In this term paper we will discuss about how molecules are transported through membranes.

All cells are bounded by a delicate plasma membrane. Moreover, most organelles in the cytoplasm of eukaryotic cells are membrane bound. Besides holding in the contents of the cell or organelle, these membranes play other vital roles in the life of the cell. They control internal concentrations of substances essential to cell life and regulate the entrance and exit of water, wastes, nutrients, and other important substances.

Because not all substances pass through cell membranes with equal ease, membranes are described as differentially or selectively permeable, that is, they allow the passage of some substances, while blocking the passage of others. Some membranes are semipermeable – they are permeable to certain solvents the fluid portion of a solution but impermeable to solutes-substances that dissolve in solvents. Although semipermeable membranes can be constructed in the laboratory, none are known to exist in living systems.

Passive Transport:

i. Diffusion:


Diffusion is a process of passive transport by which molecules and other small particles move spontaneously from a region in which they are in high concentration to one in which their concentration is lower. This process is familiar to anyone who has opened a container of a substance such as ammonia.

The ammonia molecules gradually become equally distributed throughout the room. Another example of diffusion can be observed when a drop of dye solution is placed in a jar of water. The solute, dissolved dye in this case, diffuses through the water and eventually becomes equally dispersed throughout the jar.

Mechanism of Diffusion:

To understand the mechanism of diffusion it is necessary to understand the motion of molecules. Theoretically, at a temperature of absolute zero (0 Kelvin), which is equivalent to -273° Celsius (C), molecules have no motion at all. At any temperature above absolute zero, molecules move or vibrate at a rate proportional to the temperature and inversely proportional to the substance’s molecular weight, the smallest molecules moving the fastest.


For example, at 90°C, hydrogen molecules (H2) move at an average of 45 kilometers (km) per minute (28 miles per minute), sulfur dioxide molecules (SO2) at an average of 19 km/min, and the molecules of all the gases in air at an average of 32 km/min. Heat thus provides molecules with the energy of motion, or kinetic energy.

Kinetic Energy provides the Basis for Diffusion:

Molecules in motion tend to move in random directions but in straight lines. Typically, whenever such molecules collide with one another, they rebound and move off in new directions. If ammonia vapour is released at one end of a chamber from which the air has been evacuated, it can be detected at the other end of the chamber almost immediately.

In a vacuum, few collisions occur among the molecules at the edge of the mass of released vapour; consequently, the molecules are unimpeded and quickly reach the other end of the chamber. However, vacuums are uncommon. If ammonia molecules are released at one end of a classroom, it takes quite some time before they can be detected at the other end of the room.


This delay occurs because the released molecules collide not only with each other but also with the molecules of nitrogen, oxygen, carbon dioxide, and with molecules of water vapour and any other substances in the room air. A given ammonia molecule can travel only a short distance in any direction before colliding with other molecules. This explains why diffusion is usually a slow process.

The diffusion of any substance is normally independent of the concentration of any other substance in the area. A high concentration of ammonia at one end of a room, for example, is not balanced by a high concentration of some other substance at the other end. Both substances will diffuse and in time become equally distributed throughout the room.

The greater the initial difference between the concentrations of a substance in any two areas, the more rapid will be the initial rate of its diffusion. As the concentrations of the substance in the two areas become equalized, the rate of diffusion slowly decreases. Finally, when the concentrations in the two areas have become equal, diffusion ceases.

The individual molecules of the substance continue to move and to collide at the same rate as before, but the number of molecules that move out of an area now equals the number that move in. At this point, the molecules are said to be in a state of equilibrium. The equilibrium they have achieved is a dynamic equilibrium, not a static equilibrium. A chemical system in dynamic equilibrium is similar to a college that maintains a constant level of enrollment over a period of years, with students entering and leaving in equal numbers each semester.


Solutes Diffuse Very Slowly:

The behavior of dissolved particles in solution parallels that of the gas molecules described above. But solute particles diffuse much more slowly than do vapor molecules. One reason, of course, is their frequent collisions with the more abundant molecules of the solvent in which they are dissolved. The movement of solute molecules is also usually impeded by attractive forces that normally exist between them and the molecules of solvent.

