Bio 131 Exam 1 - Membrane Transport

What is the main question of membrane transport?

How cells regulate what enters and leaves the cytoplasm.

What is the plasma membrane?

A selectively permeable boundary separating the cytoplasm from the extracellular environment.

What does selectively permeable mean?

Some substances cross more readily than others; transport proteins help control passage.

What is the fluid mosaic model?

A membrane is a fluid phospholipid bilayer containing a mosaic of proteins and other components.

What does fluid mean in the fluid mosaic model?

Many membrane lipids and proteins can move laterally within the membrane.

What does mosaic mean in the fluid mosaic model?

The membrane contains varied proteins, lipids, and carbohydrate-bearing components.

What are the most abundant lipids in most membranes?

Phospholipids.

What does amphipathic mean?

Having both hydrophilic and hydrophobic regions.

How are phospholipids oriented in a membrane?

Hydrophilic heads face the aqueous environments; hydrophobic tails face inward.

Why can a phospholipid bilayer form a stable boundary between watery compartments?

Heads interact with water while tails are sheltered inside the bilayer.

How does membrane lipid composition relate to environmental conditions?

Differences in lipid composition can adapt membranes to conditions such as temperature.

Where are membrane carbohydrates located?

On the extracellular surface of the plasma membrane.

What is a glycolipid?

A lipid with a covalently attached carbohydrate.

What is a glycoprotein?

A protein with a covalently attached carbohydrate.

What is a major function of membrane carbohydrates?

Cell-cell recognition; surface carbohydrates help identify cells.

Do all cells have identical surface carbohydrates?

No. They vary among species, individuals, and cell types.

What is an integral membrane protein?

A protein that penetrates the hydrophobic interior of the lipid bilayer.

What is a transmembrane protein?

An integral protein that spans the entire membrane.

Are all integral proteins transmembrane proteins?

No. Integral proteins enter the bilayer, but not all span it completely.

What is a peripheral membrane protein?

A protein loosely attached to the membrane surface rather than embedded in its hydrophobic interior.

What amino acids occur in the membrane-spanning regions of integral proteins?

Mostly nonpolar amino acids that interact with hydrophobic lipid tails.

What are the six main membrane protein functions?

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

What is signal transduction by a membrane receptor?

An outside signal binds a receptor and triggers a response inside the cell.

How do membrane proteins help support cell structure?

They attach to the cytoskeleton and extracellular matrix.

Where are many membrane proteins and lipids synthesized and processed?

They are synthesized in the endoplasmic reticulum and transported through the Golgi apparatus.

How do vesicles deliver membrane components to the plasma membrane?

Vesicles fuse with the plasma membrane, adding their lipids and membrane proteins.

Where do carbohydrate groups facing the vesicle lumen end up after exocytosis?

On the extracellular surface of the plasma membrane.

Which molecules cross the lipid bilayer most readily?

Small nonpolar molecules such as O2 and CO2.

Why can nonpolar molecules cross the bilayer easily?

They can dissolve in its hydrophobic interior.

Why do ions cross the pure lipid bilayer poorly despite their small size?

Their charge makes passage through the hydrophobic interior unfavorable.

Why does glucose cross the pure lipid bilayer poorly?

It is relatively large and polar.

Can water cross the lipid bilayer without aquaporins?

Yes, slowly; aquaporins greatly increase water permeability.

Why can water and urea cross more readily than glucose?

They are smaller polar molecules, though they still cross less readily than small nonpolar molecules.

What properties affect passage through the lipid bilayer?

Size, polarity, and charge.

What do transport proteins allow to cross membranes?

Specific hydrophilic molecules or ions that cannot readily cross the lipid interior.

What does a channel protein do?

Provides a hydrophilic passageway for specific molecules or ions.

What does a carrier protein do?

Binds a substance and changes shape to move it across the membrane.

What are aquaporins?

Channel proteins that facilitate water movement.

What is a gated ion channel?

A channel that opens or closes in response to a stimulus.

Why is transport protein specificity important?

Each protein transports particular substances rather than allowing everything through.

Does the presence of a transport protein prove that transport is active?

No. Proteins also mediate passive facilitated diffusion.

What is diffusion?

Random molecular movement producing net movement from higher to lower concentration.

What is a concentration gradient?

A difference in the concentration of a substance between regions.

What is passive transport?

Movement across a membrane without energy investment by the cell.

What are the main forms of passive transport in this lecture?

Simple diffusion, osmosis, and facilitated diffusion.

What is simple diffusion?

Passive movement directly through the lipid bilayer down a concentration gradient.

What is dynamic equilibrium?

Molecules continue moving, but equal movement in both directions produces no net movement.

Do molecules stop moving at equilibrium?

No. They continue moving randomly.

How does the gradient of one uncharged solute affect another solute's diffusion?

Each diffuses down its own concentration gradient.

What is osmosis?

Net diffusion of water across a selectively permeable membrane.

In a simple osmosis example with nonpenetrating solute, which way does water move?

