What is Chloroplast? {Structure, Photosynthetic Pigments, Chlorophylls Explained}

Chloroplasts – The Site of Photosynthesis

All green parts of a plant have chloroplasts; however, the leaves are the significant sites of photosynthesis in a lot of plants.

Chloroplasts are present in large numbers, about half a million per square millimetre of the leaf surface.

Chloroplasts are present primarily in the cells of mesophyll tissue inside the leaf. Each mesophyll cell has about 20-100 chloroplasts.



Structure of Chloroplast

Chloroplast has a double membrane envelope that confines dense fluid-filled region, the stroma which consists of most of the enzymes needed to produce carbohydrate molecules. Another system of membranes is suspended in the stroma.

These membranes form a sophisticated interconnected set of flat, disc-like sacs called thylakoids. The thylakoid membrane encloses a fluid-filled ‘thylakoid interior space’ or lumen, which is separated from the stroma by thylakoid membrane. In some locations, thylakoid sacs are stacked in columns called grana (sing granum). Chlorophyll and other photosynthetic pigments are found embedded in the thylakoid membranes and impart green colour to the plant.


Electron acceptors of photosynthetic ‘Electron Transport Chain’ are likewise parts of these membranes. Thylakoid membranes are therefore associated with ATP synthesis by chemiosmosis.

Chlorophyll (and other pigments) soak up light energy, which is converted into chemical energy of ATP and NADPH, the products which are used to make sugar in the stroma of the chloroplast.

Photosynthetic Pigments

Light can operate in chloroplasts only if it is absorbed. Pigments are the substances that captivate visible light (380-750 nm in wavelength). Different pigments absorb light of different wavelengths (colours), and the wavelengths that are absorbed disappear.


An instrument called Spectrophotometer is used to measure the relative capabilities of different pigments to take in different wavelengths of light.

Absorption spectrum

A chart outlining the absorption of light of different wavelengths by a pigment is called absorption spectrum of the pigment.


Thylakoid membranes consist of a number of sort of pigments; however, chlorophylls are the primary photosynthetic pigments. Other, accessory photosynthetic pigments present in the chloroplasts consist of yellow and red to orange carotenoids; carotenes are mainly red to orange and xanthophylls are yellow to orange. These widen the absorption and utilization of light energy.


There are known lots of different kinds of chlorophylls. Chlorophyll a, b, c, and d are discovered in eukaryotic photosynthetic plants and algae, while the other is discovered in photosynthetic bacteria and are known as bacteriochlorophylls.

Chlorophylls usually take in generally violet-blue and orange-red wavelengths. Green, yellow, and indigo wavelengths are least absorbed by chlorophylls and are sent or reflected back, although the yellows are frequently masked by darker green colour. For this reason, plants appear green, unless masked by other pigments

Structure of Chlorophyll Molecule

A chlorophyll molecule has two main parts: One flat, square, light taking in hydrophilic head and the other long, anchoring, hydrophobic hydrocarbon tail.

The head is an intricate porphyrin ring which is comprised of four joined up with smaller pyrrole rings made up of carbon and nitrogen atoms. An atom of magnesium is present in the centre of porphyrin ring and is coordinated with the nitrogen of each pyrrole ring. That is why magnesium shortage triggers yellowing in plants.

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Haem portion of haemoglobin is also a porphyrin ring however containing an iron atom instead of magnesium atom in the centre.

Phytol: Long hydrocarbon tail which is attached to one of the pyrrole rings is phytol (C20 H39). The chlorophyll molecule is embedded in the hydrophobic core of the thylakoid membrane by this tail.

Chlorophyll a and b

Chlorophyll a and chlorophyll b differ from each other in only one of the functional groups bonded to the porphyrin; the methyl group (- CH3) in chlorophyll a is replaced by a terminal carbonyl group (- CHO) in chlorophyll b.

Molecular Formulae of both chlorophyll molecules:

Chlorophyll a: C55H72O5N4Mg

Chlorophyll b: C55H70O6N4Mg

Due to this slight difference in their structure, the two chlorophylls reveal a little different absorption spectrum and for this reason different colours. Some wavelengths not soaked up by chlorophyll a are extremely effectively absorbed up by chlorophyll b and vice-versa. Such differences in the structure of different pigments increase the variety of wavelengths of the light taken in. Chlorophyll a is blue-green while chlorophyll b is yellow-green.

