What is Chloroplast? A Comprehensive Guide

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  • Chloroplast is an organelle found in all photosynthetic cells of plants.
  • These are also found in some protists, for example, Euglena.
  • Chloroplasts are the most common type of plastid.
  • These are absent in those plants which are not exposed to light.
  • Chloroplast is derived from the Greek word “chloros”. It means green, and plastic, which means form. 
  • Chloroplasts were first observed by Antony Von Leeuwehoek in 1679. 
  • The term chloroplast was given by Schimper (1883 A.D.)
  • Chloroplast originated from proplastids found in the growing region of plants and are surrounded by two membranes.
  •  When its size increases (up to 1µm) the inner membrane invaginates to form vesicles in the presence of sunlight. 
  • They are very important for plants, because photosynthesis by which complex organic food is manufactured, takes place in them. 
  • In the cytoplasm of plant cells, the chloroplast is well distributed homogeneously. however, it is concentrated around the nucleus. 
  • Similarly, in certain cells, it is present just beneath the plasma membrane.
  • They have a green color which helps to distinguish them from other types of plastid. The green color is produced from the presence of two pigments, chlorophyll a and chlorophyll b.
  • Other types of plastids such as leucoplast and the chromoplast do not carry out photosynthesis and have low concentrations of chlorophyll.
  • Other pigments, carotenoids are also present in chloroplast which serve as accessory pigments. Carotenoid trapping solar energy and passing it to chlorophyll.
  • Like mitochondria, chloroplast has its own extra-cellular DNA, which is thought to be inherited from the ancestor a photosynthetic cyanobacterium that was engulfed by an early eukaryotic cell.
  • They also produced lipids and proteins essential for the production of chloroplast membrane.
  • Chloroplast is moved around within plant cells, circulates, and is occasionally compressed in two to reproduce.

Chloroplast Morphology

  • Its size and shape vary from species to species. In higher plants chloroplast are generally biconvex or planoconvex. 
  • However, in different plant cells, it may have various shapes such as filamentous, saucer, ovoid, discoid, spheroid, star-like girdle-shaped, spiral ribbon-like, reticulate, or cup-shaped. 
  • The size of the chloroplast is generally measured at about 5-10 µm in diameter and 2-3 µm in thickness.
  • The chloroplast of cells of polyploid and shade plants are comparatively larger than the chloroplast of cells of diploid and sun plants.
  • From cell to cell,  chloroplast’s numbers differ from one to another.
  • It depends on the physiological state of the cell also. For example, Chlamydomonas has only one chloroplast however 1-16 chloroplast in Spirogyra.
  • According to a calculation, Ricinus communis leaf contains about 400,000 chloroplasts per square millimeter of surface area. 
  • The number of chloroplast gets increased by division when it’s inadequate in number.
  • Similarly, degeneration takes place when it’s excessive in number.

Types of pigments

  1. Chlorophyll
  • Chlorophyll is a green pigment located within the chloroplast. More specifically, it is found in the thylakoid membranes.
  • The chlorophyll consists of 75% of chlorophyll a and 25% of chlorophyll b.
  • The chlorophyll absorbs energy from sunlight and the synthesis of food molecules in the chloroplast.
  1. Carotenoids
  • Carotenoids are the pigments present in chlorophylls which are located in the thylakoid membrane. Pigments like yellow and orange are present in it.
  • Carotenoids are related to vitamin A.
  • They are important because they can absorb a certain wavelength of light that can not be absorbed by chlorophylls.
  • Carotenoids are involved in a function known as photoprotection.
  1. Xanthophylls
  • The carotenoids are carotenes and xanthophylls. Xanthophylls are present in the brown and green algae.
  1. Phycobilin
  • Phycobilin is found only in red algae and Cyanobacteria. It has a relatively narrow distribution.
  • Phycoerythrin and phycocyanin are other accessory pigments belonging to this family. 
  • Phycoerythrin makes red algae commonly red and phycocyanin causes the Cyanobacteria to appear blue-green.

Chloroplast Structure

A chloroplast consists of three main components.

