Chlorophyll

Question 1

How did the introduction of photosynthesis alter Earth?

Answer 1

-oxygen photosynth by cyanobact started to increase O2 conc in atmosphere
-as aerobic resp became more widespread and complex multicellular life developed, O2 conc cont to increase
-with the development of plants, the ozone layer was formed
-evolution of lignin decreased the O2 conc (as fungi and bacteria took longer to break down lignin; trees became stronger and taller)

Question 1

What is the structure of chlorophyll?

Answer 1

-tetrapyrrole ring containing a Mg2+ ion
-conjugated 𝜋 e- system in pyrrole ring (responsible for light absorption)
-hydrophobic phytyl tail (branched hydrocarbon chain with no double bonds)

Question 2

How are energetically unfavourable (+ve ΔG) reactions driven in photosynthesis?

Answer 2

light energy
-allows e- to be transferred from donor with +ve redox pot to acceptor with more -ve redox pot

Question 3

What is the Z-scheme in photosynthesis?

Answer 3

the redox changes
-endergonic steps of light excitation in PS2 and PS1
-exergonic steps

Question 4

What does the antenna complex (in photosystems) do?

Answer 4

Captures light energy and concentrates it to the reaction centre special pair chlorophylls
-hundreds of pigments transfer excitation energy via FRET to special pair

N/B: antenna proteins bound to pigments

Question 5

How is the spectral cross-section of light energy absorbed and transferred to reaction centre broadened?

Answer 5

Combination of different pigment types
-incl chlorophyll a, chlorophyll b, carotenoids

Question 6

Why are photosystems modular systems?

Answer 6

So that there can be more LHCs in low light and less LHCs in high light

Question 7

Why are multiple antenna proteins needed in a photosystem?

Answer 7

To provide large spatial cross-section for light absorption

Question 8

Why does PS2 have an oxygen evolving complex?

Answer 8

for water oxidation

Question 9

What do Jablonski diagrams show?

Answer 9

electron states (ground states and excited states) and electron transfer

Question 10

What does the Jablonski diagram for chlorophyll show?

Answer 10

-ground state (S0) and 2 excited states (S1 and S2)
-one excited state from absoption at blue end of absorption spectra, other at red end (therefore reflects green!)
-femtosecond timescale light absorption