In plants, the main excretory product produced during metabolism is

Analyze the question The question asks for the main excretory product of plants. Excretion is the removal of waste products of metabolism, so we need to find which option is the chief metabolic waste in plants.

Explain the concept Plants carry out two key gas-exchange processes:

• Photosynthesis: CO₂ + H₂O → glucose + O₂ (during the day). Oxygen is the by-product.
• Respiration: glucose + O₂ → CO₂ + H₂O + energy (all the time). Carbon(IV) oxide is the by-product.

Plants also make other waste substances, such as alkaloids, tannins, resins, gums, latex and mucilage. These are produced in smaller amounts and are often stored in dead tissues, leaves, bark or fruits.

Work through the options

• A. Oxygen: This is the by-product of photosynthesis. It is produced in large quantities and is released through the stomata. Textbooks (e.g. in Nigerian syllabi) treat it as the main excretory product of plants, since photosynthesis is the dominant metabolic process, and in the light, more CO₂ is used by photosynthesis than is made by respiration.
• B. Alkaloids: These are nitrogenous waste compounds (e.g. quinine, caffeine, nicotine), but they are secondary, minor wastes.
• C. Carbon(IV) oxide: This is also excreted, from respiration. In daylight, however, the CO₂ from respiration is reused in photosynthesis, so it is not the net main waste.
• D. Mucilage: This is a minor, sticky plant secretion and not a main excretory product.

Identify the answer By the usual reasoning, oxygen is the main excretory product of plants, since it is released in the largest quantity as a by-product of photosynthesis.

A note on fairness: some textbooks name CO₂ as an excretory product too, since it comes from respiration. But the question asks for the main product, and in plants that is oxygen, as plants are mostly photosynthesising.

Exam tip Remember: Animals excrete CO₂ mainly; plants excrete O₂ mainly. If a JAMB question asks for the "main" excretory product of plants and lists oxygen, choose it. Alkaloids, tannins, resins and latex are the "other" plant wastes.