BIOL2 The Variety of Life

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The Structure of Haemoglobin Molecules

  • Primary structure - sequence of amino acids in four chains of amino acids
  • Secondary structure - coiled into a helix, 2 alpha 2 beta
  • Tertiary structure - chain folded into precise shape to allow it to carry oxygen
  • Quaternary structure - four polypeptide chains linked to form an almost spherical molecule. Each is associated with a haem group which contains iron which combines with a single oxygen. 
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The Role of Haemoglobin

  • Transport oxygen
  • To be efficient, it must readily associate at gas-exchange surfaces and dissociate at respiring tissues
  • Haemoglobin changes its affinity for oxygen under different conditions because its shape changes in the presence of certain substances
  • In the presence of CO2, haemoglobin changes shape and binds more loosely with oxygen so it is more easily dissociated
  1. First oxygen binds to a haem group so distorts the shape of the haemoglobin molecule so the second, third and fourth oxygen molecules are taken up increasingly quickly
  2. Fourth oxygen molecule is taken up several hundred times more quickly than the first
  3. At respiring tissues with low pO2, first oxygen is released very quickly but others are released much more slowly
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Why have different haemoglobins?

  • Haemoglobins with a high affinity for oxygen take up oxygen quickly but release it less readily and vice versa
  • Organisms living in environments with little oxygen require haemoglobin with a high affinity for oxygen provided their metabolic rate is not very high, so the body can absorb enough oxygen but does not use it up too quickly
  • Organisms with high metabolic rate need haemoglobin with a low affinity for oxygen so it is dissociated quickly and they receive the evergy they require 
  • Lugworm's are not very active but live in areas with low pO2, so they need a high affinity for oxygen
  • Mice have a large SA:V ratio so lose heat rapidly and need a high metabolic rate to compensate for their temperature regulation
  • Haemoglobins have different affinities for oxygen because they have different shapes because they have slightly different base sequences and therefore a different arrangement of amino acidics in their primary structures
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Oxygen Dissociation Curves

  • The further to the left the curve, the greater the haemoglobin's affinity for oxygen
  • The further to the right the curve, the lower the haemoglobin's affinity for oxygen
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Effects of Carbon Dioxide Concentration

  • Haemoglobin has a reduced affinity for oxygen in the presence of CO2
  • The Bohr Effect: Tthe greater the concentration of CO2, the more rapidly haemoglobin releases its oxygen
  • When CO2 dissolves in the blood, it produces carbonic acid which lower pH values
  • Low pH causes oxygen to dissociate more quickly
  • Curve shifts to the right
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Starch

  • Polysaccharide
  • Found in plants as small grains, especially in storage organs like potato tubers
  • Major energy source for food
  • Made up of chains of alpha glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions
  • Unbranched chai is wound into a tight coil that makes the molecule very compact
  • Starch is suited for storage because:
  1. It is insoluble and therefore does not affect osmosis
  2. It is insoluble so does not easily diffuse out of cells
  3. It is compact so lots is stored in a small space
  4. When hydrolysed it forms alpha glucose which is easily transported and used in respiration
  • The human equivalent of starch is glycogen
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Glycogen

  • Similar structure to starch but chains are shorter and it is more branched
  • It animals it is storedd as small granules in the muscles and liver
  • It is even more readily hydrolysed to alpha glucose because of the shorter chains
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Cellulose

  • Made of monosaccharides of beta glucose rather than alpha glucose
  • To form glycosidic links, each beta glucose molecule must be rotated by 180 degrees compared to its neighbour so the -CH2OH group alternates between being above and below the chain
  • Does not form a coiled chain like starch, but forms straight, unbranched chains 
  • Chains run parallel to one another so hydrogen bonds form cross-links between them
  • Hydrogen bonds are very weak but the sheer number of them strengthens cellulose
  • Cellulose moleculesare grouped together in microfibrils which are arranged in parallel groups called fibres
  • Cellulose is a major component of plant cell walls and provides rigidity for the plant cell
  • The cell wall also stops the cell from bursting when water enters it during osmosis
  • As a result, living plant cells are turgid so stems and leaves provide maximum SA for photosynthesis
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Leaf Palisade Cell

  • Long, thin cells that form a continuous layer to absorb sunlight
  • Lots of chloroplasts that arrange themselves in the best positions to catch maximum sunlight
  • Large vacuole that pushes the cytoplasm and chloroplasts to the edge of the cell to keep it turgid
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Chloroplasts

Features:

  • Chloroplast envelope: double plasma membrane that surrounds the organelle that is highly selective in what it allows to enter and leave the chloroplast
  • Grana: stacks of up to 100 thylakoids where the first stage of photosynthesis takes place
  • Thylakoids: contain the photosynthetic pigment chlorophyll and may also have tubular extensions that join with thylakoids in adjacent grana 
  • Stroma: fluid-filled matrix where the second stage of photosynthesis takes place which also contains other structures like starch grains

Adapted to their function because:

  • Granal membranes provide a large SA for the chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis 
  • Fluid of the stroma possesses all the enzymes needed to carry out the second stage of photosynthesis
  • Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some proteins needed for photosynthesis
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Cell Wall

  • Consists of microfibrils of cellulose embedded in a matrix which have considerable strength so contribute to the strength of the cell wall
  • Thin layer called the midle lamella which marks the boundary between adjacent cell walls and cements cells together
  • Cell wall provides mechanical strength in order to prevent the cell bursting during osmosis
  • Gives mechanical strength to the cell as a whole
  • Allows water to pass along it so contributes to the movement of water through the plant
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Plant Cells vs. Animal Cells

  • Cellulose cell wall surrounds the cell as well as well-surface membrane
  • Only cell-surface membrane surrounds the cell
  • Chloroplasts are present in large numbers in most cells
  • Chloroplasts are never present
  • Normally hav a large central vacuole filled with cell sap
  • If vacuoles are present they are small and scattered throughout the cell
  • Starch grains are used for storage
  • Glycogen granules are used for storage
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Root Hair Cells

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