Chloroplasts Definition, Characteristics, Structure, Location, Functions, and Diagram

Definition of Chloroplasts 

Chloroplasts are plant cell organelles that convert sunlight’s photon energy into stable chemical energy. This reaction is made possible by the presence of the photosynthetic pigment chlorophyll in a plant cell’s chloroplast. Hence, chloroplasts contribute to the creation of oxygen and other organic compounds from water.

• The term “chloroplast” is derived of the Greek words chloros, which translates to green and also plastes which refers to “the one who forms”.
• Chloroplasts are membrane-bound plastids which have membranes that are embedded in a liquid matrix.
• They also contain the pigment that is a photosynthetic one called chlorophyll.
• This pigment provides a green-colored color to plant tissues and helps to absorb the energy of light.
• Chloroplasts can be seen in the cells of mesophyll found in leaves of plants. The average is 30-40 per mesophyll cell.
• In a plant cell’s cytoplasm, chloroplasts are present and distributed. The existence of chloroplasts is one of the most distinguishing characteristics between plant and animal cells.
• Moreover, chloroplasts conduct a variety of functions connected to a plant’s metabolism. It is capable of synthesising several chemical molecules, including fatty acids, membrane lipids, and others.
• These organelles, which contain their DNA and hereditary information, are known as guard cells. Moreover, chloroplasts have the potential to reproduce without a partner within a plant cell, meaning they replicate independently.

Discovery of Chloroplasts 

The first formal description of the chloroplast (Chlorophyllkornen, “grain of chlorophyll”) was provided by Hugo von Mohl in 1837 as separate bodies inside the cell of green plants. In 1883 Andreas Franz Wilhelm Schimper identified the bodies “chloroplastids” (Chloroplastiden). The year 1884 saw Eduard Strasburger adopted the term “chloroplasts” (Chloroplasten).

Characteristics of chloroplasts

• Chloroplasts are an example of a or plastid. It is a round, oval or disk-shaped body involved in the production and storage of food products.
• Chloroplasts differ in comparison to other types of plastids because of their green color that is due to their presence in two different pigments: chlorophyll A and chlorophyll B.
• The function of these pigments is that they absorb energy from light in the photosynthesis process. 
• Other pigments, like carotenoids are also found in chloroplasts. They serve as supplementary pigments, capturing sunlight and transferring the energy to chlorophyll.
• Chloroplasts are found in plant tissues.
• They are found in all green tissues but they are concentrated in parenchyma cells within mesophyll of the leaf.
• Chloroplasts range from 1-2 mm (1 millimeter equals 0.001 millimeters) thick and about 5-7 millimeters in diameter. They are enclosed within the chloroplast envelope. It is composed of two membranes, with both layers, with an intermembrane space. An internal membrane which is extremely folded and distinguished by the existence in closed disks (or thylakoids) are called the thylakoid layer. 
• In the majority of higher plants the thylakoids are placed in tight stacks, referred to as the grana (singular the granum). Grana are linked via stromal lamellae or extensions that connect one granum through the stroma and into another granum.
• The thylakoid stroma surrounds an aqueous central region, called the thylakoid lumen. The space between the outer membrane and the thin thylakoid membrane filled with stroma. It is a matrix with dissolving proteins, starch grains along with copies of the genome of the chloroplast.

Structure of Chloroplasts

Chloroplasts in higher plants are typically biconvex or planoconvex-shaped. However, in different plants chloroplasts could differ in shape, ranging from spheroid to filamentous saucer discoid, or ovoid-shaped. They can be found within mesophyll cells in leaves of plants. They are vesicular, and have the colorless center. The typical dimension of the chloroplast ranges from 5–7 μm in diameter, and 1–2 μm in thickness.

The chloroplast is comprised of an inner and an outer membrane that has an unfilled space between. Within the chloroplast, there are thylakoids that form a stack known as grana, as well being stroma, the thick fluid that is inside the chloroplast. The thylakoids are a source of chlorophyll, which is essential to allow the plant to undergo photosynthesis. The space that chlorophyll takes up is known as the thylakoid area.

The chloroplast has therefore the following components:

1. Envelope (Outer membrane)

• It’s a semi-porous membrane, and it is permeable small molecules and ions that diffuse easily.
• The membrane’s outer layer isn’t permeable to larger proteins.

2. Intermembrane Space

• It’s usually a small inter-membrane area of 10-20 nanometers in size and located between the outer and inner membranes in the chloroplast.

3. Inner membrane

• The membrane inside the chloroplast is a border that connects to the stroma. It regulates the flow of material into and out of the chloroplast.
• Alongside the regulation function, fat acids, lipids and carotenoids are made within the membrane of the chloroplast’s inner.

