Chapter 8 - Study Points

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  • Chapter 8 - Study points
    • Problems associated with increase in size
      • Diffusing molecules change direction every time they collide. if the pathway through a cell is too long, molecules change direction so often that it would take too long to supply enough oxygen or remove enough carbon dioxide for the cell to function. Therefore, there is an upper size limit to the size of a cell
    • Oxygen dissolves in the moisture on the worm's surface before diffusing into the capillaries
    • Gas exchange in vertebrate groups
      • There are three main types of respiratory surface in animals
        • Gills: in aquatic insects, the amphibian larvae (tadpoles) and fish
        • Lungs: these evolved from the common ancestor of terrestrial animal groups including birds, reptiles and mammals and adult amphibians also have them
        • Tracheae: the air filled tubes in terrestrial insects
      • For efficient gas exchange, the more advanced multicellular organisms need:
        • A ventilation mechanism
        • An internal transport system, to move gases between the respiratory surface and respiring cells
        • A respiratory pigment in the blood to increase its oxygen-carrying capacity
      • It is important to remember the problems to overcome when an animal respires on land. As lungs are internal, they are protected and lose less water and less heat from the respiratory surface than if they were external
    • Gas exchange in fish
      • Carbon dioxide diffuses down a concentration gradient in the opposite direction from oxygen, from the blood into the water
      • The function of a ventilation mechanism is to move the respiratory medium, air or water, across the respiratory surface. This gives the respiratory surface a fresh supply of oxygen, removes carbon dioxide and maintains a diffusion gradient
    • The human breathing system
      • The cartilage rings around the trachea are not complete at the back. This allows the oesophagus, behind it, to bulge as a bolus of food is swallowed, without meeting a hard structure, which would prevent the food from moving past
      • The main cause of air being forced out of the lungs during normal breathing is the elastic recoil of the lungs
      • The diffusion pathway is short as the walls of both the alveoli and the capillary wall are one cell thick
    • Gas exchange in insects
      • Chitin is impermeable to water, so water cannot evaporate across the tracheole
      • Insects do not have a blood circulation to carry oxygen
    • Gas exchange in plants
      • All aerobic organisms use oxygen day and night. It is produced only by plants and only during the day
      • The diffusion  pathway for gases is short in leaves because leaves are thin
      • The diffusion gradients of oxygen and carbon dioxide between the inside and outside of a leaf are maintained by mitochondria carrying out respiration and by chloroplasts carrying out photosynthesis
      • Malate and K+ ions lower the water potential of guard cells
    • Practical exercises
      • To find the area of the field of view
        • 3. Calculate the area          pi x r squared
        • 1. Convert the diameter, measured in eyepiece units, to um, using the calibration value
        • 2. Calculate the radius:       r = diameter/2
      • The most commonly used stains will give the xylem vessels red cell walls, as they contain lignin. The cell walls of the phloem sieve tube elements will stain blue as they contain cellulose, not lignin

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