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Photosynthesis and Respiration 18 Photosynthesis and Respiration ATP is the energy currency of the cell Goal To understand how energy from sunlight is harnessed to Cells need to carry out many reactions that are energetically unfavorable. generate chemical energy by photosynthesis and You have seen some examples of these non-spontaneous reactions in respiration. earlier chapters: the synthesis of nucleic acids and proteins from their corresponding nucleotide and amino acid building blocks and the transport Objectives of certain ions against concentration gradients across a membrane. In many cases, unfavorable reactions like these are coupled to the hydrolysis of ATP After this chapter, you should be able to: in order to make them energetically favorable under cellular conditions; we • Explain the concepts of oxidation and have learned that for these reactions the free energy released in breaking reduction. the phosphodiester bonds in ATP exceeds the energy consumed by the • Explain how light energy generates an uphill reaction such that the sum of the free energy of the two reactions is electrochemical gradient. negative (ΔG < 0). To perform these reactions, cells must then have a way • Explain how an electrochemical of generating ATP efficiently so that a sufficient supply is always available. gradient generates chemical energy. The amount of ATP used by a mammalian cell has been estimated to be on the order of 109 molecules per second. In other words, ATP is the principal • Explain how chemical energy is harnessed to fix carbon dioxide. energy currency of the cell. • Explain how glucose is used to generate How does the cell produce enough ATP to sustain life and what is the source ATP anaerobically. of the energy required to drive the uphill reactions needed to form ATP? A • Explain how glucose is used to generate biochemist by the name of Peter Mitchell considered these questions in the ATP aerobically. 1950s and 1960s and made the surprising discovery that membranes play a critical role (see Box 1). Specifically, this work showed, and as we shall see, that the energy released by transporting protons across the membranes of certain specialized organelles—chloroplasts and mitochondria—drives the synthesis of ATP from ADP and inorganic phosphate (Pi). Figure 1 Chapter 18 Photosynthesis and Respiration 2 Box 1 Peter Mitchell and the Chemiosmotic Theory In the 1950s, it was a mystery how the phosphorylation of ADP to ATP was mechanistically connected to the oxidation of glucose. Two main theories emerged—promoted by two men who were educated in the Biochemistry department at Cambridge in the years after World War II. Edward Charles Slater developed methods during the war to measure vitamin concentrations in food and later studied small molecule inhibitors of oxidative phosphorylation. These studies led him to suggest that the oxidation of glucose in some way led to the production of a high energy intermediate metabolite. This theory suggested that that intermediate was then used to convert ADP to ATP. Slater spent years trying to identify and isolate this proposed high energy intermediate without success. Around the same time, Peter Mitchell began working on the same problem. Peter Mitchell During the war, Mitchell helped develop a treatment for skin lesions caused Nobel Prize in Chemistry 1978 by a chemical warfare agent, Lewisite. The molecule they developed would not cross biological membranes and would scavenge the chemical agent from blood, taken from Nobelprize.org "Peter Mitchellallowing - Facts". Nobelprize.org. it to be Nobel excreted. Media AB 2014.After Web. 27the Apr 2018.war, Mitchell studied the mechanism of action of penicillin and the bilayer <http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1978/mitchell-facts.html>structure of bacterial membranes. His work on membranes and the transport of metabolites across them led him to propose the chemiosmotic theory of oxidative phosphorylation. This theory suggests that the energy in glucose is in some way stored in an electrochemical gradient across a biological membrane. The energy released upon movement of an ion down that gradient is then used to convert ADP and Pi to ATP. This was a radical idea at the time and Slater and Mitchell debated their theories in print for many years until Mitchell’s gained widespread acceptance. He was awarded the Nobel Prize in Chemistry in 1978 for his work on the chemiosmotic theory. Mitchell and his colleague, Jennifer Moyle, performed many of the definitive experiments on chemiosmosis at a research facility he built in a neglected mansion, Glynn House, in Cornwall, England. After stepping down from his position at Edinburgh University for health reasons, he spent several years restoring the mansion and building the laboratory, which was named the Glynn Research Institute. Mitchell continued to work there with a small group of research associates until 1987. depicts the structure of these organelles and the following sections explain how these organelles capture energy from sunlight or energy from specific chemicals (e.g. carbohydrates) and convert it to the cell’s energy