Study Notes for Transport Across Cell Membranes

Lipid Bilayers: Impermeable to ions and most uncharged polar molecules.
Membrane Transport Proteins: Facilitate the movement of select substances across cell membranes.
Ion Concentrations: Inside a cell are vastly different from those outside, leading to a membrane potential.
Concentration Differences: In inorganic ions create a potential across the cell membrane.
Transport Mechanisms:
- Solutes can cross membranes by either passive or active transport.
- Both the concentration gradient and membrane potential influence the passive transport of charged solutes.
- Water moves across cell membranes down its concentration gradient, known as Osmosis.

The rate at which a solute crosses a protein-free, artificial lipid bilayer by simple diffusion depends on its size and solubility:
1. Small Nonpolar Molecules: Diffuse rapidly (e.g., O₂, CO₂).
2. Small Uncharged Polar Molecules: Can diffuse across the membrane if small enough.
3. Large Uncharged Polar Molecules: Have difficulty crossing (e.g., glucose).
4. Ions: Cannot cross the membrane regardless of size.

Channels:
- Form pores that allow specific ions or small polar molecules to diffuse quickly across membranes.
- Operate only when open; opening is stimulus-controlled.
- Discriminate based on size and electric charge of the molecules.
Transporters:
- Undergo conformational changes to move solutes across membranes.
- Are highly selective and slower than channels, depending on the specificity of their binding site.

Differences in concentrations create a resting membrane potential, typically negative: between -20 mV to -200 mV.
Major Ion Concentrations:
- Na⁺: Most abundant extracellular cation.
- K⁺: Most abundant intracellular cation.
- Ca²⁺, Mg²⁺, Cl⁻: Most abundant extracellularly.

Passive Transport:
- Movement of solutes down their concentration gradients, does not require ATP.
- Characteristics: Small nonpolar molecules (e.g., CO₂) utilize simple diffusion.
Active Transport:
- Moves solutes against their concentration gradients, requires energy from ATP hydrolysis.
- Uses transporters called pumps.

Influences the passive transport of charged solutes such as ions:
1. Concentration Gradient: Difference in solute levels between inside and outside of the cell.
2. Membrane Potential: Voltage across the membrane, usually more negative inside than outside.
Sodium and Potassium Movement:
- For Na⁺: Both electrochemical forces align, driving it into the cell.
- For K⁺: Membrane potential opposes its concentration gradient, reducing its net movement out of the cell.

Definition: Osmosis is a special type of diffusion that involves the movement of water across a semipermeable membrane.
Mechanism: Water moves from an area of low solute concentration (more water) to an area of high solute concentration (less water).
Aquaporins: Specialized water channels that enhance the speed and efficiency of water transport.

Passive transporters move solutes along their electrochemical gradient.
Conformational States:
1. Outward-open: Solute-binding site faces outside the cell.
2. Occluded: Binding site not exposed to either side.
3. Inward-open: Binding site faces the inside of the cell.
The transporter can transition between these states, resulting in movement of molecules, often driven by concentration gradients.

Na⁺-K⁺ ATPase (Na⁺-K⁺ Pump):
- Utilizes ATP hydrolysis to pump Na⁺ out and K⁺ into animal cells.
- Maintains Na⁺ concentration in the cytosol 10-30 times lower than extracellular fluid and K⁺ concentration 10-30 times higher intracellularly.
Ca²⁺ Pumps:
- Act as ATP-driven pumps to keep cytosolic Ca²⁺ concentrations low.
- When stimulated, Ca²⁺ floods into the cytosol from the sarcoplasmic reticulum, triggering muscle contraction, and must be pumped back into the SR post-contraction.

Mechanism: Transport of glucose into cells is coupled with the transport of Na⁺.
1. Pump opens to the outside, Na⁺ binds first due to higher concentration outside.
2. Glucose binds after Na⁺.
3. Pump transitions inward, releasing Na⁺ and glucose into the cell.
4. Reset to the outward-opening state to begin again.
- Key Concept: Glucose is cotransported with Na⁺, using the sodium gradient as the driving force.

Ion Selectivity: Designed to permit the transport of specific ions only, possessing a selectivity filter.
Gated Mechanism: Require a stimulus to switch from a closed to an open state. They allow rapid transport (millions of ions can pass per second).
Ion channels can rapidly fluctuate between open and closed states.

Definition: Rapid signaling changes that allow long-distance communication along axons.
Triggered by a depolarization of the neuron's plasma membrane:
1. Neuron at -60 mV (resting) depolarizes to -40 mV (threshold), activating Na⁺ channels.
2. Rapid depolarization continues, reaching +40 mV.
3. K⁺ outflow via voltage-gated channels results in repolarization, returning the membrane to resting potential.

An action potential reaches a nerve terminal, opening voltage-gated Ca²⁺ channels allowing Ca²⁺ to flow into the terminal.
Increased Ca²⁺ stimulates synaptic vesicles to fuse with the plasma membrane, releasing neurotransmitters into the synaptic cleft (exocytosis).
Resulting ion flows alter the postsynaptic cell's membrane potential, converting the chemical signal back into an electrical signal.
Neurotransmitter Types: Can be excitatory (stimulating) or inhibitory (suppressing), with psychoactive drugs often altering receptor activity.