B3//B3a, B3b, B3c//OCR Gateway B//Higher Tier

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  • B3
    • Molecules of life
      • Cell Structure:
        • The number of mitochondria in the cytoplasm of a cell depends on the activity of the cell, as respiration occurs in mitochondria.
        • Cells such as liver or muscle cells have large numbers of mitochondria, as the liver carries out many functions and muscle cells need to contract.
        • Ribosomes are smaller than mitochondria and are also found in the cytoplasm.
          • They are too small to be seen with a light microscope and are the site of protein synthesis.
      • DNA and the genetic code:
        • The nucleus contains genes. Each gene: is a section of a chromosone made of DNA; codes for a particular protein.
        • DNA is made of two strands coiled to form a double helix, each strand containing chemicals called bases.
          • There are four different types of bases, with cross links between the strands formed by pairs of bases.
            • Each gene contains a different sequence of bases.
        • The four bases in DNA are called A, T, C and G.
          • The cross links holding the two strands together are always between the same bases, A-T and C-G. This is called complementary base pairing.
        • The DNA base code controls which protein is made. This is because the base sequence in the DNA codes for the amino acid sequence in the protein.
          • Each amino acid is coded for by a sequencing of three bases.
        • The code needed to produce a protein is carried from the DNA to the ribosomes by a molecule called messenger RNA, or mRNA.
        • Many of the proteins that are made are enzymes, which can control the activity of a cell.
      • Discovering the structure of DNA:
        • Watson and Crick built a model of DNA using data from other scientists. Two of the important pieces of data they used were:
          • Photographs taken using x-rays which showed that DNA had two chains wound in a helix; data indicating that the bases occurred in pairs.
        • Watson and rick worked out the structure of DNA in 1953 and shared the Nobel prize for this discovery in 1962.
          • There is often such a delay between a discovery and the award of prizes because other scientists need to check the discovery to make sure that it is correct.
    • Proteins and mutations
      • Types of proteins:
        • All proteins are made of long chains of amino acids joined together.
        • Proteins have different functions. Some examples are:
          • Structural proteins used to build cells and tissues, e.g. collagen.
          • Hormones, which carry messages to control a reaction, e.g. insulin controls blood sugar levels.
          • Carrier proteins, e.g. haemoglobin, which carries oxygen.
          • Enzymes.
      • Enzymes:
        • Enzymes speed up reactions in the body and so are called biological catalysts.
          • They catalyse chemical reactions occurring in respiration, photosynthesis and protein synthesis of living cells.
        • The substrate m0lecule fits into the active site of the enzymes like a key fitting into a lock.
          • This is why enzymes are described as working according to the 'lock and key mechanism'.
          • It also explains why each enzyme can only work on a particular substrate. This is called specificity and it happens because the substrate has to be the right shape.
        • Enzymes all work best at a particular temperature and pH.
          • This is called the optimum. An change away from the optimum will slow down the reaction.
        • Enzyme activity is affected by pH and temperature:
          • At low temperatures molecules are moving more slowly and so the enzyme and the substrate are less likely to collide.
          • At very high or low pH values and at high temperatures the enzyme active site changes shape. This is called denaturing. The substrate cannot fit, so cannot react so quickly.
        • It is possible to work out how temperature alters the rate of reaction by calculating the temperature coefficient, called Q10.
          • This is done for a 10 degrees Celsius change in temperature, using: Q10 = rate at higher temperature/rate at lower temperature.
      • Mutations:
        • When they occur, mutations may lead to the production of different proteins.
          • They are also often harmful but may have no effect; and occasionally they might give the individual an advantage.
        • Mutations may occur spontaneously but can be made to occur more often by radiation or chemicals.
        • Although every cell in the body has the same genes it does not mean that all the same proteins are made.
          • This is because different genes are switched off in different cells. This allows different cells to perform different functions.
        • Gene mutations alter or prevent the production of the protein that is normally made, because they change the base code of DNA, and so change the order of amino acids in the protein.
    • Respiration
      • Why is respiration important?
        • Respiration releases energy from food and this energy is trapped in a molecule called ATP.
        • ATP can then be used to provide the energy for many different proccesses in living organisms.
      • Aerobic respiration:
        • Aerobic respiration involves the use of oxygen. The symbol equation for aerobic respiration is: C6H12O6 + 6O2 --> 6CO2 + 6H2O
      • Anaerobic respiration:
        • During exercise, despite an increase in breathing rate rate and heart rate, the muscles often do not receive sufficient oxygen.
          • They start to use anaerobic respiration in addition to aerobic respiration.
        • The word equation for anaerobic respiration is: glucose --> lactic acid (+energy).
        • Anaerobic respiration has two main disadvantages over aerobic respiration.
          • The lactic acid that is made by anaerobic respiration builds up in muscles, causing pain and fatigue.
          • Anaerobic respiration releases much less energy pre glucose molecule than aerobic respiration
        • The incomplete breakdown of glucose resulting in the build up of lactic acid is called oxygen debt.
        • During recovery the breathing rate and heart rate stay high so that:
          • Rapid blood flow can carry lactic acid away to the liver.
          • Extra oxygen can be supplied, enabling the liver to break down the lactic acid.
      • Measuring respiration rate:
        • It is possible to set up different experiments to measure the rate of respiration. Two ways to do this involve:
          • Measuring how much oxygen is used up - the faster it is consumed, the faster the respiration rate.
          • The rate at which carbon dioxide is made.
        • Scientists can use these results to calculate the respiratory quotient.
          • This is worked out using the formula: RQ=carbon dioxide produced/oxygen used.
        • The metabolic rate is described as the sum of all the reactions that are occurring in the body.
          • If the metabolic rate is high, more oxygen is needed, as aerobic respiration is faster.
        • Changes in temperature and pH can also change the respiration rate because they affect enzymes, and respiration is controlled by enzymes.

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