All cells, regardless of the tissue or species from which they are derived, use broadly similar methods to perform such functions as the conversion of chemical bond energy into free energy for cellular processes, the transmission of information from one generation of cells to another, and for the translation of this information into terms of protein structure. Most cells fall within a fairly narrow size range. Thus, the majority of cells have diameters ranging from 0–5 to 20μm. At the lower end of the scale are the bacteria; their size is determined by the number of proteins required to perform the essential functions of the cell and the quantity of DNA required to code for the synthesis of these proteins. The number of different types of protein that are needed has been estimated to be between 500 and 1000, and each of these will require for its specification about 20 times its own weight of DNA. The upper limit of cell size is determined by such physical factors as the ratios of the surface area of the cell membrane to the cytoplasmic volume, and of the nuclear and cytoplasmic volumes. Cells fall into two categories: (1) prokaryotes, which comprise the bacteria and blue-green algae, and (2) eukaryotes, which include protozoal, fungal, plant, and animal cells. In eukaryotic cells, there is a clearly defined nucleus set in what appears under the light microscope to be an amorphous gel, that is, the cytoplasm. Electron microscopy has shown that the cytoplasm is permeated by a network of protein filaments that constitute the cytoskeleton and that it contains a variety of physically and chemically distinct membrane-bound organdies. These include the endoplasmic reticulum, Golgi complex, mitochondria, lysosomes, secretory granules, and peroxisome.
All cells, regardless of the tissue or species from which they are derived, use broadly similar methods to perform such functions as the conversion of chemical bond energy into free energy for cellular processes, the transmission of information from one generation of cells to another and for the translation of this information into terms of protein structure.
Most cells fall within a fairly narrow size range. Thus the majority of cells have diameters ranging from 0·5 to 20 μm. At the lower end of the scale are the bacteria; their size is probably determined by the number of proteins required to perform the essential functions of the cell, and the quantity of DNA required to code for the synthesis of these proteins. The number of different types of protein that are needed has been estimated to be between 500 and 1000, and each of these will require for its specification about twenty times its own weight of DNA. The upper limit of cell size is probably determined by such physical factors as the ratios of the surface area of the cell membrane to the cytoplasmic volume and of the nuclear and cytoplasmic volumes.
In every cell provision must be made for the orderly flow within it of raw materials, energy and information, and for this membranes, enzymes and nucleic acids are required. For such flows to be effective they must be directional and the various intracellular systems must be precisely located. Coordinating mechanisms are required to regulate the activities of single cells both in relation to themselves and to their neighbours and the control of many cellular activities is believed to depend on the existence of various compartments within the cell which ensure that the components of a system are sometimes freely accessible to one another and, at other times, are effectively segregated.
Cells fall into two categories, namely prokaryotes, which comprise the bacteria and blue–green algae, and eukaryotes, which include protozoal, fungal, plant and animal cells. In eukaryotic cells there is a clearly defined nucleus set in what appears under the light microscope to be an amorphous gel, i.e. the cytoplasm. Electron microscopy has shown that, in fact, the cytoplasm is permeated by a network of protein filaments which constitute the cytoskeleton and that it also contains a variety of physically and chemically distinct membrane-bound organelles. These include the endoplasmic reticulum, Golgi complex, mitochondria, lysosomes, secretory granules and peroxisomes.
The endoplasmic reticulum, which is a prominent feature of the cytoplasm, is an extensive system of membranes which permeates it throughout and typically consitutes more than half the total membranes of the cell. It plays an important part in the biosynthesis of macromolecules which are required for the construction of the various cell components. The endoplasmic reticulum consists of sheets of thin paired membranes surrounding cavities or cisternae which divide the cytoplasm into an inner compartment which is enclosed by the membrane and the outer compartment which represents the cytoplasmic matrix or cytosol. Part of the membrane appears smooth while other parts are studded with a large number of granules known as ribosomes in which case it is known as rough endoplasmic reticulum. Ribosomes also occur free in the cytoplasm.
Although the various subcellular structures just described are present in most cells, the degree of their development shows considerable variation in different cell types. Since no single type of cell, e.g. liver, muscle or glandular, can be said to be more typical than another, the features of a hypothetical generalized cell are illustrated in Figure 15.1.
Various methods have been used in attempts to correlate the structure and function of the different cell components. Comparison of the fine structure of cells with differing functions shows that the number of mitochondria is correlated with metabolic activity and that the number of ribosomes relates to protein-synthesizing ability. Cytochemical techniques, including histochemistry, autoradiography and the use of fluorescent antibodies, have also contributed to our knowledge of the localization of particular enzymes, hormones and reactions. While these techniques have their limitations, they possess the great advantage that they cause little disturbance of the cell structure. On the other hand, cell fractionation techniques, which are widely used by biochemists, require the total disruption of cells. This is followed by the isolation and analysis of the released components.
Cells are usually disrupted by homogenization which breaks up the cell membrane, and liberates the contents which can be separated by differential centrifugation. By progressively increasing the speed of centrifugaron the following sequence of fractions may be obtained: nuclei, mitochondria, lysosomes, a microsomal fraction, the cytosol.
Membranes are the only structural elements that are found in all living organisms. They are essential for keeping the components of living systems together and preventing them from equilibrating with their environment. Biological membranes also control the passage of materials in and out of the system. They are sheath-like structures, composed of lipids and proteins held together by non-covalent forces.
In addition to the external or plasma membrane which bounds every cell there are various intracellular membranes including the endoplasmic reticulum and the membranes which surround the various organelles, e.g. the nucleus, mitochondria and lysosomes. It has been estimated that membranes may comprise up to 80% of the dry mass of certain cells.
