Human Physiology - Exocytosis and Membrane Potential
Human Physiology - Topic 2: Exocytosis and Membrane Potential
Exocytosis
Definition: Exocytosis is almost the reverse of endocytosis.
Purposes:
Secretion of large polar molecules: For example, hormones or enzymes.
Addition of components to the membrane: Such as channels or receptors.
Mechanism:
Secretory vesicles are produced by the Golgi complex and bud off from it.
These vesicles deliver their products to target sites by fusing with the plasma membrane and releasing their contents outside the cell.
Substances released via exocytosis includes:
Hormones
Proteins
Balance of Endocytosis and Exocytosis
Regulation: The rate of endocytosis and exocytosis is regulated to maintain a constant membrane surface area and stable cell volume.
Selectivity: Cells are selective about what enters and leaves, ensuring proper internal conditions and function.
Chapter in Perspective: Focus on Homeostasis
Homeostasis definition: The ability of cells to perform functions essential for their survival and specialized tasks that maintain homeostasis within the body.
Dependence: Successful, cooperative operation of the plasma membrane and intracellular components is crucial.
Energy Requirement: Many specialized tasks performed by cells that contribute to homeostasis require production of cellular energy and regulation of the intracellular environment.
Separation of Opposite Charges
Definition: The plasma membrane of all cells is polarized electrically, known as membrane potential.
Influence: Membrane potential is influenced by the permeability of certain important ions.
Concentration and Permeability of Ions
*Membrane potential in cells:
Constant membrane potential exists in both non-excitable and excitable cells at rest.
Measurement: Membrane potential is measured in millivolts.
Typical resting membrane potential: -70 ext{ mV} , indicating the inside of the cell is negative compared to the outside.
Summary of Ion Movement's Effect on Membrane Potential
Na-K Pump:
Contributes to resting membrane potential directly by transporting 3 Na⁺ out of the cell for every 2 K⁺ it pumps in.
The cell loses more positive charges than it gains, leading to a negative inside relative to outside.
Primary role: Actively maintain Na⁺ and K⁺ concentration gradients.
Movement of Potassium:
Potassium concentration is higher inside the cell.
This gradient favors movement of K⁺ outside down its concentration gradient.
As K⁺ leaves, the inside of the cell becomes more negative.
If K⁺ were the only ion moving, it would establish an equilibrium potential of -90 ext{ mV} .
Movement of Sodium:
Sodium concentration is high outside the cell.
This gradient favors movement of Na⁺ into the cell, down its concentration gradient.
Na⁺ acting alone would establish an equilibrium potential of +60 ext{ mV} .
Chloride Movement:
Cl⁻ is highly concentrated in the extracellular fluid.
Its equilibrium potential matches the typical resting membrane potential of -70 ext{ mV} , resulting in minimal net movement of Cl⁻ across the membrane at rest.
Specialized Use of Membrane Potential in Nerve and Muscle Cells
Excitable cells: Neurons and muscle cells can change their resting membrane potential to produce electrical signals.
Neurons: Use signals to receive, process, initiate, and transmit messages.
Muscle Cells: Use signals to initiate muscle contractions.
Depolarization and Hyperpolarization
Polarization: Any state where the membrane potential differs from 0 ext{ mV} .
Depolarization: Membrane becomes less polarized than at resting potential.
Repolarization: Membrane returns to resting potential after having been depolarized.
Hyperpolarization: Membrane becomes more polarized than at resting potential.
Electrical Signals and Ion Movement
Changes in membrane potential are caused by ion movement.
Ion movement changes due to changes in membrane permeability triggered by:
Electrical field alterations
Interaction with chemical messengers
Stimuli
Spontaneous changes of potential caused by leak-pump cycles.
Types of Ion Channels
Leak Channels:
Always open, allowing passive movement of ions down concentration gradients.
Help maintain resting membrane potential, particularly K⁺ leak channels.
Gated Channels:
Open or close in response to specific signals, including:
Voltage-gated: Activated by change in membrane potential.
Chemically gated (Ligand-gated): Open when a chemical messenger binds.
Mechanically gated: Open due to physical deformation (e.g., stretch or pressure).
Graded Potentials
Definition: Occur in a small, specialized area of excitable cell membranes.
Characteristics:
Magnitude of graded potential varies directly with the strength of the triggering event.
Usually produced by specific triggering events that cause gated channels (mostly Na⁺ channels) to open.
Passive Currents: When graded potentials occur:
Local depolarization occurs in the nerve or muscle cell membrane while inactive areas remain at resting potential.
Graded potential diminishes as it moves away from the active area due to the current loss across the plasma membrane as ions leak through open channels.
Action Potentials
Definition: Brief, rapid, large changes in membrane potential, where potential reverses.
Characteristics:
Involves only a small part of the overall excitable cell membrane.
Action potentials do not decrease in strength as they propagate from their initiation point.
Serve as long-distance signals in the nervous system.
Reversal of Membrane Potential: Depolarization slowly progresses until it reaches a critical level known as threshold potential.
After Hyperpolarization Phase: Following repolarization, potential may overshoot, resulting in a brief phase called after hyperpolarization.
Changes in Membrane Permeability
Action potentials arise from significant alterations in Na⁺ and K⁺ permeability, enabling quick ion fluxes down their electrochemical gradients.
Voltage-gated Channels:
Voltage-gated Na⁺ channels consist of an activation gate and an inactivation gate.
Voltage-gated K⁺ channels contain only a single gate which can either be closed or open.
Action Potential Mechanism
Signal Nature: A brief electrical signal travelling along nerve or muscle cells caused by ion movements.
At resting potential, channels are closed, and ions do not move.
Na⁺ channels' activation gates open during depolarization due to positive feedback.
Sodium channels inactive, allowing K⁺ channels to open, leading to membrane repolarization.
Restoration of Concentration Gradient: The Na-K pump gradually restores the disrupted concentration gradients following action potentials:
Na⁺ is pumped into the extracellular fluid (ECF).
K⁺ is pumped into the intracellular fluid (ICF).
Propagation: Action potentials are propagated from the axon hillock to the axon terminals.
Basic Parts of Neuron (Nerve Cell)
Cell Body: Houses the nucleus and organelles.