The cellular, or plasma, membrane is a protective barrier allowing delicate organelles found within a cell to be shielded from the harsh conditions outside. The job is not simply to protect though, it’s also responsible for:
- Transport of molecules into and out of the cell
- Giving the cell its shape
- Providing signals to and from the cell
This is just a few of the roles; there are a range of more specific uses. A few of these roles are discussed in more detail below to give a broader picture of how important the cellular membrane is to a cells function.
To fully understand how the cellular membrane works, it’s important to first appreciate the different biological molecules that make it up:
1. Phospholipid bilayer
Phospholipids are small molecules comprised of a hydrophilic phosphate head and two hydrophobic fatty acid tails. These molecules align next to each other, as well as tail-to-tail, forming the two distinct layers known as the ‘phospholipid bilayer’.
The small number of molecules that can passively diffuse through the phospholipid bilayer means that another passageway is required – proteins offer this. There are two main kind of proteins found in the cellular membrane:
Intrinsic (integral) – these proteins span the entire membrane and allow a range of substances to pass through into the cell – ion channels are an example of this.
Extrinsic (peripheral) – these are loosely bound to the surface of the cellular membrane and often undergo reversible interactions with small molecules, such as signalling compounds.
Cholesterol is the last large subsection of molecules found within the cellular membrane and has a variety of roles, such as cell-to-cell recognition, but predominantly cholesterol helps to reduce fluidity in order to rigidify the membrane.
Fluid Mosaic Model
The term ‘fluid mosaic’ describes how the different molecules making up the membrane combine together to form one cohesive system. It means nothing is fixed in place, allowing the proteins, phospholipids and other molecules to continually move around – this helps to regulate the large amount of substances continually interacting with the membrane.
1. Passive transport
The cellular membrane protects the cell from the outside, but molecules also need to get in and out. One way this can occur is if there is if there is a concentration gradient - meaning there is a higher concentration out a substance one side of the membrane than the other.
Diffusion – is where molecules such as oxygen passively move across the membrane, following the concentration gradient.
Osmosis – is similar to diffusion but with water molecules moving from an area of high water potential, to an area of low.
Facilitated diffusion – still using a concentration gradient, but this is for molecules that require a carrier to transverse the membrane – such as glucose.
2. Active transport
Sometimes it’s beneficial for the cell to move molecules against a concentration gradient – from low to high – such as certain ions. This requires energy in the form of ATP binding to membrane proteins to transport the molecules.