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

Membrane Structure and Function

Overview of Plasma Membrane Function

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Its structure and associated proteins enable the cell to maintain homeostasis and communicate with its environment.

• Selective Permeability: The membrane allows some substances to cross more easily than others, maintaining the internal environment of the cell.

• Transport Mechanisms: The plasma membrane uses several mechanisms to regulate traffic:

  • Passive Transport: Movement of small molecules (e.g., O2, CO2) down their concentration gradient, without energy input. May involve diffusion or transport proteins.

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

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

Structure of Cellular Membranes

Phospholipids and the Fluid Mosaic Model

Cellular membranes are primarily composed of a phospholipid bilayer with embedded proteins, carbohydrates, and other lipids. The fluid mosaic model describes the dynamic and heterogeneous nature of this structure.

• Amphipathic Molecules: Phospholipids have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, causing them to form bilayers in aqueous environments.

• Bilayer Arrangement: Hydrophobic tails face inward, shielded from water, while hydrophilic heads face outward toward the cytosol and extracellular fluid.

• Fluid Mosaic: Proteins and lipids move laterally within the layer, creating a flexible, dynamic structure. Proteins are not randomly distributed but often form functional groups.

Membrane Fluidity

Membrane fluidity is essential for proper function and is influenced by lipid composition and temperature.

• Unsaturated vs. Saturated Fatty Acids:

  • Unsaturated fatty acids (with double bonds) prevent tight packing, increasing fluidity, especially at lower temperatures.

  • Saturated fatty acids (no double bonds) pack tightly, making the membrane more viscous and less fluid.

• Cholesterol: In animal cells, cholesterol acts as a fluidity buffer:

  • At warm temperatures, cholesterol restrains phospholipid movement, reducing fluidity.

  • At cool temperatures, it prevents tight packing, maintaining fluidity.

Membrane Proteins

Proteins embedded in the membrane determine most of its specific functions.

• Peripheral Proteins: Loosely bound to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

• Functions of Membrane Proteins:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix

Membrane Carbohydrates and Cell Recognition

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

• Cell Recognition: Cells recognize each other by binding to surface molecules, often carbohydrates, which are important for immune response and tissue formation.

Membrane Permeability and Transport

Selective Permeability of the Lipid Bilayer

The plasma membrane allows some substances to cross more easily than others, based on size, polarity, and charge.

• Hydrophobic (Nonpolar) Molecules: Such as hydrocarbons, O2, and CO2 can dissolve in the lipid bilayer and cross easily.

• Hydrophilic (Polar) Molecules: Such as ions and sugars, have difficulty crossing the hydrophobic core and require transport proteins.

Transport Proteins

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

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

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane. Highly specific for their cargo.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient, without energy input.

• Dynamic Equilibrium: When the concentration of molecules is equal on both sides, movement continues but with no net change.

• Concentration Gradient: Represents potential energy for diffusion.

Osmosis

Osmosis is the diffusion of free water across a selectively permeable membrane.

• Water moves toward higher solute concentration until equilibrium is reached.

Effects of Osmosis on Cells

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

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

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

Osmoregulation

• Organisms use mechanisms to control water balance, such as contractile vacuoles in Paramecium.

Plant Cells and Water Balance

• Turgid: Firm, healthy state in hypotonic solution.

• Flaccid: Limp, in isotonic solution.

• Plasmolyzed: Membrane pulls away from cell wall in hypertonic solution.

Facilitated Diffusion

Facilitated diffusion is passive transport aided by proteins, allowing polar molecules and ions 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

Mechanism and Importance

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

• Sodium-Potassium Pump: Maintains high K+ and low Na+ inside animal cells by pumping Na+ out and K+ in, using ATP.

$\text{Na}^+/\text{K}^+\ \text{pump:}\ 3\ \text{Na}^+\ \text{out},\ 2\ \text{K}^+\ \text{in},\ \text{per ATP hydrolyzed}$

Membrane Potential and Electrochemical Gradients

The membrane potential is the voltage difference across a membrane, created by the unequal distribution of ions.

• Electrochemical Gradient: Combination of the chemical gradient (concentration) and electrical gradient (charge difference) that drives ion movement.

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

Co-Transport (Coupled Transport)

Co-transport occurs when the downhill diffusion of one solute drives the uphill transport of another.

• Example: In plants, the proton pump creates an H+ gradient used to drive the uptake of sucrose via a co-transporter.

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

Exocytosis is the process by which cells export large molecules by fusing vesicles with the plasma membrane.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

Endocytosis is the process by which cells import large molecules by engulfing them in vesicles formed from the plasma membrane.

• Phagocytosis: "Cell eating"; the cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; the cell takes in extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Table: Types of Endocytosis

Type	Main Function	Specificity
Phagocytosis	Uptake of large particles or cells	Non-specific
Pinocytosis	Uptake of extracellular fluid and solutes	Non-specific
Receptor-Mediated Endocytosis	Uptake of specific molecules via receptor binding	Highly specific

Key Terms and Concepts

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Fluid Mosaic Model: Describes the membrane as a fluid structure with a "mosaic" of various proteins embedded in or attached to a bilayer of phospholipids.

• Osmoregulation: Control of solute concentrations and water balance.

• Membrane Potential: Voltage across a membrane due to ion distribution.

• Electrochemical Gradient: The combined effect of an ion's concentration gradient and electrical gradient across a membrane.

Example Application: If a cell is placed in a hypertonic solution, water will leave the cell, causing it to shrink. If placed in a hypotonic solution, water will enter the cell, causing it to swell (and possibly burst in animal cells).