Chapter 8 Photosynthesis Lecture Outline Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1
8.1 Overview of Photosynthesis Photosynthesis converts solar energy into chemical energy Organisms that carry on photosynthesis are called autotrophs Heterotrophs are organisms that feed on other organisms
8.1 Overview of Photosynthesis Autotrophs and heterotrophs use organic molecules produced by photosynthesis Pigments allow photosynthetic organisms to capture solar energy
8.1 Overview of Photosynthesis Flowering Plants as Photosynthesizers Photosynthesis occurs in the green parts of plants Particularly leaves Leaves contain mesophyll tissue specialized for photosynthesis Water is taken up by roots and transported to leaves by veins
8.1 Overview of Photosynthesis Carbon dioxide enters through openings in the leaves called stomata Light energy is absorbed by chlorophyll and other pigments in thylakoids of chloroplasts
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cuticle Leaf cross section upper epidermis mesophyll CO 2 lower epidermis leaf vein inner membrane outer membrane O 2 stomata stroma stroma granum Chloroplast Chloroplast, micrograph 37,000x thylakoid space thylakoid membrane Grana channel between thylakoids Dr. George Chapman/Visuals Unlimited
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cuticle Leaf cross section upper epidermis mesophyll CO 2 lower epidermis Leaf vein O2 stomata
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. inner membrane outer membrane stroma stroma granum Chloroplast Chloroplast, micrograph 37,000x thylakoid space thylakoid membrane channel between thylakoids Dr. George Chapman/Visuals Unlimited Grana
8.1 Overview of Photosynthesis Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. solar energy CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 Glucose and oxygen are the products of photosynthesis The oxygen given off comes from water CO 2 gains hydrogen atoms and becomes a carbohydrate
8.1 Overview of Photosynthesis Photosynthesis consists of two sets of reactions Photo refers to capturing light Synthesis refers to producing a carbohydrate The two sets of reactions are called the: Light Reactions (light-dependent) Calvin Cycle Reactions (light-independent)
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H 2 O CO 2 solar energy ADP + P Light reactions NADP + NADPH ATP Calvin cycle reactions thylakoid membrane stroma O 2 CH 2 O
8.2 Solar Energy Capture Solar energy can be described in terms of its wavelength and its energy content White or visible light is only a small portion of the spectrum 380 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gamma rays Increasing wavelength Increasing energy X rays UV Infrared visible light Microwaves Radio waves 500 600 750 Wavelengths (nm)
8.2 Solar Energy Capture Visible Light Contains various wavelengths Violet light Shortest wavelength but high energy Red light Longest wavelength but lowest energy Only about 42% of the solar radiation that hits Earth s atmosphere ever reaches the surface of Earth most in the visible-light range
Relative Absorption 8.2 Solar Energy Capture Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The photosynthetic pigments in chlorophylls a and b and the carotenoids can absorb specific portions of visible light Chlorophyll a Chlorophyll b carotenoids 380 500 600 750 Wavelengths (nm)
8.2 Solar Energy Capture Green light is reflected and only minimally absorbed Leaves appear green Other plant pigments become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered
8.2 Solar Energy Capture Light Reactions Takes place in thylakoid membrane Light reactions consist of two pathways: Noncyclic electron pathway Cyclic electron pathway Both pathways transform solar energy to chemical energy Both pathways produce ATP Only the noncyclic pathway also produces NADPH
