Permeability & Transport

Question 1

What is the primary function of the plasma membrane ?

Answer 1

• Fat containing barrier that separates the internal chemical components of the cell from the external environment
• No membranes = no cells = no life!
• Import of nutrients/molecules, export of waste/molecules
• Relay of cell signals through ‘sensor’ proteins
• Permits motility through flexible size and shape

Question 2

What is the common structure of the cell membrane ?

Answer 2

• Two closely opposed sheets – lipid bilayer

- Interspaced with membrane spanning proteins with varying functions

Question 3

Membrane lipids are ?

Answer 3

Amphipathic

Question 4

What is the most abundant lipids in membrane ?

Answer 4

Phospholipids

Question 5

Structure of phosphatidylcholine ?

Answer 5

Choline joined to glycerol via phosphate with two hydrocarbon chains

Question 6

what does a double bond between two carbon atoms produce ?

Answer 6

‘Kink’ in hydrocarbon chain – important for membrane fluidity

Question 7

Hydrophilic molecules contain charged atoms or polar groups with uneven distribution of positive and negative charges. Charged atoms can form ?

Answer 7

Favourable electrostatic or hydrogen bond interactions with water

Question 8

Hydrophobic molecules contain uncharged atoms or non polar groups. Therefore, cannot ?

Answer 8

Form favourable interactions with water

Question 9

Lipid bilayer is a 2D fluid. Explain the movements ?

Answer 9

• Molecules move rapidly within a layer – thermal motions
• Exchange of phospholipids between layers ‘flip-flop’ is rare
• Crucial for membrane function and integrity

Question 10

How do we know that there is membrane fluidity?

Answer 10

FRAP = Fluorescence recovery after photobleaching

Question 11

What is a membrane fluidity factor ?

Answer 11

All about how close and regular are the packing of the hydrocarbon tails:
• Regular and close = less fluid
• Irregular and loose = more fluid

Question 12

What is an another membrane fluidity factor?

Answer 12

The tail length

• Long tail length = less fluid
• Short tail length = more fluid

Question 13

What is an another membrane fluidity factor?

Answer 13

Double bond number

• Less double bonds = less fluid
• More double bonds = more fluid

Question 14

What is an another membrane fluidity factor?

Answer 14

Cholesterol content

• Fill spaces left by kinks in unsaturated hydrocarbon tails
• Increases membrane rigidity and reduces permeability
• Also roles in cell signalling (lipid rafts)

Question 15

Explain how the membrane assembly begins in ER ?

Answer 15

• ER synthesises new phospholipids from free floating fatty acids
• New phospholipids deposited exclusively in cytosolic layer
• Phospholipids transferred to opposite layer through scramblases
• Some new membrane retained in ER, rest transferred to other membranes

Question 16

How is asymmetry maintained in the bilayer?

Answer 16

• Golgi membrane contains enzymes called flippases
• Flippase enzymes maintain bilayer asymmetry
• Catalyses the transfer of specific phospholipids from exterior to interior layer

Question 17

Protein directly attached to lipid bilayer – integral membrane proteins – can only be removed by?

Answer 17

Disrupting with detergents (e.g. Triton X-100, SDS)

Question 18

Remaining proteins known as peripheral membrane proteins – removed with ?

Answer 18

More gentle extraction procedures

Question 19

Explain the Membrane reinforcement – Cell cortex?

Answer 19

• Membranes thin and fragile – require support
• Support framework provided by meshwork of fibrous proteins
• Attached to cytosolic surface of membrane – affects shape, strength and motility

Question 20

What is a glycoprotein ?

Answer 20

Protein with bound oligosaccharides

Question 21

What is a proteoglycans?

Answer 21

Other proteins with one or more polysaccharide chains

Question 22

All sugar containing molecules on outside of cells form carbohydrate layer. Why is this crucial ?

Answer 22

Crucial for lubrication (motility, protection) and cell recognition

Question 23

How would you avoid cell damage by electrical damage ?

Answer 23

Electrical charge must be balanced on both inside and outside of cell

Question 24

Briefly explain a membrane transporter proteins ?

Answer 24

• Undergoes conformational changes
• Molecules/ions bind sites
• Highly selective

Question 25

Briefly explain a membrane ion proteins ?

Answer 25

• Discriminates solutes on size and charge
• Forms pore across bilayer in open or closed confirmation
• Opening controlled by external stimulus or cell conditions

Question 26

Explain the different transporter mechanisms ?

Answer 26

• Direction of transport often depends on relative concentrations of solute
• Simple diffusion - molecules naturally flow down a concentration gradient
• Passive transport (facilitated diffusion) – molecules move down a concentration gradient facilitated by channels and transporters
• Active transport – molecules move against a concentration gradient in energy- (ATP) dependent process using transporters (pumps)

Question 27

Simple diffusion – Oxygen/Carbon dioxide diffusion

Answer 27

• Simple movement of O2 or CO2 to areas of lower partial pressure (reciprocal directions)
• Particularly important on alveoli membranes where gaseous exchange takes place

Question 28

Passive transport – Glucose

Answer 28

• Transporter crosses membrane 12 times
• Changes in conformational state facilitate glucose binding, transport and release
• Highly specific for glucose isoform (D-glucose, not L-glucose)
• Important in liver cells where glycolysis takes place

Question 29

Active transport – Na+-K+ pumps

Answer 29

• To maintain osmotic balance and correct electrochemical gradient Na+ and K+ ions need to be pumped against a massive concentration gradient
• Outward transport of Na+ coupled with inward transport of K+ requiring conformational change
• ATP hydrolysed to ADP + phosphate – pump serves as ATPase