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Endosymbiosis Theory


Evolutionary origin of chloroplasts

Chloroplasts are believed to have originated from photosynthetic prokaryotic organisms (blue algae), which settled in aerobic eukaryotic primitive cells by endosymbiosis.

This symbiosis, about 1.2 billion years ago, would have given rise to red algae, then brown and green algae, and higher vegetables.

During the evolutionary process, chloroplast precursor bacteria transferred part of their genetic material to the host cell DNA, thus relying on the host cell genome to produce many of its proteins.
This origin is similar to mitochondria, but there are differences such as the size of organelles, chloroplast is much larger than mitochondria, and the energy source is different, chloroplast uses light energy while mitochondria uses chemical energy.

Chemical composition of chloroplasts

Chloroplasts are the most evident organelles of plant cells. It is made up of 50% protein, 35% lipid, 5% chlorophyll, water and carotenoids. Some of the proteins are synthesized by the cell nucleus, but lipids are synthesized within the organelle itself.
The number of chloroplasts is regulated by the cell. There are cells that contain only one chloroplast, but most cells that perform photosynthesis contain about 40 to 200 chloroplasts, which move as a function of light intensity and cytoplasmic current.
Similar to mitochondria, chloroplasts are surrounded by two membranes, one highly permeable external, and one internal that requires specific proteins for metabolic transport, and an intermembrane space.

Inside the organelle is an amorphous matrix called stroma which contains various enzymes, starch grains, ribosomes and DNA.

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However, the chloroplast inner membrane is not folded into ridges and does not contain an electron transport chain. Dipped in the stroma, there is a membrane system (bilayer) that forms a set of disc-shaped flat bags called tilacoid membrane (from Greek thylakos, bag).

The stacked disk pool is named after granum. The lumen of the tilacoid membrane is called the tilacoid space. In the membrane exposed to the stroma are located chlorophylls participating in photosynthesis.

Pigments bound to different proteins and lipids in the membranes of granar and stromal tilacoids form complex protein-chlorophyll systems called photosystems. There are two types of photosystems:

Photosystem I: located in the stromal membrane region, are the smallest intramembranous particles.

Photosystem II: located in granar tilacoids, formed by larger particles.

Chloroplast Genetic System

The plastid genome consists of a small molecule of Circular DNA, with characteristics very similar to those of mitochondria and bacteria.

Plastid DNA occurs in larger amounts and is more complex than mitochondria. exist 30 to 200 copies of DNA per organelle containing approximately 120 genes.
Genetic sequencing of chloroplasts from various plants has led to the identification of many of these genes. They transcribe all Ribosomal RNAs that make up the plastoribosomes and 30 different types of RNA transporters.

This genome also encodes 20 ribosomal proteins, 30 photosynthesis proteins and some RNA polymerase subunits (proteins involved in gene expression).

But even synthesizing their own proteins, about 90 percent of chloroplast proteins are encoded by the nuclear genes that are imported from cytosol into the organelle.