Of all the chlorophylls, chlorophyll a is the most abundant and the most crucial photosynthetic pigment as it takes part straight in the light-dependent reactions which convert solar energy into chemical energy. It is present in all photosynthetic organisms other than photosynthetic bacteria. Chlorophyll itself exists in numerous types differing slightly in their red absorbing peaks e.g. at 670, 680, 690, 700 nm.

Chlorophyll b is found along with chlorophyll a in all green plants(embryophytes) and green algae.

Chlorophylls are insoluble in water however soluble in organic solvents, such as carbon tetrachloride, alcohol and so on.

Carotenoids-accessory pigments

Carotenoids are yellow and red to orange pigments that take in strongly the blue violet variety, different wavelengths than the chlorophyll absorbs. So, they expand the spectrum of light that provides energy for photosynthesis. These and chlorophyll b are called accessory pigments because they absorb light and transfer the energy to chlorophyll a, which then starts the light reactions. It is normally believed that the order of transfer of energy is:


Some carotenoids safeguard chlorophyll from intense light by taking in and dissipating excessive light energy, instead of moving energy to chlorophyll. The studies reveal comparable carotenoids may also be safeguarding the human eye.

Q/A about Chloroplast

  1. What is the primary function of chloroplasts?
    • Chloroplasts are the sites of photosynthesis in plant cells, where they convert light energy into chemical energy, producing carbohydrates.
  2. Where are chloroplasts primarily located within plant cells?
    • Chloroplasts are present in the cells of the mesophyll tissue inside the leaves, with about half a million per square millimeter of the leaf surface.
  3. How is the structure of a chloroplast organized?
    • Chloroplasts have a double membrane envelope, containing a dense fluid-filled region called the stroma. Thylakoid membranes, arranged in stacks called grana, are suspended in the stroma.
  4. What is the role of chlorophyll in photosynthesis?
    • Chlorophyll, the primary photosynthetic pigment, absorbs light energy, converting it into chemical energy (ATP and NADPH) used to produce sugars in the stroma of the chloroplast.
  5. How do pigments contribute to photosynthesis?
    • Pigments, such as chlorophylls and carotenoids, absorb specific wavelengths of light. These pigments broaden the spectrum of light used for photosynthesis, enhancing the efficiency of energy absorption.
  6. What is an absorption spectrum in the context of photosynthetic pigments?
    • An absorption spectrum is a chart illustrating the absorption of light at different wavelengths by a pigment. It helps understand the pigment’s ability to absorb specific colors of light.
  7. What are the main types of chlorophylls, and how do they differ?
    • Chlorophylls a and b are the main types found in eukaryotic photosynthetic plants and algae. They differ in one functional group, resulting in slightly different absorption spectra and colors.
  8. Why are carotenoids considered accessory pigments?
    • Carotenoids are accessory pigments because they absorb light at different wavelengths than chlorophylls. They transfer the absorbed energy to chlorophyll, enhancing the overall range of light utilized in photosynthesis.
  9. How does chlorophyll protect itself from excessive light?
    • Some carotenoids protect chlorophyll from intense light by absorbing and dissipating excess light energy, preventing potential damage to the photosynthetic apparatus.
  10. Are chlorophylls soluble in water?
    • Chlorophylls are insoluble in water but soluble in organic solvents like carbon tetrachloride and alcohol.
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Wrap-Up: Understanding Chloroplasts and Photosynthetic Pigments

In this comprehensive exploration of chloroplasts, their structure, and the intricacies of photosynthetic pigments, we’ve delved into the fascinating world of plant biology. Let’s summarize the key points covered:

