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In plant cells that are exposed to light, such as leaf cells, for example, proplasts grow into chloroplasts.
The need for light for their formation explains why there are no chloroplasts in the cells of unenlightened parts of plants, such as the roots or the inner parts of the stems.
If we let a seed germinate in the dark, the leaves of the newborn plant will be yellowish, and in their cells no mature chloroplasts will be found, but stioplast.
Chloroplast and Stioplast
Amyloplasts or Starch Grains
In certain situations, chloroplasts or leukoplasts may accumulate large amounts of starch, a polysaccharide synthesized from glucose.
Starch can completely occupy the interior of the organelle, which turns into a structure known as starch or starch grain. Amyloplasts are large reservoirs of starch, which in times of need (if glucose is lacking) can be converted into glucose and used.
The ability of plasts to multiply and their biochemical similarities to present-day prokaryote beings suggest that these organelles had as their ancestors primitive photosynthetic bacteria, which hundreds of millions of years ago established a cooperative relationship with eukaryotic cells. In the course of the evolutionary process, the dependence between the two types of organisms would have become so great that the photosynthetic bacteria and the host eukaryote cell lost their ability to live in isolation.