Gas exchange

• Diffusion is the process by which gas exchange occurs
• Respiration is a form of gas exchange as it is the intake of oxygen and release of CO2
• Gas exchange is present in photosynthesis as CO2 diffuses in and oxygen diffuses out
• Adaptations of the leaf for gas exchange:

  • Thin: short diffusion distance
  • Flat: large SA:V ratio
  • Air spaces: allow gases to reach palisade mesophyll layer
  • Stomata: allow gas movement in and out of the leaf
  • Thin cell walls: gases can move in easily
• Stomata open and close during gas exchange to let gases diffuse in and out
• Stomata open when water moves by osmosis into the guard cells causing them to become turgid
• Stomata close when the guard cells lose water and the stomata become flaccid
• Stomata also open in sunlight and close in low light availability
• During day time, plants both photosynthesise and respire
• During the nighttime, plants only respire
• Investigating the effect of light on gas exchange in plants

  1. Measure out 20 cm3 hydrogencarbonate indicator into 4 boiling tubes

  2. Put some cotton wool into each boiling tube

  3. Label the boiling tubes A-D and set them up as follows:

  4. Tube A - No leaf (control tube)

  5. Tube B - Place a leaf in the tube and leave in the light

  6. Tube C - Place a leaf in the tube and wrap it in aluminium foil to block out the light

  7. Tube D - Place a leaf in the tube and wrap it in gauze to allow partial light

  8. Put a bung into the top of each tube

  9. Leave all 4 tubes in the light for 30 minutes
• Hydrogen carbonate indicator

  Turns yellow when there is an increase in CO2 levels and purple when there is a decrease in CO2
• Lungs

  • Large surface area to allow faster diffusion of gases across the surface
  • Thin walls to ensure diffusion distances remain short
  • Good ventilation with air so that diffusion gradients can be maintained
  • Good blood supply to maintain a high concentration gradient so diffusion occurs faster
• Ribs

  • Bone structure that protects internal organs such as the lungs
• Intercostal muscles

  • Muscles between the ribs which control their movement causing inhalation and exhalation
• Diaphragm

  • Sheet of connective tissue and muscle at the bottom of the thorax that helps change the volume of the thorax to allow inhalation and exhalation
• Trachea

  • Windpipe that connects the mouth and nose to the lungs
• Larynx

  • Also known as the voice box, when air passes across here we are able to make sounds
• Bronchi

  • Large tubes branching off the trachea with one bronchus for each lung
• Bronchioles

  • Bronchi split to form smaller tubes called bronchioles in the lungs connected to alveoli
• Alveoli

  • Tiny air sacs where gas exchange takes place
• Pleural cavity

  • The fluid filled space between the pleural membranes which reduces friction and allows the lungs to move freely
• Cilia and mucus

  • The passages down to the lungs are lined with ciliated epithelial cells
  • Cilia cells have tiny hairs on the end of them that beat and push mucus up the passages towards the nose and throat where it can be removed
  • The mucus is made by special mucus-producing cells called goblet cells because they are shaped like a goblet, or cup
  • The mucus traps particles, pathogens like bacteria or viruses, and dust and prevents them from getting into the lungs and damaging the cells there
• Ventilation

  1. During inhalation: The diaphragm contracts and flattens, The external set of intercostal muscles contract to pull the ribs up and out, This increases the volume of the chest cavity (thorax), Leading to a decrease in air pressure inside the lungs relative to outside the body, Air is drawn in

  2. During exhalation: The diaphragm relaxes it moves upwards back into its domed shape, The external set of intercostal muscles relax so the ribs drop down and in, This decreases the volume of the chest cavity (thorax), Leading to an increase in air pressure inside the lungs relative to outside the body, Air is forced out
• Alveoli:

  • There are many rounded alveolar sacs which give a very large surface area to volume ratio
  • Alveoli (and the capillaries around them) have thin, single layers of cells to minimise diffusion distance
  • Ventilation maintains high levels of oxygen and low levels of carbon dioxide in the alveolar air space
  • A good blood supply ensures constant supply of blood high in carbon dioxide and low in oxygen
  • A layer of moisture on the surface of the alveoli helps diffusion as gases dissolve
• Smoking

  Causes chronic obstructive lung disease (COPD), coronary heart disease and increased risks of several different types of cancer, including lung cancer
• Chemicals in cigarettes

  • Tar - a carcinogen (a substance that causes cancer)
  • Nicotine - an addictive substance which also narrows blood vessels
  • Carbon monoxide - reduces the oxygen-carrying capacity of the blood
• Nicotine

  • Narrows blood vessels leading to an increased blood pressure
  • Increases heart rate
• Carbon monoxide

  • Binds irreversibly to haemoglobin, reducing the capacity of blood to carry oxygen
  • Puts more strain on the breathing system as breathing frequency and depth need to increase in order to get the same amount of oxygen into the blood
  • Puts more strain on the circulatory system to pump the blood faster around the body and increases the risk of coronary heart disease and strokes
• Tar

  • Is a carcinogen and is linked to increased chances of cancerous cells developing in the lungs
  • Contributes to COPD, which occurs when chronic bronchitis and emphysema (two different diseases which are frequently linked to smoking) occur together
  • Stimulates goblet cells and mucus glands to enlarge, producing more mucus
  • Destroys cilia and mucus (containing dirt, bacteria and viruses) builds up blocking the smallest bronchioles and leading to infections
• Investigating the effects of breathing on exercise

  1. Work out student A's breathing rate at rest

  2. Student A should then exercise for a set time (at least 4 minutes)

  3. Immediately after exercising, count the breaths taken in 15 seconds and multiply by 4 to obtain the breathing rate per minute

  4. Compare the result to the breathing rate at rest in order to work out the change in breathing rate as a result of exercise

  5. Repeat this last step every minute after exercise for 5 minutes

  6. Repeat the process for student B

  7. Finally, repeat the whole investigation for each student after a period of rest