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The word thylakoid comes from the Greek word thylakos which means "sack". [133] Suspended within the chloroplast stroma is the thylakoid system, a highly dynamic collection of membranous sacks called thylakoids where chlorophyll is found and the light reactions of photosynthesis happen. [16]
Jan 29, 2024 · The term “chloroplast” comes from the Greek words “chloros,” meaning green, and “plastes,” meaning formed or molded, highlighting their characteristic green color and intricate structure. Organisms with Chloroplasts. Chloroplasts are primarily found in plants and algae. They are absent in animals and fungi.
- Overview
- Characteristics of chloroplasts
- The photosynthetic machinery
- Chloroplast genome and membrane transport
A chloroplast is an organelle within the cells of plants and certain algae that is the site of photosynthesis, which is the process by which energy from the Sun is converted into chemical energy for growth. A chloroplast is a type of plastid (a saclike organelle with a double membrane) that contains chlorophyll to absorb light energy.
Where are chloroplasts found?
Chloroplasts are present in the cells of all green tissues of plants and algae. Chloroplasts are also found in photosynthetic tissues that do not appear green, such as the brown blades of giant kelp or the red leaves of certain plants. In plants, chloroplasts are concentrated particularly in the parenchyma cells of the leaf mesophyll (the internal cell layers of a leaf).
Why are chloroplasts green?
Chloroplasts are green because they contain the pigment chlorophyll, which is vital for photosynthesis. Chlorophyll occurs in several distinct forms. Chlorophylls a and b are the major pigments found in higher plants and green algae.
Do chloroplasts have DNA?
Chloroplasts are a type of plastid—a round, oval, or disk-shaped body that is involved in the synthesis and storage of foodstuffs. Chloroplasts are distinguished from other types of plastids by their green colour, which results from the presence of two pigments, chlorophyll a and chlorophyll b. A function of those pigments is to absorb light energy for the process of photosynthesis. Other pigments, such as carotenoids, are also present in chloroplasts and serve as accessory pigments, trapping solar energy and passing it to chlorophyll. In plants, chloroplasts occur in all green tissues, though they are concentrated particularly in the parenchyma cells of the leaf mesophyll.
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Chloroplasts are roughly 1–2 μm (1 μm = 0.001 mm) thick and 5–7 μm in diameter. They are enclosed in a chloroplast envelope, which consists of a double membrane with outer and inner layers, between which is a gap called the intermembrane space. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids), is known as the thylakoid membrane. In most higher plants, the thylakoids are arranged in tight stacks called grana (singular granum). Grana are connected by stromal lamellae, extensions that run from one granum, through the stroma, into a neighbouring granum. The thylakoid membrane envelops a central aqueous region known as the thylakoid lumen. The space between the inner membrane and the thylakoid membrane is filled with stroma, a matrix containing dissolved enzymes, starch granules, and copies of the chloroplast genome.
The thylakoid membrane houses chlorophylls and different protein complexes, including photosystem I, photosystem II, and ATP (adenosine triphosphate) synthase, which are specialized for light-dependent photosynthesis. When sunlight strikes the thylakoids, the light energy excites chlorophyll pigments, causing them to give up electrons. The electrons then enter the electron transport chain, a series of reactions that ultimately drives the phosphorylation of adenosine diphosphate (ADP) to the energy-rich storage compound ATP. Electron transport also results in the production of the reducing agent nicotinamide adenine dinucleotide phosphate (NADPH).
ATP and NADPH are used in the light-independent reactions (dark reactions) of photosynthesis, in which carbon dioxide and water are assimilated into organic compounds. 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 first step of carbon fixation in the Calvin cycle (also called Calvin-Benson cycle), the primary pathway of carbon transport in plants. Among so-called C4 plants, the initial carbon fixation step and the Calvin cycle are separated spatially—carbon fixation occurs via phosphoenolpyruvate (PEP) carboxylation in chloroplasts located in the mesophyll, while malate, the four-carbon product of that process, is transported to chloroplasts in bundle-sheath cells, where the Calvin cycle is carried out. C4 photosynthesis attempts to minimize the loss of carbon dioxide to photorespiration. In plants that use crassulacean acid metabolism (CAM), PEP carboxylation and the Calvin cycle are separated temporally in chloroplasts, the former taking place at night and the latter during the day. The CAM pathway allows plants to carry out photosynthesis with minimal water loss.
The chloroplast genome typically is circular (though linear forms have also been observed) and is roughly 120–200 kilobases in length. The modern chloroplast genome, however, is much reduced in size: over the course of evolution, increasing numbers of chloroplast genes have been transferred to the genome in the cell nucleus. As a result, proteins encoded by nuclear DNA have become essential to chloroplast function. Hence, the outer membrane of the chloroplast, which is freely permeable to small molecules, also contains transmembrane channels for the import of larger molecules, including nuclear-encoded proteins. The inner membrane is more restrictive, with transport limited to certain proteins (e.g., nuclear-encoded proteins) that are targeted for passage through transmembrane channels.
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Apr 27, 2017 · Chloroplast Definition. The chloroplast, found only in algal and plant cells, is a cell organelle that produces energy through photosynthesis. The word chloroplast comes from the Greek words khloros, meaning “green”, and plastes, meaning “formed”. It has a high concentration of chlorophyll, the molecule that captures light energy, and ...
Jun 26, 2023 · chloroplast (n.) chloroplast. (n.) type of membrane in plants that conducts photosynthesis, 1887, from German chloroplast (1884, Eduard Strasburger), shortened from chloroplastid "a chlorophyll granule" (1883, Andreas Franz Wilhelm Schimper); see chloro- + -plast. also from 1887.
Jun 24, 2019 · Chloroplasts are tiny plant powerhouses that capture light energy to produce the starches and sugars that fuel plant growth. They are found inside plant cells in plant leaves and in green and red algae as well as in cyanobacteria. Chloroplasts allow plants to produce the complex chemicals necessary for life from simple, inorganic substances ...
Aug 14, 2017 · This new study shows that the chloroplast lineage split from their closest cyanobacterial ancestor more than 2.1 billion years ago in low salinity environments. It took another 200 million years ...
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