1.3 cell membrane structure and transport

• What are the two main components of cell membranes?

  Phospholipids and protein
• What are the chemical components of phospholipids?

  Glycerol, two fatty acids, and a phosphate group
• How do fatty acids behave in relation to water?

  They are hydrophobic and repel water
• What is the nature of the phosphate group in phospholipids?

  It is hydrophilic and attracts water
• What are the 'heads' and 'tails' of phospholipids?

  Hydrophilic 'head' and hydrophobic 'tails'
• What is the chemical structure of a phospholipid?

  • Glycerol backbone
  • Two fatty acid tails (hydrophobic)
  • One phosphate group (hydrophilic)
• In the phospholipid structure, what does the black circle represent?

  The charged phosphate group
• In the phospholipid structure, what does the green rectangle highlight?

  The two fatty acid components
• A simple structure of a phospholipid is a glycerol backbone with two fatty acid tails

  
• The simplest form of a phospholipid

  
• If isolated phospholipids are placed on water, they form a monolayer with the hydrophilic heads in the water and the tails which repel water sticking out into the air.

  
• In cells, as cytoplasm is watery and cells are bathed in watery solutions, phospholipids arrange themselves in a bilayer – with the hydrophilic heads next to the water on both sides and the hydrophilic tails in the centre.

  
• The other main component of a cell membrane is protein.

  Proteins can be intrinsic or extrinsic. Intrinsic proteins span the membrane from one side to the other. Extrinsic proteins are found in one side of the bilayer or on the surface of the bilayer. Some intrinsic proteins form channels through the membrane as shown in the diagram.

  
• Proteins have hydrophilic and hydrophobic portions depending on the location of the R group in the primary sequence. Hydrophilic parts with polar/charged R groups are located around the phospholipid heads or on the outside. Hydrophobic portions with non-polar R groups are arranged near/between the hydrophobic tails. Channel proteins often have charged/polar R groups lining the channel.

  
• What is the nature of cell membranes?

  Cell membranes are selectively permeable.
• Why are cell membranes described as selectively permeable?

  Due to the hydrophobic layer's impermeability.
• Which types of molecules can cross the membrane by simple diffusion?

  Non-polar molecules like oxygen and carbon dioxide.
• What role do channel proteins play in cell membranes?

  They allow specific charged or polar particles to diffuse.
• What are protein carriers involved in?

  Active transport and facilitated diffusion.
• What is the function of specific receptors in cell membranes?

  They bind hormones or neurotransmitters.
• What is ATP synthetase classified as?

  An intrinsic protein embedded in mitochondria.
• What are glycoproteins?

  Proteins with carbohydrate groups attached.
• What are glycolipids?

  Phospholipids with carbohydrate groups attached.
• What is the glycocalyx?

  The carbohydrates projecting outside of the cell.
• What is the primary function of the glycocalyx?

  Cell to cell recognition.
• Where is the glycocalyx found?

  On the side of the bilayer facing out of the cell.
• What role do specific antigens play in the cell membrane?

  They are unique to each individual.
• How does cholesterol affect the cell membrane?

  It regulates the fluidity of the membrane.
• What happens if there is too little cholesterol in the membrane?

  The membrane becomes too fluid.
• What happens if there is too much cholesterol in the membrane?

  The membrane becomes too rigid.
• Who proposed the fluid mosaic model of membrane structure?

  Singer and Nicolson in 1972.
• What does the 'fluid' aspect of the fluid mosaic model refer to?

  The lateral movement of molecules in the membrane.
• What does the 'mosaic' aspect of the fluid mosaic model refer to?

  The random pattern of proteins in the membrane.
• What evidence supports the mosaic aspect of the fluid mosaic model?

  Images from freeze fracture electron microscopy.
• How is the fluidity of the membrane evidenced?

  Through experiments like cell hybridization.
• What are the main functions of proteins and phospholipids in cell membranes?

  • Selectively permeable barrier
  • Facilitate diffusion and active transport
  • Serve as receptors for signaling
  • Provide structural support
• What are the components of the fluid mosaic model?

  • Fluid: lateral movement of molecules
  • Mosaic: random arrangement of proteins
  • Evidence: freeze fracture microscopy and hybridization experiments
• The evidence for fluidity comes from experiments. One such experiment is hybridisation of cells; this is where two cells are fused together.

  An hour after fusion, the red and green dyed proteins have dispersed evenly across the membrane. This shows that the proteins move laterally within the membrane.

  Different experiments involve dyeing either phospholipid or protein molecules, or attaching fluorescent markers and bleaching an area. Over time the bleached areas ‘disappear’, as the bleached and marked molecules mix.

  
• What trend is observed in beetroot color as temperature increases?

  Color changes from light to dark
• How does a colorimeter improve measurement accuracy?

  It provides numerical data for graphs