Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PHOTOSYNTHESIS

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Process That Feeds the Biosphere Photosynthesis is the process that converts solar energy (light energy from the sun) into chemical energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Autotrophs sustain themselves without eating anything derived from other organisms Producers Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide (inorganic) Autotrophs – self feeders

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Producers of the biosphere Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also the entire living world

Examples of Producers Plants Unicellular protist Multicellular algaeCyanobacteria Purple sulfur bacteria 10 µm 1.5 µm 40 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterotrophs obtain their organic material from other organisms Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen Heterotrophs - Consumers

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Basics of Photosynthesis Chloroplasts: Sites of Photosynthesis

LE 10-3 Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm Photosynthesis takes place in the chloroplasts/found in several organisms 1.Leaves are the major location for photosynthesis 2.Green color is from chlorophyll- the pigment in chloroplasts – absorbs light energy 3.Through microscopic pores (stomata) CO2 enters and O2 leaves (stoma means “mouth”) 4.Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf. A typical mesophyll has chloroplasts 6. In the inner membrane encloses a second compartment which is filled with thick fluid called stroma. Suspended in the stroma are the thylakoids, which contains the thylakoids space. Thylakoids can be concentrated in stacks called grana 5.The chlorophyll is in the membranes of thylakoids

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Equation of Photosynthesis Photosynthesis can be summarized as the following equation: 6 CO 2 + 6H 2 O + Light energy  C 6 H 12 O O 2 What do you notice about this equation compared to respiration? Requires a lot of energy that comes from sunlight Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules, releasing oxygen.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis is a Redox Process, like cellular respiration Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced The redox process takes the form of hydrogen transfer from water to CO2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis Equation Water molecules are split apart, yielding O2, and are oxidized. They lose electrons along with hydrogen ions. CO2 is reduced to sugar as electrons and hydrogen ions are added to it. The food-producing redox reactions of photosynthesis involve an uphill climb The light energy captured by chlorophyll molecules in the chloroplast provides the boost for the electrons.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

LE 10-5_1 H2OH2O LIGHT REACTIONS Chloroplast Light Light energy is absorbed by chlorophyll and drives the transfer of electrons and hydrogen from water to the electron acceptor…. The Light Reactions – converts solar energy to chemical energy Take place in the thylakoids Split water

LE 10-5_2 H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 Stores energized electrons Electron acceptor is NADP+ Reduced to NADPH Oxygen is released ATP is formed NADPH is an electron carrier.

LE 10-5_3 H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar) The Calvin Cycle The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of ATP and NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Nature of Sunlight Sunlight is a form of electromagnetic energy, also called radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength = distance between crests of waves Light also behaves as though it consists of discrete particles, called photons

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Visible radiation The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis

LE 10-6 Visible light Gamma rays X-rays UVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm10 6 nm 1 m (10 9 nm) 10 3 m nm Longer wavelength Lower energy Shorter wavelength Higher energy The amount of energy in a photon is inversely related to its wavelength

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Animation: Light and Pigments Animation: Light and Pigments

LE 10-7 Chloroplast Light Reflected light Absorbed light Transmitted light Granum

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorophyll a is the main photosynthetic pigment – Participates directly in the light reactions Accessory pigments, such as chlorophyll b, – Absorbs mainly blue and orange light (reflects yellow-green) Broadens the range of light that a plant can use Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll – Absorb mainly blue-green light (reflect yellow- orange) Chloroplast Pigments

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excitation of Chlorophyll by Light Light behaves as discrete packets of energy called photons – A photon is a fixed quantity of light energy When a pigment absorbs a photon, the electron has been raised from a ground state to an excited state, which is very unstable When excited electrons fall back to the ground state, photons are given off (electrons lose energy), and emit an afterglow called fluorescence

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem consists of a reaction center surrounded by light- harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosystem A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

