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Cooper G.M. Molecular Approach to the Cell.2002.Sudbury, MA: Sinauer Associates.



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Compared to mitochondria, chloroplasts, the organelles responsible for photosynthesis, have many similarities.These organelles have evolved through endosymbiosis, have their own genetic systems, and replicate by division.While mitochondria generate ATP, chloroplasts perform several critical tasks in addition to that.A key function of chloroplasts is the photosynthetic conversion of CO2 into carbohydrates.Additionally, chloroplasts synthesize amino acids, fatty acids, and the lipid components of their membranes.As part of the incorporation of nitrogen into organic compounds, chloroplasts also reduce nitrite (NO2-) to ammonia (NH3).Chloroplasts are just one type among a few types of related organelles (plastids) which play various roles inside plant cells.


The Structure and Function of Chloroplasts

Unlike mitochondria, chloroplasts are surrounded by membranes called chloroplast envelopes (5-10 *m long).A third membrane system, known as the thylakoid membrane, is present within chloroplasts as well as the outer and inner membranes of the envelope.The thylakoid membrane consists of flatened discs called thylakoids, which are stacked to form grana.A three-membrane structure gives chloroplasts a more complex internal organization than mitochondria.There are three different compartments within chloroplasts determined by the three membranes: (1) the intermembrane space between the two membranes of the chloroplast envelope; (2) the stroma, which lies within the envelope but outside the thylakoid membrane; and (3) the thylakoid lumen.


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The structure of a chloroplast.The chloroplast contains a third internal membrane system, besides the inner and outer membranes of the envelopes: the thylakoid membranes.This membrane partitions each chloroplast into three compartments.FIGURE 4 (Electron micrograph..


While chloroplasts are more complex, their membranes have certain functional similarities to those of mitochondria-as expected, given the role that both organelles play in chemiosmotic ATP synthesis.Chloroplast envelopes, like mitochondrial envelopes, are made of porins, making them freely permeable to small molecules.Compared to the outer membrane, the inner membrane is impervious to ions and metabolites, which can only pass through specific membrane transporters.As with mitochondria, the inner and outer membranes of chloroplasts restrict the passage of molecules between the cytosol and the inside of the organelle.During photosynthesis, the chloroplast stroma is exactly the same as the mitochondrial matrix in functioning: It contains both the chloroplast genetic system and metabolic enzymes, including those involved in CO2 conversion to carbohydrates.

There is a major difference between the structure and function of mitochondria and chloroplasts, and that is the thylakoid membrane.Chloroplasts hold a central place in electron transport and chemiosmotic ATP generation, where this membrane plays an integral role in electron transport and mitochondrial membrane function (Figure 10.14).The chloroplast inner membrane, which is not folded into cristae, is not involved in photosynthesis.Instead, the electron transport system in chloroplasts is located in the thylakoid membrane, which pumps protons between the stroma and the lumen.The electrochemical gradient triggers ATP synthesis as protons enter the stroma.The thylakoid membrane plays a similar role in the generation of metabolic energy in chloroplasts as it does in mitochondria.


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In chloroplasts and mitochondria chemiosmosis produces ATP. .The proton gradient is (more..)


The Chloroplast Genome

Unlike mitochondria, chloroplasts contain their own genetic code, reflecting their evolutionary descent from photosynthetic bacteria.The chloroplast genome is similar to the mitochondria genome in that it consists of circular DNA molecules in multiple copies within each organelle.Chloroplast genomes, on the other hand, are larger and more complex than mitochondrial genomes, containing up to 120 genes and ranging from 120 to 160 kb.

Plant chloroplast genomes have been fully sequenced, leading to the identification of many genes contained in the organelle DNAs.No rewrites were found.The chloroplast genome also contains the ribosomal RNA and transfer RNAs that are required for chloroplast translation.Among them are four rRNA genes (23S, 16S, 5S, and 4.5S) and 30 tRNA genes.Contrary to the smaller number of mitochondrial tRNAs, the chloroplast tRNAs are sufficient to translate all the codons in mRNA according to the universal genetic code.In addition to these RNA pieces of the translation process, the chloroplast genome encodes about 20 proteins that make up a third of chloroplast ribosomal proteins.The chloroplast encodes part of RNA polymerase, even though additional RNA polymerase subunits and other factors crucial to chloroplast gene expression are encoded by the nucleus.


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Furthermore, the chloroplast genome encodes approximately 30 proteins that are involved in photosynthesis, including components of photosystems I and II, of the cytochrome bf complex, and of ATP synthase.Additionally, DNA encoding one of the subunits of ribulose bisphosphate carboxylase (rubisco) is found in chloroplasts.The Calvin cycle happens when CO2 is added to ribulose-1,5-bisphosphate by the Rubisco enzyme (see Figure 2.39).It is not only the principal component of the chloroplast stroma, but it is also regarded as the most abundant protein on Earth, so it is notable that one of its subunits is encoded by the chloroplast genome.


