Introduction
We are living in a diverse and colorful world. Earth is the only planet in our solar system where life sustains. The flora and fauna of the earth are very diverse. There are so many kinds of animals, plants, and microorganisms living on the earth. Among these organisms, only plants and some algae can produce food on their own. This is only possible because of the chlorophyll pigment in their cells. The chlorophyll pigment is located in the chloroplast of the plant cell. Chloroplasts are one of the cell organelles of plant cells. Chloroplast structure and function are discussed below (2).
Chloroplast
Meaning
The word chloroplast comes from the Greek words khloros and plastes. Khloros means “green”, and the word plastes means “formed”. Due to high chlorophyll molecules in chloroplasts, they give green color to many plants and algae (2).
Definition
The cell is the structural and functional unit of the plant. These cells are made up of different cell organelles. Chloroplasts are a type of cell organelle. The chloroplast is the kitchen of the plant cell. The bi-membraned cytoplasmic organelles present in plant cells that help photosynthesis and the organelles that turn the leaves green are called chloroplast. Chloroplasts are mainly present in the mesophyll tissue of leaves, in the skin of young stems and young fruits, and in the petals of flowers. Chloroplasts are also found in seeds in embryos and in parenchyma tissue cells of plants. The chlorophyll pigment present in the chloroplasts participates in the process of photosynthesis (2 & 5).
Why chloroplasts are green?
Chloroplasts are present in all the cells of the plants. Due to the presence of chloroplasts, the leaves, young branches, stems, green fruits, etc. of the plant look green. Chloroplasts are mainly composed of chlorophyll a, chlorophyll b, keratin, and xanthophyll. But chloroplasts contain high amounts of chlorophyll a and chlorophyll b. This chlorophyll a and chlorophyll b plays an important role in providing green color to plant organs. Since all the elements that provide green color are present in chloroplasts, therefore, the chloroplasts are green in color. And the leaves of the tree are green because of the presence of chloroplasts (4).
Properties of chloroplast
1. Chloroplasts are round, oval, lattice, spiral stripes, discs, etc.
2. Chloroplasts present in plant cells participate in photosynthesis and produce food in the plant body.
3. Chloroplasts are located in the mesophyll tissue of leaves.
4. The diameter of the chloroplast is 3 to 6 m. And these are 1 to 2 mm thick.
5. Like other cell organelles, chloroplasts have chromosomes. The chromosomes contain genes. This gene causes photosynthesis and other activities in plant cells.
6. This cellular organelle consists of chlorophyll a, chlorophyll b, keratin, and xanthophyll (1 & 5).
The main component of the chloroplast
Chlorophyll is the main component of the chloroplast therefore, is one of the most important components of photosynthesis. It is mainly of five types, chlorophyll a, b, c, d, and e. However, chlorophyll a (C₅₅H₇₂O₅N₄Mg) and chlorophyll b (C₅₅H₇₀O₆N₄Mg) are mainly present in green plants and algae.
Location
Chlorophyll is present in thylakoids of chloroplasts in all green plants.
Role of chlorophyll in plant cells
Chlorophyll present in chloroplasts plays an important role in plant cells.
1. Chlorophyll is heated by absorbing photon particles emitted from sunlight and dissolves the water absorbed by the plant into H⁺ and OH⁻ ions.
2. It binds solar energy to ATP as chemical energy.
3. Chlorophyll (a and b) mainly absorbs red and purple-blue light of the spectrum, which is the main function
4. The chlorophyll present in the chloroplast of leaves of the plant absorbs the red and purple-blue light and reflects the green wave light. So the color of the leaves is green (2 & 3).
Ultra-structure of chloroplast
The chloroplasts of plant cells are made up of several parts.
1. Outer membrane
Each chloroplast is surrounded by two membranes. The outermost membrane of the chloroplast consists of phospholipids. And is accessible to small organic molecules. The outer membrane contains 48% phospholipids, 46% galactolipids, and 7% sulfolipids.
2. Inner membrane
The inner membrane is the inner part of the outer membrane made of phospholipids. It contains 16% phospholipids, 79% galactolipids, and 5% sulfolipids. The inner membrane of the chloroplast assists in the transport of proteins.