In biological systems, diffusion is effective for the transport of materials over a distance of only about half a millimeter. Despite this limitation, it is of major importance in moving materials within cells, between cells, and in exchanges between cells and their immediate environment.

ii. Osmosis:


The principles that govern the diffusion of molecules in an open system also operate when a membrane permeable to both solute and solvent is interposed between solutions with different concentrations of a solute. However, if the membrane is impermeable to the solute but permeable to the solvent, the result is different, as can be demonstrated by an apparatus. In this device, a glass U tube is divided at its base by an artificial membrane that is permeable to water but is impermeable to some solutes, such as sugar.

Under these conditions, the water molecules are able to move through the membrane in either direction. However, the water molecules on the right, where there are no solute molecules, are less restricted in their movement than are those on the left, whose movement is impeded by the dissolved sugar molecules. The result is a net movement of water molecules into the sugar solution. Such movement is termed osmosis.

In this example, the process of osmosis is retarded for at least three basic reasons:

1. Because of the mutual attraction that exists between water molecules and the solute molecules dissolved in them, solute molecules in water, or aqueous, solutions develop a close association with the nearest water molecules; they are said to be hydrated, that is, combined with water. Thus, the presence of the solute molecules restricts the movement-and hence the diffusion-of the water molecules.


2. Solute molecules take up space that water molecules might otherwise occupy. In the sugar solution, for example, some of the water molecules that might have passed through the membrane do not because solute molecules are in their way.

3. In many solutions, especially concentrated ones, a given volume of solution (solvent plus solute) contains fewer water molecules than does an equal volume of pure solvent. In such cases the solution can be thought of as having a lower concentration of water than does pure water alone.

More water usually crosses membranes during osmosis than can be accounted for by these factors. Apparently other forces are also involved.

For these reasons, although water molecules move in both directions across a semipermeable membrane, at any given time more molecules will be moving from the side of the U tube containing water into the side containing 1-percent sugar solution than in the reverse direction. In other words, a net movement of water occurs from the distilled water into the 1-percent sugar solution.

Osmosis Does Not Require Pure Solvent on One Side of the Membrane:

If enough sugar molecules are added to the distilled water side of the tube to make a 2-percent solution, the osmotic flow would be reversed because the greater restriction of water molecule movement would occur on the side of the membrane containing the higher concentration of sugar molecules.

The more highly concentrated, 2-percent solution of sugar, with its lower concentration of water, would then gain water at the expense of the 1-percent sugar solution, with its higher concentration of water. Osmosis can therefore be defined as the movement of water or another solvent by diffusion from a region of low or no solute concentration, across a membrane, to a region of higher solute concentration.

Note that osmosis applies only to the passage of water and other solvents through the membrane. If a salute to which the membrane is permeable, for example, sodium chloride, is introduced into one arm of the U tube, it diffuses through the membrane until it becomes equally distributed on both sides. In this case, its passage through the membrane is an example of diffusion, not of osmosis.

Osmotic Pressure is Proportional to the Concentration of Dissolved Particles:

Consider again the example, in which a semipermeable membrane separates a 1-percent sugar solution from water. As the sugar solution gains water osmotically, the column of water on the left side will rise. Eventually, the weight of this rising column will become great enough to force water molecules back through the membrane at the same rate at which they entered the sugar solution by osmosis.

However, if a watertight piston is placed in the left side of the tube before osmosis begins and just enough pressure is applied to the piston to prevent any upward movement of water, the force generated by the difference between a 1- percent sugar solution and pure water can be measured by an appropriate device. This force is referred to as osmotic pressure and is usually measured in atmospheres, the units used for air or water pressure.

A solution’s osmotic pressure is best defined as the tendency of a solvent to enter it by osmosis through a perfectly semipermeable membrane from pure solvent, which contains no dissolved solute particles. A solution’s osmotic pressure is directly proportional to the concentration of all solute particles it contains. Thus, describing a solution as 1-percent sugar says little about its osmotic pressure. For example, if the sugar in question is glucose, a 1-percent solution (1 gram of glucose in 100 grams of solution) would generate about twice the osmotic pressure as a 1-percent solution of sucrose.

