Membrane Structure and Function: Study Notes (Campbell Biology, Chapter 7)

Membrane Structure and Function

Overview of Plasma Membrane Regulation

The plasma membrane is a dynamic structure that controls the movement of substances into and out of the cell. It achieves this regulation through several mechanisms, ensuring cellular homeostasis and communication with the environment.

• Passive Transport: Movement of small molecules across the membrane without energy input, often via diffusion or transport proteins.

• Active Transport: Movement of small molecules against their concentration gradient, requiring energy (usually ATP) and transport proteins.

• Bulk Transport: Movement of large molecules via vesicles, including exocytosis (out of the cell) and endocytosis (into the cell).

Amphipathic Nature of Membrane Components

Cellular membranes are primarily composed of amphipathic phospholipids, which have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This dual nature is essential for forming the bilayer structure of membranes.

• Phospholipid Bilayer: Hydrophobic tails face inward, shielded from water, while hydrophilic heads face outward toward the aqueous environment.

• Proteins: Integral and peripheral proteins are embedded or attached to the membrane, with hydrophilic regions exposed to water and hydrophobic regions interacting with the lipid core.

Fluid Mosaic Model

The fluid mosaic model describes the membrane as a mosaic of protein molecules floating in a fluid bilayer of phospholipids. This model explains both the flexibility and the functional diversity of membranes.

• Fluidity: Lipids and some proteins can move laterally within the membrane; rarely, lipids may flip-flop between layers.

• Mosaic: Proteins are not randomly distributed but often form functional groups.

Membrane Fluidity: Role of Fatty Acids and Cholesterol

Membrane fluidity is influenced by the types of fatty acids present and by cholesterol, which acts as a buffer against temperature changes.

• Unsaturated Fatty Acids: Increase fluidity due to kinks in their tails, preventing tight packing.

• Saturated Fatty Acids: Decrease fluidity by allowing tight packing of phospholipids.

• Cholesterol: At moderate temperatures, reduces fluidity by restraining phospholipid movement; at low temperatures, prevents solidification by disrupting packing.

Types of Membrane Proteins

Membrane proteins are crucial for various cellular functions and are classified based on their association with the membrane.

• Integral Proteins: Penetrate the hydrophobic core; many are transmembrane proteins spanning the bilayer.

• Peripheral Proteins: Bound to the surface of the membrane, often attached to integral proteins or the cytoskeleton.

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Role of Membrane Carbohydrates

Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) serve as identification markers for cell recognition and communication.

• Cell Recognition: Cells bind to specific carbohydrate markers on other cells, facilitating immune response and tissue formation.

• Glycolipids and Glycoproteins: Carbohydrate chains covalently bonded to lipids or proteins, respectively.

Selective Permeability of the Membrane

The plasma membrane exhibits selective permeability, allowing some substances to cross more easily than others.

• Hydrophobic (Nonpolar) Molecules: Pass through the lipid bilayer rapidly (e.g., O2, CO2).

• Hydrophilic (Polar) Molecules: Cross slowly or require transport proteins (e.g., sugars, ions, water).

Transport Proteins

Transport proteins facilitate the movement of hydrophilic substances across the membrane.

• Channel Proteins: Provide hydrophilic tunnels for specific molecules or ions (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane; highly specific.

Passive Transport: Diffusion and Osmosis

Passive transport is the movement of substances across the membrane without energy input, driven by concentration gradients.

• Diffusion: Movement of particles from high to low concentration until equilibrium is reached.

• Osmosis: Diffusion of free water across a selectively permeable membrane toward higher solute concentration.

Equation for Diffusion Rate:

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

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

Osmotic Environments and Cell Response

Cells respond differently to osmotic environments based on the presence or absence of cell walls.

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

• Hypertonic: Higher solute concentration outside; cell loses water and shrivels.

• Hypotonic: Lower solute concentration outside; cell gains water and may burst (animal cells) or become turgid (plant cells).

Environment	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Facilitated Diffusion

Facilitated diffusion is passive transport aided by proteins, allowing specific molecules to cross the membrane more efficiently.

• Channel Proteins: May be gated, opening in response to stimuli.

• Carrier Proteins: Undergo shape changes to move substances down their concentration gradient.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually from ATP).

• Carrier Proteins: Use ATP to transport ions and molecules.

• Sodium-Potassium Pump: Maintains high K+ and low Na+ inside animal cells.

Equation for Sodium-Potassium Pump:

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

Membrane Potential and Electrochemical Gradients

The membrane potential is the voltage across a cell's membrane, created by differences in ion distribution. It influences the movement of charged substances.

• Electrochemical Gradient: Combination of chemical (concentration) and electrical (charge) forces driving ion movement.

• Electrogenic Pumps: Transport proteins that generate voltage across the membrane (e.g., sodium-potassium pump in animals, proton pump in plants).

Bulk Transport: Exocytosis and Endocytosis

Bulk transport moves large molecules across the membrane via vesicles, requiring energy.

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

• Endocytosis: Plasma membrane engulfs material, forming a vesicle to bring substances into the cell.

Types of Endocytosis

Type	Description
Phagocytosis	Cell engulfs large particles or cells; forms a food vacuole.
Pinocytosis	Cell "gulps" extracellular fluid and dissolved solutes; non-specific.
Receptor-Mediated Endocytosis	Specific molecules bind to receptors, triggering vesicle formation.

• Example: LDL cholesterol uptake via receptor-mediated endocytosis; defects can lead to hypercholesterolemia.

Additional info: These notes are based on Campbell Biology, Chapter 7, and cover essential concepts for understanding membrane structure and function in college-level General Biology.