Plasma Membrane Structure and Function

Question 1

Why must the interior of the cell be physically separated from the surrounding environment?

Answer 1

To maintain the differences between intracellular and extracellular environment and to keep undesirable substances out (impermeable barrier)
To control the influx and efflux of molecules (transport)
To allow communication between cells (signalling)

Question 2

Name the function of the plasma membrane?

Answer 2

Barrier
Transport of ions, nutrients and other small molecules
Cell-cell interactions, cell signalling (receptors)
Sites of metabolic activities
Cell shape

Question 3

What evidence showed the existence of the plasma membrane?

Answer 3

Until Electron Microscopy was utilised (1950s) PM not seen (PM is too thin to be visualised by light microscope)

1) Cell swelling [hypotonic] & shrink [hypertonic]
2) Escape of cell contents if mechanically ruptured
3) Compartmentalization needed for biochemical activity of cells

Question 4

Expand on the timeline of the development of the plasma membrane?

Answer 4

• Charles Overton in the 1890s: understanding the lipid nature of membrane
• Irving Langmuir in 1900s: lipid monolayer
• Gorter and Grendel in 1920s: lipid bilayer
• Davson and Danielli in 1940s: lipid bilayer plus protein sheets -Robertson in 1960s: unit membrane (electron microscope)
• Singer and Nicolson in 1970-80s: Fluid Mosaic Model

Question 5

How much mass do lipids constitute in the plasma membrane?

Answer 5

50%

Question 6

Define the 2 properties of plasma membrane?

Answer 6

Asymmetrically distributed

Amphiphilic

Question 7

How much mass do membrane lipids constitute?

Answer 7

30-80%

Question 8

What components are included in membrane lipids?

Answer 8

Phospholipids
Glycolipids
Sterols

Question 9

Define phospholipids?

Answer 9

Two hydrocarbon (lipid) tails-   Tails differ in length between 14 -24 carbon atoms long- 
If there are cis-double bonds in one tail, this creates a kink and the phospholipid fills more space.
Lipid molecules spontaneously aggregate to keep their hydrophobic tails in the interior and expose their hydrophilic heads to water

Question 10

What are glycerol based phospholipids called?

Answer 10

phosphoglyceride

Question 11

Define the structure of a phospholipids?

Answer 11

Polar head (usually: choline, ethanolamine, serine, inositol)
Glycerol or sphingosine
Lipid tail(s)

Question 12

How much glycolipids make up membrane lipids?

Answer 12

5-10%

Question 13

Define the structure of glycolipids?

Answer 13

Carbohydrate head group
Lipid backbone
Glycerol or sphinogosine based

Question 14

How does sterols affect plasma membranes?

Answer 14

increase permeability barrier properties of PM
Maintains stability & integrity of PM
Affects PM fluidity

Question 15

Explain how the fluidity of the plasma membrane is affected?

Answer 15

Temperature
Cholesterol
Cis and Trans

Question 16

What are the three ways that phospholipids move within the plasma membrane?

Answer 16

Rotation about its long axis
Lateral diffusion by exchanging places with neighbouring molecules in the same monolayer
Transverse diffusion, or “flip-flop” from one monolayer to the other (rare)

Question 17

How much membrane proteins make up the plasma membrane?

Answer 17

20-60%

Question 18

What are the 3 types of proteins embedded in the plasma membrane?

Answer 18

Integral
Peripheral
Lipid anchored

Question 19

Define integral proteins?

Answer 19

Embedded within bilayer

Hydrophobic segments have affinity for hydrophobic interior of bilayer

Question 20

Define peripheral proteins?

Answer 20

Located on surface of PM

Not an intrinsic part of the membrane, attached through ionic interactions

Question 21

Define Lipid anchored proteins?

Answer 21

Proteins have a modification, are attached to a glycolipid

Lipid part of glycolipid anchor is embedded within the hydrophobic region of the membrane

Question 22

What are lipid rafts?

Answer 22

Clusters/associations of lipids and proteins within membrane
Cholesterol rich, saturated fatty acid tails.
Associated with signalling units.
May be transient.