Question

relative workload is a measure of exercise difficulty. vo₂ max is a measure of oxygen used during exercise.
what is the best explanation for the increase in lactic acid (lactate) in the graph?
○ as relative workload decreases, pyruvate builds up in blood.
○ between 0 - 60% of vo₂ max cells are using fermentation and after 60% of vo₂ max, cells are using aerobic respiration
○ between 0 - 60% of vo₂ max, cells are using aerobic respiration, then switch to anaerobic respiration after 60% of vo₂ max.
○ cells in this graph are probably using aerobic respiration for all levels of vo₂ max.

Explanation:

1. Analyze Option 1: The graph shows relative workload increasing (x - axis increases), not decreasing. Also, pyruvate build - up isn't the direct cause of lactic acid increase as described here. Eliminate this option.
2. Analyze Option 2: Aerobic respiration is more efficient and produces more ATP. Cells use aerobic respiration when oxygen is sufficient (lower workloads, 0 - 60% VO₂ max) and fermentation (anaerobic) when oxygen is limited (higher workloads, after 60% VO₂ max). This option has the processes reversed. Eliminate this option.
3. Analyze Option 3: At lower relative workloads (0 - 60% VO₂ max), there is enough oxygen for aerobic respiration, so lactic acid (lactate) levels are low. As the workload increases beyond 60% VO₂ max, the oxygen demand exceeds the oxygen supply, and cells switch to anaerobic respiration (fermentation), which produces lactic acid. This matches the graph where lactic acid levels start to rise more rapidly after the anaerobic threshold (around 60% VO₂ max).
4. Analyze Option 4: If cells were using aerobic respiration for all levels, lactic acid levels would not show a sharp increase after 60% VO₂ max, as aerobic respiration does not produce lactic acid as a by - product in large amounts. Eliminate this option.

Answer:

C. Between 0 - 60 % of \(VO_2\) max, cells are using aerobic respiration, then switch to anaerobic respiration after 60% of \(VO_2\) max.