Biomolecules II: Lipids and Membranes – Structure, Properties, and Transport

Lipids

Definition and Characteristics

Lipids are a diverse group of macromolecules essential for biological systems, primarily due to their hydrophobic nature and structural roles in membranes.

• Not polymers: Lipids consist of long chains of hydrocarbons, but unlike proteins or nucleic acids, they are not formed by repetitive monomer units.

• Hydrophobic/non-polar: Lipids cannot form hydrogen bonds with water, making them insoluble in aqueous environments.

• Structural variety: Lipids vary in size, structure, and functional groups, including fatty acids, triglycerides, phospholipids, waxes, and carotenoids.

Fatty acids are amphipathic molecules, containing a hydrophobic hydrocarbon tail and a polar functional group at one end.

Degree of Saturation and Stability

The physical properties of lipids are determined by the degree of saturation and the length of their hydrocarbon chains.

• Saturated fatty acids: Only single bonds between carbons; straight chains; solid at room temperature (e.g., butter, wax).

• Unsaturated fatty acids: One or more double bonds; can be cis (kinked) or trans (straight); liquid at room temperature (e.g., olive oil).

• Chain length: Longer chains increase melting point and stability.

Example: Butter (short, saturated), wax (long, saturated), olive oil (short, unsaturated).

Major Types and Functions of Lipids

• Triglycerides: Energy storage.

• Phospholipids: Major component of cell membranes.

• Waxes and oils: Waterproofing.

• Carotenoids: Pigments and antioxidants.

Lipid Bilayers

Phospholipids and Amphipathic Nature

Phospholipids are amphipathic molecules, meaning they have both hydrophobic and hydrophilic regions. This property drives their spontaneous arrangement in biological membranes.

• Structure: Fatty acid tail (hydrophobic/non-polar) + phosphate head (hydrophilic/polar).

• Bilayer formation: In water, phospholipids self-assemble into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Bilayer Structure and Stability

• Hydrophilic heads: Interact with water via hydrogen bonds.

• Hydrophobic tails: Interact with each other via van der Waals forces.

• Micelles, liposomes, and bilayers: Increasing numbers of phospholipids in water form these structures.

Degree of Saturation and Membrane Fluidity

• Unsaturated tails: Increase fluidity due to kinks preventing close packing.

• Saturated tails: Increase viscosity and decrease fluidity.

• Homeoviscous adaptation: Cells adjust lipid composition to maintain optimal membrane fluidity under changing temperatures.

Role of Cholesterol

• Cholesterol: Stabilizes membrane by preventing excessive fluidity at high temperatures and rigidity at low temperatures.

• Without cholesterol: Membranes may become too rigid (cold) or too fluid (hot), losing structural integrity.

Membrane Permeability and Transport

Selective Permeability of Lipid Bilayers

The lipid bilayer is selectively permeable, allowing some substances to cross more easily than others.

• High permeability: Small, non-polar molecules (O2, CO2, N2).

• Moderate permeability: Small, uncharged polar molecules (H2O, glycerol).

• Low permeability: Large, uncharged polar molecules (glucose, sucrose).

• Very low permeability: Ions (Na+, K+, Cl-).

Type of Molecule	Permeability	Example
Small, non-polar	High	O2, CO2
Small, polar	Moderate	H2O
Large, polar	Low	Glucose
Ions	Very low	Na+, K+

Simple Diffusion

• Passive process: Molecules move down their concentration gradient (from high to low concentration).

• Equation: $J = -D \frac{dC}{dx}$ (Fick's Law of Diffusion)

• Hydrophobic molecules: Freely diffuse across the bilayer.

Osmosis

• Definition: Movement of water across a selectively permeable membrane to balance solute concentrations.

• Direction: Water moves from areas of low solute concentration to high solute concentration.

• Example: Animal and plant cells in hypotonic, isotonic, and hypertonic solutions.

Fatty Acid Length and Permeability

• Short, unsaturated tails: Increase membrane permeability.

• Long, saturated tails: Decrease membrane permeability.

Plasma Membranes

Fluid Mosaic Model

Plasma membranes are dynamic structures composed of a mosaic of lipids, proteins, and carbohydrates.

• Fluidity: Components move laterally; membrane is non-rigid and adaptable.

• Mosaic: Made of multiple biomolecules (phospholipids, cholesterol, proteins, carbohydrates).

Proteins in Membranes

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the membrane surface; involved in cell signaling and structural support.

• Functions: Signal receptors, transporters, channels, pores, anchors.

Critical Functions of Plasma Membranes

• Barrier: Separates internal and external environments.

• Selective transport: Allows some substances to enter or leave the cell.

• Signal recognition and response: Detects and responds to external signals.

• Attachment: Connects to other cells and extracellular structures.

Transport Across Membranes

Passive Transport

• Simple diffusion: Movement of non-polar or small polar molecules down their concentration gradient.

• Facilitated diffusion: Movement of ions and polar molecules through channel or carrier proteins; still passive.

• Aquaporins: Specialized channel proteins that facilitate rapid water movement (osmosis).

Active Transport

• Definition: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

• Carrier proteins: Change shape to transport specific molecules across the membrane.

• Example equation: $\text{Na}^+/\text{K}^+ \text{ATPase}$

Summary Table: Types of Membrane Transport

Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Additional info: These notes expand on the original slides by providing definitions, examples, and equations relevant to General Biology topics on lipids and membranes. The content covers key concepts from Chapter 6 (Lipids, Membranes, and the First Cells) and Chapter 7 (Inside the Cell) of a typical college biology curriculum.