Cell Membranes and Transport - 6

Plasma Membrane Function

The plasma membrane regulates what enters and leaves the cell.

Selective Permeability

The plasma membrane allows some substances to pass while blocking others.

Diffusion

Diffusion is the passive movement of molecules from high to low concentration.

Passive Transport

Passive transport does not require energy.

Equilibrium

In diffusion, equilibrium occurs when concentrations are equal and net movement stops.

Factors Affecting Diffusion Rate

Diffusion rate depends on concentration gradient, molecule size, and membrane permeability.

Simple Diffusion

Small nonpolar molecules diffuse directly through the lipid bilayer.

Examples of Simple Diffusion

Oxygen, carbon dioxide, and small water molecules diffuse freely.

Hydrophilic Molecules and Membranes

Charged and large polar molecules cannot cross the lipid bilayer easily.

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane.

Solute

A solute is a dissolved substance.

Solvent

The solvent is the liquid that dissolves the solute.

Osmotic Potential

Osmotic potential depends on the total solute concentration.

Direction of Osmosis

Water moves from low solute concentration to high solute concentration.

Hypertonic Solution

A hypertonic solution has a higher solute concentration than the cell.

Effect of Hypertonic Solution

Cells lose water and shrink in hypertonic solutions.

Hypotonic Solution

A hypotonic solution has a lower solute concentration than the cell.

Effect of Hypotonic Solution

Cells gain water and may burst in hypotonic solutions.

Isotonic Solution

An isotonic solution has equal solute concentration to the cell.

Effect of Isotonic Solution

No net water movement occurs in isotonic solutions.

Osmosis in Animal Cells

Animal cells may burst or shrink because they lack cell walls.

Contractile Vacuole

A contractile vacuole pumps excess water out of some protists.

Osmosis in Plant Cells

Plant cells become turgid when placed in hypotonic solutions.

Turgor Pressure

Turgor pressure is the pressure of water pushing against the cell wall.

Plasmolysis

Plasmolysis occurs when plant cells lose water and the membrane pulls away from the wall.

Facilitated Diffusion

Facilitated diffusion uses transport proteins but no energy.

Transport Proteins

Transport proteins are specific and selective.

Carrier Proteins

Carrier proteins change shape to move molecules across membranes.

Channel Proteins

Channel proteins form pores that allow specific ions to pass.

Saturation of Transporters

Facilitated diffusion reaches a maximum rate when all transporters are occupied.

Glucose Transport

Glucose enters cells via facilitated diffusion.

Glucose Phosphorylation

Phosphorylation traps glucose inside the cell.

Insulin and Transporters

Insulin increases the number of glucose transporters in the membrane.

Type I Diabetes

Type I diabetes results from the inability to produce insulin.

Type II Diabetes

Type II diabetes results from insulin resistance.

Active Transport

Active transport moves substances against their gradient using energy.

ATP in Active Transport

ATP provides energy for active transport.

Sodium-Potassium Pump

The Na+/K+ pump maintains ion gradients in cells.

Na+/K+ Pump Step 1

Three sodium ions bind inside the cell.

Na+/K+ Pump Step 2

ATP phosphorylates the pump causing a shape change.

Na+/K+ Pump Step 3

Sodium ions are released outside the cell.

Na+/K+ Pump Step 4

Two potassium ions bind from outside.

Na+/K+ Pump Step 5

Potassium binding causes phosphate release.

Na+/K+ Pump Step 6

Potassium ions are released inside the cell.

Electrochemical Gradient

The Na+/K+ pump works against both concentration and charge gradients.

Cotransport

Cotransport moves two substances across a membrane simultaneously.

Symport

Symport moves substances in the same direction.

Antiport

Antiport moves substances in opposite directions.

Hydrogen-Sucrose Pump

A proton gradient drives sucrose transport in plants.