angelfirenm.info: Cell Structure: Chloroplasts
Learn about the role chloroplasts play in allowing plants to convert Photosynthesis occurs in eukaryotic cell structures called chloroplasts. A detailed account of the structure and functions of chloroplasts has Starch synthesized through photosynthesis is stored in the stroma in the. Chloroplasts, the organelles responsible for photosynthesis, are in many Both chloroplasts and mitochondria function to generate metabolic energy, The major difference between chloroplasts and mitochondria, in terms of both structure.
Search term Chloroplasts and Other Plastids Chloroplaststhe organelles responsible for photosynthesisare in many respects similar to mitochondria.
Both chloroplasts and mitochondria function to generate metabolic energy, evolved by endosymbiosiscontain their own genetic systems, and replicate by division. However, chloroplasts are larger and more complex than mitochondria, and they perform several critical tasks in addition to the generation of ATP.
Most importantly, chloroplasts are responsible for the photosynthetic conversion of CO2 to carbohydrates. In addition, chloroplasts synthesize amino acids, fatty acidsand the lipid components of their own membranes. The reduction of nitrite NO2- to ammonia NH3an essential step in the incorporation of nitrogen into organic compounds, also occurs in chloroplasts. Moreover, chloroplasts are only one of several types of related organelles plastids that play a variety of roles in plant cells.
In addition to the inner and outer membranes of the envelope, chloroplasts have a third internal membrane system, called the thylakoid membrane. The thylakoid membrane forms a network of flattened discs called thylakoids, which are frequently arranged in stacks called grana. Because of this three-membrane structure, the internal organization of chloroplasts is more complex than that of mitochondria.
In particular, their three membranes divide chloroplasts into three distinct internal compartments: In addition to the inner and outer membranes of the envelope, chloroplasts contain a third internal membrane system: These membranes divide chloroplasts into three internal compartments. Despite this greater complexity, the membranes of chloroplasts have clear functional similarities with those of mitochondria —as expected, given the role of both organelles in the chemiosmotic generation of ATP.
The outer membrane of the chloroplast envelope, like that of mitochondria, contains porins and is therefore freely permeable to small molecules. In contrast, the inner membrane is impermeable to ions and metabolites, which are therefore able to enter chloroplasts only via specific membrane transporters. These properties of the inner and outer membranes of the chloroplast envelope are similar to the inner and outer membranes of mitochondria: In both cases the inner membrane restricts the passage of molecules between the cytosol and the interior of the organelle.
The chloroplast stroma is also equivalent in function to the mitochondrial matrix: It contains the chloroplast genetic system and a variety of metabolic enzymesincluding those responsible for the critical conversion of CO2 to carbohydrates during photosynthesis.
The major difference between chloroplasts and mitochondriain terms of both structure and function, is the thylakoid membrane. This membrane is of central importance in chloroplasts, where it fills the role of the inner mitochondrial membrane in electron transport and the chemiosmotic generation of ATP Figure The inner membrane of the chloroplast envelope which is not folded into cristae does not function in photosynthesis.
Instead, the chloroplast electron transport system is located in the thylakoid membrane, and protons are pumped across this membrane from the stroma to the thylakoid lumen.
The resulting electrochemical gradient then drives ATP synthesis as protons cross back into the stroma. In terms of its role in generation of metabolic energy, the thylakoid membrane of chloroplasts is thus equivalent to the inner membrane of mitochondria. Chemiosmotic generation of ATP in chloroplasts and mitochondria. In mitochondria, electron transport generates a proton gradient across the inner membrane, which is then used to drive ATP synthesis in the matrix.
In chloroplasts, the proton gradient is more The Chloroplast Genome Like mitochondriachloroplasts contain their own genetic system, reflecting their evolutionary origins from photosynthetic bacteria. The genomes of chloroplasts are similar to those of mitochondria in that they consist of circular DNA molecules present in multiple copies per organelle. However, chloroplast genomes are larger and more complex than those of mitochondria, ranging from to kb and containing approximately genes.
