Photosynthesis Chapter 5

Overview of Photosynthesis Converts solar energy (sunlight) into chemical energy which is used to build carbohydrate molecules 6CO2 + 6H2O + energy (sunlight)  C6H12O6 +6O2 Energy used to hold the carbohydrate molecules together is released by cellular respiration C6H12O6 +6O2  6CO2 + 6H2O + energy (ATP)

Overview of Photosynthesis Photosynthesizers Autotrophs Auto = self Troph = feeder Includes plants, algae from Kingdom Protista, cyanobacteria, and some archaebacteria Major producers for most food webs Algae Kingdom Protista Volvox Cyanobacteria Anabaena

Overview of Photosynthesis Non photosynthesizers Heterotrophs Hetero = other Troph = feeder Rely on producers directly (herbivore) or indirectly (carnivore) Includes fungi, animals, some Protists, and some bacteria Kingdom Protista Stentor and Amoeba

Main Structures of Photosynthesis Photosynthesis occurs in chloroplasts contained in photosynthetic cells within the leaves (or other green parts of the plant)

Main Structures of Photosynthesis Leaves Leaves have multiple cell layers and tissues Upper and lower epidermis (non-photosynthetic) Palisade and spongy mesophyll (photosynthetic) Bundle sheath cells surrounding veins Xylem (transports water and dissolved nutrients) Phloem (transports photosynthetic products) Most photosynthesis occurs in mesophyll cells

Main Structures of Photosynthesis Leaves Leaves have stomata Small, regulated openings through the leaf epidermis Gas exchange occurs through stomata CO2 enters H2O and O2 exit

Main Structures of Photosynthesis Chloroplasts Structure Surrounded by a double membrane (envelope) The inner semi-fluid matrix is called the stroma The thylakoid membrane is a third internal membrane Extensively folded into thylakoid disks tightly stacked in grana Function Light is harvested and converted into chemical energy over the thylakoid membrane Sugars are built in the stroma

Main Structures of Photosynthesis Thylakoid membrane Contains pigments and photosystems

Main Structures of Photosynthesis Thylakoid membrane Pigments absorb wavelengths of visible light Part of the electromagnetic spectrum from the sun

Main Structures of Photosynthesis Thylakoid membrane Pigments absorb wavelengths of visible light Can be described and measured as waves Wavelength is inversely related to energy level Long wavelength = low energy and short wavelength = high energy

Main Structures of Photosynthesis Thylakoid membrane Pigments absorb wavelengths of visible light Organized as packets of energy called photons

Main Structures of Photosynthesis Thylakoid membrane Pigments absorb wavelengths of visible light Chlorophyll a Main photosynthetic pigment Absorbs violet and red light Reflects green light

Main Structures of Photosynthesis Thylakoid membrane Pigments absorb wavelengths of visible light Accessory pigments Chlorophyll b Carotenoids (reflects yellow, orange, or red) Lycopenes (reflects yellow, orange, or red) Xanthophyls (reflects yellow or brown) Phycobilins (reflects red)

Main Structures of Photosynthesis Thylakoid membrane Contains photosystems Groups of pigments and other molecules There are two photosystems Photosystem II (absorbs light at 680nm) Photosystem I (absorbs light at 700nm)

Questions What is a photoautotroph? What organelle is used for photosynthesis? Photosynthetic organelles are found in __________ cells What happens in the stroma? What is the chloroplast’s third membrane called? Which pigment is the main photosynthetic pigment? Chlorophyll a is green. What does this mean in terms of absorbed and reflected light?

