1.6 Plasma Membrane

Why Cells Need a Membrane?

  • Separate interior from exterior - protection, support

  • Very thin boundary

  • Allows some materials to cross more easily - selectively permeable

What is it Made of?

Phospholipids

  • Spontaneously arrange into bilayer (two parallel layers)

  • Amphipathic due to lipophilic and hydrophilic regions

  • Bilayer held together by weal hydrophobic interactions between tails

Proteins

  • Proteins may be permanently or temporarily attached

  • Diverse in structure, position, and function

  • Functions include: junctions, enzymes, transport, recognition, anchorage, and transduction

Integral Proteins – permanently attached to membrane and are typically transmembrane (span across bilayer)

Peripheral Proteins – temporally attached to noncovalent interactions and associate with one surface

  • Non-polar amino acids are located within membrane

  • Anchors protein into membrane

  • Polar amino acids located on outer surface of membrane

  • Extend into extracellular fluid & cytosol

Carbohydrates

Glycolipid

  • lipid with a carbohydrate chain attached

  • help cell to recognize other cells of body

Glycoprotein

  • protein with carbohydrate chain attached

  • help in cell to cell communication and transport

Cholesterol

  • Amphipathic molecule found in animal membranes

  • Immobilizes outer surface - reducing fluidity

  • Makes less permeable to small water-soluble molecules

  • Separates phospholipid tails - prevents crystallization

Fluid Mosaic Model

  • Singer-Nicolson Model

  • Membranes are fluid - not fixed in position (can adopt

  • amorphous shapes)

  • Fluid - phospholipids can move

  • Mosaic - embedded with proteins

Fluidity

Fluidity increases with temperature and shorter fatty acid tails - changes to kinetic energy and decreased viscosity

Fluidity increases with double bonds - harder to pack

At high temps cholesterol stabilizes membrane, at low temps it prevents clustering of phospholipids

Davson-Danielli Model

Lipo-protein sandwich’, lipid layer sandwiched between two protein layers - trilaminar appearance

Falsification of this model led to Singer-Nicolson model

Proteins are embedded within lipid bilayer rather than existing as separate layer