Cell membranes

• What are cell membranes composed of?

  Lipids (mainly phospholipids), proteins & carbohydrates (attached to proteins or lipids).
• Fluid mosaic model:

  Describes arrangement of molecules in membrane. Phospholipid molecules form continuous bilayer- it's fluid as phospholipids constantly moving. Proteins scattered through bilayer- include channel & carrier proteins that allow large molecules & ions to pass through membrane. Receptor proteins allow cell to detect chemicals released from other cells & cell can respond. Glycoproteins & glycolipids present. Cholesterol molecules present in bilayer. Some proteins can move sideways through bilayer.
• What do phospholipids do?

  Form barrier to dissolved substances. Hydrophilic head & hydrophobic tail. Heads face out towards water on either side of membrane. Centre of bilayer hydrophobic so water-soluble substances (ions & polar molecules) can't diffuse through. Small, non-polar substances & water can diffuse through. (Water is small).
• What is cholesterol?

  Lipid present in cell membranes (except bacterial). Gives membrane stability. Fits between phospholipids. Binds to hydrophobic tails of phospholipids causing them to pack more closely together. Restricts movement of phospholipids making membrane less fluid & more rigid. Helps maintain shape of animal cells as they don't have cell walls. Has hydrophobic regions to create further barrier. Very important for cells not supported by other cells e.g, red blood cells.
• How do temperatures below 0.C affect cell membranes?

  Phospholipids don't have much energy so can't move very much. Packed closely together & membrane is rigid. Channel & carrier proteins denature, increasing permeability of membrane. Ice crystals may form & pierce membrane making it highly permeable when it thaws.
• How do temperatures between 0.C & 45.C affect cell membranes?

  Phospholipids can move around & aren't packed as tightly together. Membrane partially permeable. As temperature increases, phospholipids move more as they have more energy- increases permeability of membrane.
• How do temperatures above 45.C affect cell membranes?

  Phospholipid bilayer starts to melt & membrane becomes more permeable. Water inside cell expands, putting pressure on membrane. Channel & carrier proteins in membrane denature, so can't control what enters & leaves cell. Increases permeability of membrane.
• Investigating how temperature affects beetroot membrane permeability method:

  Use scalpel to cut 5 equal sized beetroot pieces & rinse to remove any pigment released during cutting. Add 5 pieces to 5 different test tubes each with 5cm3 water. Place each tube in water bath at different temperature (10,20,30,40 & 50.C) for same length of time. Remove pieces leaving coloured liquid. Use colorimeter & measure absorbance of each solution. Higher absorbance= more pigment released so higher permeability.
• How do you use the colorimeter?

  Switch on & allow 5 mins for it to stabilise. Use blue filter. Add distilled water to cuvette so it's 3/4 full. Put cuvette into colorimeter. Calibrate machine to 0. Transfer sample of liquid from 1st tube using pipette into clean cuvette so 3/4 full. Put into colorimeter & record absorbance. Repeat for each liquid.
• What is simple diffusion?

  Net movement of particles from an area of higher concentration to an area of lower concentration. Passive process. Molecules diffuse directly through cell membrane.
• What is the concentration gradient?

  Path from an area of higher concentration to an area of lower concentration (particles diffuse down it).
• Factors affecting rate of simple diffusion:

  Concentration gradient, thickness of exchange surface & surface area.
• How does concentration gradient affect rate of diffusion?

  Steeper it is, faster the rate. As diffusion takes place, difference in concentration between 2 sides of membrane decreases until it reaches an equilibrium (diffusion slows over time).
• What does facilitated diffusion involve?

  Carrier & channel proteins to aid diffusion of large molecules & charged particles. Normally they would diffuse slowly due to size and water-solubility.
• How do carrier proteins work?

  Move large molecules across membrane down their concentration gradient. Different carrier proteins facilitate diffusion of different molecules. Large molecule attaches to carrier protein in membrane. Protein changes shape- this releases molecule on opposite side of membrane.
• How do channel proteins work?

  Form pores in membrane for charged particles to diffuse through. Different channel proteins facilitate diffusion of different charged particles.
• Factors affecting rate of facilitated diffusion:

  Concentration gradient: steeper, faster rate. As equilibrium reached rate levels off. Number of channel or carrier proteins.
• How do you calculate rate of diffusion?

  Draw tangent & calculate gradient.
• What is a phospholipid?

  Triglyceride in which 1 of the 3 fatty acid molecules is replaced by a phosphate molecule.
• What is osmosis?

  Diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential.
• What is water potential?

  Potential of water molecules to diffuse out of or into a solution.
• What is the water potential of pure water?

  Zero
• What is a hypotonic solution?

  Has higher water potential than inside cell, so cell swells.
• What is a hypertonic solution?

  Has lower water potential than inside cell so cell shrinks.
• What is an isotonic solution?

  Solution has same water potential as inside cell.
• Factors affecting rate of osmosis:

  Water potential gradient (higher water potential, faster rate. As osmosis goes on, difference in water potential on either side of membrane decreases, so rate levels off over time), thickness of exchange surface & surface area.
• How do you investigate water potential?

  Make serial dilutions, measure change in mass of potato chips & produce calibration curve (plot percentage change in mass against concentration of sucrose solution. Determine water potential of potato cells).
• How do you make the serial dilutions?

  Line 5 test tubes. Add 10cm3 2M sucrose solution to 1st tube & 5cm3 distilled water to other 4 tubes. Using pipette, transfer 5cm3 of 1st tube solution to 2nd tube & mix. (This is 1/2 concentrated as 1st tube). Repeat process x3.
• How do you measure the change in mass of the potato chips?

  Use cork borer to cut potatoes into identically sized chips (1cm diameter). Divide chips into groups of 3 & measure mass of each group using mass balance. Place 1 group into each sucrose solution & leave 20mins. Remove chips & pat dry. Weigh each group again & record results. Calculate % change in mass for each group. Chips will gain water (& mass) in solutions with higher water potential.