Photosynthesis AP Biology Ms. Haut

Introduction Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world Photosynthesis—process in which some of the solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers). 6CO2 + 6H2O C6H12O6 + 6O2 Light energy enzymes

Glucose is the main source of energy for all life Glucose is the main source of energy for all life. The energy is stored in the chemical bonds. Cellular Respiration— process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities. C6H12O6 + 6O2 6CO2 + 6H2O + energy enzymes

Autotroph —organisms that synthesize organic molecules from inorganic materials (a.k.a. producers) Photoautotrophs —use light as an energy source (plants, algae, some prokaryotes) Chemoautotrophs —use the oxidation of inorganic substances (some bacteria) Heterotroph —organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)

Thylakoids trap sunlight Sunlight = electromagnetic energy Wavelike properties Particlelike properties (photon) Light may be reflected, transmitted, or absorbed when it contacts matter

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

Photosynthetic Pigments Pigments -substances that absorb light (light receptors) Wavelengths that are absorbed disappear Wavelengths that are transmitted and reflected as the color you see Chlorophyll —absorbs red and blue light but reflects green, hence the green color of leaves

Accessory Pigments Absorb light of varying wavelengths and transfer the energy to chlorophyll a Chlorophyll b -yellow-green pigment Carotenoids -yellow and orange pigments

An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

Photosynthesis: redox process Endergonic redox process; energy is required to reduce CO2 Light is the energy source that boosts potential energy of electrons (e-) as they are moved from water to glucose When water is split, e- are transformed from the water to CO2, reducing it to sugar

oxidation C6H12O6 + 6O2 6CO2 + 6H2O + energy reduction

The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) Light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

Photosynthesis: 2 processes Light reactions —convert light energy to chemical bond energy in ATP and NADPH Occurs in thylakoids in chloroplasts NADP+ reduced to NADPH—temporary energy storage (transferred from water) Give off O2 as a by-product Generates ATP by phosphorylating ADP

Photosynthesis: 2 processes Calvin Cycle —carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate Occur in the stroma of the chloroplast Do not require light directly, but requires products of the light reactions Incorporates into existing organic molecules and then reduces fixed carbon into carbohydrate NADPH provides the reducing power ATP provides chemical energy

Interdependent Reactions Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced

Photosystems: light-harvesting complexes in thylakoid membrane Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane form a light gathering antennae that absorb photons and pass energy from molecule to molecule Photosystem I —specialized chlorophyll a molecule, P700 Photosystem II —specialized chlorophyll a molecule, P680

Noncyclic Electron Flow Light drives the light reactions to synthesize NADPH and ATP Includes cooperation of both photosystems, in which e- pass continuously from water to NADP+

When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor. This leaves a hole in the P680

To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2

Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain. e- are transferred to plastoquinone (Pq) and plastocyanin (Pc) (e- carriers)

As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP (PHOTOPHOSPHORYLATION) This ATP is used to make glucose during Calvin cycle

When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I. Pre-existing hole was left by former e- that was excited

When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor. e- are transferred to ferredoxin (Fd) (e- carrier) NADP+ reductase transfers e- from Fd to NADP+, storing energy in NADPH (reduction reaction) NADPH provides reducing power for making glucose in Calvin cycle

Cyclic Electron Flow Only photosystem I is used Only ATP is produced animation

Chemiosmosis Energy released from e- transport chain is used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid. Creates concentration gradient across thylakoid membrane Process provides energy for chemisomostic production of ATP

The light reactions and chemiosmosis: the organization of the thylakoid membrane REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+ Figure 10.17

Calvin Cycle Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose) The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar For the net synthesis of this sugar, the cycle must take place 2 times

Calvin Cycle

Calvin Cycle

Calvin Cycle

Calvin Cycle Carbon Fixation: 3 CO2 molecules bind to 3 5-Carbon sugars, ribulose bisphosphate (RuBP) using enzyme called RuBP carboxylase (rubisco) Produces 6 molecules of a 3-carbon sugar, 3-phosphoglycerate

Calvin Cycle Carbon Fixation Reduction: 6 ATP molecules transfer phosphate group to each molecule of 3-phos. to make 1,3-diphosphoglycerate 6 molecules of NADPH reduce each molecule of 1,3-diphosph. to make glyceraldehyde 3-phosphate (G3P) One of the G3P exits the cycle to be used by the plant; the other 5 molecules are used to regenerate the CO2 acceptor, RuBP: 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP

Calvin Cycle 3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle. 2 G3P molecules can be used to make glucose

Interdependent

Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis The closing of stomata reduces access to CO2 and causes O2 to build up These conditions favor a seemingly wasteful process called photorespiration

Photorespiration: An Evolutionary Relic? In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2 Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

Photorespiration: An Evolutionary Relic? Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle Corn Crab Grass

C4 Plants PEP carboxylase-high affinity to CO2 and no affinity for O2, thus no photorespiration possible

CAM Plants CAM plants open their stomata at night, incorporating CO2 into organic acids Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle http://ecology.botany.ufl.edu/ecologyf02/graphics/saguaro.GIF

LE 10-20 Bundle- sheath cell Mesophyll Organic acid C4 CO2 CALVIN CYCLE Sugarcane Pineapple Organic acids release CO2 to Calvin cycle CO2 incorporated into four-carbon organic acids (carbon fixation) CAM Sugar Spatial separation of steps Temporal separation of steps Day Night

The CAM and C4 pathways: Are similar in that CO2 is first incorporated into organic intermediates before it enters the Calvin cycle Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO2