Photosynthesis

Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Autotrophs Photosynthesizers are autotrophs – organisms that produce organic molecules from CO 2 & inorganics from environment. Photoautotrophs - plants, algae, some other protists, and some prokaryotes Chemoautotrophs – oxidize inorganics (S, NH 3 ). Unique to bacteria.

Heterotrophs Live on products of other organisms Consumers Decomposers Completely dependent on autotrophs for byproducts of photosynthesis

Location of Photosynthesis Chloroplasts – any green part of plant, primarily leaves ½ million chloroplasts/mm 2 of leaf surface Green color derived from pigment chlorophyll Chlorophyll important in light absorption (more on that later)

Chloroplasts in Elodea

Location of Chloroplasts Found mainly in mesophyll cells – interior of leaf O 2 exits and CO 2 enters leaf through stomata Stomata in close proximity to chloroplasts – WHY? Veins deliver H 2 0 from roots and carry off sugar to other areas where needed

Typical mesophyll cell has chloroplasts Chloroplast structure – remember? Thylakoids/gran a, stroma

Leaf cross section

Photosynthesis – Redox Rxn Recall - RESPIRATION C 6 H 12 O O H 2 O  6 CO H 2 O + Energy OR C 6 H 12 O O 2  6 CO H 2 O + Energy Glucose is oxidized to form CO 2 Oxygen is reduced, forming water Reaction is EXERGONIC

Photosynthesis Equation Photosynthesis reverses aerobic respiration Net process of photosynthesis is: 6CO 2 + 6H 2 O + light energy -> C 6 H 12 O 6 + 6O 2 Water split and e - transferred to CO 2, reducing it to sugar Byproduct: 6O 2 Reaction is ENDERGONIC

Free Oxygen Plants give off O 2, split from H 2 O not CO 2 C.B. van Neil, studies with H 2 S in bacteria Later scientists used radioactive tracer 18 O to confirm van Neil’s H 2 O hypothesis

Photosynthesis Equation Where does the energy to power the reaction come from? In reality, photosynthesis adds one CO 2 at a time (carbon fixation) CO 2 + H 2 O + light energy -> [CH 2 O]* + O 2 *CH 2 O represents the general formula for a sugar.

Photosynthesis: A closer look… Two major components LIGHT REACTIONS (PHOTOPHOSPHORYLATION) - conversion of light energy to chemical energy CALVIN CYCLE (DARK REACTIONS) – transforms atmospheric CO 2 to organic molecule; uses energy from light rxn to reduce to sugar.

Light Reactions: Overview Light energy absorbed by chlorophyll in thylakoids Drives the transfer of e - to NADP + (nicotinamide adenine dinucleotide phosphate), forming NADPH Generates ATP by photophosphorylation

What is LIGHT? Electromagnetic energy – travels in waves Distance between waves is the wavelength ↓ wavelength = ↑ energy ↑ wavelength = ↓ energy Measured via electromagnetic spectrum Visible light = 380 – 750 nm

Photons Direct particles of energy Intensity inversely related to wavelength Purple/blue light carries much more energy than orange/red range of spectrum

When light meets matter, it is either reflected, transmitted or absorbed Different pigments absorb photons of different wavelengths WHY ARE LEAVES GREEN?

Spectrophotometer Measures pigment’s ability to absorb wavelengths Uses transmittance Absorption spectrum

Thylakoids - 3 major pigments Chlorophyll a – dominant pigment. Red & blue absorption Chlorophyll b and carotenoids Slightly different absorption Funnel energy to chloro a PHOTOPROTECTION (carotenoids)

Action Spectrum All the pigments together determine “action spectrum” for photosynthesis

Action spectrum ≠ absorption spectrum of ONE pigment Engelmann 1883 – aerobic bacteria indic. O 2 & absorption

Capturing Light Energy Molecule absorbs photon Causes e - to elevate to orbital with more potential energy “Ground” state to “excited” state Molecules absorb photons that match the energy difference between ground and excited state of e - Corresponds to specific wavelengths, absorption spectrum

Photons are absorbed by clusters of pigment molecules in thylakoid membranes Energy of photon converted to potential energy of e - raised from ground state to excited state In chlorophyll a and b, an electron from Mg in the porphyrin ring is excited

Chlorophyll “head”

Excited e - unstable Drop to ground state in billionth of a second, releasing heat energy Chlorophyll & other pigments release photon of light (fluorescence) without an e - acceptor

Photosystems In thylakoid membrane, chlorophyll organized photosystems Acts like a light-gathering “antenna” Hundreds of chloro a, b, and carotenoids Some proteins, other small organic molec.

Photon absorbed by any antenna molecule Transmitted from molecule to molecule until reaches reaction center At reaction center is a primary electron acceptor Removes an excited e - from chloro a in reaction center This starts the light reactions

Photosystem I & II Photosystem I has an absorption peak at 700nm - its rxn center is called the P700 center Photosystem II - rxn center at 680nm. Differences between reaction centers due to the associated proteins Photosystems work together to generate ATP and NADPH.

Cyclic & Noncyclic Electron Flow During light rxn, e - can flow 1 of 2 ways: Noncyclic electron flow, the predominant route, produces both ATP and NADPH Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow

Noncyclic Pathway Similar to oxidative phosphorylation

1. Photosystem II absorbs light, captures an excited electron (rxn ctr oxidized) 2. Enzyme extracts e - from H 2 O and donates to oxidized reaction center P680 is the strongest oxidizing agent known – it must be filled with e - 3. Photoexcited e - pass along ETC from PSII to PSI

Electron carriers: Pq (plastoquinone), a cytochrome complex Pc (plastocyanin), a protein 4. Exergonic fall of e - provides energy for ATP synthesis Meanwhile - in Photosystem I…

5. PS I rxn center excited, releasing photoexcited electron e - captured by acceptor creating an e - hole in P700 center Hole filled by e- from the PS II ETC 6. 2 nd ETC in PS I. Electron carrier is Fd (ferredoxin), a protein 7. NADP reductase transfers e - from Fd to NADP, reducing to NADPH

Cyclic Pathway Under certain conditions, photoexcited e - from PS I (not PS II), take an alternative pathway, cyclic electron flow “Short circuit” – no NADPH or O 2 produced Excited e - cycle from rxn center to primary acceptor, along ETC, and return to oxidized P700 chlorophyll As e - flow along ETC, they generate ATP by cyclic photophosphorylation.

Benefits of Cyclic Pathway Noncyclic e - flow produces ATP and NADPH in roughly equal quantities Calvin cycle consumes more ATP than NADPH Cyclic electron flow allows chloroplast to generate extra ATP to satisfy the Calvin cycle

Chemiosmosis Chloroplasts and mitochondria generate ATP by the same mechanism ETC pumps protons across membrane as e - are passed along a series electronegative carriers. Builds proton-motive force in the form of H + gradient ATP synthase harness this force to generate ATP as H + diffuses back across membrane.

The proton gradient, or pH gradient, across thylakoid membrane is substantial When illuminated, the pH in thylakoid space drops to about 5 and the pH in stroma increases to about 8, a thousandfold difference in H+ concentration Produces ATP and NADPH on the stroma side of the thylakoid

Overall products Noncyclic flow pushes e - from H 2 O to NADPH, where they have high potential energy This process also produces ATP Oxygen is a byproduct Cyclic flow converts light energy to chemical energy in the form of ATP