Transportation in Plasma Membrane

Plasma Membrane Transport 
The plasma membrane allows only certain molecules to enter and exit the cytoplasm freely; therefore, the plasma membrane is said to be selectively permeable. Transport of molecules across the membrane can be active or passive. Active transport requires the use of ATP energy; passive trans­port does not. Passive transport mechanisms include diffu­sion, osmosis, and filtration.
Passive Diffusion
Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentra­tion. For example, if a perfume bottle is opened in one cor­ner of a room, the perfume's scent will soon be apparent throughout the room because the perfume molecules have diffused from an area of high concentration (the corner) to areas oflower concentration. Another example of diffusion is putting a tablet of dye into water. The water eventually takes on the color of the dye as the tablet dissolves.
In the body, oxygen enters the blood from the alveoli (air sacs) ofthe lungs by diffusion. During kidney dialysis, waste molecules diffuse across a membrane from the area of higher concentration (the blood) to the area of lower concentration (the dialysate). Cells do not expend any energy when sub­stances can simply diffuse across the plasma membrane.
Some molecules that cannot cross the phospholipid bilayer of the plasma membrane diffuse through plasma membrane protein channels. Passive transport of this sort is a form of facilitated diffusion.
Osmosis 
Osmosis (oz-mo'sis) is the diffusion of water across a plasma membrane. It occurs whenever the concentrations of water on either side of a selectively permeable mem­brane are unequal. Normally, body fluids are isotonic (i"so-ton'ik) to cells, that is, the concentrations of substances (solutes) and water (solvent) on either side of the plasma membrane are equal and cells, therefore, maintain their usual size and shape. For this reason, most intravenous solutions are also isotonic to cells.
If red blood cells are placed in a hypotonic (hi "po­ton'ik) solution, which has a higher concentration of water (lower concentration of solute) than do the cells, water will enter the cells, and they will swell to bursting. Bursting of red blood cells is called hemolysis. On the other hand, if red blood cells are placed in a hypertonic (hi"per-ton'ik) solution, which has a lower concentration of water (higher concentration of solute) than do the cells, water will leave the cells, and they will shrink. The shrinking of red blood cells is called crenation.
Filtration 
Because capillary walls are only one cell thick, small mole­cules (water, small solutes) tend to passively diffuse across these walls, from areas of higher concentration to those of lower concentration. However, blood pressure aids matters by pushing water and dissolved solutes out of the capillary. This process is called filtration.
Filtration is easily observed in the laboratory when a so­lution is poured past filter paper into a flask. Large substances stay behind, but small molecules and water pass through.
Filtration of water and substances in the region of cap­illaries is largely responsible for the formation of tissue fluid, the fluid that surrounds the cells. Filtration is also at work in the kidneys when water and small molecules move from the blood to the inside of the kidney tubules.
Active Transport 
In active transport, substances accumulate either inside or outside the cell in the region of higher concentration. For ex­ample, iodine collects in the cells of the thyroid gland; sugar is completely absorbed from the gut by the cells lining the digestive tract; and sodium is sometimes almost completely withdrawn from uri ne by cells lining the kidney tubules.
Carrier proteins and an expenditure of energy are both needed to transport substances from an area of lower concentration to an area of higher concentration. A carrier is a plasma membrane protein that specializes in combining with and transporting substances across the plasma membrane. Because ATP energy is needed to cause a carrier to combine with the substance to be transported, cells primarily involved in active transport, such as kidney cells, have a large number of mitochondria near the mem­brane where active transport is occurring.
During active transport. vvhich requires plasma membrane carriers and ATP energy. substances move against a concentration gradient and accumulate in the area of higher concentration.
Endocytosis and Exocytosis
At times, substances are taken into cells by vesicle forma­tion. This is called endocytosis (en"do-si-to'sis). When the material taken in is quite large, the process is called phagocytosis (fag" o-si-to' sis) (cell eating). Phagocy­tosis is common to amoeboid-type cells, such as macro­phages. These white blood cells are called the body's scavengers because they engulf worn-out red blood cells and other types of debris. When cells take in material that is small enough to be dissolved or suspended in water, the process is called pinocytosis (pi" no-si- to' sis) (cell drinking).
Vesicles within the cytoplasm of the cell can fuse with the plasma membrane and release their contents to the outside of the cell. This is called exocytosis (ex" o-si-to'sis). Some cells of the nervous system release substances in­volved in the transfer of nerve impulses between adjacent cells via exocytosis.

Posted in Human Anatomy and Physiology