A microorganism’s habitat provides necessary substances—some abundant, others scarce—that must still be taken into the cell for nutrition. The reverse is also true: cells must transport waste materials out of the cell (and into the environment). Whatever the direction, transport occurs across the cell membrane, the structure specialized for this role. This is true even in organisms with cell walls (bacteria, algae, and fungi), because the cell wall is usually only a partial, nonselective barrier. In this section, we discuss some important physical forces in cell transport.
Diffusion and Molecular Motion
All atoms and molecules, regardless of being in a solid, liquid, or gas, are in continuous movement. As the temperature increases, the molecular movement becomes faster due to an increase in kinetic energy. In any solution, including cytoplasm, these moving molecules cannot travel very far without having collisions with other molecules, and they will bounce off each other like millions of colliding pool balls every second (figure 1). As a result of each collision, the directions of the colliding molecules are altered, and the direction of any one molecule is unpredictable and considered random. An example would be a situation in which molecules of a substance are more concentrated in one area than in another. Just by random thermal movement, the molecules will become dispersed away from an area of higher concentration to an area of lower concentration. Over time this will evenly distribute the molecules in the solution. This net movement of molecules down their concentration gradient by random thermal motion is known as diffusion. It can be demonstrated by a variety of simple observations. A drop of perfume released into one part of a room is soon smelled in another part, or a dye spreads through a beaker of water without stirring (figure 1).
Fig1. Diffusion of molecules in aqueous solutions. A brown dye crystal diffuses down a concentration gradient, from high concentration, near the crystal, to low concentration (farther away). As diffusion continues (left to right), the dye eventually is spread evenly throughout the aqueous phase, and a gradient ceases to exist. The system is now said to be at equilibrium. (left): McGraw Hill; (middle and right): Holly Curry/McGraw Hill
Diffusion is a driving force in cell activities, but its effects are greatly controlled by membranes. Two special cases are osmosis and facilitated diffusion. Both processes are considered a type of passive transport. This means that the cell does not expend extra energy for them to function. The inherent energy of the molecules moving down a gradient does the work of transport.
The Diffusion of Water: Osmosis
Diffusion of water through a selectively permeable membrane, a process called osmosis, is a physical phenomenon that is easily demonstrated in the laboratory with nonliving materials. Simple experiments provide a model of how cells deal with various solute concentrations in aqueous solutions (figure 2). In an osmotic system, the membrane is selectively, or differentially, permeable, having passageways that allow free diffusion of water but can block certain dissolved molecules. When this type of membrane is placed between solutions of differing concentrations where the solute (protein, for example) cannot pass across, then under the laws of diffusion, water will diffuse at a faster rate from the side that has more water to the side that has less water. As long as the concentrations of the solutions differ, one side will experience a net loss of water and the other a net gain of water until equilibrium is reached and the rate of diffusion is equalized.
Fig2. Model system to demonstrate osmosis. Here we have a solution enclosed in a membranous sac and attached to a hollow tube. The membrane is permeable to water (solvent) but not to solute. The sac is immersed in a container of pure water and observed over time.
Osmosis in living systems is similar to the model shown in figure 2. Living membranes generally block the entrance and exit of larger molecules and permit free diffusion of water. Because most cells contain and are surrounded by some sort of aqueous solution, osmosis can have far-reaching effects on cellular activities and survival. Depending on the water content of a cell as compared with its environment, a cell can gain or lose water, or it may remain unaffected. Terms that we use for describing these conditions are isotonic, hypotonic, and hypertonic (figure 3).
Fig3. Cell responses to solutions of differing osmotic content.
Under isotonic conditions, the environment is equal in solute concentration to the cell’s internal environment; and because diffusion of water proceeds at the same rate in both directions, there is no net change in cell volume. Isotonic solutions are generally the most stable environments for cells, because they are already in an osmotic steady state with the cell. Parasites living in host tissues are most likely to be living in isotonic habitats.
Under hypotonic conditions, the solute concentration of the external environment is lower than that of the cell’s internal environment. Pure water provides the most hypotonic environment for cells because it has no solute. Because the net direction of osmosis is from the hypotonic solution into the cell, cells without walls swell and can burst when exposed to this condition.
Slight hypotonicity is tolerated quite well by most bacteria because of their rigid cell walls. The light flow of water into the cell keeps the cell membrane fully extended and the cytoplasm full. This is the optimum condition for the many processes occurring in and on the membrane.
A cell in a hypertonic environment is exposed to a solution with higher solute concentration than its cytoplasm. Because hypertonicity will force water to diffuse out of a cell, it is said to create high osmotic pressure or potential. In cells with a wall, water loss causes shrinkage of the protoplast away from the wall, a condition called plasmolysis. Although the whole cell does not collapse, this event can still damage and even kill many kinds of cells. The effect on cells lacking a wall is to shrink down and usually to collapse (figure 3). The growth-limiting effect of hypertonic solutions on microbes is the principle behind using concentrated salt and sugar solutions as preservatives for food, such as in salted hams and fish. Such solutions effectively remove moisture from the food, which inhibits the growth of microbial contaminants.
