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

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