Lecture 8: Photosynthesis

rutgers genbio

carbon fixation

CO₂ → C based molecules

redox

H₂O is oxidized and CO₂ is reduced

endergonic process →

boost from light energy

equation of photosynthesis

Energy + 6CO₂ + 6H₂O → C₆H₁₂O₆ + O₂

photoautotrophs

fix inorganic carbons (CO₂); producers

heterotrophs

obtain carbon material from organic sources (other organisms); consumers

light

form of electromagnetic energy

photo

small particle of light energy; wave

what happens when a molecule absorbs a photon of light energy?

electron becomes energized → shifts to higher energy

options for what molecule absorbing photon of light energy can do

return to lower energy orbital 
or
leave atom →captured by an electron acceptor → acceptor is reduced

what type of light do plants absorb?

visible light

why are leaves green?

chlorophyll reflects and transmits green light

chloroplasts

photosynthetic organelle

main site of photosynthesis

leaves

chloroplasts

primary eukaryotic photosynthetic organelle

thylakoid

inner membrane

stroma

inner “cytosol”

how many outer membranes in chloroplasts?

two

pigment

substance that absorbs visible light

photosynthetic pigment

captures light energy for photosynthesis

location of photosynthetic pigment

embedded in thylakoid

chlorophyll a

main chlorophyll

is H₂O oxidized or reduced?

oxidized

is CO₂ oxidized or reduced?

reduced

is photosynthesis endergonic or exogonic?

exergonic; it requires energy input from sunlight

light reactions

1. split H₂O and release O₂

2. reduce NADP⁺ to NADPH

3. generate. ATP from ADP

how do light reactions occur?

on thylakoid membrane
two photosystems (PSII and PSI) 

1. trap sun energy then

2. convert it to NADPH, ATP, linear electron flow

goal of light reaction

build ATP and NADPH

ATP

cellular energy

NADPH

electron carrier

photosystems II and I

light-harvesting complexes that capture light (photon energy) and use it to excite an electron; each have their own specific wavelength of light

electron transport chains (ETC)

protein chain; pass electrons from one protein to the next

ATP synthase

protein; proton (H⁺) channel

what does the H⁺ flow through the ATP synthase from the lumen to the stroma catalyze?

ADP + Pi → ATP

linear electron flow

both photosystems involved, same 3 things happen in each: excite electrons, use energy in electrons, replace electrons; many steps involve redox reactions

1. boost PSII electron

photon hits pigment in PSII; electron absorbs energy; excited electron transferred to 1^o electron acceptor

2. use energy in electron passed down ETC (to make ATP)

electrons from 1^o electron acceptor goes through ETC; passed from protein to protein and this causes H⁺ to be pumped into the thylakoid (generates H⁺ gradient inside thylakoid space); H⁺ diffuses through ATP synthase (facilitated diffusion) → ATP synthesis

3. replace PSII electron (from H₂O)

PSII is an extremely strong oxidizing agent → H₂O is oxidized (photolysis); electron transferred to PSII; O₂ released as a byproduct; this is where atmospheric O₂ comes from

4. boost PSI electrons

light energy excites electrons in pigments → PSI oxidized, 1^o acceptor reduced

5. use energy in electrons (this time to make NADPH)

electrons transferred to the enzyme NADP⁺ reductase; NADP⁺ + e^- + H⁺ → NADPH

6. replace PSI electrons with PSII electrons

after PSII electrons travel down ETC, energy has been used; now low-energy electrons donated to PSI → replaces lost electrons