Quiz 9

Membrane Transport

The membrane plays a crucial role in:
- Transport across the membrane
- Transport within the cell
- Providing a framework within the cell

Chapter 12 focuses on membrane transport, specifically diffusion.
- Diffusion is the simplest form of transport.
- It allows for selective passage through the cell membrane.

Channel Proteins vs. Carrier Proteins:
- Two main types of proteins facilitate transport across the membrane.
Passive and Active Transport:
- Transport can occur with or without energy input.
Protein conformational change is often involved in carrier protein transport.
Symport and Antiport:
- These are types of coupled transport where multiple molecules are transported together.
Osmosis:
- The diffusion of water across a membrane.
Channels can be triggered to open, allowing for regulated transport.

Neuronal signaling relies heavily on transport mechanisms.
- Action Potential generation and propagation
- Synaptic transmission
Psychoactive drugs often target these transport processes.

Two primary methods:
- Directly through the phospholipid membrane (diffusion)
- Through a special protein (transporter) on the membrane
  - Two types of transporters: carrier and channel proteins
Diffusion:
- Small, non-charged hydrophobic molecules can diffuse through the membrane by dissolving in the phospholipids.
- Small, non-charged hydrophilic molecules can also diffuse through the membrane.
- Particles migrate from higher concentration to lower concentration.
  - Relies solely on the "Concentration Gradient" without any energy input.
- Larger molecules and charged ions cannot be diffused through the membrane; they require special channels or carriers.
Pure Phospholipid Bilayer:
- Without channels, some particles can still pass through via simple diffusion.
Key Requirements for Diffusion:
- Small size
- Non-charged nature
- Presence of a concentration gradient

Carrier Proteins:
- Molecules pass by binding to a specific binding site.
- Specificity in binding selects molecules that can fit into the binding site for passage (similar to enzyme-substrate interaction).
- Can actively carry molecules through the membrane.
Channel Proteins:
- Select molecules for passage based on size and electric charge.
- As long as molecules are small enough and carry the right charge, they can pass when the channel is open.
- Specificity differs from carrier proteins as binding is not required.
Key Difference:
- Carrier proteins require binding, while channel proteins do not.
Mechanism of Carrier Protein Transport:
1. Molecules bind to a specific binding site in the carrier protein.
2. The carrier protein undergoes a conformational change, opening the other side.
3. Molecules are released on the inner side of the cell, following the concentration gradient.
  - Carrier proteins can transport molecules along or against the concentration gradient.

Passive Transport:
- Occurs without energy input; molecules move across the membrane based on the concentration gradient.
- Can be facilitated by carrier proteins, channel proteins, or simple diffusion.
Active Transport:
- Requires energy to pump molecules against the concentration gradient.
- Exclusively performed by carrier proteins.
Active Transports rely on different energies:
- Coupled Transporters:
  - One target goes along the concentration gradient and the other one against.
  - The first one fuel the other one to go against the gradient.
- ATP-driven pumps:
  - Use the energy from ATP hydrolysis to pump molecules.
- Light-driven pumps:
  - Utilize energy from light (e.g., Bacteriorhodopsin).

Couple the transport of two solutes using one carrier.
- Unlikely to occur with channels directly.
Antiport:
- Two solutes are transported in opposite directions.
Symport:
- Two solutes are transported in the same direction.

State A:
- When Na^+ binds to the protein, it increases the protein's affinity for glucose.
- Glucose then binds to the protein.
State B:
- When Na^+ leaves the protein, the protein loses its affinity for glucose.
- Glucose disengages and leaves the protein.
Since Na^+ concentration is higher in the extracellular space than in the cytosol, it's easier for Na^+ to enter, driving glucose entry.
A coupled transporter can involve one active and one passive transport mechanism.
- Active transport helps passive transport without ATP or other energy input.
Glucose carrier in intestinal epithelium:
1. A Na^+-driven symport pumps glucose into the high-glucose cells.
2. Glucose is released to the other side of the epithelium through a passive uniport.
  - Against gradient, energy is required.
  - Along the gradient, energy is not required.
ATP-driven transporters can function as symporters or antiporters.
- Since both solutes rely on ATP for energy, they don't depend on each other's gradients.

Diffusion of water across a membrane along the concentration gradient.
- Passive process, not requiring ATP.
Consequences of Osmosis:
- In single-cell organisms, extracellular water concentration is typically higher.
  - Water diffuses into cells via osmosis.
  - Eventually, cells can burst.
Counteracting Osmosis:
- Animal cells maintain intracellular solute concentration by pumping out ions and balancing concentrations.
- Plant cells pump excess water to central vacuoles.
- Protozoan cells pump excess water out through contractile vacuoles.

Selectively allow certain ions to pass through.
- Selection is less specific than with carrier proteins.

Neurons include:
- Cell body
- Dendrites (receive signals)
- Axon (transmits signals)
Dendrites receive signals from other neurons.
Axons transmit signals down to the neuron terminal.

Achieved by depolarization of the neuron's membrane (axon).
- Resting potential exists when gates are closed, and ions flow across the membrane once opened.
Depolarization:
- Normally, the membrane is positively charged outside and negatively charged inside.
- A neuronal signal causes Na^+ influx, canceling out the polarization.
- V = -70 mV

Synapses transmit signals to the next neuron
A nerve impulse (action potential) triggers the opening of Ca^{2+} channels at the terminus.
- Ca^{2+} influx into the nerve terminal.
- triggers the release of neurotransmitters
Neurotransmitter receptors are also channels, opened by neurotransmitter reception.
Neurotransmitter Receptors:
- Neurotransmitter receptors are ion channels, opened when binding with neurotransmitters.
After neurotransmitters bind to receptors, ion channels open, allowing ions to flow in and initiate another action potential.

Psychoactive drugs exert their effects by influencing these synaptic transmission processes.