PHYSL 212 Lecture 2 - Membrane Transport (Questions)

Describe the movement in simple membrane diffusion and why certain molecules prefer to move this way.

Random, constant motion of molecules across the lipid bilayer to reach diffusion equilibrium. Moves in the direction of their net flux

Nonpolar molecules are able to dissolve in the lipid bilayer and move across quickly. Polar molecules cannot dissolve and diffusion is impeded

According to Fick’s First Law of Diffusion, what are the factors that affect diffusion?

1. Fixed permeability coefficient of the molecule (different molecules diffuse at different rates)

2. Difference in concentration between the inside and outside of the cell (the larger size of the concentration gradient, the faster diffusion will be driven)

3. Surface area of the membrane (greater surface area → more area for diffusion to take place → faster net flux)

Between the two types of facilitated diffusion/protein mediated transport, which is quicker? Why?

Ion channels: act as a bridge/tunnel with free flow, unblocked. Simpler and quicker

Protein transporters: time is taken in saturating the protein, conformational changes, unloading of molecules, etc. More complicated, slower

What forces contribute to the flux and movement of ions across ion channels in facilitated diffusion?

Electrochemical gradient:

1. Ion concentration inside and outside of the cell

2. Membrane potential

Why do ions prefer to diffuse via ion channels? Describe their structure. How are they selective to certain ions?

Ion channels are comprised of protein subunits bunched together to form pores. They are aqueous and small in diameter, only allowing small ions (polar) to pass through these pores

Selective permeability of ions is dependent on:

1. Channel diameter (selects for ions of different sizes)

2. Charge of the polypeptide (+ or - charges attract certain ions and repel others)

3. Number of water molecules associated with the ion

How is movement across ion channels regulated?

Channel gating: ion channels will close or open depending on the environmental conditions of the cell, allowing ions to move when needed and to control if unneeded

Chemically, electrically, and physically gated → a threshold must be met

Describe the structure of membrane transporters. How do they move molecules across the membrane?

1. Solute binds to site exposed to extracellular fluid, which has a high affinity for the molecule

2. Saturation leads to conformational change in the protein, exposing the site to intracellular fluid

3. Binding affinity of the site changes to low, causing solute to dissociate into the intracellular fluid

What forces contribute to the flux and movement of molecules across transporters in facilitated diffusion?

1. Solute concentration (net flux determined by size of gradient)

2. Affinity of the transporter for solute (exact matches, similar amino acid compositions, completely different composition will all have different rates)

3. Number of transporters in the membrane (more transporters → more molecules transported → faster)

4. Rate at which transporter goes through conformational change

Compare the limitations in flux of solute into cells for direct diffusion compared to transporter-facilitated diffusion. Assume constant gradient of diffusion.

Direct: flux into cells is directly proportional to solute concentration, theoretically limitless if solute concentration keeps increasing

Transporter: proteins are saturatable and in finite numbers. Once all proteins are saturated/in use, increasing the amount of solute will not change the flux at which they are being moved

Describe the structure activity of membrane transport proteins used in primary active transport. How do they move molecules across the membrane?

“ATPase” transporters hydrolyze ATP, releasing energy and an inorganic phosphate. The phosphate is added to the transporter (phosphorylated) to cause a conformational change, and the energy released powers movement of the solute against its concentration gradient

Describe how the sodium potassium pump transports sodium and potassium ions against their gradient, including affinity of sites and conformational changes.

1. Transporter in its Na+ affinity state, sites face the inside of the cell. 3 Na+ bind to the high affinity binding sites on intracellular surface of ATP bound protein

2. Binding of sodium activates ATPase activity of the transporter, hydrolyzing ATP to ADP and an inorganic phosphate. Surface of protein is phosphorylated with inorganic phosphate, using energy from hydrolysis

3. Phosphorylation causes conformational change → transporter changes to K+ affinity state and sites face outside of cell. Binding sites lose sodium affinity, and Na+ is released outside the cell

4. Binding sites have affinity for K+ outside the cell, which bind to the protein sites. Binding of K+ kicks off phosphate, causing a conformational change of the protein back to its Na+ affinity state

5. Conformational change → binding sites face inside of the cell, transporter no longer has affinity for K+ and releases it inside the cell

Describe how a secondary active transporter could use a sodium gradient to power movement of molecules against its gradient.

Sodium flows into the cell with its concentration gradient, releasing energy from favourable energetic movement. The solute molecule is cotransported with it at the same time, using the energy from the diffusion of sodium to go against its gradient

Does secondary active transport directly hydrolyze ATP? What is required for it to keep going?

No ATP consumed, but ultimately depends on ATP hydrolysis by primary active transport proteins to power it.

Ex. sodium gradient powered transport needs a consistently low amount of Na+ in the cell for diffusion inside to happen. Sodium potassium pump maintains this gradient

Compare the types of endocytosis by function, method of uptake, and solute specificity.

Pinocytosis: ingestion of fluid. A “furrow” is created in the membrane to uptake the fluid. Nonspecific uptake

Phagocytosis: immune function (unique to phagocytes). Membrane “reaches out” to enclose the solute and bring it into the cell. Specific uptake of bacteria or damaged tissue

Receptor-mediated: specific solute intake depending on function. Cluster of receptors recruits cytosolic clathrin to form a coated pit, which then breaks off into a vesicle. Specific binding of high-affinity ligands

What happens to the molecules transported by endocytosis once they are inside the cell?

Pinocytosis: vesicle fuses with lysosome to be hydrolyzed

Phagocytosis: phagosome fuses with vesicle where contents are hydrolyzed

Receptor-mediated: clathrin-coated vesicle can fuse with endosomes or lysosomes among other fates depending on function. Receptor is recycled back to surface

What are the functions of exocytosis?

1. Allows secretion of membrane impermeable molecules into extracellular fluid (move cargo)

2. Replaces cell surface membrane lost during endocytosis (doesn’t require cargo)

How can endocytosis allow for rapid secretion of molecules commonly found in signalling?

Secretory vesicles can be stored/built up, making them available for rapid release when signalled to do so

Ex. presynaptic neuron

Describe how the transcellular pathway of epithelial transport couples multiple mechanisms of transport.

Passive transport: protein mediated diffusion with concentration gradient (ex. Na+ concentration high in organ lumen, low in epithelial cell. No energy needed)

Primary active transport: higher concentration of sodium in blood than in epithelial cell, protein pump required to move sodium out into blood