Membrane Dynamics and Transport Mechanisms (Chapter 5 Study Guide)

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Membrane Dynamics

Fluid Compartments and Homeostasis

The human body maintains distinct fluid compartments separated by cell membranes, which are essential for physiological homeostasis. Homeostasis refers to the maintenance of a stable internal environment despite external changes.

Intracellular fluid (ICF): Fluid within cells.
Extracellular fluid (ECF): Fluid outside cells, including plasma and interstitial fluid.
Homeostasis: Dynamic equilibrium of physiological variables (e.g., temperature, pH, ion concentrations).
Equilibrium vs. Disequilibrium: Equilibrium is a balanced state; disequilibrium occurs when there is an imbalance, such as ion gradients across membranes.
Example: Sodium (Na+) is higher in ECF, potassium (K+) is higher in ICF.

Types of Membrane Transport

Passive vs. Active Transport

Transport across cell membranes can be classified as passive (no energy required) or active (energy required).

Passive Transport: Movement of substances down their concentration or electrochemical gradient without energy input.
Active Transport: Movement of substances against their gradient, requiring energy (usually ATP).
Major energy source for active transport: ATP hydrolysis.
Example: Sodium-potassium pump (Na+/K+ ATPase).

Types of Passive Transport

Simple Diffusion: Movement of molecules from high to low concentration directly through the lipid bilayer.
Facilitated Diffusion: Movement via membrane proteins (channels or carriers).
Osmosis: Diffusion of water across a selectively permeable membrane.

Fick's Law of Diffusion

Describes the rate of diffusion across a membrane:

J: Rate of diffusion
D: Diffusion coefficient
dC/dx: Concentration gradient

Osmosis and Tonicity

Osmosis is the movement of water in response to solute concentration differences. Tonicity describes the effect of a solution on cell volume.

Isotonic: No net water movement; cell volume remains unchanged.
Hypotonic: Water enters the cell; cell swells and may burst.
Hypertonic: Water leaves the cell; cell shrinks.
Example: Red blood cells in different solutions (see diagram).

Facilitated Diffusion

Channel Proteins vs. Carrier Proteins

Facilitated diffusion uses membrane proteins to transport substances.

Channel Proteins: Form water-filled pores for rapid transport (e.g., ion channels).
Carrier Proteins: Bind and transport molecules by changing shape (e.g., glucose transporter).
Example: Aquaporins (water channels), GLUT transporters (glucose).

Active Transport Mechanisms

Primary vs. Secondary Active Transport

Primary Active Transport: Direct use of ATP to move substances (e.g., Na+/K+ ATPase).
Secondary Active Transport: Uses energy from the gradient created by primary active transport (e.g., Na+-glucose symporter).

Major Secondary Active Transporters

Symport Carrier	Substrates
Na+-glucose (SGLT)	Na+, glucose
Na+-amino acid	Na+, amino acids
Na+-HCO3-	Na+, HCO3-
Na+-Cl-	Na+, Cl-
Na+-K+-2Cl-	Na+, K+, 2Cl-
Antiport Carrier	Substrates
Na+-H+	Na+, H+
Na+-Ca2+	Na+, Ca2+

Vesicular Transport

Endocytosis and Exocytosis

Large molecules and particles are transported via vesicles.

Endocytosis: Uptake of substances into the cell by vesicle formation.
Exocytosis: Release of substances from the cell via vesicle fusion with the membrane.
Example: Neurotransmitter release, hormone secretion.

Transcytosis

Transcytosis is the transport of macromolecules across the interior of a cell, often seen in capillary endothelium.

Example: Movement of antibodies across epithelial cells.

Membrane Potential and Electrical Gradients

Resting Membrane Potential

The cell membrane separates electrical charges, creating a voltage difference known as the resting membrane potential (typically -70 mV in neurons).

Electrochemical Gradient: Combination of concentration and electrical gradients.
Equilibrium Potential: The membrane potential at which there is no net movement of a particular ion.
Nernst Equation: Used to calculate equilibrium potential for an ion:

z: Charge of the ion
[ion]out: Extracellular concentration
[ion]in: Intracellular concentration

Summary Table: Types of Membrane Transport

Type	Energy Required?	Example
Simple Diffusion	No	O2, CO2
Facilitated Diffusion	No	Glucose via GLUT
Osmosis	No	Water via aquaporins
Primary Active Transport	Yes (ATP)	Na+/K+ ATPase
Secondary Active Transport	Yes (gradient)	Na+-glucose symporter
Endocytosis/Exocytosis	Yes (ATP)	Neurotransmitter release

Additional info:

Membrane transport and electrical gradients are fundamental for processes such as nerve impulse transmission and hormone secretion.
Disequilibrium of ions across membranes is essential for cell signaling and muscle contraction.