Chapter 08 Lecture Outline See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8.1 Overview of Photosynthesis Photosynthesis converts solar energy into chemical energy of carbohydrates Organisms that carry on photosynthesis are called autotrophs Plants, algae, and cyanobacteria are organisms capable of photosynthesis 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 Most photosynthetic organisms contain the pigment chlorophyll Another common pigment group are carotenoids

Flowering Plants as Photosynthesizers Photosynthesis occurs in the green parts of plants Particularly leaves, contain chlorophyll and other pigments Leaves contain mesophyll tissue specialized for photosynthesis Raw materials are water and CO2

8.1 Overview of Photosynthesis Water is taken up by roots and transported to leaves by veins Carbon dioxide enters through openings in the leaves called stomata Light energy is absorbed by chlorophyll and other pigments in thylakoids of chloroplasts

8.1 Overview of Photosynthesis Chloroplast structure The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane Thylakoids are a different membrane system within the stroma that form flattened sacs Thylakoids are stacked together to from grana Thylakoid space is formed by a continuous connection between individual thylakoids

Figure 8.2 cuticle Leaf cross section upper epidermis mesophyll lower Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cuticle Leaf cross section upper epidermis mesophyll lower epidermis CO2 O2 Leaf vein stomata Figure 8.2

© Dr. George Chapman/Visuals Unlimited 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 Grana channel between thylakoids © Dr. George Chapman/Visuals Unlimited Figure 8.2

© Dr. George Chapman/Visuals Unlimited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. cuticle Leaf cross section upper epidermis mesophyll lower epidermis CO2 O2 leaf vein stomata inner membrane outer membrane stroma stroma granum Chloroplast Chloroplast, micrograph 37,000x thylakoid space thylakoid membrane Grana Figure 8.2 channel between thylakoids © Dr. George Chapman/Visuals Unlimited

Photosynthetic Reaction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glucose and oxygen are the products of photosynthesis The oxygen given off comes from water CO2 gains hydrogen atoms and becomes a carbohydrate solar energy CO2 + 6 H2O C6H12O6 + 6 O2 pigments

Two Sets of Reactions The two sets of reactions are called the: Photosynthesis consists of two sets of reactions Photo refers to capturing solar energy Synthesis refers to producing a carbohydrate The two sets of reactions are called the: Light Reactions (light-dependent) Calvin Cycle Reactions (light-independent) Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions

Figure 8.3 H2O CO2 solar energy ADP + P NADP+ Calvin cycle reactions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy ADP + P NADP+ Calvin cycle reactions Light reactions NADPH ATP thylakoid membrane stroma O2 CH2O Figure 8.3

8.2 Plants as Solar Energy Converters During the light reactions, different pigments within the thylakoid membranes absorb energy Solar energy can be described in terms of its wavelength and energy content

8.2 Plants as Solar Energy Converters The electromagnetic spectrum extends from very short gamma rays to very long radio waves White or visible light is only a small portion of the spectrum Visible light is further divided into wavelengths between 380 and 750 nm Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Increasing wavelength Increasing energy Gamma rays Micro- waves Radio waves X rays UV Infrared visible light 380 500 600 750 Wavelengths (nm) Figure 8.4

Visible Light Visible light contains various wavelengths The colors of visible light range from: Violet light Shortest wavelength but high energy Red light Longest wavelength but lowest energy Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range Higher wavelengths are screened by the ozone layer

Visible Light Most photosynthetic pigments in cells are chlorophylls a and b and the carotenoids Can absorb specific various portions of visible light The absorption spectrum shown in figure on the right Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chlorophyll a Chlorophyll b carotenoids Relative Absorption 380 500 600 750 Wavelengths (nm) Figure 8.5

Visible Light 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

Light Reactions Light Reactions Take 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 produces NADPH

Noncyclic Electron Pathway Noncyclic electron pathway, named because electron flow is traced from water to NADP+ Uses two photosystems (Photosystems I and II) A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane The pigment complex can be described as a “antenna” for gathering solar energy

Noncyclic Electron Pathway Noncyclic Electron Pathway begins with photosystem II (PSII) Pigment complex absorbs solar energy Energy passes from one pigment to another until it is concentrated in reaction center Chlorophyll a molecule Electrons in the reaction center chlorophyll become so energized Escape from the reaction center and move to a nearby electron acceptor

Noncyclic Electron Pathway Photosystem II would disintegrate without replacement electrons Electrons provided by splitting water Releases oxygen (O2) to atmosphere which benefits all organisms that use O2 Hydrogen ions (H+) stay in the thylakoid space Contribute to formation of hydrogen ion gradient

