Cell Membrane Transport and Homeostasis: Study Guide

Chapter 5: Cell Membrane Transport and Homeostasis

Overview

This chapter covers the fundamental concepts of membrane transport, the maintenance of homeostasis, and the mechanisms by which substances move across cell membranes. Understanding these processes is essential for comprehending how cells interact with their environment and maintain internal stability.

Fluid Compartments and Homeostasis

Fluid Compartments

• Intracellular Fluid (ICF): The fluid within cells, making up about two-thirds of total body water.

• Extracellular Fluid (ECF): The fluid outside cells, including plasma and interstitial fluid.

Homeostasis, Equilibrium, and Disequilibrium

• Homeostasis: The maintenance of a stable internal environment despite external changes. It involves dynamic processes that detect and respond to deviations from set points.

• Equilibrium: A state where opposing forces or influences are balanced. In physiology, true equilibrium is rare; most systems are in a state of dynamic disequilibrium.

• Disequilibrium: A condition where concentrations of substances are not equal across compartments, but are maintained by active processes (e.g., Na+ and K+ gradients across the cell membrane).

Types of Membrane Transport

Active vs. Passive Transport

• Passive Transport: Movement of substances across the membrane without energy input. Driven by concentration or electrochemical gradients.

• Active Transport: Movement of substances against their gradient, requiring energy (usually from ATP).

• Major Energy Source: ATP (adenosine triphosphate) is the primary energy source for active transport.

Types of Passive Transport

• Simple Diffusion: Movement of molecules from high to low concentration directly through the lipid bilayer.

• Facilitated Diffusion: Movement of molecules via membrane proteins (channels or carriers) down their concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Types of Active Transport

• Primary Active Transport: Direct use of ATP to move substances (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., Na+-glucose symporter).

• Vesicular Transport: Movement of large particles or fluids via vesicles (endocytosis, exocytosis, transcytosis).

Diffusion and Fick's Law

Definition and Principles

• Diffusion: The net movement of molecules from an area of higher concentration to an area of lower concentration.

• Fick's Law of Diffusion: Describes the rate of diffusion across a membrane.

$\text{Rate of diffusion} = (\text{surface area}) \times (\text{concentration gradient}) \times (\text{membrane permeability})$

• Membrane Permeability: Depends on lipid solubility, molecular size, membrane thickness, and composition.

Factors Affecting Diffusion

• Concentration gradient

• Membrane surface area

• Membrane permeability

• Molecular size and lipid solubility

Osmosis and Tonicity

Osmosis

• Osmosis: The movement of water across a selectively permeable membrane from an area of low solute concentration to high solute concentration.

Tonicity: Hypotonic, Isotonic, and Hypertonic Solutions

• Hypotonic Solution: Lower solute concentration than the cell; water enters the cell, causing it to swell or lyse.

• Isotonic Solution: Equal solute concentration; no net water movement, cell remains unchanged.

• Hypertonic Solution: Higher solute concentration than the cell; water leaves the cell, causing it to shrink (crenate).

Red Blood Cell Responses

• In hypotonic solutions: cells swell and may burst.

• In isotonic solutions: cells maintain normal shape.

• In hypertonic solutions: cells shrink.

Facilitated Diffusion: Channels and Carriers

Channel Proteins

• Channel Proteins: Form water-filled passages for specific ions or molecules. Can be gated (voltage, ligand, or mechanically gated).

• Examples: Voltage-gated Na+ channels in neurons.

Carrier Proteins

• Carrier Proteins: Bind to specific molecules and undergo conformational changes to transport them across the membrane.

• Types: Uniport (one substance), Symport (two substances, same direction), Antiport (two substances, opposite directions).

• Examples: Glucose transporters (GLUT), Na+/K+ pump.

Active Transport Mechanisms

Primary vs. Secondary Active Transport

• Primary Active Transport: Direct use of ATP (e.g., Na+/K+ ATPase).

• Secondary Active Transport: Uses the gradient established by primary active transport to move other substances.

Examples of Secondary Active Transporters

Symport Carriers	Antiport Carriers
Na+-glucose (SGLT)	Na+/Ca2+ (NCX)
Na+-amino acids	Na+/H+
Na+-phosphate	Cl-/HCO3-
Na+-iodide	Additional info: Other antiporters may include K+/H+ exchangers.

Vesicular Transport

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., neurotransmitter release).

• Endocytosis: Cell engulfs substances into vesicles (includes phagocytosis, pinocytosis, receptor-mediated endocytosis).

• Transcytosis: Transport of substances across a cell via vesicles.

Membrane Potential and Electrochemical Gradients

Resting Membrane Potential

• Resting Membrane Potential: The electrical potential difference across the cell membrane at rest, typically around -70 mV in neurons.

• Generated by differences in ion concentrations and selective permeability of the membrane.

Electrochemical Gradient

• Electrochemical Gradient: The combined effect of the concentration gradient and electrical gradient on ion movement.

Equilibrium Potential and Nernst Equation

• Equilibrium Potential: The membrane potential at which there is no net movement of a particular ion across the membrane.

• Nernst Equation: Calculates the equilibrium potential for an ion:

$E_{ion} = \frac{61}{z} \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Where z is the charge of the ion, and [ion]out and [ion]in are the extracellular and intracellular concentrations, respectively.

Integrated Membrane Transport

• Membrane transport processes are integrated in physiological functions, such as beta cells in the pancreas releasing insulin in response to glucose uptake.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (gradient)	Against gradient	Na+-glucose symporter
Osmosis	No	Water down gradient	Water movement in kidneys
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Key Terms

• Homeostasis

• Equilibrium

• Disequilibrium

• Diffusion

• Osmosis

• Tonicity

• Facilitated Diffusion

• Active Transport

• Electrochemical Gradient

• Resting Membrane Potential

• Nernst Equation

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard academic context.