The cell (plasma) membrane is a barrier specifically designed to prevent random objects from crossing at will. Yet cells must constantly exchange materials with their outside environment if they are to survive. How do some substances get across the cell membrane barriers when other do not? As we are about to see, there are four principle ways in which materials are moved into and out of cells.
Diffusion
The process of diffusion is the random movement of substances from a region of high concentration to a region of low concentration until no differences in concentration remain. For example, consider what happens when you make a cup of tea. The tannin molecules, which give the tea its golden brown color, are very concentrated in a tea bag but absent from the hot water. As they start to dissolve, the molecules inside the bag begin to move faster and faster. One molecule bumps into anotherabd then into a second and a third and so on until the molecules gradually spread out and leave the vicinity of the tea bag, and turn the water a marvelous golden color. Slowly, the tannin molecules have moved away from a region of high concentration near the tea bag and into a region of low concentration in the surrounding water until eventually the tannin molecules are at equal concentrations throughout the liquid.
As the tannin molecules randomly disperse themselves throughout the liquid, they are obeying the second law of thermodynamics, and the amount of disorder in the universe is increased.
Osmosis
Like other substances, water diffuses from regions where it is in high concentration to regions where it is in low concentrarion. One important region of high water concentrarion is just outside the cell where the fluid is mainly water that contains few dissolved substances. On the other hand, water is at a lower concentration inside the cell because of the presence of sugars, amino acids, proteins and organelles as well as other substances. Water therefore, diffuses from its high concentration outiside the cell to the low concentration inside the cell, passing through pores in the hydrophobic cell membrane.
The cell membrane, however, is a very selective barrier not permeable to many other substances found at high concentrations inside the cell. It is said to be semipermeable; that is, water can move freely across the cell membrane, but other materials cannot. As a result, there is a net flow of molecules into the cell as water diffuses in and other materials are prevented from moving out. Passage of water from a dilute solution to a more concentrated solution across a semipermeable membrane is called osmosis. If a cell does not compensate for the effects of osmosis, it will gradually swell with water and burst.
How do cells compensate for the effects of osmosis? Cell walls surrounding plants, fungi and most prokaryotes are strong and rigid. Such cells can expand only until the swollen cell presses up against this immovable barrier. Single-celled, freshwater animals contain pumps. These organelles, called contractile vacuoles, specialize in collecting excess water and expelling it from the cell. In multicellular organisms like ourselves, each cell is surrounding by fluid that contains salts, sugars and so forth at the same concentration as those found inside the cell. Osmosis is not a problem, therefore, unless the balance of materials outside the cells is disturbed, asi it is in some medical conditions.
Transport
Diffusion plays an important role in moving small molecules short distances within and around cells. However for large molecules and many others diffusion is too slow and nonselective. If they are to obtain vital ingredients in a timely manner, cells must have a way of speeding up the movement of molecules into and out of their cytoplasm.
Also, the cell membrane is impermeable to many of the larger substances needed by cells. Sugars, amino acids, etc. Cannot simply diffuse from one side of the membrane to the other. Cells, therefore, transport these needed molecules across the membrane using special carrier proteins. These proteins, located in the membrane, are very specific and attach only to certain types of molecules. In this way, a cell can carefully regulate the amount and types of molecules that pass into and out of it.
Passive transport
Carrier proteins can move molecules only in the same direction they woul normally move by diffusion. This process, called facilitated diffusion, allows the cell to control what materials are transported across the membrane and in what quantity. No energy is required.
As its name suggests, diffusion is still important in this transport mechanism. Molecules, like sugars, reach carrier proteins in the membrane by diffusion and are then moved across the membrane from a region of high concentration to one of low concentration. For example, if a cell is placed in a sugar solution, sugar molecules will diffuse and make contact with carrier proteins within the membrane. Carrier proteins will then transport these sugars across the membrane and into the cell, but only as long as the concentration of sugar is greater outside the cell than inside.
Active transport
There are many instances, however, in which the cell must move substances from regions of low concentration to regions that are already highly concentrated. For example, a particular sugar may be in dilute solution outside the cell as well as in high concentration inside the cell, but it would be beneficial for the cell to accumulate more of this valuable resource. Built into the membrane are carrier proteins that attach to sugar molecules outside the cell and then use energy to pump these molecules against the normal direction of diffusion and into the cell. This system is known as active transport. Energy is required for active transport because work is being done against a natural force of diffusion.
Endocytosis and Exocytosis
For larger particles, cell must use other processes. Endocytosis is a general term for the process whereby very large particles of material are wrapped with plasma membrane and moved into the cell in form of vesicles of vacuoles. None of the trapped material actually moves through the membrane, but it remains on the other side of the original membrane, even while the vacuole is inside the cell.
Exocytosis is the reverse of endocytosis. Quantities of material are expelled from the cell without ever passing through the membrane as individual molecules. By using the processes fo endocytosis and exocytosis, some specialized types of cells move large amounts of bulk material into and out of themselves.
Solid particles are engulfed by phagocytosis (cell eating) a process that begins when solids make contact with the outer cell surface, triggering the movement of the membrane. The desired particles are then enclosed within a small piece of plasma membrane, which forms a sac called vacuole (vesicle), with the food particles inside. This vacuole is the moved to the interior of the cell. Strictly speaking, the food particles are not yet part of the cell because they are still surrounded by membrane.
Before food can be used, it must be broken down into smaller pieces and those pieces moved into the cytoplasm. Digestion occurs when the food vacuole is fused with a second vacuole called a lysosome that contains power digestive enzymes. Food is degraded, its nutrients are absorbed by the cell, and its waste products are left in the digestive vacuole, which may then leave the cell by exocytosis.
Phagocytosis occurs in the scavenging white blood cells of our body. They prowl around looking for invading bacteria and viruses, which they engulf and destroy. Pinocytosis (cell drinking) is almost the same process as phagocytosis, except it involves liquids instead of solids.
During exocytosis, a vacuole containing material to be excreted from the cell moves to the plasma membrane and fuses with it. The vacuole membrane becomes part of the plasma membrane and the contents are released to the outside. Cells use this method to eliminate wastes left from digestion and metabolism and also to release a whole variety of materials that have been synthesized inside the cell but are needed outside the cell. Release of hormones and digestive enzymes, found in multicellular animals, are two examples of this process.
