Cell Membrane

All living cells are surrounded by a thin boundary known as the cell membrane this acts as a barrier and controller of what is able to pass through the into and out of the cell. Cell organelles also have a membrane which is very similar to the cell membrane.

Structure of the membrane 

The plasma membrane is known to consist of both proteins and phospholipids the phospholipids are able to constantly move, making the membrane a fluid-mosaic structure.

Phospholipid bilayer

As we know from the previous chapter - lipids, the phospholipid has a hydrophilic head and a hydrophobic tail. This arrangement causes the phospholipids to arrange themselves in to a bilayer.
as we can see from the diagram on the left, the heads form the barrier this is both intercellular (inside the cell) and extracellular (outside the cell). This then means that the water-insoluble tales point towards each other in the middle.

This arrangement makes the membrane fluid yet stable, it also allows tiny molecules such as water to pass through quickly but larger molecules such as glucose need porous proteins to pass through.


Proteins play an important role in cell membranes as described below:
  • Some provide structural support, which strengthens the membrane.
  • Some may contain channels which allows the transfer of small molecules through the membrane.
  • Some act as carriers and will actively transport certain molecules across the membrane.
  • There are enzymes, which appear on both sides of the membrane, these catalyse certain reactions.
  • There may be hormone receptors present.
  • Others may also be antigenic markers to help recognise 'foreign' and 'self' cells.
Membranes differ from cell to cell and organelle to organelle, for example: the membrane of a nucleus has a double porous membrane where as the membrane of a mitochondrion is convoluted. Although they are different, they all play the same role and that is to act as selective barriers

Cell membranes also play an important role in regulating the cell's mechanism and keeping its internal environment fairly constant. It manages to regulate the internal environment by it being selectively permeable.

Here is a short video explaining the phospholipid bilayer:

Phospholipid Bilayer

There are different modes of transport across the cell membrane these include:
  • Diffusion 
  • Osmosis
  • Active Transport
  • Endocytosis/Exocytosis
Diffusion - this is the transport of molecules from a level of high concentration to a level of low concentration, it therefore diffusion occurs on the concentration gradient. As we know from above the membrane has many tiny pores, allowing small molecules such as water and CO2 through easily, it also contains larger pores which allow proteins to diffuse in and out of the cell. Therefore the membrane is said to be selectively permeable. 

osmosis is the transfer of water from a level of high concentration to a level of low concentration. It works in the same way as diffusion as this movement happens rapidly.

Osmosis in red blood cells (RBC) - if a red blood cell is placed in water, the water will diffuse into the cell's cytoplasm, causing to burst (cell lysis). If it is placed in a salt solution with a low water concentration then the RBC will shrink. If it is placed in an isotonic solution (equal to its concentration) the cell will remain unaltered.

Active Transport - this is when molecules are moved from a level of low concentration to a level of high concentration, against the concentration gradient. Active transport requires energy (ATP). Active transport is affected by factors such as temperature, oxygen, and the availability of a respiratory substrate (glucose), all of these also directly effect respiration.

We have only talked about the movement of small molecules across the membrane, during endo/exocytosis, movement of large molecules occurs:

Endocytosis - this is when a cell engulfs a large quantity of a material. The membrane forms a pouch, this then becomes an intracellular vesicle.  There is two types of endocytosis, these are phagocytosis (cell eating) and pinocytosis (cell drinking).

Exocytosis as the name suggests is the opposite of endocytosis and involves a large quantity of a material being expelled from the cell.