Plastids- Definition, Structure, Types, Functions and Diagram


Plastids are specialized organelles that are found in the cells of plants and some algae. They are involved in various functions such as photosynthesis, food storage, pigment synthesis, and cell differentiation. Plastids have a double membrane-bound structure and contain their own DNA and ribosomes.

The term "plastid" was coined by Ernst Haeckel in 1866, who derived it from the Greek word "plastos", meaning "formed" or "molded". He used it to describe the granules of starch and other substances that he observed in plant cells. However, he did not distinguish between different types of plastids or their origin.

The first clear definition of plastids was given by A. F. W. Schimper in 1883, who recognized them as independent units of the cell that can multiply by division and can differentiate into various forms. He also proposed that plastids originated from a common ancestor, the proplastid, which he considered to be a primitive form of cyanobacterium that was incorporated into a eukaryotic cell through endosymbiosis.

Schimper`s theory of plastid evolution was later supported by molecular and genetic evidence, which showed that plastids share a common origin with cyanobacteria and have retained some of their ancestral genes and metabolic pathways. Plastids are also closely related to mitochondria, another type of endosymbiotic organelle that originated from an ancient bacterium.

Plastids are highly diverse and dynamic organelles that can change their shape, size, number, and function depending on the developmental stage and environmental conditions of the cell. They can also interconvert between different types or revert back to their undifferentiated state. The main types of plastids are chloroplasts, chromoplasts, leucoplasts, and gerontoplasts, which will be discussed in the next section.