Study Notes on Lipids, Membranes, and the First Cells

LIPIDS, MEMBRANES, AND THE FIRST CELLS

• Key Theme: Key milestone in life is the formation of self-replicating membranes enclosed in plasma membranes.

• Plasma Membrane Functions:

  • Serves as a selective barrier.

  • Allows entry of necessary materials.

  • Keeps harmful substances out.

  • Facilitates essential chemical reactions by sequestering chemicals.

PART 1: LIPID STRUCTURE AND FUNCTION

• Definition of Lipids:

  • Diverse carbon-containing compounds defined mainly by their insolubility in water.

• Functional Groups of Lipids:

  1. Amino Groups: Act as bases by attracting protons.

  2. Carboxyl Groups: Act as acids by releasing protons.

  3. Carbonyl Groups: Link molecules into complex compounds.

  4. Hydroxyl Groups: Act as weak acids.

  5. Phosphate Groups: Usually carry two negative charges.

  6. Sulfhydryl Groups: Can form disulfide bonds.

• Hydrophobic Nature of Lipids:

  • Rich in nonpolar C-C and C-H bonds.

  • Hydrocarbons (C and H only) are hydrophobic substances.

HYDROCARBON STRUCTURE & FUNCTION

• Lipid Structure Impact:

  • Isoprenoids: Nonpolar hydrocarbons serving as pigments and precursors for vitamins and hormones.

• Fatty Acids:

  • Comprised of hydrocarbon chains attached to a polar carboxyl group.

  • Chain lengths range from 14-20 carbon atoms and can be saturated or unsaturated.

• Saturation Effects on Structure:

  • Saturated Fatty Acids: Only single bonds between carbons leading to maximum hydrogen.

  • Unsaturated Fatty Acids: One or more double bonds resulting in kinks in the chain.

  • Polyunsaturated: Multiple double bonds.

LIPID PHYSICAL STATE

• Implications of Saturation:

  • Saturated lipids are solid at room temperature (e.g., butter), while unsaturated are liquid (e.g., oils).

  • Highly saturated fats are denser and form stiff solids.

• Hydrogenation: Can convert unsaturated fats to saturated by breaking double bonds and adding hydrogen.

TYPES OF LIPIDS

• Steroids:

  • Characterized by a bulky four-ring structure, differing by functional groups.

• Fats (Triglycerides):

  • Composed of three fatty acids linked to glycerol; major function is energy storage due to high-energy bonds.

  • Formed via dehydration reactions creating ester linkages.

• Phospholipids:

  • Composed of glycerol, phosphate group, and two hydrocarbon chains.

  • Essential for forming cell membranes.

MEMBRANE INTERACTIONS

• Amphipathic Nature of Phospholipids:

  • Contain both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

  • Create micelles and bilayers in aqueous solutions which serve as defining barriers of cells.

PART 2: LIPID BILAYERS

• Phospholipid Bilayer Properties:

  • Composed of two layers of phospholipids with hydrophilic heads outwards and hydrophobic tails inwards.

  • Form spontaneously in water without energy input.

• Artificial Liposomes:

  • Lab-created vesicles that demonstrate lipid bilayer properties and are used in experiments.

• Selective Permeability:

  • Permeability Scale:

  • High: $O2$, $CO2$, $N_2$

  • Medium: $H_2O$, glycerol

  • Low: $CI^{-}$, $K^{+}$, $Na^{+}$

MEMBRANE PERMEABILITY

• Factors Influencing Permeability:

  • Hydrocarbon tail length, saturation state, and cholesterol presence.

  • Unsaturation leads to weaker barriers due to kinks reducing tight packing.

  • Longer saturated tails create denser membranes, lowering permeability.

• Cholesterol Effects: “Increases” density of hydrophobic section of membranes leading to decreased permeability.

• Temperature Effects:

  • Lower temperatures reduce fluidity and permeability as tails pack tighter.

NEXT CLASS

• Continuation on Lipid Bilayers.