Import and Sorting of Chloroplast Proteins

Chloroplasts still encode about 90% of their proteins from nuclear genes, despite encoding more of their own proteins than mitochondria.Proteins like these are synthesized on cytosolic ribosomes and then imported into chloroplasts as polypeptide chains.The proteins must then be sorted into their appropriate compartments within chloroplasts-a task even more difficult than protein sorting in mitochondria. This is because chloroplasts contain three different membranes that divide them into distinct compartments inside.

The import of protein into chloroplasts is generally similar to mitochondrial import (Figure 10.15). .In chloroplasts, transit peptides are recognized by the Toc complex, and proteins are transported across the membrane by means of this complex.Translocated to the inner membrane's translocation complex (the Tic complex), they are then transported there.Like in mitochondria, molecular chaperones on both cytosolic and stromal sides of the envelope are needed to facilitate protein import, which requires ATP.Transit peptides are not positively charged as are mitochondrial import sequences, and the translocation of polypeptide chains into chloroplasts does not require an electric potential across a membrane.


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The process of importing proteins into chloroplast stroma.Upon importation into chloroplasts, proteins are targeted for import through transit peptides at their amino termini.The transit peptide directs the translocation of polypeptides through the Toc complex in the chloroplast outer membrane (more..)


In the thylakoid lumen, proteins are transported in two steps (Figure 10.16).A second hydrophobic signal sequence - exposed after cleavage of the transit peptide - directs them to translocate across the thylakoid membrane after they are initially imported into the stroma as previously described.The hydrophobic signal sequence directs the translocation of the polypeptide across the membrane and it is ultimately eliminated via proteolytic cleavage within the lumen of the thylakoid.


Proteins are imported into the thylakoid lumen.Import of proteins into thylakoids is divided into two steps. .This is followed by the cleavage of a second hydrophobic (more..)


The pathways for sorting proteins into the other four compartments of chloroplasts-the inner and outer membranes, thylakoid membrane, and intermembrane space-are less well-established.Proteins appear to be inserted directly into the outer membrane of the chloroplast envelope, as they are in mitochondria.Proteins destined for the thylakoid membrane, as well as the chloroplast envelope, aren't initially targeted for import into the stroma, but are rather injected into the nucleus by transit peptides.Following transit peptide cleavage, these proteins are then targeted for insertion into appropriate membranes by other sequences, which are yet to be fully characterized.Neither the sequences that direct proteins to the intermembrane space nor the pathways by which they reach this location have been identified.


Other Plastids

Chloroplasts are only one organelle of a family of plant organelles called plastids, though they are one of the most prominent.The chloroplast genome is present in all plastids, but their structure and function differ.The chloroplasts are responsible for photosynthesis and feature the thylakoid membrane system within.Phylloplasts involved in other aspects of plant cell metabolism lack thylakoid membranes and other components of the photosynthetic apparatus, but they are bound by two membranes of the plastid envelope.

Plasmids are frequently classified according to the type of pigments they contain.They are called chloroplasts because chlorophyll is present.The chloroplasts (Figure 10.17A) lack chlorophyll but contain carotenoids. This pigment produces yellow, orange, and red colors in some flowers and fruits, but their exact role in the metabolism of cells remains unclear.The nonpigmented plastids called leucoplasts store a variety of energy sources in the tissues that do not photosynthesis.Among leucoplasts, amyloplasts (Figure 10.17B) and elaioplasts store starch and fat, respectively.


Electrified micrographs of chromoplasts and amyloplasts.As discussed in part A, chromoplasts contain lipid droplets that contain carotenoid pigments.In (B), Amyloplasts contain large granules of starch.Biophoto Associates/Photo Researchers, Inc.; Dr. Jeremy Burgess/Photo (more..)


Plastids, including chloroplasts, form from proplastids, small, undifferentiated organelles in the rapidly dividing cells of roots and shoots of plants.Depending on the differentiation requirements of differentiated cells, the proplastids become various types of mature plastids.Plastids can also change from one type to another as they mature.A fruit develops chloroplasts from chloroplasts, for example, during ripening (e.g., tomatoes).New types of carotenoids are synthesized while chlorophyll and thylakoid membranes are degraded.

.Photosynthesis occurs in photosynthetic cells, such as those of leaves (Figure 10.18).Throughout this process, vesicles form from the plastid envelope inner membrane, and the different components of the photosynthetic apparatus are synthesized and assembled.Chloroplasts, however, develop only in the presence of light. .As soon as dark-grown plants are exposed to light, their etioplasts develop into chloroplasts.The coordination of plastid and nuclear gene expression explains this dual control of plastid development.In plant molecular biology, understanding how such coordinated gene expression occurs is one of the biggest challenges.


Creation of chloroplasts.A chloroplast develops from a proplastid in photosynthetic cells.


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