CH 8: Photosynthesis Overview Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic from C and other inorganic Plants are photoautotrophs, using the energy of sun to make organic Heterotrophs obtain their organic material from other organisms Heterotrophs are the consumers of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most everything else (a) Plants (d) Cyanobacteria Concept 8.: Photosynthesis converts energy to the chemical energy of food The structural organization of photosynthetic cells includes enzymes and other grouped together in a Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria (c) Unicellular eukaryotes (b) Multicellular alga (e) Purple sulfur bacteria Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf Each mesophyll cell contains 0 0 chloroplasts C enters and exits the leaf through microscopic pores called stomata The chlorophyll is in the s of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense interior fluid Granum Stroma Mesophyll Leaf cross section Chloroplasts Stomata Vein C Chloroplast Mesophyll cell Outer Inter Inner 20 µm µm
The Splitting of Water Photosynthesis is a complex series of reactions that can be summarized as the following equation 6 C 2 energy C 6 6 6 6 Chloroplasts split into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar and releasing oxygen as a by-product Reactants: 6 C 2 Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which is oxidized and C is reduced Photosynthesis is an endergonic process; the energy boost is provided by becomes reduced becomes oxidized Products: C 6 6 6 6 The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the reactions (the photo part) and cycle (the synthesis part) The reactions (in the thylakoids) Split Release Reduce the electron,, to Generate from by adding a phosphate group, photophosphorylation Synthesis The cycle (in the stroma) forms sugar from C, using and The cycle begins with carbon fixation, incorporating C into organic Video: Photosynthesis Figure 8.5 The Nature of Sun C is a form of electromagnetic energy, also called electromagnetic radiation Chloroplast Reactions P i [C] (sugar) Like other electromagnetic energy, travels in waves - Wavelength is the distance between crests of waves Wavelength determines the type of electromagnetic energy The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible consists of wavelengths that produce colors we can see also behaves as though it consists of discrete particles, called photons 2
Figure 8.6 0 5 nm 0 nm nm 0 nm 0 6 nm Gamma rays X-rays Shorter wavelength Higher energy UV Infrared Visible Microwaves m (0 9 nm) 0 m Radio waves 80 50 500 550 600 650 700 750 nm Longer wavelength Lower energy Photosynthetic s: The Receptors s are substances that absorb visible Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green Chloroplast Absorbed Transmitted Granum Reflected Figure 8.8 Technique A spectrophotometer measures a pigment s ability to absorb various wavelengths White Refracting prism Chlorophyll solution 2 Photoelectric tube Galvanometer This machine sends through pigments and measures the fraction of transmitted at each wavelength Slit moves to pass of selected wavelength. Green The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green. Blue The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue. An absorption spectrum is a graph plotting a pigment s absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red work best for photosynthesis Accessory pigments include chlorophyll b and a group of pigments called carotenoids An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process Figure 8.9 Results Absorption of by chloroplast pigments Rate of photosynthesis (measured by release) Chlorophyll a Chlorophyll b Carotenoids 00 500 600 700 Wavelength of (nm) (a) Absorption spectra 00 500 600 700 (b) Action spectrum Aerobic bacteria Filament of alga 00 500 600 700 (c) Engelmann s experiment
The action spectrum of photosynthesis was first demonstrated in 88 by Theodor W. Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess He used the growth of aerobic bacteria clustered along the alga as a measure of production Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis A s structural difference between chlorophyll a and chlorophyll b causes them to absorb sly different wavelengths Accessory pigments called carotenoids absorb excessive that would damage chlorophyll Figure 8.0 CH CH in chlorophyll a CHO in chlorophyll b Excitation of Chlorophyll by Porphyrin ring: -absorbing head of molecule; note magnesium atom at center When a pigment absorbs, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off and heat Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid s of chloroplasts; H atoms not shown Figure 8. Energy of electron e Chlorophyll molecule Excited state (fluorescence) Ground state Heat A Photosystem: A Reaction-Center Complex Associated with -Harvesting Complexes A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by -harvesting complexes The -harvesting complexes (pigment bound to proteins) transfer the energy of photons to the reaction center (a) Excitation of isolated chlorophyll molecule (b) Fluorescence