4. Stroma

• Stroma, an alkaline Aqueous Fluid that is protein-rich and is found within the membrane that surrounds the chloroplast.
• The space that is outside of the thylakoid is known as the Stroma.
• The chloroplast DNA chloroplasts ribosomes, as well as the thylakoid th grains and a variety of proteins, can be located in the stroma.

5. Thylakoid System

• The thylakoid organ system is suspended within the stroma.
• The thylakoid system comprises composed of membranous sacs referred to as Thylakoids.
• The chlorophyll is present within the thylakoids and provides the spectre to allow the photosynthesis-related light reactions to occur.
• Thylakoids are placed in a grana-like stack. Each granum has around 10-20 Thylakoids.

6. Peripheral Reticulum

• The chloroplasts in certain plants have another set of tubules membranous known as peripheral reticulum which is derived from the membrane that forms the inside that forms the outer envelope.
• Tiny vesicles grow from the outer membrane of the chloroplast before assembling to form tubules of the peripheral reticulum.

What is the function of chloroplasts? – Functions of Chloroplast

• Chloroplasts are sites for photosynthesis. It is an array of light-dependent and light-independent processes to capture solar energy and convert the energy into chemical power.
• The components of the chloroplast play an important role in a variety of roles in the regulation of cells and also in photorespiration.
• Chloroplasts are also involved in a variety of biochemical activities for plant cells that include the production of membrane lipids, fatty acids isoprenoids and tetrapyrroles. hormones, and starch.
• All plant cells are involved in the plant immune response.
• The chloroplasts, which are connected to the nucleus, cell membrane along with the ER are the most important organelles in the defense against pathogens.
• Chloroplasts are cells that act as sensors.

Location of chloroplasts

Distribution of chloroplasts in a plant

Some cells of multicellular plants have chloroplasts. The green components of every plant have chloroplasts. The chloroplasts and more specifically, the chlorophyll inside these cells are what makes the photosynthetic elements of plants green. Plant cells that contain chloroplasts typically are parenchyma cell types, but they can also be located in collenchyma tissues. Plant cells with chloroplasts is called an achromatoma cell. A typical cell of a plant’s land contains between 10 and 100 chloroplasts.

In certain plants, such as Cacti, there are chloroplasts in the stems, but in the majority of plants, chloroplasts reside in leaves. Each square millimeter in leaf tissue could contain a half million chloroplasts. In a leaf, chloroplasts can be located in the mesophyll layer of the leaf, as well as those guard cells that make up the stomata. The mesophyll cells of palisade can have 30 to 70 chloroplasts in a single cell, while guard cells of stomatal contain just 8-15 cells in addition to less chlorophyll. Chloroplasts may also be seen inside the cells of the bundle in leaves, particularly in C4 plants, that perform the Calvin cycle within the bundle cells of their sheaths. They are usually removed from the epidermis leaves.

Movement of Chloroplast 

The chloroplasts of plant as well as algal cells are able to orient themselves to make the most of available light. When the light is dim they spread as a sheet, maximizing the surface area that can absorb light. In the presence of intense sunlight, they seek shelter by aligning vertical columns on the cell wall of the plant or turning their sides so that light hits them from the edge. This minimizes the risk of exposure and helps protect them from damage from photooxidative radiation. 

The ability to disperse chloroplasts to hide behind one another or spread out could be the reason plants of the land evolved to possess a variety of smaller chloroplasts instead of just a few large ones. Chloroplast movement is thought to be to be one of the most closely controlled stimulus-response mechanisms that can be observed in plants. Mitochondria also have been found to follow chloroplasts while they move.

In more arid plants the movement of chloroplasts is controlled by phototropins blue light photoreceptors that are responsible for phototropism in plants. In certain algae, mosses flowers, and ferns the movement of chloroplasts is influenced by red light , in addition to blue light, but long wavelengths of red interfere with the movement instead of increasing it. Blue light usually makes chloroplasts seek shelter, whereas red light is able to draw them out to increase light absorption.

Research on Vallisneria gigantea the aquatic flowering plant has revealed that chloroplasts begin moving in just five minutes after light exposure, but they initially don’t show any apparent directionality. They can be moving along microfilament tracks as well as the way that the microfilament mesh alters shape to create an intricate honeycomb around the chloroplasts following their move. been moved indicates that the microfilaments can aid in anchoring chloroplasts into the right place.

