Cytoplasm

‘Cytoplasm’ is used to describe all intracellular contents outside the nucleus. It consists of the cytosol and the organelles.

Cytosol

Cytosol refers to the intracelluar fluid containing proteins and electrolytes. Intracellular fluid forms 40% of body weight and possesses an electrolyte composition, which is discussed in Chapter 11.

Organelles

A summary of the organelles and their functions is shown in Figure 10.4.

Cytoskeleton

This system of microscopic fibres maintains the cell structure and enables cell movement to occur. Its main components are:

Cellular motion, shape changes and ciliary or flagellar movement all involve molecular motor mechanisms based on the action of ATPases. These form moving flexible cross-bridges between cytoskeletal components and membranes or organelles. The actin–myosin mechanism responsible for muscle contraction is a molecular motor mechanism that occurs universally in other cells. Other examples of molecular motors include dynamin and kinesin, which act on microtubules.

Mitochondria

These are sausage-shaped structures with outer and inner membranes. Their main function is to produce chemical energy in the form of ATP by oxidative phosphorylation. The inner membrane is folded to form cristae, which are studded with units containing the oxidative phosphorylating and ATP-synthesising enzymes (Figure 10.5). The matrix contains enzymes required to drive the citric acid cycle, which in turn provides the substrate for oxidative phosphorylation. Mitochondria also contain a small amount of DNA, which is solely of maternal origin.

Figure 10.5

Mitochondrion structure

Endoplasmic reticulum (ER)

This membranous structure is composed of complex folds and tubules. In its granular form, ribosomes are attached to the cytoplasmic surfaces and are the primary site for protein synthesis in the cell. Agranular ER is free from ribosomes and is the site of steroid synthesis and detoxification.

Ribosomes

Ribosomes are about 32 nm in diameter, with large and small subunits. They are composed of 65% RNA and 35% protein, and are the sites of protein synthesis. Free ribosomes exist in the cytoplasm and synthesise haemoglobin, peroxisomal and mitochondrial proteins.

Centrosome

This is composed of two centrioles at right angles to each other. The centrioles are cylindrical structures in which nine triplets of microtubules form the walls. A cylinder of pericentriolar material surrounds these structures. The whole structure is situated near the nucleus and is activated at the start of mitosis. Initially the centrioles replicate, and then the centriole pairs separate to form the mitotic spindle.

Golgi apparatus

This consists of flattened membranous sacs or cisterns that are stacked together to form a polarised structure with cis and trans ends, separated by a middle region. The Golgi apparatus prepares proteins for secretion (via exocytosis) by receiving the proteins from the ER at the cis side, coding them for destination and finally producing secretory granules or vesicles at the trans side (Figure 10.6).

Figure 10.6

Golgi apparatus and protein processing

Intercellular junctions

The organisation of cells into tissues involves the formation of specialised junctions between the cells and the formation of an extracellular matrix surrounding the cells. Connective tissues such as bone and tendon are predominantly extracellular matrix with the cells anchored to the matrix rather than forming intercellular junctions. Epithelial tissues consist of layers of cells anchored to a basement membrane with little extracellular matrix, but dense specialised intercellular junctions determining the functions of the epithelium.

Some basic types of junction (Figure 10.7) are:

Figure 10.7

Intercellular junctions

Cilia

These projections on the luminal surface of epithelial cells are motile processes that move secretions and other substances across the surface of the cell. They are composed of nine pairs of microtubules arranged circumferentially around a central pair of microtubules. Each cilium is attached to a basal granule that has a structure similar to that of a centriole. Ciliary movement is produced by molecular motor mechanisms that cause the microtubules to slide relative to each other.

Cell adhesion molecules (CAMs)

Cell adhesion molecules form a large group of membrane proteins with several subdivisions and a wide range of functions. Some of the processes cell adhesion molecules are involved in include:

An outline of the different CAMs is summarised in Figure 10.9.

G proteins

This group of membrane proteins has a high affinity for guanine nucleotides. Over 16 G proteins have been identified, composed of α, β and γ subunits, suggesting the existence of many more. Activation of the G protein enables it to bind guanosine triphosphate (GTP) and interact with an effector protein. The activated G protein then deactivates itself by intrinsic GTPase activity (Figure 10.10). This reduces the GTP to GDP, thus deactivating the G protein.

Figure 10.10

G protein activation and deactivation

Activation of the G protein system can result in various effects. It can control the release of second messengers via Gs- and Gi-type proteins that have stimulatory or inhibitory effects on enzymes such as adenylyl cyclase. Some G proteins are directly coupled to ion channels, and thus control membrane permeability to ions. Others can increase intracellular calcium concentrations and activate intracellular kinases.

Second messenger enzymes

The production of second messengers – cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), inositol triphosphate and diacylglycerol – takes place at the cell membrane. The enzymes responsible for their production are adenylyl cyclase, guanylyl cyclase and tyrosine kinase. The activities of these enzymes are controlled by various pathways, which involve both activation and inhibition. Second messenger effects are multiple and widespread intracellularly.