Lipids and Membrane Structure: Chapter 6 Study Notes

Lipids and Membrane Structure

Overview

This chapter explores the structure and function of lipids, their role in forming biological membranes, and the mechanisms by which substances move across these membranes. The plasma membrane, a defining feature of life, is primarily composed of lipids and proteins.

• Lipid Structure and Function: Understanding the chemical nature and biological roles of lipids.

• Formation of Bilayers: How lipids spontaneously organize into bilayer structures.

• Membrane Transport: Mechanisms such as diffusion, osmosis, and the role of membrane proteins.

Lipid Structure and Properties

What are Lipids?

Lipids are a diverse group of mostly nonpolar organic molecules. Unlike proteins and nucleic acids, lipids are not polymers because they are not made from repeating monomer units.

• Key Types of Lipids:

  • Fats (Triglycerides)

  • Phospholipids

  • Steroids

  • Waxes

• General Structure: Lipids are mostly composed of carbon and hydrogen atoms, making them hydrophobic (water-repelling).

Fatty Acids and Glycerol

Most biologically important lipids are formed from glycerol and fatty acids through dehydration reactions (condensation reactions).

• Fatty Acid: A hydrocarbon chain with a carboxylic acid group at one end.

• Glycerol: A three-carbon alcohol with hydroxyl groups.

• Ester Linkages: The bond formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid, releasing water.

Example: Formation of a triglyceride (fat) involves three fatty acids linked to one glycerol molecule.

Chemical Nature of Lipids

Lipids are nonpolar due to their hydrocarbon structure. The covalent bonds between carbon and hydrogen have similar electronegativities, resulting in little charge separation.

• Hydrocarbons: Chains of carbon atoms bonded to hydrogen atoms.

• Nonpolarity: Makes lipids insoluble in water.

Saturated vs. Unsaturated Fatty Acids

The presence or absence of double bonds in the hydrocarbon chain affects the properties of fatty acids.

• Saturated Fatty Acids:

  • No double bonds between carbon atoms.

  • Maximum number of hydrogen atoms.

  • Usually solid at room temperature (e.g., butter).

• Unsaturated Fatty Acids:

  • One or more double bonds in the hydrocarbon chain.

  • Causes kinks, preventing tight packing.

  • Usually liquid at room temperature (e.g., oils).

Example: Olive oil contains mostly unsaturated fatty acids, while lard contains mostly saturated fatty acids.

Phospholipids and Membrane Structure

Phospholipids

Phospholipids are the primary component of cell membranes. They are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Structure:

  • Glycerol backbone

  • Two fatty acid tails (hydrophobic)

  • Phosphate group with a polar head (hydrophilic)

• Amphipathic Nature: Drives the formation of bilayers in aqueous environments.

Lipid Bilayers

When placed in water, phospholipids spontaneously form bilayers with hydrophobic tails facing inward and hydrophilic heads facing outward. This structure is the foundation of biological membranes.

• Fluid Mosaic Model: Membranes are not rigid; lipids and proteins can move laterally within the layer.

• Membrane Fluidity: Influenced by fatty acid composition (saturated vs. unsaturated) and cholesterol content.

Membrane Permeability and Transport

Permeability of Lipid Bilayers

The ability of molecules to cross the membrane depends on their size, polarity, and charge.

• High Permeability: Small, nonpolar molecules (e.g., O2, CO2).

• Low Permeability: Large, polar molecules and ions (e.g., glucose, Na+).

• Cholesterol: Reduces membrane permeability and increases stability.

Diffusion

Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, driven by random molecular motion.

• No energy input required.

• Continues until equilibrium is reached.

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 water across a selectively permeable membrane. Water moves from areas of low solute concentration to high solute concentration.

• Occurs when solutes cannot cross the membrane, but water can.

• Results in changes in cell volume depending on the environment.

Osmotic Environments

Solution Type	Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Equal inside and outside	Normal	Flaccid
Hypertonic	Higher outside	Shriveled	Plasmolyzed
Hypotonic	Lower outside	Lysed (bursts)	Turgid (normal)

Membrane Proteins and Transport Mechanisms

Types of Membrane Proteins

• Integral (Transmembrane) Proteins: Span the entire bilayer; amphipathic.

• Peripheral Proteins: Attached to the membrane surface; do not penetrate the bilayer.

Protein Functions in Membranes

• Transport: Channel and carrier proteins facilitate movement of substances.

• Enzymatic Activity: Some proteins catalyze reactions at the membrane surface.

• Cell Signaling: Receptors detect and transmit signals.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via specific transmembrane proteins.

• Channel Proteins: Provide corridors for specific molecules or ions to cross.

• Carrier Proteins: Change shape to shuttle molecules across the membrane.

• No energy required; movement is down the concentration gradient.

Active Transport

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

• Pumps: Membrane proteins that use energy to transport ions or molecules.

• Example: Sodium-potassium pump ($Na^+/K^+$-ATPase) moves $3Na^+$ out and $2K^+$ into the cell per ATP hydrolyzed.

Equation:

$ATP \rightarrow ADP + P_i + \text{energy}$

Secondary Active Transport

Uses the energy stored in electrochemical gradients created by primary active transport to move other substances against their gradients.

• Example: Sucrose-H+ cotransporter uses the proton gradient to import sucrose into the cell.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Protein Involved?	Example
Simple Diffusion	No	High to Low	No	O2, CO2
Facilitated Diffusion	No	High to Low	Yes	Glucose, Ions
Active Transport	Yes (ATP)	Low to High	Yes	Na+/K+ pump
Osmosis	No	Water: Low to High Solute	Sometimes (aquaporins)	Water movement

Additional info:

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

• Essential fatty acids (omega-3 and omega-6) must be obtained from the diet.

• Membrane proteins can be studied using freeze-fracture electron microscopy, which reveals their distribution within the bilayer.