Structure of a cell

All cells are made of a substance called protoplasm, which is jelly-like, opaque and colourless, and consists mainly of water with other substances in solution (Fig. 2.1).

Fig. 2.1 A cell.

The word cytoplasm is commonly used to describe the protoplasm which forms the bulk of the cell, the prefix ‘cyto-’ being derived from the Greek word for cell. The cytoplasm contains molecules of ribonucleic acid (RNA), which acts as a messenger carrying information out from the nucleus to the cytoplasm.

The plasma membrane plays a very important role in holding the contents of the cell together, and yet is flexible enough to allow cells to conform to many shapes. The plasma membrane also regulates the passage of materials such as nutrients, electrolytes and waste products in and out of the cell. In this way the plasma membrane is described as being selectively permeable or semipermeable.

The plasma membrane (Fig. 2.2) is composed of protein and a substance called phospholipid. The structure of phospholipid molecules, which makes them hydrophobic (repelled by water) at one end and hydrophilic (attracted to water) at the other end, leads to the unique structure of the plasma membrane, a double layer known as a phospholipid bilayer. The protein molecules are embedded in this bilayer and it is the protein molecules that confer the selective permeability on the plasma membrane. The protein molecules can form channels or carriers for the transport of certain substances across the plasma membrane.

Fig. 2.2 The plasma membrane.

Other functions of the protein in the plasma membrane are to act as sites of recognition between cells and to act as receptors whereby substances such as hormones (see Chapter 7) can attach to cells and influence their ability.

Transport processes across plasma membranes

Movement of substances across cell membranes takes place by several processes:

Osmosis

The processes of osmosis and diffusion, whereby water and substances in solution, respectively, move across semipermeable membranes, work across the plasma membrane. If a substance with large molecules, e.g. sugar, is made into a solution and is separated from a weaker sugar solution by a semipermeable membrane, only water will pass through the membrane, from the weaker solution to the stronger solution, because the sugar molecules are too large to pass through. This passage of water across a semipermeable membrane is called osmosis (Fig. 2.3).

Fig. 2.3 Osmosis.

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