Describe the basic layout of a cell

Whilst each cell has specialist functions, there are many structural features common to all (Figure 2.1). Each cell has three main parts:

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  The cell surface membrane, a thin barrier that separates the interior of the cell from the extracellular fluid (ECF). Structurally, the cell membrane is a phospholipid bilayer into which are inserted glycoproteins akin to icebergs floating in the sea. The lipid tails form a hydrophobic barrier that prevents the passage of hydrophilic substances. The charged phosphate-containing heads of the lipids are hydrophilic and thereby form a stable lipid–water interface. The most important function of the cell membrane is to mediate and regulate the passage of substances between the ECF and the intracellular fluid (ICF). Small, gaseous and lipophilic substances may pass through the lipid component of the cell membrane unregulated (see Chapter 4). The transfer of large molecules or charged entities often involves the action of the glycoproteins, either as channels or carriers.

  
  
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  The nucleus, which is the site of the cell’s genetic material, made up of deoxyribonucleic acid (DNA). The nucleus is the site of messenger ribonucleic acid (mRNA) synthesis by transcription of DNA and thus coordinates the activities of the cell (see Chapter 3).

  
  
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  The cytoplasm, the portion of the cell interior that is not occupied by the nucleus. The cytoplasm contains the cytosol (a gel-like substance), the cytoskeleton (a protein scaffold that gives the cell shape and support) and a number of organelles (small, discrete structures that each carry out a specific function).

Figure 2.1 Layout of a typical cell.

Describe the composition of the cell nucleus

The cell nucleus contains the majority of the cell’s genetic material in the form of DNA. The nucleus is the control centre of the cell, regulating the functions of the organelles through gene – and therefore protein – expression. Almost all of the body’s cells contain a single nucleus. The exceptions are mature red blood cells (RBCs; which are anuclear), skeletal muscle cells (which are multinuclear) and fused macrophages (which form multinucleated giant cells).

The cell nucleus is usually a spherical structure situated in the middle of the cytoplasm. It comprises:

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  The nuclear envelope, a double-layered membrane that separates the nucleus from the cytoplasm. The membrane contains holes called ‘nuclear pores’ that allow the regulated passage of selected molecules from the cytoplasm to the nucleoplasm, as occurs at the cell surface membrane.

  
  
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  The nucleoplasm, a gel-like substance (the nuclear equivalent of the cytoplasm) that surrounds the DNA.

  
  
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  The nucleolus, a densely staining area of the nucleus in which RNA is synthesised. Nucleoli are more plentiful in cells that synthesise large amounts of protein.

The DNA contained within each nucleus contains the individual’s ‘genetic code’, the blueprint from which all body proteins are synthesised (see Chapter 3).