It is becoming increasingly apparent that membranes are dynamic structures; they are rapidly and continually synthesized and degraded and are inherently implicated in vital processes. Membranes from different sources show wide variations in their composition and properties.
The main components of membranes are lipids and proteins although it is usual to find some carbohydrate as well. Lipids usually constitute about 25–50% of the dry weight of plasma membranes and consist mainly of phospholipids, glycolipids and cholesterol. The nature of the fatty acids does not seem to be fixed and may vary according to the temperature and other conditions. It appears to be essential that the fluidity of the membrane is maintained.
The lipid character of membranes explains their high electrical resistance, their ready destruction by lipolytic agents, and why they are permeable to lipid-soluble materials but have a low permeability for ions and most polar molecules. The presence of protein accounts for their mechanical strength, low surface tension and high specific conductance. The membrane proteins are also responsible for the specific properties of membranes and different types of protein are found in different membranes.
In aqueous solutions most phospholipids and glycolipids will assemble themselves into double layers in which the polar ends of the molecules are directed outwards and the long fatty acid chains are directed inwards as befits their amphiphilic nature (Figure 8.2b). Bimolecular lipid structures of this type are an essential feature of membrane structure.
Membranes also contain relatively large amounts of cholesterol which is thought to increase the mechanical stability and to help regulate the fluidity of the membrane. The relatively flat steroid ring is inserted between the phospholipid molecules with the polar −OH group adjacent to the polar heads of the phospholipids (Figure 15.2).
The most widely applicable model of membrane structure which allows for the physicochemical properties of membrane lipids and proteins and accounts for the dynamic nature and versatility of membranes is the fluid mosaic model (Figure 15.3). According to this the membrane is composed of a two-dimensional array of phospholipid and globular protein molecules in which the protein molecules are floating in a phospholipid continuum. About 70–80% of the protein molecules are integral proteins, which are held in association with the lipid bilayer by hydrophobic interactions. They, like the lipids, are amphiphilic and form an essential part of the structure. Other proteins known as peripheral or surface proteins can be dissociated from the membrane by the addition of salts. From this it is concluded that they are bound to the membrane surface by hydrogen bonds and salt linkages.
The functions of membrane proteins are many, varied and specific; they include transport, communication and energy transductions. Membranes are asymmetrical and the properties of their two surfaces are different.
All eukaryotic cells have carbohydrate groupings on their external surface. These are mainly the oligosaccharide side chains of glycoproteins and glycolipids. In addition the ‘cell coat’ may contain secreted glycoproteins and proteoglycans (page 407). The cell surface carbohydrates contain mannose, fucose and sialic acid in addition to glucose, glucosamine, galactose and galactosamine. The sialic acid is usually found at the ends of the side chains which are often branched, and it is the sialic acid which is believed to be mainly responsible for the negative charge found on the surface of all eukaryotic cells. The cell surface carbohydrates play an important role in recognition processes and cellular interactions.
Structural studies on integral proteins suggest that the portions of polypeptide chain which are located within the lipid bilayer take up an α-helical conformation. Thus the membrane-crossing regions of these proteins have a relatively rigid rod-like structure. Two structurally distinct classes of integral protein occur. In the first class, which could be described as anchored proteins, the active site of the protein is held in the aqueous environment at the membrane surface while a single helical segment of protein traverses the membrane anchoring the protein within it (Figure 15.4a). Examples of this type of protein are the intestinal hydrolases as well as hormone receptors and antigens such as the histocompatibility antigens on which recognition sites are presented at the membrane surface. The integral proteins which function in membrane transport and as ‘gates’ have a different type of structure. The part within the membrane which forms the transporting region is complex and is made up of a cluster of transmembrane helices (Figure 15.4b). The sodium pump and the calcium pump of muscle are proteins of this type.
Biological membranes are selectively permeable. They allow free diffusion of certain materials but act as a barrier towards others. In addition, they possess special pumping or active transport mechanisms which are responsible for the uptake or elimination of particular substances and which ensure that the composition of the enclosed compartment is closely controlled.
The physical forces responsible for the movement of materials across membranes, although based on diffusion and osmosis and the existence of electrochemical gradients, are extremely complex since they are affected not only by the composition of the membrane but also by such factors as the molecular size, lipid solubility, charge and degree of hydration of the penetrant as well as on its relative concentrations on the two sides of the membrane. Furthermore, many different substances are moving at the same time and the movement of one may have an effect on the movement of others.
The distribution of ions on the two sides of a membrane is also affected by the presence of large charged non-diffusible molecules, e.g. proteins and nucleic acids, which may themselves be unequally distributed.
Without considering in detail the factors mentioned above, it may be stated that most membranes will allow the passage of small uncharged molecules by simple diffusion and that the rate of their passage is related to their lipid solubility. The passage of other materials may occur as a result of the presence of pores, polar regions and gates over a small area of the membrane surface. Certain evidence suggests that the average pore is about 0·3–0·4 nm in diameter. A molecule that is larger than this will not cross the membrane passively, and for reasons given above neither will some smaller molecules.
Whereas synthetic membranes such as cellophane are useful in the study of passive distribution processes resulting solely from physical effects, biological membranes are notably different in that they contain special in-built transport mechanisms. It appears that the transported molecule binds specifically and reversibly with a carrier molecule, present in the membrane, which operates between the two surfaces of the membrane, alternately picking up and releasing the transported substance.
1. Passive transport or facilitated diffusion. In this form no energy is expended and movement only takes place from a region of higher to one of lower concentration. However, owing to the presence of the carrier, substances can pass more rapidly along the concentration gradient existing across the membrane than would be expected from simple physicochemical considerations. The uptake of glucose into most cells occurs by passive transport of this sort.