Cell Membranes, Transport Mechanisms, and Cell Junctions: Study Notes for Anatomy & Physiology

Cell Membranes and Membrane Proteins

Structure and Function of the Cell Membrane

The cell membrane (also called the plasma membrane) is a selectively permeable barrier that surrounds the cell, maintaining homeostasis and mediating communication with the environment.

• Phospholipid bilayer: Composed of hydrophilic heads and hydrophobic tails, forming a double layer.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, enzymes, and anchors.

• Cholesterol: Stabilizes membrane fluidity and structure.

• Carbohydrates: Attached to proteins and lipids, forming glycoproteins and glycolipids for cell recognition.

Functions:

• Physical barrier

• Regulation of exchange with the environment

• Communication and signaling

• Structural support

Types of Membrane Proteins

• Channel proteins: Form pores for passive movement of ions and water.

• Carrier proteins: Bind and transport specific substances across the membrane.

• Receptor proteins: Bind signaling molecules and initiate cellular responses.

• Enzymes: Catalyze reactions at the membrane surface.

• Anchoring proteins: Attach the membrane to cytoskeleton or extracellular matrix.

• Cell identity markers: Glycoproteins that help cells recognize each other.

Transport Across Cell Membranes

Passive Transport Mechanisms

Passive transport does not require cellular energy (ATP) and relies on concentration gradients.

• Simple diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated diffusion: Movement of larger or polar molecules via channel or carrier proteins.

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

Factors affecting diffusion rate:

• Concentration gradient

• Temperature

• Molecule size

• Membrane permeability

Equation for rate of diffusion:

$J = -D \frac{dC}{dx}$ Where: $J$ = flux (rate of movement) $D$ = diffusion coefficient $\frac{dC}{dx}$ = concentration gradient

Active Transport Mechanisms

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Primary active transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary active transport: Uses energy from the movement of one substance down its gradient to move another substance against its gradient (e.g., symport and antiport systems).

• Vesicular transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Example: Sodium-Potassium Pump

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

• Maintains electrochemical gradients essential for nerve and muscle function.

Equation for sodium-potassium pump:

$3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Tonicity and Osmosis

Definitions and Effects of Tonicity

Tonicity describes the effect of a solution on cell volume due to water movement.

• Isotonic solution: No net movement of water; cell volume remains unchanged.

• Hypotonic solution: Water enters the cell; cell swells and may lyse.

• Hypertonic solution: Water leaves the cell; cell shrinks (crenates).

Clinical relevance: Intravenous fluids must be isotonic to prevent cell damage.

Osmosis and Osmotic Pressure

• Osmosis: Movement of water from an area of low solute concentration to high solute concentration.

• Osmotic pressure: Pressure required to prevent osmosis.

Equation for osmotic pressure:

$\Pi = iMRT$ Where: $\Pi$ = osmotic pressure $i$ = van 't Hoff factor (number of particles per molecule) $M$ = molarity $R$ = gas constant $T$ = temperature (Kelvin)

Cell Junctions

Types and Functions of Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, providing communication and structural integrity.

• Tight junctions: Seal adjacent cells to prevent passage of substances between them; found in epithelial tissues.

• Desmosomes: Anchor cells together, providing mechanical strength; common in skin and heart tissue.

• Gap junctions: Allow direct communication between cells via connexon channels; important in cardiac and smooth muscle.

Comparison of Cell Junctions

Junction Type	Main Function	Location
Tight Junction	Barrier to solute movement	Epithelium (e.g., intestines)
Desmosome	Mechanical attachment	Skin, heart
Gap Junction	Cell-cell communication	Cardiac muscle, smooth muscle

Additional Info

• Endocytosis: Process by which cells engulf substances into vesicles (includes phagocytosis and pinocytosis).

• Exocytosis: Release of substances from cells via vesicles.

• Clinical application: Disorders of membrane transport can lead to diseases such as cystic fibrosis (defective chloride channel).