Chapter 10 Photosynthesis

Energy needs of life All life needs a constant input of energy Heterotrophs (Animals) get their energy from “eating others” organic molecules make energy through cellular respiration Autotrophs (Plants) get their energy sunlight Build their own organic molecules (food) from CO2 synthesize sugars through photosynthesis Then get energy from the sugars they made by performing cellular respiration.

How are they connected? Heterotrophs  Autotrophs  making energy & organic molecules from ingesting organic molecules glucose + oxygen  carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP  + exergonic Autotrophs So, in effect, photosynthesis is respiration run backwards powered by light. Cellular Respiration oxidize C6H12O6  CO2 & produce H2O fall of electrons downhill to O2 exergonic Photosynthesis reduce CO2  C6H12O6 & produce O2 boost electrons uphill by splitting H2O endergonic making energy & organic molecules from light energy + water + energy  glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy  + endergonic

Sites of Photosynthesis Mesophyll cells: chloroplasts mainly found in these cells of leaf stomata: pores in leaf (CO2 enter/O2 exits) chlorophyll: green pigment in thylakoid membranes of chloroplasts

Plant structure Chloroplasts Thylakoid membrane contains double membrane stroma fluid-filled interior thylakoid sacs grana stacks Thylakoid membrane contains chlorophyll molecules electron transport chain ATP synthase H+ gradient built up within thylakoid sac A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana.

Tracking atoms through photosynthesis Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis (C.B. van Niel) Reactants: Products: 6 CO2 12 H2O C6H12O6 6 H2O 6 O2

Photosynthesis = Light Reactions + Calvin Cycle

Light Reactions  produces ATP produces NADPH H2O ATP O2 light energy  + NADPH H2O sunlight produces ATP produces NADPH releases O2 as a waste product Energy Building Reactions NADPH ATP O2

Calvin Cycle  builds sugars uses ATP & NADPH CO2 C6H12O6  + NADP ATP NADPH ADP CO2 builds sugars uses ATP & NADPH recycles ADP & NADP back to make more ATP & NADPH ADP NADP Sugar Building Reactions NADPH ATP sugars C6H12O6

Putting it all together CO2 H2O C6H12O6 O2 light energy  + H2O CO2 Plants make both: Energy (ATP & NADPH) sugars sunlight ADP NADP Energy Building Reactions Sugar Building Reactions NADPH ATP sugars C6H12O6 O2

Why do Calvin Cycle? Because light reaction only produces 1 ATP The glucose made from Calvin cycle goes to cells of plant for cellular respiration (yes, the plants do BOTH photosynthesis and cellular respiration) to produce lots of ATP!

Light Reactions

The light reactions convert solar energy to chemical energy of ATP and NADPH Nature of sunlight Light = energy = electromagnetic radiation Shorter wavelength (λ): higher E Visible light - detected by human eye Light: reflected, transmitted or absorbed

Photosynthetic pigments Pigments absorb different λ of light chlorophyll – absorb violet-blue/red light, **reflect green chlorophyll a (blue-green): light reaction, converts solar to chemical E chlorophyll b (yellow-green): conveys E to chlorophyll a carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis

(absorption of chlorophylls a, b, & carotenoids combined) Action Spectrum: plots rate of photosynthesis vs. wavelength (absorption of chlorophylls a, b, & carotenoids combined) Engelmann: used bacteria to measure rate of photosynthesis in algae; established action spectrum Which wavelengths of light are most effective in driving photosynthesis?

Interaction of light with chloroplasts

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering “antenna complex” Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths. antenna pigments

Proton motive force generated by an Electron Transport Chain H+ from water H+ pumped across by cytochrome Removal of H+ from stroma when NADP+ is reduced

Calvin Cycle

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar Uses ATP, NADPH, CO2 Produces 3-C sugar G3P (glyceraldehyde-3-phosphate) Three phases: Carbon fixation Reduction Regeneration of RuBP (CO2 acceptor)

To G-3-P and Beyond! Glyceraldehyde-3-P G-3-P = important intermediate end product of Calvin cycle energy rich 3 carbon sugar “C3 photosynthesis”…..C3 Plants G-3-P = important intermediate G-3-P   glucose   carbohydrates   lipids   amino acids   nucleic acids

Accounting The accounting is complicated 1 turn of Calvin cycle = 1 G3P 3 CO2 1 G3P (3C) 2 turns of Calvin cycle = 1 C6H12O6 (6C) 6 CO2 1 C6H12O6 (6C) 18 ATP + 12 NADPH  1 C6H12O6 any ATP left over from light reactions will be used elsewhere by the cell

Evolutionary Adaptations On hot dry days when a plant is forced to close its stomata to prevent excess water loss, CO2 levels inside the leaf become low. If the plant continues to attempt to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations. Less sugar is made….bad news for the plant. This is called Photorespiration. To prevent this process, two specialized biochemical additions have been evolved in the plant world: C4 and CAM metabolism

C4 Plants: CO2 fixed to 4-C compound Ex. corn, sugarcane, grass Hot, dry days  stomata close 2 cell types = mesophyll & bundle sheath cells mesophyll : PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath bundle sheath: CO2 used in Calvin cycle ↓photorespiration, ↑sugar production WHY? Advantage in hot, sunny areas

CAM Plants: Crassulacean acid metabolism (CAM) NIGHT: stomata open  CO2 enters  converts to organic acid, stored in mesophyll cells DAY: stomata closed  light reactions supply ATP, NADPH; CO2 released from organic acids for Calvin cycle Ex. cacti, pineapples, succulent (H2O- storing) plants WHY? Advantage in arid conditions

Importance of Photosynthesis Plant: Glucose for respiration Cellulose Global: O2 Production Food source

Review of Photosynthesis and Cellular Respiration

Comparison RESPIRATION PHOTOSYNTHESIS Plants + Animals Needs O2 and food Produces CO2, H2O and ATP, NADH Occurs in mitochondria membrane & matrix Oxidative phosphorylation Proton gradient across membrane Plants Needs CO2, H2O, sunlight Produces glucose, O2 and ATP, NADPH Occurs in chloroplast thylakoid membrane & stroma Photorespiration Proton gradient across membrane