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Chapter 10: Photosynthesis - Life from Light
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Energy needs of life All life needs a constant input of energy
Heterotrophs get their energy from “eating others” consumers of other organisms consume organic molecules Autotrophs get their energy from “self” get their energy from sunlight use light energy to synthesize organic molecules Chemoautotrophs Harvest energy from oxidizing inorganic substances such as sulfur and ammonia Unique to bacteria Heterotrophs consumers animals fungi most bacteria Autotrophs producers plants photosynthetic bacteria (blue-green algae)
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making energy & organic molecules from ingesting organic molecules
How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules glucose + oxygen carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP + 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 +
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The Great Circle of Life!
Energy cycle sun Photosynthesis glucose O2 H2O CO2 Cellular Respiration The Great Circle of Life! Where’s Mufasa? ATP
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What does it mean to be a plant
Need to… collect light energy transform it into chemical energy store light energy in a stable form to be moved around the plant & also saved for a rainy day need to get building block atoms from the environment C,H,O,N,P,S produce all organic molecules needed for growth carbohydrates, proteins, lipids, nucleic acids
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Plant structure Obtaining raw materials sunlight CO2 H2O nutrients
leaves = solar collectors CO2 stomates = gas exchange regulation Found under leaves H2O uptake from roots nutrients
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Structure of the Leaf Mesophyll Stomata
Tissue forming the interior of the leaf; site of most chlorophyll Stomata Microscopic pores in the leaf; allows for gas exchange
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Each Mesophyll Cell: Has approx. 30-40 chloroplasts
Each chloroplast is equipped to carry out photosynthesis
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Plant structure Chloroplasts Chlorophyll & ETC in thylakoid membrane
double membrane stroma thylakoid sacs grana stacks Chlorophyll & ETC in thylakoid membrane H+ gradient built up within thylakoid sac A typical mesophyll cell has 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. H+
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Structure of the Chloroplast
Chlorophyll Photosynthetic pigment found in the thylakoid Thylakoid Membranous sacs filled with fluid and chlorophyll – Site of the LIGHT REACTIONS Granum = Stack of Thylakoids Stroma Fluid portion of the chloroplast; sight of the LIGHT INDEPENDENT REACTIONS (Calvin-Benson Cycle)
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Chloroplasts split water molecules
Evidence Discovery that the O2 given off by plants comes from H2O not CO2 Before the 1930’s the hypothesis was that photosynthesis occurred in two steps: 1. CO2 C + O2 2. C+ H2O CH2O
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Chloroplasts Split Water Molecules: Changing the Hypothesis
Studying bacteria, C.B. van Neil challenged the hypothesis: H2S was used, not water Proposed the following Rxn CO2 + 2H2S CH2O +2S Applied the same principle to plants CO2 + 2H20 + light CH2O +O2
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Pigments of photosynthesis
Why does this structure make sense? chlorophyll & accessory pigments “photosystem” embedded in thylakoid membrane structure function Orientation of chlorophyll molecule is due to polarity of membrane.
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Pigments of Photosynthesis Continued
Leaves look green because they absorb red and blue light, while transmitting and reflecting green light Chlorophyll A Dominant pigment – absorbs red/blue Chlorophyll B Directs photons to chlorophyll A Carotenoids Funnel energy from other wavelengths to Chlorophyll A (mostly orange/yellow)
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Light: Absorption Spectra
Photosynthesis performs work only with absorbed wavelengths of light chlorophyll a — the dominant pigment — absorbs best in red & blue wavelengths & least in green other pigments with different structures have different absorption spectra
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Photosynthesis overview
Light reactions – Light Dependent Rxns convert solar energy to chemical energy ATP Calvin cycle – Light Independent Rxns uses chemical energy (NADPH & ATP) to reduce CO2 to build C6H12O6 (sugars)
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Photosystems Photosystems 2 photosystems in thylakoid membrane
collections of chlorophyll molecules 2 photosystems in thylakoid membrane 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 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.
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Light reactions Similar to ETC in cellular respiration
membrane-bound proteins in organelle electron acceptors NADP+ (Oxygen in cellular respiration) proton (H+) gradient across inner membrane ATP synthase enzyme Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.
