Cell Membrane Transport and Neuronal Action Potentials: Study Notes

Cell Membrane Transport Mechanisms

Simple Diffusion

Simple diffusion is the passive movement of solute molecules across the plasma membrane down their concentration gradient, without the aid of transport proteins or energy input.

• Definition: Movement of solute from an area of higher concentration to an area of lower concentration through the lipid bilayer.

• Key Features:

  • No energy (ATP) required

  • No transport protein required

  • Solutes must be small, nonpolar, or lipid-soluble

• Examples: Oxygen, Carbon Dioxide, Lipids

Facilitated Diffusion

Facilitated diffusion is a passive process where solutes move down their concentration gradient with the help of membrane proteins.

• Definition: Movement of solute with its concentration gradient via a channel or carrier protein; no energy required.

• Key Features:

  • Transport proteins required (channels or carriers)

  • No ATP required

  • Allows passage of ions and polar molecules

• Examples: Sodium ions, Potassium ions, Calcium ions, Glucose, Amino acids

Osmosis

Osmosis is the passive movement of water molecules across a selectively permeable membrane.

• Definition: Movement of water from an area of lower solute concentration to an area of higher solute concentration through a selectively permeable membrane.

• Key Features:

  • Water moves to balance solute concentrations

  • No energy required

• Examples: Water absorption from the intestinal lining, Water reabsorption from the kidneys, Water movement between extracellular fluid (ECF) and blood vessels

Primary Active Transport

Primary active transport uses energy from ATP hydrolysis to move solutes against their concentration gradients.

• Definition: Movement of solute against its concentration gradient using ATP.

• Key Features:

  • Direct use of ATP

  • Transport proteins (pumps) required

• Example: Na+/K+ ATPase pump moves 3 Na+ out and 2 K+ into the cell against their gradients.

Secondary Active Transport

Secondary active transport uses the energy stored in the concentration gradient of one solute (often Na+) to drive the movement of another solute against its gradient.

• Definition: An ATPase pump drives a solute out of (or into) the cell, establishing a concentration gradient. Movement of another solute is coupled to this gradient, allowing transport against its own gradient without direct ATP use.

• Key Features:

  • Indirect use of ATP

  • Symporters and antiporters involved

• Example: Sodium-glucose symporter uses Na+ gradient to bring glucose into the cell.

Vesicular Transport

Phagocytosis

Phagocytosis is a form of endocytosis where large particles are engulfed by the cell.

• Definition: "Cell eating"; ingestion of large particles via a phagosome; ATP required.

• Examples: Ingestion of bacteria and cell debris by phagocytes

Pinocytosis

Pinocytosis is a form of endocytosis where the cell engulfs extracellular fluid and dissolved solutes.

• Definition: "Cell drinking"; bringing substances in the ECF into the cell via a protein-coated vesicle; ATP required.

• Example: Nutrient transport

Receptor-Mediated Endocytosis

This process allows cells to selectively take in specific molecules using receptor proteins.

• Definition: Bringing a specific substance into a transport vesicle using receptors on the plasma membrane; ATP required.

• Examples: Cholesterol, iron, and hormone transport

Exocytosis

Exocytosis is the process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Definition: Release of substances from the cell via an exocytic transport vesicle that fuses with the plasma membrane; ATP required.

• Examples: Release of proteins and glycoproteins into the ECF, secretion of hormones, neurotransmitters, and enzymes, adding components to the plasma membrane

Osmosis and Tonicity: Effects on Red Blood Cells

Osmosis affects cell volume depending on the tonicity of the surrounding solution. Tonicity describes the ability of a solution to change the volume of a cell by osmosis.

Solution	Sketch of RBC	Change in Cell Volume	Tonicity
0.9% NaCl Solution	Normal, biconcave shape	No change	Isotonic
Distilled Water	Swollen, may burst (lyse)	Increase in cell volume	Hypotonic
10% NaCl Solution	Shriveled (crenated)	Decrease in cell volume	Hypertonic

Structure of a Neuron

Neurons are specialized cells for communication in the nervous system. They have distinct structural regions that support their function.

• Dendrite: Receives incoming signals from other neurons.

• Cell Body (Soma): Contains the nucleus and organelles; integrates signals.

• Axon Hillock: Initiates action potentials.

• Axon: Conducts action potentials away from the cell body.

• Myelin Sheath: Insulates the axon, increasing conduction speed.

• Node of Ranvier (Myelin Sheath Gap): Gaps in myelin where action potentials are regenerated.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Action Potential: Generation and Propagation

An action potential is a rapid, temporary change in membrane potential that travels along the axon of a neuron, enabling neural communication.

1. Resting State: The plasma membrane is at resting membrane potential (typically around -70 mV). Voltage-gated Na+ and K+ channels are closed.

2. Depolarization: A stimulus depolarizes the membrane to threshold, voltage-gated Na+ channels open, and Na+ enters the cell, making the inside more positive.

3. Repolarization: Na+ channels inactivate, voltage-gated K+ channels open, and K+ exits the cell, restoring negativity inside.

4. Return to Resting State: K+ channels remain open, allowing continued K+ efflux, and the membrane potential returns to resting levels.

5. Hyperpolarization: The membrane may become more negative than resting potential before K+ channels close, after which the membrane returns to resting potential.

Key Terms

• Threshold: The critical level to which a membrane potential must be depolarized to initiate an action potential.

• Depolarization: Decrease in membrane potential difference (inside becomes less negative).

• Repolarization: Return of the membrane potential to resting value after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than the resting potential.

Relevant Equation

The Nernst equation can be used to calculate the equilibrium potential for an ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Where: R = gas constant T = temperature (Kelvin) z = charge of the ion F = Faraday's constant [ion]outside = concentration of ion outside the cell [ion]inside = concentration of ion inside the cell

Additional info: The notes above integrate and expand on the provided diagrams and text, ensuring a comprehensive overview of membrane transport and neuronal action potentials for Anatomy & Physiology students.