The reason is, of course, that sucrose molecules are about twice the molecular weight of glucose molecules, and therefore 1 gram of sucrose contains only about half the number of molecules as 1 gram of glucose. This suggests that a convenient index to a solution’s osmotic pressure would be its molarity, the concentration of molecules it contains. However, this will not suffice because some substances, called electrolytes, undergo dissociation into ions when dissolved in water. A dilute solution of sodium chloride (NaCl), for example, dissociates almost completely into sodium ions (Na+) and chloride ions (CI).

Thus, a 0.1-molar solution of NaCl would be expected to generate about twice the osmotic pressure of a 0.1-molar solution of glucose. A 0.1-molar solution of calcium chloride (CaCl2), which contains one calcium ion (Ca2+) and two chloride ions (CI) for each of the CaCl2 molecules, would generate about three times the osmotic pressure of an equimolar solution of a nonelectrolyte such as glucose.

Osmolality Defines Particle Concentration:

An osmole is 6.02 x 1023 molecules of a solute or mixture of solutes of any kind. An osmole of solute dissolved in 1,000 ml (1 liter) of solution constitutes a 1 osmolar solution. The important consideration in determining the osmolar concentration of a solution is the effective concentration of all of its particles. A 0.1-osmolar solution could be a mixture of salts, ions, sugars, amino acids, and protein molecules.

Large constituents, such as red blood cells and very large molecules, may not contribute to a solution’s effective osmolality; such components are osmotically inert or inactive. The osmolarity of a solution is always measured in an ideal situation, for example, with the solution separated from pure water by a perfectly semipermeable membrane.

Osmolarity is one of the factors that determines the osmotic activity of various fluids of biological interest, such as soil water, lake water, seawater, cytoplasm, and blood. For example, the osmolarities of root cell cytoplasm and of soil water can be used to predict whether osmosis will occur between roots and soil water, and in which direction.

Two solutions are isosmolar if their effective particle concentrations are equal. If solution A has a higher concentration of osmotically active particles than solution B, A is hyperosmolar to B, and B is hypoosmolar to A.

Tonicity Indicates Whether Osmosis Will Occur in Specific Cases:

Osmolarity of an unknown solution can be measured by using a semipermeable membrane; however, such membranes are not known to exist in biological systems. Instead, biological membranes are often differentially permeable: they permit the fret passage of some solutes, allow others to pass in small amounts, and are completely impermeable to yet others. The mere presence of some solutes can change a membrane’s permeability to various other solutes and even to water.

Thus, the fact that two solutions separated by a membrane are isosmolar does not necessarily mean that no osmosis will occur. The osmotic effectiveness of a given solution in a specific situation is indicated by whether a given cell or other membrane-bounded structure swells, shrinks, or does neither when placed in the solution. For example, if human red blood cells are placed in a 0.16-molar solution of NaCl, they neither swell nor shrink.

The 0.16-molar salt solution is therefore said to be isotonic to the cells’ cytoplasm. If distilled water is added to the salt solution, diluting it, water enters the cells by osmosis and they swell. Lacking the support of a rigid cell wall such as that of plant cells, the plasma membrane becomes stretched to the breaking point and the cell contents-consisting mostly of the protein hemoglobin-flow out. In red blood cells, this process is called hemolysis because the cells burst, or lyse.

In this case, the dilute salt solution is hypotonic to the cell contents, and the cell contents are hypertonic to the dilute salt solution. If salt is added to the 0.16-molar solution, the cells shrink; the solution is thus hypertonic to the cell contents, and the cell contents are hypotonic to the more concentrated salt solution.

In hypertonic solutions, red blood cells become crenated, or wrinkled, in a characteristic way. Some cells have evolved mechanisms that enable them to live in environments characterized by high salt concentrations. Some plants, such as Spartina grass, are specialized to grow in or near a salt water environment.

Osmotic Pressure is Important to Living Systems:

The cytoplasm of nearly all freshwater and terrestrial plants and algal protists is maintained at a higher osmolar concentration than the solute concentration of the lake, stream, or soil water in which they live. Therefore, their cells tend to absorb water from their environment. Because these cells are surrounded by fairly rigid cell walls, excess swelling is prevented once the cell has become turgid.

In hypertonic solutions, however, the plant cell protoplast—all the material bounded by the plasma membrane— shrinks away from the cell wall as a result of water loss. This process is called plasmolysis. If plasmolysis is prolonged, the plant will wilt and die. This is why plants die along roadways to which salt is applied in winter and why a heavy application of fertilizer to a lawn kills the grass.

