Chapter 3 Part A: The Plasma Membrane and Membrane Transport

Plasma Membrane Structure and Function

Overview of Plasma Membrane Functions

The plasma membrane is a dynamic structure that surrounds the cell, separating the internal environment from the external environment. It plays a critical role in maintaining cellular homeostasis by regulating the movement of substances into and out of the cell.

• Physical barrier: Separates cell contents from the extracellular fluid.

• Selective permeability: Controls entry and exit of ions, nutrients, and waste products.

• Communication: Contains receptors for signal transduction.

• Cell recognition: Glycoproteins and glycolipids allow cells to recognize each other.

Selective Permeability

Selectively permeable (or semipermeable) means that the plasma membrane allows some substances to pass through while restricting others.

• Permits passage of small, nonpolar molecules (e.g., O2, CO2).

• Restricts large or charged molecules unless specific transport mechanisms are present.

Plasma Membrane Structure

Major Components and Their Functions

• Phospholipids: Form the basic structure as a lipid bilayer, with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stabilizes membrane fluidity and integrity.

• Glycolipids: Lipids with attached carbohydrate chains; involved in cell recognition.

• Membrane proteins: Embedded or attached proteins with various functions (see below).

Types of Membrane Proteins

• Integral membrane proteins: Span the lipid bilayer; involved in transport, signaling, and cell adhesion.

• Peripheral membrane proteins: Loosely attached to the membrane surface; function as enzymes or structural attachments.

• Glycoproteins: Proteins with carbohydrate chains; play roles in cell recognition and immune response.

Examples of Membrane Proteins and Their Functions

• Ion channels: Allow specific ions to cross the membrane (e.g., sodium channels in neurons).

• Enzymes: Catalyze reactions at the membrane surface (e.g., ATPase pumps).

• Receptors: Bind signaling molecules (e.g., insulin receptors on muscle cells).

• Transporters (carriers): Move substances across the membrane (e.g., glucose transporters in red blood cells).

Membrane Transport Mechanisms

Passive vs. Active Transport

• Passive transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis).

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

Factors Increasing Rate of Diffusion

• Higher temperature

• Greater concentration gradient

• Smaller molecule size

• Increased membrane surface area

• Increased membrane permeability

Definitions of Key Transport Terms

• Simple diffusion: Movement of molecules from high to low concentration directly through the lipid bilayer (e.g., O2, CO2).

• Facilitated diffusion: Movement of molecules via membrane proteins (channels or carriers), still down the concentration gradient (e.g., glucose transport).

• Primary active transport: Direct use of ATP to move substances against their gradient (e.g., Na+/K+ pump).

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

• Endocytosis: Bulk transport of substances into the cell via vesicles.

  • Receptor-mediated endocytosis: Specific uptake of molecules after binding to receptors (e.g., LDL cholesterol uptake).

  • Phagocytosis: "Cell eating"; engulfment of large particles (e.g., macrophages ingesting bacteria).

  • Bulk-phase endocytosis (pinocytosis): "Cell drinking"; nonspecific uptake of extracellular fluid.

• Exocytosis: Vesicular transport of substances out of the cell (e.g., neurotransmitter release).

• Transcytosis: Transport of substances across a cell by vesicle formation (e.g., antibody transport across endothelium).

• Osmosis: Diffusion of water across a selectively permeable membrane from low solute to high solute concentration.

Examples in the Body

• Simple diffusion: Oxygen entering red blood cells in the lungs.

• Facilitated diffusion: Glucose uptake into muscle cells via GLUT transporters.

• Primary active transport: Na+/K+ ATPase in nerve cells.

• Secondary active transport: Na+-glucose symport in intestinal cells.

• Phagocytosis: White blood cells engulfing bacteria.

• Exocytosis: Release of insulin from pancreatic beta cells.

• Osmosis: Water reabsorption in kidney tubules.

Osmosis and Tonicity

Hypertonic, Hypotonic, and Isotonic Solutions

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic solution: Higher solute concentration outside the cell; water moves out, causing cell shrinkage (crenation).

• Hypotonic solution: Lower solute concentration outside the cell; water moves in, causing cell swelling or lysis.

Body Situations Causing Hypertonic or Hypotonic Environments

• Hypertonic environment: Severe dehydration increases blood osmolarity, making extracellular fluid hypertonic to cells.

• Hypotonic environment: Overhydration or rapid IV infusion of pure water can make extracellular fluid hypotonic, leading to cell swelling.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 into cells
Facilitated Diffusion	No	Down gradient	Glucose into muscle
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against gradient	Na+-glucose symport
Osmosis	No	Water down gradient	Kidney water reabsorption
Endocytosis	Yes (ATP)	Into cell	LDL uptake
Exocytosis	Yes (ATP)	Out of cell	Neurotransmitter release

Key Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Osmotic Pressure Equation:

$\Pi = iMRT$

• Where $\Pi$ is osmotic pressure, i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature in Kelvin.

Additional info: Academic context and examples have been added to clarify and expand upon the original study guide points.