Cell physiology - Transport across cell membrane

Question 1

Only form of transport that is not carrier-mediated

Answer 1

Diffusion

Question 2

Characteristics of the different types of transport (simple diffusion, facilitated diffusion, primary active transport, cotransport, countertransport)

• Gradient
• Metabolic energy
• Na⁺ gradient

Answer 2

• Gradient
  
  • Simple diffusion
    
    • Downhill
  • Facilitated diffusion
    
    • Downhill
  • Primary active transport
    
    • Uphill
  • Cotransport
    
    • Uphill
  • Countertransport
    
    • Uphill
• Metabolic energy
  
  • Simple diffusion
    
    • No
  • Facilitated diffusion
    
    • No
  • Primary active transport
    
    • Yes
  • Cotransport
    
    • Indirect
  • Countertransport
    
    • Indirect
• Na⁺ gradient
  
  • Simple diffusion
    
    • No
  • Facilitated diffusion
    
    • No
  • Primary active transport
    
    • N/A
  • Cotransport
    
    • Yes, same direction
  • Countertransport
    
    • Yes, opposite direction

Question 3

Diffusion equation

Answer 3

• Memory aid:*
• Say it as, Jay, pa-C1-C2 ka naman!

Question 4

What is the P in the equation for diffusion, its definition and factors that increase it?

Answer 4

• Permeability
  
  • Ease with which a solute diffuses through a membrane
  • Factors that increase permeability
    
    • ↑ Oil/water partition coefficient → ↑ solubility of lipid
    • ↓ Radius (size) of the solute → ↑ the diffusion coefficient + diffusion speed
    • ↓ Membrane thickness → decreases the diffusion distance
• Note: The concentration difference of the solute has no effect on permeability*
• Memory aid:*
• PORT

Question 5

Substances that have the highest permeabilities in lipid membranes

Answer 5

Small hydrophobic solutes (e.g., O2)

Question 6

If the solute is an ion (is charged), then its flux will depend on

Answer 6

Both the concentration difference and the potential difference across the membrane

Question 7

Examples of carrier-mediated transport

Answer 7

• Facilitated diffusion
• Primary and secondary active transport

Note: So, carrier-mediated transport is either passive or active

Question 8

Characteristics of carrier-mediated transport and explanation

Answer 8

1. Stereospecificity
   
   • D-glucose (the natural isomer) is transported by facilitated diffusion, but the L-isomer is not
2. Saturation
   
   • Transport rate increases as the concentration of the solute increases, until the carriers are saturated
3. Competition
   
   • Structurally related solutes compete for transport sites on carrier molecules; for example, galactose is a competitive inhibitor of glucose transport in the small intestine

Question 9

Characteristics of facilitated diffusion

• Passive or active
• Electrochemical gradient
• Speed
• Carrier-mediated or not

Answer 9

• Passive
• Occurs down an electrochemical gradient
• More rapid than simple diffusion
• Carrier-mediated and thus exhibits stereospecificity, saturation, and competition

Question 10

Example of facilitated diffusion

Answer 10

Glucose transport in muscle and adipose cells is “downhill,” is carrier-mediated, and is inhibited by sugars such as galactose

• Memory aid:*
• Sugars are kinda big; so they need a transporter ⇒ carrier-mediated

Question 11

Characteristics of primary active transport

• Passive or active
• Electrochemical gradient
• Carrier-mediated or not

Answer 11

• Active
• Against an electrochemical gradient (“uphill”)
• Carrier-mediated and thus exhibits stereospecificity, saturation, and competition

Question 12

Examples of primary active transport

Answer 12

1. Na⁺-K⁺-ATPase (or Na⁺–K⁺ pump)
2. Ca²⁺-ATPase (or Ca²⁺ pump)
3. H⁺-K⁺-ATPase (or proton pump)

• Memory aid:*
• SPC actively pumps – _s_odium potassium, _p_roton, _c_alcium pumps

Question 13

Na⁺-K⁺ pump location, function and inhibition

Answer 13

• Location
  
  • Cell membrane
• Function
  
  • Transports Na⁺ from ICF to ECF and K⁺ from ECF to IC
  • Maintains low intracellular [Na⁺] and high intracellular [K⁺]
• Inhibition
  
  • Cardiac glycoside drugs ouabain and digitalis

Question 14

Proton pump location, function and inhibition

Answer 14

• Location
  
  • Gastric parietal cells
• Function
  
  • Transports H+ into the lumen of the stomach against its electrochemical gradient
• Inhibition
  
  • PPI

Question 15

Ca²⁺ pump location and function

Answer 15

• Location
  
  • Sarcoplasmic reticulum (SR) or cell membrane
• Function
  
  • Transports Ca²⁺ against an electrochemical gradient

Question 16

Why is Na+-K⁺ pump not considered a secondary active transport

Answer 16

Because both the Na and K are transported against their gradient

Question 17

SERCA stands for

Answer 17

Sarcoplasmic and endoplasmic reticulum Ca²⁺-ATPase

Question 18

Characteristics of secondary active transport

Answer 18

• The transport of two or more solutes is coupled
• One of the solutes (usually Na+) is transported “downhill” and provides energy for the “uphill” transport of the other solute(s)

Note: Therefore, they use ATP indirectly → will fail if Na⁺/K⁺ pump fails

Question 19

Relationship of secondary active transport and Na/K pump

Answer 19

• Metabolic energy is not provided directly, but indirectly from the Na⁺ gradient that is maintained across cell membrane
• Thus, inhibition of Na⁺-K⁺-ATPase will decrease transport of Na+ out of the cell, decrease the transmembrane Na⁺ gradient, and eventually inhibit secondary active transport

Question 20

Example of symport and where found

Answer 20

• Na⁺-glucose cotransport in the small intestine
• Na⁺-K⁺-2Cl^– cotransport in the renal thick ascending limb
• Memory aid:*
• Naki-carpool [symport] si sweet [glucose] KC kay sodium!

Question 21

Examples of countertransport, exhange or antiport

Answer 21

Examples

• Na^₊-Ca²⁺ exchange
• Na⁺-H⁺ exchange
• Memory aid:*
• ex_CH_ange

Question 22

Na⁺-glucose cotransport

• Location
• Direction of Na⁺ and glucose
• Source of energy

Answer 22

• Location
  
  • Luminal membrane of intestinal mucosal and renal proximal tubule cells
• Direction of transport
  
  • Glucose is transported “uphill”
  • Na⁺ is transported “downhill”
• Where is the energy derived?
  
  • From the “downhill” movement of Na⁺

Note: The inwardly directed Na⁺ gradient is maintained by the Na⁺–K⁺ pump on the basolateral (blood side) membrane

Question 23

Na⁺-Ca²⁺ exchange function

Answer 23

Transports Ca²⁺ “uphill” from low intracellular Ca²⁺ to high extracellular Ca²⁺

• Note: Ca²⁺ and Na⁺ move in opposite directions across the cell membrane*
• Memory aid:*
• Normally, there is low iCA - low intracellular Ca²⁺

Question 24

Effect if Na-K pump is inhibited

Answer 24

Increased intracellular Na+ concentration decreases the Na⁺ gradient across the cell membrane → inhibiting Na⁺–Ca²⁺ exchange → increase in intracellular Ca²⁺ concentration