The chloroplast genomes of several plants have been completely sequenced, leading to the identification of many of the genes contained in the organelle DNAs. These chloroplast genes encode both RNAs and proteins involved in gene expression, as well as a variety of proteins that function in photosynthesis Table In contrast to the smaller number of tRNAs encoded by the mitochondrial genome, the chloroplast tRNAs are sufficient to translate all the mRNA codons according to the universal genetic code.
In addition to these RNA components of the translation system, the chloroplast genome encodes about 20 ribosomal proteins, which represent approximately a third of the proteins of chloroplast ribosomes. Some subunits of RNA polymerase are also encoded by chloroplasts, although additional RNA polymerase subunits and other factors needed for chloroplast gene expression are encoded in the nucleus.
The chloroplast genome also encodes approximately 30 proteins that are involved in photosynthesisincluding components of photosystems I and II, of the cytochrome bf complex, and of ATP synthase. In addition, one of the subunits of ribulose bisphosphate carboxylase rubisco is encoded by chloroplast DNA.
Rubisco is the critical enzyme that catalyzes the addition of CO2 to ribulose-1,5-bisphosphate during the Calvin cycle see Figure 2. Not only is it the major protein component of the chloroplast stroma, but it is also thought to be the single most abundant protein on Earth, so it is noteworthy that one of its subunits is encoded by the chloroplast genome.
As with mitochondria, these proteins are synthesized on cytosolic ribosomes and then imported into chloroplasts as completed polypeptide chains. It is this pigment that imparts a green color to plant parts, and serves to capture light energy. A detailed account of the structure and functions of chloroplasts has been provided below.
Structure Chloroplasts are located in the parenchyma cells of plants as well as in autotrophic algae. Although their dimensions are almost similar in all plants, the algal chloroplasts show a variation in their size as well as shape.
The major components of a chloroplast are as illustrated and explained below. Envelope The chloroplast envelope is double-membrane structure comprising an outer and an inner membrane. Each of these membranes is a phospholipid bilayer, and is 6 - 8 nm thick. A 10 - 20 nm thick space present between the two membranes is known as intermembrane space.
Stroma The aqueous matrix present inside this double-membrane envelope is called the stroma. The internal components as well as several solutes are dispersed into the stroma.
Chloroplasts and Other Plastids - The Cell - NCBI Bookshelf
The stroma is especially rich in proteins, and contains several enzymes necessary for vital cellular processes. The chloroplast DNA is also present in the stroma along with ribosomes and other molecules required for protein synthesis. Starch synthesized through photosynthesis is stored in the stroma in the form of granules.
Thylakoids In addition to the two membranes that form the envelope, chloroplasts contain a third internal membrane system called thylakoid membrane. Thylakoids are the internal, membrane-bound compartments formed by such thylakoid membranes.
The internal portion of the thylakoid is called the thylakoid lumen, and contains plastocyanins and other molecules required for the transport of electrons. Grana Some of the thylakoids are arranged in the form of discs stacked one above the other.
These stacks are termed grana, and are connected to each other through inter grana thylakoids and stroma thylakoids.
Chloroplast: Structure and Function
Photosystems Present in the thylakoid membranes, these are the structural and functional units for harnessing solar energy. A photosystem comprises a reaction center surrounded by light-harvesting or antenna complexes that contain chlorophyll, carotenoids, and other photosynthetic pigments, as well as the associated proteins.
Peripheral Reticulum The chloroplasts of certain plants contain an additional set of membranous tubules called peripheral reticulum that originate from the inner membrane of the envelope.
Tiny vesicles bud off from the inner membrane of the chloroplast, and assemble to form the tubules of peripheral reticulum. Functions Chloroplasts are the sites for photosynthesis, which comprises a set of light-dependent and light-independent reactions to harness solar energy and convert it into chemical energy.