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

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.


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In Chapter 18, we saw that oxidative phosphorylation, the predominant means of generating ATP from fuel molecules, was compartmentalized into mitochondria. Likewise, photosynthesis, the means of converting light into chemical energy, is sequestered into organelles called chloroplasts, typically 5 μm long. Like a mitochondrion, a chloroplast has an outer membrane and an inner membrane, with an intervening intermembrane space (Figure 19.3). The inner membrane surrounds a stroma, which is the site of the carbon chemistry of photosynthesis (Section 20.1). In the stroma are membranous structures called thylakoids, which are flattened sacs, or discs. The thylakoid sacs are stacked to form a granum. Different grana are linked by regions of thylakoid membrane called stroma lamellae. The thylakoid membranes separate the thylakoid space from the stroma space. Thus, chloroplasts have three different membranes (outer, inner, and thy-lakoid membranes) and three separate spaces (intermembrane, stroma, and thylakoid spaces). In developing chloroplasts, thylakoids are believed to arise from invaginations of the inner membrane, and so they are analogous to the mitochondrial cristae. Like the mitochondrial cristae, they are the site of coupled oxidation-reduction reactions that generate the proton-motive force.

Photosynthetic catastrophe-

If photosynthesis were to cease, all higher forms of life would be extinct in about 25 years. A milder version of such a catastrophe ended the Cretaceous period 65.1 million years ago when a large asteroid struck the Yucatan Peninsula of Mexico. Enough dust was sent into the atmosphere that photosynthetic capacity was greatly diminished, which apparently led to the disappearance of the dinosaurs and allowed the mammals to rise to prominence.


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Figure 19.3

Diagram of a Chloroplast.


19.1.1. The Primary Events of Photosynthesis Take Place in Thylakoid Membranes

The thylakoid membranes contain the energy-transducing machinery: light-harvesting proteins, reaction centers, electron-transport chains, and ATP synthase. They have nearly equal amounts of lipids and proteins. The lipid composition is highly distinctive: about 40% of the total lipids are galactolipids and 4% are sulfolipids, whereas only 10% are phospholipids. The thylakoid membrane and the inner membrane, like the inner mitochondrial membrane, are impermeable to most molecules and ions. The outer membrane of a chloroplast, like that of a mitochondrion, is highly permeable to small molecules and ions. The stroma contains the soluble enzymes that utilize the NADPH and ATP synthesized by the thylakoids to convert CO2 into sugar. Plant leaf cells contain between 1 and 100 chloroplasts, depending on the species, cell type, and growth conditions.


19.1.2. The Evolution of Chloroplasts

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Chloroplasts contain their own DNA and the machinery for replicating and expressing it. However, chloroplasts are not autonomous: they also contain many proteins encoded by nuclear DNA. How did the intriguing relation between the cell and its chloroplasts develop? We now believe that, in a manner analogous to the evolution of mitochondria (Section 18.1.2), chloroplasts are the result of endosymbiotic events in which a photosynthetic microorganism, most likely an ancestor of a cyanobacterium (Figure 19.4), was engulfed by a eukaryotic host. Evidence suggests that chloroplasts in higher plants and green algae are derived from a single endosymbiotic event, whereas those in red and brown algae arose from at least one additional event.


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Figure 19.4

Cyanobacteria. A colony of the photosynthetic filamentous cyanobacteria Anabaena shown at 450× magnification. Ancestors of these bacteria are thought to have evolved into present-day chloroplasts.


The chloroplast genome is smaller than that of a cyanobacterium; however, like that of a cyanobacterium, it is circular with a single start site for DNA replication, and its genes are arranged in operons—sequences of functionally related genes under common control (Chapter 31). In the course of evolution, many of the genes of the chloroplast ancestor were transferred to the plant cell"s nucleus or, in some cases, lost entirely, thus establishing a fully dependent relation.

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