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:

  • Transporters: These undergo sequential conformational changes to transport the small molecules.
  • Channels: It forms narrow pores allowing passive transmembrane movement.
  • 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:

  • Passive transport: All channels and many transporters allow solute to cross the membrane only passively.
  • Active transport: Here, it requires a certain form of energy to catalyze and

  actively help the solute cross the membrane.

  • Endocytosis: (bulk transport)
  • Pinocytosis: ingestion of liquid material.
  • Phagocytosis: ingestion of solid complex materials.
  • 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 membrane’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.

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