Photosynthesis: Life from Light and Air

Energy needs of life All life needs a constant input of energy Heterotrophs get their energy from “eating others” eat food = other organisms = organic molecules make energy through respiration Autotrophs produce their own energy (from “self”) convert energy of sunlight build organic molecules (CHO) from CO2 make energy & synthesize sugars through photosynthesis consumers producers

*How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules glucose + oxygen  carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP  + oxidation = exergonic Autotrophs making energy & organic molecules from light energy 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 + water + energy  glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy  + reduction = endergonic

*Plant structure Obtaining raw materials sunlight CO2 H2O nutrients leaves = solar collectors CO2 stomates = gas exchange H2O uptake from roots nutrients N, P, K, S, Mg, Fe…

stomate transpiration gas exchange

*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 builds up within thylakoid sac outer membrane inner membrane granum stroma thylakoid 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.

It’s not the Dark Reactions! Photosynthesis *Light reactions light-dependent reactions energy conversion reactions convert solar energy to chemical energy ATP & NADPH *Calvin cycle light-independent reactions sugar building reactions uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 It’s not the Dark Reactions!

Light reactions Electron Transport Chain thylakoid chloroplast H+ Light reactions H+ ATP Electron Transport Chain like in cellular respiration proteins in organelle membrane electron acceptors NADPH proton (H+) gradient across inner membrane find the double membrane! ATP synthase Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

*ETC of Respiration Mitochondria transfer chemical energy from food molecules into chemical energy of ATP use electron carrier NADH NADH is an input ATP is an output O2 combines with H+’s to form water Electron is being handed off from one electron carrier to another -- proteins, cytochrome proteins & quinone lipids So what if it runs in reverse?? generates H2O

*ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP use electron carrier NADPH Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide generates O2

ATP Synthesis *sunlight breakdown of C6H12O6 moves the electrons photosynthesis respiration *sunlight breakdown of C6H12O6 H+ ADP + Pi moves the electrons runs the pump pumps the protons builds the gradient drives the flow of protons through ATP synthase bonds Pi to ADP generates the ATP ATP

Pigments of photosynthesis How does this molecular structure fit its function? *Chlorophylls & other pigments embedded in thylakoid membrane arranged in a “photosystem” collection of molecules structure-function relationship

A Look at Light The spectrum of color V I B G Y O R

*Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a absorbs best in red & blue wavelengths & least in green accessory pigments with different structures absorb light of different wavelengths chlorophyll b, carotenoids, xanthophylls Why are plants green?

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering molecules *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

ETC of Photosynthesis *Photosystem II *Photosystem I chlorophyll a chlorophyll b *Photosystem I Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide

ETC of Photosynthesis e e O split H2O H+ sun sun to Calvin Cycle ATP Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide O to Calvin Cycle split H2O ATP

ETC of Photosynthesis ETC uses light energy to produce ATP & NADPH go to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” need to replace electron in chlorophyll enzyme extracts electrons from H2O & supplies them to chlorophyll splits H2O O combines with another O to form O2 O2 released to atmosphere and we breathe easier!

Memorization of the steps in the Calvin cycle, the structure of the molecules and the names of enzymes (with the exception of ATP synthase) are beyond the scope of the course and the AP Exam.

Experimental evidence Where did the O2 come from? radioactive tracer = O18 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 1 6CO2 6H2O C6H12O6 6O2 light energy  + 6CO2 6H2O C6H12O6 6O2 light energy  + Experiment 2 Proved O2 came from H2O not CO2 = plants split H2O!

Noncyclic Photophosphorylation Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle! ATP

Cyclic photophosphorylation If PS I can’t pass electron to NADP…it cycles back to PS II & makes more ATP, but no NADPH coordinates light reactions to Calvin cycle Calvin cycle uses more ATP than NADPH  ATP 18 ATP + 12 NADPH  1 C6H12O6

Photophosphorylation cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP

Photophosphorylation Cyclic – used to generate ATP in “ancient” organisms, like bacteria NADPH and sugars are not made Noncyclic – used to generate NADPH for the Calvin Cycle

Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems Figure 6.20 Noncyclic Electron Transport Uses Two Photosystems As chlorophyll molecules in the reaction centers of photosystems I and II absorb light energy, they pass electrons into a series of redox reactions, ultimately producing NADPH and ATP. The term “Z scheme” describes the path (blue arrows) of electrons as they travel through the two photosystems. In this scheme the vertical positions represent the energy levels of the molecules in the electron transport system.

Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP Figure 6.21 Cyclic Electron Transport Traps Light Energy as ATP Cyclic electron transport produces ATP but no NADPH.

Occurs in the stroma of the chloroplast. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates Calvin cycle: the energy in ATP and NADPH is used to “fix” CO2 in reduced form in carbohydrates Occurs in the stroma of the chloroplast. Each reaction is catalyzed by a specific enzyme. The cycle is composed of three distinct processes.

Figure 6.22 The Calvin Cycle 1. Carbon Fixation 2. Reduction 3. Regeneration Figure 6.22 The Calvin Cycle The Calvin cycle uses the ATP and NADPH generated in the light reactions to produce G3P from CO2. The G3P is used as a starting material for the production of glucose and other carbohydrates. Six turns of the cycle are needed to produce one molecule of the hexose glucose.

Figure 6.23 RuBP Is the Carbon Dioxide Acceptor 1. Fixation of CO2 3PG Figure 6.23 RuBP Is the Carbon Dioxide Acceptor The enzyme rubisco adds CO2 to the five-carbon compound RuBP. The resulting six-carbon compound immediately splits into two molecules of 3PG. Rubisco: ribulose bisphosphate carboxylase the most abundant enzyme in the world!

2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P). Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 2. 3PG is reduced to form glyceraldehyde 3- phosphate (G3P).

3. The CO2 acceptor RuBP is regenerated from G3P. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates 3. The CO2 acceptor RuBP is regenerated from G3P. Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose). When glucose accumulates, it is linked to form starch, a storage carbohydrate.

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis The closing of stomata reduces access to CO2 and causes O2 to build up These conditions favor an apparently wasteful process called photorespiration

Photorespiration: An Evolutionary Relic? In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate) In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar C3 plants: rice, wheat, soybeans

Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle 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 Utilize enzyme PEP carboxylase in an alternate pathway to fix carbon PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Photosynthetic cells of C4 plant leaf PEP carboxylase Figure 10.20 C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell Oxaloacetate (4C) PEP (3C) Vein (vascular tissue) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell Stoma CO2 Calvin Cycle Figure 10.20 C4 leaf anatomy and the C4 pathway. Sugarcane, corn, tropical grasses Sugar Vascular tissue 36

CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO2 into organic acids (storage) Stomata close during the day, when light reactions generate ATP and NADPH Then, CO2 is released from organic acids and used in the Calvin cycle Pineapple, agave