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

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.

Spectrophotometer

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.

Chlorophylls

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.

 

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