Lipids, Membranes & Cell Transport

Structure & Behavior of Lipids

  • Fatty Acids (FAs) are amphipathic

    • Carboxylic acid “head” with long hydrophobic “tails”

    • hydrophobic effect & van der Waals interactions prevail between tails

Trigylcerides (TAGs)

  • Fats in the diet stored in the adipocytes as triglycerides

  • The alkyl chain can be saturated or different degrees of unsaturation

Adipocytes store TAGs

  • Adipocyte cells in animal cells are specialized for fat storage

  • 3 distinct functions:

    • Energy production

    • Heat production

    • Insulation

Waxes

  • Complex mixtures of ester of long-chain carboxylic acids and long chain alcohols

  • Protective coatings for plants and animals

  • Perilipin

    • When fat stores are needed by the body, hormonal binding to receptor stimulates the phosphorylation of perilipin

    • Phosphorylated perilipin allows fat to be removed from the fat droplet for use by other cells in the body

Membrane lipids

  • Selectively permeable cellular boundaries and barriers

  • Form an amphipathic bilayer in aqueous environments

  • Contain transport systems that allow cells to take up specific molecules and remove unwanted ones

  • Major components of cellular membranes:

    • Phosphoglycerides

    • Sphingolipids

    • Glycosphingolipids

    • Glycoglycerolipids

    • Cholesterol

Sphingolipids - Ceramides

  • Sphingolipids are built upon a sphingosine core which contains a fixed hydrophobic “tail”

  • amphipathic but requires another FA to be bound at the amine to be suitable for the membrane = ceramide

  • Abundant in myelin sheath around nerve cells

Glycosphingolipids

  • A glycolipid derived from sphingosine where C1 is bound to a carbohydrate

    • Bound to 1 or more sugar

    • Makes the ABO blood type

  • Type and number of carbohydrates classify them

  • 2 important types are cerebrosides and gangliosides important in the cell membranes of brain and nerve cells

  • Common in plant and bacterial membranes but less so in animals

  • Monogalactosyl diglyceride constitutes half the protein volume in the chloroplast

Cholesterol

  • Isoprenoid class

  • 4 fused hydrocarbon rings

  • Hydroxyl group is only polar part

  • Not found in prokaryotes

  • Varying amount in animal membranes

    • Makes up 25% of lipid content in nerve cells

  • Stored in cells as cholestryl esters

Types of lipid - Characteristic

  • Fatty acids - Long hydrocarbon chain with carboxylic acid group

  • Waxes - Esters of a fatty acid and a long-chain alcohol

  • Triacylglycerols - triesters of glycerol and fatty acids

  • Glycerophospholipids - Triesters of glycerol with two fatty acid molecules and a phosphate bonded to an amino alcohol

  • Sphingolipids - Sphingosine molecule bonded to a fatty acid and a sugar

  • Steroids - nucleus of four fused carbon rings

  • Glycoglycerolipids - Diesters of glycerol with two fatty acid molecules and a sugar bound by an ther

Self-Assembly process

  1. Lipid bilayers have an inherent tendency to be extensive

  2. Lipid bilayers will tend to close on themselves so that they are no edges with exposed hydrocarbon chains, so they form compartments

  3. Lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavorable

  • Lipid bilayers have 2 roles

    • Solvent for integral membrane proteins

    • Permeability barrier

Integral and Peripheral Proteins

  • Membrane lipids form permeability barrier and establish compartments

  • Integral membrane proteins interact extensively with hydrocarbon chains of lipids and most span the entire lipid bilayer

  • Peripheral membrane proteins are bound mainly by electrostatic and hydrogen-bond interactions with head groups of lipids

Fluid Mosaic Model

  • Describes membranes as solutions of oriented lipids and globular proteins interacting with each other

    • “fluid” refers to lipid bilayer and “mosaic” refers to proteins

Membrane Fluidity

  • Controlled by fatty acid composition and cholesterol content

  • FAchains in membrane bilayers can be in an ordered, more rigid state or disordered, fluid state

Lipid Rafts

  • Cholesterol can form specific complexes with sphingolipids & proteins

  • Complexes within small but highly dynamic regions within membranes

  • Moderation of membrane fluidity making membranes less fluid but also less subject to phase transitions