Membranes
Assumed Knowledge
• Membrane components• Membrane structure• Membrane properties• Membrane functions• Membrane-bound organelles
Membrane components
• Composed of a phospholipid bilayerPhosphate ‘head’- hydrophilicLipid ‘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 to Not permeable to
- Hydrophobic molecules e.g. O2 - 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