Questions Name each indicated structure E F

Light Dependent Reactions Light energy is converted into high-energy chemical carriers that will be used in the Calvin Cycle ATP and NADPH Occurs over the thylakoid membrane

Light Dependent Reactions Photon energy is absorbed by pigments and passed along a pathway to other chlorophyll molecules Energy is transferred to electrons enabling them to break free from the chlorophyll molecule Chlorophyll is an electron donor (oxidized) Electrons are accepted by the first molecule of the electron transfer chain (reduced)

Light Dependent Reactions Replacement electrons are pulled from water molecules Water is split into oxygen, electrons, and hydrogen ions (H+) H2O  ½ O2 + 2e- + 2H+ This is referred to as photolysis Hydrogen ions accumulate in the thylakoid space* Oxygen diffuses out of the cell as O2

Light Dependent Reactions High energy electrons enter the electron transport chain (ETC) Each ETC molecule accepts the electrons (reduced) and then donates them (oxidized) to the next molecule

Light Dependent Reactions Energy released by the electrons as they move through the chain is used to pump H+ from the stroma into the thylakoid space Active transport H+ concentration increases* The electrons are ultimately accepted by a pigment molecule in photosystem I

Light Dependent Reactions Photons are captured by photosystem I re-energizing the electrons

Light Dependent Reactions High-energy electrons leave photosystem I where they are ultimately donated to NADP+ to form NADPH NADP+ + 2 e- + H+  NADPH NADPH is a high-energy molecule that will be used to make sugar molecules The process results in a decrease of H+ in the stroma and thus an increase in the H+ concentration in the thylakoid space*

Light Dependent Reactions The build-up of H+ in the thylakoid space creates an electrochemical gradient Due to photolysis, ETC active transport of H+, and production of NADPH Chemical because of the concentration difference of H+ Electrical because of the difference in the charge across the membrane

Light Dependent Reactions The electrochemical gradient propels H+ across the thylakoid membrane through ATP synthase Chemiosmosis The flow of H+ has enough force to cause the synthase to attach a phosphate to ADP forming ATP Phosphorylation ATP is a high-energy molecule that will be used to make sugar molecules

Light Dependent Reactions Summary The light-dependent reactions capture and convert the sun’s radiant energy into ATP (chemical bond energy) and NADPH ATP and NADPH will be used in the light-independent reactions (Calvin- Benson Cycle) to build sugar molecules H2O is split to provide the electrons for the electron transport chain Releases O2

Questions For each indicated location Name any structures or molecules involved Explain what is happening

Questions Where do the light-dependent reactions occur? How are electrons replaced in photosystem II? (what is the process called?) Energy from electrons is used for what process by the electron transport chain? What molecule do the high-energy electrons finally end up in? What processes result in the H+ electrochemical gradient? What structure catalyzes phosphorylation of ADP to ATP?

Light Independent Reactions During the light-independent reactions (Calvin-Benson Cycle) chemical energy from ATP and NADPH are used to build glucose from CO2 Fixes inorganic CO2 into organic molecules Occurs in the stroma

Light Independent Reactions Calvin-Benson Cycle Major molecules involved CO2 carbon dioxide RuBP ribulose-1,5-bisphosphate PGA 3-phosphoglyceric acid PGAL/G3P glyceraldehyde-3-phosphate

Light Independent Reactions Calvin-Benson Cycle phases Carbon Fixation Reduction Regeneration

Light Independent Reactions Calvin-Benson Cycle Carbon Fixation (RuBP  PGA) CO2 is attached to RuBP (a 5-carbon molecule) Catalyzed by the enzyme RuBisCO A 6-carbon intermediate is formed which is converted immediately into two 3-carbon molecules called PGA CO2 is “fixed” from its inorganic form into organic molecules

Light Independent Reactions Calvin-Benson Cycle Reduction (PGA  PGAL) ATP provides energy Releases ADP NADPH donates high energy electrons and H+ Reduction Releases NADP+ ADP and NADP+ return to the light- dependent reactions

Light Independent Reactions Calvin-Benson Cycle Reduction (PGA  PGAL) PGAL/G3P is produced Some is released to make glucose 2 PGAL  Glucose (C6H12O6) The remainder is recycled in the next phase

Light Independent Reactions Calvin-Benson Cycle Regeneration (PGAL  RuBP) ATP provides energy Releases ADP Multiple bond rearrangement steps regenerate RuBP Thus the pathway is a “cycle”

