Lipids and Membranes

Fatty Acid Structure and Lipid Properties

The structure of fatty acids directly influences the physical and biological properties of lipids, affecting membrane fluidity and function.

• Saturated fatty acids: No double bonds between carbon atoms; straight chains allow tight packing, making fats solid at room temperature (e.g., butter).

• Unsaturated fatty acids: One or more double bonds (often cis), creating kinks that prevent tight packing; typically liquid at room temperature (e.g., oils).

• Trans fats: Hydrogenated unsaturated fats; straighter chains behave more like saturated fats, associated with increased risk of heart disease.

• Longer fatty acid chains: Increase van der Waals interactions, making lipids harder or more solid (e.g., waxes).

Example: Butter (saturated fat) is solid at room temperature, while olive oil (unsaturated fat) is liquid.

Phospholipids and Membrane Formation

Phospholipids are essential for forming biological membranes due to their amphipathic nature.

• Amphipathic: Possess a hydrophilic head (phosphate + glycerol) and hydrophobic tails (fatty acids).

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

Example: Cell membranes are formed by phospholipid bilayers, creating a barrier between the cell and its environment.

Cholesterol: 'Good' vs. 'Bad'

Cholesterol is a sterol that regulates membrane fluidity. Its health impact depends on how it is transported in the blood.

• LDL (Low-Density Lipoprotein): Delivers cholesterol to cells; excess can build up in arteries ('bad cholesterol').

• HDL (High-Density Lipoprotein): Carries cholesterol away from arteries to the liver for removal ('good cholesterol').

• Balance between LDL and HDL is crucial for cardiovascular health.

Example: High LDL levels are associated with atherosclerosis, while high HDL levels are protective.

Fluid Mosaic Model of the Cell Membrane

The fluid mosaic model describes the cell membrane as a dynamic, flexible structure composed of a phospholipid bilayer with embedded proteins and cholesterol.

• Phospholipid bilayer: Provides fluidity and acts as a barrier.

• Proteins: Embedded throughout, functioning as channels, carriers, or receptors.

• Cholesterol: Modulates fluidity, restraining movement at high temperatures.

Example: Membrane proteins can move laterally, allowing flexibility and dynamic cell responses.

Transmembrane Proteins

Properties Allowing Residence in the Bilayer

Transmembrane proteins are adapted to span the hydrophobic core of the membrane.

• Integral (transmembrane) proteins: Span the membrane; contain hydrophobic regions that interact with lipid tails.

• Hydrophilic regions: Extend into the aqueous environment on either side.

• Amphipathic: Structure matches membrane polarity, allowing stable insertion.

Example: Ion channels and transporters are integral proteins that facilitate movement across membranes.

General Functions of Transmembrane Proteins

Transmembrane proteins perform diverse roles essential for cell function.

• Transport: Move substances via channels, carriers, or pumps (requires ATP).

• Enzymatic activity: Catalyze reactions (e.g., metabolic pathways).

• Signal transduction: Receive and relay chemical messages (e.g., hormones).

• Cell-cell recognition: Identify other cells via glycoproteins.

• Intercellular joining (adhesion): Attach cells to each other.

• Attachment: Link the cell to the extracellular matrix (ECM) and cytoskeleton.

Example: Aquaporins are channel proteins that facilitate water transport.

Transport Across Membranes

Simple Diffusion

Simple diffusion is the passive movement of molecules from high to low concentration until equilibrium is reached.

• Passive transport: No energy required.

• Occurs for small, uncharged, nonpolar molecules (e.g., O2, CO2).

• Driven by a concentration gradient (potential energy).

Equation:

Semipermeable Membrane

A semipermeable (selectively permeable) membrane allows some substances to cross freely but restricts others, maintaining homeostasis.

• Permits small, nonpolar molecules or water.

• Restricts large or charged molecules.

Example: Cell membranes are semipermeable, controlling entry and exit of substances.

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane from low solute concentration (high water) to high solute concentration (low water).

• Occurs when solutes are nonpenetrating (cannot cross membrane).

• Goal: Equalize water potential on both sides.

Equation:

Where is water potential, is solute potential, and is pressure potential.

Osmolarity vs. Tonicity

Osmolarity and tonicity describe solute concentration and its effect on cell shape.

• Osmolarity: Total concentration of all solutes (penetrating + nonpenetrating).

• Tonicity: Depends only on nonpenetrating solutes; determines water movement and cell shape.

Facilitated Diffusion

Facilitated diffusion is passive transport of molecules across a membrane via protein channels or carriers.

• Moves polar/charged molecules (e.g., glucose, ions).

• No energy required; driven by concentration gradient.

Active Transport

Active transport moves substances against their concentration gradient (low to high), requiring energy (ATP).

• Example: Sodium-Potassium Pump (-ATPase): Pumps 3 Na+ out, 2 K+ in per ATP used.

• Creates electrochemical gradients crucial for nerve signals and muscle contraction.

Equation:

Secondary Active Transport

Secondary active transport (cotransport) uses energy stored in gradients created by primary active transport.

• Symport: Two molecules move in the same direction.

• Antiport: Two molecules move in opposite directions.

• Example: Proton (H+) pump uses ATP to create a gradient (primary active transport); sucrose-H+ symporter uses H+ gradient to move sucrose against its gradient (secondary active transport).

Key Terms and Definitions

• Triglyceride: Fat molecule made of one glycerol backbone attached to three fatty acids by ester linkages; used for long-term energy storage.

• Glycerol: Three-carbon alcohol serving as the backbone of triglycerides and phospholipids.

• Fatty acid: Long hydrocarbon chain with a carboxyl group (-COOH); can be saturated or unsaturated.

• Ester linkage: Covalent bond formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid during dehydration synthesis.

• Saturated fatty acid: Only single bonds between carbon atoms; solid at room temperature.

• Unsaturated fatty acid: One or more double bonds between carbon atoms; typically liquid at room temperature.

• Cholesterol: Type of lipid found in animal cell membranes; helps maintain membrane fluidity and stability.

• Fluid mosaic: Model describing the cell membrane as a flexible layer of phospholipids with proteins and other molecules embedded.

• Transmembrane: Describes a protein that spans the entire phospholipid bilayer, allowing substances to cross the membrane.

• Diffusion: Passive movement of molecules from high to low concentration until equilibrium.

• Osmosis: Diffusion of water molecules across a semipermeable membrane from low solute concentration to high solute concentration.

• Semipermeable: Membrane that allows some substances to pass through but restricts others.

• Osmolarity: Measure of solute concentration in a solution, expressed as osmoles per liter (Osm/L).

• Facilitated diffusion: Passive transport of molecules across a membrane through a protein channel or carrier.

• Active transport: Movement of substances across a membrane against their concentration gradient using energy (usually ATP).

• Secondary active transport: Form of active transport that uses energy stored in an ion gradient.

• Symport: Cotransport where two molecules move in the same direction across a membrane.

• Antiport: Cotransport where two molecules move in opposite directions across a membrane.