Cell Membranes

• Plasma membranes are found around cells and some organelles. 
• Plasma membranes are partially permeable barriers – selectively permeable. 
• Plasma membranes are made up of phospholipid molecules arranged in a bilayer (due to water being present inside and outside cells) and proteins.  
• Under an electron microscope you can see the bilayer as two dark bands. The distance across the membrane is about 7nm. 
• The fluid mosaic model:

  The component molecules are not bonded together so there is some movement but it is relatively stable because of the nature of the phospholipid.
• The fluid mosaic model:

  Singer and Nicholson proposed this model of how the components in the membrane are arranged in 1972.
• The fluid mosaic model:

  Fluid 

  • Phospholipids (and proteins) are free to move laterally (sideways). 

  Mosaic 

  • Proteins have a scattered arrangement within the phospholipid bilayer. 
• Phospholipids:

  • Make up most of the plasma membrane. Arranged in a bilayer. 
  • Hydrophobic tail faces inwards (away from water), hydrophilic head faces outward (towards water). 
• Role of phospholipids:

  • Act as a barrier to water soluble, polar molecules and ions. 
  • Allows lipid soluble, small, non-polar substances to diffuse through. 
  • Make the membrane flexible and self-sealing making endo and exocytosis possible. 
• Cholesterol:

  • Found in Eukaryotic cells for stability and fluidity of the cell membrane.  
• Cholesterol:

  • Steroid molecule that fits between fatty acid tails completing the membrane barrier to water and ions. 
• Cholesterol:

  • Binds to the hydrophobic tails of the phospholipids They prevent the membrane being too fluid at high temperatures but also stops the membrane solidifying at low temperatures. 
• Glycolipids:

  • Phospholipids with a carbohydrate attached.
• Roll of glycolipids:

  • Inter cell signalling and recognition 
  • Cell adhesion: helps to stick cells together and to basement membranes to form tissues. 
• Intrinsic proteins:

  • Channel or carrier proteins that allow transport of hydrophilic, polar molecules (like glucose) and ions through the phospholipid bilayer. 
• Extrinsic proteins:

  • Inter cell signalling 
  • Cell recognition  
  • Enzymes e.g. in mitochondria cristae for respiration. 
  • Receptors for signalling between cells e.g. hormone receptors. 
  • Cell adhesion  
• Glycoproteins:

  • Proteins with a carbohydrate attached.
• Glycoproteins:

  • Receptors for signalling molecules e.g. hormones, neurotransmitters 
  • Cell signalling and recognition e.g. cell surface antigens 
  • Binding cells together to basement membrane, making tissues, cell adhesion 
• Roles of plasma membranes within cells:  

  1. Separate cell components from cytoplasm COMPARTMENTALISATION 
  2. Holding the components of metabolic pathways in place e.g. enzymes for aerobic respiration in mitochondria. 
  3. Control what enters or leaves the organelle 
  4. Sites of attachment e.g. for enzymes, ribosomes 
• Roles of plasma membranes within cells:

  • Sites of attachment e.g. for enzymes, ribosomes 
• Roles of plasma membranes within cells:

  • Control what enters or leaves the organelle 
• Roles of plasma membranes within cells:

  • Holding the components of metabolic pathways in place e.g. enzymes for aerobic respiration in mitochondria. 
• Roles of plasma membranes within cells:

  • Separate cell components from cytoplasm - compartmentalisation 
• Roles of plasma membranes at the surface of cells:  

  1. Separate cell contents from the outside environment 
  2. Cell recognition and signalling 
  3. Regulating transport of materials into or out of cells 
  4. Create/ maintain concentration gradients 
• Roles of plasma membranes at the surface of cells:  

  • Create/ maintain concentration gradients 
• Roles of plasma membranes at the surface of cells:  

  • Regulating transport of materials into or out of cells 
• Roles of plasma membranes at the surface of cells:  

  • Cell recognition and signalling 
• Roles of plasma membranes at the surface of cells:  

  • Separate cell contents from the outside environment 
• Cell signaling:

  • Processes that lead to communication and coordination between cells, so that they can work together to trigger a response.
  • Cell signalling also allows cells to be recognised as self and so prevents cells from being destroyed by immune system.
  • A receptor in the cells plasma membrane detects chemical signals and brings about responses within the cell.  
• • Protein receptors found on target cells have a specific shape complementary only to a specific hormone (or neurotransmitter).
  • They will bind with the hormone to allow the cell to respond in a particular way. 
  • An example is an insulin receptor. Insulin is released when there is increased blood glucose concentration.
  • There are insulin receptors on liver and muscle cells, insulin binds to the receptors and the cells respond by taking up more glucose to reduce the blood glucose levels. 
• Plasma membranes are affected by temperature- it affects how much the phospholipids in the bilayer can move and this affects membrane structure and permeability.
• 35-50ºC = Increase in temperature increases kinetic energy of phospholipids and protein molecules in the membrane. Molecules being transported also have more kinetic energy and can diffuse across membrane more quickly. 
• 50-60 ºC = increase in temperature denatures proteins making membrane more permeable. Also, the phospholipid membrane becomes more fluid and permeable. This causes gaps to open temporarily in the bilayer and results in ions/molecules diffusing more quickly. 
• 60-70 ºC = Further increase in temperature causes the membrane to be destroyed. Ions/molecules diffuse in first few moments of experiment and reach equilibrium quickly.  
• • 35-50ºC : Increase in temperature causes an increase in kinetic energy of phospholipids and protein molecules. Molecules being transported also have more KE so move across the membrane faster.
  • 50-60ºC : Increase in temperature denatures proteins. The membrane becomes more permeable and the phospholipid bilayer becomes more fluid and permeable. Gaps open temporarily in the bilayer and ions/molecules diffuse through quickly.
  • 60-70ºC : Further increase in temperature causes the membrane to be destroyed. Ions and molecules quickly move across and equilibrium is reached quickly.