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You are watching: What do mitochondrial and thylakoid membranes have in common

Cooper GM. The Cell: A molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.

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Chloroplasts, the oribel responsible because that photosynthesis, space in numerous respects similar to mitochondria. Both chloroplasts and mitochondria role to create metabolic energy, progressed by endosymbiosis, contain their own genetic systems, and replicate by division. However, chloroplasts room larger and also more complex than mitochondria, and they carry out several vital tasks in enhancement to the generation that ATP. Most importantly, chloroplasts space responsible for the photosynthetic conversion that CO2 come carbohydrates. In addition, chloroplasts synthesize amino acids, fat acids, and the lipid materials of their own membranes. The reduction of nitrite (NO2-) come ammonia (NH3), critical step in the organization of nitrogen right into organic compounds, additionally occurs in chloroplasts. Moreover, chloroplasts room only among several varieties of associated organelles (plastids) that play a selection of roles in plant cells.

The structure and role of Chloroplasts

Plant chloroplast are large organelles (5 to 10 μm long) that, favor mitochondria, room bounded by a dual membrane dubbed the chloroplastic envelope (Figure 10.13). In addition to the inner and outer membrane of the envelope, chloroplasts have a third internal membrane system, referred to as the thylakoid membrane. The optical membrane membrane creates a network of planarization discs referred to as thylakoids, which are frequently arranged in stacks referred to as grana. As such three-membrane structure, the internal organization of chloroplasts is more complex than that of mitochondria. In particular, their three membranes division chloroplasts right into three distinct internal compartments: (1) the intermembrane room between the 2 membranes of the chloroplasts envelope; (2) the stroma, i m sorry lies within the envelope yet outside the optical membrane membrane; and also (3) the optical membrane lumen.


Figure 10.13

Structure of a chloroplast. In enhancement to the inner and outer membranes of the envelope, chloroplasts contain a third internal membrane system: the thylakoid membrane. This membranes divide chloroplasts right into three internal compartments. (Electron micrograph (more...)

Despite this higher complexity, the membrane of chloroplasts have actually clear sensible similarities through those that mitochondria—as expected, offered the role of both oribelles in the chemiosmotic generation that ATP. The external membrane that the chloroplast envelope, favor that that mitochondria, includes porins and also is as such freely permeable to tiny molecules. In contrast, the within membrane is impermeable to ions and metabolites, i beg your pardon are as such able to get in chloroplasts only via details membrane transporters. This properties the the inner and also outer membranes of the chloroplasts envelope are comparable to the inner and also outer membrane of mitochondria: In both cases the inner membrane restricts the i of molecules in between the cytosol and also the interior of the organelle. The chloroplasts stroma is also equivalent in role to the mitochondrial matrix: It has the chloroplast genetic system and also a variety of metabolic enzymes, including those responsible because that the an important conversion that CO2 come carbohydrates during photosynthesis.

The significant difference between chloroplasts and mitochondria, in regards to both structure and function, is the thylakoid membrane. This membrane is of main importance in chloroplasts, where it fills the duty of the inner mitochondrial membrane in electron transport and the chemiosmotic generation that ATP (Figure 10.14). The inner membrane the the chloroplastic envelope (which is no folded right into cristae) go not function in photosynthesis. Instead, the chloroplastic electron transport mechanism is located in the optical membrane membrane, and also protons are pumped across this membrane from the stroma come the thylakoid lumen. The resulting electrochemical gradient climate drives ATP synthesis as protons cross back into the stroma. In terms of its function in generation of metabolic energy, the optical membrane membrane of chloroplastic is for this reason equivalent to the inner membrane of mitochondria.


Figure 10.14

Chemiosmotic generation that ATP in chloroplasts and mitochondria. In mitochondria, electron transfer generates a proton gradient across the inside membrane, i m sorry is then offered to drive ATP synthesis in the matrix. In chloroplasts, the proton gradient is (more...)