The photosynthetic machinery of chloroplasts

The thylakoid membrane is home to chlorophylls, as well as a variety of protein complexes, such as photosystems I, II as well as ATP (adenosine triphosphate) synthase, all of which are designed to produce light-dependent photosynthesis. When sunlight hits the thylakoids energy excites chlorophyll pigments leading them to release electrons. The electrons are then absorbed into the chain of electron transportation, which is a sequence of reactions that eventually results in the phosphorylation of the ADP (ADP) in the form of the energetic storage chemical ATP. Electron transport also leads to producing the reduction agent, nicotinamide the adenine dinucleotide (NADPH).

ATP as well as NADPH are utilized in the light-dependent processes (dark reaction) of photosynthesis, during which water and carbon dioxide are transformed in organic compound. The light-independent reactions of photosynthesis are carried out in the chloroplast stroma, which contains the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). Rubisco catalyzes the initial stage of carbon fixation within the Calvin cycle (also known as the Calvin-Benson Cycle) the principal pathway for transporting carbon in plants.

In the case of C4 plant species, both the first carbon fixation phase along with the Calvin cycle are spatially separated. Carbon fixation is accomplished through the phosphoenolpyruvate (PEP) carboxylation of mesophyll-containing chloroplasts, whereas malate, the four-carbon byproduct of this process, is transferred to chloroplasts within bundle-sheath cell which is where the Calvin cycle is completed. Photosynthesis of C4 attempts to reduce losses of carbon dioxide through photorespiration. In plants that utilize crassulacean acid metabolism (CAM) PEP carboxylation as well as the Calvin cycle are separated by time in chloroplasts, with the first occurring in the evening and the latter during the daytime. The CAM pathway permits plants to perform photosynthesis at a low loss of water.

Chloroplast genome and membrane transport

The genome of the chloroplast is typically circular (though linear versions have been seen) and ranges from 120 to 200 kilogramases in length. The modern genome of chloroplasts however, is considerably smaller in size. During the evolution of time, growing amounts of chloroplast genes were transferred to the genome of the nucleus of cells. This has meant that the nuclear DNA encoded proteins have become vital for the function of chloroplasts. Thus, the membrane that is the outermost of the chloroplast can be freely permeable to tiny molecules, also has transmembrane channels that allow for the entry large molecules which includes nuclear encoded proteins. Its inner membrane, however, is much more restrictive and transport is restricted to specific proteins (e.g. nuclear encoded proteins) which are designed to pass through transmembrane channels.

Chloroplast Evolution

It is believed that the Endosymbiotic theory was developed to identify the genesis of chloroplasts. Organelles like chloroplasts and mitochondria were cell structures within eukaryotic cells , which emerged due to an endosymbiosis primary that took place many millions many years ago among prokaryotic eukaryotic endosymbionts host cells. The eukaryotic cells, being the largest cell has taken in the smaller prokaryotes that were photosynthetic (e.g. cyanobacteria) which allowed them to produce photosynthetic. The prokaryotes eventually evolved and became plastids specifically 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 chloroplast cpDNA, the similarities between membranes and the binary fission process as method of reproduction provide evidence for this theory.

Chloroplasts Division

The majority of chloroplasts found in cells that are photosynthetic don’t originate directly from etioplasts, proplastids or even et. In reality the typical shoot meristematic cell is made up of just 7-20 proplastids. These proplastids divide into chloroplasts, and then divide to produce the 30 to 70 chloroplasts in mature plants that are photosynthetic. In the event that the cell splits, the division of chloroplasts will provide additional chloroplasts for divide between two cells.

In single-celled algae (single-celled), chloroplast division is the sole way that new chloroplasts can be created. There is no proplastid differentiation–when an algal cell divides, its chloroplast divides along with it, and each daughter cell receives a mature chloroplast.

Most chloroplasts in cells divide, more than a few rapidly growing chloroplasts. Chloroplasts do not have a specific S-phase, meaning that their DNA replication isn’t controlled or synchronized to that of the cells they host. The majority of the information we know about the division of chloroplasts originates from studying organisms like Arabidopsis as well as the red algae Cyanidioschyzon merolae.

The division process begins at the point that the two proteins FtsZ1 and FtsZ2 are able to form filaments and, with the aid of a protein called ARC6 create a structure known as Z-rings in the chloroplast’s stroma. The Min system is responsible for the positioning of the Z-ring, making sure that the chloroplast gets cleaved in a more or less uniform manner. MinD is a protein. MinD stops FtsZ from linking and making filaments. Another protein , ARC3, may be involved, however it’s not fully identified. These proteins function at the poles of chloroplast and prevent the formation of Z-rings in the area, but at the central part of the chloroplast MinE hinders them, and allows Z-rings to develop.