8.2 Solar Energy Capture Noncyclic Electron Pathway Electron flow can be traced from water to a molecule of NADP + Uses two photosystems A photosystem consists of a pigment complex and electron acceptor molecules within the thylakoid membrane The pigment complex can be described as a antenna for gathering solar energy
8.2 Solar Energy Capture Noncyclic Electron Pathway begins with photosystem II (PSII) Pigment complex absorbs solar energy Passes from one pigment to another until it is concentrated in reaction center Chlorophyll a molecule Electrons (e - ) in the reaction center chlorophyll become so energized that they escape from the reaction center and move to a nearby electron acceptor
8.2 Solar Energy Capture Photosystem II would disintegrate without replacement electrons Provided by splitting water Releases oxygen (O 2 ) to atmosphere Hydrogen ions (H + ) stay in the thylakoid space Contribute to formation of hydrogen ion gradient
8.2 Solar Energy Capture In PSII, the electron acceptor that received the energized electrons from the reaction center sends those electrons down an electron transport chain, a series of carriers that pass electrons from one to the other Energy is released to pump hydrogen ions (H + ) into thylakoid space forming gradient When hydrogen ions flow through ATP synthase it makes ATP
8.2 Solar Energy Capture PSI comes next in noncyclic electron pathway When the photosystem I pigment complex absorbs solar energy, energized electrons leave its reaction center and are captured by a different electron acceptor Low energy PSII electrons used to replace those lost by PSI Electron acceptor in photosystem I passes its electrons to NADP + and it becomes NADPH
energy level Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sun sun electron acceptor electron acceptor e e e e NADP + H + e e NADPH reaction center pigment complex e H 2 O 2H + 2 O 2 Photosystem II 1 Photosystem I reaction center pigment complex CO 2 CH 2 O Calvin cycle reactions
energy level Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H 2 O CO 2 solar energy sun Light reactions ADP + P NADP + NADPH Calvin cycle reactions sun ATP electron acceptor thylakoid membrane O 2 CH 2 O electron acceptor e e e e NADP + H + e e NADPH reaction center pigment complex e H 2 O 2H + 2 O 2 Photosystem II 1 Photosystem I reaction center pigment complex CO 2 CH 2 O Calvin cycle reactions
8.2 Solar Energy Capture The Cyclic Electron Pathway Uses only photosystem I (PSI) Begins when PSI complex absorbs solar energy Energized electrons escape from the reaction center and travel down electron transport chain Released energy is stored in the form of a H + gradient, which causes ATP production by ATP synthase Spent electrons return to PSI (cyclic) Pathway only results in ATP production
energy level ATP from cyclic and noncyclic electron transport NADPH from cyclic electron transport Ready to be used in Calvin cycle to make carbohydrates Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. electron acceptor reaction center sun e e Photosystem I ATP Pigment complex CO 2 CH 2 O Calvin cycle reactions and other enzymatic reactions
8.2 Solar Energy Capture Organization of the Thylakoid Membrane PS II Pigment complex and electron acceptors Water is split to replace energized electrons Oxygen (O 2 ) is released Electron transport chain Carries electrons from PS II to PS I Uses energy to pump H + from the stroma into thylakoid space
8.2 Solar Energy Capture Organization of the Thylakoid Membrane PS I Pigment complex and electron acceptors Adjacent to enzyme that reduces NADP + to NADPH ATP synthase complex Has a channel for H + flow Flow drives ATP synthase to join ADP and P
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. photosystem II H + electron transport chain Pq photosystem I NADP reductase H + e e e NADP + NADPH e e H + H + H 2 O 2 + H + 1 2 O 2 H + H + A T P synthase complex thylakoid space H + A TP H + chemiosmosis P + ADP Stroma
