Chloroplasts

Hugo Von Mohl discovered the first chloroplast in 1837 in green plants. 

The first set of species which could make its food by itself was discovered 1000 million years ago. These species include red algae, green algae, and plants (with chloroplasts). Some bacteria like cyanobacteria and euglena were also able to photosynthesise and chloroplasts were found in them.

Types of Chloroplasts

The common types of chloroplasts are chlorophyll a and chlorophyll b.

These are the pigments responsible for the green colour present in plants. The range of the colour depends on the combination of the chlorophyll pigment.

There are also various types of pigments. These include: 

• Chlorophyll C
• Chlorophyll D
• Chlorophyll F, etc.

 The colours coded by the pigment are in the range of red to olive green. 

Shapes of Chloroplasts

Chloroplasts are present in different species in different shapes. These shapes increase the surface area, which helps them maximise the output of the process of photosynthesis. 

The different shapes of the chloroplast are as follows: 

• Round
• Oval,
• Disk-shaped 
• Spiral
• Reticulate
• Ribbon shape
• Girdle shape
• Cup shape
• Star-shaped, etc

Structure of Chloroplasts 

A chloroplast contains double membrane-bound organelles. It has two membranes; the external membrane is called the outer membrane, and the inner membrane is called the internal membrane.

There is space between both the membranes called peri plastidial space, filled with a plastidial fluid. 

These membranes divide the chloroplast into two chambers: the outer and inner chambers. Both these chambers are filled with fluid, which helps regulate enzymes and metabolic reactions.

The outer chamber is filled with the plastidial fluid, while the inner chamber is filled with the stroma. The stroma contains all enzymes necessary for dark reactions. 

This stroma contains some organelles of chloroplast, which include:

• 70s ribosome
• Circular coiled naked DNA
• Thylakoid
• Grana 

Chloroplasts and plastids show cytoplasmic inheritance, which helps to share the genes with the other cells during the division of transduction.

70s ribosomes help form protein, which helps in the dark reactions. 

Thylakoid is a single membrane-bound organelle. It stores all the enzymes required for the light reaction (photosynthesis). All the photopigment and the colour pigment are present in the thylakoid membrane as it acts as an accessory pigment and transport and harvests the sunlight. 

Chlorophyll pigment is divided into porphyrin head and phytol tail. In contrast, the porphyrin head is made up of 4 pyrrole rings which are joined together by the methylene bridge. 

The park first discovered quantasome, and it is present in the thylakoid membrane. It is considered the structural and functional unit of the quantasome. It is a pigment bearing portion of thylakoid, and it contains 200-300 chlorophyll molecules and about 50 carotenoid molecules.

Functions of Chloroplasts

• Chloroplast helps form food by the process called photosynthesis.
• Chloroplast helps establish the proton gradient with the help of electron carriers. 

Photosynthesis

The formation of glucose molecules by trapping the sunlight is called photosynthesis. All the animals are either directly or indirectly dependent on the plant for the intake of food molecules.

The photosynthesis process takes place in the thylakoid of the chloroplast.

The raw material required for the operation of photosynthesis is as follows: 

• Carbon dioxide
• Water
• Sunlight
• Chloroplast

There are three steps involved in the process of photosynthesis, and these are: 

1. Trapping of sunlight by the chlorophyll
2. Conversion of the sunlight energy into chemical energy.
3. Use of chemical energy to reduce carbon dioxide into glucose molecules.

Photosystem and C3 Cycle

The C3 cycle was discovered by M Calvin. There are 3 steps involved in the C3 cycle:

1. Carboxylation
2. Reduction
3. Regeneration

In Carboxylation, carbon dioxide is accepted by the RUBP and forms a triose compound which further breaks down into 2 moles of 3PGA.

In reduction, these 3 moles of PGA will change into G3P as the first stable compound of 3 carbon. This is called the C3 cycle.

In regeneration, The RuBp compound will be free for the next generation. The enzyme used in this cycle is the RUBISCO enzyme, the most abundant enzyme in the world.

C4 Cycle 

Among C4 plants, C4 cycles occur in two places. One is a bundle sheet, and the other is a mesophyll shell. The C4 cycle is a repetition of the C3 cycle and a new enzyme.

In the C4 cycle, carbon dioxide fixation occurs via phosphoenolpyruvate. In mesophyll cells, the C4 cycle operates but in bundle sheets the C3 cycle operates. C4 plants have a dimorphic chloroplast, the adrenal chloroplast found in bundle sheets, and the granite chloroplast, which is found in mesophyll.

The primary acceptor of carbon dioxide in the C4 plant is a PEP, phosphoenolpyruvate, a 3 carbon compound. The secondary and final carbon dioxide acceptor in the C4 plant is RUBP, a 5C carbon compound.

C4 plants require both the enzyme per case and the RubisCO enzyme.

The first stable product is oxalic, acetic acid, which is a 4 carbon compound after which the C4 cycle is named. 

Chemiosmotic Therapy

Biochemist Peter Mitchell postulated a theory in 1961 to describe ATP synthesis through a proton electrochemical coupling.

Photosystem -II was found to be located on the inner side of the thylakoid membrane ( towards the lumen) and after photoexcitation. It behaves as an oxidant and causes the photolysis of water.

Photolysis of water occurs on the inner side of the thylakoid membrane. After the photolysis of water, oxygen molecules evaporate into the surrounding area, and proton ions accumulate in the Intra thylakoid space.

The electron off photosystem 2 transfers to photosystem 1 and finally transfers to NADP+ to form NADP-

As the electrons transfer through the photosystem, protons are transported across the membrane, which happens due to the primary acceptor of electrons located outside the membrane.

The molecule transfers its electron to an electron carrier located on the inner side of the membrane, and the resulting protons are released into the lumen or inner side of the thylakoid membrane.

Due to the accumulation of protons in the Intra thylakoid space, it established a proton gradient, and after the establishment of the proton gradient and later, it diffused from the Intra thyroidal space to the stroma.

Conclusion

So, it can be concluded that chloroplasts are plant cell organelles that use the photosynthetic process to transform light energy into relatively stable chemical energy. They are preserving life on Earth by doing so. Chloroplast is the critical organelle for the formation of food, and it is not replaceable. Chloroplast can also be a prokaryotic endosymbiont because of prokaryotic organelles.