Lecture 23

Question 1

Photosynthesis

Answer 1

converts light energy into chemical energy. The chemical energy is then used to synthesize carbohydrates from C02

Thus, ultimately, all life on earth depends on the sun.

Question 2

So far:

Answer 2

Cells use chemical energy to synthesize ATP, which in turn, is used to fuel all endergonic reactions to sustain life.

Question 3

chloroplast

Answer 3

Note, during photosynthesis, protons will be pumped from the stroma into the thylakoid lumen. Compare to mitochondria!`

Question 4

Light reactions

Answer 4

oxidation of water! Water is very stable.
That means it is reluctant to donate electrons, it is a LOUSY REDUCTANT.
An incredibly STRONG OXIDANT is required to do this!

2H2O -light-> 4H+ + 4e- + O2

Question 5

Dark reactions

Answer 5

should be called “Light-independent” reactions
The catalyze the reduction of CO2
The reducing equivalent used is NADPH

4H+ + 4e- + CO2 –> (CH2O) + H2O

Question 6

What is “light energy”?

Answer 6

– The energy of photons.
E = h x ν

h = Planck constant = 6.63 x 10-34 J x s
ν = frequency = c / λ
c = speed of light = 3 x 108 m/s
λ = wavelength [nm]

Photon energy: The higher the frequency, the shorter the wavelength, the higher the energy

Question 7

How to “capture” light? Pigments absorb photons Chlorophyll

Answer 7

tetrapyrrole (ring I-IV)
ring IV is reduced
cyclopentanone (ring V)
central ion: Mg2+
(Pheophytin has protons instead of a metal ion)
R1, R2, R3: differ in different chlorophylls
R4: hydrophobic, anchors molecule in membrane
Compare to heme!

Question 8

Photosystem II

Answer 8

> 30 chlorophylls

Question 9

Photo-oxidation

Answer 9

Chlorophyll -light–> Chlorophyll+ + electron -

Question 10

Summary of light reactions (part I)

Answer 10

Chlorophyll -light–> Chlorophyll+ + electron -
— electron (high E & high reducing power)

1. Similar to the ETC, the transfer of electrons is used to generate a proton gradient, which is used to generate ATP.
2. These electrons will ultimately reduce NADP+ + H+ to NADPH

Thus, the light reactions convert light energy to chemical energy in form of ATP and NADPH

Question 11

Summary of light reactions (part II)

Answer 11

Chlorophyll –light-> Chlorophyll+
1/2H2O –> H+ + 1/4 O2

Chlorophyll+ = strong oxidant that is capable of oxidizing water!
When it returns to its ground state it has a very positive reduction potential

3. Reduction of chlorophyll radical by oxidation of water

Question 12

reduction potential

Answer 12

if ΔE (E0’ acceptor – E0’ donor) > 0,
then the reaction is spontaneous because ΔG < 0

The more positive E0’, the higher the affinity of the oxidized form of the redox-couple to accept electrons and to become reduced.

Question 13

how often is o2 released

Answer 13

every fourth flash

Question 14

Light reactions in plants

Answer 14

Electrons from water travel through the photosystems synthesize NADPH. Like in the mitochondrial electron transport chain, a proton gradient is formed, which is used to synthesize ATP. Production of ATP and NADPH is uncoupled by the “cyclic” pathway, so the cell can adjust for different needs of ATP and NADPH.

Question 15

efficiency

Answer 15

12 protons –> 4 ATP

2 NADPH –> potential energy equivalent to 5 - 6 ATP

Question 16

Why do photosystems absorb photons of low energy (680 and 700 nm)?

Answer 16

– Antennae pigments collect photons of ALL wavelengths and transfer energy to photosystems. Increase of efficiency: from 1 hit / sec to 10,000,000,000 / sec (90% of photons are now used to activate a photosystem)

Question 17

This is fast:

Answer 17

It takes 10 -100 picoseconds from the time when an antenna pigment absorbs a photon to when the energy excites the reaction center chlorophyll.

Question 18

Key concepts for Photosynthesis- Light Reactions

Answer 18

• Be familiar with the meaning of reduction potential.
• Water is oxidized to O2 by an Oxygen Evolving Center that transfers the electrons to photo-oxidized chlorophyll.
• FAD, Iron-sulfur clusters, Plastoquinone, Cu ions and Plastocyanin act as electron carriers.
• Protons from water and from the Q cycle translocate from the stroma to thylakoid lumen.
• Energy is stored in a proton gradient and is used to make ATP
• Like in the respiratory cascade the Q cycle allows a 2 electron carrier to transfer one electron at a time to a 1 electron carrier.
• NADP+ is reduced to NADPH by electrons ultimately derived from water.
• Plastoquinone and Plastocyanin are “couriers” between complexes (analogous to ubiquinone and cytochrome c in respiration).
• Antenna pigments help to collect energy and distribute it to the special pair of chlorophylls.