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  • B3
    • Cell Division
      • Becoming Multicellular
        • Humans are multicellular. The advantages of being multicellular are:
          • It allows an organism to become larger and more complex
          • It allows different cells to take on different jobs, which is known as 'Cell Differentiation'
        • When an organism becomes multicellular, it needs systems that can:
          • Allow communication between all cells in the body
          • Supply all cells with enough nutrients
          • Control exchanges with the environment such as heat and gases
      • Mitosis
        • The process that produces cells for growth
        • Cells made are genetically identical
        • Before cells divide,  DNA replication must take place in order for each cell to have 2 copies of each chromosome
        • Body cells in mammals have 2 copies of each chromosome, so are diploid cells
        • Before mitosis happens, DNA must be replicated. This involves:
          • 2 strands of DNA molecule 'unzipping' to form single strands
          • new double strands form by DNA bases lining up in complementary pairings
        • Mitosis occurs:
          • Chromosomes line up along the centre of the cell and divide. The copies then move to the opposite poles (ends) of the cell.
      • Meiosis
        • Type of cell division which produces gametes.
        • Gametes are haploid cells because they contain only one chromosome from each pair
          • This means that the zygote only receives one copy of a gene from one parent and another copy from the other parent
            • Producing 'Genetic Variation'
        • The structure of a sperm cell is adapted to its function. It has:
          • many mitochondria, to provide energy for swimming to the egg
          • an acrosome which releases enzymes to digest the egg membrane
        • In meiosis, there are 2 divisions:
          • First the single chromosomes are joined to form X shaped chromosomes, and chromosomes with the same genes pair up. Then:
            • In the first division: 1 chromosome from each pair move to opposite poles of the cell
            • In the second division: the copies of each chromosome come apart and move to opposite poles of the cell
    • Respiration
      • The incomplete breakdown of glucose, resulting in the build up of lactic acid, is known as oxygen debt
      • C6H12O6+ 6O2 = 6CO2+6H2O
        • Aerobic Respiration
      • Measuring Respiration rate
        • Respiratory Quotient
          • RQ= CO2 Produce/O2 Used
        • Experiments to measure the rate of respiration
          • Measure the amount of O2 used up
            • The faster it's consumed, the faster the respiration rate
          • Measure the rate that CO2 is made
        • Metabolic Rate
          • The sum of all reactions which occur in the body
          • If the metabolic rate is high, more O2 is needed, as aerobic respiration is faster
        • Changes in temperature and pH also affect the rate of respiration, as they affect enzymes, which control respiration process
      • Disadvantages on Anaerobic respiration
        • 1) The lactic acid made and built up in muscles causes pain and fatigue. 2) Releases much less energy per glucose molecule
      • Glucose = Lactic acid (+energy)
        • Anaerobic Respiration
      • Releases energy from food and this is trapped in a molecule called ATP
      • During exercise, despite an increase in breathing rate and heart rate, the muscles do not receive sufficient oxygen
        • Respire anaerobically in addition to aerobically


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