1.6 - structure + function of membranes

Question 1

tonicity

Answer 1

is the osmotic force exerted by a solution

term used to describe the osmolarity of a solution compared with the osmolarity of a blood (isotonic, hypertonic e.g. severe dehydration, hypotonic)

Question 2

what does the direction of diffusion depend on for ions

Answer 2

determined by electrical gradient AND chemical gradient (opposing)

The balance is determined by the Nernst potential

e.g K+ tend to diffuse out of cells via chemical gradient but are drawn back in due to electrical gradient (negative membrane potential left behind)

Question 3

What is ficks law

Answer 3

Question 4

Why can gases like O₂ and CO₂ freely pass through the lipid bilayer?

Answer 4

Gases are uncharged and lipid-soluble, enabling them to freely pass through the lipid bilayer.

Question 5

Why is water theoretically unable to easily pass through the lipid bilayer?

Answer 5

Water is polar and has low lipid solubility, which should prevent it from passing easily through the membrane.

Question 6

How does water actually move across the lipid bilayer?

Answer 6

Despite low lipid solubility, water moves rapidly and in an organized manner across the membrane, indicating specialized transport mechanisms beyond simple leakage. = aquaporins

Question 7

What is an artificial lipid membrane?

Answer 7

An artificial lipid membrane is a model membrane composed purely of a lipid bilayer, used to study permeability.

Question 8

Why is the permeability of ions and molecules compared between artificial and real membranes?

Answer 8

This comparison helps identify additional components, like proteins, in real membranes that affect permeability.

Question 9

What does the comparison between artificial lipid bilayers and real membranes reveal?

Answer 9

shows that real membranes have other components enabling functions that wouldn’t be possible with just a lipid bilayer.

Question 10

types of membrane transport

Answer 10

porins
channels
carriers

Question 11

types of channels

Answer 11

voltage gated
ligand gated
stretch activated

Question 12

types of carriers

Answer 12

primary active transport
secondary active transport
facilitated diffusion

Question 13

What role do aquaporins play in the membrane?

Answer 13

Aquaporins are water channels that facilitate the rapid and organized transport of water across the membrane.

Question 14

How does perforin function in cytotoxic T-lymphocytes?

Answer 14

Cytotoxic T-lymphocytes release perforin monomers, which form large channels to kill target cells.

Perforin monomers polymerize and assemble like staves in a barrel, forming doughnut-like channels that disrupt the target cell membrane.

Question 15

what are porins

Answer 15

Transmembrane pores that are ALWAYS open

Question 16

What are channels in the context of cell membranes?

Answer 16

Channels are aqueous-filled gated pores that can switch between open and closed states to regulate ion flow across the membrane.

Question 17

what do channels allow the passage of

Answer 17

ions and some organic osmolytes e.g. taurine via passive diffusion

Question 18

What are leak channels, and how do they function?

Answer 18

Leak channels are constitutively open channels that flicker rapidly between open and closed states without biological regulation, such as K⁺ leak channels that help control membrane potential.

Question 19

What types of gating mechanisms can control channel opening?

Answer 19

Voltage-gated: changes in membrane potential

Ligand-gated: extracellular binding of a ligand to the channel or to an associated receptor e.g. G-protein coupled receptor (e.g., nAChR or GPCR)

Secondary messenger-gated: intracellular binding of a secondary messenger (e.g., cGMP)

Stretch-activated: membrane deformation

Question 20

What are gap junctions, and what is their function?

Answer 20

Gap junctions are channels connecting the cytoplasm of adjacent cells, allowing diffusion of small molecules and ions to mediate cell-cell communication.

non-selective channel for electrical and chemical communication = e.g in heart. cardiac myocytes

Question 21

How are gap junctions structured?

Answer 21

Each gap junction is formed by two connexon pores, each made of six connexin proteins, which align to create a non-selective channel for electrical and chemical communication.

Question 22

Why can K⁺ ions pass through K⁺ channels, but Na⁺ ions cannot?

Answer 22

K⁺ channels have pore linings that efficiently replace the water molecules that shield K⁺ ions. Na⁺ ions are too small to interact effectively with these pore linings, preventing their passage.

