UNIT 3 Solute Diffusion/Transport

What is selective permeability?

The membrane allows some substances across more easily than others.

Which crosses a protein-free bilayer most easily: CO2 or Ca2+?

CO2. Small nonpolar molecules cross much more easily.

Which crosses more easily: glycerol or glucose?

Glycerol; it is smaller and less polar than glucose.

Which crosses more easily: glucose or lactose?

Glucose; lactose is larger and more polar.

Which crosses more easily: lysine or CO2?

CO2. Lysine is charged; CO2 is small and nonpolar.

Which crosses least easily: Ca2+, lysine, glucose, glycerol, CO2?

Ca2+ is among the least permeable because it is charged.

Why can't a 20-bp DNA sequence freely cross a bilayer?

DNA is large, highly polar, and negatively charged.

Which listed molecules require transport proteins?

Ca2+, lysine, glucose, lactose, and DNA.

What are HDL and LDL?

Lipoprotein particles that transport lipids through aqueous blood.

What is phosphatidylserine?

A negatively charged phospholipid commonly enriched on the cytosolic leaflet.

How is membrane charge distributed asymmetrically?

Anionic lipids are enriched on the cytosolic side.

How does asymmetric charge affect membrane potential?

It contributes to an electrical gradient across the membrane.

What is a glycoprotein?

A protein with covalently attached carbohydrate.

Where are membrane carbohydrates located?

On the extracellular/noncytosolic surface.

Why is membrane glycosylation asymmetric?

Glycosylation occurs in the ER/Golgi lumen, becoming extracellular.

What are functions of membrane carbohydrates?

Cell recognition, adhesion, protection, and signaling.

What is a lipid raft?

A membrane domain enriched in cholesterol and sphingolipids.

How do lipid rafts differ physically from surrounding membrane?

They are more ordered and often less fluid than surrounding lipid.

What molecules are enriched in lipid rafts?

Cholesterol, sphingolipids, and selected membrane proteins.

What is a solute?

A substance dissolved in a solvent.

What is a solvent?

The medium that dissolves a solute.

What is a concentration gradient?

A difference in solute concentration across space or a membrane.

What is simple diffusion?

Passive movement directly through the lipid bilayer down a gradient.

What is passive transport?

Transport down an electrochemical gradient without energy input.

What is active transport?

Transport against an electrochemical gradient using energy.

For spontaneous membrane transport, what must ΔG be?

ΔG must be less than 0.

Why are membranes impermeable to many ions?

Charged ions cannot readily enter the hydrophobic membrane core.

How does facilitated transport help energetically?

Transport proteins lower the activation barrier for membrane crossing.

Does facilitated transport make an unfavorable ΔG favorable?

No. It lowers the barrier but does not change ΔG.

How does a concentration gradient affect passive transport?

Passive transport moves solute from high to low concentration.

How does active transport move solutes?

It can move solutes from low to high concentration.

What is osmosis?

Net movement of water across a selectively permeable membrane.

What is isotonic?

A solution with equal effective osmotic concentration across the membrane.

Why does water enter cells containing many inorganic ions?

Solutes lower water's chemical potential, drawing water into the cell.

How can cells prevent excessive water uptake?

They export ions/solutes and regulate water movement.

What is an electrochemical gradient?

The combined chemical concentration and electrical gradients.

What is an electrical gradient?

A difference in charge/voltage across a membrane.

What is a transporter?

A membrane protein that binds solute and changes conformation to move it.

What is a channel?

A membrane protein forming a hydrophilic pore for specific solutes.

How do channels and transporters differ?

Channels form pores; transporters bind solute and undergo conformational changes.

What do passive channels and transporters have in common?

Both can move solutes down electrochemical gradients without energy input.

Why are channels gated?

To prevent uncontrolled ion/solute flow and regulate cell activity.

What can regulate channel opening?

Voltage, ligand binding, mechanical force, or other signals.

