Lipids: Phospholipids, Sphingolipids, and Glycolipids

Phospholipids

• Phospholipids are a type of lipid composed of two fatty acids, glycerol, phosphoric acid, and a functional group attached to the phosphate.
• The functional group determines the specific properties of the phospholipid.
• Common functional groups include ethanolamine, choline, inositol, and serine.

Sphingolipids

• Sphingolipids are another type of lipid composed of sphingosine (a long-chain amino alcohol), a fatty acid, and optionally phosphoric acid.
• They also incorporate a functional group like ethanolamine, choline, or serine.
• Sphingolipids are categorized into sphingomyelins and glycolipids.
• Sphingomyelins comprise sphingosine, a fatty acid, phosphoric acid, and a functional group (serine, ethanolamine, or choline).
• Important components of brain matter, neural tissue, and the myelin sheath.

Glycolipids

• Glycolipids are lipids composed of sphingosine, a fatty acid, and one or more sugar molecules.
• Cerebrosides are the simplest type, containing a single sugar molecule (glucose or galactose).
• Gangliosides are more complex, containing up to seven sugar residues.

Sterols

• Sterols are alcohols classified within the steroid family.
• Cholesterol is the most important sterol in animals.
• Cholesterol is a cyclic compound with a branched side chain.

Structure of Membrane Lipids

• Membrane lipids usually contain one or two fatty acid residues with an even number of carbon atoms (often 16-18).
• At least one bond in these fatty acid residues can be unsaturated, adding to their spatial characteristics.
• Membrane lipid molecules are amphiphilic, having both a hydrophilic ("water-loving") polar head and a hydrophobic ("water-fearing") nonpolar tail.
• The hydrophilic head can be electrically charged or an electric dipole.
• All membrane lipids share a hydrophilic head and one or two hydrophobic tails.

Lipid Bilayer

• The cell membrane is a phospholipid bilayer composed of two layers of phospholipids.
• Hydrophilic (polar) heads face the aqueous environments inside and outside the cell.
• Hydrophobic (nonpolar) hydrocarbon chains are oriented towards the interior of the bilayer.
• Surrounding water prevents membrane lipids from escaping, maintaining membrane integrity.

Membrane Proteins

• Membrane proteins are categorized by how they interact with the lipid bilayer: integral, peripheral, and surface.
• Integral proteins are embedded within the membrane.
  • Monotopic: attached to one side.
  • Transmembrane: span the entire membrane
  • Polytopic: cross the membrane multiple times.
• Peripheral proteins are located on the membrane surfaces, bound by electrostatic, hydrogen, or van der Waals forces.
• Surface proteins are on the membrane's outer surface, linked via anchor motifs like protein loops or lipids.
• Functions: transport, structural support, signaling, and enzymatic activity.

Cell Membrane Properties

• Selective permeability: Regulates the passage of molecules across the membrane.
• Fluidity: Influenced by composition (cholesterol content, unsaturated fatty acids) and temperature.
• Asymmetry: Different lipid and protein composition between the two membrane layers (leaflets).
• Heterogeneity: Not uniform in structure, with specialized regions like lipid rafts and caveolae.

Passive Transport

• Osmosis: Movement of water across a membrane from high to low concentration. Facilitated by aquaporins.
• Simple diffusion: Solutes move down their concentration gradient; affected by factors like concentration, membrane permeability, and temperature.
• Facilitated diffusion: Passive transport using membrane proteins (channels or transporters).

Ion Channels

• Protein structures creating water-filled pathways between intracellular and extracellular spaces.
• Selective: Only specific ions pass through, based on channel structure.
• Function: Temporarily increase membrane permeability to specific ions.
• Opening and closing controlled by stimuli like temperature, electrochemical gradient, and concentration.

Transporters

• Responsible for moving small water-soluble organic molecules and some inorganic ions.
• Highly selective: Usually transport only one type of solute.
• Mechanism: Alternate between different conformational states (e.g., outward open, closed, inward open).
• Affected by concentration gradient, speed of conformational changes, and factors like insulin or glucocorticoids.

Coupled Transport

• Symport: Two substances transported in the same direction.
• Antiport: Two substances transported in opposite directions.