Noncyclic Electron Pathway In PSII, an electron acceptor receives energized electrons from the reaction center It sends those electrons down an electron transport chain, (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

Noncyclic Electron Pathway PSI comes next in noncyclic electron pathway When the photosystem I complex absorbs solar energy, energized electrons leave 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

Calvin cycle reactions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sun sun electron acceptor electron acceptor energy level e– e– e– e– NADP+ electron transport chain H+ e– e– NADPH reaction center reaction center pigment complex pigment complex Photosystem I e– Photosystem II H2O CO2 CH2O Calvin cycle reactions Figure 8.6 2H+ 1 2 – O2

Calvin cycle reactions Calvin cycle reactions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy sun ADP + P sun NADP+ Calvin cycle reactions cycle Light reactions NADPH ATP electron acceptor electron acceptor thylakoid membrane O2 CH2O energy level e– e– e– e– NADP+ electron transport chain H+ e– e– NADPH reaction center reaction center pigment complex pigment complex Photosystem I e– Photosystem II H2O CO2 CH2O Calvin cycle reactions Figure 8.6 2H+ 1 2 – O2

Cyclic Electron Pathway Uses only photosystem I (PSI) and 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

Calvin cycle reactions and other enzymatic reactions Energized electrons leave the photsytem I reaction center and return to photosystem by an electron transport chain ATP from cyclic electron transport used in Calvin cycle to make carbohydrates Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sun electron acceptor energy level ATP electron e– transport e– chain CO2 CH2O reaction center Calvin cycle reactions and other enzymatic reactions Pigment complex Photosystem I Figure 8.7

The Organization of the Thylakoid Membrane The following molecular complexes are present in the thylakoid Membrane: PS II Pigment complex and electron acceptors Water is split to replace energized electrons Oxygen (O2) is released Electron transport chain Carries electrons from PS II to PS I Uses energy to pump H+ from the stroma into thylakoid space

The 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. photosystem II electron transport chain H+ photosystem I NADP reductase H+ Pq e– e– e– NADP+ NADPH e– e– H+ H+ H2O 2 H+ + 1 2 O2 A T P synthase H+ H+ complex A TP thylakoid space H+ H+ chemiosmosis P + ADP Stroma Figure 8.8

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. thylakoid membrane thylakoid thylakoid space granum photosystem II electron transport chain H+ stroma photosystem I NADP reductase H+ Pq e– e– e– NADP+ NADPH e– e– H+ H+ H2O 2 H+ + 1 2 O2 A T P synthase H+ H+ complex A TP thylakoid space H+ H+ chemiosmosis P + ADP Stroma Figure 8.8

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy ADP + P NADP+ Calvin cycle reactions Light reactions NADPH ATP thylakoid membrane thylakoid membrane thylakoid thylakoid space O2 CH2O granum photosystem II electron transport chain H+ stroma photosystem I NADP reductase H+ Pq e– e– e– NADP+ NADPH e– e– H+ H+ H2O 2 H+ + 1 2 O2 A T P synthase H+ H+ complex A TP thylakoid space H+ H+ chemiosmosis P + ADP Stroma Figure 8.8

ATP Production ATP Production Thylakoid space acts as a reservoir for hydrogen ions (H+) H+ from water being split within thylakoid space 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

8.3 Plants as Carbon Dioxide Fixers The Calvin Cycle (named after Melvin Calvin) Series of reactions that use CO2 from the atmosphere to produce carbohydrate Humans other most other organisms take in O2 and release CO2 Includes Carbon dioxide fixation Carbon dioxide reduction Ribulose-1,5-bisphosphate (RuBP) regeneration

Metabolites of the Calvin Cycle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H2O CO2 solar energy ADP+ P NADP+ Calvin cycle Light reactions NADPH Metabolites of the Calvin Cycle ATP RuBP ribulose-1,5-bisphosphate 3PG 3-phosphoglycerate 3 CO2 stroma BPG 1,3-bisphosphoglycerate O2 CH2O intermediate G3P glyceraldehyde-3-phosphate 3 C6 6 3PG C3 3 RuBP C5 CO2 fixation 6 ATP CO2 reduction These ATP and NADPH molecules were produced by the light reactions. Calvin cycle 3ADP + 3 P 6ADP + 6 P regeneration of RuBP 6 BPG C3 These ATP molecules were produced by the light reactions. 3 ATP 5 G3P C3 6 NADPH 6 G3P C3 6 NADP+ x 2 net gain of one G3P Other organic molecules Glucose

Fixation of Carbon Dioxide Carbon dioxide fixation is the 1st step of the Calvin cycle CO2 is attached to 5-carbon RuBP molecule This reaction occurs three times The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG) RuBP Carboxylase is the enzyme that makes this happen Comparatively slow enzyme so there is a lot of it

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+.

