Thylakoid Space

photosystem II

light

cytochrome b^/f plastocyanin \

NADPH i to the light-

independent reactions

photosystem I

light photosystem I

photosystem II

light light

Figure 10.8 The light-dependent reactions of photosynthesis, which occur in more than one way. In noncyclic photophosphorylation, involving photosystems I and II, which convert light energy to biochemical energy in the form of ATP and NADPH, water molecules are split, releasing electrons, protons, and oxygen gas. The electrons are subsequently used to produce NADPH, whereas the protons are used, in part, to enable production of ATP. Oxygen gas is a by-product of this noncyclic photophosphorylation, although aerobic organisms rely upon this gas for respiration. The ATP and NADPH are used in the carbon-fixing reactions that convert CO2 to sugars (see Fig. 10.10). Only photosystem I is involved in cyclic photophosphorylation. In this relatively simple system, electrons boosted from a photosystem I reaction-center molecule are shunted back into the reaction center via the electron transport system. ATP is produced from ADP, but no NADPH or oxygen is produced.

II received its "II" designation because it was discovered after photosystem I, but we know now that the events of photosynthesis that take place in photosystem II come before those of photosystem I.

While both photosystems produce ATP, only organisms that have photosystems I and II can produce NADPH and oxygen as a consequence of electron flow. It is likely that evolutionary events led to organisms that possess both photosystems. At least 2.8 billion years ago, photosynthetic forms of bacteria (cyanobacteria) are believed to have evolved from primitive bacteria with the development of chlorophyll a and photosystem II. Humans and other organisms dependent on oxygen are fortunate that photosynthetic organisms evolved the oxygen-generating steps of photosystem II because these reactions are what provide the oxygen we breathe today. Since photosynthetic organisms can generate oxygen from water, this very process in the future could be exploited to sustain human life during space travel and colonization of other planets.

Each photosynthetic unit of photosystem I consists of 200 or more molecules of chlorophyll a, small amounts of chlorophyll b, carotenoid pigment with protein attached, and a special reaction-center molecule of chlorophyll called P700. Although all pigments in a photosystem can absorb

Chapter 10

photons, the reaction-center molecule is the only one that can actually use the light energy. The remaining photosystem pigments are called antenna pigments because together they function somewhat like an antenna in gathering and passing light energy to the reaction-center molecule (see Fig. 10.9). There are also iron-sulfur proteins that are the primary electron acceptors for photosystem I (i.e., iron-sulfur proteins first receive electrons from P700).

A photosynthetic unit of photosystem II consists of chlorophyll a, P-carotene (the precursor of vitamin A) attached to protein, a little chlorophyll b, and a reaction-center molecule of chlorophyll a; these special molecules are called P680.

The letter P stands for pigment and the numbers 700 and 680 of the reaction-center molecules of chlorophyll a refer to peaks in the absorption spectra of light with wavelengths of 700 and 680 nanometers, respectively. These peaks differ slightly from those of the otherwise identical chlorophyll a molecules of the photosynthetic units. A primary electron acceptor calledpheophytin (or Pheo) is also present in photosystem II. One reaction-center molecule was found by Johann Deisenhofer and Hartmot Michel of Germany to have over 10,000 atoms. They received a Nobel Prize in 1988 for their work in determining the atomic structure.

Water-Splitting (Photolysis) When a photon of light is absorbed by a P680 molecule of a photosystem II reaction center (located near the inner surface of a thylakoid membrane), the light energy boosts an electron to a higher energy level. This is referred to as exciting an electron. Excited electrons are unstable and can rapidly revert back to their lower energy level, releasing the absorbed energy, perhaps as heat. However, during photosynthesis, the energy of excited electrons is transferred to the P680 reaction centers.

The excited electron in P680 is picked up by the primary acceptor molecule, pheophytin. This electron is shuttled to another acceptor, PQ (plastoquinone), within the thylakoid membrane.4 PQ is mobile and moves within the lipid bilayer toward the side of the stroma, unloading the electrons to the cytochromes next in line. Electrons extracted from water by a manganese-containing oxygen-evolving complex (OEC) replace the electrons lost by the P680 molecule. It has been suggested that there is an oxidation-reduction system, usually designated Z, operating between water and P680. Transfer of an electron from Z to P680 reoxidizes Z and prepares it for accepting another electron from the OEC. Recent investigations indicate that for each two water molecules that are split, one molecule of oxygen is produced, along with four protons and four electrons.

This metabolic pathway eventually evolved in photo-synthetic bacteria (cyanobacteria). The abundance of water, as an electron source, probably facilitated the mechanism

4. Except for the high speed (trillionths of a second) at which it usually carries out its function, an acceptor molecule could be compared to a pickup order telephoned to a clerk in a store in the sense that an item is picked up from a source, held temporarily, and then transferred elsewhere.

that generated oxygen as a by-product of photosynthesis. This process increased the supply of oxygen in earth's atmosphere and made it possible for energy-efficient aerobic respiration to evolve.