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ETC of Photosynthesis ETC produces from light energy
ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER. splits H2O (Photolysis!!) O2 released to atmosphere ATP is produced for later use
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Chloroplasts transform light energy into chemical energy of ATP
use electron carrier NADPH ETC of Photosynthesis 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 split H2O
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This shows Noncyclic photophosphorylation.
2 Photosystems 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! This shows Noncyclic photophosphorylation.
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ETC of Photosynthesis PS II absorbs light
Excited electron passes from chlorophyll to the primary electron acceptor Need to replace electron in chlorophyll An enzyme extracts electrons from H2O & supplies them to the chlorophyll This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2 O2 released to atmosphere Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state Electron is handed off down chain from electron acceptor to electron acceptor In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac. Flow back through ATP synthase to generate ATP.
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ETC of Photosynthesis Need a 2nd photon -- shot of light energy to excite electron back up to high energy state. 2nd ETC drives reduction of NADP to NADPH. Light comes in at 2 points. Produce ATP & NADPH
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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
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Photosynthesis summary so far…
Where did the energy come from? Where did the H2O come from? Where did the electrons come from? Where did the O2 come from? Where did the H+ come from? Where did the ATP come from? Where did the O2 go? What will the ATP be used for? What will the NADPH be used for?
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From Light reactions to Calvin cycle
Chloroplast stroma Need products of light reactions to drive synthesis reactions ATP NADPH
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From CO2 C6H12O6 CO2 has very little chemical energy
fully oxidized C6H12O6 contains a lot of chemical energy reduced endergonic Reduction of CO2 C6H12O6 proceeds in many small uphill steps each catalyzed by specific enzyme using energy stored in ATP & NADPH
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ribulose bisphosphate ribulose bisphosphate carboxylase
Calvin cycle 1C CO2 ribulose bisphosphate 1. Carbon fixation 3. Regeneration 5C RuBP Rubisco 6C unstable intermediate 3 ADP 3 ATP ribulose bisphosphate carboxylase PGAL to make glucose 3C 2x PGA 3C x2 PGAL sucrose cellulose etc. RuBP = ribulose bisphosphate Rubisco = ribulose bisphosphate carboxylase PGA = phosphoglycerate PGAL = phosphoglyceraldehyde 2. Reduction 6 NADP 6 NADPH 6 ADP 6 ATP 3C 2x
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Rubisco Enzyme which fixes carbon from atmosphere
ribulose bisphosphate carboxylase the most important enzyme in the world! it makes life out of air! definitely the most abundant enzyme
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Calvin cycle PGAL PGAL important intermediate
end product of Calvin cycle energy rich sugar 3 carbon compound “C3 photosynthesis” PGAL important intermediate PGAL glucose carbohydrates lipids amino acids nucleic acids
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Photosynthesis summary
Light reactions produced ATP produced NADPH consumed H2O produced O2 as byproduct Calvin cycle consumed CO2 produced PGAL regenerated ADP regenerated NADP
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Summary of photosynthesis
6CO2 6H2O C6H12O6 6O2 light energy + Where did the CO2 come from? Where did the CO2 go? Where did the H2O come from? Where did the H2O go? Where did the energy come from? What’s the energy used for? What will the C6H12O6 be used for? Where did the O2 come from? Where will the O2 go? What else is involved that is not listed in this equation?
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Photosynthesis Drives Evolution
Photosynthesis first evolved in prokaryotic organisms Scientific evidence supports that prokaryotic (bacterial) photosynthesis responsible for production of an oxygenated atmosphere Prokaryotic photosynthetic pathways – foundation of eukaryotic photosynthesis
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Plant Photosynthesis Adaptations Due to Weather Conditions
Most common type C4 Minimize the cost of photorespiration by incorporating CO2 into four carbon compounds in mesophyll cells 4 carbon compounds exported to bundle sheath cells, where they release CO2 used in the Calvin cycle CAM Open their stomata at night, incorporating CO2 into organic acids During the day, the stomata close and the CO2 is released from the organic acids for use in the Calvin cycle
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C4 Plants Figure 10.19 Mesophyll Mesophyll cell Photosynthetic CO2
Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19
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C4 and CAM Plants
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