Broadly speaking and substance that may be brought into the cell and utilized in chemical reactions is a nutrient material. Three classes of nutrient materials are required by cells, and by organisms: organic compound, inorganic’ materials, chiefly salts, and water. However, cells vary greatly in their relative requirements for these nutrients, especially with regard to organic compounds.
For the moment, let us ignore any nutritional distinctions between complex organisms and consider the requirements of cells themselves. Nutritionally speaking there are basically two types of cell: those which are dependent upon their environment for organic fuel molecules and these which manufacture their own.
The latter type of cell is found only in autotrophic, organisms, whereas the former type is found in all complex forms, autotrophic or otherwise. Not at cells of a multicellular green plant, carry on photosynthesis; those which do not do so depend upon photosynthetic cells within the same plant for their organic nutrients. In heterotrophic plants and in animals, of course, each cell must receive oxidizable organic compounds from a source external to the organism of which it forms a part.
Since we shall discuss the utilization of inorganic materials and water in a later topic, let us concentrate momentarily upon the organic nutrients and their role within the cell. Whether such compounds are formed within the cell which utilizes them as an energy course or whether they are transported in, they serve two major functions.
They may be chemically “shattered”, in which case free energy is made available to the cell, or the carbon chains may be used as building blocks in synthetic processes. The major organic nutrients are carbohydrates, lipids, and proteins.
Under certain circumstances, each of these types may be utilized as energy sources, or again, each of these types may be utilized as energy’ sources, or again, each may be utilized in synthetic reactions. Generally speaking, however, the carbohydrates and lipids are -preferred” by the cell as energy-yielding compounds, and protein functions chiefly in synthesis.
In actual practice, there is a rapid turnover in the cell of all organic molecules except for certain ones such as those of DNA which are somewhat isolated from the enzymatic machinery. This means that virtually everything in the cell is a fuel substance in the final analysis.
To draw an analogy, let us suppose that a certain lumber dealer finds it necessary, for some reason, to keep a fire going in a furnace. Under ordinary circumstances, he uses coal oil as fuels because they are cheap and readily available. He might use the furnace, since he has it anyway, to dispose of scrap lumber and assorted waster products, but he does not depend upon these for fuel. They are only incidentally burned.
Now let us assume that his supply of coal and oil is cut off. He might be forced to use whatever he can find that will burn, and if maintenance of his fire is sufficiently import to him, he might even be obliged to burn his own valuable lumber.
Of course, this analogy is only a rough one, since we do not ascribe consciousness and purpose to the cell as we do to a human being, but there are valid parallels. The coal and oil are analogous to carbohydrates and fats, the lumber to proteins, and the scraps of material to the various “used” molecules of the cell which are disposed of in the metabolic “fire”.
Even though we cannot draw an exact distinction between fuel and nonfuel compounds, the original distinction if valid for all practical purposes. Carbohydrates and fats are the only materials that furnish energy to the cell in very large amounts, at least under most conditions.
Proteins may or may not play a significant role in this respect, depending upon their quantity and the relative quantities of the other two fields. Any other organic molecules that are broken down may yield some energy, but in amounts that are quite insignificant when compared with that furnished by carbohydrates, fats, and proteins.
At this point, let us consider how nutrient materials fit into cells. Of necessity, these materials are present in the environment of the cell. Depending upon the particular type of cell, this environment may consist of pond water, the general atmosphere, or some form of body fluid such as blood. It may also include other cells.
Regardless of the general environment, a cell is almost always surrounded by some aqueous medium, even if this medium exists only as a thin film separating closely-packed cells. The aqueous medium is in direct contact with the cell membrane, even if the cell possesses a wall.
Most of the problems associated with understanding absorption involve a consideration of the structure and function of cell membranes. In addition, the nature and concentration of the nutrient substances which are absorbed must be considered.
The cell membrane has sometimes been pictured as a passive barrier containing pores which admits small particles and rejects large ones. That the membrane is porous to a degree seems evident, and in general, small molecules are absorbed more readily than large ones. However, this view of the membrane is highly misleading, because several factors govern the passage of substances across it.
Apparently, the major factors involved in absorption are osmosis, diffusion, electrical charge, solubility in lipids, and active transport. Osmosis may be defined as the movement of water from a region or relatively low concentration of dissolved substances into a region of relatively high concentration across a membrane which is impervious to at least some solute particles.
