Cell membrane structure and permeability

In a nutshell

Cells require different substances and need to release their waste products. They do so through their cell membrane which is partially permeable and allows the transport of small molecules. The cell membrane has a ‘fluid mosaic’ structure as it is composed of many different molecules.

Cell membrane

The cell membrane acts as a barrier between the internal cellular environment and the external environment. Cell membranes are partially permeable to allow the movement of substances across the membrane.

Fluid mosaic model

The ‘fluid mosaic’ model was proposed in $1972$ . The membrane is described as ‘fluid’ because the phospholipids move slightly from side to side. It was also described as a 'mosaic' due to the random arrangement of proteins, phospholipids and cholesterol within the membrane.

Biology; Exchange and transport; KS5 Year 12; Cell membrane structure and permeability — A: Outside the cell, B: inside the cell, 1. Phospholipid, 2. Bilayer, 3. Cholesterol, 4. Protein channel, 5. Protein, 6. Glycolipid, 7. Glycoprotein

Component	Description
Phospholipids	The phospholipids form a double layer known as a bilayer. Phospholipids have a head and a tail. The head is a phosphate group which is polar, so it is hydrophilic. The heads attract and interact with water. The tails are fatty acids which are non-polar, so they are hydrophobic. Therefore, the tails orientate themselves away from water. The phosphate group and the two fatty acids are held together with ester bonds. The phospholipids form a bilayer due to the aqueous environments on both sides of the membrane (the cytoplasm on the inside of the cell and the tissue fluid on the outside of the cell).
Cholesterol	Cholesterol is a lipid present in the cell membrane that disrupts the phospholipid bilayer and causes the phospholipids to bind more closely together. This prevents the movement of the phospholipids and reduces the fluidity of the membrane. As a result, the membrane is more rigid and more stable.
Proteins	Proteins can be on top, within or below the bilayer. They can support the membrane structure and they can also have a role in cell signalling pathways as they can act as receptor molecules. Example Channel proteins and carrier proteins involved in the transport of molecules across the membrane are found in the cell membrane.
Glycoproteins	Glycoproteins are proteins with carbohydrate chains attached. These are found on the surface of the cell membrane and have a role in cellular recognition, the immune response and membrane stabilisation. Glycoproteins act as receptors for hormones and they can allow cells to recognise one another, thus allowing immune cells to distinguish between their own cells and a pathogen.
Glycolipids	Glycolipids are lipids that are covalently bonded with a carbohydrate. The carbohydrate part of the glycolipid extends out of the bilayer and into the aqueous external environment. They act as surface receptors and help to stabilise the membrane. Example The human ABO blood system is as a result of glycolipids on the cell surface membrane.

Cell membrane permeability

For molecules to pass through the membrane it must be partially permeable. There are different factors, like temperature and alcohol concentration, that can impact the permeability of the membrane. The impact of changing the temperature on membrane permeability can be investigated using the method below.

Procedure

1.	Cut five equal sized pieces of beetroot.
2.	Rinse the pieces to remove excess pigment.
3.	Pipette $5\ cm^3$ of water to five test tubes.
4.	Place one piece of beetroot into each test tube of water.
5.	Place each tube into a water bath at a different temperature. Example $10\degree C$ , $20\degree C$ , $30\degree C$ , $40 \degree C$ and $50\degree C$ .
6.	Leave the tubes in the water for $10\ minutes$ .
7.	Remove and discard the beetroot pieces.
8.	Place the tubes into a colorimeter and measure the absorbance. *Note:* The colorimeter will need to be calibrated using water.

A colorimeter measures the absorbance of light through a solution. If the absorbance reading is higher, this means that more pigment has been released. If more pigment has been released, then the membrane permeability will have increased. You would expect results as shown in the table below.

Temperature	Observation
Below $0\degree C$	The phospholipids will not move very much and this will make the membrane rigid. However, the channel and carrier proteins may change shape at low temperatures and this will increase the permeability of the membrane. It is possible for ice crystals to form and pierce the membrane which will make the membrane more permeable.
Between $0\degree C$ and $45 \degree C$	The phospholipids move so the membrane is partially permeable. At higher temperatures, the phospholipids will have more energy and this allows them to move more. As a result, the membrane permeability increases at higher temperatures.
Above $45\degree C$	The phospholipid bilayer starts to break down which makes the membrane much more permeable. The channel and carrier proteins change shape which increases the permeability of the membrane.