The Plastid

What is a Plastid?

Plastid is one of the energy-converting organelles found in plants and some algae. It is similar to that of mitochondria and is used in the process of photosynthesis.

There is a theory behind the existence of plastids that states that they used to exist individually as bacteria and then, later on, plant cells came and ate them and then plants thought them as useful organelles to keep inside as they helped in the conversion of light into glucose. Plastids and mitochondria both are double-layered membranes in which one layer is their own and the second layer is cell membrane. There are three types of plastids in plant cell: Leucoplast, chloroplast and chromoplast.


The chloroplast is the most important organelle as it is used to convert sunlight into oxygen and glucose. Oxygen is released out as it is not useful for plants. All the green parts of the plants contain chloroplast. It has two layers: the outer layer and the inner layer. The inner layer protects the stroma and grana. Grana is the stack formed structure, and stroma is the cytoplasm consisting of various enzymes needed for the synthesis of protein and Carbohydrates. One single stack is known as the granum which contains thylakoids. Photosynthesis process occurs at the surface of thylakoid. After the formation of energy, it is transferred to the stroma where carbon is fixed, and sugar is synthesized. The stacks of thylakoids are connected to each other through stromal lamellae. This is also known as the skeleton of the chloroplast as it separates each granum from the other which results in an increase in efficiency of capturing lights.


Chloroplast’s function is to form glucose for plants. When sunlight hits the plant body, chloroplast absorbs light and converts it into chemical energy found in the components of ATP and NADPH. These chemical compounds move to stroma where carbon molecule is fixed from the carbon dioxide molecule.

Applications of chloroplast:

  • It helps in the photosynthesis process.
  • It helps in the release of oxygen which in turn is used for respiration process by all aerobic organisms.
  • It temporarily stores starch greens and pyrenoid.
  • It helps to fix carbon from carbon dioxide.

This was just a brief layout of plastid and its types; mainly chloroplast. To know more and understand the structure of plastid watch the video.

Practise This Question

The compound which has only primary and tertiary carbons is?