Light Independent Reactions Calvin-Benson Cycle The cycle continues to repeat It takes 6 CO2 molecules to produce 1 molecule of glucose

Photosynthesis Overview Process: Photo: Light-dependent reactions split H2O and produce ATP, NADPH, and O2 Synthesis: Light-independent reactions use ATP, NADPH, and CO2 to produce glucose LIGHT ENERGY C6H12O6 + 6O2 glucose 6H2O + 6CO2

Questions Light-independent reactions use ATP and NADPH to build what? What is the name of the metabolic pathway used for the light-independent reactions? What is the reaction called when CO2 binds to RuBP? What enzyme is responsible for the above reaction? Two of which intermediate are used to make glucose? How many CO2 molecules are required to make one glucose molecule?

Questions Identify the molecules 6 A C ATP 3ADP Calvin-Benson cycle 6ADP + 6Pi 3 ATP 6 D 2Pi 6NADP+ 5PGAL E 1 Pi F

Alternate Carbon-fixing Pathways C3 Plants Most plants harvest and convert light energy while fixing CO2 into organic sugars at the same time in mesophyll cells They are referred to as C3 plants because the first stable intermediate (PGA) is a 3- carbon molecule Typically found in temperate climates Inefficient on hot, dry days and suffer lower productivity due to photorespiration

Alternate Carbon-fixing Pathways C3 plants have a problem in hot climates To reduce H2O loss, they close their stomata which are used for gas exchange CO2, O2 and H2O Closed stomata means that H2O and O2 can’t exit CO2 can’t enter Reduced CO2 and increased O2 concentrations results in photorespiration Thus C3 plants are faced with two bad choices on hot days Wilt due to water loss or Have reduced productivity due to photorespiration Stomata surrounded by guard cells H2O O2 CO2

Alternate Carbon-fixing Pathways Photorespiration Without CO2 no PGAL/G3P will be synthesized Rubisco binds O2 instead of CO2 forming a useless molecule Wastes CO2 and energy The result: No glucose is synthesized Reduced or no productivity (growth) The adaptation Use alternate carbon-fixing pathways

Alternate Carbon-fixing Pathways Alternate Pathways Some plants that live in hot climates have adaptations to circumvent photorespiration When they can take in CO2 they create a chemical which stores the CO2 to be used later or in a different cell type in the Calvin- Benson Cycle

Alternate Carbon-fixing Pathways Alternate Pathways: C4 plants When the stomata are open CO2 is fixed into a 4 carbon intermediate in mesophyll cells (thus called C4 plants) The enzyme used ignores O2, so no photorespiration occurs Provides a “storage” of CO2 for the plant

Alternate Carbon-fixing Pathways Alternate Pathways: C4 plants The 4 carbon molecule moves into the bundle-sheath cells CO2 is released from the C4 molecule and used in the Calvin-Benson pathway to produce sugar

Alternate Carbon-fixing Pathways Alternate Pathways: C4 plants Divides photosynthesis between different cells Typically found in tropical climates

Alternate Carbon-fixing Pathways Alternate Pathways: CAM plants The stomata are only open at night when it is cooler CO2 is fixed into a 4 carbon molecule at night Crassulacean acid metabolism (CAM) The cell “stores” CO2 in the central vacuole as part of a C4 intermediate During the day CO2 is shuttled back into the chloroplast for the Calvin/Benson cycle

Alternate Carbon-fixing Pathways Alternate Pathways: CAM plants Divides photosynthesis between night and day Typically found in desert climates

Questions Stomata A. Fixes CO2 at night Photorespiration B. Openings on leaves Rubisco C. Temperate plants C3 plant D. Uses bundle sheath cells C4 plant E. Enzyme used to fix CO2 CAM plant F. Location of Calvin Cycle in C4 plants Bundle sheath cells G. Result of too much O2 and not enough CO2

Summary Photosynthesis structures Light dependent reactions Chloroplasts Pigments Light dependent reactions Light independent reactions Calvin-Benson Cycle Alternate pathways