AQA Biology Unit 2: 10 Variety of Life

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10.1 Haemoglobin

  • Primary structure of a protein is the sequence of amino acids determined by DNA that makes up a polypeptide chain
  • It is this sequence that determines how the polypeptide chain is shaped into its tertiary structure
  • Haemoglobins: group of protein molecules that have a quaternary structure

Haemoglobin Molecules

  • Haemoglobins are a group of chemically similar molecules found in organisms
  • The structure is made up of:
    • Primary Structure: consisting of four polypeptide chains
    • Secondary Structure: Each polypeptide chain is coiled into a helix
    • Tertiary Structure: Each polypeptide chain is folded into a precise shape allowing ability to carry oxygen
    • Quaternary Structure: All four polypeptides are linked together. Each polypeptide is associated with a haem group which contains a ferrous (Fe2+) ion. Each Fe2+ ion can combine with an O2 molecule
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Haemoglobin Cont

The Role of Haemoglobin

  • Transport oxygen efficiently
    • It can readily associate with oxygen at the surface where gas exchange occurs
    • It can readily dissociate from oxygen at tissues requiring it
  • These two requirements are achieved by haemoglobin being able to change its affinity for oxygen under different conditions
  • Haemoglobin can change shape in the presence of certain substances such as CO2
  • With CO2 present haemoglobin molecule binds more loosely to oxygen

Why have Different Haemoglobin

  • Haemoglobin with high affinity for oxygen: take up O2 easily but release it less easily
  • Haemoglobin with low affinity for oxygen: take up O2 less easily but release it easily
  • Organism living in low O2 area requires high affinity haemoglobin
  • Organism with high metabolic rate needs low affinity haemoglobin
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Haemoglobin Cont

Why different Haemoglobins have different Affinities

  • Due to slightly different amino acid sequences

Loading and Unloading Oxygen

  • Loading/Associating: process haemoglobin combines with oxygen
  • Unloading/dissociating: process haemoglobin releases oxygen
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10.2 Oxygen Dissociation Curves

  • When haemoglobin is exposed to different partial pressures of oxygen it does not absorb O2 evenly
  • Low Concentrations: Four polypeptides of haemoglobin molecule are closely united so difficult to absorb first O2 molecules
  • Once loaded the O2 causes the polypeptides to load remaining O2 easily

Graph Reading

  • The further to the left the curve the greater the affinity of haemoglobin for oxygen (it takes up oxygen readily but releases it less easily)
  • The further to the right the curve the lower the affinity of haemoglobin for oxygen
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Dissociation Curves Cont.

Effects of CO2 Concentration

  • Haemoglobin has a reduced affinity for O2 in the presence of CO2
  • The Bohr Effect: the greater the concentration of CO2 the more readily O2 is released
    • At gas exchange surfaces CO2 level is low due to it being expelled. The affinity of haemoglobin for oxygen is increased meaning O2 is readily loaded by haemoglobin. The reduced CO2 level shifts the graph to the left
    • In rapidly respiring tissues the CO2 level is high, thee affinity for oxygen is reduced causing the oxygen to be readily unloaded from the haemoglobin. The increased CO2 level shifts the graph to the right
  • Carbon Dioxide is acidic so lowers pH causing haemoglobin to change shape
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Dissociation Curves Cont.

Loading, Transport and Unloading Oxygen

The higher the rate of respiration->the more carbon dioxide the tissue produces->the lower the pH->the greater the haemoglobin shape change->the more readily oxygen is unloaded-> the more oxygen available for respiration

  • In humans haemoglobin becomes saturated in the lungs so they carry the 4 oxygen molecules
  • When the haemoglobin reaches a low respiratory rate tissue one of the oxygen molecules is released
  • At a very active tissue 3 oxygen molecules will be unloaded from each haemoglobin
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10.3 Starch, Glycogen and Cellulose


  • Found in plants: seeds and storage organs
  • Forms important part of diets as an energy source
  • Made up of alpha-glucose monosaccharides linked by glycosidic bonds
  • Glycosidic bonds formed by condensation reactions
  • Unbranched chain wound into a coil so compact

Suited as a energy store:

  • Insoluble so doesn't draw in water by osmosis
  • Doesn't diffuse out of cells easily as insoluble
  • Compact: lots can store in small space
  • Hydrolysed into alpha glucose used in respiration
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  • Similar structure to starch
  • Short chain and highly branched
  • Major carbohydrate storage in animals
  • Stored as small granules in muscles and liver
  • Readily hydrolised due to small chains
  • Not found in plant cells
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  • Made of Beta-Glucose monomers: position of -H group and -OH group on single carbon atom are reversed
  • -OH group is above the ring so to form glycosidic bonds the monomers are rotated 180 degrees
  • The -CH2OH group on each Beta-Glucose molecule alternates
  • Forms straight unbranched chain that runs parallel allowing hydrogen bonds to form cross links between adjacent chains
  • Cellulose is stregthened by the multiply hydrogen bonds
  • The cellulose molecules group to form microfibrils which form fibres
  • Major component of plant cell walls as provides rigidity
  • Prevents cell bursting when water enters as it exerts an inward pressure
  • Herbaceous parts of plants are semi-rigid as the cells push against each other
  • Important in stems and leaves
  • Provide maximum surface area for photosynthesis
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10.4 Plant Cell Structure

  • Plant cells are eukaryotic cells: have distinct nucleus and membrane bound organelles like mitochondria and chloroplasts

Leaf Palisade Cell

  • Function: Photosynthesis
  • Features:
    • Long, thin cells form continuous layer (Absorb Sunlight)
    • Chloroplast that arrange themselves for maximum light
    • Chloroplast carries out the photosynthesis
    • Large vacuole pushes cytoplasm and chloroplast to edge of cell
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  • Typically disc shaped


  • Chloroplast Envelope: double plasma membrane surrounds organelle, selects what enters and leaves the cell
  • Grana: Stacks of discs called thylakoids, thylakoids contain chlorophyll. The first stage of photosynthesis occurs here
  • Stroma: fluid filled matrix that contains other structures such as starch grains. The second stage of photosynthesis occurs here

Adaptations for Photosynthesis:

  • Granal membranes provide large surface area for chlorophyll attachment, electron carriers ans enzymes that carry out 1st stage of photosynthesis
  • Fluid of stroma possess enzymes needed for 2nd stage of photosynthesis
  • Chloroplast contains DNA and ribosomes that allow manufacture of proteins for photosynthesis
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Cell Wall

  • Consists of microfibrils of cellulose embedded in a matrix


  • Consists of many polysaccharides
  • Middle Lamella marks boundary between adjacent cell walls that sticks them together


  • Provide mechanical strength to stop cell bursting
  • Mechanical strength to whole plant
  • Allow water to pass along it
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Root Hair Cell

  • Absorb water and mineral ions
  • Water absorbed by osmosis
  • Roots have high concentration of ions and sugar compared to soil
  • Uptake of mineral ions is against the concentration gradient so used active transport
  • Special carrier proteins use ATP to absorb mineral ions
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Xylem Vessels

  • Transport water
  • Thick cell walls
  • Formed from dead cells
  • Lignin often forms rings around the vessel
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Really helpful thanks ;)

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