Photosynthesis and Respiration - Harvard University

18

Photosynthesis and Respiration

Goal

To understand how energy from sunlight is harnessed to generate chemical energy by photosynthesis and respiration.

Objectives

After this chapter, you should be able to:

? Explain the concepts of oxidation and reduction.

? Explain how light energy generates an electrochemical gradient.

? Explain how an electrochemical gradient generates chemical energy.

? Explain how chemical energy is harnessed to fix carbon dioxide.

? Explain how glucose is used to generate ATP anaerobically.

? Explain how glucose is used to generate ATP aerobically.

ATP is the energy currency of the cell

Cells need to carry out many reactions that are energetically unfavorable. You have seen some examples of these non-spontaneous reactions in earlier chapters: the synthesis of nucleic acids and proteins from their corresponding nucleotide and amino acid building blocks and the transport of certain ions against concentration gradients across a membrane. In many cases, unfavorable reactions like these are coupled to the hydrolysis of ATP in order to make them energetically favorable under cellular conditions; we have learned that for these reactions the free energy released in breaking the phosphodiester bonds in ATP exceeds the energy consumed by the uphill reaction such that the sum of the free energy of the two reactions is negative (G < 0). To perform these reactions, cells must then have a way of generating ATP efficiently so that a sufficient supply is always available. 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 energy currency of the cell.

How does the cell produce enough ATP to sustain life and what is the source of the energy required to drive the uphill reactions needed to form ATP? A biochemist by the name of Peter Mitchell considered these questions in the 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

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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, t"aPkeetnerfrMomitchNeoalbll-elFloparcwitzse"i..onNrgogbeilptrtizoe.obrge. Neobxecl Mreedtiea AdB.2A01f4t. Weerbt. 2h7eApwr 2a01r8,. Mitchell studied the mechanism of action of penicillin and the bilayer 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 A

outer membrane B inner membrane mtheymlabkroainde

Photosynthesis and Respiration

3

stroma thylakoid space

matrix inner membrane intermspeamcebrane outer membrane

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 membrane. Both TEM images were collected by George E. Palade ().

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 the thylakoid 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

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Figure 2 Light-harvesting photo- A

B

Light

system complexes in the chloroplast thylakoid membrane contain many bound chlorophyll mole-

Light-absorbing porphyrin ring

N

N

Mg

N

N

cules, which absorb light energy and funnel it to the reaction center

O

O

OO

OCH3

Reaction Center

(A) The chemical structure of the chlorophyll molecule contains a light-absorbing porphyrin ring and a hydrophobic tail. A single photosystem can contain hundreds of chlorophyll molecules. (B) The trajectory of energy transfers between chlorophyll molecules in the photosystem is shown. Any one of the antenna chlorophyll molecules (green hexagons) can absorb light and then transfer it to its neighbors. The individual chlorophyll molecules are bound by the proteins in slightly different ways that allow the light energy to be transferred towards the reaction center. When the pair of chlorophyll molecules at the reaction center (dark green hexagons) receive the energy, they eject electrons, initiating an electron transport chain.

Hydrophobic

tail

e-

Antenna Complexes

transfer their absorbed energy to a neighboring chlorophyll molecule. This second chlorophyll molecule, in turn, transfers its absorbed energy to yet another neighbor in a chain reaction that continues until the energy reaches the reaction center of the photosystem complex. The electrons in the pair of chlorophyll molecules in the reaction center also move to a higher state when they absorb the energy from the neighboring antenna chlorophyll, but they then transfer those high energy electrons to an acceptor molecule. In other words, the reaction center chlorophyll molecules transfer electrons with their absorbed energy rather than just transferring the energy (as the antenna chlorophyll molecules do). This electron transfer is the step in photosynthesis that converts light energy into electrical energy.

The transfer of electrons from the reaction center creates a charge separation in which the chlorophyll molecules now carry positive charges. This leaves us with two questions: How do the special chlorophyll molecules regain their electrons and what is the fate the electrons that had been ejected from the reaction center? We shall address the fate of the ejected electrons after we explain the answer to the first question.

The chlorophyll molecules regain their proper number of electrons by extracting them from water molecules in the thylakoid space. Reactions of this type, in which electrons are transferred from one molecule to another, are called redox reactions. The molecule that loses electrons is said to be "oxidized," and the molecule that gains electrons is said to be "reduced." As shown below, we can write the oxidation and reduction steps of a reaction as two separate "half reactions," in which we explicitly show the lost or gained electrons as products or reactants, respectively.

Half Reactions:

Oxidation: A A+ + e-

Reduction: B+ + e- B

Complete Redox Reaction:

A + B+ A+ + B

In photosynthesis, the oxygen atoms in water molecules undergo oxidation, transferring some of their electrons to reduce the special chlorophyll molecules in the reaction center and in the process the water breaks down to

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Figure 3 The electron transport

Light

chain in the chloroplast thylakoid

membrane.

2 H2O

O2 + 4 H+

Two photosystems harvest light energy

2 H+

and eject high energy electrons, which are

then transferred to other electron carrier

molecules in the chain. As the electrons move from water to NADPH, six protons are released into the thylakoid space, creating a

ePhotosystem

II

e-

proton gradient across the membrane. For

a more detailed description of each of the electron transfers in this chain, see Box 2.

2 H+

Light

e-

eePhotosystem I

THYLAKOID SPACE

e-

STROMA

NADP+ + H+ NADPH

form O2 and H+ ions (Figure 3). The oxidation and reduction half reactions for this process are:

Half Reactions:

Oxidation: 2H2O 4H+ + O2 + 4 e-

Reduction: 2 chlorophyll2+ + 4 e- 2 chlorophyll

Complete Redox Reaction:

2H2O + 2 chlorophyll2+ 4H+ + O2 + 2 chlorophyll

This oxidative water splitting reaction starts the process of generating an electrochemical gradient across the thylakoid membrane; as the water molecules react, they produce more protons on the inside of the membrane (in the thylakoid space) than there are on the outside (in the stroma).

The electrons that are released from the special reaction center chlorophyll molecules initiate a series of electron transfer reactions that are energetically favorable--that is, the electrons become more stable with each transfer from one molecule to the next in the chain (Figure 3). The electrons pass from the first photosystem (which is counterintuitively named photosystem II) through a set of intermediate electron carriers. Some of the energy that is released by these downhill electron transfers is used to transport two protons from the stroma into the thylakoid space, increasing the proton gradient across the membrane. The electrons then arrive at a second photosystem complex (named photosystem I) where they receive another input of light energy. The antenna chlorophyll molecules of this complex again absorb light and funnel its energy to the special reaction center chlorophyll molecules, which then lose electrons. These high energy electrons are transferred through another set of intermediates on the stromal side of the membrane to the ultimate acceptor in the chain, NADP+, generating NADPH. NADPH is a small molecule that, by carrying high energy electrons, functions as another energy currency in the cell; see Box 3 for a description of the chemistry of NADPH and other similar molecules.

Overall, electrons are transferred through the chloroplast electron transport chain from H2O to NADP+, generating O2, protons, and NADPH in the process. The "Z-scheme" summarizes the energetic changes associated with

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