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| Extended Lecture Outline |
Chapter 10: Photosynthesis |
10.1 Photosynthesis in eucaryotes occurs in chloroplasts.
a. A plant's leaves appear green due to the presence of small bodies called chloroplasts (Figure 10.1).
1. Chloroplasts of plant cells are typically small, numerous, disc-shaped, and about 1 micrometer thick and 2—4 µm wide.
2. Chloroplasts are bound by a double membrane, but most of their volume is occupied by an extensive third membrane, the thylakoid.
3. The thylakoid membrane divides the chloroplast into two parts: an internal space called the lumen and an external space called the stroma.
4. The thylakoid membrane is folded into many disc-shaped vesicles in stacks called grana, which are connected by irregular sections of membrane.
5. The light-dependent reactions of photosynthesis are those in which light energy is absorbed and stored.
6. The light-independent reactions of photosynthesis are those in which the energy acquired in the light-dependent reactions is used to reduce CO2 to organic molecules, a process called CO2 fixation.
b. What is the product of photosynthesis?
1. Inside plant chloroplasts, triose phosphates can be converted into hexose phosphates and stored as starch through a pathway that is essentially the reversal of glycolysis.
2. Triose phosphates are commonly converted into hexose sugars, in starch or sucrose, so the product of photosynthesis is represented as hexose (C6H12O6).
c. The two phases of photosynthesis are remarkably parallel to the two phases of respiration, but they run in opposite directions.
1. In the first phase of respiration, glucose is oxidized to CO2, with the reduction of NAD+ to NADH.
2. In the second phase of respiration, NADH is oxidized in an electron transport system (ETS) to make ATP, using the reduction of oxygen to water as a final electron (or hydrogen) acceptor.
3. In the light-dependent reactions of photosynthesis, water donates protons and electrons to an ETS that reduces NADP+, which is functionally the opposite of the respiratory scheme.
4. In the light-independent reactions of photosynthesis, the reduction of CO2 to sugar is the opposite of respiration, though it uses entirely different pathways.
d. Procaryotic phototrophs are very important ecologically and they carry out photosynthesis without chloroplasts.
1. The cyanobacteria (blue-green bacteria) are common in water, soil, and on the bark of trees.
2. These bacteria draw electrons not from water but from other oxidizable inorganic substances such as H2S or in some cases from organic compounds.
3. The phototrophic bacteria develop internal sacs and lamellae that are functionally equivalent to the thylakoid membrane of chloroplasts (Figure 10.2).
10.2 Molecules absorb light through activation of their electrons.
a. Why are leaves green?
1. Natural, white light is a mixture of all colors.
2. Pigments absorb part of the light and reflect the rest. We see the colors of the light that they reflect.
3. Leaves absorb red and blue light, so what we see is mostly green light; hence, leaves appear to be green.
b. Light is one form of electromagnetic (EM) radiation.
1. Light is energy that behaves like both a particle and an oscillating wave.
2. As a particle, each unit of EM radiation is called a photon.
3. The frequency and wavelength are related inversely such that when one increases, the other decreases. This is represented by c = ln, where c = velocity (3 x 108 m/sec in a vacuum), n = vibrations per second, and l = wavelength.
4. The energy of EM radiation is proportional to its frequency, and the EM spectrum ranges from very high-energy, short-wave gamma rays and X-rays through visible light of intermediate wavelengths, to low-energy, long-wavelength radio waves (Figure 10.3).
c. EM radiation of different energies interacts with matter in characteristic ways.
1. Microwaves, with wavelengths in the range of 1 mm to 1 meter, make molecules move and vibrate faster, so people use them to cook food.
2. Infrared radiation, with wavelengths of 2—15 µm, has only enough energy to make the bonds between atoms stretch and bend. Infrared radiation is commonly experienced as heat.
3. Radiation in both the visible light range (about 380—760 nm) and the ultraviolet range (about 200—380 nm) has just enough energy to make electrons jump from one level to another in an atom or molecule (Figure 10.4).
d. White light is a mixture of photons with all the energies in the visible range.
1. Every substance absorbs light with a characteristic series of wavelengths, as displayed in an absorption spectrum that shows the light it absorbs at each wavelength (Figure 10.5).
2. The absorption spectrum of chlorophyll a has strong peaks at 680—700 nm (red light) and at 430 nm (blue light).
3. Absorption spectra are used to identify substances.
4. In photosynthesis, an excited electron in chlorophyll is drawn away by a nearby molecule and started on its path through a system of electron carriers that make ATP.
5. When a material emits light of a slightly longer wavelength than the light it absorbs, fluorescence is the result.
10.3 Chlorophylls are the major pigments used in photosynthesis.
a. Most of the light that drives photosynthesis is absorbed by chlorophylls (Figure 10.6).