Sea Urchin Eggs as Osmometers:

The membrane that encloses a sea urchin egg shows little permeability to most solutes but is very permeable to water. When sea urchin eggs are placed in various dilutions of seawater, they absorb water osmotically and adjust their volume in each dilution until their cytoplasm is isotonic to that dilution. When they are returned to full-strength seawater, their volume returns to normal.

Changes in the Volume of Sea Urchin Eggs in Various Dilutions of Seawater

The magnitude of the response of a sea urchin egg to different dilutions of seawater is not exactly what would have been predicted from the egg’s volume, considering that the cell contents are isotonic to 100 percent seawater and that the egg’s outer membrane shows little permeability to solutes. However, when a correction is made for the osmotically inactive 12.5 percent of the cell contents, composed of insoluble proteins and lipids, the cells behave as nearly perfect osmometers devices for measuring osmotic pressure.

iii. Membrane Permeability:

Using a variety of methods, it is possible to measure the rate of penetration of various substances into cells and to arrive at some conclusions about the permeability of plasma membranes and the mechanisms by which materials are transported across them. Of the many factors that determine whether a particular solute can cross a membrane, three appear to be of major importance.

First, the rate of penetration of a substance is primarily related to its solubility in lipids. This factor is known as the solubility coefficient of a penetrating substance. The more soluble a substance is in lipid or in lipid solvents, the more readily the substance will penetrate the plasma membrane.

In a series of compounds with decreasing solubility in lipids, such as methyl alcohol, glycerol ethyl ether, glycerol methyl ether, glycerol, and erythritol, the relative rates of penetration through plasma membranes decreases in the same order. This was one of the observations that led to the concept of the lipoidal nature of the plasma membrane.

A second factor affecting membrane permeability to solutes is the molecular size of the solute. Small molecules usually pass through the plasma membrane more readily than do large molecules of the same type. Among a series of compounds with increasing molecular size, such as urea, glycerol, arabinose, glucose, and sucrose, the rate of transport through a plasma membrane decreases in the same order.

However, the lipid solubility of a molecule appears to be more important than its size in determining its rate of penetration. Thus, a large molecule with a high lipid solubility will probably penetrate the plasma membrane more readily than a smaller molecule with a low lipid solubility.

Third, plasma membranes are more permeable to uncharged particles than they are to those that are charged. Electrolytes thus generally penetrate plasma membranes more slowly than do nonelectrolytes of the same size. Furthermore, strong electrolytes— those that tend to ionize completely in solution-generally pass through membranes more slowly than do weak electrolytes; moreover, the greater the charge of an ion, the slower its rate of penetration.

Of the two types of ions, anions (negatively charged particles) penetrate membranes far more readily than do cations (positively charged particles). Reduced permeability to charged particles appears to be due both to the fact that ions in aqueous solution are intimately surrounded by clouds of water molecules called hydration shells, which increase their effective size, and to the fact that ions are generally less soluble in lipid than are uncharged particles.

These three factors affecting membrane permeability have a great bearing on the physiological functioning of the cells. Many substances essential for the survival of cells are not soluble in lipids. Among them are such molecules as sugars and amino acids, and several important elements, such as K, Na, CI, Ca, Mg, P, all of which are available to cells only in an ionized state (as anions or cations). Plasma membranes have specialized mechanisms that facilitate the movement of these vital ions into and out of cells.

It was assumed that the plasma membrane had specialized channels or pores that worked as gates; this would be a means by which charged molecules that are insoluble in lipids could cross the membrane. We now know that such pores exist, but their structure is different.

Facilitated Diffusion:

Certain amino acids, sugars, and other compounds can move across plasma membranes in response to a concentration gradient. However, the rate of movement is proportional to the difference in concentration of the substances on the two sides of the membrane only up to a point. After that point, further increases in the concentration difference do not increase the rate of transport across the membrane.

This observation suggests that the transport of such compounds is not a case of simple diffusion but is facilitated by some membrane mechanism that becomes saturated at high solute concentrations. In other words, diffusion appears to be facilitated by proteins in a mechanism analogous to that of enzyme reactions.

