Membranes

• Compartmentalisation • Providing a selectively permeable barrier • Transporting solutes • Communication and intracellular interactions • Scaffold for biochemical pathways and activities • Scaffold for attachments to cytoskeleton and adjacent cells

• separates organelles from the cytoplasm so each organelle is doing its own function simultaneously • increases the efficiency of subcellular processes by confined spaces in cells • it allows the creation of specific microenvironments

• it prevent unrestricted exchange of molecules • allows communication between compartments

Helps organelles to communicate with each other and the external environment

1) simple diffusion - - passive - no specialised proteins required - goes down a concentration gradient 2) facilitated diffusion - - passive - carrier proteins needed - goes down a concentration gradient 3) osmosis - passive - specific to water - goes down a concentration gradient 4) active transport - active (requires energy from ATP) - goes against a concentration gradient - requires specialised channel and carrier proteins

External stimuli is transmitted into cells via specific receptors in the plasma membrane and so cells need to carry out specific responses Stimuli includes nutrients, ions, hormones, viruses and toxins Responses include endocytosis, intracellular signalling cascades , mitosis and changes in gene expression

• Membranes are sites of chemical reactions where enzymes involved are embedded in membrane structure • Membranes provide scaffolding within which components can be ordered for effective interaction • Membranes bring enzymes and substrates closer to each other to increase their collisions • Membranes are sites of energy transduction (chemical gradients are transformed into electrical gradients)

• the membrane supports the cytoskeleton by providing attachment sites • the cytoskeleton interacts with the extracellular matrix via transmembrane molecules providing cell shape and strength

• it is a sheet like bilayer composed of polar amphipathic lipids • it is 6-10 nm thick • all bilayers have a cytosolic face (faces the cytoplasm) and exoplasmic face (faces the external environment)

It is held together by cooperative non-covalent interactions - • hydrophobic interactions - these ensure the hydrocarbon tails are isolated from aqueous surroundings • van der waals forces - attraction between hydrocarbon tails favours close packing • electrostatic and hydrogen bonding - interactions between polar heads and water molecules keeps the bilayers together • bilayers 1 - close in on themselves to form compartments • bilayers 2 - invagination (endocytosis) and evagination (exocytosis)

• proposed by Singer and Nicholson in 1972 • it describes how the phospholipids, cholesterol molecules, proteins and carbohydrates are distributed across membranes • mosaic : the membrane is fluid which makes it impossible for it to form an impenetrable barrier • lipids can diffuse laterally, rotate and flip flop • proteins can only diffuse laterally and rotate but never flip flop

Membrane composition varies with cell type and it defines a cell’s function. The bilayer components are distributed unequally between the 2 surfaces to create asymmetry. • lipids and sterols form the permeable barrier • proteins carry out majority of the functions • polysaccharides linked to lipids make glycolipids and polysaccharides linked to proteins make glycoproteins used for signalling

Structure : lipid 76% - protein 18% - carbohydrate 3% Function : it insulates axons of peripheral nerves and has a high stability and so a low metabolic activity

Structure : lipid 43% - protein 49% - carbohydrate 8% Function : stable and metabolically active so it can transfer oxygen to tissues and remove carbon dioxide

Structure : lipid 24% - protein 76% - carbohydrate - 0% Function : high metabolic activity for the electron transport chain in aerobic respiration

• higher percentage of lipids = more stable • greater amounts of protein (enzymes) = high metabolic activity • polysaccharides are absent from internal membranes • cholesterols reduce the permeability of the membrane and increase compressibility

Lipids can undergo rapid lateral diffusion and transverse diffusion - transverse diffusion is the movement of the phospholipid from one face of the bilayer to the other face - lateral diffusion gives the membrane fluidity

Scramblase enzymes ensure both monolayers remain equally populated Flippase enzymes move the phospholipids from one face to the other face for asymmetry

The asymmetrical disposition of the lipids helps to give symmetry

• The phospholipids could separate from each other leaving gaps in the membrane allowing the passage of unwanted substances • The phospholipids could get closer together and prevent the passage of important substances • Could result in diseases such as Alzheimer’s

• only found on the exoplasmic leaflet • unable to show transverse diffusion • asymmetrically distributed • protect the cell from mechanical and chemical damage

They are grouped into 3 classes in regards to their relationship with the phospholipid bilayer

• intrinsic or integral • extrinsic or peripheral • lipid anchored

• penetrate hydrophobic core of the bilayer • single pass or multi pass transmembrane proteins • totally embedded in the membrane • function as receptors, channels, transporters and electron transfer agents • they are amphipathic - have hydrophobic and hydrophilic parts