SSC
Which metal atom is present at the centre of the chlorophyll molecule?
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Answer: Option B — Magnesium
Detailed Solution
Chlorophyll is the green pigment found in the chloroplasts of plants, algae and cyanobacteria. It absorbs light energy from the Sun and uses it for photosynthesis, the process by which plants make their food (glucose) from carbon dioxide and water, releasing oxygen.
The chlorophyll molecule has a head and a tail. The head is a flat, ring-shaped structure called a porphyrin ring, made of four smaller nitrogen-containing rings (pyrrole rings). At the centre of this ring is a single atom of magnesium (Mg), held by four nitrogen atoms. The long tail is a hydrocarbon chain (phytol), which anchors the molecule in the membranes of the chloroplast.
Chlorophyll absorbs mainly red and blue (violet) light and reflects green light, which is why leaves look green.
Because magnesium is part of chlorophyll, it is an essential nutrient for plants. Magnesium deficiency causes chlorosis, a yellowing of leaves between the veins, especially in older leaves, because the plant cannot make enough chlorophyll.
Interestingly, the structure of chlorophyll is very similar to that of haem, the part of haemoglobin in our red blood cells that carries oxygen. The main difference is that haem has an iron (Fe) atom at its centre instead of magnesium.
Types of chlorophyll:
- Chlorophyll a: the main pigment of photosynthesis, found in all photosynthetic plants, algae and cyanobacteria.
- Chlorophyll b: an accessory pigment found in plants and green algae, which helps capture more light.
- Chlorophylls c and d: found in some algae.
Other pigments, such as carotenoids (yellow, orange and red) and xanthophylls, also help absorb light and protect chlorophyll from damage. In autumn, when chlorophyll breaks down, these pigments become visible, giving leaves their autumn colours.
The structure of chlorophyll was worked out by Hans Fischer, and its complete chemical synthesis was achieved by Robert Burns Woodward in 1960.
Why not the other options?
- ✗Iron is at the centre of haem in haemoglobin.
- ✗Copper is found in haemocyanin, the blue blood pigment of some molluscs and arthropods.
- ✗Zinc is a micronutrient for plants, not the central atom of chlorophyll.
Quick tip
Remember: Chlorophyll = magnesium at the centre (porphyrin ring); haemoglobin = iron; haemocyanin = copper; magnesium deficiency causes chlorosis.
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Plant Physiology & Nutrition in Plants — Preparation
This question tests your understanding of Plant Physiology & Nutrition in Plants, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers how plants live and grow: photosynthesis, transpiration, respiration in plants, the transport of water and food through xylem and phloem, plant hormones, modes of nutrition in plants, and plant tissues. Questions usually ask what is needed for photosynthesis and what it produces, where it takes place (in the chloroplasts), which tissue carries water (xylem) and which carries food (phloem), and what each plant hormone does, such as auxins for growth and ethylene for fruit ripening. Insectivorous plants such as the pitcher plant are also commonly tested. Practise the plant physiology questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.