Question

CELL BIOLOGY

Chapter 10

The function of a lipid bilayer

How is the lipid bilayer formed

Know the different types of phospholipids and any unique features and locations (if there are any)

Know the overall structure of a phospholipid

How does a lipid membrane stay fluid?

How to lipid move in a membrane? Just as important, know the movement lipids are unable to do in the absence of flipase.

Alberts.-.Molecular.Biology.Of.The.Cell.5th.Ed IS THE TEXT

Answer 1

The lipid bilayer is important for maintaining the shape of a cell and for selective permeability. Therefore, it is vital for the survival and function of the cell. This article explains what the lipid bilayer is and how its structure contributes to cell viability.

Functions of the lipid bilayer

• Maintain an internal environment: the external cellular environment is often different to the cellular environment (ion, protein concentrations etc). The cell can control its environment by controling gene expression of transmembrane proteins (see 'crossing the membrane')
• Allows the cell to control what enters/exits: transporter proteins and channels enable the cell to selectively uptake molecules that it requires whilst excluding those that may be harmful.
• Protection: the lipid bilayer creates an envelope - housing the cellular components and offering some protection. Some cells, such as plant cells, have a cell wall which offers further protection.
• Contains important components involved in cell recognition, communication, signalling etc: the lipid bilayer is the boundary between the internal and external environment and must mediate communication between the two.
• Gives the cell shape (anchors with the cytoskeleton): in some cells the shape is important. One example is the red blood cell. Its concave shape increases its surface area allowing it to bind oxygen much more efficiently.

Structure of the lipid bilayer

The lipid bilayer is made up of many phospholipids that align together. Each phospholipid is made up of a hydrophilic head and a hydrophobic tail.

The most common phospholipid is phosphatidylcholine which contains a choline molecule bound to phosphate and glycerol. The hydrophilic head is polar allowing it to form hydrogen bonds with water molecules, whereas the tail region - made from two hydrocarbon chains - is non-polar or hydrophobic.

It is this combination of both a hydrophobic and hydrophilic region (amphiphilic) that gives phospholipids such an important function within the cell. When placed in water, the phospholipid molecules naturally align into a bilayer, allowing the hydrophobic tails to avoid water whilst the hydrophilic heads form hydrogen bonds with water molecules.