Plasma Membrane

Question 1

What is the function of the cell membrane?

Answer 1

• let substances in and out of the cell
• compartmentalise cell contents and organelles
• communication

Question 2

What is meant by partially permeable?

Answer 2

• only allows some molecules through

Question 3

Why is the membrane described as a fluid mosaic model?

Answer 3

• fluid: individual phospholipid and protein molecules move around within their layer
• mosaic: pattern produced by scattered protein molecules which vary in shape, size and position when surface membrane is viewed from above
• model: agreed structure is based on chemical and experimental evidence so classed as a model

Question 4

Where are phospholipid bi-layers found?

Answer 4

• cell surface/plasma membrane
• organelles
• vesicles

Question 5

What is compartmentalism?

Answer 5

• formation of separate membrane-bound areas in a cell

Question 6

Why is compartmentalism vital to a cell?

Answer 6

• contains reactions in separate parts of the cell
• allows specific conditions required for said reactions, such as chemical gradients to be maintained
• protects vital cell components

Question 7

What is the function of a phospholipid?

Answer 7

• forms partially permeable bilayer due to hydrophobic fatty acid tails and hydrophilic heads
• form membrane bound compartments
• acts as a barrier
• allows movement of small, soluble, lipids only
• regulates fluidity

Question 8

How are membranes formed?

Answer 8

• from a phospholipid bilayer
• hydrophilic phosphate heads of the phospholipid form both the inner and outer surface of a membrane
• they sandwich the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane

Question 9

What is cholesterol’s function?

Answer 9

• helps regulate fluidity of membrane
• prevents phospholipids forming too closely and becoming solid
• adds stability to membranes without them becoming too rigid

Question 10

What is the structure of cholesterol?

Answer 10

• lipid with hydrophobic and hydrophilic end
• hydrophilic ends interact with hydrophilic phosphate heads
• hydrophobic end interacts with hydrophobic fatty acid tails
• positioned between phospholipids

Question 11

What are glycolipids?

Answer 11

• lipid attached to carbohydrate (sugar) chain of varying length

Question 12

What are the roles of glycolipids?

Answer 12

• cell markers/antigens on cell surfaces: white blood cells recognise the shape of glycolipid as “self” so don’t start an immune response

Question 13

What are glycoproteins?

Answer 13

• protein attached to carbohydrate (sugar) chain of varying length
• intrinsic protein
• embedded in cell surface membrane

Question 14

What are the roles of glycoproteins?

Answer 14

• cell adhesion
• receptors involved in chemical signalling:
  e. g. neurotransmitters binding to receptors on the postsynaptic neuron triggering a response
  e. g. hormone receptors such as those for insulin and glucagon which are present on the cells and liver
• some drugs act by binding to a receptor to block or even trigger its activity

Question 15

What are intrinsic proteins?

Answer 15

• embedded within the membrane
• are transmembrane proteins
• stay in place due to hydrophobic R-groups on their external surfaces which interact with hydrophobic core of bilayer

Question 16

What are channel proteins?

Answer 16

• intrinsic/integral proteins

- have hydrophilic channel allowing passive moment of molecules that are not small/and or lipid soluble

Question 17

What are carrier proteins?

Answer 17

• intrinsic/integral proteins
• allows molecules that aren’t small and/or lipid soluble to move
• can either be towards or against concentration gradient (active/passive)
• often change shape during transport

Question 18

What are extrinsic proteins?

Answer 18

• present on one side of membrane but can move between layers
• stay in place due to hydrophilic R-groups on the external surfaces which interact with the polar heads of the phospholipids
• site of chemical reactions e.g. enzymes
• can also interact with intrinsic proteins to act as receptors

Question 19

What’s the difference between intrinsic end extrinsic proteins?

Answer 19

Intrinsic:
- embedded in both sides of bilayer 
- e.g. channel or carrier protein 
Extrinsic:
- embedded in one side of bilayer
- e.g. enzyme

Question 20

Alcohol, caffeine, and nicotine are all lipid-soluble molecules, having an almost instant and widespread effect on the body. Why?

Answer 20

• lipid soluble molecules can pass through membrane
• by simple diffusion
• so diffuse quickly through whole body

Question 21

Membranes, particularly those present in mitochondria, are often highly folded. What are the advantages of the folding?

Answer 21

• process occur across membranes
• process is enzyme controlled
• folding gives increased surface area
• so more enzymes increased rate of reaction
• therefore increase rate of ATP production