1. All eucaryotic phototrophs have chlorophyll a (abbreviated Chl a), plus smaller amounts of either Chl b (in green algae and plants) or Chl c (in chromists such as golden-brown and brown algae).
2. All phototrophs also use secondary pigments, especially carotenoids, which include carotenes and more oxidized and modified molecules called xanthophylls (Figure 10.6).
3. While chlorophyll absorbs red and blue light, carotenoids absorb maximally in the blue-violet range, so we see the yellow-orange-red light they don't absorb.
4. Carotenoids are very similar to retinal, the major visual pigment that absorbs light in the eye (Figure 10.6).
b. An action spectrum shows the rate of some process at each wavelength of light and tells whether light of a particular wavelength is actually used.
1. The action spectrum for photosynthesis is broadly similar to the absorption spectra of the chlorophylls plus carotenoids, showing that light absorbed by both kinds of pigments contributes to photosynthesis (Figure 10.7).
2. In 1882 Theodor Engelmann, a German plant physiologist, conducted several simple experiments that demonstrated the action spectrum of photosynthesis (Figures 10.8).
10.4 Photosynthesis requires a reducing agent, which is generally water.
a. In the beginning of the nineteenth century, scientists worked to understand the chemistry behind photosynthesis.
1. In 1931, C. B van Niel confirmed that bacteria depend on light and that they oxidize hydrogen sulfide (H2S) and other reduced sulfur compounds to elemental sulfur and sulfate, instead of producing oxygen.
2. C. B van Niel proposed that there were several kinds of photosynthesis with one common feature: using light to split a reduced compound, H2X. The X is a by-product and the H atoms combine with CO2 to make carbohydrates.
3. In 1937, Robert Hill showed that illuminated chloroplasts produce oxygen if they are simply given an appropriate electron acceptor.
4. The Hill reaction demonstrated that illuminated chloroplasts can generate reducing power through the light-driven splitting (photolysis) of water.
10.5 Two photosystems cooperate in plant photosynthesis.
a. The light-dependent reactions of photosynthesis, as they occur in a plant, consist of three main events.
1. The primary event of photosynthesis occurs when a chlorophyll molecule absorbs a photon of light, thus exciting one of its electrons.
2. A primary electron acceptor removes the excited electron, so it becomes negative and leaves a positively charged chlorophyll molecule. By creating positive and negative centers through this process of charge separation, the system has made a minute battery and an energized electron capable of moving from one center to the other.
3. ATP synthesis in photosynthesis is called photophosphorylation.
b. The photosynthetic pigments and proteins of the thylakoid membrane are organized into complexes called photosystems (PS): PS I and PS II.
1. The Emerson enhancement effect explains that photosynthesis requires the interaction of two light reactions.
2. PS I absorbs most of the far-red light while PS II absorbs most of the shorter-wavelength light.
3. Each photosystem is organized around a photosynthetic unit of about 200—300 chlorophyll molecules, packed together in a single, large, light-collecting structure; this is called an antenna complex (Figure 10.9).
4. A reaction center is a unique place where energy can be trapped. The reaction-center molecules are identified at P700 in PS I and P680 in PS II. Each reaction center is a special pair of chlorophyll molecules associated with certain proteins and electron carriers.
10.6 Cyclic photophosphorylation creates only ATP.
a. The process of cyclic photophosphorylation, which is restricted to PS I, is a mechanism for converting light energy to chemical energy (Figure 10.10).
1. When a photon of light excites an electron of a chlorophyll molecule, the energy is passed on to the reaction center, P700, designated P700+ in its excited state.
2. Light effectively acts like a pump to raise electrons to a high enough potential to initiate the whole process, by giving P700 the energy to reduce the chain of electron carriers.
3. This system is therefore very similar to the oxidative phosphorylation system of mitochondria, including a similar ATP synthase, but here the protons are pumped to the inside of the membrane instead of the outside.
10.7 Noncyclic photophosphorylation creates both ATP and NADPH.
a. In addition to producing ATP, chloroplasts must produce a store of reducing power in the form of NADPH.
1. The reduction of NADP+ to NADPH requires a noncyclic pathway and the absorption of a second photon of light by photosystem II.
2. Reducing P700+ requires an electron from an external source and the energy of a second photon of light, and PS II supplies both.
3. P680, the reaction-center molecule in PS II, absorbs a photon of light and is activated.
4. P680 is oxidized to P680+ as it donates an electron to a noncyclic pathway of electron carriers (Figure 10.11).
5. Electron transport through the noncyclic pathway, from one electron carrier to another, ferries protons across the thylakoid membrane, adding to the proton gradient that drives ATP synthesis (Figure 10.12).
