knowledge-9 GE

• Experiment of surface area to volume ratio on diffusion rate:

  Using an indicator that changes from a colour to colorless will show the effect best:

  1. make agar cubes containing coloured indicator.
  2. cut three cubes of increasing size and submerge them in an acidic or alkaline solution.
  3. time how long it takes each cube to change colour.
  4. the effect of surface area to volume ratio on the rate of diffusion will be visible.
• Most substances cross cell plasma membrane to enter or leave an organism , in single celled organisms , simple diffusion is adequate for gas exchange.
• In large multi-cellular organisms , most cells are in some form of tissue fluid , too far from the exchange membranes or surface substances to diffuse them. Mass transport systems , such as blood in a circulatory system, carry substances between exchange surfaces.
• Mass transport maintains concentration gradients at exchange surfaces thereby keeping a relatively stable environment in the tissue fluid.
• Manometers, three-way taps and simple respirometers are used to measure the volumes of air involved in gas exchange.

  • small invertebrates or seeds are placed in the experiment chamber , with a control such as glass beads in the opposite chamber.
  • carbon dioxide is absorbed by soda lime so only oxygen uptake is measured.
  • Any decreases in the volume of oxygen can be measured using the manometer tube where a coloured liquid will be pulled toward the respiring organism as oxygen is used and not replaced.
• Pulmonary ventilation rate (PVR):

  PVR = tidal volume x breathing rate

  tidal volume is the normal volume of air displaced by the lungs at rest

  breathing rate is number of breaths taken per minute.
• Human gas exchange structure:

  Human gas exchange occurs in the lungs.

  Air is pulled into the lungs through the trachea. The trachea divides into two bronchi , which further divide into bronchioles , until they terminate into millions of sacs , the alveoli.

  Gas exchange between the blood and the air takes place in the alveoli.
• Human ventilation:

  The external and internal intercostal muscles work in antagonistic pairs.

  when the volume of lungs decreases , the pressure increases , and causes air to be pushed out.

  when the volume of the lung increases , it decreases the pressure causing air to be drawn in.
• Process (inhalation):

  • the external intercostal muscles contract and move up whereas the internal intercostal muscles relax.
  • the diaphragm contracts and flattens
  • lung volume increases
  • pressure in thoracic cavity decreases
  • movement of air into the lungs.
• Process (exhalation):

  • external intercostal muscles relax and move ribs down.
  • diaphragm relaxes and moves up.
  • pressure in the thoracic cavity increases.
  • lung volume decreases.
  • movement of air into the lungs.
• Features of the alveolus:

  alveolus maximize gas exchange by:

  • Having very large surface area
  • being moist to aid diffusion of gases
  • having rich blood supply to maintain conc gradient
  • the alveolar epithelium and the capillaries are very thin so the diffusion distance between air in alveoli and red blood cells in capillaries is short.
• lung diseases decrease the surface area of the lungs , reducing oxygen uptake.
• Asthma: air pollution can result in asthma, which causes the airways to narrow.
• Bronchitis: a lung infection , causing inflammation of the linings, excess mucus production and coughs.
• Emphysema: phagocytes cross the alveoli walls and break down proteins , causing alveoli to burst. this reduces number of alveoli and the number of capillaries decreasing oxygen uptake.
• Gas exchange in single celled organisms :

  Single-celled organisms have a large surface area to volume ratio and so can rely on diffusion for substances to move into and out of them.

  organisms can move to areas of different oxygen concentration to improve the concentration gradient.
• Gas exchange in leaves (mesophyll):

  • differentiated into columnar palisade cells , which have many chloroplasts and irregularly shaped spongy parenchyma cells.
  • large, moist surfaces to absorb oxygen and carbon dioxide , facilitating diffusion.
  • air spaces between cells to allow gases to diffuse
  • conc gradient formed as gases are absorbed or released.
• Gas exchange in leaves (stomata):

  • allow gases to pass in and out of pores in the leaf surface.
  • Gases diffuse due to concentration gradient between inside and outside of the leaf.
  • stomata can close to reduce water loss.
• Adaptations of xerophytes:

  xerophytes are plants that grow in dry habitats. Adaptations to reduce water evaporation from leaves , whilst enabling gas exchange and photosynthesis, include:

  • thicker cuticle
  • reduced leaf surface area
  • sunken and fewer stomata
  • rolled leaves.
• Gas exchange in insects:

  Many insects have spiracles along their thorax.

  spiracles have valves to allow air in and out by diffusion.

  Inside, the trachea (tubes) divide until they reach cells called tracheoles. Diffusion of oxygen and carbon dioxide occurs between body cells and the thin wall of the tracheoles.

  terrestrial insects use their spiracles valves to balance water loss and gas exchange.
• Adaptations of gills:

  • large surface area of lamellae and filaments when in water.
  • rich blood supply by mass flow to the gills
  • concentration of oxygen along the whole length of the lamellae by countercurrent principle.
• Countercurrent principle:

  Blood in capillaries flows in the opposite direction to water flowing over them.

  by allowing the blood to flow in the opposite direction to the water , an oxygen conc gradient is maintained , where the oxygen concentration in water is always higher than in the blood.

  this maximizes the amount of oxygen that can diffuse into the bloodstream and prevents oxygen diffusing back into the water again once the blood is oxygen rich.