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. granum thylakoid membrane thylakoid space thylakoid photosystem II H + electron transport chain stroma Pq photosystem I NADP reductase H + e e e NADP + NADPH e e H + H + H 2 O 2 + H + 1 2 O 2 H + H + A T P synthase complex thylakoid space H + A TP H + chemiosmosis P + ADP Stroma
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H 2 O CO 2 solar energy Light reactions ADP + P NADP + NADPH ATP Calvin cycle reactions thylakoid membrane O 2 CH 2 O granum thylakoid membrane thylakoid space thylakoid photosystem II H + electron transport chain stroma Pq photosystem I NADP reductase H + e e e NADP + NADPH e e H + H + H 2 O 2 + H + 1 2 O 2 H + H + A T P synthase complex thylakoid space H + A TP H + chemiosmosis P + ADP Stroma
8.2 Solar Energy Capture ATP Production Thylakoid space acts as a reservoir for hydrogen ions (H + ) H + from water being split Pumped in by electron transport chain More H + in thylakoid space than stroma Electrochemical gradient H + can only flow through ATP synthase Energy powers making ATP by chemiosmosis
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8.3 Calvin Cycle Reactions The Calvin Cycle Series of reactions that use CO 2 from the atmosphere to produce carbohydrate Includes Carbon dioxide fixation Carbon dioxide reduction Ribulose-1,5-bisphosphate (RuBP) regeneration
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolites of the Calvin Cycle RuBP ribulose-1,5-bisphosphate 3 CO 2 intermediate 3PG BPG G3P 3-phosphoglycerate 1,3-bisphosphoglycerate glyceraldehyde-3-phosphate 3 C 6 3 RuBP C 5 CO 2 fixation 6 3PG C 3 6 ATP Calvin cycle CO 2 reduction 6ADP + 6 P These ATP and NADPH molecules were produced by the light reactions. 6 BPG C 3 6 NADPH 6 G3P C 3 6 NADP + Other organic molecules net gain of one G3P x 2 Glucose
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolites of the Calvin Cycle RuBP ribulose-1,5-bisphosphate 3 CO 2 intermediate 3PG BPG G3P 3-phosphoglycerate 1,3-bisphosphoglycerate glyceraldehyde-3-phosphate 3 C 6 3 RuBP C 5 CO 2 fixation 6 3PG C 3 6 ATP 3ADP + 3 These A T P molecules were produced by the light reactions. P 3 ATP regeneration of RuB 5 G3P C 3 Calvin cycle 6 G3P C 3 CO 2 reduction 6 BPG C 3 6 NADPH 6 NADP + 6ADP + 6 P These ATP and NADPH molecules were produced by the light reactions. net gain of one G3P x 2 Other organic molecules Glucose
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H 2 O CO 2 solar energy Light reactions ADP+ P NADP + NADPH ATP Calvin cycle reactions Metabolites of the Calvin Cycle RuBP ribulose-1,5-bisphosphate O 2 CH 2 O stroma 3 CO 2 intermediate 3PG BPG G3P 3-phosphoglycerate 1,3-bisphosphoglycerate glyceraldehyde-3-phosphate 3 C 6 3 RuBP C 5 CO 2 fixation 6 3PG C 3 6 ATP 3ADP + 3 These A T P molecules were produced by the light reactions. P 3 ATP regeneration of RuB 5 G3P C 3 Calvin cycle 6 G3P C 3 CO 2 reduction 6 BPG C 3 6 NADPH 6 NADP + 6ADP + 6 P These ATP and NADPH molecules were produced by the light reactions. Other organic molecules net gain of one G3P x 2 Glucose
8.3 Calvin Cycle Reactions Fixation of Carbon Dioxide CO 2 is attached to 5-carbon RuBP molecule This results in a 6-carbon molecule that splits into two 3-carbon molecules (3PG) RuBP Carboxylase is the enzyme that makes this happen Comparatively slow enzyme so there is a lot of it
8.3 Calvin Cycle Reactions Reduction of Carbon Dioxide Each 3PG molecules undergoes reduction to G3P in two steps Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)
Reduction of Carbon Dioxide Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ATP ADP + P 3PG BPG G3P NADPH NADP + As 3PG becomes G3P ATP becomes ADP + P, and NADPH becomes NADP +.