It is believed that the diffusing molecules combine to form complexes with these specific carrier protein molecules within the membrane. We now know that membrane-bound carrier-protein complexes form channels through which the diffusing molecules pass. For this reason, they are sometimes referred to as permeases. First, the shape of the protein changes in response to the diffusing molecule – the protein parts, so to speak, allowing the molecule to cross the membrane to the other side.

Various Types of Transports Across the Membrane

This shape, or conformational, change also lowers the affinity of the protein for the diffusing molecule once it has reached the other side, allowing the molecule to be released. Once the diffusing molecule is released, the protein resumes its original shape and can bind to another molecule. Thus, facilitated diffusion, as this process is called, enables molecules to cross an otherwise impermeable or poorly permeable membrane.

Except for the limitations imposed by the temporary union with a membrane protein, facilitated diffusion proceeds according to the principles of ordinary diffusion: transport takes place only from a region of high to a region of low concentration and requires no expenditure of energy. Because diffusion is a passive process, it can proceed in either direction, depending on which side of the membrane has the higher concentration of the diffusing substance.

Active Transport:

The chemical composition of a cell differs from that of its surroundings in many ways. The amount of potassium inside a cell is higher than that in the surrounding environment. Similarly, the concentration of sugars and amino acids inside a cell is usually higher than that in the medium in which the cell grows, at least in the protists and free-living prokaryotes. On the other hand, the amount of sodium in the cell is lower than that in its surroundings.

Comparison of the Kinetics of Transport

Thus, it is clear that cells are capable of moving molecules and ions against concentration gradients that normally exist in their environments. Since such transport requires energy, it is called active transport. Active transport may also take place in the direction favoured by a concentration gradient. In such cases, the gradient serves to facilitate the active transport process.

Active transport can be distinguished from simple or facilitated diffusion by its dependence on energy. Thus, it is relatively simple to determine whether the transport of a particular substance across a membrane is truly active – if the process continues in the absence of an energy source, such as adenosine triphosphate (ATP), it cannot be active transport.

i. Sodium-Potassium Pump:

Among the most intensively studied examples of active transport is the mechanism by which sodium ions (Na+) and potassium ions (K+) are transported across the plasma membranes of animal cells. Plant and animal cells tend to maintain low internal concentrations of Na+ and high internal concentrations of K+ in spite of the fact that the surrounding fluids are high in Na+ and low in K+.

The maintenance of differential ionic concentrations is vital to cells, and about one-third of the energy expended by most cells is used to maintain Na+ and K+ gradients. Enzyme activity, protein synthesis, the conduction of nerve impulses, and the contraction of muscle all require different concentrations of these ions.

Diagram of the Na+ K+ ATPase in the Cell Membrane

The active transport system that maintains high levels of K+ and low levels of Na+ within the cell is called the sodium-potassium pump (Na+/K+ pump). Although understanding of this pump is based largely on studies of the mechanism in red blood cells, this knowledge is applicable to all Na+/K+ pumps.

An important step in elucidating this mechanism was the discovery of the enzyme sodium-potassium adenosine triphosphatase (Na+/K+ ATPase) in the red blood cell membrane.

This enzyme is capable of hydrolyzing ATP to adenosine diphosphate (ADP) and inorganic phosphate (Pi) only in the presence of sodium, potassium, and magnesium ions by the following reaction:

The demonstration of the presence of this enzyme in the plasma membrane led to the idea that the hydrolysis of ATP is probably coupled to the active transport of K+ into and Na+ out of the cell. This role for the enzyme is strongly suggested by the fact that Na+/K+ ATPase activity and the functioning of the Na+/K+ transport system in intact cells are specifically inhibited by a plant extract called ouabain. For example, ouabain interferes with normal heart function because it inhibits active transport of cations.

Concentrations of Na+ and K+ for Various Cell Types

The functioning of the Na+/K+ carrier enzyme has the following characteristics:

1. The pump functions only if both Na+ and K+ are present, and the same pump transports both ions.

2. The pump works only in one direction for a given ion type – K+ must be present on the outside and Na+ on the inside of the membrane so that K+ is pumped into the cell and Na+ is pumped out.

