Which one of the following processes takes place in the cytoplasm?

The correct answer is Option (1) → Breakdown of Glucose into Pyruvate and Energy

Note: The given answer is as per the official NTA Answer Key. However, the question can have more than one correct answer, i.e. Option (1), Option (2) and Option (3), as explained below.

Respiration breaks down glucose in steps. The first step is the same in every living cell: in the cytoplasm, a molecule of glucose (with six carbon atoms) is broken into two molecules of pyruvate (three carbon atoms each), and a little energy is released. This step, called glycolysis, needs no oxygen.

What happens to the pyruvate next depends on the cell and on the oxygen available:

• In yeast, without oxygen: pyruvate gives ethanol, carbon dioxide and energy (fermentation), the process in Option (3).

• In muscle cells short of oxygen: pyruvate gives lactic acid and energy, the process in Option (2); the lactic acid causes muscle cramps.

• With oxygen, in the mitochondria: pyruvate is broken down fully into carbon dioxide, water and much more energy, the process in Option (4).

Strictly, the processes of Options (2) and (3) also take place in the cytoplasm: lactic acid formation in muscle cells and alcoholic fermentation in yeast both happen there, without entering the mitochondria. Only the process of Option (4) takes place in the mitochondria. Option (1) is the step that every cell carries out in the cytoplasm, which is why it is the best answer.

Advice for the exam: When a question has more than one possible answer, mark the one you worked out yourself. If NTA’s answer key gives a different answer, challenge it when NTA opens the window for challenges.