1.3 Membrane Structure

Question 1

Some substances are attracted to water –

Answer 1

hydrophilic (or polar)

Question 2

Some molecules are not attracted to water –

Answer 2

hydrophobic (non-polar)

Question 3

Phospholipids are amphipathic molecules:

Answer 3

Polar head (hydrophilic) containing glycerol and a phosphate molecule – a.k.a. phosphate head

Two non-polar tails (hydrophobic) made of fatty acid chains (hydrocarbons) – a.k.a. hydrocarbon tail

Question 4

When put into water, an emergent property of phospholipids is that

Answer 4

they will self-organize to keep their heads “wet” and tails “dry”

Question 5

Phospholipids spontaneously arrange into a

Answer 5

bilayer when in water

Question 6

Phospholipid molecules can flow past each other laterally but

Answer 6

cannot move vertically

This fluidity allows for spontaneous breaking and reforming of membranes
Cell division
Releasing vesicles (exocytosis)
Taking in vesicles (endocytosis)

Question 7

Gorter and Grendel (1920’s)

Answer 7

The Model: plasma membrane are composed of a lipid bilayer

Evidence: analyzed red blood cells
Extracted phospholipids from the plasma membrane

Calculated that there were twice as many phospholipids than would be needed for monolayer —> must be a bilayer
Many errors to methods, but luckily cancelled out to still get “true” results.
Model did not account for proteins

Question 8

Davson-Danielli (1930’s)

Answer 8

The Model: “sandwich” model in which the lipid bilayer is “sandwiched” by protein molecules

Lipid bilayer composed of phospholipids whose outer surface are coated by proteins

Proteins do not permeate the lipid bilayer

Evidence: electron micrographs showed “railroad track” appearance
Proteins appear dark and phospholipids appear light

This explains: despite being very thin membranes are an effective barrier to the movement of certain substances.

Question 9

Assumptions of the Davson-Danielli model

Answer 9

All membranes were the same thickness and would have a constant lipid-protein ratio

All membranes would have symmetrical internal and external surfaces

Question 10

Freeze-etched electron micrographs

Answer 10

Rapid freezing of cells and fracturing them

Evidence:
Fracture reveals an irregular rough surface inside the phospholipid bilayer
Able to view “globular structures” interpreted as transmembrane proteins

Conclusion
Refutes the point that they are only found on the outside of the membrane

Question 11

Structure of membrane proteins

Answer 11

Improvement in biochemical techniques allowed proteins to be extracted from membranes

Evidence
Proteins extracted found to be different in size and shape
Conclusion
Refutes uniform/continuous layer hypothesis

Evidence
Proteins found to all have hydrophobic section –> would embed in “tails”
Conclusion
Refutes proteins found only on outer layer

Question 12

Fluorescent antibody tagging

Answer 12

Red or green fluorescent markers attached to antibodies which would bind to membrane proteins
The membrane proteins of some cells were tagged with red markers and other cells with green markers.

Evidence
The cells were fused together.
Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.

Conclusion
Membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer.

Question 13

Singer-Nicolson fluid mosaic model

Answer 13

Our current model of the cell membrane

This model was first proposed in by Singer-Nicolson in 1972

Question 14

Integral Proteins

Answer 14

Exposed to aqueous environments on both sides, used to transport molecules across the membrane
Partially hydrophobic and embed in the centre of the membrane
Monotopic or polytopic (transmembrane)

Question 15

Peripheral Proteins

Answer 15

Hydrophilic – located on surface
Some attached to integral protein
Some attached to hydrocarbon chain that anchors to membrane
Monotopic

Question 16

Glycoproteins

Answer 16

proteins with an oligosaccharide (oligo = few, saccharide = sugar) chain attached

They are important for cell recognition by the immune system and as hormone receptors

Question 17

Protein content of a membrane varies depending on the function of the cell

Answer 17

Myelin sheath around nerve cells insulate —> 18% protein content

Most plasma membranes —> 50%
Mitochondria (cell respiration) and chloroplasts (photosynthesis) —> 75%

Question 18

Functions of membrane proteins

Answer 18

Hormone binding sites (receptors)

Enzymatic activities (e.g. ETC)

Cell adhesion

Cell-to-cell communication/ recognition

Channels for passive transport

Pumps for active transport

Question 19

Animal cells also contain

Answer 19

cholesterol (type of lipid)

Question 20

Hydroxyl group (-OH) makes

Answer 20

the head polar and hydrophilic - attracted to the phosphate heads on the outer layer of the membrane.

Question 21

Carbon rings

Answer 21

– it’s not classed as a fat or an oil, cholesterol is a steroid

Question 22

Non-polar (hydrophobic) tail

Answer 22

– attracted to the hydrophobic tails of phospholipids in the centre of the membrane

Question 23

cholesterol amount varies in animal cells

Answer 23

Membranes of vesicles that hold neurotransmitters at synapses (space between neurons) 🡪 30% content

Question 24

Overall the membrane is…

Answer 24

fluid —> components can move freely

Question 25

It is important to regulate the degree of fluidity:

Answer 25

Fluid enough that the cell can move

Fluid enough that the required substances can move across the membrane

If too fluid however the membrane could not effectively restrict the movement of substances across itself

Question 26

Cholesterol’s role in membrane fluidity

Answer 26

Reduces membrane fluidity by restricting motion of phospholipids molecules

Disrupts tight packing of hydrophobic tails in the bilayer —> increase flexibility by preventing the tails from crystallising and hence behaving like a solid.

Reduces permeability to some solutes (hydrophilic)

Helps membranes curve into a concave shape —> formation of vesicles during endocytosis