6. ATP synthesis through this pathway is called noncyclic photophosphorylation.
b. For contrast, Figure 10.13 shows the photosynthetic apparatus of purple sulfur bacteria, which produce both ATP and NADH with only a single photosystem.
1. The pathway of electron transport is essentially cyclic, with the creation of a chemiosmotic potential that generates ATP.
2. The component that accepts the excited electron from the reaction center does not have a high enough potential to reduce NAD+ directly, so NADH is formed through a partial reversal of electron transport.
10.8 CO2 is reduced to organic compounds in the Calvin cycle.
a. The light-independent reactions, which reduce CO2 to organic compounds, are used by all autotrophs, including the chemoautotrophs that generate ATP and NADPH by oxidizing inorganic materials.
1. Melvin Calvin and his associates tried to determine the pathway of carbon dioxide reduction to sugar by using radioactive CO2 (Figure 10.14).
2. They were able to determine that CO is incorporated through a complex cycle of reactions, now called the Calvin cycle or photosynthetic carbon reduction cycle (PCR).
3. The use of the abbreviation PCR should not be confused with polymerase chain reaction which is also often abbreviated as PCR.
b. Figures 10.15 and 10.16 highlight the key reactions of the Calvin cycle.
c. The synthesis of one molecule of sugar requires 6 CO2, 18 ATP, and 12 NADPH.
1. Phototrophs have an enormous potential source of energy in sunlight.
2. Photosynthesis is a highly efficient process that supplies energy and materials for the cells that are actually photosynthesizing.
3. A whole plant contains nonphotosynthetic parts such as roots, and to supply their needs the photosynthetic cells produce the disaccharide sucrose, which is carried away in the plant sap.
4. These sugars may be used in other tissues in respiration, or they may be stored as starch.
10.9 Some plants use an alternative pathway for CO2 fixation.
a. M. D. Hatch and C. R. Slack's demonstration of a new pathway of CO fixation came as a surprise in the 1960s.
1. Photosynthesis that uses the Calvin cycle generates a C3 product and is called C3 photosynthesis.
2. The Hatch-Slack pathway generates C4 compounds first and is called C4 photosynthesis (Figure 10.17).
3. C4 photosynthesis is used by at least a hundred species of plants, primarily those that grow in hot, dry conditions.
4. Most plants that conduct C4 photosynthesis have kranz anatomy, with a layer of bundle-sheath cells surrounding the veins that carry materials in and out of the leaf.
5. The mesophyll cells transport the malate they have made into the bundle-sheath cells, where it is decarboxylated; the resulting CO2 is used to operate the standard Calvin cycle.
b. C4 photosynthesis allows C4 plants, such as corn and sugar cane, to grow very well in hot climates (Figure 10.18).
1. All vascular plants exchange gasses through pores in their leaves known as stomata, which admit enough CO2 to support photosynthesis, while allowing O2 to escape.
2. Plants also maintain a continuous flow of water, which evaporates through the stomata, and when water is limited the stomata close, as a water-conservation mechanism.
3. When the stomata close, the supply of CO2 is cut off and the supply of O2 increases. The higher levels of O2 stimulate photorespiration and wastes energy.
4. C4 plants avoid the problem of photorespiration, though their cost of fixing one CO2 can be raised from 3 ATPs to 5 ATPs depending on the temperature and CO2 concentration.
5. C4 plants make excellent use of the CO2 they can obtain with limited stomatal openings, and they transfer this CO2 to the bundle-sheath cells, where the CO2/O2 ratio can be kept high and where classical C3 photosynthesis can go on uninhibited.
6. Studies in N. America, Australia, and Europe have shown that the percentage of C4 plants increases with the minimum temperature during the growing season and with aridity (Figure 10.19).
c. Many plants use a variation on C4 metabolism called crassulacean acid metabolism (CAM) because it is common on cacti and similar fleshy-leaved plants of the family Crassulaceae, such as the kalanchoes often grown as house plants.
1. The plant opens its stomata at night, taking in CO2 and fixing it with a C4 process to make malic acid and some related acids.
2. During the day the plant closes its stomata and the accumulated acids are decarboxylated to make CO2, which is incorporated via the common C3 pathway.
3. CAM differs from C4 photosynthesis in two ways: it is correlated with a daily stomatal cycle, and each cell accumulates the C4 acids in its vacuole and then converts them with C3 photosynthesis in its chloroplasts rather than transporting these acids to another cell.
4. CAM can be induced by environmental conditions such as water-limits or salt-stress.
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