  1. Chloroplasts – The Hub of Photosynthesis:
    • Found in all green parts of plants, chloroplasts are especially concentrated in leaves, serving as the primary sites of photosynthesis. With about half a million per square millimeter of leaf surface, chloroplasts are crucial for energy conversion.
  2. Structural Insights:
    • Chloroplasts boast a double membrane envelope enclosing a fluid-filled stroma, rich in enzymes for carbohydrate production. Thylakoid membranes, organized in stacks called grana, house chlorophyll and other pigments essential for capturing light energy.
  3. Photosynthetic Pigments and Light Absorption:
    • Pigments, such as chlorophylls and carotenoids, play a vital role in absorbing visible light during photosynthesis. The absorption spectrum, measured by a spectrophotometer, outlines the wavelengths absorbed by these pigments.
  4. Chlorophyll Varieties and Functions:
    • Chlorophyll a and b, the primary photosynthetic pigments in eukaryotic plants, exhibit slight structural differences, leading to varied absorption spectra. Chlorophyll a is particularly crucial for the light-dependent reactions, converting solar energy into chemical energy.
  5. Carotenoids as Accessory Pigments:
    • Carotenoids, additional pigments alongside chlorophylls, broaden the spectrum of light absorbed for photosynthesis. They act as accessory pigments, transferring energy to chlorophyll and protecting it from excessive light.
  6. Structure of Chlorophyll Molecule:
    • The chlorophyll molecule comprises a complex porphyrin ring with a magnesium atom, a hydrophilic head, and a hydrophobic hydrocarbon tail (phytol). Its structure plays a key role in light absorption.
  7. Varieties of Chlorophyll and Their Significance:
    • Different forms of chlorophylls (a, b, c, and d) exist in various photosynthetic organisms, contributing to the diversity of light absorption. Chlorophyll a remains the most abundant and vital pigment in the process.
  8. Carotenoids’ Protective Role:
    • Carotenoids not only aid in light absorption but also protect chlorophyll from intense light by dissipating excess energy.



Multiple Choice Questions (MCQs) on Chloroplasts and Photosynthetic Pigments:

1. Where are chloroplasts primarily located in a plant?
A. Roots
B. Stems
C. Leaves
D. Flowers

Answer: C

2. What is the approximate number of chloroplasts per square millimeter of the leaf surface?
A. 100,000
B. 250,000
C. 500,000
D. 750,000

Answer: C

3. Which part of the chloroplast contains most of the enzymes for carbohydrate production?
A. Thylakoid membrane
B. Stroma
C. Grana
D. Porphyrin ring

Answer: B

4. What is responsible for the green color of plants?
A. Carotenoids
B. Thylakoids
C. Chlorophyll
D. Stroma

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Answer: C

5. What instrument is used to measure the relative capabilities of different pigments to absorb light of different wavelengths?
A. Microscope
B. Spectrophotometer
C. Chromatograph
D. Photometer

Answer: B

6. What term is used to describe the chart outlining the absorption of light by a pigment at different wavelengths?
A. Absorption chart
B. Spectrum chart
C. Pigment chart
D. Absorption spectrum

Answer: D

7. Apart from chlorophyll, what are the other primary photosynthetic pigments found in chloroplasts?
A. Carotenoids
B. Xanthophylls
C. Both A and B
D. None of the above

Answer: C

8. Which wavelengths are generally least absorbed by chlorophylls?
A. Red
B. Blue
C. Green
D. Violet

Answer: C

9. What is the structure responsible for the light-absorbing function of chlorophyll?
A. Thylakoid
B. Porphyrin ring
C. Phytol tail
D. Stroma

Answer: B

10. Which chlorophyll molecule is most abundant and crucial for the light-dependent reactions in photosynthesis?
A. Chlorophyll a
B. Chlorophyll b
C. Chlorophyll c
D. Chlorophyll d

Answer: A

11. What is the primary role of carotenoids in photosynthesis?
A. ATP synthesis
B. Light absorption
C. Chlorophyll protection
D. CO2 fixation

Answer: C

12. In which part of the chloroplast is chlorophyll embedded?
A. Stroma
B. Thylakoid membrane
C. Grana
D. Porphyrin ring

Answer: B

13. What metal is present in the center of the porphyrin ring of chlorophyll?
A. Iron
B. Magnesium
C. Copper
D. Zinc

Answer: B

14. Which chlorophyll molecule is responsible for the yellow-green color in plants?
A. Chlorophyll a
B. Chlorophyll b
C. Chlorophyll c
D. Chlorophyll d

Answer: B

15. What is the role of the hydrophobic tail (phytol) in the chlorophyll molecule?
A. Light absorption
B. ATP synthesis
C. Anchoring in the thylakoid membrane
D. Carbohydrate production

Answer: C