currency. Chloroplasts harvest light energy to generate electrical energy The ultimate source of chemical energy in the biosphere is the sun. Photosynthetic organisms capture the sun’s energy and use it to generate chemical fuels (simple sugars), which are directly or indirectly consumed through the food chain. This conversion of sunlight into chemical energy takes place in three steps. First, photosynthetic organisms use light energy to generate an electrochemical gradient across a membrane, converting energy from light into electrical potential energy. A remarkable protein machine then converts this potential energy into chemical energy in the form of ATP. Finally, the ATP is used to drive the formation of carbohydrates by fixation Chapter 18 Photosynthesis and Respiration 3 A outer membrane B inner membrane thylakoid membrane stroma thylakoid space matrix inner membrane intermembrane outer membrane space Figure 1 The membranes of chloroplasts and mitochondria play a critical role in the process that generates ATP in eukaryotes (A) Transmission electron microscopy (TEM) image of a chloroplast from a green alga. Chloroplasts contain three membranes: an outer membrane, an inner membrane, and a thylakoid membrane. These create three aqueous compartments: the intermembrane space between the inner and outer membranes, the stroma between the inner and thylakoid membranes, and the thylakoid space inside the thylakoid membrane. The thylakoids pack against each other into stacks called grana. (B) TEM image of a mitochondrion from a guinea pig pancreas. The organelle contains two membranes: an inner and an outer membrane. The inner membrane is folded into extended internal structures called cristae, which give the inner membrane a larger surface area than the outer membrane. The membranes create two internal aqueous compartments: the intermembrane space (IMS) between the two membranes and the matrix inside the inner mem- brane. Both TEM images were collected by George E. Palade (cellimagelibrary.org). of carbon dioxide from the atmosphere. In what follows, we will consider these three phases of photosynthesis: the generation of an electrochemical gradient, the synthesis of ATP, and the conversion of carbon dioxide to carbohydrate. The molecules that absorb light in plants are found in thethylakoid membranes of their chloroplasts. Chloroplasts have three membrane bilayers: outer and inner membranes, which surround the organelle, and a thylakoid membrane, which folds back and forth upon itself into stacks of membranes called grana (Figure 1). Light-absorbing protein complexes called photosystems span the thylakoid bilayer and consist of several integral membrane proteins and hundreds of small pigment molecules. The most important of these pigments is chlorophyll—the molecule that gives plants their green color. Most of the chlorophyll molecules are bound to the proteins at the periphery of the photosystem in what are called antenna complexes, but two special chlorophyll molecules are also found in the middle of the photosystem in the reaction center (Figure 2). Photosystems convert light energy into electrical energy at this reaction center. Chlorophyll molecules contain a chemical functional group, a porphyrin ring, that is capable of absorbing sunlight in the red region of the visible spectrum (Figure 2). When one of the chlorophyll molecules in the antenna complexes absorbs light, the electrons in its porphyrin ring move to a higher energy state. As the electrons return to their normal, lower energy state, they Chapter 18 Photosynthesis and Respiration 4 Figure 2 Light-harvesting photo- A B Light system complexes in the chloro- Light-absorbing N N plast thylakoid membrane contain Mg porphyrin ring N N many bound chlorophyll mole- Reaction Center cules, which absorb light energy O O OCH O O 3 and funnel it to the reaction center (A) The chemical structure of the chloro- Hydrophobic phyll molecule contains a light-absorbing - tail e porphyrin ring and a hydrophobic tail. A single photosystem can contain hundreds of chlorophyll molecules. (B) The trajecto- ry of energy transfers between chlorophyll Antenna Complexes molecules in the photosystem is shown. Any one of the antenna chlorophyll mole- cules (green hexagons) can absorb light and transfer their absorbed energy to a neighboring chlorophyll molecule. This then transfer it to its neighbors. The indi- vidual chlorophyll molecules are bound by second chlorophyll molecule, in turn, transfers its absorbed energy to yet the proteins in slightly different ways that another neighbor in a chain reaction that continues until the energy reaches allow the light energy to be transferred to- the reaction center of the photosystem complex. The electrons in the pair wards the reaction center. When the pair of chlorophyll molecules in the reaction center also move to a higher state of chlorophyll molecules at the reaction when they absorb the energy from the neighboring antenna chlorophyll, center (dark green hexagons) receive the energy, they eject electrons, initiating an but they then transfer those high energy electrons to an acceptor molecule.
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