DIFFUSION & OSMOSIS

Diffusion is the result of the random motion of molecules.

According to Newton's laws of motion, objects will remain in motion and travel in a straight line unless acted upon by an unbalanced force. Thus we have molecules constantly moving back and forth, colliding with each other, bouncing off, and spreading outward away from the middle. This results in a gradual spreading of molecules from areas of great concentration to areas of lesser concentration. Many things can diffuse. Odors diffuse through the air. Salt diffuses through water. Nutrients diffuse from the blood to the body tissues.

The difference between the greater concentration and the lesser concentration is called a concentration gradient. Just as water will always flow from high to low, molecules will always diffuse down the gradient, from high to low. However, when there are equal numbers of molecules in each area, a state of equilibrium has been reached. At this point molecules continue to move, but they now move back and forth in equal amounts.

Diffusion of substances and their absorption by the body is an important part of many of our body functions. The regulation and maintenance of a steady balance of substances in the body is referred to as homeostasis. The cell membrane is the most important part of the cell in regard to homeostasis of substances. In living organisms diffusion is sometimes complicated by the fact that the cell membrane will allow some substances to pass through it but not others. If a substance can pass through the membrane we say that the membrane is permeable to it. So the cell membrane is described as being selectively permeable. Water can almost always diffuse through the membrane although its dissolved solutes may not always do so. This may create concentration gradients (areas of high and low concentration) on either side of the membrane. This diffusion of water through a cell's selectively permeable membrane is termed osmosis.

The main direction in which the water flows is determined by the concentration gradient. The concentration gradient of solutions is indicated by use of the terms hypotonic, hypertonic, and isotonic.

The prefix hypo means "less than". The prefix hyper means "greater than". The prefix iso means "equal".

ANSWER QUESTIONS 1-6

In a hypotonic solution the concentration of dissolved solutes is less than that of the cell. This means that the water concentration will be higher outside the cell and more water will move into the cell. Cells placed in this type of solution may swell, build up pressure, or burst. The bursting of cells, cytolysis, happens more often in animal cells because they have no thick cell walls. In order to prevent this, some unicellular organisms pump out the extra water using organelles called contractile vacuoles. Plant cells, with their thick cell walls, may build up a pressure called turgor.

EXAMPLE:

Blood cells have a salt concentration of 0.9% salt. This means that they are approximately 99% water (This is higher than most cells). When blood cells are placed in distilled water (pure water, 100% water), more water diffuses into the membrane than diffuses out. Think of it in this way. Since the substance outside the membrane is 100% water molecules (and the membrane is permeable to water) all of the molecules that try to diffuse through, will be able to pass into the cell. However, only 99% of the molecules on the inside of the cell are water molecules. Thus only 99% of the molecules that try to diffuse out will be able to. Therefore there are 100 molecules going in, for every 99 that come out. Eventually the increase in water may cause the blood cells to burst (cytolysis.)



ANSWER QUESTIONS 7-11

In a hypertonic solution, the concentration of dissolved solutes is greater than that of the cell. This means that the water concentration outside the cell will be less than the concentration inside. More water will diffuse out of the cell than in. When this occurs the cell shrinks or shrivels. This is called plasmolysis.

Example: When salt is poured on a snail, a hypertonic solution is created because the concentration of salt is greater outside his body than inside the cells. If the salt solution outside is 20% salt, then the water concentration is 80%. If the water concentration inside the snail's cells is in the 90% range, more water diffuses out of the cell than in. In reality water diffuses both directions. But for every 80 molecules that diffuse in, 90 molecules diffuse out. As a result the cells of the snail shrink or shrivel (plasmolysis).

In an isotonic solution, the concentration of the dissolved solutes is the same inside and outside the cell. If the water concentration of the snail cells is 90% (10% solutes) and they are placed into a 10% salt solution (90% water), then equal numbers of water molecules will diffuse in and out of the cells.



ANSWER QUESTIONS 12-16

OTHER TYPES OF TRANSPORT:

Substances may pass through the membrane by diffusion, moving from areas of higher concentrations to lower concentrations without using any energy from the cell. These types of movment requiring no use of cell energy are called Passive Transport. They include simple diffusion, osmosis, and a type of assisted diffusion called Facilitated Diffusion. To "facilitate" means to make easier. Facilitated diffusion is used to speed up the diffuion of larger molecules, like glucose, through the membrane. In order to do this, special protein molecules imbedded in the lipid membrane combine with glucose and then change shape, pulling the glucose through the membrane. These special protein molecules are called carrier molecules. Facilitated diffusion works only when the diffusion is from high to low.

Cells also have special protein molecules called "gated-channels" that can be turned on or off as needed to make the membrane permeable or impermeable to certain substances. These open up at specific times to allow substances to diffuse through.

Sometimes the cell needs to move substances through the membrane against the concentration gradient (from low to high). Going against the concentration gradient requires the use of energy (like paddling upstream). The use of energy to transport substance across the membrane is called Active Transport. One common use of active transport is used to maintain high levels of sodium ions outside the cell and high levels of potassium ions inside the cell. This is called the sodium-potassium pump. During active transport ATP molecules cause carrier molecules to move 3 sodium molecules out of the cell and bring 2 potassium molecules into the cell. This unequal distribution positive ions creates an electrical potential that is responsible for the generation of nerve impulses. Other pumps are also used elsewhere in the body.



ANSWER QUESTIONS 17-20

Molecules that are too large for gated channels, pores, or carrier molecules, such as food materials and waste materials may enter and leave the cell through processes called Endocytosis and Exocytosis. During endocytosis, the membrane encloses a large object (such as food) in an inward pouch and then pinches off to the inside where cellular enzymes may digest the food. When endocytosis involves large solid particles it is referred to as phagocytosis. If endocytosis involves movement of dissolved solutes or fluids into the cell, it is referred to as pinocytosis. Amebas feed by using phagocytosis. White blood cells may also destroy bacteria in the body by phagocytosis.

Exocytosis is the process of transporting substances out of the cell that are too large to pass through the membrane. It is the exact reverse of endocytosis. One common example is the export of packaged proteins out of the cell. The ribosome manufactures the proteins which are transported to the golgi apparatus where they are surrounded by a membrane forming a bubble or vesicle. These vesicles move to the membrane, fuse with it, and empty their contents out of the cell.



ANSWER QUESTIONS 21-25