From lower solute concentration toward higher solute concentration.

What is tonicity?

The ability of a surrounding solution to cause a cell to gain or lose water.

What is an isotonic solution?

A solution with the same effective nonpenetrating solute concentration as the cell, producing no net water movement.

What is a hypertonic solution?

A solution with a higher effective nonpenetrating solute concentration than the cell; water leaves the cell.

What is a hypotonic solution?

A solution with a lower effective nonpenetrating solute concentration than the cell; water enters the cell.

What happens to an animal cell in a hypotonic solution?

It swells and may lyse, or burst.

What happens to an animal cell in an isotonic solution?

It maintains its normal volume.

What happens to an animal cell in a hypertonic solution?

It loses water and shrivels.

What happens to a plant cell in a hypotonic solution?

It becomes turgid; the cell wall resists further expansion.

What happens to a plant cell in an isotonic solution?

It becomes flaccid because it lacks strong turgor pressure.

What happens to a plant cell in a hypertonic solution?

It loses water and becomes plasmolyzed.

What is plasmolysis?

Water loss causes the plasma membrane to pull away from the plant cell wall.

Which environment generally suits animal cells without a cell wall?

An isotonic environment.

Which environment generally suits plant cells?

A hypotonic environment, which promotes turgor.

What is facilitated diffusion?

Passive movement down a gradient through specific channel or carrier proteins.

Does facilitated diffusion require ATP?

No.

Can carrier proteins participate in passive transport?

Yes. Some carriers mediate facilitated diffusion.

What is active transport?

Energy-dependent movement of substances against their concentration or electrochemical gradients.

Why do cells use active transport?

To establish or maintain gradients different from their surroundings.

What is a common energy source for active transport?

ATP; some transport instead uses energy stored in an ion gradient.

What does the sodium-potassium pump move per cycle?

Three Na+ out of the cell and two K+ into the cell, using one ATP.

Where are Na+ and K+ typically more concentrated in the pump diagram?

Na+ is more concentrated outside; K+ is more concentrated inside.

Why is the sodium-potassium pump active transport?

It uses ATP to move Na+ and K+ against their gradients.

How does ATP help the sodium-potassium pump change shape?

Phosphorylation changes its shape; phosphate release helps restore its original shape.

What is the net charge movement per sodium-potassium pump cycle?

One net positive charge moves out of the cell.

What is membrane potential?

The voltage across a membrane caused by unequal charge distribution.

What is an electrochemical gradient?

The combined effects of an ion's concentration gradient and the electrical potential across the membrane.

What are the two forces affecting ion movement?

A chemical force from concentration differences and an electrical force from charge differences.

Is concentration alone enough to predict the direction of ion movement?

No. The membrane potential also matters.

What does a proton pump move?

Hydrogen ions, H+, across a membrane using energy.

What is cotransport?

Coupling downhill movement of one solute to uphill movement of another.

How does the plant H+/sucrose cotransport example work?

An ATP-powered proton pump builds an H+ gradient; H+ moving back into the cell drives sucrose uptake.

Does the H+/sucrose cotransporter directly hydrolyze ATP in the diagram?

No. It uses the H+ gradient created by the ATP-powered proton pump.

Why can cotransport count as active transport?

One solute moves uphill using energy stored in another solute's gradient.

What is bulk transport?

Energy-requiring movement of large molecules or particles using vesicles.

What is exocytosis?

A vesicle fuses with the plasma membrane and releases its contents outside the cell.

What is endocytosis?

The plasma membrane folds inward to form a vesicle containing material from outside the cell.

What are the three types of endocytosis covered?

Phagocytosis, pinocytosis, and receptor-mediated endocytosis.

What is phagocytosis?

Cellular eating: engulfing a large particle into a vesicle.

What is pinocytosis?

Cellular drinking: uptake of extracellular fluid and dissolved substances in small vesicles.

What is receptor-mediated endocytosis?

Selective uptake of substances that bind specific membrane receptors.

How do cells take up LDL particles?

LDL binds membrane receptors and enters through receptor-mediated endocytosis.

What does LDL stand for?

Low-density lipoprotein.

What defect is described in familial hypercholesterolemia?

LDL receptors may be defective or missing, reducing LDL uptake and contributing to cholesterol accumulation in blood.

If a cell has 2 percent nonpenetrating solute and its surroundings have 5 percent, which way does water move?

Out of the cell; the surroundings are hypertonic.

If a cell has 2 percent nonpenetrating solute and its surroundings have 1 percent, which way does water move?

Into the cell; the surroundings are hypotonic.

A solute crosses through a carrier from high to low concentration without ATP. What transport is this?

Facilitated diffusion.

A pump uses ATP to move a solute from low to high concentration. What transport is this?

Active transport.

If ATP production stops, must all diffusion immediately stop?

No. Passive movement can continue while suitable gradients remain.

Can opening more channels reverse a solute's electrochemical gradient?

No. Channels provide a pathway; the gradient determines the direction of net passive movement.