Figure 8.2 Photosystem STROMA - Reaction- harvesting center electron complexes complex e - Transfer Special pair of of energy chlorophyll a THYLAKOID SPACE (INTERIOR OF THYLAKOID) Chlorophyll Protein subunits (a) How a photosystem harvests (b) Structure of a photosystem STROMA THYLAKOID SPACE A primary electron in the reaction center accepts excited electrons and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron is the first step of the reactions There are two types of photosystems in the thylakoid Figure 8.UN02 C functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 Reactions [C] (sugar) Linear Flow Linear electron flow involves the flow of electrons through both photosystems to produce and using energy Linear electron flow can be broken down into a series of steps. A photon hits a pigment and its energy is passed among pigment until it excites 2. An excited electron from is transferred to the primary electron (we now call it ). is split by enzymes, and the electrons are transferred from the hydrogen atoms to, thus reducing it to ; is released as a by-product. Each electron falls down an electron from the primary electron of PS II to PS I 5. Energy released by the fall drives the creation of a proton gradient across the thylakoid ; diffusion of H (protons) across the drives synthesis 6. In PS I (like PS II), transferred energy excites P700, causing it to lose an electron to an electron (we now call it P700 ) P700 accepts an electron passed down from PS II via the electron 7. Excited electrons fall down an electron from the primary electron of PS I to the protein ferredoxin (Fd) 8. The electrons are transferred to, reducing it to, and become available for the reactions of the cycle This process also removes an H from the stroma 5
Figure 8.- Figure 8.-2 2 H / 2 Figure 8.- Figure 8.- 2 H / 2 H 2O Pq Cytochrome complex 5 Pc 2 H / 2 H 2O Pq Cytochrome complex 5 Pc e P700 6 Photosystem I (PS I) Figure 8.-5 Figure 8. Mechanical analogy 2 H / 2 H 2O Pq Cytochrome complex 5 Pc e P700 7 Fd reductase 6 8 H Mill makes carriers Pq (plastoquinone) Pc (plastocyanin) Photosystem I (PS I) Photosystem I 6
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Figure 8.5 Chloroplasts and mitochondria generate by chemiosmosis but use different sources of energy Mitochondria transfer chemical energy from food to ; chloroplasts transform energy into the chemical energy of Different but similar In mitochondria, protons are pumped to the inter and drive synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the thylakoid and drive synthesis as they diffuse back into the stroma MITOCHONDRION STRUCTURE Key Inter Inner Matrix Higher [H ] Lower [H ] synthase H P i Diffusion H CHLOROPLAST STRUCTURE Stroma Figure 8.5a MITOCHONDRION STRUCTURE Inter Inner H Diffusion CHLOROPLAST STRUCTURE and are produced on the side facing the stroma, where the cycle takes place In and increase the potential energy of electrons by moving them from to synthase Matrix Key P i Higher [H ] H Lower [H ] Stroma Summary, reactions Figure 8.6 Reactions C H H 2O / O 2 2 THYLAKOID SPACE 2 H (high H concentration) Cytochrome complex Pq 2 H Pc Photosystem I Fd reductase H To STROMA (low H concentration) synthase P i H [C] (sugar) 7
Concept 8.: The cycle uses the chemical energy of and to reduce C to sugar The cycle, like the citric acid cycle, regenerates its starting material after enter and leave the cycle Unlike the citric acid cycle, the cycle is anabolic It builds sugar from smaller by using and the reducing power of electrons carried by Figure 8.UN0 Reactions C [C] (sugar) Carbon enters the cycle as C and leaves as a sugar named glyceraldehyde -phospate (GP) For net synthesis of one GP, the cycle must take place three times, fixing three of C The cycle has three phases Carbon fixation Reduction Regeneration of the C Phase, carbon fixation, involves the incorporation of the C into ribulose bisphosphate (RuBP) using the enzyme rubisco Phas, reduction, involves the reduction and phosphorylation of -phosphoglycerate to GP Phase, regeneration, involves the rearrangement of GP to regenerate the initial C receptor, RuBP GP =Glyceraldehyde -phosphate Figure 8.7- Input Rubisco as C Phase : Carbon fixation Evolution of Alternative Mechanisms of Carbon Fixation in Hot, Arid Climates Phase : Regeneration of RuBP P P RuBP 5 P GP P 6 P i 6 P P,-Bisphosphoglycerate 6 P GP Output 6 P -Phosphoglycerate 6 P GP P Glucose and other organic compounds 6 6 6 Phas: Reduction Adaptation to dehydration is a problem for land plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves but also limits photosynthesis The closing of stomata reduces access to C and causes to build up These conditions favor an apparently wasteful process called photorespiration 8
In most plants (C plants), initial fixation of C, via rubisco, forms a three-carbon compound (- phosphoglycerate) In photorespiration, rubisco adds instead of C in the cycle, producing a two-carbon compound Photorespiration decreases photosynthetic output by consuming,, and organic fuel and releasing C without producing any or sugar Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less and more C Photorespiration limits damaging products of reactions that build up in the absence of the cycle C Plants CAM Plants C plants minimize the cost of photorespiration by incorporating C into a four-carbon compound An enzyme in the mesophyll cells has a high affinity for C and can fix carbon even when C concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release C that is then used in the cycle Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating C into organic acids Stomata close during the day, and C is released from organic acids and used in the cycle Figure 8.8 The Importance of Photosynthesis: A Review The energy entering chloroplasts as sun gets stored as chemical energy in organic compounds C Sugarcane Mesophyll cell Bundlesheath cell Organic acid Sugar C Pineapple Organic acid Sugar C C 2 C 2 Night Day CAM Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic of cells Plants store excess sugar as starch in the chloroplasts and in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the in our atmosphere (a) Spatial separation of steps (b) Temporal separation of steps 9