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Cell Structure & Physiology
This page focuses on understanding basic cell structure and physiology. The material on this page not included in the Wilderness Medicine Handbook.

Cells are the smallest form of life and, as the basic building blocks for the human body, they are responsible for creating tissues, organs, and organ systems. While there are numerous types of cells within the body, all respond to internal and external stimuli in order to maintain homeostasis. The shape of a cell reflects its function. Each cell is surrounded by a lipid membrane (stuff sack) that separates the contents of the cell from the extracellular fluid and contains the cell’s organelles. Each organelle has its own stuff sack and is held in place by structural proteins. Additional proteins are embedded in the cell membrane and act as receptors, channels, enzymes, molecular carriers, anchors, or identifiers. Individual organelles are responsible for specific functions within each cell: mitochondria process glucose and store energy as ATP. The ribosomes on the endoplasmic reticulum synthesize and store proteins, carbohydrates, and lipids. Lysosomes are responsible for digestion, defense, and recycling. Golgi apparatus package secretions and enzymes in small vesicles and maintain the cell membrane. The nucleus controls metabolism, reproduction and protein synthesis. Some cells produce hair-like extensions of their cellular membrane to increase their surface area and speed absorption (microvilli), to move liquids across their surface (cilia), or to move through tissue and body fluids (flagella).
In order to perform efficiently, all cells require a constant supply of nutrients and the steady elimination of waste products. Both nutrients and wastes are transported to and from the cells via the circulatory system. They are either suspended within the blood or lymph as solutes or bound to carrier cells. They must be able to freely pass through the semipermeable membranes of the capillaries and cells. Passage generally depends on a combination of their molecular size, shape, electrical charge and lipid solubility. It is through the extracellular fluid that the cells either absorb nutrients or dispose of wastes. Movement of both nutrients and wastes across the cell membranes takes place by filtration, diffusion, active transport or vesicular transport. Filtration happens when particles are forced through the cell membrane by hydrostatic pressure.
Diffusion occurs when solutes move across a membrane from an area of higher concentration to an area of lower concentration. Carbon dioxide and oxygen are transported across cell membranes by diffusion. Glucose is also transported across the cell membrane by diffusion; however, its entry is facilitated by the presence of the hormone, insulin. Osmosis is a form of diffusion, wherein water, not a solute, diffuses through a semipermeable membrane to equalize the concentration on each side of the membrane. A working knowledge of osmosis is necessary to prevent, assess, and treat heat-related illnesses.
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Active transport occurs when a cell uses energy, that is, converts ATP into ADP, to move solutes across its membrane, often against the concentration gradient. For example, the sodium/potassium pump uses ATP to move three sodium ions out of the cell for every two potassium ions moved into the cell to maintain a higher concentration of sodium in the extracellular fluid and a higher concentration of potassium inside the cell that allows different charges to exist on either side of a cell membrane at the same time allows nerve cells to communicate with the remainder of the body. (See nerve impulses here).

In vesicular transport, nutrients, products, waste, or pathogens are packaged in extensions of the cell membrane for transport into or out of the cell.
As stated previously, all cells require oxygen and glucose in order to survive. Cells break down glucose to produce chemical energy, primarily in the form of ATP, and heat: C6H12O6 + 6O2 —> 6CO2 + 6H2O + ATP. The energy is used by the cell to carry out its functions. A cell deprived of oxygen and glucose will eventually die. Carbon dioxide is a waste product of cellular metabolism. It is picked up by the blood and eliminated through the lungs. Other cellular waste must be transported to the kidneys for removal. If waste products build to toxic levels, cellular function will decrease and the cell will die. In order for the transportation of both nutrients and wastes to be effective, fluid levels within the human body must remain within tight parameters. Cellular function will significantly decrease or stop if fluids fall below acceptable levels.
As stated previously, all cells require oxygen and glucose in order to survive. Cells break down glucose to produce chemical energy, primarily in the form of ATP, and heat: C6H12O6 + 6O2 —> 6CO2 + 6H2O + ATP. The energy is used by the cell to carry out its functions. A cell deprived of oxygen and glucose will eventually die. Carbon dioxide is a waste product of cellular metabolism. It is picked up by the blood and eliminated through the lungs. Other cellular waste must be transported to the kidneys for removal. If waste products build to toxic levels, cellular function will decrease and the cell will die. In order for the transportation of both nutrients and wastes to be effective, fluid levels within the human body must remain within tight parameters. Cellular function will significantly decrease or stop if fluids fall below acceptable levels.