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GM Cooper. A Molecular Approach to the Cell.2.Minneapolis (Minn.): Sinauer Associates, 2000.



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Organelles responsible for photosynthesis, chloroplasts, are similar to mitochondria in many ways.In plants, chloroplasts and mitochondria produce metabolic energy, evolved by endosymbiosis, contain their own genetic systems, and replicate by division.Although chloroplasts generate ATP, they perform a variety of other tasks that are critical to the function of the cell.It is chloroplasts that convert CO2 to carbohydrates through photosynthetic processes.Moreover, chloroplasts produce amino acids, fatty acids, as well as lipids for their own membranes.In chloroplasts, nitrite (NO2-) is converted to ammonia (NH3), another essential step in nitrogen incorporation into organic compounds.Additionally, chloroplasts are only one of several types of related organelles (plastids) found in plants.


The Structure and Function of Chloroplasts

Plant chloroplasts are large organelles (5 to 10 μm long), which are bounded by a double membrane called the chloroplast envelope (Figure 10.13).The thylakoid membrane is an external membrane system, in addition to the inner and outer membranes of the envelope.Thylakoids are flattened discs formed throughout the thylakoid membrane. These discs are arranged in stacks of four called grana. .There are three distinct compartments in chloroplasts, namely: (1) the intermembrane space between the two membranes of the chloroplast envelope; (2) the stroma, which is located inside the envelope, but external to the thylakoid membrane; and (3) the thylakoid lumen.


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Anatomy of a chloroplast.As well as the membrane in the envelope, chloroplasts also have a third membrane inside: the thylakoid membrane.Thylakoid membranes divide chloroplasts into three internal cavities.(Photograph (more..)


.As with mitochondria, the outer membrane of chloroplasts is porin-rich and thus includes pores for free permeability to small molecules. .Similar to the inner and outer membranes of mitochondria, the chloroplast envelope's inner membrane inhibits molecule movement between its interior and the cytosol.

.A chloroplast membrane has a central role in electron transport and chemiosmotic ATP generation in the same way as the inner mitochondrial membrane (Figure 10.14).The inner membrane of the chloroplast envelope (which is not folded into cristae) is not involved in photosynthesis. .ATP is then generated by the electrochemical gradient as the protons enter the stroma.Thylakoid membranes of chloroplasts are therefore equivalent to mitochondrial inner membranes in their function in generating metabolic energy.


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Chloroplasts and mitochondria produce ATP via chemiosmotic reactions.In mitochondria, electron transport leads to a proton gradient crossing the inner membrane, which is used to drive ATP synthesis in the matrix.In chloroplasts, the proton gradient is (read on..)


The Chloroplast Genome

The chloroplasts contain their own genetic system, reflecting their evolutionary origins as photosynthetic bacteria.Chloroplast genomes resemble those of mitochondria in that they consist of circular DNA molecules that are present in multiple copies in each organelle.Although chloroplast genomes range from 120 to 160 kb and contain about 120 genes, they are bigger and more complex than mitochondrial genomes.

.In addition to RNAs that function in gene expression, chloroplast genes encode a variety of proteins that function in photosynthesis (Table 10.2).Ribosomal RNA and transfer RNA are encoded within chloroplast genomes.Among these are four rRNAs (23S, 16S, 5S, and 4.5S) and 30 types of tRNA.While mitochondrial genomes encode fewer tRNAs, chloroplast tRNAs are sufficient to translate the codons in mRNA according to universal genetic code.As well as these RNA components of the translation system, the chloroplast genome encodes about 20 ribosomal proteins, which make up about one third of chloroplast ribosome proteins.There are some of the subunits of RNA polymerase encoded in chloroplasts, but other subunits and factors needed for chloroplast gene expression are encoded in the nucleus.


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.Additionally, chloroplast DNA encodes one of the subunits of ribulose bisphosphate carboxylase (rubisco). .It is not only the major protein of the chloroplast stroma, but it is also thought to be the most abundant protein on earth, so it is noteworthy that one of its subunits is encoded by the chloroplast genome.


Import and Sorting of Chloroplast Proteins

Despite having more of their own proteins encoded by chloroplasts than mitochondria, about 90% are still encoded by nuclear genes.On the same basis as mitochondria, these proteins are synthesized on cytosolic ribosomes and then imported into chloroplasts as polypeptide chains.Then they must be sorted to their proper location within chloroplasts, which is an even more complex task than sorting protein in mitochondria, since chloroplasts have three separate membranes that divide them into three compartments.

