2.2 Lipid bilayer membranes & Membrane transport

phospholipids

glycerol-phosphate with two hydrocarbon chains

Eukaryotes have membrane lipid innovations that are not found in either Archaea or Bacteria: ______ and ______

sterols, sphingolipids

Sterols

any organic compound with a skeleton closely related to cholestan-3-ol and having a hydroxyl group at carbon 3

sphingolipids

a class of lipids containing a backbone of sphingoid bases, which are a set of aliphatic amino alcohols that includes sphingosine.

lipid rafts

the plasma membranes of cells that contain combinations of glycosphingolipids, cholesterol and protein receptors organized in glycolipoprotein lipid microdomains

Great Oxygenation Event

a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere and shallow seas first experienced a rise in the concentration of free oxygen

hopanoids

the bacterial equivalent of membrane sterols. they have 5 rings and do not require oxygen for their biosynthesis.

saturated fatty acids

fatty acids with no double bonds in the hydrocarbon chain

unsaturated fatty acids

fatty acids that have one or more double bonds and therefore have fewer hydrogens (2 fewer per double bond)

Trans-unsaturated fatty acids

fatty acids with the hydrogens on opposite sides, result in a nearly straight hydrocarbon chain like a saturated fatty acid.

Osmosis

the diffusion of solvent (water) molecules across a membrane

isotonic solution environment

environments where the solute concentrations inside the cell and outside the cell are in balance, so there is no net movement of water across the cell membrane

hypotonic solution environment

environments where the solute concentration outside the cell is lower than inside the cell, so water will enter the cell to try to reduce the internal solute concentration

hypertonic solution environment

environments where the solute concentration outside is higher than inside the cell, so water will exit and the cell will shrivel up

facilitated diffusion

the movement of molecules through protein channels

Active transport

transport of molecules against their concentration gradient requires energy, often in the form of hydrolysis of ATP

Fluid Mosaic Model

describes membranes as a phospholipid bilayer with hydrophilic heads and hydrophobic tails, embedded with proteins, glycoproteins, glycolipids, and carbohydrates. Lipids and proteins move laterally within the bilayer (fluid), but rarely flip between leaflets. The mosaic refers to the diverse composition of lipids, proteins, and carbohydrates.

Membrane Lipids in the 3 Domains of Life

• Bacteria & Eukarya: Fatty acid tails ester-linked to D-glycerol.

• Archaea: Branched isoprene chains, L-glycerol backbone, ether linkages.

• Eukaryotes (unique): Sterols (cholesterol), sphingolipids.

• Bacteria (unique): Hopanoids (sterol analogs).

Factors Affecting Membrane Fluidity & Integrity

• Temperature: High → more fluid/permeable; Low → rigid.

• Sterols: Buffer fluidity (reduce at high T, prevent rigidity at low T).

• Fatty acid chain length: Longer → less fluid, more rigid.

• Saturation: Unsaturated (double bonds) → more fluid; Saturated → less fluid.

Diffusion Across Membranes

• Freely diffuse: Small, nonpolar, uncharged molecules (O₂, CO₂).

• Slowly diffuse: Water (small but polar).

• Do not diffuse readily: Large, polar, or charged molecules (Na⁺, glucose).

Types of Membrane Transport

• Simple Diffusion: Direct movement through bilayer, down concentration gradient, no proteins, no energy, linear kinetics.

• Facilitated Diffusion: Protein-mediated, down gradient, no energy, saturable kinetics (limited by transporters).

• Active Transport: Protein-mediated, against gradient, requires energy (ATP), saturable kinetics.

Osmosis & Water Transport

• Isotonic: Equal solute concentrations → no net water movement.

• Hypotonic: Lower solute outside cell → water enters cell.

• Hypertonic: Higher solute outside cell → water leaves cell.