A balance of water between the cell and its environment seems to be at least partially explicable on the basis of osmosis. If the two solutions are equal to each other in concentration of dissolved particles, they are said to be isotonic, in which case there is no net movement of water across the membrane.
In order to demonstrate that living cells are subject to pressure changes associated with osmosis, let us consider a cell such as that of Spirogyra, a filamentous green alga which thrives in ponds of fresh water. Although its surrounding medium is never absolutely pure water, it is nevertheless hypotonic to the protoplasmic contents of the cell. There is thus a continual tendency for osmosis to occur inward. Why, it might be asked, does the cell not increase in size until it bursts? The reason lies in the rigidity of the cellulose cell wall, which is sufficient to resist bursting under these conditions.
Enough pressure builds up inside the cell to establish an osmotic equilibrium. Whenever a cell exhibits an internal pressure due to osmosis, it is said to be turgid, and such pressure is called turgor pressure.
Now suppose that a filament of Spirogyra is put into a solution prepared by dissolving five grams of sodium chloride in sufficient water to make the entire solution equal one hundred milliliters.
This solution proves to be hypertonic to the protoplasm of the cells, and within amatter of seconds sufficient water leaves a given cell to cause a shrinking of the protoplasm. The cell wall, being rigid, remains in place and the plasma membrane actually draws away from it.
This loss of turgidity on the part of the cell due to osmosis is called plasmolysis, and a cell whose turgidity is less than that experienced in its normal environment is said to be flaccid. Unless plasmolysis has occurred to a critical degree, normal turgidity may be restored to the cells of the Spirogya filament by replacing the sodium chloride solution with pond or tap water, thus reversing the direction of osmosis.
Many freshwater organisms which do not possess rigid cell walls manage to withstand turgor pressures by “bailing out” excess water. Were it not for this mechanism, such delicate cells would soon burst. In higher animals, cells are surrounded by fluids which are isotonic to the protoplasm.
Although water itself is not an energy-yielding substance, it is of paramount important to the metabolic life of cells, and hence to organisms, in that it serves as a solvent for and a carrier of a variety of compounds both inside and outside the cell.
Not only is this the case, but it makes possible enzymatic reactions which could not occur otherwise. Furthermore, its molecules actually enter into certain metabolic reactions.
Diffusion is the net movement of particles resulting from their tendency to be distributed evenly throughout a given space. In a sense, it is exemplification of the second law of thermodynamics in that particles tend toward as random distribution within their particular system.
It has been observed that charged particles enter the cell less readily than uncharged ones. As arule, the greater the charge the less freely a particle moves across the cell membrane. For instance, such monovalent ions as K+ and CI- enter the cell more readily than divalent or multivalent ones.
This phenomenon is probably best explained by the nature of the proteins present in the cell membrane. In general, protein molecules are highly charged, and thus tend to repel charged particles.
Still another factor known to be involved in absorption is the degree to which a substance is soluble in lipids. It has been postulated that since lipid materials constitute a large proportion of the cell membrane, such substances as the higher alcohol are more readily accepted by the membrane than less lipid-soluble materials.
Of all the factors which influence the passage of nutrients through cell membranes, active transport is probably the most important, and at the same time, the least understood. As the name implies, active transport involves the passage of a substance through the cell membrane with an expenditure of energy on the part of the cell.
In other words, a metabolic process is involved, and the cell is obliged to perform work in transporting many substances to its interior. There are numerous examples of ions and large molecules exhibiting low solubility in lipids being transported into the cell against an electrical and a concentration gradient.
Furthermore, if in such cases, which indicates that transport of these materials is dependent upon the processes of metabolism. Work is required on that part of the ell in keeping such ions from establishing equilibrium on both sides of the membrane.
Although relatively little is known about the precise mechanisms involved in active transport; it appears that it is a very complex phenomenon, and one that is highly important in a wide variety of absorption phenomena.
In addition to the five factors which we have discussed, cells occasionally take in relatively large droplets of materials by a process called pinocytosis.
Furthermore, cells of multicellular animals have been observed to ingest relatively large particles of organic matter in similar fashion to the feeding habits of an amoeba. This phenomenon is called phagocytosis. Pinocytosis and phagocytosis may be considered to be special cases of active transport.