Nutrition and Nutrient Transport to Cells
Living organisms are dynamic systems always in need of energy and nutrients that are indispensable to multiple vital processes such as growth, maintenance and repair of body structures, protection and detoxification of cells, and reproduction. 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 (from sugar cane), lactose (from dairy products), and starches (from grains and its derivates). Lipids are found in butter, cheeses, margarines, whole milk, eggs, nuts, olive oil, meats, fish, and other sources. The main source of proteins is animal products (milk, eggs, and meats), although they are also present in soybeans, fruits, and nuts. Moreover, small quantities of micronutrients such as essential minerals and vitamins are also indispensable for both structural repair and crucial metabolic functions in cells, tissues and organs. The best sources of vitamins are whole milk, whole grains, raw vegetables, and fruits. Trace minerals are present in practically every kind of food in physiological amounts.
Nutrition starts with the intake of foods and the process of digestion that will reduce them to molecules suitable for absorption by the gastrointestinal mucosa. The basic process involved in digestion is hydrolysis, which reduces carbohydrates to monosaccharides and water. The dietary lipids are usually neutral fats (triglycerides) constituted by three molecules of fatty acids condensed with a single glycerol molecule. Hydrolysis adds three molecules of water to each triglyceride molecule, and separates the fatty acid from the glycerol. From protein, the digestion process ultimately obtains the amino acids needed to a wide range of physiologic functions, such as the synthesis of body enzymes and proteins, nucleic acids, and regulation of metabolic processes in every tissue. Proteins are first digested through the action of pepsin, an enzyme present in the stomach, and then by several pancreatic enzymes delivered in the upper small intestine, duodenum, and jejunum. A final stage occurs in the intestinal lumen, mediated by peptidase enzymes. The resultant products are smaller proteins (dipeptides, tripeptides) and some free amino acids that can be absorbed by the intestinal membranes.
Absorption of nutrients by the gastrointestinal mucosa may take place by active transport or by diffusion. Active transport involves the spending of body energy, whereas diffusion occurs simply through random molecular movement and, therefore, without the use of body energy. Water for example, is transported through the intestinal mucosa by diffusion (isosmotic absorption); on the other hand, the absorption of sodium, involves active transport. Nutrients absorbed through the intestinal membranes are delivered in the blood circulation and transported to all tissues and organs, where they are absorbed by the cells.
The transport of most nutrients through the cell membrane depends on its solubility in lipids, because cell membranes are constituted by a double lipid layer, with the polar ends (hydrophilic ends) on the external side and the apolar chains (hydrophobic ends) in the interior face. Therefore, fatty acids and steroid hormones can easily penetrate through the membrane, whereas other substances will be transported through 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 an extra cellular solution. When the concentration of a given solute inside the cell decreases, passive diffusion from the outside takes place through osmosis, without energy spent. However, the active transport through cell membranes is done with the release of energy from ATP (adenosine triphosphate), which is converted into ADP (adenosine diphosphate) in the process. Another form of transport of nutrients to cells, termed facilitated diffusion, occurs in the delivery of several substances, such as glucose, galactose, and certain amino acids. In this process, permeases, or transporting molecules present in the plasmatic membrane, are associated with the molecules of the substance to be transported. Cells also have the ability to transfer great quantities of macromolecules (proteins, polysaccharides, polynucleotides) to their interior through endocytosis. Endocytosis may occur under the form of phagocytosis or pinocytosis. Phagocytosis is the process by which cells envelop a solid particle by modifying its membrane to form a sac or phagosome around the particle. Phagosomes are then pulled by the cytoskeleton motion to the cytosol, where they are incorporated into one or more lysosomes for digestion of the particles. Pinocytosis is used to transport liquid substances (solutes) into the cell. Several pockets occur in a given area of the cell membrane, capturing the liquid and forming vesicles, which are then pulled by the cytoskeleton motion to the cytoplasm. Pinocytosis may be selective or non-selective.
Selective pinocytosis occur in two stages: 1) the liquid substance first adheres to membrane receptors at the membrane surface; and 2), the membrane then sinks and the substance is transferred to vesicles that leave the surface and transport their content to the cytoplasm. In non-selective pinocytosis, the vesicles envelop all the solutes eventually present in the extra cellular fluid.
The ultimate goal of nutrition is the conversion of nutrients into bioenergy. Energy is indispensable for muscular and neural activity, synthesis of enzymes and proteins, repair of DNA and tissues, absorption of nutrients in the gastrointestinal tract, cell proliferation, excretion of toxic metabolites from the cells, body temperature maintenance, and many other physiological functions.
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