Q.The energy yield in terms of ATP is higher in aerobic respiration than anaerobic respiration. Why anaerobic respiration occurs even in organisms that live in aerobic condition like human beings and angiosperms?

Even organisms that normally live and respire under aerobic conditions can face brief situations of local oxygen shortage, and anaerobic respiration lets them keep making some ATP through those moments.

• In humans, vigorously working muscle can use up oxygen faster than it can be delivered, so the cell temporarily switches to lactic acid fermentation to keep glycolysis (and some ATP production) going.
• In angiosperms, tissues such as germinating seeds or waterlogged roots can face similarly restricted oxygen availability, and fermentation offers a fallback pathway under such anaerobic conditions. …

Aerobic respiration yields far more ATP per glucose than fermentation, so it might seem strange that aerobic organisms such as humans and angiosperms ever use the anaerobic route at all. The reason lies in the fact that oxygen availability at the cellular level is not always guaranteed, even in an organism living in an oxygen-rich environment overall.

In humans, muscle cells performing intense exercise can consume oxygen faster than the circulatory system can deliver it. Rather than stopping ATP production entirely while waiting for more oxygen, the muscle cell switches its pyruvic acid to lactic acid fermentation, reoxidising NADH + H+ back to NAD+ so glycolysis can keep running and continue to make some ATP.

Method 1 — Weigh yield against availability

1. Note that aerobic respiration gives far more ATP per glucose than fermentation.
2. But aerobic respiration absolutely requires a steady O2 supply at the cell/tissue level.
3. Identify situations where O2 delivery can't keep pace with demand even in an aerobic organism: exercising muscle (humans); germinating seeds/waterlogged roots (angiosperms). …