4.1 Cell surfaces

The cell surface is functionally very important, because it is the cell's interface with its environment.

One major difference between different eukaryotes is the presence or absence of a cell wall.

Plant cells (Figure 1b) and fungal cells have a cell wall, which provides support and determines cell shape, while protists are diverse in this respect; some, such as amoebae, do not have a rigid cell wall, but many do. All cells, however, have a cell membrane, which acts as a barrier and an interface with the external environment.

The cell membrane is composed predominantly of phospholipids, arranged in a bilayer. A schematic diagram illustrating the main features of a typical animal cell membrane is shown in Figure 7. Within the bilayer are embedded a variety of proteins and glycoproteins (proteins that have sugars attached). These proteins, many of which span the membrane, play crucial roles in the interactions of the cell with its environment. Some membrane proteins act as transporters or channels that allow selective movement of ions, nutrients or other molecules into the cell. Others, often glycoproteins, act as receptors, which respond to specific molecular changes in the extracellular environment and transduce information about the environment to the inside of the cell, thereby allowing appropriate responses to be initiated.

Figure 7 A schematic diagram of the animal cell membrane. The membrane consists of a fluid phospholipid bilayer with proteins embedded in it. Integral membrane proteins interact with the hydrophobic 'tails' of lipid molecules within the lipid bilayer; some of these proteins are transmembrane proteins that act as channels (pores) for transport of molecules into and out of the cell. Peripheral proteins interact only with the outer hydrophilic 'heads' of the lipid molecules. Membrane lipids and proteins are often glycosylated, i.e. attached to sugar chains.

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The diagram shows the double layer of phospholipid molecules: the polar head of each phospholipid molecule is shown as a blue sphere and the two hydrophobic or non-polar tails attached to the head are shown as zig-zag yellow ribbons. The tails in each layer are oriented in parallel and are facing, but not touching, the tails of the opposing layer in the bilayer structure. Interspersed between the phospholipid molecules are so-called integral protein molecules (shown here in red) which span the width of the membrane and provide aqueous channels or pores in the membrane. Other protein molecules, the peripheral membrane proteins, are attached to the cytosolic surface of the membrane. Some of the proteins and lipids at the extracellular surface of the membrane have short, sometimes branched sugar chains attached; these are called glycoproteins and glycolipids, respectively.

Figure 7 A schematic diagram of the animal cell membrane. The membrane consists of a fluid phospholipid bilayer with proteins embedded in...

Yet other glycoproteins act as recognition molecules, and can promote adhesion between adjacent cells in a tissue. The cell membrane is also linked to proteins on its cytoplasmic surface (intracellular proteins). These include components of the cytoskeleton, which have a structural role, maintaining the shape of the cell.