Cell physiology - Transport across cell membrane Flashcards
Only form of transport that is not carrier-mediated
Diffusion
Characteristics of the different types of transport (simple diffusion, facilitated diffusion, primary active transport, cotransport, countertransport)
- Gradient
- Metabolic energy
- Na+ gradient
- Gradient
- Simple diffusion
- Downhill
- Facilitated diffusion
- Downhill
- Primary active transport
- Uphill
- Cotransport
- Uphill
- Countertransport
- Uphill
- Simple diffusion
- Metabolic energy
- Simple diffusion
- No
- Facilitated diffusion
- No
- Primary active transport
- Yes
- Cotransport
- Indirect
- Countertransport
- Indirect
- Simple diffusion
- Na+ gradient
- Simple diffusion
- No
- Facilitated diffusion
- No
- Primary active transport
- N/A
- Cotransport
- Yes, same direction
- Countertransport
- Yes, opposite direction
- Simple diffusion
Diffusion equation
- Memory aid:*
- Say it as, Jay, pa-C1-C2 ka naman!
What is the P in the equation for diffusion, its definition and factors that increase it?
- 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
Substances that have the highest permeabilities in lipid membranes
Small hydrophobic solutes (e.g., O2)
If the solute is an ion (is charged), then its flux will depend on
Both the concentration difference and the potential difference across the membrane
Examples of carrier-mediated transport
- Facilitated diffusion
- Primary and secondary active transport
Note: So, carrier-mediated transport is either passive or active
Characteristics of carrier-mediated transport and explanation
- Stereospecificity
- D-glucose (the natural isomer) is transported by facilitated diffusion, but the L-isomer is not
- Saturation
- Transport rate increases as the concentration of the solute increases, until the carriers are saturated
- 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
Characteristics of facilitated diffusion
- Passive or active
- Electrochemical gradient
- Speed
- Carrier-mediated or not
- Passive
- Occurs down an electrochemical gradient
- More rapid than simple diffusion
- Carrier-mediated and thus exhibits stereospecificity, saturation, and competition
Example of facilitated diffusion
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
Characteristics of primary active transport
- Passive or active
- Electrochemical gradient
- Carrier-mediated or not
- Active
- Against an electrochemical gradient (“uphill”)
- Carrier-mediated and thus exhibits stereospecificity, saturation, and competition
Examples of primary active transport
- Na+-K+-ATPase (or Na+–K+ pump)
- Ca2+-ATPase (or Ca2+ pump)
- H+-K+-ATPase (or proton pump)
- Memory aid:*
- SPC actively pumps – _s_odium potassium, _p_roton, _c_alcium pumps
Na+-K+ pump location, function and inhibition
- 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
Proton pump location, function and inhibition
- Location
- Gastric parietal cells
- Function
- Transports H+ into the lumen of the stomach against its electrochemical gradient
- Inhibition
- PPI
Ca2+ pump location and function
- Location
- Sarcoplasmic reticulum (SR) or cell membrane
- Function
- Transports Ca2+ against an electrochemical gradient
Why is Na+-K+ pump not considered a secondary active transport
Because both the Na and K are transported against their gradient
SERCA stands for
Sarcoplasmic and endoplasmic reticulum Ca2+-ATPase
Characteristics of secondary active transport
- 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
Relationship of secondary active transport and Na/K pump
- 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
Example of symport and where found
- 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!
Examples of countertransport, exhange or antiport
Examples
- Na+-Ca2+ exchange
- Na+-H+ exchange
- Memory aid:*
- ex_CH_ange
Na+-glucose cotransport
- Location
- Direction of Na+ and glucose
- Source of energy
- 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
Na+-Ca2+ exchange function
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+
Effect if Na-K pump is inhibited
Increased intracellular Na+ concentration decreases the Na+ gradient across the cell membrane → inhibiting Na+–Ca2+ exchange → increase in intracellular Ca2+ concentration
