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, maintaining cellular homeostasis. It achieves this regulation through several mechanisms, each suited for different types and sizes of molecules.

• 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 specific transport proteins.

• Bulk Transport: Movement of large molecules (such as proteins and polysaccharides) via vesicles, including exocytosis (outbound) and endocytosis (inbound).

Fluid Mosaic Model of Membrane Structure

The fluid mosaic model describes the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. This model explains both the flexibility and selective permeability of biological membranes.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer with tails facing inward and heads facing outward.

• Proteins: Embedded within or attached to the bilayer, performing various functions such as transport, signaling, and structural support.

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

Membrane Fluidity

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

• 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: Acts as a fluidity buffer; at moderate temperatures, it restrains movement of phospholipids, while at low temperatures, it prevents tight packing and solidification.

Example: Animal cell membranes contain cholesterol to maintain optimal fluidity across temperature changes.

Types of Membrane Proteins

Membrane proteins are crucial for the diverse functions of the plasma membrane. They are classified based on their association with the lipid bilayer.

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

• 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 in Cell Recognition

Carbohydrates on the cell surface are key for cell-cell recognition and communication.

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: Carbohydrates covalently bonded to proteins.

• Function as markers for cellular identification, important in immune response and tissue organization.

Selective Permeability of the Lipid Bilayer

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

• Hydrophobic (nonpolar) molecules: Pass through the bilayer rapidly (e.g., O2, CO2).

• Hydrophilic (polar) molecules: Pass slowly or require transport proteins (e.g., glucose, ions).

Transport Proteins

Transport proteins facilitate the movement of specific 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.

Example: Glucose carrier proteins transport only glucose, not its structural isomer fructose.

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:

$\text{Net diffusion} = \text{Movement down concentration gradient}$

Effects of Osmosis on Cells

Osmosis affects water balance in cells, depending on the tonicity of the surrounding solution.

• 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).

Example: Paramecium uses a contractile vacuole to expel excess water in a hypotonic environment.

Facilitated Diffusion

Facilitated diffusion is passive transport aided by proteins, allowing substances to move down their concentration gradients.

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

• Carrier Proteins: Undergo shape changes to move solutes across the membrane.

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:

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

Membrane Potential and Ion Pumps

Membrane potential is the voltage across a cell membrane, created by differences in ion distribution.

• Electrogenic Pumps: Generate voltage across membranes (e.g., sodium-potassium pump in animals, proton pump in plants).

• Electrochemical Gradient: Combination of chemical and electrical forces driving ion movement.

Equation:

$\text{Electrochemical gradient} = \text{Concentration gradient} + \text{Membrane potential}$

Coupled Transport (Cotransport)

Cotransport occurs when the downhill movement of one solute drives the uphill transport of another.

• Example in Plants: Proton pumps create an H+ gradient, which is used to transport sucrose into cells.

• Example in Animals: Glucose is transported into intestinal cells via cotransport with Na+.

Bulk Transport: Exocytosis and Endocytosis

Bulk transport moves large molecules across the membrane via vesicles.

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

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

Types of Endocytosis

• Phagocytosis: Cell engulfs large particles or cells, forming a food vacuole.

• Pinocytosis: Cell "gulps" extracellular fluid and dissolved solutes into small vesicles; nonspecific.

• Receptor-Mediated Endocytosis: Specific molecules bind to receptors, triggering vesicle formation; used for uptake of substances like cholesterol (via LDLs).

Type of Transport	Energy Required?	Direction	Example
Passive Transport	No	Down concentration gradient	O2 diffusion
Facilitated Diffusion	No	Down concentration gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against concentration gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	In or out of cell	Exocytosis, Endocytosis

Additional info: These notes expand on the original slides and handwritten content to provide a comprehensive, self-contained study guide suitable for General Biology students.