Membranes

• State 5 roles of membranes within cells and at the surface of cells

  CompartmentalisationControls what enters/exits the cellControls what enters/exits organellesCell communication/signallingSite of chemical reactions
• Define compartmentalisation and explain why it is useful to cells

  The formation of separate membrane bound areas in a cell. Keeps incompatible reactions separate, maintains necessary chemical conditions for different reactions and protects other organelles.
• Define partially permeable

  Membrane that allows some substances to cross but not others
• Define cell signalling

  A complex system of intercellular communcication
• Define phospholipid bilayer

  Arrangement of phospholipids found in cell membranes; the hydrophilic phosphate heads form both the inner and outer surface of the membrane, sandwiching the fatty acid tails to form a hydrophobic core
• Explain what is meant by 'fluid' and 'mosaic' in the fluid mosaic model of membrane structure

  Phospholipids within the bilayer are free to move (fluid) and proteins of various shapes and sizes are embedded in various positions
• Glycoprotein

  Intrinsic proteins embedded in cell surface membrane with attached carbohydrate chains. Cell adhesion + cell signalling - act as receptors for chemical signals
• Phospholipid

  Phosphate head and 2 fatty acid tails. Form bilayer, can mover around each other
• Cholesterol


  Lipid with hydrophilic and hydrophobic end. Regulates membrane fluidity
• Glycolipid

  Lipid with a carbohydrate chain attached. Cell markers or antigens recognised by immune system as self or non-self
• Channel protein


  Provide hydrophobic channel through membrane so polar molecules and ions can diffuse through
• Carrier proteins

  Involved in passive and active transport. For active the shape of the protein often changes.
• Intrinsic


  Through both layers
• Extrinsic


  One side of bilayer only
• Phospholipid diagram (components and properties)

  
• Diagram of fluid mosaic model

  
• State 3 pieces of evidence/experiments that support the fluid mosaic model of the membrane

  The fluid property of functional biological membranes had been determined through labeling experiments, x-ray diffraction, and calorimetry. These studies showed that integral membrane proteins diffuse at rates affected by the viscosity of the lipid bilayer in which they were embedded, and demonstrated that the molecules within the cell membrane are dynamic rather than static.Frye and Edidinused used Sendai virus to force human and mouse cells to fuse and form a heterokaryon. Using antibody staining, they were able to show that the mouse and human proteins eventually diffused and over time the border between the two halves was lost. Lowering the temperature slowed the rate of this diffusion by causing the membrane phospholipids to transition from a fluid to a gel phase.The fluid mosaic model explains changes in structure and behavior of cell membranes under different temperatures, as well as the association of membrane proteins with the membranes. Recent advances in fluorescence microscopy and structural biology have validated the fluid mosaic nature of cell membranes.
• Define cell adhesion

  When cells join together to form tight junctions in certain tissues
• Explain how cholesterol affects the fluidity and stability of membranes

  Cholesterol positioned between phospholipids - hydrophobic end interacts with tails and hydrophilic with heads pulling them together. Also prevents crystallising by stopping them coming too close.
• Explain the importance of membrane bound proteins in chemical reactions

  Have to be held in specific positions for reactions to take place e.g. Electron carriers and enzyme ATP synthase must be in correct positions in cristae for the production of ATP within mitochondria and enzymes for photosynthesis found held in membrane stacks of chloroplasts
• Explain how the binding of a molecule to a membrane bound glycoprotein can cause effects within a cell

  Binding of neurotransmitter triggers/prevents impulse in next neurone at synapseHORMONES
• Describe and explain the effect of changing temperature on the permeability of cell membranes

  Increased temp. increases permeability of cell membrane as phospholipids have more kinetic energy therefore move more. This more fluid membrane loses structure and at high enough temp. will break down completely
• Describe and explain the effect of solvent concentration on the permeability of cell membranes

