Chapter 5 Summary: Transport by Simple Diffusion

What is passive solute transport?

Passive solute transport only moves solutes toward electrochemical equilibrium.

For uncharged solutes, this is the same as concentration equilibrium.

For charged solutes, electrochemical equilibrium occurs when the electrical gradient exactly balances the concentration gradient.

What is simple diffusion?

The most basic form of passive transport.

• Random molecular motion causes more molecules to move out of regions of high concentration than into them, resulting in movement toward equal concentrations.

How does diffusion differ for charged and uncharged solutes?

Uncharged solutes diffuse according to concentration gradients alone. Charged solutes are influenced by both concentration gradients and electrical forces (attraction and repulsion from voltage differences).

Why can voltage differences exist across cell membranes?

Cell membranes act as capacitors, allowing unequal distributions of positive and negative charges on opposite sides of the membrane, creating a voltage difference.

Do voltage differences affect diffusion in bulk solutions?

No

• Bulk solutions are electrically neutral, so voltage differences do not influence simple diffusion within them.

How do lipid-soluble molecules cross cell membranes?

Lipid-soluble molecules such as steroid hormones and fatty acids dissolve in the membrane's lipid bilayer and cross by simple diffusion.

Why can't ions cross membranes by dissolving in the lipid bilayer?

Ions are poorly soluble in lipids and therefore cannot diffuse directly through the membrane lipid layer.

How do inorganic ions cross cell membranes?

Inorganic ions typically cross membranes through ion channels via simple diffusion.

What are gated ion channels?

Gated ion channels can open or close in response to specific signals, such as ligand binding, altering ion movement across the membrane.

What determines membrane permeability to lipid-soluble substances?

Permeability depends on how easily the solute dissolves in and moves through the membrane lipid layer.

What determines membrane permeability to ions?

Permeability depends on the number of ion channels present and whether those channels are open or closed.

What is active transport?

Active transport uses energy derived from metabolism to move solutes away from electrochemical equilibrium. It is also called uphill transport or pumping.

Can O₂ and H₂O be actively transported?

No

• Active transport mechanisms are known for many solutes, but not for oxygen (O₂) or water (H₂O).

Why is active transport considered carrier-mediated transport?

Active transport requires solutes to bind noncovalently to transporter proteins that move them across membranes.

What is facilitated diffusion?

Facilitated diffusion is carrier-mediated transport that uses transporter proteins but cannot use metabolic energy, so it only moves substances toward equilibrium.

What is primary active transport?

Primary active transport uses ATP directly as its energy source through ATPase transporter proteins.

What is the Na⁺/K⁺-ATPase?

Na⁺/K⁺-ATPase is a primary active transport protein found in all animal cells. It uses ATP directly to pump ions across membranes and is especially important in epithelial cells.

What types of substances are moved by primary active transport?

Primary active transport primarily pumps ions.

What is secondary active transport?

Secondary active transport uses energy stored in an electrochemical gradient rather than ATP directly.

What proteins are used in secondary active transport?

Secondary active transport uses cotransporters and countertransporters that transport two solutes simultaneously.

What types of substances are commonly moved by secondary active transport?

Secondary active transport commonly pumps organic solutes and ions.

What is the difference between electrogenic and electroneutral transport?

Electrogenic transport generates a voltage difference across a membrane, while electroneutral transport does not.

What are colligative properties?

Colligative properties depend on the number of dissolved particles in a solution, not on the chemical identity of those particles.

What are the three most important colligative properties in biology?

Osmotic pressure, freezing point, and water vapor pressure.

Why are osmotic pressure, freezing point, and water vapor pressure important?

They are quantitatively related, meaning any one of them can be used to calculate the others.

What is an osmolar (Osm)?

A 1-osmolar solution behaves osmotically as if it contains 1 Avogadro's number (6 × 10²³) of dissolved particles per liter.

How do physical chemists measure osmotic pressure?

Osmotic pressure is measured using hydrostatic pressure.

What is osmosis?

Osmosis is the passive movement of water across a membrane.

In which direction does water move during osmosis?

Water moves from a solution with lower osmotic pressure (more available water) to a solution with higher osmotic pressure (less available water).

Why does osmosis occur?

Osmosis occurs to move the system toward equilibrium by equalizing osmotic pressures across a membrane.

What does isosmotic mean?

Two solutions are isosmotic when they have the same osmotic pressure.

What does hyposmotic mean?

A solution with lower osmotic pressure relative to another solution.

What does hyperosmotic mean?

A solution with higher osmotic pressure relative to another solution.

Does a single solution generate hydrostatic pressure because of its osmotic pressure?

No.

• A single solution does not generate hydrostatic pressure by itself.

How can hydrostatic pressure be generated by osmosis?

Hydrostatic pressure develops when two solutions interact across a membrane and water moves between them by osmosis.

How can water cross membranes that lack aquaporins?

Water can dissolve in the membrane and move through by simple diffusion.

What are aquaporins?

Aquaporins are specialized membrane channels that allow highly specific water transport across membranes.

How do aquaporins affect the rate of osmosis?

Aquaporins increase the rate of osmosis approximately 5- to 50-fold compared to membranes without aquaporins.

Where are aquaporins commonly found?

Aquaporins are found in membranes such as red blood cells and certain kidney tubule cells.