biology 2.5

• what is compartmentalisation?

  cell membranes seperate all different areas within the cel or within the organelle
• why are cell membranes important?

  sites of chemical reactions

  • allow chemical gradients to be maintained
  • eg: mitochondria, cytoplasm, chloroplasts

  sites of signalling

  • cells communicate with each other via receptors on membranes
• why is signaling specific?

  only the correctly shaped molecule can trigger a response
• what are plasma membranes made of?

  phospholipids

  proteins

  cholesterol
• why is it called a fluid mosaic?


  fluid - because individual phospholipids and proteins can move freely within the layer

  mosaic - meaning made of pieces
• what are intrinsic/integral proteins?

  span the entire bilayer

  have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane keeping them in place
• what are extrinsic/peripheral proteins?


  • smaller proteins on one side of the bilayer only
  • can be present in either layer and some move between layers
  • have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins
• what are the functions of proteins?

  can act as enzymes

  transport

  receptors/cell signaling

  • glycoproteins and glycolipids
• what are channel proteins?


  provide hydrophilic channels that alloallowws the passive movement of polar memoleculescules and ions down a concentration gradient through membranes
• what are glycoproteins?


  intrinsic proteins with carbohydrate side chains attached

  • cell adhesion
  • cell signaling
• what are glycolipids?


  lipids with carbohydrate chains attached

  • act as antigens
  • cells of the immune system can use them to recognise self and non-self cells
• what does cholesterol do?


  lipid like steriod

  has polar and non-polar regions

  reduces membrane permeability

  increases membrane stability
• what factors affect membrane permeability?


  temperature

  solvents (alcohol/water)
• what are the effects of temperature on cell membranes?


  as the temperature increases the kinetic energy of the phospholipids increases

  the molecules move around more creating gaps that causes the pigments and other molecules to leak out

  • membrane becomes more permeable
• what happens if the temperature continues to rise?


  proteins denature

  • channel and carrier proteins stop working which affects the permeability of the membrane to polar molecules
  • also creates more gaps in the membrane so more leakage occurs

  eventually the membrane will break down completely
• what are the effects of solvents on cell membranes?


  many solvents are less polar than water (eg: alcohol) or non-polar (eg: benzene)

  organic solvent will dissolve membranes disrupting cells
• what are the effects of alcohol on cell membranes?


  pure/very strong alcohol is toxic and will destroy cells

  less concentrated solutions (alcoholic drinks) will not dissolve membranes but will cause damage

  to the alcohol molecules that can enter the membrane disrupting its structure
• what effects does alcohol have on nerve cells?


  nerve cells need intact membranes for nervous impulses to be sent correctly

  alcohol destroys receptor cells so not as many signals are picked up slowing down the nervous system therefore slowing down brain activity
• what is ATP?


  short term store of energy

  its unstable but easy to form by the phosphorylation of ADP so is made as needed
• why are large stores of ATP not needed?


  the interconversion of ATP and ADP is happening continuously
• what is osmosis?


  the net movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane due to the random movement of molecules
• what is water potential?


  the tendency of water molecules to move from one region to another

  • water molecule used to dissolve solute are not free to move
  • pure water has the highest water potential
  • measured in units of pressure - Pascals (Pa)
• what happens to plant cells in pure water?

  there is a higher water potential outside the cell

  hypotonic solution

  water moves into the cell so it swells and becomes turgid
• what stops the cell from bursting?

  cellulose cell wall
• what happens to animal cells in pure water?

  higher water potential outside the cell

  • hypotonic solution

  water moves into the cell so it swells and bursts

  • cytolysis
• what is the structure of a cell membrane?


  membranes are formed by a phospholipid bilayer.

  the hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane putting the fatty acid tails of the phospholipids inside to form a hydrophobic core inside the membrane
• what is a phospholipid bilayer made up of?


  hydrophilic phosphate heads - interact with water

  hydrophobic tails
• what is the cell membrane theory?


  membranes were seen for the first time following the invention of electron microscopy.

  Images taken in the 1950's showed the membrane as two black parallel lines supporting an earlier theory that membranes were composed of a lipid bilayer
• who are the scientists who came up with the model about the lipid bilayer model?

  Singer and Nicolson proposed a model in which proteins occupy various positions in the membrane.

  model is known as the fluid-mosaic model because the phospholipids are free to move within the layer relative to each other giving the membrane flexibility.
• what are the components of a cell membrane?

  glycoprotein

  glycolipids

  cholesterol

  protein molecule partially embedded (extrinsic protein)

  hydrophobic tails of phospholipid

  protein molecule spanning the phospholipid layer (intrinsic protein)

  protein molecule lying on the surface (extrinsic protein)

  pore

  hydrophilic heads
• what does the glycoprotein do in the cell membrane?


  branching carbohydrate portion of a protein which acts as a recognition site for chemicals eg: hormones
• what does the glycolipid do in the cell membrane?


  acts as a recognition site
• what does the cholesterol do in the cell membrane?


  for stability/ flexibility
• what are membrane proteins?

  have important roles in various functions of membranes have important roles in various functions of membranes

  there are two types - intrinsic and extrinsic
• what are carrier proteins?


  have an important role in both passive transport and active transport (down and against a conc gradient) into cells

  often involves the shape of the protein changing
• what are some examples of intrinsic proteins?

  carrier proteins

  channel proteins

  glycoproteins

  glycolipids
• what is cell signalling?


  when the chemical binds to the receptor it elicits a response from the cell - this may cause a direct response or set off a cascade of events
• what are some examples of cell signaling?


  receptors for neurotransmitters such as acetylcholine at nerve cell synapses. the binding of the neurotransmitters triggers or prevents an impulse in the next neurone

  receptors for peptide hormones including insulin and glucagon which affect the uptake and storage of glucose by cells
• what is cholesterol?


  • a lipid with a hydrophilic end and a hydrophobic end like a phospholipid. it regulates the fluidity of membranes
  • positioned between phospholipids in a membrane bilayer with the heads and the hydrophobic end interacting with the tails pulling them together adding stability
  • prevent the membranes becoming too solid
• how does cholesterol stop the cell membranes from becoming too solid?


  stopping the phospholipid molecules from grouping too closely and crystalising