photosynthesis. plants capture light energy from the sun energy is converted to chemical energy...

Photosynthesis

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.

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/mm2 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

• 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

• Typical mesophyll cell has 30-40 chloroplasts

• Chloroplast structure – remember? • Thylakoids/

grana, stroma

Leaf cross section

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

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

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

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.

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.

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. O2

& 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 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

• 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. 2nd 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 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.

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 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