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Energy Conversion: Mitochondria and Chloroplasts

Chloroplasts and Photosynthesis25


The Chloroplast Is One Member of a Family of Organelles That Is Unique to Plants - the Plastids

Chloroplasts Resemble Mitochondria But Have an Extra Compartment

Two Unique Reactions in Chloroplasts: The Light-driven Production of ATP and NADPH and the Conversion of CO2 to Carbohydrate

Carbon Fixation Is Catalyzed by Ribulose Bisphosphate Carboxylase

Three Molecules of ATP and Two Molecules of NADPH Are Consumed for Each CO2 Molecule That Is Fixed in the Carbon-Fixation Cycle

Carbon Fixation in Some Plants Is Compartmentalized to Facilitate Growth at Low CO2 Concentrations

Photosynthesis Depends on the Photochemistry of Chlorophyll Molecules

A Photosystem Contains a Reaction Center Plus an Antenna Complex

In a Reaction Center, Light Energy Captured by Chlorophyll Creates a Strong Electron Donor from a Weak One

In Plants and Cyanobacteria Noncyclic Photophosphorylation Produces Both NADPH and ATP

Chloroplasts Can Make ATP by Cyclic Photophosphorylation Without Making NADPH

The Electrochemical Proton Gradient Is Similar in Mitochondria and Chloroplasts

Like the Mitochondrial Inner Membrane, the Chloroplast Inner Membrane Contains Carrier Proteins That Facilitate Metabolite Exchange with the Cytosol

Chloroplasts Carry Out Other Biosyntheses

Figure 14-39: The chloroplast

Figure 14-40: Electron micrographs of chloroplasts

Figure 14-41: Comparison of a mitochondrion and a chloroplast
Section References
Bogorad, L.Chloroplasts. J. Cell Biol. 91:256s-270s, 1981 [PubMed]

Clayton, R.K.Photosynthesis: Physical Mechanisms and Chemical Patterns. Cambridge, UK: Cambridge University Press, 1980. (Excellent general treatment.)

Haliwell, B.Chloroplast MetabolismThe Structure and Function of Chloroplasts in Green Leaf Cells. Oxford, UK: Clarendon Press, 1981.

Hoober, J.K.Chloroplasts. New York: Plenum Press, 1984.
Cramer, W.A.; Widger, W.R.; Herrmann, R.G.; Trebst, A.Topography and function of thylakoid membrane proteins. Trends Biochem. Sci. 10:125-129, 1985

Miller, K.R.The photosynthetic membrane. Sci. Am. 241(4)102-113, 1979.
Chloroplasts Resemble Mitochondria But Have an Extra Compartment27

    Chloroplasts carry out their energy interconversions by chemiosmotic mechanisms in much the same way that mitochondria do, and they are organized on the same principles (Figures 14-39 and 14-40). They have a highly permeable outer membrane, a much less permeable inner membrane, in which special carrier proteins are embedded, and a narrow intermembrane space. The inner membrane surrounds a large space called the stroma, which is analogous to the mitochondrial matrix and contains various enzymes, ribosomes, RNA, and DNA.
    There is, however, an important difference between the organization of mitochondria and that of chloroplasts. The inner membrane of the chloroplast is not folded into cristae and does not contain an electron-transport chain. Instead, the electron-transport chain as well as the photosynthetic light-absorbing system and an ATP synthase are all contained in a third distinct membrane that forms a set of flattened disclike sacs, the thylakoids (see Figure 14-39). The lumen of each thylakoid is thought to be connected with the lumen ofother thylakoids, thereby defining a third internal compartment called the thylakoid space, which is separated from the stroma by the thylakoid membrane.
    The structural similarities and differences between mitochondria and chloroplasts are illustrated in Figure 14-41. Superficially, the chloroplast resembles a greatly enlarged mitochondrion in which the cristae have been converted into a series of interconnected submitochondrial particles in the matrix space. The knobbed end of the chloroplast ATP synthase, where ATP is made, protrudes from the thylakoid membrane into the stroma, just as it protrudes into the matrix from the membrane of each mitochondrial crista.

© 1994 by Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson.