Structure of the cell-surface membrane

Question 1

What is the general name for all membranes?

Answer 1

Plasma membranes.

Question 2

What is a cell-surface membrane?

Answer 2

It’s the name for the plasma membrane which specifically surrounds cells and forms the boundary between the cell cytoplasm and the environment.
It allows different conditions to be established inside and outside of a cell.

Question 3

What are the features of a cell-surface membrane?

Answer 3

Phospholipids 
Proteins
Cholesterol
Glycolipids
Glycoproteins

Question 4

What does the phospholipid do in the cell surface membrane? Draw the structure of a phospholipid bilayer.

Answer 4

• Phospholipids form a bilayer in the cell surface membrane.
• The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane, attracted by the water.
• The hydrophobic tails of both phospholipid layers point into the center the of cell surface membrane, repelled by the water.
• They are said to be amphipathic because they have both hydrophobic and hydrophilic regions.

Question 5

What are the functions of the phospholipids in the membrane?

Answer 5

• Allow lipid-soluble substances to enter and leave the cell.
• Prevent water-soluble substances from entering and leaving the cell.
• Make the membrane flexible and self-sealing.

Question 6

How are proteins embed themselves in the phospholipid bilayer?

Answer 6

• Some occur in the surface of the bilayer and never extend completely across it.
• They give mechanical support to the membrane
• In conjunction with glycolipids, as cell receptors for molecules e.g. hormones.
• Some completely span the phospholipid bilayer from one side to another.
• Some are protein channels - form water-filled tubes to allow water-soluble ions to diffuse across the membrane.
• Some are carrier proteins that bind to ions or molecules e.g. glucose and amino acids, then change shape so they can move these molecules across the membrane.

Question 7

What are the functions of proteins in the membrane?

Answer 7

• Provide structural support.
• Act as channels transporting water-soluble substances across the membrane.
• Allow active transport across the membrane through carrier proteins.
• Form cell-surface receptors to identify cells.
• Stick cells together.
• Act as receptors e.g. for hormones.

Question 8

Where can cholesterols be found?

Answer 8

Within the phospholipid bilayer of the cell-surface membrane.

Question 9

What do the cholesterols do?

Answer 9

• Add strength to the membranes.
• Very hydrophobic so they can prevent loss of water and dissolved ions from the cell.
• They pull together the fatty acid tails of the phospholipid molecules, limiting their movement and other molecules but without making the membrane too rigid.

Question 10

What are the functions of cholesterols in the membrane?

Answer 10

• Reduce lateral movement of modules including phospholipid, so the membrane can obtain its shape and are able to function.
• Make the membrane less fluid at high temperatures.
• At lower temperature, cholesterols interfere with the interactions between fatty acid chains and it prevents membranes from freezing and maintains the fluidity.
• Prevent leakage of water and dissolved ions from the cell.

Question 11

How does plasma membranes vary in different cells?

Answer 11

They all have the same phospholipid bilayer structure but as the substances they contain varies e.g. proteins, it causes them to have a different property.

Question 12

What are glycolipids?

Answer 12

• Made up of a carbohydrate covalently bonded with a lipid.
• The carbohydrate portions extend from the phospholipid bilayer to the watery environment outside of the cell where it acts as a cell-surface receptor for specific chemicals.
  e. g. ABO blood systems operate due to glycolipids on the cell surface membrane.

Question 13

What are the functions of glycolipids in the membrane?

Answer 13

• Acts as a recognition site.
• Help to maintain the stability of the membrane by forming hydrogen bonds with the water molecules both inside and outside the cell surface membrane.
• Help cells to attach to each other so it can form tissues.

Question 14

What are intrinsic and extrinsic proteins?

Answer 14

Some proteins in the membrane are ‘intrinsic’ (channel proteins). This means that they completely span the bilayer. Others are ‘extrinsic’ and are partly embedded in the bilayer.

Question 15

What is a glycoprotein?

Answer 15

They act as cell-surface receptors, more specifically for hormones and neurotransmitters.
Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the membrane.

Question 16

What are the functions of glycoproteins in the membrane?

Answer 16

• Act as recognition sites as they are able to recognise other cell’s specific glycoproteins and glycolipids (very important in helping WBC in recognising and destroying foreign pathogens.)
• Help cells to attach to one another to form tissues.
• Allow cells to recognise on another e.g. lymphocytes can recognise self-cells.

Question 17

Why do most molecules not able to freely diffuse across the cell surface membrane?

Answer 17

• Not soluble in lipids so cannot go past the phospholipid bilayer.
• Too large to go through the channels in the membrane.
• Has the same charge as the protein channels so even if they are small enough to pass through, they would repel due to their charges.
• Some are electrically charged (polar) so it’s hard to go pass the non-polar hydrophobic tails in the phospholipid bilayer.

Question 18

What are the roles of the membranes within cells?

Answer 18

• Control entry and exit of materials in discrete organelles e.g. mitochondria, chloroplast.
• Separate organelles from cytoplasm so specific metabolic reactions can take place within them.
• Provide an internal transport system. e.g. ER
• Isolate enzymes that might damage the cell e.g. lysosomes.
• Provide surfaces for reactions to happen e.g. protein synthesis by using the ribosomes on RER.

Question 19

What are the 3 main factors which influences the membrane’s fluidity?

Answer 19

1) Temperature - The temperature will affect how the phospholipids move and how close together they are found. When it’s cold they are found closer together and when it’s hot they move farther apart.
2) Cholesterols - They are randomly distributed around the membrane, it helps them to stay fluid in different environments. They hold the phospholipid to they do not separate too far and let unwanted substances to go through and if, without cholesterols, the phospholipid will be too close and restrict the movements within the membrane.
3) Saturated and unsaturated fats - Saturated with single bonds will allow the fats to be easily compacted whereas the unsaturated fat will have double bonds and create kinks in the chain so it’s not as easy to pack them together. The kink will create spaces between the phospholipids so it will affect the fluidity.

Question 20

What is a fluid mosaic model?

Answer 20

• It’s a model describing all the various molecules combined into the structure of the cell-surface membrane.
• Fluid because of the individual phospholipid molecules can move from one to another so this gives the membrane a flexible structure which is constantly changing shape.
• Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern.