Chapter 20 Photosynthesis and Carbohydrate Synthesis in Part II Bioenergetics and Metabolism

Chapter 20 PHOTOSYNTHESIS AND CARBOHYDRATE SYNTHESIS IN PLANTS

An illustration depicts the chapter opener

We have now reached a turning point in our study of cellular metabolism. Thus far in Part II we have described how the major metabolic fuels — carbohydrates, fatty acids, and amino acids — are degraded through converging catabolic pathways that lead to the citric acid cycle and yield their electrons to the respiratory chain, driving ATP synthesis by oxidative phosphorylation. We now turn to reductive, anabolic, divergent processes fueled by energy from the sun that take place in photosynthetic organisms, and in all other organisms, driven ultimately by the photosynthetic reduction of ${CO}_{2}$ $CO Subscript 2$ .

As we examine this process, these principles will emerge:

The capture of solar energy by photosynthetic organisms and its conversion to the chemical energy of reduced organic compounds is the ultimate source of nearly all biological energy and organic nutrients for all of the nonphotosynthetic organisms, including humans. It is arguably the most important biochemical process in the biosphere.
Photosynthetic organisms use tightly organized light-harvesting complexes to absorb sunlight and capture its energy in chemical form: a separation of positive and negative charge leading to electron flow. The energy from an absorbed photon moves from one antenna chlorophyll to another and another until it arrives at the reaction center where it promotes the photochemical reaction that sends electrons through a series of electron carriers.
The light-driven flow of electrons through specialized protein carriers is coupled to ATP synthesis. A strong reducing agent (NADPH) is also produced, and simultaneously, water is oxidized to $O_{2}$ $upper O Subscript 2$ , which is released into the atmosphere.
Evolution yielded a universal mechanism for coupling ATP synthesis to the flow of electrons. A proton gradient created by electron flow is used to energize the ATP-synthesizing enzyme in microorganisms, animals, and plants.
The ATP and NADPH produced in the light-dependent reactions of photosynthesis provide the energy and the reducing power to convert atmospheric ${CO}_{2}$ $bold upper C upper O bold 2$ into simple organic compounds. High concentrations of ATP and NADPH allow the chloroplast to carry out redox reactions that are thermodynamically unfavorable.

Photosynthesis encompasses two processes: the light-dependent reactions, in which sunlight provides the energy for the synthesis of ATP and NADPH, and the ${CO}_{2}$ $bold upper C upper O bold 2$ -assimilation reactions, in which ATP and NADPH are used to reduce ${CO}_{2}$ $CO Subscript 2$ to form triose phosphates via a set of reactions known as the Calvin cycle (Fig. 20-1). We heterotrophs are alive because the enormous energy of sunlight has been captured and tamed by autotrophs by photosynthesis and made available to us as fuel, vitamins, and building blocks. How do they do it?

A figure shows how the assimilation of C O 2 through the light-dependent reactions of photosynthesis allows plants to produce a range of carbon compounds. — FIGURE 20-1 Assimilation of ${CO}_{2}$ $bold upper C upper O bold 2$ provides all of the carbon a plant needs. The light-driven synthesis of ATP and NADPH provides energy and reducing power for the fixation of ${CO}_{2}$ $CO Subscript 2$ into trioses in the Calvin cycle. All of the carbon-containing compounds of the plant cell are synthesized from this fixation of ${CO}_{2}$ $CO Subscript 2$ .

FIGURE 20-1 Assimilation of ${CO}_{2}$ $bold upper C upper O bold 2$ provides all of the carbon a plant needs. The light-driven synthesis of ATP and NADPH provides energy and reducing power for the fixation of ${CO}_{2}$ $CO Subscript 2$ into trioses in the Calvin cycle. All of the carbon-containing compounds of the plant cell are synthesized from this fixation of ${CO}_{2}$ $CO Subscript 2$ .

A cycle of reactions is shown with A T P, N A D P H at the top, a clockwise curved arrow down to A D P, N A D P plus below, and a clockwise curved arrow back up to A T P, N A D P H. A yellowish-orange arrow labeled, light-dependent reactions of photosynthesis points down to the arrow curving up from A D P, N A D P plus to A T P, N A D P H. The curved downward arrow from A T P, N A D P H to A D P, N A D P plus meets a vertical arrow to the right labeled C O 2-assimilation reactions (Calvin cycle) that points from points down from C O 2, H 2 O to triose phosphates below. Upward- and downward-pointing arrows extend from triose phosphates to hexose phosphates below, indicating a reversible reaction. Three similar sets of arrows point down from triose phosphates to three yellow boxes below labeled from left to right as sucrose (transport), starch (storage), and cellulose (cell wall). Another set of arrows points from hexose phosphates to pentose phosphates at the upper right. A bracket to the right of pentose phosphate joins an arrow pointing right to metabolic intermediates, from which four arrows point to D N A, R N A, protein, and lipid in a yellow box.

All vascular plants, as well as algae and cyanobacteria, carry out the same basic process of photosynthesis, but some are more amenable to study than others. Algae and cyanobacteria have been extensively studied because of the relative ease of culturing and manipulating them in the laboratory. Spinach is a vascular plant commonly used for studies of photosynthesis because of the ease of obtaining large amounts of material; and for genetic approaches, the small plant Arabidopsis thaliana is a favorite. What we say here about photosynthesis is essentially true of photosynthesis in all of these organisms.

After looking at photosynthesis, we will discuss the conversion of trioses produced in the Calvin cycle to sucrose (for sugar transport) and starch (for energy storage) (see Fig. 20-1). This conversion is accomplished by mechanisms analogous to those used by animal cells to make glycogen. We also describe the synthesis of the cellulose of plant cell walls. Finally, we consider how carbohydrate metabolism is integrated within a plant cell and throughout the plant.

Although strikingly different on the surface, the processes of photophosphorylation in the chloroplast and oxidative phosphorylation in the mitochondrion are closely similar at the molecular level, and the mechanism for ATP synthesis is virtually identical: a proton gradient drives rotary catalysis by a remarkable ATP synthase.