The related topics of photosynthesis and respiration deal with the chemical steps of energy transformation in plants and animals. During our discussion on these topics, we will repeatedly refer to topics from chapter four. Make sure that you have a good understanding of energy, gradients, enzymes, metabolic pathways, and the role of ATP. These concepts are necessary for an understanding of photosynthesis and respiration.

Photosynthesis is the broad term for the transformation of energy from sunlight into chemical-energy storage molecules like sugars. In overview, plants can harvest the energy from sunlight and use it to from a proton (H+) gradient. This gradient is then used to synthesize ATP. The ATP is subsequently used to drive the very unfavorable (energy requiring) reactions involved in taking carbon dioxide from the air and making it into sugars. When the sun is not shining, the plant can use these sugars to make ATP in the process of respiration, or oxidative phosphorylation. Animals, in contrast, have access to only the second pathway - the use of sugars to generate ATP. Photosynthesis uses carbon dioxide and releases oxygen while respiration uses oxygen and releases carbon dioxide. See Section 6.7 (page 110) for a good overview of metabolism.

Photosynthesis begins inside the inner or thylakoid membrane of the chloroplast ) see figure 5.2. In this process, chlorophyll and carotenoid pigments absorb energy from sunlight. This energy is used to strip electrons from water, releasing oxygen and protons (Figure 5.7). Oxygen is released to the atmosphere while the protons accumulate inside the thylakoid space, generating a gradient. The pumping of additional electrons in the process of electron transport further strengthens the gradient. Then, as the protons flow out of the thylakoid space, the energy in the gradient is used to generate ATP. These reactions take place only in the light.

ATP generated in the light reactions is then available for use in sugar synthesis. When sugars are burned for energy in respiration, carbon dioxide is released. The dark reactions of photosynthesis (see figure 5.10) are the reverse of this process, and require the input of large amounts of energy from ATP. These reactions are the source of most energy used by humans. Exceptions include nuclear, photovoltaic, and wind power.

Aerobic respiration - the burning of sugars and other organic molecules in the presence of oxygen to generate oxygen - is common to all eukaryotes. Figure 6.2 gives a good overview of this process. Glucose (a sugar) is broken down and the energy stored in this molecule is used to generate a few molecules of ATP and a large number of reducing equivalents (NADH, FADH). Carbon dioxide is released. Then, the reducing equivalents from the preceding steps are used to generate a proton gradient across the inner membrane of the mitochondrion. Finally, this gradient is used, as in photosynthesis, to generate large amounts of ATP.

The initial steps of this pathway, glycolysis (see figure 6.3) take place in the cytoplasm, and involve the input of two ATP and the output of four ATP for a net gain of two ATP per sugar molecule. One molecule of NADH is also generated. The second stage of the pathway, the Krebs cycle, takes place in the mitochondrion, and releases the carbon of the original sugar molecule as carbon dioxide. In this process, several reducing equivalents are generated. The largest amount of ATP synthesis occurs when the reducing equivalents from the Krebs cycle are used by proteins in the inner mitochondrial membrane to form a gradient in which protons accumulate in the intermembrane space. When these protons flow back into the inner space of the mitochondrion, ATP is synthesized (See Figure 6.7). It is this part of the pathway that requires oxygen. In the absence of oxygen, reducing equivalents cannot donate electrons to the electron transport system, and the Krebs cycle does not function. NADH generated in glycolysis is turned over in fermentation to generate either ethanol (yeast) or lactate (humans and others). As shown in figure 6.11, this pathway is the ultimate destination for most of the carbon in our food.