Membrane Structure and Function – Chapter 7 Study Notes

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, either by diffusion or via 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 (outbound) and endocytosis (inbound).

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 membrane structure and function.

• 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 the dynamic nature and selective permeability 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 tighter 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 membrane surface, often attached to integral proteins or cytoskeletal elements.

• 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 markers for cell recognition and play a role in immune response and tissue organization.

• Cell Recognition: Cells identify each other by binding to carbohydrate markers on the membrane surface.

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

Selective Permeability of the Membrane

The plasma membrane is selectively permeable, allowing certain substances to cross more easily than others.

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

• Hydrophilic (Polar) Molecules: Pass slowly or not at all; 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 molecules or ions (e.g., aquaporins for water).

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

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 Osmotic Potential:

$\Psi = \Psi_s + \Psi_p$

Where $\Psi$ is the water potential, $\Psi_s$ is the solute potential, and $\Psi_p$ is the pressure potential.

Effects of Tonicity on Cells

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• 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).

Osmoregulation: Mechanisms to control water balance, such as contractile vacuoles in protists.

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 Electrogenic Pumps

Membrane potential is the voltage across a membrane, created by differences in ion distribution. Electrogenic pumps generate this potential, storing energy for cellular work.

• Sodium-Potassium Pump: Main electrogenic pump in animal cells.

• Proton Pump: Main electrogenic pump in plants, fungi, and bacteria; actively transports H+ out of the cell.

Coupled Transport (Cotransport)

Cotransport occurs when the active transport of one solute indirectly drives the transport of another. The downhill movement of one substance is coupled to the uphill transport of another.

• Example: In plants, the proton gradient drives the uptake of sucrose via a cotransporter.

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: The membrane forms a vesicle to bring substances into the cell. Types include:

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

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

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

Table: Comparison of Transport Mechanisms

Transport Type	Energy Required?	Direction	Examples
Passive Transport	No	Down concentration gradient	Diffusion, Osmosis, Facilitated Diffusion
Active Transport	Yes (ATP)	Against concentration gradient	Sodium-Potassium Pump, Proton Pump
Bulk Transport	Yes	In or out of cell	Exocytosis, Endocytosis (Phagocytosis, Pinocytosis, Receptor-Mediated)

Example Applications

• Paramecium: Uses contractile vacuole for osmoregulation in hypotonic environments.

• Pancreatic Cells: Secrete insulin via exocytosis.

• Familial Hypercholesterolemia: Defective LDL receptor proteins impair cholesterol uptake, leading to cardiovascular disease.

Additional info: These notes expand on the provided slides and handwritten content, filling in academic context and definitions for clarity and completeness.