gaseous exchange

• To maximise the rate of exchange, gas exchange surfaces tend to have the following features:

  1. Large surface area
  2. Moist- so that gases can dissolve before they diffuse
  3. Diffusion gradient present for both gases
  4. Permeable
  5. Thin to provide a short diffusion distance
• what happens to rate of photosynthesis and respiration under different light levels?

  high levels of light: - rate of photosynthesis is higher than the rate of respiration. So more carbon dioxide taken in than given out. Hydrogen carbonate indicator turns purple.

  No light: - No photosynthesis taking place, so some carbon dioxide produced by respiration. Increased levels of carbon dioxide so hydrogen carbonate indicator will turn yellow.

  Compensation point: - no net movement of carbon dioxide. Rate of respiration and photosynthesis is equal. Hydrogen carbonate indicator will remain Red.
• leaf diagram

  A) waxy cuticle

  B) upper epidermis

  C) Palisade mesophyll layer

  D) spongy mesophyll layer

  E) lower epidermis

  F) guard cells

  
• The leaf is a highly adapted for the process of photosynthesis and gas exchange. These adaptations include:

  • Thin - large surface area to volume ratio and short diffusion distance
  • Moist - allows gases to dissolve before they diffuse to the appropriate cells
  • Spongy mesophyll - Contains air spaces and the cells are loosely arranged to provide a large surface area over which gases can diffuse
  • Stomata - present to allow diffusion of gases into and out of the leaf
  • Air spaces - in spongy mesophyll to facilitate diffusion of gases
• Stomata:

  These will open during the daytime when the diffusion of gases into and out of the leaf is at its greatest. The stomata will then close at night when there is no photosynthesis to ensure that there is enough oxygen for respiration to take place. This will also reduce water loss at night.
• Adaptations of lungs for gas exchange

  • Thin layer of squamous epithelial cells in alveoli for short diffusion distance
  • Close proximity of capillaries to alveolar wall for efficient gas exchange
  • Red blood cells pass through narrow capillaries, reducing diffusion pathway
  • High number of alveoli (350 million per lung) for large surface area
  • Well-developed capillary network for maintaining concentration gradient
  • Ventilation of lungs creates steep concentration gradient for efficient gas exchange
• Surfactant secreting cells in the alveolar wall produce surfactant that:

  • Reduces surface tension in the moisture coating of the alveoli.
  • Prevents alveoli collapsing during exhalation.

  
• Macrophages:

  These originate from monocytes and can be found in the alveoli. These protect against infection by carrying out phagocytosis (microbes: surrounded, engulfed and digested)
• Lungs diagram

  A) Larynx

  B) ring of cartilage

  C) thorax

  D) lung

  E) heart

  F) diaphragm

  G) pleural membranes

  H) exterior intercostal muscle

  I) interior intercostal muscle

  J) left bronchus

  K) rib

  L) pleural liquid

  M) bronchiole

  N) pulmonary artery

  O) pulmonary vein

  P) alveoli

  Q) capillary network

  
• Inspiration (breathing in(advanced))

  So the...

  The volume of thorax increases, causing the pressure between the pleural surfaces to decrease. The lungs expand to fill thoracic cavity. Air pressure in alveoli is less than atmospheric pressure. air is forced in by the higher external atmospheric pressure.
• Expiration (breathing out(advanced))

  So the...

  Volume of thorax decreases causing an increase in the pressure around the lungs. The lung tissue recoils causing the air pressure in alveoli to be more than atmospheric pressure, air is forced out. The natural elasticity of the lungs produces an elastic recoil, which helps force air out of the lungs during expiration.
• Inspiration (breathing in(basic))

  1. external intercostal muscles contract (internal intercostal muscles relax)
  2. ribs move up and out
  3. width of thorax increases causing the volume to increase
  4. diaphragm contracts and moves down, flattening
  5. depth of thorax increases top to bottom.
• Expiration (breathing out(basic))

  1. external intercostal muscles relax (internal intercostal muscle contracts)
  2. ribs move down and in
  3. width of thorax decreases, decreasing volume of thorax
  4. diaphragm relaxes and moves up, returning to dome shape
  5. depth of thorax decreases, decreasing volume of thorax
• Tar – is a collection of toxic chemicals in cigarette smoke, many of which are carcinogens (cancer causing). Tar damages the DNA in the epithelial cells lining the lungs and causes lung cancer. This means some of cells divide uncontrollably producing a tumour. A tumour can block airways or damage the lungs. There is a correlation between the risk of lung cancer and number of years as a smoker and the number of cigarettes smoked per day.
• Bronchitis- Caused by inflammation of the bronchial tubes narrowing them. Mucus production increases reducing air flow in and out of the lung. Tar will also paralyse the cilia which removes mucus and microbes, increasing the risk of infection. The symptoms are breathlessness, coughing and increased susceptibility to infections.
• Emphysema - caused by destruction of the walls of the alveoli. Smoking tar destroys the elastin fibres that give the alveoli their elastic properties. As a result they lose their ability to return to their original size when exhaling. They become overinflated with air so there is not enough space for fresh air to enter. Symptoms include shortness of breath, wheezing and chronic coughing.