Where it starts:? Photosynthesis
• Before photosynthesis
evolved,
– Earth’s atmosphere had
little free oxygen
• Oxygen released during
photosynthesis changed
the atmosphere
Then
– Favored evolution of new
metabolic pathways,
including aerobic
respiration
Now
Sunlight as an energy source
• Visible light
– A small part of a spectrum
of electromagnetic energy
radiating from the sun
• Electromagnetic energy
– Travels in waves
– Is organized as photons
Photosynthetic Pigments
• Photosynthesis begins
when photons are
absorbed by
photosynthetic pigment
molecules
• Pigment molecules absorb
only light of particular
wavelengths
– Photons not captured are
reflected as color
Major Photosynthetic Pigments
• Chlorophyll a
– Main photosynthetic pigment
– Absorbs violet and red light (appears
green)
• Chlorophyll b, carotenoids, phycobilins
– Absorb additional wavelengths
• Collectively, photosynthetic pigments
absorb almost all of wavelengths of
visible light
Overview of photosynthesis
• Photosynthesis proceeds in two
stages
– Light-dependent reactions
– Light-independent reactions
6H2O + 6CO2
6O2 + C6H12O6
Sites of photosynthesis: chloroplasts
• Light-dependent
reactions occur at a muchfolded thylakoid
membrane
– a single, continuous
compartment inside the
stroma (chloroplast’s
semifluid interior)
• Light-independent
reactions occur in the
stroma
Products of light-dependent reactions
• Typically, sunlight energy
pass to photosystems (I, II)
• pull electrons from water
– release them
– drives formation of ATP and
NADPH
• Oxygen is released from the
chloroplast (and the cell)
Light independent reactions:
the sugar factory
• Light-independent reactions proceed in the
stroma
• Carbon fixation: Enzyme rubisco attaches
carbon from CO2 to RuBP to start the Calvin–
Benson cycle
Calvin–Benson Cycle
• Cyclic pathway makes glucose
– uses energy from ATP, hydrogen and electrons from NADPH
– carbon and oxygen from CO2,
• Reactions use glucose to form photosynthetic products
(sucrose, starch, cellulose)
Plant adaptations to environment
• Environments differ
– Plants have different details of
sugar production in lightindependent reactions
• On dry days, plants conserve
water by closing their stomata
– O2 from from photosynthesis
cannot escape
• Photorespiration reduces
efficiency of sugar production
Plant adaptations to environment
• C4 plants
– Carbon fixation occurs
twice
– First reactions release CO2
near rubisco, limit
photorespiration when
stomata are closed
Plant adaptations to environment
• CAM plants
– Open stomata and fix
carbon at night
A burning concern
• Photoautotrophs remove CO2
from atmosphere; metabolic
activity of organisms puts it back
• Human activities disrupt the
carbon cycle
– Add more CO2 to the atmosphere
than photoautotrophs can remove
• Imbalance contributes to global
warming