Membrane structure

Phospholipid bilayers

Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules

• molecular structure of a phospholipid

• important to emulate the degree of fluidity

  Though it is difficult to determine whether the membrane is truly either a solid or liquid it can definitely be said to be fluid.

  • Membranes need to be fluid enough that the cell can move
  • Membranes need to be fluid enough that the required substances can move across the membrane
  • If too fluid however the membrane could not effectively restrict the movement of substances across itself

• Davson and Danielli model 1930s

  The evidence: In high magnification electron micrographs membranes appeared as two dark parallel lines with a lighter coloured region in between. Proteins appear dark in electron micrographs and phospholipids appear light - possibly indicating proteins layers either side of a phospholipid core.

  The model:

  • A protein-lipid sandwich
  • Lipid bilayer composed of phospholipids(hydrophobic tails inside, hydrophilic heads outside)
  • Proteins coat outer surface
  • Proteins do not permeate the lipid bilayer

Problems with model:

Freeze-etched electron micrographs

• this technique involves rapid freezing of cells and then fracturing them
• The fracture occurs along lines of weakness, including the centre of membranes.
• The fracture reveals an irregular rough surface inside the phospholipid bilayer
• structures of proteins being extracted later, they figure the varied in size can’t be what sandwich model said form plate layers
• Fluorescente antibody tagging
• The globular structures were interpreted as trans-membrane proteins.

Membrane Proteins

Membrane proteins are diverse in terms of structure, position in the membrane and function

• Functions of membrane proteins
  • Hormone binding sites (receptors)
  • Immobilized enzymes with the active site on the outside
  • cell adhesion to form tight junctions between groups of cells in tissues and organs
  • Cell-to-cell communication
  • Channels for passive transport to allow hydrophilic particles across by facilitated diffusion
  • Pumps for active transport which use ATP two move particles across the membrane

Integral protein

• hydrophobic on at least part of their surface
• embedded in the hydrocarbon chains in the centre of membrane
• usually extend across the membrane

Peripheral proteins

• hydrophilic on their surface so are not embedded in membrane

• mostly attached to surface of integral proteins

• attachment is often reversible

• types of protein

  • Receptor proteins: each binds to a specific molecule outside the cell which triggers a change inside the cell. Can be integral/peripheral (insulin receptor protein)

  • Enzyme proteins: promote chemical reactions that synthesize or break apart biological molecules. Can be integral/peripheral (ATP Synthase)

  • Adhesion proteins: Anchors the cell membrane to the inner cytoskeleton or proteins outside the cell as well as to other cells. Can be integral/peripheral (Cadherins, presence of calcium binds cells within tissues together)

  • Recognition proteins: Serve as identification tags on the surface of a cell. Often times these are glycoproteins (proteins with an attached sugar molecule) Can be integral/peripheral (MHC)

  • Glycoproteins

    Protein produced by ribosome on RER—> add carbohydrates on protein to form glycoprotein/add fatty acid form lipoproteins by Golgi apparatus

    • Carbohydrate groups may help position or orientate glycoproteins in membrane (prevent rotation in membrane)
    • Carbohydrate groups may act as markers that determine the destination of a glycoprotein within the cell or for export (it may be removed after the protein has reached its destination)
    • Important in intercellular recognition, interaction of different cells to form tissues, and detection of foreign cells by immune system

  • Channel proteins: Serve as pores/tunnels for large or hydrophilic molecules to be passively (no energy required) transported into/out of the membrane. All are integral

  • Pump Protein: Serve as pores/tunnels for large or hydrophilic molecules to be actively (energy required) transported into/out of the membrane. All are integral

Cholesterol in membranes

Cholesterol is a component of animal cell membranes

• structure of cholesterol

• type of steroids, liquid
• most of molecule is hydrophobic so attracted to hydrophobic hydrocarbon tails, but contain hydroxyl group (—OH) which is hydrophilic
• Hydrophilic group attracted to phosphate heads

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Role of cholesterol in membranes

Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes

• hydrocarbon tails usually behave as liquid, phosphate heads act more like solid. Cell membranes do not correspond exactly to any of the 3 states.
• Overall membrane is fluid as components of the membrane are free to move
• Fluidity need to be control in membrane, too fluid—> less able to control what substances pass through, less fluid—> movement of cell and substances within it would be restricted
• Cholesterol disrupts regular packing of the hydro carbon tails, prevents them crystallizing and behaving as a solid
• It also restricts molecular motion and therefore fluidity of the membrane
• It also reduces the permeability to hydrophilic particles e.g. Na+
• Due to its shape cholesterol can help membranes to curve into a concave shape, which helps in the formation of vesicles during endocytosis