Which one among the following is the source of energy in cells?
- (a)ADP
- (b)ATP
- (c)AMP
- (d)NAD
Correct — B, ATP (adenosine triphosphate). ATP is the cell's universal energy currency. It stores energy in two high-energy phosphate bonds; when the terminal phosphate is split off by hydrolysis (ATP -> ADP + inorganic phosphate), the released energy powers muscle contraction, active transport, biosynthesis and nearly every other cellular process.
- (a)ADP — Adenosine DIphosphate is the lower-energy product left AFTER ATP has already given up its terminal phosphate and released its energy. It is recharged back to ATP during respiration — it is not itself the ready source of energy.
- (c)AMP — Adenosine MONOphosphate has only one phosphate and even less stored energy than ADP. It is a further breakdown product, not the cell's energy currency.
- (d)NAD — NAD (nicotinamide adenine dinucleotide) is a coenzyme that acts as an electron/hydrogen CARRIER (NAD+/NADH) in respiration. It ferries electrons to the electron transport chain but is not the direct energy source that does cellular work.
Cells run on ATP — adenosine triphosphate — made mainly in the mitochondria during cellular respiration. Energy released by oxidising glucose is captured by adding a phosphate to ADP, forming ATP; the energy is later released on demand by breaking that phosphate bond. This constant ATP <-> ADP cycling is how cells store and spend energy.
Distinguish the energy 'currency' (ATP) from the electron 'carrier' (NAD/FAD) and from ATP's lower-energy relatives (ADP, AMP). The number of phosphates is the clue: three phosphates (ATP) hold the most usable energy; losing phosphates gives ADP then AMP with progressively less.
- ATP = adenosine + three phosphate groups; it is the cell's 'energy currency'.
- Energy is released when ATP is hydrolysed to ADP + Pi (inorganic phosphate).
- ATP is regenerated from ADP mainly in the mitochondria during aerobic respiration.
- NAD+/NADH is an electron carrier (coenzyme), not the direct energy source; ADP and AMP have fewer phosphates and less energy.
- Choosing ADP or AMP — they are lower-energy breakdown products, not the source.
- Choosing NAD — it is an electron carrier/coenzyme, not the energy currency.
Asked as 'energy currency of the cell' or 'source of energy in cells' — the answer is always ATP.
No directly related past PYQ was found.
- practice — not a real PYQ
ATP is often called the 'energy currency' of the cell because it
- (a)stores genetic information
- (b)releases energy when its terminal phosphate bond is broken
- (c)carries oxygen to cells
- (d)builds the cell membrane
Answer(b) releases energy when its terminal phosphate bond is broken — powering cellular work.
- practice — not a real PYQ
In the cell, ATP is produced mainly in the
- (a)nucleus
- (b)ribosome
- (c)mitochondrion
- (d)lysosome
Answer(c) mitochondrion — the 'powerhouse' where aerobic respiration generates most ATP.