In an experiment, mould and yeast cells were transferred into an environment with low oxygen concentration…

Step 1: Analyze the question Mould and yeast are both fungi, and neither can photosynthesize because they have no chlorophyll. Both were placed in low oxygen. The mould died, but the yeast survived. We need the best explanation for the difference.

Step 2: The concept

• Aerobic respiration needs oxygen. It releases a lot of energy: glucose + oxygen → carbon dioxide + water + energy (about 38 ATP).
• Anaerobic respiration (fermentation) does not need oxygen. In yeast, it gives: glucose → ethanol + carbon dioxide + a small amount of energy (2 ATP).
• Yeast is a facultative anaerobe, so it can switch between aerobic and anaerobic respiration. Most moulds are strict (obligate) aerobes, so they need oxygen to get enough energy.

Step 3: Evaluate the options

• A. Wrong. Yeast is a fungus and has no chlorophyll, so it cannot photosynthesize.
• B. Wrong. It is a true statement only in the sense that photosynthesis needs light, water and CO₂, not the absence of oxygen. It also does not explain why yeast survived while mould died.
• C. Correct. Yeast can respire anaerobically and gets energy without oxygen, so it survives. The mould cannot, so it dies.
• D. Wrong. Mould does respire, but it does so aerobically, and that is why it died when oxygen was low.

Step 4: Answer The correct answer is C: respiration can take place in yeast cells in the absence of oxygen.

Tip: Remember "Yeast = ferments." Whenever a question compares an organism that survives with one that dies in low oxygen, look for the option about anaerobic respiration. Also remember that fungi never photosynthesize, which lets you eliminate any option that says they do.