Membranes

Assumed Knowledge Membrane components Membrane structure Membrane properties Membrane functions Membrane-bound organelles

Membrane components Composed of a phospholipid bilayer  Phosphate ‘head’- hydrophilic  Lipid ‘tail’ – hydrophobic Arrangement prevents contact between the lipid core and water in the extracellular/intracellular environment

Carbohydrates – mainly glycoproteins (glyco- meaning sugar) Proteins  Mainly transmembrane proteins – these span the entire phospholipid bilayer e.g. ion channels, G- protein coupled receptors Cholesterol

Membrane structure The phospholipid bilayer gives the membrane fluidity This fluidity means proteins and other components e.g. cholesterol can move across the bilayer Cholesterol makes the membrane rigid – more cholesterol, more rigid Proteins and other molecules in the membrane give it an mosaic effect The fluid mosaic model

Membrane Properties Semi permeability: this means the membrane is permeable to some molecules but not all: Permeable toNot permeable to - Hydrophobic molecules e.g. O 2 - Large molecules e.g. glucose - Small uncharged molecules - Ions e.g. K +, H +, Cl -

Asymmetry: the extracellular side and the intracellular side of the membrane differ in their composition – this allows for recognition between the inside and outside environment

Membrane Functions Separation of the extracellular and intracellular environments Anchorage of the cytoskeleton maintaining the shape of the cell (the cytoskeleton is an intracellular network of proteins that act like a scaffolding) Attachment of the extracellular matrix for grouping of cells together to form tissues Selective permeability Endocytosis (absorption of molecules by engulfing them) Exocytosis (removal of membrane bound molecules by fusion with the plasma membrane) Transport via proteins in the plasma membrane

Endocytosis Exocytosis

Membrane bound organelles Within the cell there are numerous membrane bound organelles Organelles are structures within the cell with a unique function -Endoplasmic reticulum: synthesis of proteins and lipids -Golgi apparatus: protein modification, packaging of molecules for exocytosis or use in the cell -Lysosome: break down of cellular waste and debris -Nucleus: storage and replication of DNA -Mitochondria: ATP synthesis