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

Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all living cells, responsible for regulating the movement of substances into and out of the cell. Its unique composition allows for selective permeability, maintaining homeostasis and enabling communication with the environment.

Major Ways the Plasma Membrane Regulates Inbound and Outbound Traffic

• Passive Transport: Movement of small molecules 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 (outbound) and endocytosis (inbound).

Membrane Structure

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, carbohydrates, and other lipids. The components are not static; they move laterally within the layer, giving the membrane fluidity and flexibility.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer that serves as the basic structural framework.

• Proteins: Integral and peripheral proteins are interspersed throughout the bilayer, contributing to various functions such as transport, signaling, and cell recognition.

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

Amphipathic Nature of Phospholipids

• Amphipathic: Molecules that possess both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Phospholipids arrange themselves so that hydrophobic tails face inward, shielded from water, while hydrophilic heads face outward toward the aqueous environment.

Membrane Fluidity

Membrane fluidity is essential for proper function, affecting the movement of proteins and lipids and the ability of the cell to change shape.

• Unsaturated fatty acids: Increase fluidity due to kinks in their tails that prevent tight packing.

• Saturated fatty acids: Decrease fluidity by allowing tighter packing of phospholipids.

• Cholesterol: Acts as a fluidity buffer, restraining movement at high temperatures and preventing solidification at low temperatures.

Comparison of Fatty Acid Types

Type	Effect on Fluidity
Unsaturated	Increases fluidity
Saturated	Decreases fluidity
Cholesterol	Buffers fluidity (prevents extremes)

Membrane Proteins

Types and Functions

• Integral Proteins: Penetrate the hydrophobic core of the bilayer; often span the membrane (transmembrane proteins).

• Peripheral Proteins: Bound to the surface of the membrane; not embedded in the lipid bilayer.

• 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 proteins (glycoproteins) or lipids (glycolipids) serve as identification markers for cell recognition.

• These markers are crucial for immune response and tissue organization.

Selective Permeability

Lipid Bilayer Permeability

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

• Hydrophilic (polar) molecules: Pass through slowly or not at all; require transport proteins.

Transport Proteins

• 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

Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• No energy input required.

• At equilibrium, movement continues but there is no net change in concentration.

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

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

• Water moves toward higher solute concentration.

• Continues until solute concentrations are equal on both sides.

Tonicity and Water Balance

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

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, causing it to swell.

Effects on Animal and 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.

• Includes channel and carrier proteins.

• Does not require energy; moves substances down their concentration gradient.

Active Transport

Mechanism

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

• Carrier proteins are involved.

• Maintains concentration differences essential for cell function.

Sodium-Potassium Pump

The sodium-potassium pump is a classic example of active transport in animal cells.

• Transports 3 Na+ ions out and 2 K+ ions into the cell per ATP molecule hydrolyzed.

• Maintains electrochemical gradients necessary for nerve impulse transmission and muscle contraction.

Equation:

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

Membrane Potential and Ion Pumps

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

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

• Electrochemical gradients drive the movement of ions.

Coupled Transport (Cotransport)

Cotransport occurs when the active transport of one solute indirectly drives the transport of another solute.

• Example: In plants, proton pumps create an H+ gradient that drives the uptake of sucrose via a cotransporter.

• In animal cells, glucose uptake in the intestine is coupled to Na+ diffusion.

Bulk Transport

Exocytosis

• Transport vesicles fuse with the plasma membrane, releasing their contents outside the cell.

• Used for secretion of large molecules (e.g., insulin from pancreatic cells).

Endocytosis

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

• Three types:

  • Phagocytosis: Cell engulfs large particles or cells.

  • Pinocytosis: Cell "gulps" extracellular fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

Comparison of Endocytosis Types

Type	Specificity	Example
Phagocytosis	Non-specific	White blood cell engulfing bacteria
Pinocytosis	Non-specific	Cell taking in extracellular fluid
Receptor-mediated	Highly specific	LDL cholesterol uptake

Key Terms and Definitions

• Selective Permeability: Ability of the membrane to allow some substances to pass while blocking others.

• Concentration Gradient: Difference in concentration of a substance across a space.

• Osmoregulation: Control of solute concentrations and water balance.

• Turgor Pressure: Pressure exerted by water inside the cell against the cell wall.

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required?	Direction	Example
Passive (Diffusion)	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 (Exocytosis/Endocytosis)	Yes (various)	Large molecules	Insulin secretion, LDL uptake

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).

Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and tables have been inferred and formatted for study purposes.