Chapter 11: Membrane Transport of Small Molecules and the Electrical Properties of Membranes

Transportation of molecules across lipid membrane

  • The lipid layer restricts the passage of most polar molecules.
  • This barrier often allows the balance in concentrations of solutes in the cytosol and the external environment.
  • To regulate and transport materials across the membrane. It uses specialized membrane proteins.
  • Cells often transfer larger molecules (macromolecules), that take place with a different mechanism.

Principles of Membrane Transport

  • The main classes of membrane proteins include:
    Ā Ā * Transporters: These undergo sequential conformational changes to transport the small molecules.
    Ā Ā * Channels: It forms narrow pores allowing passive transmembrane movement.
    Ā Ā * Transport through channels occurs much faster than transport mediated by transporters.
  • Protein-Free Lipid bilayers are impermeable to ions. The diffusion rate depends on the size of the molecule.
  • Transportation across a gradient:
    Ā Ā * Passive transport: All channels and many transporters allow solute to cross the membrane only passively.
    Ā Ā * Active transport: Here, it requires a certain form of energy to catalyze and

Ā Ā Ā Ā actively help the solute cross the membrane.
Ā Ā * Endocytosis: (bulk transport)
Ā Ā Ā Ā * Pinocytosis: ingestion of liquid material.
Ā Ā Ā Ā * Phagocytosis: ingestion of solid complex materials.
Ā Ā * Exocytosis: egestion of waste materials from cells through the plasma membrane.

  • Due to the concentration gradient and the potential difference across the membrane, there exists the membraneā€™s potential difference which also influences the transport.
Membrane Structure:
  • Cell membranes are composed of a phospholipid bilayer with embedded proteins.
  • The hydrophobic interior of the membrane acts as a barrier to the movement of polar molecules.
Selective Permeability:
  • Membranes are selectively permeable, allowing certain substances to pass while restricting others.
  • Permeability is determined by factors such as molecular size, charge, and lipid solubility.
Passive Transport:
  • Passive transport occurs without the input of energy.
  • Diffusion is the primary mechanism of passive transport, allowing molecules to move from an area of high concentration to an area of low concentration.
  • Simple diffusion involves the movement of small, non-polar molecules directly through the lipid bilayer.
  • Facilitated diffusion relies on carrier proteins or channel proteins to facilitate the movement of specific molecules across the membrane.
Active Transport:
  • Active transport requires the expenditure of energy (usually ATP) to move molecules against their concentration gradient.
  • Primary active transport involves the direct use of ATP to pump molecules across the membrane, often against a concentration gradient.
  • Secondary active transport utilizes the energy stored in an ion gradient to transport molecules against their gradient.
Endocytosis and Exocytosis:
  • Endocytosis is the process by which cells engulf substances from the extracellular environment by forming vesicles.
  • Exocytosis is the reverse process, where vesicles fuse with the membrane, releasing their contents into the extracellular space.
Membrane Potential:
  • Membrane potential refers to the voltage difference across the cell membrane, usually maintained by ion concentration gradients.
  • It is crucial for various cellular functions, such as nerve impulse transmission and muscle contraction.
Transporters:
  • Transporters are integral membrane proteins that facilitate the movement of specific molecules across the membrane.
  • They can operate through active or passive mechanisms, depending on the energy requirement and directionality of transport.
Specialized Transport Processes:
  • Some specialized transport processes include symport, antiport, and uniport.
  • Symport involves the transport of two different molecules in the same direction.
  • Antiport involves the transport of two different molecules in opposite directions.
  • Uniport refers to the transport of a single molecule or ion.

Channels and Electrical Properties of Membranes

Ion Channels
  • Ion channels are specialized membrane proteins that allow the selective passage of ions across the cell membrane.
  • They play a crucial role in maintaining the electrical properties of membranes.
Types of Ion Channels:
  • Voltage-gated ion channels: These channels open or close in response to changes in the membrane potential.
  • Ligand-gated ion channels: These channels open or close in response to the binding of specific molecules (ligands) to the channel.
  • Mechanically-gated ion channels: These channels open or close in response to mechanical stimuli such as pressure or stretch.
  • Leak channels: These channels are always open, allowing a small and constant flow of ions across the membrane.
Ion Selectivity:
  • Ion channels exhibit selectivity for specific ions based on their size, charge, and hydration.
  • For example, potassium channels are highly selective for potassium ions, while sodium channels preferentially allow the passage of sodium ions.
Conductance and Permeability:
  • Conductance refers to the ability of an ion channel to allow the flow of ions.
  • Permeability is the measure of the ease with which ions can pass through a channel.
  • Channels with high conductance and permeability facilitate the movement of ions more effectively.
Resting Membrane Potential:
  • Resting membrane potential is the electrical potential difference across the cell membrane when the cell is at rest.
  • In most cells, the resting membrane potential is around -70 millivolts (mV).
  • It is primarily determined by the differential distribution of ions (such as potassium, sodium, and chloride) across the membrane.
Action Potentials:
  • Action potentials are rapid and transient changes in the membrane potential that allow for long-range communication in excitable cells such as neurons and muscle cells.
  • They are initiated by a depolarization event that reaches a threshold, leading to the opening of voltage-gated ion channels.
Depolarization and Repolarization:
  • Depolarization refers to a change in the membrane potential towards a more positive value, typically caused by the influx of positively charged ions.
  • Repolarization is the process of returning the membrane potential back to its resting state after depolarization.
  • This is achieved by the efflux of positively charged ions or the influx of negatively charged ions.
Refractory Period:
  • After an action potential, there is a refractory period during which the membrane is temporarily unresponsive to further depolarization stimuli.
  • The refractory period ensures the proper propagation of action potentials and prevents overlapping signals.
Saltatory Conduction:
  • In myelinated neurons, action potentials jump from one node of Ranvier to the next, a process known as saltatory conduction.
  • This increases the speed and efficiency of electrical signal transmission along the axon.
Synaptic Transmission:
  • Synaptic transmission involves the release of neurotransmitters from the presynaptic neuron, which then bind to receptors on the postsynaptic neuron.
  • This binding can result in the opening or closing of ion channels, leading to changes in the postsynaptic membrane potential.