Photophosphorylation The high-energy acceptor molecule PQ releases an electron to an electron transport system, which functions something like a downhill bucket brigade that temporarily moves electrons from H2O to a storage molecule, nicotinamide adenine dinucelotide phosphate (NADP), an electron acceptor for photosystem I. The electron transport system consists of iron-containing pigments called cytochromes and other electron transfer molecules, plus plas-tocyanin—a protein that contains copper. While electrons pass along the electron transport system and protons move across the thylakoid membrane by chemiosmosis (see the next section, "Chemiosmosis"), ATP molecules are being formed from ADP and phosphate in the process of photophosphorylation.

A somewhat similar series of events occurs in photosystem I. When a photon of light is absorbed by a P700 molecule in a photosynthetic unit, the energy excites an electron, which is transferred to an iron-sulfur acceptor molecule designated Fe-S. This then passes the electron to another iron-sulfur acceptor molecule, ferredoxin (Fd), which, in turn, releases it to a carrier molecule called flavin adenine dinu-cleotide (FAD). FAD contains flavoprotein, which assists in the reduction of NADP to NADPH. The electrons removed from the P700 molecule are replaced by electrons from photosystem II via the electron transport system outlined previously. This overall movement of electrons from water to photosystem II to photosystem I to NADP is called non-cyclic electron flow, because it goes in one direction only. The production of ATP that correspondingly occurs is designated as noncyclic photophosphorylation.

Photosystem I can also work independently of photosystem II. When it does, the electrons boosted from P700 reaction-center molecules (of photosystem I) are passed from ferredoxin to plastoquinone (instead of NADPH) and back into the reaction center of photosystem I. This process, which does not occur in most plants, is cyclic electron flow. ATP generated by cyclic electron flow is called cyclic photophosphorylation, but water molecules are not split and neither NADPH nor oxygen are produced in this process.

Chemiosmosis The oxygen-evolving complex on the inside of a thylakoid membrane catalyzes the splitting of water molecules, producing protons and electrons, as well as oxygen gas. These electrons used to replenish those excited in chlorophyll are then transferred in bucket-brigade fashion through an electron transport system, ultimately reducing NADP to NADPH. As electrons travel through this transport system, additional protons move from the stroma to the inside of the thylakoid membrane in specific oxidation-reduction reactions when electrons pass from photosystem II to PQ. These protons join with the protons from the split water molecules and thereby contribute to an accumulation of four protons toward the inside of the thylakoid membrane (an area also known as the thylakoid lumen).

-granum

-granum

Pigments Thylakoid Photosystem

thylakoid space

Figure 10.Q Organization of the thylakoid membrane showing relative positions of antenna complexes, photosystems, and protein complexes. Proteins of photosystems within the thylakoid membrane pump hydrogen ions from the stroma into the thylakoid space (lumen). When hydrogen ions flow back out of the thylakoid into the stroma through the ATP synthase complex, ATP is produced. Source: From Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.

thylakoid space

Figure 10.Q Organization of the thylakoid membrane showing relative positions of antenna complexes, photosystems, and protein complexes. Proteins of photosystems within the thylakoid membrane pump hydrogen ions from the stroma into the thylakoid space (lumen). When hydrogen ions flow back out of the thylakoid into the stroma through the ATP synthase complex, ATP is produced. Source: From Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.

Stern-Jansky-Bidlack: I 10. Plant Metabolism I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Stern-Jansky-Bidlack: I 10. Plant Metabolism I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Thylakoid Lumen Mcgraw Hill

wareness

Jigs? i

Photosynthesis and Pizza

The next time you order pizza, imagine all the things that went into making that pizza. Of course, the very bread used to make the crust came from wheat seeds, produced as a result of photosynthesis. You can probably ascertain that any fruits or vegetables, including tomatoes, green peppers, onions, and spices also were produced because photosynthesis took place. The cheese, pepperoni, sausage, bacon, and, perhaps, even anchovies on the pizza came to exist as a result of animals that ate plants produced from photosyn-

thesis. Even the pizza box itself was made as a direct result of photosynthesis, which produces the cellulose we need for paper products, clothing, and wood. As you turn the pages (also made from plants) of this chapter, try to envision the remarkable processes that enabled conversion of light energy into biological energy. It is this energy, derived from the photosynthetic light reactions, that fuels the light-independent reactions of photosynthesis and, hence, the synthesis of organic molecules needed for life on earth.

Although some protons are used in the production of NADPH on the stroma side of the thylakoid membrane, there is still a net accumulation of protons in the thylakoid lumen from the splitting of water molecules and electron transport. This establishes a proton gradient, giving special proteins in the thylakiod membrane the potential for moving protons from the thylakoid lumen back to the stroma. Movement of protons across the membrane is thought to be a source of energy for the synthesis of ATP. The action has been described as similar to the movement of molecules during osmosis and has been called chemiosmosis, or the Mitchell theory, after its author, Peter Mitchell. In this concept, protons move across a thylakoid membrane through protein channels called ATPase. With the proton movement, ADP and phosphate (P) combine, producing ATP (Fig. 10.9). This chemiosmotic mechanism for connecting electron flow with conversion of ADP to ATP is essentially the same as that of oxidative phosphorylation in mitochondria (see Fig. 10.14), except that in mitochondria, oxygen (instead of NADP) is the terminal electron acceptor.

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