Cell Biology- Chapter 12

Transport Proteins & Selectivity

• Transport proteins grant selectivity to lipid bilayers.

• Movement of molecules across membranes is governed by concentration and properties.

• The hydrophobic membrane layer is a barrier to some molecules.

• Larger, polar molecules have a harder time crossing than smaller, non-polar ones.

• Ions cannot cross at all without transport mechanisms

Hierarchy of Membrane Permeability

1. Small nonpolar molecules

2. Small uncharged polar molecules

3. Larger uncharged polar molecules

4. Ions (cannot cross without assistance)

Channels vs. Transporters

• Channels: Create continuous open paths for solutes to flow rapidly.

  • Can have open/closed states.

  • Allow movement according to gradients.

• Transporters: Move fixed amounts of solutes at a time via conformational changes.

  • Slower than channels.

  • Can participate in facilitated diffusion or active transport.

  • Both are selective.

Ion Movement & Membrane Voltage

• Unequal ion distribution creates a charge difference (membrane potential).

• Movement is governed by chemical and electrical gradients.

Water Transport via Aquaporins

• Water diffusion across membranes is facilitated by aquaporin channels.

• While some water crosses passively, aquaporins allow rapid transport.

• Osmosis: Water moves according to solute concentrations.

• Cells manipulate solute concentrations to control water movement.

Water Balance in Cells

• Contractile vacuoles (single-celled organisms): Pump solutes into vacuoles, drawing in water, which is then expelled.

• Plant cells: Use rigid walls to withstand osmotic pressure.

• Animal cells: Export solutes to manage osmotic balance

Passive Transporters

• Alternate between conformations independent of solute binding.

• Solute movement is bidirectional, driven by concentration gradients.

• Example: Glucose transporters move glucose inward when extracellular concentration is higher.

Active Transport & Pumps

• Gradient-driven: Uses a concentration gradient for movement.

• ATP-driven: Uses ATP hydrolysis to transport against gradients.

• Light-driven: Uses light energy to drive transport.

Sodium-Potassium Pump (Na+/K+ Pump)

• Uses ATP to maintain sodium and potassium gradients.

• Essential for cell function and membrane potential regulation.

Calcium Pump & Sarcoplasmic Reticulum (SR)

• First crystallized pump.

• SR stores calcium in muscle cells.

• Calcium release triggers contraction; ATP-powered pumps restore levels.

Types of Transporters

• Symport: Moves two solutes in the same direction.

• Antiport: Moves two solutes in opposite directions.

• Coupled transport: Uses one gradient to drive movement of another solute.

Na+/Glucose Symporter in Intestinal Cells

• Glucose uptake is coupled to sodium inflow.

• ATP-powered sodium pump maintains gradient.

• Used by intestinal cells to absorb glucose and other nutrients.

Glucose Transport in the Gut

• Active Na+/Glucose symporter moves glucose into epithelial cells.

• Passive glucose transporters move glucose into the bloodstream.

• Sodium-potassium pumps prevent sodium accumulation.

Ion Channels & Membrane Potential

• Animal cells: Prefer sodium pumps.

• Plant cells: Prefer proton pumps.

• Both types can exist in all cells.

Ion Channel Selectivity

• Selectivity filters determine which ions pass.

• Example: Sodium channels block potassium.

• Vestibule positions polar amino acids to strip water shells, ensuring selectivity.

Ion Channel States

• Channels fluctuate between open and closed states.

• Some switch rapidly, others are gated by signals.

Membrane Potential & Ion Gradients

• Unequal charge distribution creates a membrane potential.

• Only a small number of ions influence the potential.

Nernst Equation & Equilibrium Potential

• Describes the contribution of individual ions to membrane potential.

• Equilibrium potential: When electrical and chemical forces are balanced.

Potassium Leak Channels & Resting Potential

• Major contributor to resting membrane potential (~ -70mV).

• K+ leaves the cell, making the inside more negative.

• Sodium-potassium pumps help maintain balance.

Patch-Clamp Experiments

• Measure activity of individual ion channels.

• Uses a fine pipette to isolate a membrane patch.

• Shows how stimuli influence channel activity.

Types of Ion Channel Gating

1. Voltage-gated: Open/close in response to voltage changes.

2. Ligand-gated (extracellular): Open/close when a molecule binds outside the cell.

3. Ligand-gated (intracellular): Open/close when a molecule binds inside the cell.

4. Mechanically-gated: Open/close due to physical force.

Mechanically-Gated Ion Channels in Hearing

• Hair cells in the ear detect vibrations.

• Stereocilia movement opens mechanically-gated ion channels.

• Ion flow triggers signals to the auditory nerve.

Ion Channels in Plant Movement

• Venus flytrap: Uses electrical potential changes to close leaves.

• Mimosa (sensitive plant): Touch activates ion channels, folding leaves.

Neurons & Signal Transmission

• Dendrites: Receive inputs (excitatory/inhibitory).

• Axons: Propagate electrical signals.

• Nerve terminals: Communicate with target cells.

Squid Giant Axon & Neuron Function

• Used to study action potentials.

• Larger axons conduct signals faster.

Action Potentials

• Triggered by sufficient depolarization.

• Sodium inflow through voltage-gated Na+ channels drives depolarization.

• "All-or-nothing" response.

Sodium & Action Potentials

• Extracellular sodium concentration affects depolarization intensity.

• Sodium entry leads to depolarization; potassium outflow repolarizes.

Threshold Potential & Na+ Channels

• Threshold potential: Voltage at which Na+ channels open.

• At peak action potential, inactivation gate blocks sodium flow.

• Refractory period: Prevents immediate reopening.

Action Potential Propagation

• Na+ channel inactivation ensures one-way travel.

• Stronger stimuli increase action potential frequency, not intensity.

Neurons & Synaptic Transmission

• Action potential arrival triggers neurotransmitter release.

• Voltage-gated Ca2+ channels open at the synapse.

Neurotransmitter Receptors & Target Cells

• Govern response to stimulation.

• Example: Acetylcholine receptors are ligand-gated sodium channels.

Psychoactive Drugs & Synaptic Signaling

• Stimulants (e.g., nicotine): Enhance synaptic transmission.

• Sedatives (e.g., cannabis): Reduce synaptic activity.

Neural Integration

• A single neuron receives multiple inputs.

• Whether it fires depends on the sum of excitatory/inhibitory signals.

Optogenetics & Light-Activated Ion Channels

Uses channelrhodopsin to study neuron function with light.