Green plants are autotrophs and can synthesize all the molecules they need from simple starting materials: CO2, H2O, phosphate, sulfate, and ammonium ions (NH4+). NH4+ is needed for amino acids and comes either from the conversion of nitrogen-containing molecules in soil water taken up by the plant's roots, or from the bacterial conversion of N2 gas from the atmosphere.
The light reactions of photosynthesis generate ATP and NADPH, which are used to synthesize carbohydrates. These compounds can then be used in cellular respiration to provide energy for processes such as active transport and an-abolism. Both cellular respiration and fermentation can occur in plants, although the former is far more common. Plant cellular respiration, unlike photosynthesis, takes place both in the light and in the dark. Because glycolysis occurs in the
cytosol, respiration in the mitochondria, and photosynthesis in the chloroplasts, all these processes can proceed simultaneously.
Photosynthesis and respiration are closely linked through the Calvin-Benson cycle (Figure 8.18). The partitioning of G3P is particularly important:
► Some G3P from the Calvin-Benson cycle can be converted to pyruvate, the end product of glycolysis. This pyru-vate can be used in cellular respiration for energy, or its carbon skeletons can be used anabolically to make lipids, proteins, and other carbohydrates (see Figure 7.17).
► Some G3P can enter a pathway that is the reverse of gly-colysis (the gluconeogenic pathway). In this case, sucrose is formed and transported to the nonphotosyn-thetic tissues of the plant, such as the root.
Energy flows from sunlight to reduced carbon in photosynthesis to ATP in respiration. Energy can also be stored in the bonds of macromolecules such as polysaccharides, lipids, and proteins. For a plant to grow, energy storage (as body structures) must exceed energy release; that is, overall carbon fixation by photosynthesis must exceed respiration. This principle is the basis of the ecological food chain, as we will see in later chapters.
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