3.5 Intracellular membranes

The small transport vesicles moving to and from the plasma membrane in exocytosis and endocytosis are parts of a dynamic system of intracellular membranes, which includes the endoplasmic reticulum, the Golgi complexes, the nuclear envelope, and a variety of small vesicles such as lysosomes and peroxisomes. (Fig. 6)

Figure 6. High-magnification electron micrographs of a sectioned cell show rough endoplasmic reticulum, studded with ribosomes, smooth endoplasmic reticulum, and the Golgi complex.

The smooth endoplasmic reticulum (ER) is a system of membranes found throughout the cell, forming a cytoplasmic skeleton. It is an extension of the outer nuclear membrane with which it is continuous.

The functions of the ER may thus be summarized as:

1. Providing a large surface area for chemical reactions.

2. Providing a pathway for the transport of materials through the cell.

3. Producing proteins, especially enzymes.

4. Producing lipids and steroids.

5. Collecting and storing synthesized material.

6. Providing a structural skeleton to maintain cellular shape.

3.6 Movement in and out of cells

The various organelles and structures within a cell require a variety of substances in order to carry out their functions. In turn they form products, some useful and some wastes. Most of these substances pass in and out of the cell. They do this by diffusion, osmosis, active transport, phagocytosis and pinocytosis.

Diffusion

Diffusion is the process by which a substance moves from a region of high concentration of that substance to a region of low concentration of the same substance. Diffusion occurs because the molecules are in random motion (kinetic theory). The rate of diffusion depends upon:

1. The concentration gradient - The greater the difference in concentration between two regions of a substance the greater the rate of diffusion.

2. The distance over which diffusion takes place - The shorter the distance between two regions of different concentration the greater the rate of diffusion. Any structure in an organism across which diffusion regularly takes place must therefore be thin. Cell membranes for example are only 7.5 nm thick and even epithelial layers such as those lining the alveoli of the lungs are as thin as 0.3 µm across.

3. The area over which diffusion takes place - The larger the surface area the greater the rate of diffusion. Diffusion surfaces frequently have structures for increasing their surface area and hence the rate at which they exchange materials. These structures include villi and microvilli.

4. The nature of any structure across which diffusion occurs - Diffusion frequently takes place across epithelial layers or cell membranes. Variations in their structure may affect diffusion. For example, the greater the number and size of pores in cell membranes the greater the rate of diffusion.

5. The size and nature of the diffusing molecule - Small molecules diffuse faster than large ones. Fat-soluble molecules diffuse more rapidly through cell membranes than water-soluble molecules.

Osmosis

Osmosis is a special form of diffusion which involves the movement of solvent molecules. The solvent in biological systems is invariably water. Most cell membranes are permeable to water and certain solutes only. Osmosis in living organisms can therefore be defined as: the passage of water from a region where it is highly concentrated to a region where its concentration is lower, through a partially permeable membrane.

Osmosis occurs not only when a solution is separated from its pure solvent by a partially permeable membrane but also when such a membrane separates two solutions of different concentrations. In this case, water moves from the more dilute, or hypotonic, solution, to the more concentrated, or hypertonic, solution. When a dynamic equilibrium is established and both solutions are of equal concentration they are said to be isotonic

Active transport

Diffusion and osmosis are passive processes, i.e. they occur without the expenditure of energy. Some molecules are transported in and out of cells by active means, i.e. energy is required to drive the process.

The energy is necessary because molecules are transported against a concentration gradient, i.e. from a region of low concentration to one of high concentration. It is thought that the process occurs through the proteins that span the membrane. These accept the molecule to be transported on one side of the membrane and, by a change in the structure of the protein, convey it to the other side.

Phagocytosis

Phagocytosis (phago - ‘feeding’, cyto – ‘cell’) is the process by which the cell can obtain particles that are too large to be absorbed by diffusion or active transport. The cell invaginates to form a cup-shaped depression in which the particle is contained. The depression is then pinched off to form a vacuole. Lysosomes fuse with the vacuole and their enzymes break down the particle, the useful contents of which may be absorbed. The process only occurs in a few specialized cells (called phagocytes), such as white blood cells where harmful bacteria can be ingested.

Pinocytosis

Pinocytosis or ‘cell drinking’ is very similar to phagocytosis except that the produced vesicles are smaller. The process is used for intake of liquids rather than solids.

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