Cell Biology Part IV: Membrane Transport

Membrane Structure and Transport

Basic Membrane Structure

• The cell membrane is crucial for:
  • Transporting molecules across the membrane.
  • Facilitating transport within the cell.
  • Providing a structural framework within the cell.

Membrane Transport Overview

• Chapter 12: Membrane Transport
• Diffusion is the simplest form of transport.
• Membranes allow selective passage of molecules.
• Various transport systems exist, including:
  • Channel Proteins
  • Carrier Proteins
  • Passive Transport
  • Active Transport
  • Symport
  • Antiport
  • Osmosis
• Channel opening can be triggered by various stimuli.
• Neuronal signaling relies on membrane transport, including:
  • Action Potential
  • Synapses
  • Psychoactive drugs

Transport Mechanisms

• Transport across the membrane occurs via:
  • Direct diffusion through the phospholipid membrane (without a transporter).
  • Transport through specialized membrane proteins (transporters).
• Two main types of transporters:
  • Carrier Proteins
  • Channel Proteins
• Small, non-charged hydrophobic molecules can diffuse directly through the membrane by dissolving in the phospholipids.
• Small, non-charged hydrophilic molecules can also diffuse through the membrane.
• Diffusion: Movement of particles from an area of higher concentration to an area of lower concentration.
  • Driven solely by the concentration gradient (no energy input required).
• Larger molecules and charged ions cannot diffuse directly through the membrane; they require special channels or carriers.
• In a pure phospholipid bilayer (without proteins or channels), some particles can still pass through via simple diffusion.
• Diffusion relies on the concentration gradient.
• The concentration gradient can be considered a driving force.
• Molecules that rely on simple diffusion:
  • Small
  • Non-charged

Carrier Proteins vs. Channel Proteins

• Carrier Proteins: Transport molecules by binding to a specific binding site. This binding is highly specific, similar to enzyme-substrate interactions.
• Channel Proteins: Select molecules based on size and electric charge. If a molecule is small enough and has the appropriate charge, it can pass through when the channel is open.
• Key difference: The method of solute selection, which affects specificity.
  • Carrier proteins require binding.
  • Channel proteins do not require binding.
• Carrier proteins transport molecules via binding and conformational changes:
  1. Molecules bind to a specific binding site on the carrier protein.
  2. The carrier protein undergoes a conformational change, opening on the other side of the membrane.
  3. Molecules are released and move into the cell, following the concentration gradient.
  • Carrier proteins can transport molecules along or against the concentration gradient.

Passive vs. Active Transport

• Passive Transport: Does not require energy; molecules move across the membrane based on the concentration gradient.
  • Can occur via carrier proteins, channel proteins, or simple diffusion.
• Active Transport: Requires energy to pump molecules against the concentration gradient.
  • Only performed by carrier proteins.
• Active transport relies on different energy sources:
  • Coupled Transporters: One solute moves along its concentration gradient, providing the energy for another solute to move against its gradient.
  • ATP-driven pumps: Use energy from ATP hydrolysis to pump molecules.
  • Light-driven pumps: Use light energy (e.g., Bacteriorhodopsin).

Coupled Transporters

• Couple the transport of two solutes using one carrier protein.
  • Antiport: Two solutes are transported in opposite directions.
  • Symport: Two solutes are transported in the same direction.

Example of Coupled Transport

• State A: When Na^+ binds to the protein, it increases the protein's affinity for glucose, leading to glucose binding.
• State B: When Na^+ leaves the protein, the protein loses its affinity for glucose, causing glucose to disengage.
• Since Na^+ concentration is higher in the extracellular space, it readily enters the cell, driving the entry of glucose as well.
• Coupled transport can involve one active and one passive transport process.
• Active transport can facilitate passive transport without direct ATP or energy input.

Glucose Transport in Intestinal Epithelium

1. A Na^+-driven symport pumps glucose into cells, resulting in high intracellular glucose concentration.
2. Glucose is released to the other side of the epithelium through a passive uniport.

• Active transport moves molecules against their concentration gradient and requires energy.
• Passive transport moves molecules along their concentration gradient and does not require energy.

ATP-Driven Transporters

• Can function as symports or antiports.
• Both solutes rely on ATP for energy and do not depend on each other's concentration gradients.

Osmosis

• The diffusion of water across a membrane, moving from an area of higher water concentration to lower water concentration.
• Passive process (does not require ATP).

Osmosis in Single-Celled Organisms

• Typically, the extracellular water concentration is higher, causing water to diffuse into the cell via osmosis.
• Uncontrolled water influx can cause cells to burst.
• Mechanisms to remove excess water are necessary:
  • Animal cells: Maintain intracellular solute concentration by pumping out ions to balance the concentration across the membrane.
  • Plant cells: Pump excess water into a central vacuole.
  • Protozoan cells: Pump excess water out using contractile vacuoles.

Ion Channels

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

Triggering Channel Opening

• Once a channel is open, molecules flow through, driven by the concentration gradient.
• Voltage-gated channels: Open and close in response to changes in voltage across the membrane.
• Ligand-gated channels: Open and close in response to ligand binding.

Neuronal Signaling

• Neurons consist of:
  • Cell body
  • Dendrites
  • Axon
• Neurons send and receive signals.
• Dendrites: Receive signals from other neurons.
• Axon: Transmits signals to the neuron terminal.

Action Potential

• Achieved by depolarization of the neuron's membrane (axon).
• When the gate opens, ions flow in (resting potential).
• Depolarization: Normally, the membrane is positively charged outside and negatively charged inside. When a neuronal signal arrives, Na^+ ions flux in, neutralizing the polarity.
• Resting membrane potential: V = -70 mV

Synapses

• Synapses transmit signals to the next neuron.
• When an action potential reaches the terminus, it triggers the opening of Ca^{2+} channels, causing an influx of Ca^{2+} into the nerve terminal.
• Neurotransmitter receptors are also channels that open upon neurotransmitter binding.
• The influx of Ca^{2+} triggers the release of neurotransmitters, which bind to neurotransmitter receptors.
• Neurotransmitter receptors are ion channels that open upon binding with neurotransmitters, allowing ions to flow in and initiate another action potential.

Psychoactive Drugs

• Application of understanding membrane transport in the context of psychoactive drugs.