Cell Membrane structure 1.3

Question 1

What is the structure of a phospholipid?

Answer 1

• Consists of a hydrophobic, polar head, composed of glycerol + phosphate molecule
• Consists of two non-polar, hydrophilic tails composed of fatty acid (hydrocarbon chains)
• Is amphipathic (hydrophobic + hyrophilic)

Question 2

How are phospholipids arranged in membranes?

Answer 2

• Spontaneously arrange into bilayer
• Hydrophobic tails face inwards (away from surrounding polar fluids), while hydrophilic heads face outwards (associate with cystolic + extracellular fluids respectively)

Question 3

What are properties of the phospholipid bilayer?

Answer 3

• Bilayer = held together by weak hydrophobic interactions between the tails
• Hydrophobic + hydrophilic layers restrict passage of many substances
• Individual phospholipids can move within the bilayer -> allowing for membrane fluidity + flexibility
• This fluidity allows for the spontaneous breaking + reforming of membranes (endocytosis/exocytosis)

Question 4

What are the two main types of proteins in the phospholipid bilayer?

Answer 4

• Integral proteins = Permanently attached to the membrane + are typically transmembrane (span across the bilayer)
• Peripheral proteins = Temporarily attached by non-covalent interactions + associate with one surface of the membrane

Question 5

Structure of membrane proteins?

Question 6

What are the functions of membrane proteins?

Answer 6

• Junctions = connect + join two cells together
• Enzymes = fixing to membranes localises metabolic pathways
• Transport = responsible for facilitated diffusion + active transport
• Recognition = may function as marker for cellular identification
• Anchorage = attachment points for cytoskeleton + extracellular matrix
• Transduction = function as receptors for peptide hormones
  (JET RAT)

Question 7

Why is cholesterol absent in the membranes of plant cells?

Answer 7

Because these plasma membranes are surrounded + supported by a rigid cell wall made of cellulose
- Whereas are necessary in animal cells to maintain integrity + functional stability

Question 8

Why is a cholesterol molecule amphipathic like a phospholipid?

Answer 8

• Cholesterol’s hydroxyl (-OH) group is hydrophilic + aligns towards the phosphate heads of phospholipids
• The remainder of the molecule (steroid ring + hydrocarbon tail) is hydrophobic + associates with phospholipid tails

Question 9

What does it mean when saying that a phospholipid bilayer is fluid?

Answer 9

Meaning that the phospholipids are in constant movement relative to one another

Question 10

How does cholesterol interact with the fatty acid tails of phospholipids to moderate the properties of the membrane?

Answer 10

• Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity
• It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
• It functions to separate phospholipid tails and so prevent crystallisation of the membrane
• It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Question 11

What are components of the plasma membrane?

Answer 11

• Phospholipids - form a bilayer with phosphate heads facing outwards + fatty acid tails facing inwards
• Cholesterol - Found in animal cell membranes + functions to improve stability + reduce fluidity
• Proteins - May be either integral (transmembrane) or peripheral and serve a variety of roles

Question 12

Was the fluid-mosaic model the first scientifically accepted paradigm to describe membrane structure?

Answer 12

No

Question 13

Who proposed the first accepted model that attempted to describe the positions of proteins within a bilayer, and when?

Answer 13

Hugh Davson and James Danielli in 1935

Question 14

What kind of characteristic appearance do membranes exhibit when viewed under a transmission electron microscope?

Answer 14

A trilaminar appearance

Question 15

What does a ‘trilaminar’ appearance mean?

Answer 15

It means that there are 3 layers (2 dark outer layers + a lighter inner region)

Question 16

What did Davson and Danielli propose?

Answer 16

A model where two layers of protein flanked a central phospholipid bilayer
- Model was segmented as a ‘lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers
- The dark segments seen under the electron microscope were identified (wrongly) as representing two protein layers

Question 17

What were the problems with the lipo-protein sandwich model proposed by Davson and Danielli?

Answer 17

• It assumed that all membranes were of a uniform thickness + would have a constant lipid-protein ratio
• It assumed all membranes would have symmetrical internal + external surfaces (i.e. not bifacial)
• It did not account for permeability of certain substances (did not recognise the need for hydrophilic pores)
• The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Question 18

How did the discovery of membrane proteins being insoluble in water falsify the lipo-protein sandwich model?

Answer 18

Membrane proteins being discovered to be insoluble in water (indicating hydrophobic surfaces) + the variations in size
- As such proteins would not be able to form a uniform + continuous layer around the outer surface of a membrane

Question 19

How did fluorescent antibody tagging of membrane proteins falsify the lipo-protein sandwich model?

Answer 19

Showed that they were mobile + not fixed in place
- Membrane proteins from 2 different cells were tagged with red and green fluorescent markers respectively
- When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
- This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)

Question 20

How did freeze fracturing falsify the Davson-Danielli lipo-protein sandwich model?

Answer 20

Freeze fracturing was used to split open the membrane + revealed irregular rough surfaces within the membrane
- These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure

Question 21

When was the new membrane model proposed and by whom?

Answer 21

A new model was proposed by Seymour Singer and Garth Nicolson in 1972

Question 22

What did Singer and Nicolson’s model suggest?

Answer 22

According to this model, proteins were embedded within the lipid bilayer, rather than existing as separate layers

Question 23

What is the name of Singer and Nicolson’s newer model?

Answer 23

It is known as the fluid-mosaic model, and remains the model preferred by scientists today (with refinements)