Then, two plastid-dividing rings or PD rings are formed. The plastid-dividing ring inside is situated on the inner portion of the chloroplast’s internal membrane and is created first. The outer plastid-dividing ring can be situated around the outside of the chloroplast membrane. It is made up of filaments that measure 5 nanometers wide, laid out into rows 6.4 nanometers apart. It shrinks as it squeezes the chloroplast. This is when the constriction of the chloroplast starts.

In some species, such as Cyanidioschyzon merolae and Cyanidioschyzon, the chloroplasts are equipped with an additional plastid-diverging ring in the chloroplast’s intramembrane space.

In the final phase of constriction the dynamin proteins form a ring around the plastid-dividing ring that is outside and help to push the chloroplast. In the meantime, the Z-ring as well as the inner plastid-dividing rings are broken down. In this phase there is a break in the chloroplast DNA plasmids that are floating in the stroma get separated and dispersed to the two chloroplasts that form daughter chloroplasts.

The dynamins then move through the plastid’s outer rings, and into directly contact with the chloroplast’s exterior membrane, and thereby cleave the chloroplast into two daughter chloroplasts.

A small portion of the plastid’s outer division ring remains suspended in between two chloroplasts of the daughter and a speck of the dynamin rings remains connected to one of the daughter chloroplasts.

Of the six or five rings involved in chloroplast division only the plastid-dividing outer rings is present during the entire division and constriction phase. While the Z-ring first forms the constriction process does not start until the plastid-dividing ring that surrounds it is formed.

How are mitochondria and chloroplasts similar?

Mitochondria and chloroplasts are similar in several ways:

1. Both are organelles found in eukaryotic cells. Mitochondria are found in almost all eukaryotic cells, while chloroplasts are found in plants and some algae.
2. Both organelles are involved in energy production. Mitochondria are known as the “powerhouse” of the cell, as they are responsible for generating the majority of the cell’s ATP (adenosine triphosphate), which is the molecule that stores energy. Chloroplasts are responsible for photosynthesis, which is the process by which plants and some algae convert light energy into chemical energy in the form of organic compounds.
3. Both organelles have their own DNA and can reproduce independently of the cell. This is due to the endosymbiotic theory, which suggests that both mitochondria and chloroplasts were originally free-living bacteria that were engulfed by eukaryotic cells and eventually evolved into organelles.
4. Both organelles have a double membrane structure. This is also thought to be a result of their origin as free-living bacteria, as the inner membrane is thought to have originated from the bacterial plasma membrane, while the outer membrane is thought to have originated from the host cell’s membrane.

Overall, while mitochondria and chloroplasts have different functions in the cell, they share several key similarities due to their common evolutionary origin.

Facts about chloroplasts

• Chloroplasts are organelles found in eukaryotic cells, primarily those of plants and some algae.
• Photosynthesis, the process by which plants and some algae transform light energy into chemical energy in the form of organic compounds, is mediated by chloroplasts.
• During photosynthesis, chloroplasts contain pigments called chlorophylls that are responsible for the absorption of light energy.
• It is believed that the double membrane structure of chloroplasts is the result of their genesis as free-living photosynthetic bacteria that were ingested by eukaryotic cells.
• Similar to bacterial DNA and ribosomes, chloroplasts have their own DNA and ribosomes. This is believed to be another consequence of their origin as free-living microorganisms.
• Chloroplasts are extremely dynamic organelles capable of modifying their shape, size, and number in response to various environmental signals.
• In addition to photosynthesis, chloroplasts are involved in a vast array of metabolic processes, including the synthesis of amino acids, lipids, and secondary metabolites.
• It is believed that chloroplasts played a crucial role in the evolution of life on Earth by creating oxygen during photosynthesis, a crucial component in the genesis of aerobic respiration.
• Chloroplasts can coordinate cellular operations by communicating with organelles such as the nucleus and mitochondria.
• Chloroplasts are prone to damage from environmental stresses such as high light intensity, drought, and temperature extremes; thus, plants have evolved a variety of defensive mechanisms to limit this damage.

FAQ

References

• Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Chloroplasts and Other Plastids. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9905/
• Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Chloroplasts and Photosynthesis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26819/
• https://micro.magnet.fsu.edu/cells/chloroplasts/chloroplasts.html
• https://www.toppr.com/guides/biology/cell-the-unit-of-life/chloroplast-definition-structure-functions/
• https://www.nagwa.com/en/explainers/812162636509/
• https://www.twinkl.co.in/teaching-wiki/chloroplast
• https://platoonline.com/chloroplasts
• https://www.britannica.com/science/chloroplast
• https://www.sciencedirect.com/topics/medicine-and-dentistry/chloroplast
• https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/2.19/primary/lesson/chloroplasts-bio/
• https://www.sciencefacts.net/chloroplast.html