gen bio - lecture 8: photosynthesis

equation for photosynthesis

light energy + 6 CO₂ + 6 H₂O = C₆H₁₂O₆ + 6O₂

• H₂O is oxidized

• CO₂ is reduced

• endergonic reaction (energy input from sunlight)

carbon fixation

conversion of CO₂ into carbon based molecules

photoautotrophs

fix inorganic carbons (CO₂)

• producers

heterotrophs

obtain their carbon material from organic sources (other organisms)

• consumers

light is a form of _________ energy

electromagnetic

photon

• small particle of light energy

• wave of light

energy = inversely proportional to wavelength

when a molecule absorbs a photon of light energy… (2 things)

1. electron becomes energized and shifts to a higher energy orbital

2. @ this outermost orbital it can…

• return to a lower energy orbital

OR

• leave the atom and become captured by an electron acceptor —> acceptor becomes reduced

chloroplasts + chloroplast structure

primary eukaryotic photosynthetic organelle

• has 2 outer membranes:

  • inner membrane: thylakoid

  • inner “cytosol”: stroma

pigment + photosynthetic pigment

• pigment: substance that absorbs visible light

• photosynthetic pigment: captures energy for photosynthesis

• embedded in thylakoid membrane

  • chlorophyll a: main photosynthetic pigment

    • absorbs red + violet light

light-dependent reactions

• where do they occur

• what 3 main things happen

• how do these happen (general overview)

• overall goal of light-dependent reaction

• where: in THYLAKOID

• 3 main parts:

1. split H₂O and release O₂

2. reduce NADP+ (electron carrier) to NADPH

3. generate ATP from ADP

• how does this occur:

  • 2 photosystems (PSII and PSI)

    • trap sun energy + convert it to NADPH, ATP through a linear electron flow

• overall goal:

  • TO BUILD ATP (cellular energy) AND NADPH (electron carrier)

light-dependent reactions (six steps)

• step one: boost PSII electron (3 steps)

1. photon hits pigment in PSII (photosystem 2)

2. electron absorbs energy

3. excited electron is transferred to the primary electron acceptor (redox reaction)

light-dependent reactions (six steps)

• step two: use energy in electron passed down ETC to make ATP (3 steps)

1. electron from primary electron acceptor goes through ETC (electron transport chain)

2. electron passes from protein to protein which causes H+ to be pumped into thylakoid

• GENERATES H+ GRADIENT inside thylakoid space

3. H+ diffuses through ATP synthase (facilitated diffusion)

• ATP SYNTHESIS

light-dependent reactions (six steps)

• step three: replace PSII electron from H₂O (5 steps)

1. PSII is an extremely strong oxidizing agent —> H₂O is oxidized through photolysis (means that H₂O is broken down into electrons, hydrogen atoms, and oxygen)

2. electron is transferred to PSII

3. PSII returns to it’s reduced form

4. O₂ is released as a byproduct (atmospheric O₂)

5. the H+ flows from thylakoid lumen to the stroma through ATP synthase

• catalyzes ADP + Pi = ATP

• NOW electron is very low energy

light-dependent reactions (six steps)

• step four: boost PSI electron (1 step)

1. light energy excites electron in pigments

• PSI is oxidized and the primary electron acceptor is reduced

light-dependent reactions (six steps)

• step five: use energy in electron to make NADPH (1 step)

1. electron transferred to enzyme NADP+ reductase

NADP+ + electron + hydrogen = NADPH

light-dependent reactions (six steps)

• step six: replace PSI electron with PSII electron (2 steps)

1. after PSII electron travels down ETC, energy has been used up

2. now low energy electron is donated to PSI to replace the lost electron

explain linear electron flow (same 3 steps happen)

1. excite electron

2. use energy in electron

3. replace electron

carbon fixation reactions (where do they occur and why)

• where? stroma

• why? reduce CO₂ into sugar using NADPH and ATP

  • step one: carbon fixation

  • step two: carbon reduction

  • step three: regeneration

carbon fixation reactions

• step one: carbon fixation (3 steps)

1. creating a carbon compound from CO₂

2. CO₂ + 5 carbon RuBP (ribulose biphosphate) are catalyzed by rubisco (enzyme)

3. this forms unstable 6 carbon compound —> splits into two 3 carbon PGA

carbon fixation reactions

• step two: carbon reduction (1 step)

PGA is reduced by…

• NADPH —> NADP+

• ATP —> ADP + Pi

into…

• two 3 carbon G3P molecules (will become sugar)

carbon fixation reactions

• step three: regeneration of RuBP (2 steps)

1. five 3 carbon G3Ps continue in the cycle

2. add ATP to convert them into RuBP

process restarts!!