Lecture 13: Fluid Mosaic Model, Membrane Proteins

Morris: Chapter 5 sections 5.1-5.2 and figure 5.8

Cell membranes

• cells are defined by membranes

• lipids are the main component of cell membranes

• E-book - BIOL*1090 F24 - Macmillan Learning Achieve

• https://courselink.uoguelph.ca/d2l/le/content/896731/viewContent/3842265/View

Function of biological membranes

1. define cell boundary (organelles; mitochondria, Golgi, etc)

2. define enclose compartments

3. control movement of material into and out of cell

4. allow response to external stimuli

5. enable interactions between cells

6. provide scaffold for biochemical activities

   1. energy transduction; mitochondria/chloroplast provides scaffolding for biochemical activities producing energy

Plasma membrane

• PM: plasma membrane

• SR: sarcoplasmic reticulum

  • endoplasmic reticulum equivalent, movement of cations into muscle cells causing contractions

• Red blood cells are used to study PM b/c they don’t have nuclei or internal membrane

• Trilaminar structure made of a phospholipid bilayer

Trilaminar structure

• made of phospholipid bilayer

• ~6nm thick

• made up of phospholipids (phosphate head, hydrophobic tail, polar hear - inner layer)

  • micelle: formation of phospholipids into sphere shape (no lumen)

  • liposome: double phospholipid membrane layer with a lumen inside

Phospholipids forming the plasma membrane

lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads to water

• micelles are formed by fatty acids with only one hydrophobic tail

• amphipathic: hydrophobic (non-polar) and hydrophilic (polar) regions

Phospholipid structure

• two fatty acid chains

• esterified (ester bonds)

• stereospecific (left to right) numbering sn-1 and sn-2 of the glycerol, sn-3 has head group linked by phosphate residue

What attaches to the glycerol portion of phospholipid?

• phosphatidyl-:

  • ethanolamine

  • choline

  • serine

  • inositol

Other type of phospholipid:

• sphingolipid: mimics the shape of glycerol — has hydroxyl instead of ester

  • class of lipids containing backbone of sphingoid bases instead of glycerol which are a set of aliphatic amino alcohols

  • groups bonded to terminal oxygen:

    • phosphocholine forms sphingomyelin/SM (nervous system)

    • hydroxyl group forms a ceramide

    • glucose forms glucosesphingolipid

  • important in signal transduction and cell recognition

Phospholipid synthesis basics

• occurs at the interface of the cytosol and outer ER (which has all the enzymes for synthesis and distribution)

• multistep process

Phospholipid synthesis steps pt 1

• cytosol: fatty acids (FA) activated by attachment of CoA molecule → activated FA bonds to glycerol phosphate into ER membrane → phosphate removed by phosphate enzyme → choline is attached via choline phosphotransferase → flippases transfer phospholipid to leaflet

  • flippases → phospholipid to inner leaflet membrane

  • floppases → phospholipids to outer leaflet membrane

Phospholipid synthesis steps pt 2

vesicle containing phospholipids) leaves the ER for the cytoplasmic cellular membrane on the exterior leaflet (exocytosis via the 2 inner and outer membranes)

• different cells have different cell membranes (integral, peripheral, chloroplast, glycoprotein, glycolipid)

Fluid Mosaic Model

• fluid: individual lipids move

• mosaic: diverse ‘particles’ like proteins, carbohydrates, and cholestorol penetrate the lipid layer

• proposed by Seymour Jonathan Singer and Garth Nicolson in 1972

  • 2-dimensional liquid restricting the diffusion of membrane components

    • proteins embedded in layer, are mobile, and can interact

Dynamic of plasma membrane - Lipids

Lipids:

• move easily, laterally, within leaflet

• movement to other leaflets is slow

Dynamic of plasma membrane - Membrane Proteins

• diffuses within the bilayer

• movement is restricted

• rapid movement is spatially limited (small area it can move fast)

• long range diffusion is slow

• biochemical modification can alter protein mobility → important for signal transduction

Frye-Edidin Experiment (1970)

• inspired Singer and Nicolson’s Mosaic

• fused mouse and human cells → discover surface proteins diffuse around the unified membrane

Different biological membranes

Cell plasma membranes contain combinations of glycosphingolipids, cholesterols, and protein receptors which are organized into microdomains called lipid rafts

• microdomains can compartmentalize cellular processes by organizing centers of the assembly of signaling models -> allowing closer interactions between protein receptors and effectors to promote kinetically favorable interactions that are necessary for signal trandcution

Example of differences in biological membranes

oligodendrocytes are types of neurons surrounded by myelin sheath:

• myelin sheath have very few types of transmembrane protein

  • consists of layers of plasma membrane wrapping an axon

  • increases speed of electrical signals

the inner membrane of the mitochondria has a very high concentration of protein necessary for ETC and ATP synthesis

Three classes of membrane proteins

1. integral membrane proteins spans across the lipid bilayer

   • transport nutrients/ions, cell-cell communication, attachment

2. peripheral membrane proteins associate with the surface of the bilayer

3. lipid-anchored proteins are attached to a lipid in the bilayer

Symmetry of biological membranes

• asymmetrical

• two leaflets have distinct lipid composition in many plasma membranes

  • outer contains glycolipids and glycoproteins

What changes fluidity

warmer and unsaturated lipids increase fluidity → liquid crystal

cooling and saturated lipids decrease fluidity → crystalline gel

How cholesterol modulates membrane fluidity

• bidirectional regulator that either stabilizes/raises melting point vs intercalates the membranes

  • added to liquid crystal = fluidity decrease

  • added to crystalline gel = fluidity increase

Countering temperature changing fluidity

1. desaturation of lipids

2. exchange of lipid chains