  Solvents, e.g. ethanol, dissolve membranes making them more permeable as the structure is disrupted. Many organic solvents are less polar than water.
• Describe a method using a colorimeter to investigate the effect of temperature (or ethanol concentration) on the permeability of cell membranes

  Cut pieces of beetroot and place them in distilled water.Label a set of test tubes (one for each temperature of water bath) with the temperature. Add exactly 5 cm3 of distilled water to each test tube and place the tubes, one in each water bath, for 5 minutes to equilibrate to the water bath temperature.Remove the beetroot cores from the distilled water and blot gently on a paper towel. Place one 2 cm beetroot core into each test tube and leave in the water bath for 30 minutes.After 30 minutes, shake the test tubes gently to make sure any pigment is well-mixed into the water, then remove the beetroot cores.Set a colorimeter to respond to a blue/ green filter (or wavelength of 530 nm) and to measure absorbance. Check the colorimeter reading for distilled water.Measure the absorbance of each tube and plot a graph of absorbance against temperature.
• Define diffusion and explain the difference between simple and facilitated.

  The net movement of molecules or ions from an area of high concentration to an area of low concentration.The term simple diffusion refers to a process whereby a substance passes through a membrane without the aid of an intermediary such as a integral membrane protein. Facilitated diffusion requires the aid of a protein but is still a passive process (does not require ATP)
• Define osmosis

  The net movement of water by diffusion, through a partially permeable membrane from a solution of higher water potential to a solution of lower water potential down a concentration gradient.
• Explain the difference between passive and active transport

  Passive - does not require or use energyActive transport is the movement of particles across a plasma membrane against a concentration gradient. Energy is required.
• Define endocytosis

  The bulk transport of materials into cells by the invagination of the cell surface membrane forming a vesicle
• Define exocytosis

  The bulk transport of material out of a cell. Vesicles containing the material fuse with the cell-surface membrane and the contents are released to the outside of the cell.
• Define pinocytosis

  Endocytosis of liquid materials
• Define phagocytosis

  Process by which white blood cells called phagocytes recognise non-self cells, engulf them and digest them within a vesicle called a phagolysosome.
• Explain the difference between a channel protein and a carrier protein and link this to their functions


  Channel proteins are a hydrophobic channel allowing diffusion of polar molecules and ions whereas carrier proteins take part in active transport, changing shape to move molecules into the cell using ATP
• State three particular examples of channel proteins and 3 carrier proteins and for each describe the role that they preform

  Channel proteins:Examples of channel proteins include chloride, sodium, calcium, and potassium ion channelsCarrier proteins:GLUT1 - transports glucose across the bilayerCytochromes - transport electrons as part of 'electron transport chain'Cysteine carrier proteins - removing cysteine from urine and back into blood
• Explain the importance of the structure of ATP in active transport

  ATP binds to carrier protein and is hydrolysed into ADP and a phosphate group which also binds to the protein, changing its shape to allow the molecule being transported into the cell
• List the factors affecting the rate of diffusion of a molecule

  TemperatureConcentration difference
• Explain how substances can move across a membrane by simple diffusion

  Small molecules can pass through the phospholipid bilayerFactors affecting rate across the membrane:Thickness of membraneSurface area of membrane
• State which type of molecules can move across membranes by simple diffusion

  Small non-polar non-charged moleculesSome polar molecules e.g. water (but slowly)Smaller molecules more easily than larger molecules
• Explain how molecules move across membranes by facilitated diffusion

  Channel proteins are tunnels through which polar molecules or ions can pass from a higher concentration to a lower. Are often specific to the molecule or ion. Rate increases with more proteins.
• Explain how substances move across a membrane by active transport

  Molecule binds to outside of carrier protein, ATP to inside. ATP hydrolyses to ADP and phosphate which binds to protein, changing its shape and opening it to the inside of the cell so the substance is allowed in. ADP and phosphate group reform ATP and carrier protein returns to original shape. Specific substances are transported by specific proteins - it is a selective process.
• Explain how substances move into cell by endocytosis

  E.g. Phagocytosis