Fluid Mosaic Membranes (CIE A Level Biology)

In 1935 a model describing the structure of the cell surface membrane was proposed by the biologists Davson and Danielli. Fig. 1 shows a representation of their proposal.

Fig. 1

The model shown in Fig. 1 was replaced in 1972 when two other scientists, Singer and Nicolson, proposed the fluid mosaic model to describe the structure of the cell surface membrane.

Describe what is meant by the term 'fluid mosaic'.

Compare the model of membrane structure shown in Fig. 1 with the fluid mosaic model that replaced it.

Explain how the features of a phospholipid contribute to the structure of a cell surface membrane.

State two features of cell membranes other than the structure that can be explained by the fluid mosaic model.

Some components of the cell surface membrane include:

• Phospholipid molecules
• Protein molecules
• Cholesterol molecules

Draw a labelled diagram of a section through a cell surface membrane to show how the components mentioned above are organised within the membrane. Include other named components and label the inner and outer surfaces of the membrane.

The cell surface membrane is said to be partially permeable.

Explain how the structure of a phospholipid prevents some substances from diffusing directly between the phospholipids that make up the cell surface membrane.

High temperatures can denature membrane proteins.

Suggest the effect this would have on a cell surface membrane.

State one role of phospholipid bilayers within the cell.

Fig. 1 is an electron micrograph of part of a eukaryotic cell.

A ruler is shown in the image for scale. 

Fig. 1

State how it is possible to deduce that Fig. 1 is a transmission electron micrograph and not a scanning electron micrograph.

Both the Golgi body and the rough endoplasmic reticulum are part of the network of membranes inside cells.

Outline structural features shown in Fig. 1 that identify G as the Golgi body and not the rough endoplasmic reticulum.

Calculate the actual diameter, X–Y, of the mitochondrion labelled in Fig. 1

Write down the formula that you will use to make your calculation. Give your answer to the nearest whole nanometre (nm).

Many of the cell structures in Fig. 1 are surrounded by membranes.

Membranes are approximately 6 nm to 7 nm wide.

[3]

ii)

The inner membrane of the mitochondrial envelope is much less permeable than the outer membrane.

Suggest one way in which the structure of the inner membrane of the mitochondrion may differ from that of the outer membrane to produce a less permeable inner membrane.

[1]

Glucagon is a hormone that affects the activity of liver cells, leading to an increase in blood glucose levels. Glucagon acts as a ligand.

Suggest the features of glucagon that make it an example of a ligand.

Using your knowledge of the cell surface membrane:

[1]

[1]

Glycoproteins play an important role in cell signalling.

Explain the role of glycoproteins in initiating a response inside a cell.

Linoleic acid is an unsaturated fatty acid that is found in some triglycerides and some phospholipids.

Phospholipids are components of cell membranes.

Fig. 1 shows a molecule of linoleic acid.

Fig. 1

The composition of cell membranes of plants changes in response to changes in temperature.

At the start of the cold season, there is an increase in the proportion of phospholipids with unsaturated fatty acids in the chickpea, Cicer arietinum. Chickpea plants that do not make this change do not survive.

Suggest how the increase in the proportion of phospholipids with unsaturated fatty acids helps plants, such as chickpea, survive decreases in temperature.

State why triglycerides and phospholipids cannot be described as polymers.    

State two differences in the structure of triglycerides and phospholipids.

Platelets metabolise linoleic acid to produce a molecule known as thromboxane.

Thromboxane is released by platelets when blood loss occurs. Thromboxane acts on smooth muscle cells in the walls of arteries. This causes arteries to constrict, which reduces blood flow.

Explain why the constriction of arteries following blood loss is an example of cell signalling.

Fig. 1 shows the molecular diagram of part of the fluid mosaic structure of the plasma membrane. These molecules play an important role in maintaining the stability of the plasma membrane.

Fig. 1

Identify the part of the fluid mosaic structure of the plasma membrane represented by the molecular diagram in Fig.1.

Scientists have found that wheat crops adapted to grow in winter have increased unsaturated phospholipid content.

Suggest why the presence of unsaturated phospholipids would be advantageous.

Pieces of phospholipid bilayer were analysed from two different mammalian cell surface membranes. Sample X contained phospholipid molecules at a density of 4.2 x 10⁶ molecules µm^-2, whereas sample Y contained phospholipid molecules at a density of 5.5 x 10⁶ molecules µm^-2. One sample was from an exocrine pancreatic cell and the other was from a skin cell. Identify, with reasons, which cell type corresponds to samples X and Y.  

Researchers have discovered that an individual phospholipid molecule can exchange places with its neighbouring phospholipid molecule in a monolayer as frequently as 10⁷ times per second.

By contrast, phospholipid molecules almost never exchange places with each other from one monolayer to the other within a bilayer, referred to as a ‘flip-flop’ exchange. The ‘flip-flop’ takes place around once a month for a typical phospholipid molecule.

Suggest why there is this difference in molecular behaviour.