Figure 10.15 shows that chloroplast protein import is similar to mitochondrial protein import. .The transit peptides are recognized and transported across the membrane by the translocation complex of the outer chloroplast member (the Toc complex).After being transferred to the Tic complex, they are transported across the inner membrane to the stroma.Like mitochondria, molecular chaperones are required on both the cytoplasmic and stromal sides of the envelope for protein import, which requires energy in the form of ATP.In contrast to the sequence of mitochondrial import, however, transit peptides are not positively charged and the translocation of polypeptide chains into chloroplasts requires no electric potential across the membrane.


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Chloroplast stroma are able to import proteins.Transport peptides at their amino termini direct proteins to chloroplasts for import.During chloroplast outer membrane translocation through the Toc complex, the transit peptide directs polypeptides (more..)


A protein incorporated into the thylakoid lumen is transported to its destination in two steps (Figure 10.16).They are imported into the stroma, as already described, and then targeted for translocation across the thylakoid membrane by a second hydrophobic signal sequence revealed upon cleavage of the transit peptide.Located on the hydrophobic side of the signal sequence, the signal directs polypeptide translocation across the thylakoid membrane. It is then removed from the lumen by second proteolytic cleavage.


Proteins are imported into the thylakoid lumen.In the thylakoid lumen, proteins are imported in two steps.First, the transit peptide must be imported into the chloroplast stroma, as shown in Figure 10.15.Following cleavage of the transit peptide, a second hydrophobic molecule (read more..) is exposed.


.Like mitochondria, proteins are believed to be inserted directly into the outer membrane of chloroplasts.Contrary to this, proteins that are destined for either the thylakoid membrane or the chloroplast envelope's inner membrane are first targeted for import into the stroma by N-terminal transit peptides. .Additionally, neither the sequences that direct proteins to the intermembrane space nor the pathways that guide them there have been identified.


Other Plastids

The chloroplast is only one of a larger family of organelles in plants known as plastids.Despite sharing the same genome as chloroplasts, plastids differ both in structure and function.In addition to being specialized for photosynthesis, chloroplasts are unique in that they contain an internal thylakoid membrane system.

Phylloids are typically classified according to the pigments they contain.The chloroplast is so named because it contains chlorophyll.Despite a lack of chlorophyll, chloroplasts contain carotenoids, which are responsible for the yellow, orange, and red colors of some flowers and fruits, although their exact role in cell metabolism is unknown.The nonpigmented plastids, leucoplasts, store multiple types of energy in nonphotosynthesis tissues.The amyloplast and the elaioplast are examples of leucoplasts that store starch and lipids, respectively.


The chromoplasts and amyloplasts under electron microscopy.As a consequence, carotenoid pigments are stored in lipid droplets within chromoplasts.The amyloplasts contain large starch granules.(A, Biophoto Associates, Inc.; B, Dr. Jeremy Burgess, Inc., Photo Researchers, Inc.


In quickly dividing cells of roots and shoots, chloroplasts are formed from proplastids, which are small undifferentiated organelles (0.5 to 1 *m in diameter).In response to the needs of differentiated cells, proplastids develop into diverse types of mature plastids.Moreover, mature plastids can be transformed into different kinds of cells.A chloroplast develops into a chloroplast as ripening fruit develops (e.g. tomatoes).Chrorophyll and thylakoid membranes are degraded during this process, while carotenoids are synthesized.

Plastids' development is partially influenced by environmental signals as well as by intrinsic programs.Figure 10.18 illustrates how proplastids in the photosynthetic cells of leaves become chloroplasts.It is in this process that thylakoids are formed by budding vesicles from the plastid envelope, and the various components of the photosynthetic apparatus are synthesized and assembled.However, chloroplasts develop only when light is present. .Dark-grown plants exposed to light see their etioplasts develop into chloroplasts.It is noteworthy that the dual control of plastid development can be attributed to coordination of genes within the plastid and nuclear genomes.In plant molecular biology, understanding the mechanisms responsible for such coordinated gene expression remains a challenging problem.


Developing chloroplasts.Plant leaves contain chloroplasts, which develop from proplastids.During vesicle budding from the inner membrane, the thylakoid membrane is formed. The proplastid consists of the inner and outer envelope membranes.


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