The chloroplasts Genome

Like mitochondria, chloroplasts contain your own genetic system, reflecting their evolutionary beginnings from photosynthesis bacteria. The genomes of chloroplastic are comparable to those that mitochondria in that they covers circular DNA molecules existing in multiple duplicates per organelle. However, chloroplast genomes are larger and more facility than those of mitochondria, ranging from 120 come 160 kb and containing roughly 120 genes.

The chloroplastic genomes of several plants have actually been fully sequenced, resulting in the to know of countless of the genes had in the organelle DNAs. These chloroplast genes encode both RNAs and proteins associated in gene expression, and also a range of protein that duty in photosynthesis (Table 10.2). Both the ribosomal and transfer RNAs provided for translate in of chloroplast mRNAs are encoded by the organelle genome. These incorporate four rRNAs (23S, 16S, 5S, and also 4.5S) and also 30 tRNA species. In comparison to the smaller variety of tRNAs encoded by the mitochondrial genome, the chloroplasts tRNAs are adequate to analyze all the mRNA codons follow to the universal hereditary code. In enhancement to this RNA components of the translation system, the chloroplasts genome encodes around 20 ribosom proteins, which represent approximately a 3rd of the protein of chloroplasts ribosomes. Part subunits of RNA polymerase are also encoded by chloroplasts, although added RNA polymerase subunits and other factors needed for chloroplast gene expression room encoded in the nucleus.


The chloroplast genome additionally encodes around 30 proteins that are involved in photosynthesis, including components of photosystems I and II, the the cytochrome bf complex, and also of ATP synthase. In addition, one of the subunits the ribulose bisphosphate carboxylase (rubisco) is encoded by chloroplastic DNA. Rubisco is the vital enzyme that catalyzes the addition of CO2 come ribulose-1,5-bisphosphate during the Calvin cycle (see number 2.39). Not only is it the significant protein component of the chloroplastic stroma, yet it is additionally thought to be the single most abundant protein on Earth, so the is remarkable that among its subunits is encoded by the chloroplastic genome.

Import and Sorting of chloroplasts Proteins

Although chloroplastic encode more of their own proteins than mitochondria, around 90% of chloroplast proteins room still encoded by nuclear genes. Similar to mitochondria, this proteins space synthesized on cytosolic ribosomes and also then imported into chloroplasts as completed polypeptide chains. They should then be sorted come their proper location within chloroplasts—an also more facility task than protein sorting in mitochondria, because chloroplasts save on computer three separate membranes that division them into three distinctive internal compartments.

Protein import into chloroplasts usually resembles mitochondrial protein income (Figure 10.15). Proteins room targeted because that import right into chloroplasts through N-terminal assignment of 30 come 100 amino acids, referred to as transit peptides, which direct protein translocation throughout the 2 membranes the the chloroplasts envelope and also are then eliminated by proteolytic cleavage. The transit peptides are well-known by the translocation complex of the chloroplast external member (the Toc complex), and proteins are transported v this complex across the membrane. They space then moved to the translocation complicated of the within membrane (the Tic complex) and transported across the inside membrane come the stroma. Together in mitochondria, molecular chaperones on both the cytosolic and stromal sides of the envelope are forced for protein import, i beg your pardon requires power in the type of ATP. In comparison to the presequences the mitochondrial import, however, transit peptides are not positive charged and the translocation of polypeptide chains right into chloroplasts walk not need an electric potential across the membrane.


Figure 10.15

Protein import right into the chloroplasts stroma. Proteins are targeted because that import right into chloroplasts through a transit peptide at your amino terminus. The transit peptide directs polypeptide translocation through the Toc complicated in the chloroplast external membrane (more...)

Proteins integrated into the thylakoid lumen space transported to their destination in two actions (Figure 10.16). Castle are an initial imported into the stroma, as already described, and also are then targeted because that translocation throughout the optical membrane membrane by a 2nd hydrophobic signal sequence, which is exposed complying with cleavage that the transit peptide. The hydrophobic signal succession directs translocation of the polypeptide throughout the optical membrane membrane and is finally removed by a second proteolytic cleavage in ~ the lumen.

