The breakdown of glucose in cytoplasm results in the formation of
- (a)pyruvate and energy
- (b)pyruvate and carbon dioxide
- (c)pyruvate and oxygen
- (d)pyruvate and nitrogen
Correct — A, pyruvate and energy. The breakdown of glucose in the cytoplasm is glycolysis. One six-carbon glucose molecule is split into two three-carbon pyruvate molecules, and the process releases energy that the cell traps as ATP. Glycolysis needs neither oxygen nor a mitochondrion, which is why it is the shared first step of both aerobic and anaerobic respiration; what happens to the pyruvate afterwards is what separates the two routes.
- (b)pyruvate and carbon dioxide — No carbon dioxide is released during glycolysis in the cytoplasm. Carbon dioxide appears later — when pyruvate is oxidised inside the mitochondrion, or during fermentation in yeast, which gives ethanol and carbon dioxide.
- (c)pyruvate and oxygen — Respiration consumes oxygen; it never produces it. Oxygen is a product of photosynthesis, not of the breakdown of glucose.
- (d)pyruvate and nitrogen — Glucose is made only of carbon, hydrogen and oxygen, so no step of its breakdown can release nitrogen.
Glycolysis is the first stage of cellular respiration and takes place in the cytoplasm of every cell. Glucose, a six-carbon sugar, is broken into two molecules of three-carbon pyruvate with a net release of energy captured as ATP. The pyruvate then takes one of three routes — into the mitochondrion for complete oxidation when oxygen is available, into lactic acid in our muscle cells when it is not, or into ethanol and carbon dioxide in yeast.
The trap is to answer with the products of complete respiration — carbon dioxide and water — because that is the equation most students memorise. Read the question's two constraints carefully: it fixes the site as the cytoplasm and asks only for what that step yields. Every option already gives you pyruvate, so the question is really asking what accompanies it, and the honest answer for the cytoplasmic step is energy.
- Glycolysis takes place in the cytoplasm and splits one glucose molecule into two molecules of pyruvate, releasing energy as ATP.
- Glycolysis does not require oxygen and is the first step of both aerobic and anaerobic respiration.
- Without oxygen, pyruvate becomes lactic acid in human muscle cells, or ethanol and carbon dioxide in yeast.
- With oxygen, pyruvate is broken down further inside the mitochondrion into carbon dioxide and water, releasing far more energy.
- Glucose (6 carbons) in the cytoplasm
- Glycolysis — no oxygen needed — gives 2 pyruvate (3 carbons each) + energy as ATP
- With oxygen: pyruvate enters the mitochondrion, giving carbon dioxide, water and much more energy
- Without oxygen: lactic acid in our muscle cells, or ethanol and carbon dioxide in yeast
The question stops at the second box — the cytoplasmic step, whose products are pyruvate and energy.
- Answering with the products of complete respiration when the question restricts you to the cytoplasmic step.
- Treating oxygen as a product of respiration rather than a reactant.
A single line that fixes the site (cytoplasm) and asks for the products, with pyruvate held constant and four different second products offered.
The complete conversion of glucose in the presence of oxygen into carbon dioxide and water with release of energy is called
- (a) aerobic respiration
- (b) anaerobic respiration
- (c) glycolysis
- (d) hydrolysis
Answer(a) aerobic respiration
The complementary question — UPSC asks for the name of the complete, oxygen-using breakdown, while NDA asks for the products of the partial, cytoplasmic step. Holding the two apart is exactly what stops you from answering 'carbon dioxide' here.
When yeast cells are O2 starved, fermentation serves as the source of energy. This results in the production of :
- (a) ATP + CO2 + Ethanol
- (b) ATP + O2 + Pyruvate
- (c) ATP + CO2 + Lactic acid
- (d) ATP + O2 + Acetaldehyde
Answer(a) ATP + CO2 + Ethanol
Picks up the story where this question leaves off — what happens to the pyruvate from glycolysis when yeast has no oxygen. Notice that the carbon dioxide appears at this later stage, not in the cytoplasmic step itself.
The lactic acid that gets accumulated in the muscle cells during rigorous exercise causing muscle cramps is produced from
- (a) ATP
- (b) Pyruvate
- (c) Ethanol
- (d) Glucose
Answer(b) Pyruvate
The human counterpart of the yeast question — the same pyruvate produced by glycolysis becomes lactic acid in our muscles when oxygen runs short.
- practice — not a real PYQ
Glycolysis takes place in the
- (a)mitochondrion
- (b)cytoplasm
- (c)nucleus
- (d)Golgi apparatus
Answer(b) cytoplasm — which is why it can proceed even in cells or conditions without oxygen.
- practice — not a real PYQ
In human muscle cells working without enough oxygen, pyruvate is converted into
- (a)ethanol
- (b)lactic acid
- (c)acetic acid
- (d)carbon dioxide and water
Answer(b) lactic acid — its build-up is associated with muscle cramps and fatigue.