3. Ouabain blocks cation transport only if applied to the outside of the membrane.

4. The ratio of transport of the two ions is 3 Na+ to 2 K+ for each ATP molecule hydrolyzed by the enzyme. This causes an electrical imbalance between the two sides of the membrane, the inside being negative to the outside; the electrical imbalance is maintained by the membrane’s impermeability to some ions, particularly to CI and Na+.

5. The membrane-bound protein involved in this pump is a dimer, a molecule formed by the union of two molecules of a simpler compound; this dimer consists of two separate polypeptide chains joined by a covalent bond.

6. Although the mechanism involved in this transport process is not fully understood, it apparently involves the transfer of an energy rich phosphate group from ATP to the membrane-bound protein in response to the presence of the Na+/K+ ATPase enzyme. This reaction, called phosphorylation, seems to induce conformational changes in the enzyme, resulting in a high affinity of the protein at the inner surface of the membrane for Na+.

Simultaneously, the affinity of the protein at the outer surface of the membrane for K+ also increases. Once the Na+ has moved to the outer surface of the membrane and the K+ to the inner surface, a second conformational change in the protein results in a much lowered affinity for these ions as well as to the phosphate. Then Na+ is released to the outside of the cell and K+ enters the cell and the phosphate is released as Pi. The pump is now ready for another transport cycle.

ii. Other Pumps:

Potassium and sodium ions are not the only ions transported across plasma membranes by an active transport mechanism involving a carrier molecule. Calcium ions (Ca2+), for example, are transported across the plasma membranes of skeletal muscle cells by a calcium pump that uses a calcium-activated ATPase carrier. The Ca2+ ATPase enzyme regulates the amount of free calcium near the muscle fibers and thus regulates muscle contraction.

Several other membrane carrier mechanisms regulate the transport of other ions as well as molecules such as sugars and amino acids across plasma membranes. One example is the active absorption of glucose by the cells that line the lumen of the vertebrate small intestine. Some of these mechanisms are known to involve a membrane-bound ATPase pump.


While the energy released by the hydrolysis of ATP is used directly in the active transport of Na+ and K+ in the ion pump, other systems use energy indirectly to transport a substance against a concentration gradient. For example, glucose can be actively transported across the plasma membrane of the epithelial cells lining the intestinal wall against a concentration gradient. However, the energy expended is used, not for glucose transport directly, but to pump Na+ out of the cells, where it is maintained in high concentrations.

Special proteins in the membranes of these cells facilitate the diffusion of Na+ from the lumen of the intestine into the cells. The same proteins act as carriers of glucose; they transport Na+ only if they bind and transport glucose at the same time. Because the Na+ concentration in the lumen is higher than that in the cytoplasm of the cells, Na+ ions diffuse into the cells along the gradient, carrying glucose with them, despite glucose being in higher concentration inside the cell.

This type of transport of one substance along with another one across a plasma membrane is called co-transport. The energy utilized in this process is used to pump Na’ ions out of the cell to maintain the Na+ gradient between the intestinal lumen and the cytoplasm of the epithelial cells.

Because these transport proteins move Na+ and glucose in the same direction, they are called symports. Proteins that transport one molecule into a cell while transporting another molecule out, as in the Na+/K+ pump, are called antiports. Proteins specialized to move only one kind of substance across a cell membrane are called uniports. Some cells, such as those lining the urinary tubules of the kidney, have many transport proteins of all three types associated with their plasma membranes.

Bulk Transport:

In addition to transporting small molecules and ions by regular and facilitated diffusion and active transport, the plasma membrane can transport large molecules and liquids into and out of cells. The general term for the process by which such bulk materials are transported into the cell is endocytosis. When the substance taken into a cell is a solution, such as a solution of nutrient substances, the process is called pinocytosis (Greek for “cell drinking”).

When the substances taken in are particulate, such as bacteria, the process is termed phagocytosis (Greek for “cell eating”). The transport of bulk materials out of the cell is called exocytosis. The secretion of cellular products such as hormones and the removal of cellular wastes involves exocytosis. All these processes require metabolic energy and, like active transport, can be inhibited by substances that block the generation of such energy.

i. Endocytosis:

Pinocytosis occurs in all animal cells that regularly take in large molecules. It can be induced in many cells merely by adding protein to their culture medium. Pinocytosis can be demonstrated in many cells by a tracer technique using fluorescent-dye-labeled protein as an inducing agent. Upon being irradiated with ultraviolet light in a special microscope, the labeled protein gives off a coloured light, or fluoresces.

