To Investigate How the Permeability of the Membrane Changes With Temperature.

Diagram from unknown book p224

As the cell membrane selectively allows some substances through, but not others, it is described as partially permeable.

What is the structure of the cell membrane?

Although a eukaryotic cell may have many membranes within itself the generally follow the same structure.

The following diagram1 is the fluid mosaic model of the membrane as we regard it today, developed in 1972 by Singer and Nicholson.

The basic structure of all membranes is the phospholipid bilayer. Phospholipids are amphipathic as their phosphate head is polar, and therefore hydrophilic and their two fatty-acid tails are non-polar, and therefore hydrophobic.  As the membrane is surrounded by water molecules both inside and out, the phospholipids naturally form a bilayer, with the hydrophilic heads pointing outwards towards the water, and the hydrophobic tails pointing inwards, away from the water. Because the tails are non-polar, it is difficult for polar molecules or ions to pass through this part of the membrane. Therefore, the bilayer acts as a barrier against water-soluble substances.

        Cholesterol molecules also have hydrophilic heads and hydrophobic tails. They have several functions in the cell membrane:-

1. They help control the fluidity of the membrane, preventing it from becoming too fluid, or too rigid
2.  They are important in the mechanical stability of membranes, preventing them from breaking up too easily.
3. The hydrophobic regions prevent ions or polar molecules from passing through - this is particularly important in the mycelium sheath around nerve cells, where leakage of ions would slow down nerve impulses.

Proteins also have a variety of functions in the membrane:-

1. Transmembrane channel proteins, or transport proteins, span the whole cell membrane. They provide hydrophilic channels for ions and polar molecules to pass through. There are many different types of protein channels, and each is specific to a particular substance, so they can control the type of substances allowed to enter or leave the cell.
2. Extrinsic proteins are those which are embedded on the surface of the membrane, and do not span the whole of the membrane. “They can be enzymes, specific receptor molecules, electron carriers and energy transducers in photosynthesis and respiration.” 1
3. Proteins are very important in the membrane of organelles. E.G. in mitochondria and chloroplasts, they play an important part in the respiration and photosynthesis functions of the organelles. 2

All proteins in the cell membrane have a very specific function, and a very specific shape to aid in this function. Most have a secondary structure, coiled into an α-helix, or a tertiary structure, where the α-helix itself is coiled or folded. The shape it forms is very precise, held together by various bonds forming between the amino acids 2 :-

1. Hydrogen bonds form between strongly polar groups. They can be broken by high temperatures or pH changes.

2. Disulphide bonds form between cysteine molecules. They can be broken by reducing agents.

3. Ionic bonds form between ionized amine and carboxylic acid groups. They can be broken by pH changes.

4. Hydrophobic interactions occur between polar and non-polar chains.

As the structure of proteins is specific to its function, it is imperative that the bonds are not broken. Thus the surrounding conditions, such as the temperature and pH, must remain constant; “proteins are generally adapted to work at set temperatures…high temperatures, as well as other stressful conditions effectively denatures proteins or generally renders them ‘unserviceable’. Such damaged molecules are of no use to the cell and may even be harmful.”

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