Chapter 10: Photosynthesis
Topic: Photosynthesis in Nature
1. Plants and other Autotrophs
Self-feeders and producers.
Photoautotrophs: Use light.
Heterotrophs: Other-feeders, consumers, depend on photoautotrophs.
Structurally similar and most likely evolved from photosynthetic bacteria.
Chlorophyll: Green pigment in most chloroplasts and therefore, leaves.
Found mainly in mesophyll cells. (Interior leaf tissue).
Stomata: Microscopic pores that allow CO2 and O2 to cross.
Chlorophyll in thylakoid membranes.
Topic: Pathways of Photosynthesis
1. Chloroplasts Split H2O
Carbon dioxide + water + light energy --> organic compounds + oxygen (reverse of cellular respiration).
O2 comes from H2O not CO2.
H from H2O makes carbon dioxide.
Photosynthesis – Redox process.
Reverse electron flow, from H2O(oxidized) to CO2, reducing it to sugar, electron increases potential energy, using light energy.
2. Photosynthesis Overview
The light reactions (thylakoids): split H2O, release O2, reduce NADP+ to NADPH and generate ATP from ADP by photophosphorylation.
Calvin Cycle (stroma): Forms sugar from CO2, using ATP and NADPH. Begins with carbon fixation, incorporating CO2 into Organic Molecules.
Sunlight is basically electromagnetic energy (radiation).
Visible Light: 380-750nm wavelength.
Energy inversely proportional to wavelength.
4. Photosynthetic Pigments
When pigments absorb light, those wavelengths disappear.
What we see is reflected or transmitted wavelengths.
Spectrophotometer: Measures light transmitted.
Absorption Spectrum: Graph of light absorbed vs. wavelength.
Action Spectrum: Photosynthesis Performance (Confirms red/blue).
Accessory pigments contribute.
Carotenoids: Photoprotection (absorb, dissipate excessive light that could damage chlorophyll),
5. Excitation of Chlorophyll
When a molecule absorbs a photon of light, electron is elevated to an orbital, therefore increases potential energy.
Excited state becomes unstable when drops down, get heat or light.
Light harvesting complexes of the thylakoid membrane.
Antenna pigment molecules: Absorbs light
Reaction center chlorophyll: Where first light-driven chemical reaction occurs.
Primary electron acceptor: Redox.
Photosystems I and II: Each with unique reaction center.
Photosystem I: P700 (Absorbs 700nm)
Photosystem II: P680 (Absorbs 680nm)
The primary pathway, involves both photosystems and produces ATP and NADPH using light energy.
Oxidized chlorophyll becomes strong oxidizing agent. (Electron “Hole” needs to be filled).
Electrons from the splitting of H2O fill the “hole” and form O2.
Photoexcited electrons go from Photosytem II to Photosystem I via ETC
Electron flow provides energy to --> ATP to be used in Calvin Cycle (Noncyclic Photophosphorylation)
Electrons fill electron hole in P700 chlorophyll in Photosystem I.
Electrons flow down from ETC to NADP+ --> NADPH in a redox reaction (reducing power for Calvin Cycle).
Uses Photosystem I only, no NADPH, no O2, and makes ATP. (Cyclic Photophosphorylation).
Why? Because the Calvin Cycle needs more ATP than NADPH.
An increase in NADPH may stimulate cyclic electron flow (if low on ATP, then NADPH builds up).
Electrons from food. (Chemical Energy).
ETC pumps H+ from matrix to intermembrane space.
In photosynthesis, light energy drives electrons and ETC pumps H+ from stroma to thylakoid space.
Enters with CO2 and leaves with Sugar.
Spends ATP and consumes NADPH.
Produces glyceraldehyde-3-phosphate (G3P) with 3 cycles.
2. The Three Phases of the Calvin Cycle
Phase I: Carbon Fixation: Rubisco enzyme attaches CO2 to RuBP --> (2) 3-phosphoglycerate.
Phase II: Reduction: Attaches phosphates, electrons reduce to G3P which exits.
Phase III: Regeneration of CO2 acceptor (RuBP): Uses ATP.
Topic: Alternative Carbon Fixations in Hot/Arid Climates
1. Stomata close to conserve H2O, but limits CO2 intake on hot days.
C3 Plants (Mostly): First organic product is a 3-carbon compound. (Example: Rice, Wheat, and Soybeans).
Rubisco also accepts O2 in place of CO2 --> 2-carbon compound leaves chloroplast, mitochondria and peroxisomes break it down to CO2.
Forms a 4-carbon compound. (Example: Sugarcane, Corn, and Grasses).
Open stomata at night, closed during day (conserves H2O, but less CO2 coming in)
At night takes up CO2 --> organic acids.
Mesophyll cells store organic acids in vacuoles until morning when ATP and NADPH supplied.