Following, or tracing, the movement of the fluorescent molecules pinpoints the precise location of the pinocytotic activity. Pinocytosis can also be demonstrated by another tracer technique using the iron-containing protein ferritin, which shows up in electron micrographs at those locations on the plasma membrane at which it is being taken. Because of their large number of iron atoms, ferritin molecules scatter electrons much more effectively than do most other cell components. They are therefore more readily visualized in electron micrographs.

Model of Endocytosis

Four Stages of Pinocytosis:

Through the use of tracer techniques, it has been shown that pinocytosis occurs in the four steps:

(a) Binding of the protein or other inducing molecules to the plasma membrane,

(b) Invagination of the membrane to form a channel,

(c) Formation of vesicles and their movement into the interior of the cell, and

(d) Utilization of the materials that have been brought into the cell.

The fact that the binding stage is apparently unaffected by temperature and metabolic poisons suggests that the first step in pinocytosis is purely passive. The uptake of inducing proteins and other large molecules does appear to be affected by both the size of the molecules and their electric charge. For molecules with the same positive charge, size is an important factor; larger molecules are more readily taken in by pinocytosis.

Otherwise, the size of the charge may be the determining factor, molecules with a higher positive charge being more readily absorbed into cells. The invagination stage is often accompanied by a bulging, or projection, of the cytoplasm around the invagination. Little is known about how the channels are formed except that treatment with respiratory poisons interferes with their formation, which indicates that at least this step requires a metabolic energy supply.

Pinocytotic vesicles, or pinosomes, are formed at the cell surface or are pinched off from the ends of the channels. They then migrate into the cytoplasm. Once inside the cell, pinosomes fuse with lysosomes, forming secondary lysosomes.

The enzymes of the lysosomes hydrolyze ingested material into small molecules, which diffuse out through the lysosomal membrane into the cytoplasm. Later, the vesicle, along with any residual undigested material, may be extruded from the cell by the process of exocytosis.

Many Functions of Phagocytosis:

The uptake, or phagocytosis, of large, solid particles by cells has long been known to be a widely occurring phenomenon. It forms the basis for the nutrition of many protists, particularly the Protozoa. It is readily seen by light microscopy and has been reported in a large variety of animal cells.

In mammals, phagocytosis by certain white blood cells and other phagocytes (Greek for “eating cells”) is an important means of defense against the invasion of the body by such foreign substances as bacteria, parasitic organisms, and dust particles. Besides white blood cells, mammalian phagocytes include several types of cells of the liver, spleen, lymph nodes, connective tissue, brain, bone marrow, lungs, and other tissues.

The basic steps in phagocytosis are:

(a) Attachment of the foreign particle to the cell surface;

(b) Engulfing of the particle by pseudopodia (Greek for “false feet”) extended from the cytoplasm;

(c) Pinching off of the vesicle, called a phagosome;

(d) Fusion of lysosomes with the phagosomes to form phagolysosomes;

(e) Digestion of the particles by the lysosomal enzymes; and

(f) Extrusion of the undigested debris from the cell by exocytosis.

ii. Exocytosis:

The process of exocytosis, by which cells transport substances in bulk from the cytoplasm out of the cell, is highly developed in the secretory gland cells of animals. The secretory cell products are synthesized in the rough endoplasmic reticulum and accumulate in vesicles, which are pinched off into the cytoplasm.

These vesicles move to a Golgi complex, where the secretory products are packaged into secretory vesicles. In salivary, pancreatic, and adrenal glands, the secretory vesicles move to the plasma membrane, and the vesicle membrane fuses with the plasma membrane, releasing the vesicle contents outside the cell.

An interesting variation of this method, of secretion is seen in the oil glands of mammalian skin. Secretion occurs when the cell disintegrates, releasing oil onto the surface of the skin. Such a secretory process in which the entire secreting cell, along with its accumulated secretion, forms the secreted matter is called holocrine secretion. A similar process takes place in milk- producing glands, but here only the part of the cell in which the secretions have accumulated is pinched off by a process called apocrine secretion.