to molecules suitable for absorption by the gastrointestinal mucosa. Digestion works on the major nutrients in a variety of ways. The basic mechanism involved in carbohydrate digestion is hydrolysis, whereby carbohydrates are reduced to monosaccharides and water. The dietary lipids are usually neutral fats (triglycerides) consisting of three fatty acid molecules condensed with a single glycerol molecule. Hydrolysis adds three molecules of water to each triglyceride molecule and separates the fatty acid from the glycerol. Protein digestion relies on a variety of enzymes (proteases) to obtain amino acids. These acids are needed for a wide range of physiological functions, including the synthesis of enzymes and proteins as well as the regulation of metabolic processes throughout the body. Initial protein digestion is carried out via the action of pepsin, an enzyme present in the stomach; further digestion results from several pancreatic enzymes delivered in the upper small intestine (duodenum and jejunum). Mediated by peptidase enzymes, a final stage of digestion occurs in the intestinal lumen. The result of protein digestion is an array of smaller proteins (dipeptides and tripeptides) and some free amino acids that can be absorbed by the intestinal membranes.
Absorption of nutrients by the gastrointestinal mucosa can take place by active transport or by diffusion. Active transport requires the input of energy, whereas diffusion occurs through unassisted molecular movement and, therefore, without the use of body energy. For example, water moves through the intestinal mucosa by diffusion, in a process called isosmotic absorption. The absorption of sodium, on the other hand, involves active transport. Nutrients absorbed through the intestinal membranes move into the blood; circulation transports them to the tissues and organs, where they are absorbed by the cells.
In most cases, the transport of a nutrient through the cell membrane depends on its solubility in lipids because cell membranes consist of a double lipid layer. The phospholipids that make up the membrane have two ends, each with a different molecular relationship to water. Their polar (hydrophilic) ends are on the external side of the membrane, and the apolar (hydrophobic) chains (also called tails) are in the interior. As a result of this arrangement, fatty acids and steroid hormones can easily penetrate through the membrane, whereas other substances often need to be transported by other mechanisms such as active transport or facilitated diffusion. Many molecules enter the cell through passive diffusion or osmotic transport, simply because they are dissolved in extracellular fluids. When the concentration of a given solute in the cell decreases, passive diffusion from the outside takes place by osmosis, which does not require energy from the cell. However, the active transport through cell membranes requires the release of energy from ATP (adenosine triphosphate), which is converted into ADP (adenosine diphosphate) in the process.
Several substances rely on another mechanism, called facilitated diffusion, for delivering nutrients to cells. These substances include glucose, galactose, and certain amino acids. In this process, transporting molecules present in the plasmatic membrane (permeases) are associated with the molecules of the substance that need to be transported. Cells can also transfer large quantities of macromolecules (proteins, polysaccharides, polynucleotides) to their interior through a process called endocytosis, which can take the form of phagocytosis or pinocytosis. Phagocytosis is a process by which cells envelop a solid particle, modifying the cell membrane to form a sac (phagosome) around the particle. Motions in the cytoskeleton then pull the phagosome into the cytosol, where it is incorporated into one or more lysosomes, in which the particles are digested. Pinocytosis is used to transport liquid substances (solutes) into the cell. In pinocytosis, pockets form in a given area of the cell membrane; these pockets capture the liquid and form vesicles, which are then pulled into the cytoplasm by the cytoskeleton.
Pinocytosis may be selective or nonselective. Selective pinocytosis occurs in two stages. First, the liquid substance adheres to membrane receptors at the membrane surface. The membrane then sinks, and the substance is transferred to vesicles that leave the surface and transport their contents to the cytoplasm. In nonselective pinocytosis, the vesicles envelop all the solutes from the extracellular fluid without relying on the mediation of receptors.
The ultimate goal of nutrition is the conversion of nutrients into bioenergy. Energy is indispensable for muscular and neural activity, enzyme and protein synthesis, repair of DNA and tissues, absorption of nutrients in the gastrointestinal tract, cell proliferation, excretion of toxic metabolites from the cells, body temperature regulation, and many other physiological functions.