What is an aquaporin?

A highly selective channel that rapidly transports water.

Why must channels be selective?

Cells must control which ions or molecules cross membranes.

How can channels achieve selectivity?

Specific pore chemistry and geometry favor particular solutes.

How does the K+ channel achieve ion selectivity?

Its selectivity filter precisely coordinates K+ ions.

How can active transport move solute uphill?

Coupling transport to an energy-releasing process makes overall ΔG favorable.

What is primary active transport?

Transport powered directly by ATP or another energy source.

What is secondary active transport?

Transport powered by an electrochemical gradient established by primary transport.

What is a uniport?

Transports one type of solute in one direction.

What is a symport?

Moves two solutes in the same direction.

What is an antiport?

Moves two solutes in opposite directions.

Why is the Na+/K+ pump energetically costly?

It continually maintains steep Na+ and K+ gradients using ATP.

What does the Na+/K+ pump transport per ATP?

3 Na+ out and 2 K+ into the cell.

How does ATP operate the Na+/K+ pump?

ATP phosphorylates the pump, driving conformational changes.

Why are electrochemical gradients critical?

They store usable energy for transport and electrical/cellular processes.

How does active transport maintain electrochemical gradients?

It moves ions against their gradients using energy.

In intestinal glucose absorption, what powers glucose uptake from the gut?

The Na+ electrochemical gradient powers Na+/glucose cotransport.

Which intestinal glucose transport is energetically unfavorable?

Glucose uptake into the epithelial cell against its gradient.

Which intestinal glucose transport is energetically favorable?

Glucose movement from epithelial cell into bloodstream down its gradient.

What type of transporter is the Na+/glucose transporter?

A symporter using secondary active transport.

What type of transporter releases glucose to the bloodstream?

A uniporter using facilitated diffusion.

Why must intestinal transporters be asymmetrically distributed?

They create directional movement from gut → cell → bloodstream.

What is an intestinal epithelial cell?

A cell lining the intestine that mediates nutrient absorption.

What are ABC proteins?

ATP-binding cassette proteins that use ATP to transport substances.

What is P-glycoprotein (PGP/MDR1)?

An ABC transporter that exports many drugs and other compounds.

What is PGP's normal beneficial role?

It exports potentially harmful compounds and protects tissues.

How can cancer cells exploit PGP?

They increase drug efflux, lowering intracellular anticancer drug levels.

What does PGP-mediated drug resistance demonstrate?

Transport proteins can reduce drug accumulation inside cells.

What is the blood-brain barrier (BBB)?

A selective barrier limiting substances entering brain tissue.

Why can ABC transporters contribute to BBB protection?

They export potentially harmful compounds from brain endothelial cells.

What is CFTR?

An ABC-family protein functioning as a regulated chloride channel.

What is CFTR's normal function?

It regulates Cl− transport and helps control salt and water movement.

What is the ABC domain of CFTR?

An ATP-binding domain that regulates CFTR channel activity.

Is CFTR primarily an ATP-powered transporter?

No. It is an ATP-regulated ion channel.

What does ATP binding/hydrolysis do in CFTR?

It regulates opening and closing of the chloride channel.

What is cystic fibrosis?

A disease caused by mutations that impair CFTR function.

How does defective CFTR affect secretions?

It disrupts Cl−/water transport, producing abnormally thick mucus.

What is multidrug resistance?

Resistance to multiple drugs, often through shared resistance mechanisms.

How can cancer drug resistance be broadly classified?

Reduced drug entry, increased drug removal, altered targets, or altered cell survival.

How can cancer cells reduce drug entry?

By decreasing uptake transporters or changing membrane properties.

How can cancer cells increase drug removal?

By increasing efflux transporters such as PGP.

How can cancer cells alter drug targets?

Mutations or changes can reduce drug binding/effect.

How can cancer cells survive despite drug action?

They can alter apoptosis, repair damage, or activate survival pathways.