Adaptations to Osmotic Variations in the Environment
Let us now see how specific microbes have adapted osmotically to their environments. In general, isotonic conditions place little stress on cells, so survival depends on counteracting the adverse effects of hypertonic and hypotonic environments.
An alga and an amoeba living in fresh pond water are examples of cells that live in constantly hypotonic conditions. The rate of water diffusing across the cell membrane into the cytoplasm is rapid and constant, and the cells would die without a way to adapt. As with bacteria, the majority of algae have a cell wall that protects them from bursting even as the cytoplasmic membrane becomes turgid* from pressure. The amoeba has no cell wall to protect it, so it must expend energy to deal with the influx of water. This is accomplished with a water, or contractile, vacuole that siphons excess water out of the cell like a tiny pump.
A microbe living in a high-salt environment (hypertonic) has the opposite problem and must either restrict its loss of water to the environment or increase the salinity of its internal environment. Halobacteria living in the Great Salt Lake and the Dead Sea must absorb salt to make their cells isotonic with the environment; thus, they have a physiological need for a high-salt concentration in their habitats.
The Movement of Solutes across Membranes
Simple diffusion works well for movement of small nonpolar molecules such as oxygen or lipid-soluble molecules that readily pass through membranes. But many substances that cells require are polar and ionic chemicals with greatly reduced permeability. Simple diffusion alone would be unable to transport these substances. One way that cells have adapted to this limitation involves a process called facilitated diffusion (figure4). This passive transport mechanism utilizes a carrier protein in the membrane that will bind a specific substance. This binding changes the conformation of the carrier proteins in a way that facilitates movement of the substance across the membrane. Once the substance is transported, the carrier protein resumes its original shape and is ready to transport again. These carrier proteins exhibit specificity, which means that they bind and transport only a single type of molecule. For example, a carrier protein that transports sodium will not bind glucose. A second characteristic exhibited by facilitated diffusion is saturation. The rate of transport of a substance is limited by the number of binding sites on the transport proteins. As the substance’s concentration increases, so does the rate of transport until the concentration of the transported substance causes all of the transporters’ binding sites to be occupied. Then the rate of transport reaches a steady state and cannot move faster despite further increases in the substance’s concentration.
Fig4. Facilitated diffusion. Facilitated diffusion involves the attachment of a molecule to a specific protein carrier. Bonding of the molecule causes a conformational change in the protein that facilitates the molecule’s passage across the membrane (left view). The membrane protein releases the molecule into the cell interior (right view). The cell does not have to expend energy for transport.
Other examples of passive protein carriers found in a wide variety of cells are aquaporins, also known as water channels. These openings in the membrane facilitate passive transport of water molecules following an existing osmotic gradient. They appear to be involved in regulating volume and osmotic pressure.
Active Transport: Bringing in Molecules against a Gradient
Free-living microbes exist under relatively nutrient-starved conditions and cannot rely completely on slow and rather inefficient passive transport mechanisms. To ensure a constant supply of nutrients and other required substances, microbes must capture those that are in low concentrations and actively transport them into the cell. Cells must likewise transport substances in the reverse direction to the external environment. Some microbes have such efficient active transport systems that an essential nutrient can be found in intracellular concentrations that are hundreds of times greater than in the environment surrounding the cell. Examples of substances transported actively are monosaccharides, amino acids, organic acids, phosphates, and metal ions.
Features inherent in active transport systems are:
1. the transport of nutrients against the diffusion gradient or in the same direction as the natural gradient but at a rate faster than by diffusion alone,
2. the presence of specific membrane proteins (permeases and pumps; figure 5a), and
3. the expenditure of additional cellular energy in the form of ATP-driven uptake.
Fig5. Active transport. In active transport mechanisms, energy is expended (ATP) to transport the molecule across the cell membrane.
Carrier-mediated active transport involves specific membrane proteins that bind both ATP and the molecules to be transported. Release of energy from ATP drives the movement of the molecule through the protein carrier. This can occur in either direction. Some bacteria transport certain sugars, amino acids, vitamins, and phosphate into the cell by this mechanism. Other bacteria can actively pump drugs out of the cell, thereby providing them resistance to the drugs. Another type of active transport pump can rapidly carry ions such as K+, Na+, and H+ across the membrane. This type of transport protein is, defective in cystic fibrosis patients, which is responsible for the pathology of the disease.
Another type of active transport, group translocation, couples the transport of a nutrient with its conversion to a substance that is immediately useful inside the cell (figure 5b). This method is used by certain bacteria to transport sugars (glucose, fructose) while simultaneously adding phosphate molecules that activate them in preparation for a metabolic cycle.
Endocytosis: Eating and Drinking by Cells
Some cells can transport large molecules, particles, liquids, or even other cells across their cell membranes. Because the cell usually expends energy to carry out this movement, it is also a form of active transport. The substances transported do not pass physically through the membrane but are carried into the cell by endocytosis. First, the cell encloses the substance in its membrane, simultaneously forming a vacuole and engulfing it (figure 5c). Amoebas and certain white blood cells ingest whole cells or large solid matter by a type of endocytosis called phagocytosis. Liquids, such as oils or molecules in solution, enter the cell through pinocytosis. The mechanisms for transport of molecules into cells are summarized in table 1.
Table1. Summary of Transport Processes in Cells
