cell membranes

what is the function of the cell membrane?

• separates organelles from cytoplasm - allows specific metabolic reactions to happen e.g. respiration in mitochondria, photosynthesis in chloroplasts

• controls entry/exit of materials

• provides an internal support system e.g. ER

• isolates enzymes that could cause cell damage e.g. lysosomes

• provides surface for reactions to take place e.g. RER

can you label this fluid mosaic model of the membrane?

what happens when membranes interact w/ water?

• phospholipids in water spontaneously arrange themselves in lipid bilayers

• this means that the hydrophilic head groups can interact w/ the water molecules and the hydrophobic tails cluster together away from the water molecules

what is the function of the phospholipid bilayer?

• acts as a barrier - centre of bilayer is hydrophobic so water soluble substances (ions and polar molecules) cannot pass through

• small, lipid-soluble and non-polar molecules can diffuse through

  

what are the two types of artificial structure phospholipids can form? how are they formed?

• liposome - phospholipids w/ long tails arrange themselves in a bilayer forming a small vesicle

• micelle - phospholipids w/ short tails arrange themselves in a micelle in water

  

how can phospholipid bilayers become energetically favourable?

• phospholipid bilayers spontaneously close on themselves to form closed compartments

• the closed structures is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water which energetically unfavourable

  

what is the function of an intrinsic/integral protein?

• allow large molecules, polar molecules and ions to pass through

• the transport method through a carrier/channel protein is either facilitated diffusion (diffusion facilitated by carrier proteins) or active transport

• intrinsic proteins go across the entire bilayer

  

what are the 4 types of intrinsic protein? are they active or passive?

• carrier protein (passive)

• gated-channel protein (passive)

• channel protein (passive)

• carrier protein (active)

  

what are the functions of extrinsic proteins?

• can act as receptors/antigens

• important to facilitate cellular recognition, communication and adhesion

  

what are the two types of extrinsic proteins?

• glycoprotein

• peripheral protein

what is the function of cholesterol?

• fits between the phospholipids - restricts movement and increases stability by packing phospholipids together more closely

• important to maintain shapes of cells, particularly cells that aren’t supported by other cells

  

why is the fluid mosaic model called the fluid mosaic model?

• “fluid” - phospholipids are constantly moving

• “mosaic” - proteins are scattered throughout the bilayer like tiles in a mosaic

what are the properties of cholesterol? why are they significant (for fluidity)?

• amphipathic - has both hydrophilic and hydrophobic regions (like phospholipids)

• cholesterol’s polar, hydrophilic region attracts the polar, hydrophilic head groups on the phospholipid molecules

• the rest of the cholesterol molecule is non-polar and hydrophobic, so attracts the non-polar, hydrophobic fatty acids in the phospholipids

• cholesterol interacts w/ the fatty acid tails of phospholipids and reduces membrane fluidity and permeability

what are the factors affecting membrane fluidity? how do they affect membrane fluidity?

• temperature - increase in temperature → increase in fluidity as KE of tails increases

• no. of double bonds in fatty acids - increase in double bonds → increase in fluidity

• amount of cholesterol - decrease in cholesterol → increase in fluidity

which substances are the phospholipid bilayer permeable to?

• small, non polar molecules are soluble and so can pass through e.g. oxygen, carbon dioxide

• small, non polar molecules (but w/ polar bonds) can diffuse through, e.g. water

which substances are the phospholipid bilayer permeable to?

• large molecules

• all charged molecules and ions