LE Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane Pigment molecules include chlorophyll a and b. Cluster of pigmented molecules functions as a light-gathering antenna. Photons jump from pigment to pigment and arrive at the reaction center The reaction center is a chlorophyll a molecule and a primary electron acceptor molecule Acceptor traps light excited electron from reaction center chlorophyll. Trapped energy converted to ATP and NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Photosystems in the light reactions Water Splitting (photosystem II) – Uses light energy to extract electrons from water – Releases O2 as a waste product NADPH producing (photosystem I) – Produces NADPH by transferring light- excited electrons from chlorophyll to NADP+ An electron transport chain connecting the two systems releases energy that the chloroplast uses to make ATP

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE ATP Water splitting system e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon NADPH producing system Photon NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Generating ATP in the Light Reactions Light reactions take place in the thylakoid membrane The mechanism of ATP production is similar to ATP production in cellular respiration An electron transport chain pumps hydrogen ions (H+) across the thylakoid membrane ATP synthases use the energy stored by the H+ gradient to make ATP

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Light Reactions Electrons are stored at a high state of potential energy in NADPH. This and ATP will move on to the Calvin Cycle

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review of light reactions

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. It requires more ATP than NADPH Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin Cycle The Calvin cycle has three phases: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor (RuBP) Takes place in the stroma Play

LE 10-18_1 [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input 3 CO 2 (Entering one at a time) Rubisco 3 P P Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP) CO2 is added to a five-C sugar. The reaction is catalyzed by the enzyme Rubisco splits into 2 molecules of

LE 10-18_2 [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3PP Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP 3 PP Ribulose bisphosphate (RuBP) 3 6 NADP NADPH P i 6P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds Each molecule of 3… is then phosphorylated by ATP to form 2 electrons from NADPH reduce this compound to create The cycle must turn 3 times to create a net gain of one molecule of G3P - glucose

LE 10-18_3 [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3PP Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 PP Ribulose bisphosphate (RuBP) 3 6 NADP NADPH P i 6P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds 3 3 ADP ATP Phase 3: Regeneration of the CO 2 acceptor (RuBP) P 5 G3P The rearrangement of five molecules of G3P into the 3 molecules of RuBP requires 3 more ATP 9 molecules of ATP and 6 of NADPH are required to synthesize one G3P

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Calvin Cycle review

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C4 and CAM plants All photosynthetic plants need CO2 to build sugar – CO2 binds to RuBP (in Calvin Cycle) – This is called the C3 pathway because it produces 2 – 3carbon molecules that continue in the cycle The enzyme that catalyzes this reaction is RUBISCO – Rubisco is not efficient at grabbing CO2 – If CO2 levels fall too low, rubisco grabs O2 and sugars are burned up. – This process is called photorespiration

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration - problems Problem during hot, dry days – or in these climates, plants keep stomates closed to prevent water loss Different systems evolved to deal with this problem – So stomates don’t need to be open too often

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparing C4 and CAM plants C4 During the day Takes place in mesophyll cells OAA pumped to bundle sheath (which are other cells) CO2 is then released to Calvin Cycle Ex. Corn, sugar cane, tropical grass CAM Happens at night OAA stored in vacuoles within the cell Stomates can be open in evening when cooler avoiding water loss CO2 released to Calvin Cycle in day and driven by suns energy More common: ex: cacti and succulents Each use a different enzyme to bind CO2 Harvest more CO2 without opening stomates too often Enzyme is PEP carboxylase instead of Rubisco (4 –C-molecule, hence the name) Forms OAA (oxaloacetate) CO2 comes from this to use in the Calvin Cycle

LE Bundle- sheath cell Mesophyll cell Organic acid C4C4 CO 2 CALVIN CYCLE SugarcanePineapple Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Organic acid CAM CO 2 CALVIN CYCLE Sugar Spatial separation of stepsTemporal separation of steps Sugar Day Night No structural separation like C4 plants: instead, the two processes are separated in time.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells In addition to food production, photosynthesis produces the oxygen in our atmosphere

LE Light CO 2 H2OH2O Light reactionsCalvin cycle NADP + RuBP G3P ATP Photosystem II Electron transport chain Photosystem I O2O2 Chloroplast NADPH ADP +P i 3-Phosphoglycerate Starch (storage) Amino acids Fatty acids Sucrose (export)