Membranes

Question 1

Cell membranes are composed of phospholipids.

What are these phospholipids composed of?

Answer 1

• Glycerol backbone
• 2 OH groups esterified to backbone
• Fatty acid groups

Question 2

For glycerol based phospholipids, what is special about the third glycerolic hydroxyl group?

Answer 2

Esterified to a phosphate (which is esterified to head group)

Question 3

Describe the polarity and solubility of fatty acid tails.

Answer 3

• Nonpolar due to lack of charged groups
• Dissolve poorly in polar solvents but well in organic solvents
• Hydrophobic

Question 4

Describe the solubility of the head groups of phospholipids.

Answer 4

• Charged and water soluble
• Hydrophilic

Question 5

How do the phospholipids arrange themselves when mixed in water?

Answer 5

• Hydrophobic fatty acids don’t contact with water
• Hydrophilic heads dissolve in water
• At low concentrations, form monolayers. At high concentrations, form micelles.

Question 6

Describe the phospholipid bilayer.

Answer 6

• Hydrophilic head groups orient towards water
• Hydrophobic tails orient away from water
• Tail length determines bilayer width. Head groups determine how tightly packed molecules are

Question 7

What does it mean for the structure of the bilayer to be ‘fluid’?

Answer 7

Phospholipids move within the layer in which it resides

Question 8

How does temperature influence the movement of phospholipids within the bilayer?

Answer 8

• At high temperature - sol state. Rapid lateral diffusion
• At low temperature - gel state. Slow diffusion of phospholipids
• Temperature at which bilayer converts from gel state to sol state is the transition temperature

Question 9

How do the characteristics of the phospholipid influence movement within the bilayer?

Answer 9

• Saturated fatty acids are closely packed together so require greater thermal energy to permit diffusion.
• Shorter chains and double bonds aren’t as packed so less energy required.

Question 10

How does changing the type of phospholipid influence fluidity of the membrane?

Answer 10

• Fatty acid tails are different lengths and organisations
• Interact with one another to influence moevment and rigidity

Question 11

What are the 2 major groups of phospholipids?

Answer 11

Glycerol based
Sphingolipids

Question 12

What is the effect of having cholesterol at low concentrations?

Answer 12

• Steroid rings bind to and partial immobilisation of fatty acid tails.
• Membrane rigidity increases
• Greater gel sol transition temperature

Question 13

What is the effect of having cholesterol at high concentrations?

Answer 13

• Disrupts interactions between phospholipids
• Greater fluidity
• Decreased gel-sol transition temperature

Question 14

Describe the zonation within the bilayers.

Answer 14

• Composed of several lipid types spread over a wide temperature range
• Long chain saturated fatty acids - usually in gel state
• Short chain unsaturated fatty acids - sol state

Question 15

What are the physiological functions of membranes?

Answer 15

• Separates intracellular fluid/cytosol from extracellular fluid
• Regulation of exchange with environment e.g uptake and removal
• Detects chemical messengers arriving at cell surface
• Anchors cells to cellular matrix

Question 16

Describe integral proteins.

Answer 16

• Embedded into the membrane (as opposed to peripheral proteins found on the surface)
• Usually consists of alpha helical domains
• Some have multiple transmembrane domains

Question 17

Describe the different types of proteins found in membranes.

Answer 17

• STRUCTURAL PROTEINS - connect membrane to cytoskeleton, form gap junctions
• ENZYMES
• RECEPTORS - for cell signalling
• TRANSPORTER PROTEINS - Channel and carrier proteins