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O-Level Respiration in Humans
What the O-Level syllabus expects for Respiration in Humans, and how to practise it.
What the syllabus expects
- Recognise the larynx, trachea, bronchi, bronchioles, alveoli and their associated capillaries, and give their roles in gaseous exchange in humans.
- Set out how breathing happens and the parts played by the cilia, diaphragm, ribs and the internal and external intercostal muscles.
- Account for the way an alveolus is built to carry out gaseous exchange.
- Define aerobic respiration, as it occurs in the cells of humans, to be the freeing of energy through glucose breakdown while oxygen is present, and give the equation both as words and as symbols.
- Define anaerobic respiration, as it occurs in the cells of humans, to be the freeing of energy through glucose breakdown while oxygen is absent, and give the word equation.
- Account for why cells switch to anaerobic respiration during hard exercise, producing an oxygen debt that is later cleared by fast, deep breathing once exercise stops.
How it's examined
Questions on this topic most often ask you to explain, describe. About 5% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Propose and explain how a reduced amount of carbonic anhydrase would affect gas exchange in a person.
Show the worked answer
Carbonic anhydrase in red blood cells catalyses CO2 + H2O reversibly forming carbonic acid, which dissociates to H+ and HCO3-; this lets most CO2 be carried in the blood as hydrogencarbonate. With less carbonic anhydrase, this conversion is slower in the respiring tissues, so less CO2 is loaded as hydrogencarbonate and CO2 accumulates in tissues and blood. In the lungs the reverse reaction (HCO3- back to CO2) is also slower, so less CO2 is released to be exhaled. Overall, removal of CO2 during gas exchange is reduced and CO2 builds up, lowering blood pH.
Example 2 (3 marks)
When someone exercises strenuously, the normal amount of gas Y being taken in can become insufficient to keep the muscles contracting. Describe and account for the ways the human body reacts under these conditions so that it can obtain enough of gas Y.
Show the worked answer
Gas Y is oxygen. During strenuous exercise the muscles need more oxygen for aerobic respiration to release energy for contraction. The body responds by increasing the rate and depth of breathing (ventilation), so more oxygen is taken into the lungs and absorbed into the blood per unit time. At the same time the heart rate increases, so oxygenated blood is pumped to the muscles faster. Together these deliver more oxygen to the working muscles.
Example 3 (2 marks)
Cystic fibrosis is an inherited condition that causes thick, sticky mucus to accumulate and disturb several organ systems. Explain what impact this disorder will have on a person's respiratory system.
Show the worked answer
Cystic fibrosis causes thick, sticky mucus to build up in the airways of the lungs. This mucus narrows/blocks the air passages (bronchi and bronchioles), so less air can flow in and out, reducing the amount of oxygen taken in and gas exchange. The mucus is also hard to clear and traps bacteria, so it becomes a breeding ground for microorganisms, leading to frequent chest/lung infections. Together these cause breathing difficulty and reduced gas exchange at the alveoli.
More O-Level Pure Biology topics
Cell Structure and Organisation · Movement of Substances · Biological Molecules · Nutrition in Humans · Transport in Humans · Excretion in Humans · all of O-Level Pure Biology