Cell Membrane

Question 1

What are the functions of a membrane? (5)

Answer 1

 keeping all cellular components
inside the cell
 allowing selected molecules to move in and out of the cell
 isolating organelles from the rest of the cytoplasm,
allowing cellular processes to occur separately.
 a site for biochemical reactions
 allowing a cell to change shape

Question 2

What does a cell membrane consist of?

Answer 2

 Lipids (phospholipids and cholesterol)
 Proteins
 Carbohydrates

Question 3

Structure and Properties of phospholipids.

Answer 3

One of the main components of membranes are
phospholipids, a type of lipid made from two fatty acid
chain ‘tails’ attached to a phosphate group ‘head’.
The phosphate group head is polar and hydrophilic (‘water
loving’), while the fatty acid chains of the tail are non-polar
and hydrophobic (‘water-hating’).

hydrophobic tails
hydrophilic head

The shape of the structures that phospholipids form is due to their polar nature, and the way they interact with water.

(made up of 2 fatty acids and a glycerol) - lipid + phosphate group = phospholiid

Question 4

What does amphipathic mean?

Answer 4

It’s when a molecule contains both a hydrophobic and hydrophilic section

Question 5

Structure of cell membranes.

Answer 5

 Cell membranes are formed as a bilayer of two phospholipids.

 A phospholipid is composed of glycerol, two fatty acids and a
negatively charged phosphate group.

 The phosphate group is polar and is hydrophilic.

 The fatty acids are non-polar and are hydrophobic.
 When added to water (and remember that cells contain a lot of
water) the hydrophilic part is directed towards the water and
the hydrophobic part is directed away from the water.

 Phospholipids can form micelles– spheres that form with
hydrophilic parts on the outside.

 Phospholipids can also form bilayers when the hydrophobic
fatty acids are too large to fit into the micelle.

Question 6

Role of the phospholipid bilayer.

Answer 6

 2 layers of phospholipids that make up the membrane with
the tails rotated inside (hydrophobic) and the heads outside
(hydrophilic)
 Allows lipid molecules to diffuse in the cell rapidly
 Does not allow water-soluble molecules to diffuse in the
membrane

Question 7

Role of Proteins

Answer 7

 Act as channels and pores for water and water-soluble
substances to pass through the membrane
 Act as receptors
 Important for cell recognition
 Some are enzymes

Question 8

Role of Carbohydrates

Answer 8

 Act as antigens
 Act as receptor sites
 Bind cells together to make tissues

Question 8

Role of Cholesterol in Fluidity

Answer 8

 combines with the fatty acids of the phospholipids
 holds the phospholipids together restricting the
movements of phospholipids
 this makes the membrane not to be too fluid

Question 9

What are the problems with the Davson-Danielli model?

Answer 9

 The amount and type of membrane proteins vary greatly between different cells.

 It was unclear how the proteins The model would permit the membrane to change shape without bonds being broken.

 Membrane proteins are largely hydrophobic and therefore should not be found where the model positioned them: in the aqueous cytoplasm and extracellular environment.

Question 9

Explain why the Fluid Mosaic Model is called what it is.

Answer 9

‘Fluid’ because the proteins and lipids can move around within the membrane

‘Mosaic’ because of the mosaic-like make up of the components within the membrane

Question 10

What do Glycolipids do?

Answer 10

They are thought to act as receptors and be involved in cell recognition. They may also be involved in membrane stability and cell-to-cell attachment.

Question 11

What do Glycoproteins do?

Answer 11

They have many roles, such as being involved in cell recognition. They may also be involved in membrane stability and receptor molecules

Question 12

What are integral proteins, and what do they do?

Answer 12

They are proteins which span the entire width of the cell membrane. They have many functions, particularly important in cell signalling and the transport of substances across the membrane

Question 13

When exposed to water, phospholipids form one of two
structures. name them.

Answer 13

micelle or a bilayer