Cambridge International A Level Biology Revision Guide

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Cambridge International AS Level Biology 72 Chapter 4: Cell membranes and transport Learning outcomes You should be able to: ■■ describe the structure of cell membranes ■■ explain the functions of the molecules that make up cell membranes ■■ ■■ explain how substances enter and leave cells find the water potential of plant tissues through experiment
Chapter 4: Cell membranes and transport Drug runners Liposomes are artificially prepared membrane-bound compartments (vesicles). They can be prepared by breaking up biological membranes into pieces, some of which re-seal themselves into balls resembling empty cells, though much smaller on average. Like intact cells, they are surrounded by a phospholipid bilayer and the interior is usually aqueous. They were first described in 1961. Since then they have been used as artificial models of cells and, more importantly, for medical applications. In particular, they have been used to deliver drugs. To do this, the liposome is made while in a solution of the drug, so the drug is inside the liposome. The liposome is then introduced into the body and when it reaches a target cell, such as a cancer cell or other diseased cell, it fuses with that cell’s surface membrane, delivering the drug inside the cell. Precise targeting can be achieved by inserting the correct recognition molecule – for example an antigen or antibody – into the liposome membrane. Other targeting methods also exist. A recent (2013) discovery illustrating the usefulness of liposomes is that they provide a safe way of delivering the powerful anti-cancer drug staurosporine. Although this drug has been available since 1977, it kills any cells, including healthy ones that it comes into contact with because it interferes with several cell signalling pathways. Disguising agents have been added to the outer surfaces of liposomes carrying the drug which hide the drug from the immune system and allow it to target cancer cells only. Liposomes have many other uses. For example, they are used in the cosmetics industry to deliver skin care products, such as aloe vera, collagen, elastin and vitamins A and E, when rubbed on skin. Liposome delivery of food supplements by mouth has also been tried with some success – absorption rates can be much higher than with traditional tablets. Figure 4.1 Liposomes. 73 In Chapter 1, you saw that all living cells are surrounded by a very thin membrane, the cell surface membrane. This controls the exchange of materials such as nutrients and waste products between the cell and its environment. Inside cells, regulation of transport across the membranes of organelles is also vital. Membranes also have other important functions. For example, they enable cells to receive hormone messages. It is important to study the structure of membranes if we are to understand how these functions are achieved. Phospholipids An understanding of the structure of membranes depends on an understanding of the structure of phospholipids (page 38). From phospholipids, little bags can be formed inside which chemicals can be isolated from the external environment. These bags are the membrane-bound compartments that we know as cells and organelles. Figure 4.2a shows what happens if phospholipid molecules are spread over the surface of water. They form a single layer with their heads in the water, because these are polar (hydrophilic), and their tails projecting out of the water, because these are non-polar (hydrophobic). The term ‘polar’ refers to the uneven distribution of charge which occurs in some molecules. The significance of this is explained on pages 35–36. If the phospholipids are shaken up with water, they can form stable ball-like structures in the water called micelles (Figure 4.2b). Here all the hydrophilic heads face outwards into the water, shielding the hydrophobic tails, which point in towards each other. Alternatively, twolayered structures, called bilayers, can form in sheets (Figure 4.2c). It is now known that this phospholipid bilayer is the basic structure of membranes (Figure 4.2d).
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Mary Jones, Richard Fosbery, Jennif
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Acknowledgements Meiosis” in Tuat
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Cambridge International A Level Biology Revision Guide

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