Quiz 9

Membrane Transport

Basic Membrane Structure

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

How Does the Membrane Transport?

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

Other Transport Systems

• 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 and Transport

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

Transport Across Membrane

• 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

Transporters: Carrier Proteins and Channel Proteins

• 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 and Active Transport

• 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).

Coupled Transporters

• 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.

How Coupled Transporters Work (Example: Na+/Glucose Symport)

• 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.

Osmosis

• 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.

Ion Channels

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

Gated Ion Channels

• Different stimuli trigger protein channels to open:
  • When the channel is opened, molecule will flow though, driven by concentration gradient.
  • Voltage-gated:
    • The gate opens and closes depending on voltage changes across the membrane.
  • Ligand-gated:
    • The gate opens and closes depending on ligand binding.

Neuronal Signaling

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

Action Potential

• 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

• 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.

Application: Psychoactive Drugs

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