Lesson 36 - Cell membrane structure

Question 1

Describe the structure of a phospholipid

Answer 1

Phospholipids have a hydrophilic head and a hydrophobic tail. The head is made up of a phosphate group (which is polar). This is joined to two fatty acid “tails” by a glycerol molecule. The fatty acid tails are non-polar.

Question 2

Define hydrophobic and hydrophilic

Answer 2

Hydrophilic – can interact with water /is attracted to water (because it is polar/charged)
Hydrophobic – cannot interact with water/not attracted (because it has no charge/non-polar)

Question 3

What are plasma membranes primarily made of?

Answer 3

Phospholipids

Question 4

What is the name given to the plasma membrane that surrounds a cell?

Answer 4

Cell-surface membrane

Question 5

What are the roles of membranes?

Answer 5

• Separating cell contents from the outside world.
• Separating cell components from cytoplasm.
• Cell recognition and signalling.
• Holding components for some metabolic pathways.
• Regulating transport in/out.

Question 6

Why are phospholipids important components in cell surface membranes?

Answer 6

• They can form bilayers (heads pointing towards cytoplasm (water based) on one layer and heads pointing to tissue fluid (water based) outside cell on the other)
• The phospholipid component of a membrane allows lipid-soluble molecules across, but not water soluble molecules
• They make the membrane flexible and self-sealing

Question 7

Other than phospholipids, what else is embedded in the cell membrane?

Answer 7

Proteins

Question 8

What are the 2 ways proteins can be embedded in the cell membrane?

Answer 8

• Extrinsic proteins
• Intrinsic proteins

Question 9

What are extrinsic proteins?

Answer 9

They are on either surface of the bilayer (provide mechanical support and in conjunction with glycolipids they act as cell receptors for molecules such as hormones)

Question 10

What are intrinsic proteins?

Answer 10

They extend across both layers of the phospholipid bilayer. Some are channel proteins (water filled tubes allow water-soluble ions to diffuse across membrane). Some are carrier proteins (bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across)

Question 11

What are the 2 types of intrinsic proteins?

Answer 11

Channel proteins and carrier proteins

Question 12

How do channel proteins work?

Answer 12

Channel proteins form pores that, when open, allow specific solutes (e.g. ions such as Na+ or Cl-) to pass through the membrane.

Question 13

How do carrier proteins work?

Answer 13

They bind specific solute molecules and undergo a conformational change to transfer the bound molecule from one side of the membrane to the other.

Question 14

Where is cholesterol found in the cell membrane?

Answer 14

In between the phospholipids

Question 15

What is the function of cholesterol in the cell membrane?

Answer 15

• make the membrane stronger and more rigid
• Very hydrophobic – important role in preventing loss of water and dissolved ions from cell

Question 16

Other than phospholipids, proteins and cholesterol, what other molecules are found in the membrane?

Answer 16

Glycoproteins
Glycolipids

Question 17

What are glycoproteins?

Answer 17

A protein which has a carbohydrate group attached
- Act as cell surface receptors (e.g. for hormones and neurotransmitters).
- Also help cells attach to one another to form tissues).
- Allow cells to recognise each other (e.g. lymphocytes can recognise an organism’s own cells)

Question 18

What are glycoloipids?

Answer 18

A lipid which has a carbohydrate group attached
- Carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell
- acts as a cell-surface receptor for specific chemicals (e.g. basis of ABO blood system)
- also help maintain stability of membrane and help cells attach to one another to form tissues)

Question 19

What are the main functions of plasma membranes?

Answer 19

• Forms a boundary between 2 environments (compartmentalisation) allows different conditions on either side of the membrane (eg. pH, enzymes etc) allowing different reactions to occur in different places
• A selectively permeable barrier - controls the movement of substances in and out (entry & exit)

Question 20

Describe how phospholipids are arranged in the plasma membrane

Answer 20

Bilayer (1)
Hydrophobic tails to the inside (1)
Hydrophilic tails to the outside (1)

Question 21

What are the functions of glycolipids and glycoproteins?

Answer 21

• Provide stability
• Act as cell-surface receptors for binding of molecules or recognition sites (cell-recognition)
• Also help cells to attach to each other (to form tissues)

Question 22

What can and can’t cross the phospholipid bilayer in cell membranes?

Answer 22

• It allows lipid-soluble (non-polar) substances to pass through (& small molecules eg. H2O)
• It prevents most water-soluble (polar) substances passing through

Question 23

What can affect the fluidity of plasma membranes?

Answer 23

• The more unsaturated phospholipids present (the more C=Cs) the less tightly they can pack together and the more fluid
• Less cholesterol, more fluidity
• As temperatures increase, the fluidity increases due to more kinetic energy

Question 24

Explain why the structure of a membrane is described as fluid-mosaic?

Answer 24

idea of molecules / named molecules moving = Fluid;
idea of both proteins and phospholipids = Mosaic;