The part of information is from the book "Memmler's The Human Body In Health And Disease" by Barbara Janson Cohen and Jason J. Taylor

Figure 18 The effect of osmosis on cells. Water moves through a red blood cell membrane in solutions with three different concentrations of solute. (A) The isotonic (normal) solution has the same concentration as the cell fluid, and water moves into and out of the cell at the same rate. (B) A cell placed in a hypotonic (more dilute) solution draws water in, causing the cell to swell and perhaps undergo hemolysis (bursting). (C) The hypertonic (more concentrated) solution draws water out of the cell, causing it to shrink, an effect known as crenation.

TYPE OF SOLUTION	DESCRIPTION	EXAMPLES	EFFECT ON CELLS
Isotonic	Has the same concentration of dissolved substances as the fluid in the cell	0.9% salt (normal saline); 5% dextrose (glucose)	None; cell in equilibrium with its environment
Hypotonic	Has a lower concentration of dissolved substances than the fluid in the cell	Less than 0.9% salt or 5% dextrose	Cell takes in water, swells, and may burst; red blood cell undergoes hemolysis
Hypertonic	Has a higher concentration of dissolved substances than the fluid in the cell	Higher than 0.9% salt or 5% dextrose	Cell will lose water and shrink; cell undergoes crenation

Table 6 Solutions and Their Effects on Cells

How Osmosis Affects Cells

As stated earlier, water usually moves easily through the cell membrane. Therefore, for a normal fluid balance to be maintained, the fluid outside all cells must have the same concentration of dissolved substances (solutes) as the fluids inside the cells (Fig. 18). If not, water will move rapidly into or out of the cell by osmosis. Solutions with concentrations equal to the concentration of the cytoplasm are described as isotonic. Tissue fluids and blood plasma are isotonic for body cells. Manufactured solutions that are isotonic for the cells and can thus be used to replace body fluids include 0.9% salt, or normal saline, and 5% dextrose (glucose). A solution that is less concentrated than the intracellular fluid is described as hypotonic. Based on the principles of osmosis already explained, a cell placed in a hypotonic solution draws water in, swells, and may burst. When a red blood cell draws in water and bursts in this way, the cell is said to undergo hemolysis. If a cell is placed in a hypertonic solution, which is more concentrated than the cellular fluid, it loses water to the surrounding fluids and shrinks, a process termed crenation (see Fig. 18). Fluid balance is an important facet of homeostasis and must be properly regulated for health. You can figure out in which direction water will move through the plasma membrane if you remember the saying “water follows salt,” salt meaning any dissolved material (solute). Table 6 summarizes the effects of different solution concentrations on cells.

Cell Aging
As cells multiply throughout life, changes occur that may lead to their damage and death. Harmful substances known as free radicals, produced in the course of normal metabolism, can injure cells unless these materials are destroyed. Lysosomes may deteriorate as they age, releasing enzymes that can harm the cell. Alteration of the genes, or mutations, are a natural occurrence in the process of cell division and are increased by exposure to harmful substances and radiation in the environment. Mutations usually harm cells and may lead to cancer. As a person ages, the overall activity of the body cells slows. One example of this change is the slowing down of repair processes. A bone fracture, for example, takes considerably longer to heal in an old person than in a young person. One theory on aging holds that cells are preprogrammed to divide only a certain number of times before they die. Support for this idea comes from the fact that cells taken from a young person divide more times when grown in the laboratory than similar cells taken from an older individual. This programmed cell death, known as apoptosis, is a natural part of growth and remodeling before birth in the developing embryo and in repair and remodeling of tissue throughout life (see Box 3, Necrosis and Apoptosis: Cellular Homicide and Suicide).

Box 3 Necrosis and Apoptosis: Cellular Homicide and Suicide
Cell death happens in two ways: by necrosis, because the cell is injured; or by apoptosis, because the cell is programmed to die. One way of remembering the difference is to think of necrosis as “cellular homicide” and apoptosis as “cellular suicide.” Necrosis disrupts the cell’s normal water-balancing mechanisms and stimulates autolysis. As a result, the cell swells and its organelles break down. Finally, the cell ruptures, releasing its contents into the surrounding tissue. These contents contain digestive enzymes that damage adjacent cells, producing more injury and necrosis. Apoptosis is an orderly, genetically programmed cell death triggered by the cell’s own genes. Under the right circumstances, these “suicide genes” produce enzymes called capsases that destroy the cell swiftly and neatly. The cell shrinks, and phagocytes quickly digest it. In contrast to necrosis, apoptotic cells do not cause further chaos when they die. Apoptosis is a normal bodily process. It is especially important during embryonic development because it removes unneeded cells, such as those from limb buds to form fingers and toes. Apoptosis also occurs after birth, as when cells subject to extreme wear and tear regularly undergo apoptosis and are replaced. For example, the cells lining the digestive tract are removed and replaced every 2 to 3 days.

Cells and Cancer
Certain mutations (changes) in the genetic material of a cell may cause that cell to reproduce without control. Cells that normally multiply at a fast rate, such as epithelial cells, are more likely than slower-growing cells to undergo such transformations. If these altered cells do not die naturally or get destroyed by the immune system, they will continue to multiply and may spread (metastasize) to other tissues, producing cancer. Cancer cells form tumors, which interfere with normal functions, crowding out normal cells and robbing them of nutrients.

Cancer Risk Factors
The causes of cancer are complex, involving interactions between cellular factors and the environment. Because cancer may take a long time to develop, it is often difficult to identify its cause or causes. Certain forces increase the chances of developing the disease and are considered risk factors. These include the following:
* Heredity. Certain types of cancer occur more frequently in some families than in others, indicating that
there is some inherited predisposition to the development of cancer.
* Chemicals. Certain industrial and environmental chemicals are known to increase the risk of cancer. Any chemical that causes cancer is called a carcinogen. The most common carcinogens in our society are those present in cigarette smoke. Carcinogens are also present, both naturally and as additives, in foods. Certain drugs also may be carcinogenic.
* Ionizing radiation. Certain types of radiation can produce damage to cellular DNA that may lead to cancer. These include x-rays, rays from radioactive substances, and ultraviolet rays. For example, the ultraviolet rays received from exposure to the sun are very harmful to the skin.
* Physical irritation. Continued irritation, such as the contact of a hot pipe stem on the lip, increases cell division and thus increases the chance of mutation.
* Diet. It has been shown that diets high in fats and total calories are associated with an increased occurrence of certain forms of cancer. A general lack of fiber and insufficient amounts of certain fruits and vegetables in the diet can leave one susceptible to cancers of the digestive tract.
* Viruses have been implicated in cancers of the liver, the blood (leukemias), and lymphatic tissues (lymphomas).