n., Synonyms: chloroplastid; green plastid; chloroleucite
Chloroplast definition: A plastid that contains high amounts of chlorophyll and where photosynthesis takes place
What is chloroplast? In biology, a chloroplast refers to the organelle found within the cell of plants and other photosynthetic eukaryotes that is filled with the green pigment called chlorophyll. Etymology: from Greek “chloros”, meaning “green” and “plast”, meaning “form” or “entity”. Synonyms: chloroplastid; green plastid; chloroleucite.
When a eukaryote possesses chloroplasts, it indicates that it has the capacity to produce its own food. It does so through photosynthesis. Which types of cells contain chloroplasts? Plants are examples of organisms that possess chloroplasts inside their cells. Taking a look at their cells will reveal the presence of numerous chloroplasts that spread throughout the cytoplasm (see leaf anatomy picture below). Each chloroplast contains a light-harvesting system containing chlorophylls. These green pigments absorb light in the blue and red of the electromagnetic spectrum. They, however, reflect the green portion of the spectrum. It is for this reason that plants are green. By contrast, animal cells do not contain chloroplasts. Thus, apart from the presence of a cell wall (i.e. a layer made up of cellulose and accounts for cell rigidity in plants), the presence of chloroplasts is another defining characteristic that could help identify plants from the animals. Other organisms that possess chloroplasts are the eukaryotic algae, e.g. green algae. Some bacteria that are photosynthetic (e.g. phototrophs and cyanobacteria) have chlorophylls in their cells. However, their chlorophylls will not be found inside a double membraned organelle such as the chloroplast. Rather, the chlorophyll pigments are located in the thylakoid membrane of a bacterial cell.
Plant leaf anatomy. Notice the numerous chloroplasts inside the cells of a leaf. (Image credit: Zephyris, CC BY-SA 3.0)
Characteristics of Chloroplast
The chloroplast is one of the organelles of a photosynthetic eukaryotic cell. It is a type of plastid (the other types are chromoplasts and leucoplasts). The chloroplasts are identifiable from the other plastids by its color, shape, structure, and function. The chloroplasts are green due to the chlorophyll pigments that occur in abundance. The two most common types are chlorophyll a and b. Other chlorophyll pigments are chlorophyll c, d, and f. Chlorophyll a is present in all chloroplasts whereas the other types are present (in varying amounts) depending on the species. In vascular plants, the shape resembles a lens or a disc and the size, approximately 5µm in length and ~2.5µm in width. (Ref.1) In algae, the shape may vary. They could be round, oval, or tubular.
Structure of Chloroplast
What is the structure of chloroplast? The chloroplast has at least three membrane systems: (1) outer membrane, (2) inner membrane, and (3) thylakoid system. The outer and inner membranes are the double membrane system that is a typical feature of an organelle. The thylakoids are disk-shaped structures that perform the role of harvesting or collecting photons from a light source, such as the sunlight. Embedded in the thylakoid membrane is the antenna complex consisting of proteins, and light-absorbing pigments, particularly chlorophyll and carotenoids. Hence, the job of the thylakoid is to provide a site for the light reactions of photosynthesis. The stack of thylakoids (resembling a stack of coins) is called granum (plural: grana). The matrix of the chloroplast is referred to as the stroma. It is the thick fluid in between grana. It contains enzymes, molecules, and ions. It is where the light-independent process of sugar formation takes place (the dark reactions of photosynthesis).
Similar to the mitochondria, the chloroplasts are semi-autonomous organelles. They possess their own DNA, which is referred to as chloroplast DNA or cpDNA. Thus, they do not solely rely on the genes contained in the nucleus. They produce certain proteins from their own DNA. (Ref.2)
What is the function of the chloroplast? Chloroplasts carry out the process of photosynthesis. Their main role is to provide the site for light and dark reactions. Through these organelles, inorganic sources, water, and light energy are converted into food, i.e. glucose (a sugar molecule). They are, therefore, important to photosynthetic organisms for the purpose of producing food on their own and not needing to feed on other organisms to survive. Because oxygen is one of the byproducts of photosynthesis, the chloroplasts are therefore a crucial site for producing such gas, which later is released from the cell into the environment. Oxygen is biologically important for its role, in turn, in various biochemical and physiological processes in animals.
For further description and facts on photosynthesis, read Plant Metabolism tutorial.
The Endosymbiotic theory was conceptualized to delineate the origin of chloroplasts. (Ref.3) Accordingly, organelles such as mitochondria and chloroplasts were cellular structures in eukaryotic cells that emerged as a result of a primary endosymbiosis that took place millions of years ago between the prokaryotic endosymbionts and the eukaryotic host cells. The eukaryotic cell, being the larger cell, took in the smaller photosynthetic prokaryotes (e.g. cyanobacteria) that consequently enabled them to photosynthesize. Eventually, the prokaryotes evolved and differentiated into plastids, particularly, chloroplasts. These early photosynthetic eukaryotes harboring prokaryotes-turned-organelles are presumed to be the early ancestors of modern plants and algae on Earth. The discovery of the cpDNA in chloroplasts, the similarity in membranes, and the binary fission as a means of reproduction serves as evidence that supports this theory. (Ref.4)
What is the Likely Origin of Chloroplasts? – BioTechniques. (2017, December 14). BioTechniques. https://www.biotechniques.com/molecular-biology/when-did-the-chloroplast-evolve/
- Staehelin, L. A. (2003). Chloroplast structure: from chlorophyll granules to supra-molecular architecture of thylakoid membranes. Photosynthesis Research, 76(1–3), 185–196. https://doi.org/10.1023/A:1024994525586
- Discovery of Chloroplast DNA, Genomes and Genes | Discoveries in Plant Biology. (2019). Worldscientific.Com. https://www.worldscientific.com/doi/abs/10.1142/9789812813046_0002
- Jensen, P. E., & Leister, D. (2014). Chloroplast evolution, structure and functions. F1000Prime Reports, 6. https://doi.org/10.12703/p6-40
- Evidence for endosymbiosis. (2020). Berkeley.Edu. https://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04
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