Envelope 

  • The chloroplasts are bounded by an envelope, which consists of a double membrane with outer and inner lipoprotein layers. Space or gap between two layers is known as intermembrane space.  
  • The molecules get exchanged across the double-membrane envelope. This interchange occurs between the chloroplast and the cytosol.
  • The isolated membrane of an envelope of chloroplast has a yellow color because a small number of carotenoids are present in it. However it lacks chlorophyll pigment and cytochromes.
  • They contain only 1 to 2 percent of the total protein of chloroplast. 
  • Glaucophyte algal chloroplast consists of a peptidoglycan layer between the chloroplast membrane.
Chloroplast Structure Diagram
Chloroplast Structure Diagram

Stroma

  • The major volume of chloroplast consists of a matrix or stroma which surrounds the thylakoids (grana).
  • The space between the inner membrane and thylakoid membrane is filled with stroma or matrix which is a colorless, denser, and granular ground substance.
  • The stroma mainly consists of protein (more than 50%) and also has 70 ribosomes (plastid ribosomes), circular and naked DNA molecules (0.5%), mRNA and tRNA molecules, water, minerals, and enzymes. It is the site of the dark reaction of photosynthesis.
  • Some structural proteins present in the chloroplast are synthesized by the Ribosomes and DNA of the chloroplast.
  • The stroma is the place where CO2 fixation occurs. Similarly, fatty acids, sugars, starch, and proteins are also synthesized in it.
  • In the stroma, small structure grana (singular granum) and intergrana connecting membranes remain embedded.
  • The network of membranous tubules interconnects the grana. It is known as the intergarna or stroma lamellae or frets.

Thylakoids

  • The third internal membrane is extensively folded and characterized by the presence of closed disc-shaped (or thylakoids) membranous sacs known as thylakoid membrane. 
  • Stroma and the outer surface of the thylakoid are in contact with each other.
  • Similarly, its inner surface encloses an intrathylakoid space ( i.e third compartment). 
  • Like the piles of coins, thylakoids are stacked one upon another. It then forms the grana or, they may be unstacked, intergranal, or stromal thylakoids, forming a system of anastomosing tubules that are joined to the grana thylakoids.
  • Each granum consists of disc-shaped membranous sacs called thylakoids piled one upon others.
  • In the matrix of a chloroplast, approximately 40-80 grana may be present. The number of thylakoids per granum may vary from 1 to 50 or more. For example, there may be single thylakoids (e.g., red alga), triple thylakoids, and multiple thylakoids (e.g., green algae and higher plants).
  • In the thylakoid membrane, all the enzymatic components which are essential for photosynthesis are also present.
  • They are the site of light reactions.
  • Interaction between chlorophyll and other components takes place within the thylakoid membrane.
  • The vesicle becomes arranged to form a chain of a large disc which are not connected.
  • Fusion of the discs takes place to give rise to thylakoid and stroma lamella.

Semi-autonomous nature of chloroplast

  • Like the mitochondria, they are also known as semi-autonomous cell organelles as they got their DNA and complete machinery to synthesize some of the required proteins.
  • While some other proteins depend upon nuclear DNA and cytoplasmic ribosomes.
  • Chloroplast and mitochondria are the only two organelles having their DNA.
Chloroplast Genetic Expression
Chloroplast Genetic Expression

Functions of Chloroplast

  • The most fundamental function of chloroplast is photosynthesis. They synthesize food by photosynthesis.
  • Light energy is absorbed by chloroplast and converted into chemical energy.
  • They produce NADH and oxygen for respiration for all the aerobes by photolysis of water.
  • These maintain O2 and CO2 balance in the biosphere.
  • Chlorophyll tapering solar energy is used for photosynthesis in which ATP is produced.
  • Chloroplast reduced the CO2 concentration it helps to prevent global warming.
  • They store the starch in the proteinaceous bodies called pyrenoids in algal forms.
  • The synthesis of fatty acids occurs in the chloroplast of spinach.
  • Chloroplast is responsible for natural greenery.
  • Carbon and sugar are generated during the Calvin cycle or dark reaction by using CO2 obtained from air.

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References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology(First edition). S . Chand and company Ltd.
  2. Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.
  3. Keshari, A.K (2020). A textbook of higher secondary biology. 13th edition. Vidyarthi pustak bhandar, Kathmandu,Nepal. 
  4. https://byjus.com/biology/chloroplasts/
  5. https://www.britannica.com/science/chloroplast
  6. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/chloroplast
  7. https://www.frontiersin.org/research-topics/5623/structure-and-function-of-chloroplasts
  8. https://www.toppr.com/ask/question/why-do-we-believe-chloroplast-and-mitochondria-to-be-semiautonomous-organelle/
  9. https://ucmp.berkeley.edu/glossary/gloss3/photosyn/pigments.html

About Author

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Sushmita Baniya

Sushmita Baniya is pursuing her Master’s degree in Medical Microbiology from the National College of Science and Technology (NIST), Kathmandu, Nepal. She did her Bachelor’s degree in Microbiology from Birendra Multiple Campus, Chitwan, Nepal. She is interested in Genetics and Molecular Biology.

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