Figure 10.16

Import of proteins right into the thylakoid lumen. Proteins space imported into the thylakoid lumen in 2 steps. The first step is import right into the chloroplastic stroma, as portrayed in number 10.15. Cleavage that the transit peptide then exposes a 2nd hydrophobic (more...)

The pathways the protein sorting to the other four compartments that chloroplasts—the inner and outer membranes, optical membrane membrane, and intermembrane space—are much less well established. As with mitochondria, proteins show up to it is in inserted directly into the outer membrane that the chloroplast envelope. In contrast, proteins destined for either the thylakoid membrane or the inner membrane that the chloroplasts envelope are originally targeted for import right into the stroma by N-terminal transit peptides. Adhering to cleavage that the transit peptides, these proteins space then targeted for insertion into the proper membrane by various other sequences, which are not yet well characterized. Finally, neither the sequences that target proteins to the intermembrane an are nor the pathways through which they travel to that location have been identified.

Other Plastids

Chloroplasts are only one, albeit the most prominent, member that a larger family of tree organelles dubbed plastids. Every plastids save on computer the very same genome as chloroplasts, but they differ in both structure and also function. Chloroplastic are dedicated for photosynthesis and also are unique in that they save the inner thylakoid membrane system. Other plastids, which are affiliated in different elements of plant cell metabolism, are bounded by the 2 membranes that the plastid envelope but lack both the thylakoid membranes and also other contents of the photosynthetic apparatus.

The different varieties of plastids are generally classified according to the kinds of pigments lock contain. Chloroplasts room so named due to the fact that they contain chlorophyll. Chromoplasts (Figure 10.17A) absence chlorophyll however contain carotenoids; they space responsible for the yellow, orange, and also red color of part flowers and fruits, although their an accurate function in cabinet metabolism is no clear. Leucoplasts space nonpigmented plastids, which save a range of power sources in nonphotosynthetic tissues. Amyloplasts (Figure 10.17B) and elaioplasts are examples of leucoplasts that save starch and also lipids, respectively.

Figure 10.17

Electron micrographs of chromoplasts and also amyloplasts. (A) Chromoplasts save lipid droplets in which carote-noids room stored. (B) Amyloplasts contain large starch granules. (A, Biophoto Associates/Photo Researchers, Inc.; B, Dr. Jeremy Burgess/Photo (more...)

All plastids, including chloroplasts, build from proplastids, tiny (0.5 come 1 μm in diameter) undifferentiated organelles present in the rapidly splitting cells of plant roots and also shoots. Proplastids then build into the various species of mature plastids follow to the needs of identified cells. In addition, tires plastids are able to change from one kind to another. Chromoplasts develop from chloroplasts, because that example, during the ripening of fruit (e.g., tomatoes). During this process, chlorophyll and the thylakoid membranes break down, while new types the carotenoids are synthesized.

An amazing feature the plastids is the their advance is managed both by environmental signals and also by intrinsic programs of cabinet differentiation. In the photosynthetic cells of leaves, because that example, proplastids develop into chloroplast (Figure 10.18). During this process, the thylakoid membrane is developed by engine budding from the within membrane of the plastid envelope and the various materials of the photosynthesis apparatus room synthesized and assembled. However, chloroplasts develop only in the existence of light. If tree are kept in the dark, the advance of proplastids in leaves is arrested at an intermediate stage (called etioplasts), in which a semicrystalline selection of tubular inner membranes has formed however chlorophyll has not to be synthesized (Figure 10.19). If dark-grown plants room then exposed to light, the etioplasts continue their development to chloroplasts. It is notable that this dual control the plastid advancement involves the coordinated expression of genes within both the plastid and nuclear genomes. The instrument responsible for such coordinated gene expression are mainly unknown, and their elucidation to represent a challenging problem in plant molecular biology.

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Figure 10.18

Development the chloroplasts. Chloroplasts construct from proplastids in the photosynthetic cell of leaves. Proplastids contain only the inner and outer envelope membranes; the optical membrane membrane is formed by vesicle budding indigenous the inside membrane throughout (more...)

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