Lipids, Membranes, and Membrane Transport in General Biology

Lipids: Structure and Function

Types of Lipids

Lipids are a diverse group of hydrophobic biological molecules, including fats, phospholipids, and steroids. They play crucial roles in energy storage, membrane structure, and signaling.

• Fats (Triacylglycerols): Large molecules assembled from smaller molecules via dehydration reactions. Composed of glycerol and fatty acids.

• Phospholipids: Major component of cell membranes, consisting of two fatty acids, a phosphate group, and glycerol.

• Steroids: Lipids with a carbon skeleton consisting of four fused rings (e.g., cholesterol).

Fats and Fatty Acids

Fats are constructed from glycerol and fatty acids. Fatty acids vary in length and in the number and location of double bonds.

• Saturated Fatty Acids: No carbon-carbon double bonds; saturated with hydrogen atoms. Solid at room temperature (e.g., animal fats).

• Unsaturated Fatty Acids: One or more carbon-carbon double bonds; causes kinks in the fatty acid chain. Liquid at room temperature (e.g., plant and fish oils).

• Trans Fats: Unsaturated fats that have been synthetically converted to saturated fats by adding hydrogen; associated with cardiovascular disease.

Example: Butter contains saturated fats, while olive oil contains unsaturated fats.

Functions of Fats

• Energy storage: Fats store more energy per gram than carbohydrates.

• Insulation: Adipose tissue cushions vital organs and insulates the body.

• Adipose cells: Specialized cells that store fat in mammals.

Phospholipids and Membrane Structure

Phospholipid Structure

Phospholipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions.

• Composed of two fatty acids (hydrophobic tails) and a phosphate group (hydrophilic head) attached to glycerol.

• When added to water, phospholipids self-assemble into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Example: The plasma membrane of cells is primarily composed of a phospholipid bilayer.

Membranes: Structure and Properties

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of proteins floating in or on a fluid bilayer of phospholipids.

• Membrane Fluidity: Influenced by temperature and the composition of fatty acids (unsaturated fatty acids increase fluidity).

• Cholesterol: Acts as a "fluidity buffer," stabilizing membrane fluidity across temperature changes.

Membrane Proteins

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

• Peripheral Proteins: Loosely bound to the surface of the membrane.

Functions of Membrane Proteins:

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

Membrane Transport

Selective Permeability

Cell membranes are selectively permeable, allowing some substances to cross more easily than others.

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

• Polar molecules (e.g., sugars, ions) do not cross the membrane easily.

Passive Transport

Passive transport is the movement of substances across a membrane without energy investment.

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Transport proteins speed the passive movement of molecules across the plasma membrane.

Example: Aquaporins are channel proteins that facilitate water movement across cell membranes.

Osmosis and Water Balance

• Isotonic Solution: No net movement of water; cell volume remains stable.

• Hypertonic Solution: Cell loses water and shrinks.

• Hypotonic Solution: Cell gains water and swells; may burst (lyse) in animal cells.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Sodium-Potassium Pump: Exchanges Na+ for K+ across the plasma membrane of animal cells.

• Electrogenic Pumps: Transport proteins that generate voltage across a membrane.

Equation:

$\text{Rate of diffusion} = k \times [\text{Concentration gradient}]$

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	High to Low (water)	Water via aquaporin
Active Transport	Yes (ATP)	Low to High	Sodium-potassium pump

Additional info:

• Cholesterol is a steroid that modulates membrane fluidity in animal cells.

• Plant cells use turgor pressure to maintain shape in hypotonic environments.

• ATP hydrolysis provides the energy for active transport mechanisms.