Question 23

Describe the signaling cascade of a G protein-coupled receptor (GPCR) from activation to signal termination.

Answer 23

An external ligand (e.g., hormone, neurotransmitter) binds to the GPCR’s extracellular domain, causing a conformational change.

The GPCR activates an associated G protein by promoting the exchange of GDP for GTP on the Gα subunit, causing the Gα and Gβγ subunits to dissociate.

The activated Gα and Gβγ subunits interact with target proteins, initiating different signaling pathways:

Gαs stimulates adenylyl cyclase, increasing cAMP and activating protein kinase A (PKA).

Gαi inhibits adenylyl cyclase, reducing cAMP levels.
Gαq activates phospholipase C (PLC), generating IP₃ and DAG, which release Ca²⁺ and activate protein kinase C (PKC).

The Gβγ subunits can also activate ion channels, influence other signaling molecules, and add diversity to the cellular response.

The Gα subunit hydrolyzes GTP to GDP, allowing Gα and Gβγ to reassociate, resetting the G protein. GPCR phosphorylation by GRKs and binding of arrestins can desensitize the receptor, stopping further activation.

Question 24

How do carrier proteins facilitate transport across the membrane

Answer 24

Carrier proteins binds tosolute + traps it. Its undergoes a reversible conformational change that alternates exposure of the solute-binding site from one side of the membrane to the other. Then releases it.

while channels are often open to both sides.

Question 25

Why is carrier-mediated transport slower than channel-mediated transport?

Answer 25

The binding and conformational change required for each solute molecule make carrier-mediated transport slower than the rapid passage allowed by channels.

Question 26

What does it mean for carrier-mediated transport to be “temperature sensitive”?

Answer 26

Carrier-mediated transport depends on protein conformational changes, which are affected by temperature

Question 27

What is meant by the term “saturable” in carrier-mediated transport?

Answer 27

Carrier proteins have a limited number of binding sites and can reach a maximum transport rate (Vmax) at high solute concentrations, after which transport cannot increase.

Question 28

Define Vmax in the context of carrier proteins.

Answer 28

Vmax is the maximal rate of transport by a carrier protein when all binding sites are occupied, indicating full saturation.

Question 29

What is Km in carrier-mediated transport?

Answer 29

Km is the solute concentration at which the transport rate is half of Vmax, reflecting the affinity of the carrier for its solute.

Question 30

How do passive carriers work, and give an example?

Answer 30

Passive carriers facilitate diffusion without energy input, moving solutes down their concentration gradient. An example is the GLUT transporter, which moves glucose into cells.

Question 31

When does solute movement stop in facilitated diffusion?

Answer 31

Movement stops when equilibrium is reached—either the chemical gradient (for uncharged solutes) or the electrochemical gradient (for charged solutes) is balanced.

Question 32

Describe the equilibrium condition for uncharged solutes in facilitated diffusion.

Answer 32

For uncharged solutes, transport stops when the concentration is equal on both sides, eliminating the chemical gradient.

Question 33

What is the “occluded state” in carrier-mediated transport?

Answer 33

The occluded state is an intermediate stage where the solute is temporarily enclosed within the carrier protein, shielded from either side of the membrane.

Question 34

Explain the role of glucose-6-phosphate in glucose transport.

Answer 34

Inside cells, glucose is quickly converted to glucose-6-phosphate, maintaining a low intracellular glucose concentration and driving the continuous facilitated diffusion of glucose into the cell.

Question 35

are carriers active or passive

Answer 35

can be both

Question 36

carriers vs channels

Answer 36

-carriers…

• slower
• more sensitive to temp
• saturable

Question 37

what do high and low Km suggest

Answer 37

A low Km suggests a high affinity for the solute, meaning the carrier can achieve half of its maximum transport rate (Vmax) at a low solute concentration = indicates the carrier is efficient at binding and transporting the solute even when its concentration is low

A high Km suggests a low affinity for the solute, meaning the carrier requires a higher solute concentration to reach half of its maximum transport rate (Vmax) = implies that the carrier is less efficient at binding and transporting the solute at lower concentrations.

