Click4Biology: 2.4 Membranes


2.4.1 Structure of the membrane.

2.4.2 Properties of the membrane phospholipids

2.4.3 Functions of membrane proteins .

2.4.4 Definitions of diffusion and osmosis.

2.4.5 Passive transport across membranes.

2.4.6 Active transport across the membrane.

2.4.7 Vesicle transport within the cell.

2.4.8 Membrane fluidity and transport across membrane by endocytosis and exocytosis.


2.4.1 Draw and label a diagram to show the structure of membranes (1).

Draw: To represent by means of pencil lines.

cell membrane





2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of the cell membranes (3)


Explain means to give a detailed account of causes, reasons or mechanisms.

lipid bilayer This model of the bilayer's has the proteins removed for clarity.





2.4.3 List the functions of membrane proteins (1).


List means to give a sequence of names or other brief answers with no explanation.

membrane proteins




2.4.4 Define diffusion and osmosis (1).

define means to give the precise meaning of a word, phrase or physical quantity.

Diffusion: passive movement of particles from a region of high concentration to a region of low concentration.

Osmosis is the passive movement of water molecules from a regions of lower solute concentration to a region of higher solute concentration


DIffusion ideas:


This model illustrates the main features of the process of diffusion.



This model shows diffusion through a membrane:

e.g. Source = blood oxygen. Sink= respiring cell






Osmosis ideas:






2.4.5 Explain passive transport across membranes by simple diffusion and facilitated diffusion (3).

Explain means to give a detailed account of causes, reasons or mechanisms.

The passive movement implies that there is no expenditure of energy in moving the molecules from one side of the membrane to the other:

However the molecules themselves possess kinetic energy which accounts for why they are in movement.

The membrane therefore 'allows' the molecules to pass through without needing to add any additional energy to the kinetic energy already possessed by the particles.

Particles will in fact pass in both directions but over all the emerging pattern is that molecules move from a region of their high concentration to a region of their low concentration.

simple diffusion







2.4.6 Explain the role of protein pumps and ATP in active transport across membranes(3).

Explain means to give a detailed account of causes, reasons or mechanisms.







2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum , Golgi apparatus and plasma membrane(3).

Explain means to give a detailed account of causes, reasons or mechanisms.

Cells will manufacture molecules for secretion outside of the cell.

Some of these secretion molecules are complex combinations of proteins, carbohydrates and lipids.

The base protein is coded for by a gene whose expression begins the process.

The animation below picks up the process with the protein already synthesised in the rough endoplasmic reticulum.

The animation plays very slowly so that the sequence can be followed.


1. Protein is already synthesised and present in the rER.

2. The protein is moved through the rER and modified.

3. A spherical vesicle is formed form the end of the rER with the    protein inside.

4. The vesicle migrates to the golgi apparatus.

5. Vesicle and golgi membranes fuse. The protein is released into the lumen of the golgi apparatus.

6. The golgi modifies the protein further by adding lipid or polysaccharides to the protein.

7. A new vesicle is formed from golgi membrane which then breaks away. The vesicles migrates to the plasma membrane.

8. The vesicle migrates to the plasma membrane fuses and secretes content its contents out of the cell. A process called exocytosis.





2.4.8 Describe how the fluidity of the membrane allow s it to change shape, break and re-form during endocytosis and exocytosis(2).



a) Exocytosis: vesicle membrane fuses with the plasma membrane.

b) Endocytosis:a vesicle is formed by the infolding of the plasma membrane


In each of the cases above the membranes are able to form and break without loss of the continuity of the plasma membranes. The process is very similar to the childhood game of playing with bubbles of detergent. Bubbles are produced then they can be watched readily joining together or splitting apart. The process is readily observed in cells but proof of the mechanism was not produced until 2002 by Lin Yang and Huey Huang.






Membrane fluidity:


(a) The phospholipid molecules can change places in the horizontal plane. This creates the so called fluid property of the membrane.

(b) Molecule exchange in the vertical plane does not occur. This maintains the integrity of the membrane.

(c) Cholesterol embedded in the membrane reduces its fluidity.







Mechanism for the making and breaking of the cell membrane.

membrane fusion

The Li Yang and Huey Huang model has shown that it is the proportion of other molecules in the plasma membrane that will determine whether is opens or closes.

It is suggested that the molecules that initiate membrane fusion or breakage will be:lipids;proteins and cholesterol.

They derived their model by a serendipitous observations whilst performing other experiments.

With x-ray diffraction patterns they showed how the phospholipids will form an hourglass shape at the point of contact. (click image for diffraction x-rays)

The sequence of diagrams shows how a membrane might fuse or split and yet self-seal during a wide variety of biological situations. It has not been lost on the researchers the potential uses of this knowledge and mechanism in medical therapies.

In the model:

(a) Membranes approach.

(b) Touching membranes note how the phospholipid heads flow together starting the process of fusion. As noted above this requires the presence of additional molecules.

(c) At the point of contact there is a single lipid bilayer.

(d) The pore is open and the membranes are now continuous.

Mouse-over the image for evidence x-ray diffraction images.




There are biological issues concerning membranes breaking and forming throughout this course. It is astonishing that such a common concept lacked evidence and had to wait until 2002 for Lin Yang and Huey Huang to produce the crucial evidence. This outstanding research will still take some time to reach school textbooks.