Chloroplast Structure

Of all the biological processes that occur on Earth, photosynthesis is one of the most important ones. The foundation of the food pyramid that makes life possible on our planet is the food created by the plants through process of photosynthesis. The mechanism inside Eukaryotic cells and plants that makes photosynthesis possible is chloroplast.

Chloroplast Structure and Function

Chloroplasts are cell organelles, whose prime function is to convert the energy derived from sunlight to chemical energy stored in glucose. They belong to a type of cell organelles, which are collectively known as plastids. Plastids are key organelles in eukaryotic cells and other plants, that are sites for creation and storage of important chemical compounds.

Chloroplast is a plastid, which is responsible for the complete execution of the photosynthesis process. The unique structure of chloroplast is supposed to have evolved from endosymbiotic cyanobacteria (blue green algae) over the course of millions of years.

Chloroplast Size
Chloroplasts are flat disc shaped organelles which are usually 2-10 μm in diameter with a thickness of about 1 μm. In terrestrial plants, their diameter is usually 5 μm and thickness is around 2.3 μm. They are contained within the parenchyma cells in terrestrial plants, each having about a hundred chloroplasts. Let us have a look at the structure, by considering every one of its constituent parts.

Chloroplast Membrane
The chloroplast is covered by a thick lipid bi-layer membrane, which controls the transport of materials into and out of this cell organelle.

Stroma
Inside the membrane is an aqueous fluid that permeates all the chloroplast interior and is known as stroma. It contains enzymes, ribosomes and circular strands of DNA (Deoxyriboenucleic Acid). Stroma is the site for the Calvin cycle, which is the process of carbon fixation in plants. Protein synthesis may occur at ribosomal sites within the stroma.

Thylakoids
Thylakoids are the structures inside the stroma, that are actual sites of photosynthesis. They are flat disc shaped sub-organelles of chloroplast, that are stacked vertically over each other. Every stack of thylakoids is called a granum. These grana are spread throughout the chroloplast's stroma. Every thylakoid has a membrane covering it and the photosynthesis process takes place on its surface.

Antenna Complexes
The prime function of chloroplast is absorption of light and that is executed by the thylakoid membrane mechanism. The thylakoid membrane contains light gathering structures called antenna complexes, which are made up of light absorbing pigments like chlorophyll and carotenoids, along with proteins that gel them together. There are stromal lamellae, which connect the various thylakoid stacks and create the skeleton that sustains the whole structure.

The antenna complexes are spread out, to expand the surface area of exposure to solar photons. This makes it possible for the chlorophyll and carotenoid molecules to capture photons of varying wavelengths. Through a process called 'Resonance Energy Transfer', the solar energy absorbed is transferred to the photosynthesis reaction centers in the form of excited electrons. The energy created in these reactions is used to power the Calvin cycle in the stroma.

Chloroplast is indeed a sophisticated light gathering mechanism, which forms the 'kitchens' of plants, where carbon dioxide and water is processed to create sugars like glucose through conversion of solar energy into chemical energy. As a by-product of the photosynthesis reaction that occurs within the structure, oxygen is released which is essential for the survival of all living organisms.

The sophistication of the inner chloroplast structure and its functioning is mind-boggling when you realize how precisely everything is put in place for the process of photosynthesis to occur. The biochemical machinery that makes the process work, dwarfs the most advanced engineering achievements of man!