Cell Membrane Transport and Signaling: Study Notes for Anatomy & Physiology

Cell Membrane Transport Mechanisms

Simple Diffusion

Simple diffusion is a passive process where molecules move spontaneously from areas of high concentration to low concentration, driven by their kinetic energy.

• Definition: Movement of molecules down their concentration gradient without energy input.

• Equilibrium: Achieved when there is no net movement of molecules.

• Relevant Molecules: O2, CO2, and other small nonpolar molecules diffuse easily.

• Factors Affecting Diffusion Rate:

  • Concentration: Greater difference increases diffusion rate.

  • Size: Smaller molecules diffuse faster than larger ones.

  • Temperature: Higher temperature increases kinetic energy and diffusion rate.

• Dialysis: Diffusion through a selectively permeable membrane, used in medical applications such as hemodialysis for kidney failure.

Facilitated Diffusion

Facilitated diffusion is a passive, energy-independent process where substances move across the plasma membrane via protein carriers or channels.

• Carrier-Mediated Facilitated Diffusion:

  • Carrier proteins bind to the substance and transport it across the membrane.

  • Transport rate is limited by the number of available carriers.

  • Example: Glucose transport into muscle cells.

• Channel-Mediated Facilitated Diffusion:

  • Uses specific transmembrane channel proteins.

  • Channels may be gated or open in response to stimuli.

  • Example: Ion channels for Na+, K+, Cl-.

Osmosis and Osmotic Pressure

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, typically through aquaporin channels.

• Definition: Movement of water from low solute concentration to high solute concentration.

• Osmotic Pressure: The pressure needed to prevent osmosis or the pressure osmosis creates.

• Osmolarity: The concentration of solute particles, measured in milliosmoles (mOsm).

Osmosis & Cells

• Hypotonic Solution: Lower solute concentration than the cell; water enters the cell, causing swelling or lysis.

• Hypertonic Solution: Higher solute concentration than the cell; water leaves the cell, causing shrinkage (crenation).

• Isotonic Solution: Equal solute concentration; no net water movement.

Applications: Medical use of isotonic IV fluids, effects of diabetes mellitus on urine output, and the role of diuretics.

Active Transport Mechanisms

Primary Active Transport

Primary active transport uses energy (usually ATP) to move substances against their concentration gradients via carrier proteins.

• Example: Na+/K+ pump (Na+/K+ ATPase)

  • Moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

  • Maintains the resting membrane potential.

Secondary Active Transport

Secondary active transport uses the energy from the movement of one substance down its gradient to drive another substance against its gradient.

• Symport: Both substances move in the same direction.

• Antiport: Substances move in opposite directions.

• Example: Glucose transport in the intestine coupled with Na+ movement.

Endocytosis and Vesicular Transport

Endocytosis

Endocytosis is the process of taking in large molecules or particles by engulfing them in vesicles formed from the plasma membrane.

• Phagocytosis: "Cell eating" by specialized cells (e.g., white blood cells).

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Receptor-Mediated Endocytosis: Selective uptake of specific molecules via receptor proteins.

Phagocytosis Steps

1. Phagocyte binds to the particle via pseudopodia.

2. Particle is engulfed, forming a phagosome.

3. Phagosome fuses with a lysosome for digestion.

4. Waste is expelled by exocytosis or the cell dies.

Transcytosis

Transcytosis is the movement of substances into, through, and out of cells in vesicles, important in capillary endothelial cells.

Exocytosis and Secretion

Exocytosis is the process of vesicles fusing with the plasma membrane to release their contents outside the cell.

• Examples: Hormone secretion, neurotransmitter release.

Resting Membrane Potential (RMP)

The resting membrane potential is the voltage difference across the plasma membrane of cells, especially muscle and nerve cells, due to ion distribution.

• Polarized Membrane: The inside is more negative than the outside.

• Measured in: Millivolts (mV).

• Key Ions: Na+ (higher outside), K+ (higher inside), Cl-, and proteins (anions).

• Maintained by: Active transport (Na+/K+ pump).

• Changes in RMP: Lead to nerve impulses and action potentials.

Formula:

$V_m = V_{inside} - V_{outside}$

Cell Adhesion Molecules (CAMs) & Cell Interactions

Role of the Plasma Membrane in Cell Interactions

The plasma membrane contains various proteins that mediate cell adhesion, signaling, and communication.

• CAMs: Facilitate tissue formation, cell communication, and immune responses.

• Contact Signaling: Important for development, wound healing, and immunity.

Chemical Signaling & Signal Transduction

G Protein-Coupled Receptors (GPCRs)

GPCRs are a large family of membrane receptors that mediate responses to hormones, neurotransmitters, and other signals.

• Mechanism:

  1. Ligand binds to receptor.

  2. Receptor activates G protein.

  3. G protein activates an effector enzyme (e.g., adenylyl cyclase).

  4. Second messenger (e.g., cAMP) is produced.

  5. Cellular response is triggered.

• Effects: Can include changes in metabolism, gene expression, or ion channel activity.

Example: Epinephrine binding to its receptor increases heart rate and force of contraction.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Carrier/Channel?	Direction	Example
Simple Diffusion	No	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Yes	Down gradient	Glucose, ions
Osmosis	No	Yes (aquaporins)	Down gradient	Water
Primary Active Transport	Yes (ATP)	Yes	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect	Yes	Against gradient (one substance)	Na+/glucose symport
Endocytosis	Yes	No	Into cell	Phagocytosis
Exocytosis	Yes	No	Out of cell	Neurotransmitter release

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the mechanisms of GPCRs and the summary table.

• Scientific terms were defined and examples provided for better understanding.