8.3 Calvin Cycle Reactions Regeneration of RuBP RuBP used in CO 2 fixation must be replaced Every three turns of Calvin Cycle Five G3P (a 3-carbon molecule) used To re-make three RuBP (a 5-carbon molecule) 5 3 (carbons in G3P) = 3 5 (carbons in RuBP)
Regeneration of RuBP Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5 G3P 3 RuBP 3 ATP 3 ADP + P As five molecules of G3P become three molecules of RuBP, three molecules of ATP become three molecules of ADP + P. 5 3 (carbons in G3P) = 3 5 (carbons in RuBP)
8.3 Calvin Cycle Reactions Importance of the Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules The hydrocarbon skeleton of G3P can form: Fatty acids and glycerol to make plant oil Glucose phosphate (simple sugar) Fructose (+ glucose = sucrose) Starch and cellulose Amino acids
8.4 Alternate Pathways for C 3 photosynthesis Photosynthesis First detectable molecule following fixation is a 3-carbon molecule Wheat, rice, oats Mesophyll layers of leaves in parallel layers Bundle sheath cells around the plant veins do not contain chloroplasts
8.4 Alternate Pathways for C 3 photosynthesis Photosynthesis RuBP can also bind with oxygen Photorespiration Wasteful reaction because it uses oxygen and releases carbon dioxide, decreasing the overall efficiency of the enzyme Oxygen concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO 2 mesophyll cells RuBP Calvin cycle bundle sheath cell a. C 3 Plant vein stoma G3P mesophyll cell 3PG (C 3 ) a. CO 2 fixation in a C 3 plant, tuplip The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer
8.4 Alternate Pathways for C 4 Pathway Sugarcane and corn Photosynthesis Mesophyll cells are arranged in concentric rings around the bundle sheath cells, which also contain chloroplasts CO 2 is initially fixed into a four-carbon molecule The four-carbon molecules is later broken down into a three-carbon molecule and CO 2 CO 2 enters the Calvin cycle
8.4 Alternate Pathways for C 4 Pathway Photosynthesis C 4 plants tend to be found in hot, dry climates In these climates, stomata tend to close to conserve water Oxygen then builds-up in the leaves But, RuBP carboxylase is not exposed to this oxygen in C 4 plants and photorespiration does not occur Instead in C 4 plants, the carbon dioxide is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO 2 mesophyll cell C 4 mesophyll cells bundle sheath CO 2 cell Calvin bundle sheath cell b. C 4 Plant vein stoma cycle b. CO G3P 2 fixation in a C4 plant, corn Corbis RF
8.4 Alternate Pathways for Photosynthesis When the weather is moderate, C 3 plants ordinarily have the advantage But when the weather becomes hot and dry, C 4 plants have the advantage, and we can expect them to predominate In the early summer, C 3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C 4 plant, begins to take over
8.4 Alternate Pathways for Photosynthesis CAM Pathway Prevalent among most succulent plants that grow in deserts, including the cacti CAM plants partition carbon fixation by time During the night CAM plants fix CO 2 forming C 4 molecules, The C 4 molecules are stored in large vacuoles During daylight C 4 molecules release CO 2 to Calvin cycle
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. night CO 2 C 4 day CO 2 Calvin cycle G3P c. CO 2 fixation in a CAM plant, pineapple S. Alden/PhotoLink/Getty RF
8.5 Photosynthesis Versus Cellular Respiration Both plant and animal cells carry out cell respiration In mitochondria Breaks glucose down Utilizes O 2 and gives off CO 2 Only plant cells photosynthesize In chloropalsts Builds glucose Utilizes CO 2 and gives off O 2 Both processes utilize an electron transport chain and chemiosmosis for ATP production
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP Thylakoid membrane solar energy H 2 O O 2 H 2 O CO 2 Light reactions ADP + P NADP + NADPH ATP Calvin cycle reactions thylakoid membrane O 2 CH 2 O stroma Stroma NADPH NADP + CO 2 CH 2 O Photosynthesis
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP Cristae O 2 H 2 O NADH+H + e NADH+H + e e NADH+H + e and FADH 2 e e glucose Glycolysis pyruvate Preparatory reaction Citric acid cycle Electron transport chain 2 ATP 2 ADP 4 ADP 4 ATP total 2 ATP net gain 2 ADP 2 ATP 32 ADP 32 or 34 or 34 ATP Matrix NAD + CH 2 O CO 2 NADH Cellular Respiration
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP ADP ATP Thylakoid membrane H 2 O O 2 Cristae O 2 H 2 O H 2 O CO 2 solar energy Light reactions ADP + P NADP + NADPH Calvin cycle reactions Glycolysis glucose pyruvate NADH+H + e Preparatory reaction e NADH+H + e Citric acid cycle NADH + H + and FADH 2 e e e Electron transport chain ATP thylakoid membrane O 2 CH 2 O stroma 2 ATP 2 ADP 4 ADP 4 ATP total 2 ATP net gair 2 ADP 2 ATP 32 ADP 32 or 34 or 34 ATP Stroma NADPH NADP + CO 2 CH 2 O Matrix NAD + NADH CH 2 O CO 2 Photosynthesis Cellular Respiration