3. Intermembrane space
There is an outer membrane on the outside and an inner membrane on the inside. The distance between the outer membrane and the inner membrane is about 10-20 nm. This place is called intermembrane space or periplastidial space.
4. Stroma
The next part of the inner membrane is the stroma. A transparent, homogeneous jelly-type fluid located inside the inner membrane is called the stroma. It is mainly a protein-based alkaline fluid. The stroma contains DNA, RNA, ribosome,s and various enzymes that make sugars. It plays an important role in the photosynthesis of plants. The light-independent phase of photosynthesis occurs in the stroma region of the chloroplast.
5. Thylakoid membrane
The thylakoid membrane is the third membrane located next to the stroma and is made of two-layer phospholipids.
6. Thylakoid
The stroma has many disc-type parts called thylakoids. Each thylakoid contains numerous quantasome granules. The chlorophyll pigment for which the leaves of plants are green is located inside the quantasome grains of the thylakoid. The Thylakoid lumen is the empty space inside the thylakoid.
7. Granum
One on top of the other the thylakoid is arranged to form the shape of a pile. This pile of the thylakoid is called a granum. Each chloroplast contains about 40 to 60 granum. Light energy is converted into chemical energy in granum.
Each granum contains 5 to 25 disc-shaped thylakoids. This part of the chloroplast plays a role in photosynthesis. The first process of photosynthesis, the light reaction, takes place in the grana part of the chloroplast.
Grana contains enzymes required for the dependent reaction of photosynthesis and ATP synthesis enzymes required for the synthesis of ATP molecules by ketosomosis.
Grana provides a large surface area for the attachment of chlorophyll, other photosynthetic pigments, electron carriers, and enzymes, thus assisting in photosynthesis.
8. Grana lamellae
The thylakoid membrane with which the granum is formed is called grana lamella.
9. Stroma lamellae
The stroma-lamellae are each cylindrical tubular organ connecting the two adjacent grana. Here the two granum parts are joined by a long thylakoid part. Known as stroma-thylakoid (1 & 4).
Chemical composition of chloroplast
Chloroplasts consist of DNA, RNA, different types of vitamins, various metals, proteins, lipids, carbohydrates, etc. It also contains some amount of vitamin E, K, and metals including Mg, Cu, Fe, Zn, Mn, and P (1 & 2).
| Name of component | Amount of components (%) |
| 1. Protein | 30 – 55% |
| 2. Chlorophyll | 5 – 10% |
| 3. Lipids | 20 – 35% |
| 4. Carotenoid | 2 – 4% |
| 5. Carbohydrate | 3 – 7% |
| 6. RNA | 3 – 5% |
| 7. DNA | 0.5% |
| 8. Minerals | 0.3% |
Chloroplast genetic system
Chloroplasts have their own DNA that performs replication and protein synthesis. Chloroplasts also have RNA, RNA polymerase, DNA polymerase, and ribosomes. 70s ribosomes synthesize proteins in the chloroplasts of plant cells. Chloroplasts can produce food by absorbing light emitted from sunlight. Besides, the ribosomes present in the chloroplast help in the synthesis of some amount of protein (1).
Function of chloroplast
The effectiveness of chloroplasts in plant cells is abundant. The role of chloroplasts is very important in the cellular organelles that are present in the plant body.
1. The light reaction of photosynthesis occurs in the grana region of chloroplasts. And the light-independent phase occurs in the stroma region of the chloroplast.
2. It participates in cytoplasmic inheritance due to its own DNA.
3. Chloroplasts have their own ribosomes that synthesize some proteins.
4. They can form chromoplasts and leucoplasts.
5. Chloroplasts absorb light energy emitted from the sun and convert light energy into chemical energy.
7. The function of chloroplasts is to make sugar foods (starch) in the process of photosynthesis.
8. Chloroplasts store food produced by photosynthesis for plants.
9. They make proteins and nucleic acids for their own needs.
10. Photophosphorylation occurs by chloroplast. That is, chloroplast converts ADP to ATP with the help of sunlight (1 & 4).
Reference
1. S. B Agarwal and V. K. Agarwal. Unified Botany, B.Sc Third Year. Shiva Lal Agarwal & Company Publications, Indore. Chapter: Structure and function of cell organelles. Page 229-242.
