Chapter 4 - Movement

diffusion

Movement of molecules due to random thermal motion

No

Does diffusion require ATP?

net flux

Difference between two opposing one-way fluxes.

diffusion equilibrium

Concentrations are equal and net flux is zero.

No, molecules continue moving but there is no net movement.

At diffusion equilibrium, do molecules stop moving?

The concentration gradient.

What determines the direction of net diffusion?

The size of the concentration gradient.

What determines the magnitude of diffusion?

Temperature, molecular size, surface area, and medium.

four factors affecting diffusion rate

Higher temperature increases diffusion.

How does temperature affect diffusion?

Smaller molecules diffuse faster.

How does molecular size affect diffusion?

There are fewer collisions between molecules.

Why is diffusion faster in air than in water?

Diffusion time increases with the square of distance.

How does diffusion time change with distance?

It becomes extremely slow as distance increases.

Why is diffusion ineffective over long distances?

Fick's First Law of Diffusion

J = P × A × (Co − Ci)

Diffusion rate (flux).

What does J represent in Fick's Law?

Permeability coefficient.

What does P represent?

Membrane surface area.

What does A represent?

Concentration difference across the membrane.

What does Co − Ci represent?

Permeability, surface area, and concentration difference.

Increasing which three factors increases diffusion across a membrane?

nonpolar molecules

Which molecules diffuse most readily through lipid bilayers?

O2, CO2, fatty acids, steroid hormones

Examples of molecules that diffuse directly through lipid bilayers

Greater lipid solubility increases diffusion.

How does lipid solubility affect diffusion?

They are charged and poorly soluble in lipids.

Why are ions unable to diffuse rapidly through lipid bilayers?

ion channel

Membrane protein that forms a pore allowing ions to cross the membrane.

they lack ion channels

Why are artificial lipid membranes nearly impermeable to ions?

diameter, charge, and polar surfaces

Three factors determining channel selectivity

hydration shell

Water molecules surrounding an ion in solution.

contribute to channel selectivity

Why are hydration shells important?

membrane potential

Electrical charge difference across a membrane.

millivolts

Units of membrane potential

Unequal distribution of ions across membranes.

Why do cells develop membrane potentials?

electrochemical gradient

Combined effects of membrane potential and concentration gradient.

no

Does membrane potential always favor diffusion?

electrochemical gradient

What determines the net movement of an ion?

channel gating

Opening and closing of channels.

ligand gated channel

Opens when a ligand binds.

voltage gated channel

Opens due to changes in membrane potential.

mechanically-gated channel

Opens due to membrane stretch or deformation.

mediated transport

Transport using membrane transporter proteins.

The transported substance must bind to the transporter.

What must happen before transporter-mediated movement occurs?

Four factors affecting mediated transport rate

Solute concentration, transporter affinity, transporter number, and conformational change rate.

Transporter proteins are limited in number.

Why does mediated transport show saturation?

Transport maximum (maximal flux).

Maximum transport rate achieved when all transporters are occupied

Facilitated diffusion

Passive movement through transporters down a concentration gradient.

no

Does facilitated diffusion require ATP?

no

Can facilitated diffusion move substances uphill?

It is rapidly converted to glucose-6-phosphate.

Why does glucose continue moving into many cells?

GLUT transporters.

Main glucose transporters in facilitated diffusion

Yes

Does facilitated diffusion exhibit specificity?

Yes

Does facilitated diffusion exhibit saturation?

active transport

Movement against an electrochemical gradient requiring energy.

primary and secondary active transport

Two major types of active transport

ATP directly

Primary active transport uses what energy source?

electrochemical gradients

Secondary active transport uses what energy source?

Pumps Na⁺ out and K⁺ into cells.

Function of Na+/K+ ATPase

3 out

How many Na⁺ ions are transported per ATP?

2 in

How many K⁺ ions are transported per ATP?

More positive charges leave than enter.

Why is the Na+/K+ ATPase electrogenic?

15 mM.

Intracellular Na⁺ concentration

145 nM

Extracellular Na⁺ concentration

150 nM

Intracellular K⁺ concentration

5 nM

Extracellular K⁺ concentration

Three Na⁺ ions bind intracellularly.

First step of Na+/K+ ATPase cycle

Phosphorylation-induced conformational change.

What causes release of Na⁺ outside the cell?

Dephosphorylation of the transporter.

What causes release of K⁺ inside the cell?

Pumps Ca²⁺ out of cytosol.

Ca²⁺ ATPase function

Helps regulate intracellular pH.

H⁺ ATPase function

Secretes H⁺ into the stomach lumen.

H⁺/K⁺ ATPase function

Approximately 10,000-fold.

Cytosolic vs extracellular Ca²⁺ difference

secondary active transport

Uphill transport powered by an ion gradient.

Na+

Which ion most commonly powers secondary active transport?

No

Does secondary active transport directly hydrolyze ATP?

Na+/K+ ATPase creates the Na⁺ gradient that powers it.

Why does secondary active transport depend on Na+/K+ ATPase?

Secondary active transport eventually fails.

What happens if Na+/K+ ATPase stops functioning?

Cotransport (symport)

Two substances move in the same direction.

Countertransport (antiport)

Two substances move in opposite directions.

moves down its gradient

During cotransport, what happens to Na⁺?

moves against its gradient

During cotransport, what happens to the transported solute?

osmosis

Net movement of water across a membrane.

Direction of water movement during osmosis

From lower solute concentration to higher solute concentration.

no

Does osmosis require ATP?

osmosis

Water moves from high water concentration to low water concentration.

aquaporins

Which proteins mediate rapid water movement?

they regulate water reabsorption

Why are aquaporins important in the kidney?

yes

Can cells regulate aquaporin numbers?

osmolarity

Total concentration of solute particles.

1 Osm

1 M glucose equals how many Osm?

2 Osm

1 M NaCl equals how many Osm?

3 Osm

1 M MgCl₂ equals how many Osm?

Number of particles

What determines osmolarity?

decreases

As osmolarity increases, water concentration does what?

Pressure needed to stop osmosis.

Pressure needed to stop osmosis.

no

Does osmotic pressure push water into a solution?

Higher osmolarity produces higher osmotic pressure

How does osmolarity affect osmotic pressure?

nonpenetrating solute

Solute that cannot readily cross the membrane.

they determine cell volume

Why are nonpenetrating solutes important?

300 mOsm

Typical intracellular osmolarity

300 mOsm

Typical extracellular osmolarity

isotonic solution

No change in cell volume.

hypotonic solution

Cells swell.