Respiration: Question 6
Syllabus 12.2
A cyclist pedals gently along a flat, quiet cycle path for an hour. She breathes steadily throughout and never runs out of breath. Oxygen reaches her leg muscle cells fast enough to meet all of their energy needs the whole time.
Which word equation correctly represents the type of respiration taking place in her leg muscle cells throughout this gentle ride?
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Worked solution
Step 1: Identify the type of respiration taking place
The cyclist breathes steadily and never runs out of breath, and oxygen reaches her leg muscle cells fast enough to meet all of their energy needs throughout the ride. This means her muscle cells are respiring aerobically the entire time. There is no shortage of oxygen that would force them to respire anaerobically as well.
Step 2: Recall the word equation for aerobic respiration
Aerobic respiration uses oxygen to break glucose down completely, releasing energy and producing carbon dioxide and water as waste products:
glucose + oxygen → carbon dioxide + water
Step 3: Rule out the other options
- Option B leaves out oxygen as a reactant, even though the scenario makes clear that oxygen reaches her muscle cells throughout the ride.
- Option C shows the anaerobic reaction that happens in oxygen-starved muscle, producing lactic acid. This does not apply here, since her muscles are never short of oxygen.
- Option D shows the anaerobic reaction that happens in yeast, producing alcohol and carbon dioxide without using any oxygen at all. This is the wrong organism and the wrong type of respiration for this well-oxygenated muscle.
Final answer
The word equation for the aerobic respiration taking place in her leg muscle cells is glucose + oxygen → carbon dioxide + water, option A.