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Membranes and the lipid bilayer

The lipid bilayer is a layer of phospholipids that form the main structure of the cell membrane.

The bilayer is two phospholipid molecules thick (around 6 - 10nm), with one phosphate head facing the inside of the cell and another facing outwards.

The phosphate heads are exposed to the aqueous environment while the hydrophobic tails remain hidden in the lipid bilayer.
The phosphate heads are exposed to the aqueous environment while the hydrophobic tails remain hidden in the lipid bilayer.

The amphipathic (both hydrophobic and hydrophilic) properties of phospholipids cause them to self-assemble into a continuous sphere.

The hydrophilic phosphate heads interact with the water molecules in the internal and external environment. The hydrophobic fatty acid tails interact with each other through weak intermolecular forces called Van der Waals forces.

The lipid bilayer is permeable to small, non-polar, lipid soluble molecules such as $$\ce{CO2}$$. Polar and ionic molecules cannot pass through.

The cell membrane is composed mainly of lipids and proteins.

Phospholipids form the core component of the cell membrane, making up the lipid bilayer.

Polar molecules cannot diffuse through the membrane as there is a hydrophobic boundary formed by the double layer of hydrophobic tails.

Large molecules are also unable to move through the densely packed phospholipids.

These molecules are moved across the membrane by proteins. Carrier proteins, ion channels and pores are proteins embedded in the membrane which are used to regulate the flow of molecules that cannot diffuse through the membrane.

Additionally, there are enzymes on the surface of the membrane that help to catalyse reactions.

This protein channel allows molecules to pass through the membrane.
This protein channel allows molecules to pass through the membrane.

Sterols and sugars are also present in the cell membrane.

Sterols such as cholesterol (found only in animals), regulate the fluidity of the membrane. They are scattered across the membrane and occur in varying quantities in different animal cells.

Cholesterol regulates the fluidity of the cell membrane in two ways.

  • Stiffens the membrane

    Cholesterol is a stiff molecule compared to phospholipids and helps to make the membrane stiffer

  • Prevents the membrane from being too stiff

    Cholesterol is randomly dispersed among the hydrophobic tails of phospholipids. This disrupts the orderly packing of the hydrophobic tails, preventing them from crystallizing.

Polysaccharides (sugars) associate with both lipids and proteins to form glycolipids and glycoproteins.

These sugars protrude from the external surface of the cell membrane and act as receptors. They are important in cell recognition and adhesion.

Glycoproteins allow cells to recognise other cells and form tissues (groups of similar cells). Muscle cells identify and adhere to each other to form muscle tissue.

The fluid mosaic model describes some of the properties of the cell membrane.

Fluidity: The components of the membrane are able to flow around the membrane, while maintaining the membrane structure. Their position is not static.

Mosaic: Like a mosaic, different components fit together to complete the membrane.

Scattered among the phospholipid bilayer are cholesterols and various proteins.
Scattered among the phospholipid bilayer are cholesterols and various proteins.

The fluid characteristic of the membrane is variable. Cells can regulate the fluidity of their membrane, or it can be influenced by external factors.

Temperature and lipid composition can have a significant effect on fluidity.

At low temperatures, the phospholipids are tightly packed and cannot flow easily. At higher temperatures they are more spread out, and flow with greater ease.

The degree of saturation of the fatty acid tail further influences fluidity. Unsaturated $$\ce{C=C}$$ bonds cause the tails to kink (bend), making them more spread out and unable to align fully.

Unsaturated fatty acid tails have greater fluidity, since the van der Waals forces between the tails are weaker. The bent tails also disrupt the packing of the bilayer.

The flexibility of the membrane is also determined by the type and amount of sterol present.

There are several methods by which molecules can cross the cell membrane.

The simplest methods involve passive transport. This is diffusion of a molecule along a concentration gradient (from an area of high concentration to an area of low concentration) either into or out of the cell.

In passive transport, there is no energetic investment from the cell.

Small non-polar molecules (e.g. $$\ce{CO2}$$ and $$\ce{O2}$$) are able to diffuse directly through the lipid bilayer.

Molecules that are too large, not lipid soluble, or non-polar enter the cell through a different type of passive transport called facilitated diffusion. This employs the use of protein pores.

Protein pores are present on the cell membrane to allow these molecules to passively diffuse into the cell.

Ions, and molecules such as water and glucose enter this way. The rate of facilitated diffusion saturates at high concentrations as all the pores become occupied.