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Photosynthesis

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

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Page 1: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis

Page 2: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 3: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis

• Plants capture light energy from the sun • Energy is converted to chemical energy (sugars

& organic molecule)

Page 4: 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 CO2 & inorganics from environment.

• Photoautotrophs - plants, algae, some other protists, and some prokaryotes

• Chemoautotrophs – oxidize inorganics (S, NH3).

Unique to bacteria.

Page 5: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Heterotrophs

• Live on products of other organisms • Consumers• Decomposers• Completely dependent on autotrophs

for byproducts of photosynthesis

Page 6: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 7: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Location of Photosynthesis

• Chloroplasts – any green part of plant, primarily leaves

• ½ million chloroplasts/mm2 of leaf surface• Green color derived from pigment

chlorophyll• Chlorophyll important in light absorption

(more on that later)

Page 8: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Chloroplasts in Elodea

Page 9: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Location of Chloroplasts

• Found mainly in mesophyll cells – interior of leaf

• O2 exits and CO2 enters leaf through stomata

• Stomata in close proximity to chloroplasts – WHY?

• Veins deliver H20 from roots and carry off sugar to other areas where needed

Page 10: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• Typical mesophyll cell has 30-40 chloroplasts

• Chloroplast structure – remember? • Thylakoids/

grana, stroma

Page 11: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 12: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Leaf cross section

Page 13: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 14: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 15: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis – Redox Rxn Recall -

RESPIRATION • C6H12O6 + 6 O2 + 6 H2O 6 CO2 + 12 H2O +

Energy OR

• C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy

• Glucose is oxidized to form CO2

• Oxygen is reduced, forming water• Reaction is EXERGONIC

Page 16: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis Equation• Photosynthesis reverses aerobic respiration• Net process of photosynthesis is:

6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

• Water split and e- transferred to CO2, reducing it to sugar

• Byproduct: 6O2

• Reaction is ENDERGONIC

Page 17: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 18: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Free Oxygen

• Plants give off O2, split from H2O not CO2

• C.B. van Neil, studies with H2S in bacteria

• Later scientists used radioactive tracer 18O

to confirm van Neil’s H2O hypothesis

Page 19: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis Equation

• Where does the energy to power the reaction come from?

• In reality, photosynthesis adds one CO2 at a time (carbon fixation)CO2 + H2O + light energy -> [CH2O]* + O2

*CH2O represents the general formula for a sugar.

Page 20: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photosynthesis: A closer look…

Two major components• LIGHT REACTIONS

(PHOTOPHOSPHORYLATION) - conversion of light energy to chemical energy

• CALVIN CYCLE (DARK REACTIONS) – transforms atmospheric CO2 to organic molecule; uses energy from light rxn to reduce to sugar.

Page 21: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 22: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 23: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 24: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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

Page 25: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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

Page 26: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 27: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Photons

• Direct particles of energy

• Intensity inversely related to wavelength

• Purple/blue light carries much more

energy than orange/red range of spectrum

Page 28: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• When light meets matter, it is either reflected, transmitted or absorbed

• Different pigments absorb photons of different wavelengths

• WHY ARE LEAVES GREEN?

Page 29: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Spectrophotometer• Measures

pigment’s ability to absorb wavelengths

• Uses transmittance

• Absorption spectrum

Page 30: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 31: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 32: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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)

Page 33: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Action Spectrum • All the pigments together determine “action

spectrum” for photosynthesis

Page 34: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• Action spectrum ≠ absorption spectrum of ONE pigment

• Engelmann 1883 – aerobic bacteria indic. O2

& absorption

Page 35: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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

Page 36: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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

Page 37: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 38: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Chlorophyll “head”

Page 39: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 40: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 41: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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

Page 42: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 43: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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.

Page 44: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 45: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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

Page 46: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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.

Page 47: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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

Page 48: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Noncyclic Pathway• Similar to oxidative phosphorylation

Page 49: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

1. Photosystem II absorbs light, captures an excited electron (rxn ctr oxidized)

2. Enzyme extracts e- from H2O 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

Page 50: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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…

Page 51: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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. 2nd ETC in PS I. Electron carrier is Fd (ferredoxin), a protein

7. NADP reductase transfers e- from Fd to NADP, reducing to NADPH

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Page 53: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
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Page 55: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 56: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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 O2 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.

Page 57: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 58: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 59: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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

Page 60: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

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.

Page 61: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)
Page 62: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

• 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

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Page 64: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)

Overall products

• Noncyclic flow pushes e- from H2O 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

Page 65: Photosynthesis. Plants capture light energy from the sun Energy is converted to chemical energy (sugars & organic molecule)