4.1 - Structure of the Cell Membeane

• plasma membrane

  all membranes around and within cells
• Cell surface membrane

  membrane that surrounds cells
• role of cell surface membrane

  allow certain conditions to be established inside and outside the cell

  forms boundary between cell and its environment so controls what enters and leaves the cell

  partially permeable
• molecules that make up the cell surface membrane

  phospholipids

  proteins

  cholesterol

  glycolipids

  glycoproteins
• role of membrane around organelles

  acts as a barrier between organelle and cytoplasm dividing cell into different compartments

  partially permeable
• what phospholipids formulae in the cell membrane

  a bilayer:

  hydrophilic heads point outwards attracted by water on both sides,

  hydrophobic tail points inwards repelling water so water soluble substances aren't allowed through it
• what moves through phospholipid bilayer

  lipid soluble substances

  small substances
• function of phospholipids in membranes

  allow lipid soluble substances to enter and leave the cell

  prevent water soluble substances entering and leaving the cell

  make membrane flexible and self-sealing
• proteins in membranes

  scattered in the bilayer like a mosaic
• proteins on the surface of bilayer

  give mechanical support

  act as receptors to detect chemicals from other cells
• proteins in the bilayer

  proteins channels: water filled tubes that let water soluble ions through

  carrier proteins: bind to ions and large molecules and change shape to move them across the membrane
• functions of proteins in membranes

  provide support

  allow active transport

  act as receptors

  help cells adhere together

  channels transporting water soluble substances
• cholesterol

  a type of lipid
• function of cholesterol in membranes

  hydrophobic so prevents loss of water and dissolved ions

  add strength

  helps cell maintain shape

  make membrane less fluid at high temperatures

  bind to tails of phospholipids to pull them closer together so limit their movement so membrane is more rigid
• what are glycolipids made up of

  a carbohydrate covalently bonded to a lipid
• function of glycolipids in membranes

  carbohydrate portion extends from bilayer into watery environment and acts as a receptor for specific chemicals

  act as recognition sites

  help maintain stability

  help cells attach to each other to form tissues
• what are glycoproteins made up of

  carbohydrate chains attached to extrinsic proteins on the outer surface
• function of glycoproteins in membranes

  act as recognition sites

  act as receptors for hormones and neurotransmitters

  help cells attach to one another so form tissues

  allows cells to recognise one another
• permeability of cell membranes

  most molecules don't diffuse across easily because they aren't soluble in lipids, are too large, have the same charge as proteins channels or are polar so can't pass through non-polar hydrophobic tails of lipids
• fluid mosaic structure

  describes how molecules are arranged in the cell membrane

  fluid: phospholipid molecules can move relative to one another so membrane is flexible

  mosaic: proteins embedded in bilayer vary in shape, size and pattern and are scattered throughout bilayer
• functions of membranes in cells

  control entry and exit of material in organelles

  separate organelles from cytoplasm so specific metabolic reactions take place within them

  provide internal transport system

  isolate enzymes that might damage cell

  provide surfaces where reactions can happen