Chloroplast- Definition, Structure, Types, Functions, Diagram

  • 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.

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

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