Photosynthesis that converts light energy into chemical energy by the plants, involves the use of green pigments of leaves called chlorophyll. The only structure in a leaf that hosts chlorophyll is :
- (a)Nucleus
- (b)Ribosome
- (c)Chloroplast
- (d)Chromosome
Correct — C, Chloroplast. NCERT's account of plastids says it plainly: plants prepare their food by photosynthesis in the presence of sunlight, and a green pigment called chlorophyll, present in the chloroplast, absorbs that sunlight. The chloroplast is a double-membrane organelle holding a semi-fluid stroma, and inside the stroma are disc-shaped membrane structures that carry the chlorophyll. That arrangement is why the pigment has to sit in an organelle rather than loose in the cytoplasm — light energy is captured on a membrane, and the membrane stacks give a large area for the pigment to be spread over. None of the other three structures has any pigment at all. The nucleus holds the chromosomes and directs the cell; the ribosome assembles protein; the chromosome is a thread of DNA with protein. A leaf looks green precisely because its cells are packed with chloroplasts.
- (a)Nucleus — The nucleus is the control centre of the cell and holds its chromosomes. It contains no pigment and takes no part in trapping light; a plant cell whose chloroplasts are absent, as in a root cell, still has a nucleus and is not green.
- (b)Ribosome — Ribosomes are the protein factories of the cell. They are tiny, they carry no membrane and no pigment, and they are found in every cell including those of animals, which cannot photosynthesise at all.
- (d)Chromosome — A chromosome is a thread of DNA with associated protein, carrying genes. It sits inside the nucleus, is visible only when a cell divides, and holds the instructions for making chlorophyll but not the chlorophyll itself.
Plastids are organelles found in plant cells and not in animal cells. Chloroplasts are the green plastids that carry chlorophyll and carry out photosynthesis; chromoplasts hold the other coloured pigments that make petals and fruit red, orange or yellow; leucoplasts are colourless and store starch, oil and protein. A chloroplast has two membranes, a fluid stroma, and stacks of disc-shaped membranes within the stroma where the chlorophyll sits.
The item does the reasoning for the candidate in its own stem, since it defines chlorophyll as a green pigment of leaves and then asks where it lives. The distractors are chosen from information-handling structures rather than from other plastids, which makes the item easier than it looks, but it is worth noticing why they can be dismissed together: nucleus, ribosome and chromosome all occur in animal cells too, and an animal cell has no chlorophyll. Two further points are worth carrying. A chloroplast has its own DNA and its own ribosomes, like a mitochondrion, which is the evidence behind the view that both were once free-living organisms taken in by an early cell. And a leaf looks green because chlorophyll absorbs the red and the blue of sunlight and reflects the green, so the colour we see is the light the plant did not use.
- NCERT states that the chlorophyll that absorbs sunlight is present in the chloroplast.
- A chloroplast is a double-membrane organelle enclosing a semi-fluid stroma, within which lie disc-shaped membrane structures that contain the chlorophyll.
- Chloroplast, chromoplast and leucoplast are the three kinds of plastid — green, other-coloured and colourless respectively.
- Chloroplasts, like mitochondria, carry their own DNA and their own ribosomes.
- Leaves look green because chlorophyll absorbs red and blue light and reflects green.
- Choosing the nucleus because it carries the genes for chlorophyll; the instruction and the pigment are not the same thing.
- Assuming every plant cell is green — root and many stem cells hold leucoplasts, not chloroplasts.
- Treating chlorophyll and chloroplast as interchangeable words; one is a pigment, the other the organelle that houses it.
Usually as a match of process to organelle, or as a statement item on what happens in photosynthesis; the plastid triple and the own-DNA point are the recurring extensions.
Match List I (Physiological processes) with List II (Cell organelles) and select the correct answer by using the codes given below the lists: List I — I. Photosynthesis, II. Mineral uptake, III. Respiration, IV. Protein Synthesis; List II — A) Plasma membrane, B) Chloroplast, C) Mitochondria, D) Ribosomes
- (a) I-A, II-B, III-C, IV-D
- (b) I-A, II-B, III-D, IV-C
- (c) I-B, II-A, III-C, IV-D
- (d) I-B, II-A, III-D, IV-C
Answer(c) I-B, II-A, III-C, IV-D
The same pairing inside a four-way match. Photosynthesis belongs to the chloroplast, respiration to the mitochondrion and protein synthesis to the ribosome — the very structures offered as distractors here, which is why one organelle-to-process table settles both papers.
Which one of the following statements about the process of photosynthesis is correct?
- (a) Light energy is directly converted to kinetic energy which oxidizes carbon dioxide.
- (b) Light energy is directly converted to chemical energy which causes reduction of water.
- (c) Chlorophyll absorbs the chemical energy of light which causes oxidation of carbon dioxide.
- (d) Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
Answer(d) Chlorophyll absorbs light energy which causes splitting of water molecules and reduction of carbon dioxide.
The same process asked at one level deeper. Chlorophyll absorbs the light, water is split and carbon dioxide is reduced to sugar — and all of it happens inside the chloroplast, which is where this item stops.
- practice — not a real PYQ
The colourless plastids that store starch, oil and protein in a plant cell are called
- (a)chloroplasts
- (b)chromoplasts
- (c)leucoplasts
- (d)lysosomes
Answer(c) leucoplasts — they are the colourless plastids used for storage, while chloroplasts are green and chromoplasts carry the other pigments.
- practice — not a real PYQ
A leaf appears green to the eye mainly because chlorophyll
- (a)absorbs green light and reflects red and blue light
- (b)absorbs red and blue light and reflects green light
- (c)absorbs all colours of light equally
- (d)produces a green pigment only in sunlight
Answer(b) absorbs red and blue light and reflects green light — the colour we see is the part of sunlight the pigment does not use.