Membranes

1. Membranes

2. Assumed Knowledge • Membrane components • Membrane structure • Membrane properties • Membrane functions • Membrane-bound organelles

3. 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

5. 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

7. 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

8. Membrane Properties • Semi permeability: this means the membrane is permeable to some molecules but not all: Permeable to Not permeable to - Hydrophobic molecules e.g. O2 - Large molecules e.g. glucose - Small uncharged molecules - Ions e.g. K+, H+, Cl-

9. 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

10. 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

11. Endocytosis Exocytosis

12. 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