Monocular compound microscopes are useful tools in studying and analyzing cell and tissue structure of plants and animals. Each structure can be magnified, examined and differentiated using monocular compound microscopes. Aside from the physical characteristics, the effects of various chemical processes inside a tissue or a cell can be also studied using monocular compound microscopes.
It is an indication of the dynamic role played by the cell membrane that simple sugars and polar amino acids regularly move through it, even though they are insoluble in lipid and far too large to move through pores in the membrane. Clearly, as evident by using monocular compound microscopes, there must be a special mechanism whereby some substances not soluble in the lipid bilayer can nonetheless be transported through it.
The number and kinds of protein molecules in cell membranes vary among different kinds of cells as seen when viewed under monocular compound microscopes. At least some of these proteins are thought to function as enzymelike carriers or permeases, as they are often called. Since different types of cells differ in their permeability characteristics, each type must have in its membrane permeases spe¬cific to the particular materials it takes in or releases. It is assumed that each permease is oriented in such a way that its active site is exposed at the membrane surface, and that the molecule to be trans¬ported binds to this site (probably by weak bonds). How transport is achieved after formation of the permease-substrate complex (P-S) is still not known, but according to one widely accepted model, the binding of substrate causes the permease to undergo a change in its three-dimensional folding that shifts the substrate along the permease molecule (perhaps through a channel in the molecule) from the first binding site near the pickup side of the membrane to a second binding site near the release side.
Whatever the actual mechanism, two categories of carrier-mediated transport can be distinguished. In one case the, permease merely ac¬celerates movement of a substance through the membrane by enabling it to penetrate the membrane more easily, but the direction of move¬ment is determined by the concentration gradient of the substance itself. Because the movement is with a favorable concentration gradient, and hence makes no energetic demands on the cell, this process is called passive transport, or facilitated diffusion.
In active transport, on the other hand, the substance being trans¬ported is moved against its concentration gradient, from a region where it is in lower concentration to one where it is in higher concen¬tration. Because the movement is against the concentration gradient, it is possible only through the expenditure of energy by the cell; the cell must actively perform work. For example, when seen under monocular compound microscopes, many cells perform active transport to maintain a concentration of sodium that is lower inside the cell than in the surrounding fluid. Since the mem¬brane is somewhat permeable to sodium, sodium diffuses passively into the cell, but as fast as it does so, the cell actively pumps it out again, against the concentration gradient.
Endocytosis and exocytosis
Using monocular compound microscopes, substances are seen to enter a cell without ac¬tually moving through the cell membrane. By an active process called endocytosis (endo- means “within”), the cell encloses the substance in a membrane-bounded vesicle pinched off from the cell membrane. Two types of endocytosis are recognized. The first type is phagocytosis, when the material engulfed is in the form of large particles or chunks of matter. Usually, armlike processes of the cell, called pseudopodia, flow around the material, enclosing it within a vesicle, which then becomes detached from the plasma membrane and migrates into the interior of the cell. The second type is pinocytosis, when the engulfed material is liquid or consists of very small particles. The material first becomes adsorbed on the cell membrane (i.e. becomes attached to its surface), probably at selective binding sites. Then the loaded membrane either flows inward to form deep narrow channels, at the end of which vesi¬cles take shape, or small vesicles are simply detached directly from the membrane at the cell surface.
When material is enclosed within vesicles, it has not yet entered the cell in the fullest sense. It is still separated from the cellular substance by a membrane, and it must eventually cross that membrane (or the membrane must disintegrate) if it is to become incorporated into the cell. Often the material is first acted on by enzymes in the vesicles and broken down into smaller, simpler substances that can move more easily across the vesicular membranes.
In a process essentially the reverse of endocytosis, called exocytosis, materials contained in membranous vesicles are conveyed to the pe¬riphery of the cell, where the vesicular membrane fuses with the cell membrane and then bursts, releasing the materials to the exterior.
The osmotic effects of the fluids bathing cells
It would be a mistake to suppose that the plasma membrane can completely regulate the exchange of materials between the cell and the surrounding medium and always maintain optimum conditions within the cell. Some poi¬sons can apparently move freely across the plasma membrane, much to the detriment of the cell. And some beneficial substances are lost to the cell because the membrane cannot prevent them from diffusing out, as seen using monocular compound microscopes.
Furthermore, the great permeability of the membrane to water can be harmful or even fatal to the cell. When a cell is in a medium that is hypertonic relative to it (i.e. in a medium to which it loses water by osmosis, usually because the medium contains a higher concentration of osmotically active particles), the cell tends to shrink; and if the process goes too far, it may die. Conversely, when a cell is in a medium hypotonic to it (i.e. in a medium from which it gains water, usually because the medium contains a lower concentration of os¬motically active particles), the cell tends to swell; and unless it has special mechanisms for expelling the excess water, or special struc¬tures that prevent excessive swelling (as most plant cells do), it may burst. A cell in an isotonic medium (i.e. one with which the cell is in osmotic balance, usually because it contains the same concentration of osmotically active particles) neither loses nor gains appreciable quan¬tities of water by osmosis.
The osmotic relationship between the cell and the me¬dium surrounding it is a critical factor in the life of the cell. Some cells are normally bathed by an isotonic fluid and therefore have no serious osmotic problems. Human red blood cells are an example; they are normally bathed by blood plasma, with which they are in rela¬tively close osmotic balance. Most simple oceanic plants and animals also exemplify cells in an isotonic medium; their cellular contents have an osmotic concentration close to that of seawater. All cells, however, have a higher osmotic concentration than freshwater. Freshwater organisms thus live in a hypotonic medium and face the problem of accumulating excessive water within their cells by osmo¬sis. Their very existence has depended on the evolution of ways of preventing their cells from becoming so turgid (distended by their fluid contents) that they would burst. The osmotic relationship between cells and a solution can be demonstrated in classroom experiments using monocular compound microscopes.