Regeneration of RuBP Regeneration of RuBP It takes three turns of the Calvin cycle to allow one G3P to exit For every three turns of Calvin Cycle, five G3P (3-carbon molecule) used This re-forms three RuBP (5-carbon molecule) 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

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)

Importance of the Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules These molecules meet the plant needs 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 Photosynthesis C3 Photosynthesis The leaves of C3 plants have a different structure and means of fixing CO2 than C4 plants C3 plants such as wheat, rice, oats have mesophyll cells of leaves in parallel layers Bundle sheath cells around the plant veins do not contain chloroplasts As a result, cells using Calvin cycle exposed to CO2

C3 Photosynthesis RuBP carboxylase binds O2 as well as CO2 When bound to O2, the enzyme undergoes photorespiration Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water

© The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 RuBP mesophyll cells Calvin cycle 3PG (C3) G3P vein bundle sheath cell stoma mesophyll cell a. C3 Plant a. CO2 fixation in a C3 plant, tuplip © The McGraw-Hill Companies, Inc./Evelyn Jo Johnson, photographer Figure 8.11 Figure 8.10

C4 Photosynthesis C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells They also contain chloroplasts In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2 CO2 enters the Calvin cycle

C4 Photosynthesis C4 Pathway C4 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 O2 in C4 plants and photorespiration does not occur Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the leaf air spaces

Figure 8.11 Figure 8.10 CO2 mesophyll cell C4 bundle CO2 sheath cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 mesophyll cell C4 bundle sheath cell CO2 mesophyll cells Calvin cycle vein bundle sheath cell G3P stoma b. CO2 fixation in a C4 plant, corn b. C4 Plant © Corbis RF Figure 8.11 Figure 8.10

C4 Photosynthesis When the weather is moderate, C3 plants ordinarily have the advantage. When the weather is hot and dry, C4 plants have the advantage, and can be expected to predominate. In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over.

CAM Photosynthesis CAM Pathway This pathway is prevalent among most succulent plants that grow in deserts, including the cacti. CAM plants partition carbon fixation according to time. During the night, CAM plants fix CO2, forming C4 molecules. The C4 molecules are stored in large vacuoles. During daylight, C4 molecules release CO2 to the Calvin cycle.

© S. Alden/PhotoLink/Getty RF Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO2 night C4 day CO2 Calvin cycle G3P c. CO2 fixation in a CAM plant, pineapple Figure 8.10 © S. Alden/PhotoLink/Getty RF

8.5 Photosynthesis Versus Cellular Respiration Both plant and animal cells carry out cellular respiration. Occurs in mitochondria Breaks glucose down Utilizes O2 and gives off CO2 Plant cells photosynthesize, but animal cells do not. Occurs in chloroplasts Builds glucose Utilizes CO2 and gives off O2 Both processes utilize an electron transport chain and chemiosmosis for ATP production.

Calvin cycle reactions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP Thylakoid membrane H2O O2 H2O CO2 solar energy ADP + P NADP+ Calvin cycle reactions Light reactions NADPH ATP stroma thylakoid membrane O2 CH2O NADPH NADP+ Stroma CO2 CH2O Figure 8.12 Photosynthesis

Electron transport chain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP Cristae O2 H2O NADH+H+ e– NADH+H+ e– e– e– NADH+H+ and FADH2 e– e– Glycolysis Preparatory reaction Citric acid cycle Electron transport chain glucose pyruvate 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 NAD+ NADH Matrix CH2O CO2 Cellular Respiration Figure 8.12

Calvin cycle reactions Electron transport chain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP ADP ATP Thylakoid membrane H2O O2 Cristae O2 H2O H2O CO2 solar energy NADH+H+ e– NADH+H+ e– e– e– NADH + H+ and FADH2 e– ADP + P NADP+ e– Calvin cycle reactions Glycolysis Preparatory reaction Citric acid cycle Electron transport chain glucose pyruvate Light reactions NADPH ATP 2 ATP stroma 2 ADP thylakoid membrane 4 ADP 4 ATP total O2 CH2O 2 ATP net gain 2 ADP 2 ATP 32 ADP 32 or 34 or 34 ATP NADPH NADP+ NAD+ NADH Stroma Matrix CO2 CH2O CH2O CO2 Photosynthesis Cellular Respiration Figure 8.12