Question 38

define active transport

Answer 38

the accumulation of a substance above the level that would be predicted at electrochemical equilibrium - evidence of active transport

Question 39

examples of passive carriers

Answer 39

GLUT
AE1 (Cl- shift)

Question 40

What is the role of ATP hydrolysis in carrier proteins?

Answer 40

The hydrolysis of ATP by the carrier protein initiates a conformational change that translocates bound ions across the membrane, enabling active transport.

Question 41

Describe the function of Na+/K+ ATPase.

Answer 41

The Na+/K+ ATPase maintains low intracellular sodium ([Na+]) levels by actively transporting 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP for energy.

Question 42

What is the function of Ca2+ ATPase?

Answer 42

Ca2+ ATPase extrudes Ca2+ ions from cells, helping to regulate intracellular calcium levels and maintain cellular signaling.

Question 43

What is the role of H+/K+ ATPase in the stomach?

Answer 43

H+/K+ ATPase secretes gastric acid (H+) in the stomach, contributing to the acidic environment necessary for digestion.

Question 44

What are the structural components of the Na+/K+ ATPase?

Answer 44

The Na+/K+ ATPase consists of alpha and beta subunits, where the alpha subunit has 10 transmembrane segments and mediates the active transport.

Question 45

Describe the mechanism of action of the Na+/K+ ATPase.

Answer 45

E1 Conformation: ATP binds to the pump after it releases K+ into the intracellular fluid.

3 intracellular Na+ ions bind to high-affinity sites on the pump.

ATP is hydrolyzed, phosphorylating the alpha subunit at an aspartate residue, occluding Na+ ions.

The pump transitions from E1 to E2, exposing Na+ to the extracellular fluid and decreasing its binding affinity.

The 3 Na+ ions dissociate, and the pump’s conformation changes, increasing its affinity for extracellular K+.

Extracellular K+ binds to the pump.

The acyl-phosphate bond is hydrolyzed, releasing inorganic phosphate (Pi) and occluding K+.

ATP binds again, shifting the pump back to E1 conformation, de-occluding / releasing K+ and decreasing its affinity for K+.

The cycle repeats as ATP hydrolysis continues.

Question 46

What is secondary active transport?

Answer 46

Secondary active transport uses the gradients established by primary active transport (e.g., Na+/K+ ATPase) to provide energy for the transport of other solutes against their concentration gradients.

Question 47

How do symporters and antiporters differ in secondary active transport?

Answer 47

Symporters: Co-transport two solutes in the same direction across the membrane (e.g., Na+/glucose symporter).

Antiporters: Transport one solute into the cell while transporting another solute out of the cell in the opposite direction (e.g., Na+/Ca2+ exchanger).

Question 48

Describe the function of the Na+/glucose symporter.

Answer 48

he Na+/glucose symporter is found in the cell membranes lining the gut and proximal convoluted tubule, using the steep Na+ gradient created by Na+/K+ ATPase to import glucose against its concentration gradient while Na+ moves down its gradient.

Question 49

How does the Na+/glucose symporter utilize the Na+ gradient?

Answer 49

The downhill movement of Na+ into the cell provides the energy needed for the uphill transport of glucose into the cell, allowing glucose to bind more effectively upon Na+ binding, which causes a conformational change in the protein.

Question 50

What is the stoichiometry of the Na+/glucose symporter?

Answer 50

The Na+/glucose symporter typically transports 2 Na+ ions for every 1 glucose molecule.

Question 51

Describe the Na+/Ca2+ exchanger (NCX).

Answer 51

The Na+/Ca2+ exchanger is a 3 Na+/1 Ca2+ antiporter that uses the energy from the downhill transport of Na+ into the cell to drive the uphill extrusion of Ca2+ out of the cell, particularly following cardiac myocyte contraction.

Question 52

What is the significance of the Na+/Ca2+ exchanger having a low affinity for Ca2+?

Answer 52

The low affinity allows for a fast rate of Ca2+ expulsion, making it ideal for quickly reducing intracellular Ca2+ levels when concentrations are high, which is crucial for proper muscle function and relaxation.