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Photosynthesis
Photosynthesis (Life from Light)
Energy needs of life
• All life needs a constant input of energy
o Heterotrophs (consumers)
Animals, fungi, most bacteria
Get their energy from other organisms
Consume organic molecules
o Autotrophs (producers)
Plants, photosynthetic bacteria (blue-green algae)
Get their energy from “self”, ultimately from sunlight
Use light energy to synthesize organic molecules
What does it mean to be a plant?
Need to…
• Collect light energy
• Transform it into chemical energy
• Store light energy in a stable form to be moved around the plant & also saved for a rainy day
• Need to get building block atoms from the environment (C,H,O,N,P,S)
• Produce all organic molecules needed for growth (carbohydrates, proteins, lipids, nucleic acids)
Plant structure
Obtaining raw materials
• Sunlight: leaves = solar collectors
• CO2: stomates = gas exchange
• H2O: uptake from roots
• Nutrients: uptake from roots
•
•
Chloroplasts
o double membrane
o stroma
o thylakoid sacs
o grana stacks
Chlorophyll & ETC in thylakoid
membrane
o H+ gradient built up within
thylakoid sac
Light: Absorption Spectra
• Photosynthesis performs work
only with absorbed wavelengths
of light
• chlorophyll a — the dominant
pigment —absorbs best in red &
blue wavelengths & least in
green
• other pigments with different
structures have different
absorption spectra
• Chloroplasts are green because
they absorb light wavelengths in
red & blue and reflect green back out.
Photosystems -collections of chlorophyll molecules.
• 2 photosystems in thylakoid membrane
o act as light-gathering “antenna complex”
Photosystem II
• chlorophyll a
• P680 = absorbs 680nm wavelength red light
Photosystem I
• chlorophyll b
• P700 = absorbs 700nm wavelength red light
Photosynthesis overview
• Light reactions
o Convert solar energy to chemical energy (ATP)
• Calvin cycle
o Uses chemical energy (NADPH & ATP) to reduce CO2 to build C6H12O6 (sugars).
ETC of Photosynthesis
ETC produces from light energy
o ATP & NADPH
NADPH (stored energy) goes to Calvin cycle
PS II absorbs light
o excited electron passes from chlorophyll to “primary electron acceptor”
o need to replace electron in chlorophyll
o enzyme extracts electrons from H2O & supplies them to chlorophyll
o splits H2O
o combines with another O to form O2
o O2 released to atmosphere and we breathe easier!
Experimental evidence
Where did the O2 come from?
Radioactive tracer = O18
2 Photosystems
Light reactions elevate electrons in 2 steps (PS II & PS I)
o PS II generates energy as ATP
o PS I generates reducing power as NADPH
Cyclic Photophosphorylation
If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH
o Coordinates light reactions to Calvin cycle
o Calvin cycle uses more ATP than NADPH
Where did the energy come
from?
Where did the H2O come
from?
Where did the electrons come
from?
Where did the O2 come
from?
Where did the H+ come
from?
Where did the ATP come
from?
Where did the O2 go?
What will the ATP be used
for?
What will the NADPH be
used for?
Photosynthesis: The Calvin Cycle: Life from Air
Remember what it means to be a plant…
• Need to produce all organic molecules necessary for growth
o carbohydrates, lipids, proteins, nucleic acids
• Need to store chemical energy
o in stable form
o can be moved around plant
o saved for a rainy day
Light reactions
• Convert solar energy to chemical energy
o ATP→ energy
o NADPH → reducing power
• Ultimately build stuff!!
From CO2 → C6H12O6
• CO2 has very little chemical energy
o fully oxidized
• C6H12O6 contains a lot of chemical energy
o reduced
o endergonic
• Reduction of CO2 → C6H12O6 proceeds in many small uphill steps
o each catalyzed by specific enzyme
o using energy stored in ATP & NADPH
From Light reactions to Calvin cycle
• Calvin cycle
o chloroplast stroma
• Need products of light reactions to drive
synthesis reactions
o ATP
o NADPH
Calvin Cycle
• PGAL
o end product of Calvin cycle
o energy rich sugar
o 3 carbon compound
o “C3 photosynthesis”
• PGAL → → important intermediate
PGAL → → glucose → → carbohydrates
→ → lipids
→ → amino acids
→ → nucleic acids
Rubisco
• Enzyme which fixes carbon from atmosphere
o ribulose bisphosphate carboxylase
o The most important enzyme in the world!
o It makes life out of air!
o Definitely the most abundant enzyme
The accounting is complicated:
• 3 turns of Calvin cycle = 1 PGAL
o 3 CO2 →1 PGAL (3C)
• 6 turns of Calvin cycle = 1 C6H12O6 (6C)
o 6 CO2 →1 C6H12O6 (6C)
• 18 ATP + 12 NADPH → 1 C6H12O6
• 6 ATP = left over from light reactions for cell to use elsewhere.
Photosynthesis summary
• Light reactions
o produced ATP
o produced NADPH
o consumed H2O
o produced O2 as byproduct
•
Calvin cycle
o consumed CO2
o produced PGAL
o regenerated ADP
o regenerated NADP
Controlling water loss from leaves
• Hot or dry days stomates close to conserve
water
• Guard cells
o gain H2O = stomates open
o lose H2O = stomates close
• Adaptation to living on land, but… creates
PROBLEMS!
Closed stomates
• Closed stomates lead to…
o O2 builds up (from light reactions)
o CO2 is depleted (in Calvin cycle)
causes problems in Calvin
Cycle
Inefficiency of Rubisco: CO2 vs. O2
• Rubisco in Calvin cycle
o carbon fixation enzyme
Photosynthesis
normally bonds C to RuBP
building sugars
• When O2 concentration is high:
Photorespiration
o Rubisco bonds O to RuBP
o O2 is alternative substrate
o The product splits, and a 2-C compound leaves the chloroplast.
o This product is broken down by peroxisomes back into CO2.
o It wastes ATP!!!
o Why in the world does this happen? Think evolution and atmosphere.
Reducing Photorespiration
• Separate carbon fixation from Calvin cycle
o C4 plants
physically separate carbon fixation from Calvin cycle
different enzyme to capture CO2
PEP carboxylase instead of Rubisco stores carbon in 4C compounds (Loves CO2 not O2)
Different leaf structure
o CAM plants
separate carbon fixation from Calvin cycle by time of day
fix carbon (capture CO2) during night
store carbon in organic acids
perform Calvin cycle during day
Why the C3 problem?
• Possibly evolutionary baggage
o Rubisco evolved in high CO2 atmosphere
There wasn’t strong selection against active site of Rubisco accepting both CO2 & O2
• Today it makes a difference
o 21% O2 vs. 0.03% CO2
o Photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day.
o Strong selection pressure to evolve better way to fix carbon & minimize photorespiration.