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

Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all cells, regulating the movement of substances into and out of the cell. Its unique composition and organization allow for selective permeability, communication, and compartmentalization essential for cellular life.

Plasma Membrane Regulation of Traffic

Three Main Mechanisms

• Passive Transport: Movement of small molecules (such as oxygen and carbon dioxide) across the membrane without energy input. This can occur via simple diffusion or through transport proteins.

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

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

Membrane Structure

Components of Cellular Membranes

• Phospholipids: Amphipathic molecules with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails, forming a bilayer.

• Proteins: Embedded within or attached to the membrane, responsible for transport, signaling, and structural support.

• Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins), functioning in cell recognition.

• Other Lipids: Such as cholesterol, which modulates membrane fluidity.

Amphipathic Nature of Phospholipids

• Definition: Amphipathic molecules contain both hydrophilic and hydrophobic regions.

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

Fluid Mosaic Model

• Description: The membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids.

• Protein Distribution: Proteins are not randomly distributed; they often form functional groups.

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

Membrane Fluidity

Factors Affecting Fluidity

• Saturated vs. Unsaturated Fatty Acids:

  • Unsaturated fatty acids (with double bonds) increase fluidity by preventing tight packing.

  • Saturated fatty acids (no double bonds) decrease fluidity, making the membrane more viscous.

• Temperature:

  • Low temperatures: Membranes with more unsaturated fatty acids remain fluid.

  • High temperatures: Membranes with more saturated fatty acids maintain ideal fluidity.

• Cholesterol:

  • At moderate temperatures, cholesterol reduces fluidity by restraining phospholipid movement.

  • At low temperatures, cholesterol prevents solidification by disrupting packing.

Membrane Proteins

Types and Properties

• Peripheral Proteins: Bound to the surface of the membrane.

• Integral Proteins: Penetrate the hydrophobic core; some are transmembrane proteins spanning the entire membrane.

• Transmembrane Proteins: Integral proteins that span the membrane, often with hydrophobic regions embedded in the bilayer.

• Attachment: Some proteins are anchored to the cytoskeleton or extracellular matrix for stability.

Functions of Membrane Proteins

• Transport: Facilitate movement of substances across the membrane.

• Enzymatic Activity: Catalyze reactions at the membrane surface.

• Signal Transduction: Relay signals from outside to inside the cell.

• Cell-Cell Recognition: Allow cells to identify each other.

• Intercellular Joining: Connect adjacent cells.

• Attachment: Anchor the membrane to cytoskeleton and extracellular matrix.

Role of Membrane Carbohydrates

Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids).

• These carbohydrates function as markers for cell identification.

Membrane Sidedness

Asymmetry of Membranes

• The composition and distribution of proteins, lipids, and carbohydrates are not symmetrical across the membrane.

• This sidedness is established during membrane synthesis and is essential for proper function.

Selective Permeability

Lipid Bilayer Permeability

• Hydrophobic (nonpolar) molecules: Dissolve in the lipid bilayer and pass through rapidly (e.g., hydrocarbons, O2, CO2).

• Hydrophilic (polar) molecules: Pass through slowly or not at all (e.g., sugars, water, ions).

Transport Proteins

• Channel Proteins: Provide hydrophilic tunnels for 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

• Movement of particles from high to low concentration (down their concentration gradient).

• Does not require energy input.

• At equilibrium, movement occurs equally in both directions.

Equation:

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

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

Osmosis

• Diffusion of free water across a selectively permeable membrane.

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

Effects of Tonicity on Cells

• 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).

Table: Effects of Tonicity on Animal and Plant Cells

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

Osmoregulation

• Cells in non-isotonic environments must regulate water balance (e.g., Paramecium uses contractile vacuole).

Facilitated Diffusion

Role of Transport Proteins

• Transport proteins (channels and carriers) speed passive movement of molecules across the membrane.

• Channel proteins may be gated, opening in response to stimuli.

• Carrier proteins change shape to move solutes down their concentration gradient.

Active Transport

Mechanism and Example

• Requires energy (usually ATP) to move substances against their concentration gradients.

• Carrier proteins are involved in active transport.

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

Equation:

$\text{ATP} + \text{Na}^+_{in} + \text{K}^+_{out} \rightarrow \text{ADP} + \text{Na}^+_{out} + \text{K}^+_{in}$

Membrane Potential and Ion Pumps

Electrochemical Gradients

• Membrane potential is the voltage across a membrane due to ion distribution.

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants) generate membrane potential.

• Electrochemical gradient combines chemical (concentration) and electrical (charge) forces.

Coupled Transport (Cotransport)

• Active transport of one solute indirectly drives transport of another.

• Example: In plants, proton pumps create H+ gradient used to transport sucrose into cells.

Bulk Transport

Exocytosis and Endocytosis

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

• Endocytosis: Cell takes in macromolecules by forming vesicles from the plasma membrane.

Types of Endocytosis

Type	Description	Specificity
Phagocytosis	Cell engulfs particles, forming a food vacuole	Specific
Pinocytosis	Cell "gulps" extracellular fluid into tiny vesicles	Nonspecific
Receptor-mediated endocytosis	Binding of specific solutes to receptors triggers vesicle formation	Highly specific

• Receptor-mediated endocytosis is used for uptake of cholesterol (LDL particles).

• Defective LDL receptors lead to cholesterol accumulation and cardiovascular disease.

Summary Table: Transport Mechanisms Across the Plasma Membrane

Transport Type	Energy Required?	Direction	Example
Passive Transport	No	Down gradient	O2 diffusion
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Bulk Transport	Yes (ATP)	In or out	Exocytosis of insulin

Additional info: These notes expand on the brief points and diagrams in the original materials, providing definitions, examples, and academic context suitable for college-level General Biology students.