Chloroplasts

The word chloroplast is derived from the Greek word “khloros” which means green and “plates” which means formed. They are considered to be the organelles that perform specific functions in plants, mainly photosynthesis.

Chloroplasts are round, oval or disk-shaped that help in the storage and synthesis of food and energy in plants. The main function is to absorb light energy. They occur in green leaves and stems but are mostly concentrated in the parenchyma cells of the leaf mesophyll. They have a biconvex lens like structure and are enclosed by a double-layered envelope. They contain smaller parts called thylakoids which are arranged in stock like structures called grana. There are 40-80 grana in the matrix of the chloroplast. 

The Evolution of Chloroplast 

The evolution of Chloroplasts can be understood much better through the explanation of the Endosymbiotic Theory. The theory says that chloroplasts have been embedded into the eukaryotic cell the same way as mitochondria were subdued into all eukaryotic cells: by first existing as free-living cyanobacteria that had a symbiotic relationship with a cell, along with creating energy for the cell in return for a place to live, and slowly evolving into a form that could no longer exist separately from the cell. Chloroplasts contain their own, separate DNA that is biconcave, like that of a bacterial cell, and inherited from the mother plant alga. New chloroplasts are mostly formed through a process called binary fission, or splitting, which is how bacteria generally reproduce. This is similar to the way mitochondria reproduces. Other theories suggest that chloroplasts have evolved from cyanobacteria, and mitochondria evolved from aerobic bacteria. The structure of chloroplasts is very similar to that of cyanobacteria; both have two membranes, circular DNA, thylakoids, and ribosomes.

Characteristics and Structure

Chlorophyll is shaped like biconvex lenses which are oval or disk-shaped. It contains pigments called chlorophyll that help absorb light energy. Chlorophyll is of two types- chlorophyll a and chlorophyll b. The stroma present inside the chloroplasts has colourless centres. Thylakoids are contained within the chloroplast, arranged in stock like piles. The space occupied by the thylakoids within the chloroplasts is called thylakoid space. 

Functions:

The main function of chloroplasts is photosynthesis in plants. It is the process by which plants get food by utilizing sunlight, water and carbon dioxide, releasing oxygen and producing sugar in return. The membrane of the thylakoids is the site of the light reactions.

In C₄ plants, the initial carbon fixation step and the Calvin cycle are separated, carbon fixation occurs via phosphoenolpyruvate (PEP) carboxylation in chloroplasts located in the mesophyll, and malate, the 4 carbon product of that process, is transported to chloroplasts in bundle-sheath cells, where the Calvin cycle is carried out.

Chloroplasts are also important for pathogen defence in plants and photolysis reaction of water.

Calvin Cycle:

The metabolic process through which plants incorporate carbon dioxide into carbohydrates. In the initial reaction, the Rubisco enzyme catalyzes the reaction between CO₂ and ribulose,1-5- bisphosphate and forms 6 carbon intermediate compounds. Then, 6 carbon compounds are split into 3 carbon compounds which are converted into glyceraldehyde-3-phosphate or G3P in the subsequent steps. 3 turns of the Calvin cycle are needed to make one molecule of G3P. One G3P molecule exists in the Calvin cycle and goes to the cytoplasm to make carbohydrates.  And 5 G3P go back to make 3 Rubp receptor molecules.  In the reduction step, a total of 6 ATP & 6, NADPH is required.  In regeneration, step 3 ATP is required. So, it takes six turns of the cycle, 18 ATP, and 12 NADPH, to produce one molecule of glucose.

Location 

Chloroplasts are situated in the cells of the leaves of green plants and the cells of eukaryotic algae. All green plants have a unique cell organelle called plastid, and chloroplasts are a kind of plastic. The chlorophyll in these chloroplasts is probed by sunlight which results in the conversion of solar energy into usable chemical form. And, this is how sunlight enters our living ecosystem. As mentioned earlier, the chloroplast is enveloped by a membrane. Chloroplasts have three membranes. The outer membrane is semipermeable and gives way to small molecules. The inner membrane is a slightly less porous layer, and the last and final membrane is called the thylakoid membrane which appears as a series of flattened disks piled on top of each other. Chlorophyll is mostly found in these thylakoids.Role in Photosynthesis 6CO₂ + 6H₂O + Light Energy = C₆H₁₂O₆ + 6O₂.The above reactions capture sunlight through chlorophyll and carotenoids to form adenosine triphosphate (ATP) which is the energy currency of the cell along with nicotinamide adenine dinucleotide phosphate (NADPH), which carries charged electrons. This follows the second stage which consists of the light-independent reactions, popularly known as the Calvin cycle. In the Calvin cycle, the electrons that are carried by NADPH convert inorganic carbon dioxide into organic matter in the form of carbohydrates. This process is called CO₂ fixation. The remaining carbohydrates and other organic molecules which are not used at that particular time can be stored and used later for different mechanisms of the plant. 

What happens to Plants That do not have Chloroplast? 

Chloroplasts are an essential element for the health and survival of plants and photosynthetic algae. Chloroplasts can be compared to Solar Panels. They take light energy and convert it into use for that power multiple functions. So, now what happens to plants that do not have chlorophyll? There is one such exception- Rafflesia, which is a parasitic plant that obtains nutrients from other plants, specifically, Tetrastigma vines. Because Rafflesia gets all of its energy from parasitizing the other plant, it does not need its chloroplast. Hence, the plant has gotten so used to this mechanism, that it has lost the genes that code the development of the chloroplast. It is known to be the only genus belonging to plants on land that lack chloroplasts. 

Some Facts About Chloroplast 

1. Simple cells have very few Chloroplasts, whereas complex plants can contain hundreds of them.

2. The most abundant protein in Chloroplast is called Rubisco. It is found in copious amounts.

3. Animals and humans do not need Chloroplasts, because we get our energy from eating and digesting food.

4. Chlorophyll A is the most common type, showcasing the colour green, and chlorophyll C is a golden brown colour.

5. They also fight diseases as part of the cells immune system.

6. They make amino acids for the cell. 

7. Ten percent of a chloroplasts protein is encoded by its DNA. 

8. When they degrade, they turn into chromoplast which is a colourful pigment which in turn is the reason for fruits to change colour when they ripen.