CHAPTER 10: PHOTOSYNTHESIS
PHOTOSYNTHESIS
Energy transfer from solar energy into chemical energy
Performed by “photoautotrophs,” who use the energy of sunlight to make organic molecules from water and carbon dioxide Producers Eaten by consumers (heterotrophs)
THE LEAF
CHLOROPLASTS
Likely evolved from cyanobacteria Located in leaves, mainly in the mesophyll Contain green chlorophyll
PHOTOSYNTHESIS
6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2
Light dependent reactions Chloroplasts split water into H and O
Light independent reactions Chloroplasts incorporate hydrogen from water
into sugar Redox!
H2O is oxidized to O2 CO2 is reduced to C6H12O6
OVERVIEW
SUNLIGHT
Light is electromagnetic energy Travels in waves
Distance between crests is wavelength Wavelength determines the type of
electromagnetic energy Consists of particles called photons Different pigments absorb different
wavelengths Chlorophyll absorbs everything but green light
A spectrophotometer measures a pigment’s ability to absorb wavelengths
ABSORPTION SPECTRUM
Absorption Spectrum- pigment’s light absorption vs wavelength
Action Spectrum- relative effectiveness of different wavelengths of radiation in driving photosynthesis First demonstrated by Theodor W Engelmann
and his oxygen-loving bacteria
PIGMENTS
Chlorophyll a- main photosynthetic pigment Chlorophyll b- accessory pigment, broadens
the spectrum Carotenoids- accessory pigments, absorb
excessive light that would damage chlorophyll
ABSORPTION
Light causes pigment’s electrons to go from ground state to excited state
Electrons fall down, and give off photons
PHOTOSYSTEM
Light-harvesting complexes Light-harvesting
complexes funnel the energy of photons to the reaction center
Reaction-center complex Contains a primary
electron acceptor, accepts an electron from chlorophyll a
PS II AND PS I
Photosystem II functions first Contains P680, which absorbs light at 680 nm
Photosystem I funcitons second Contains P700
Linear electron flow, or non-cyclic flow, involves both II and I, and produces ATP and NADPH
NON-CYCLIC ELECTRON FLOW
CYCLIC ELECTRON FLOW
CHEMIOSMOSIS
Mitochondria perform oxidative phosphoroyation Protons are pumped into the intermembrane
space and diffuse back into the mitochondrial matrix
Chloroplasts perform photophosphorylation Protons are pumped into the thylakoid space and
diffuse back into the stroma
ANOTHER DIAGRAM!
CALVIN CYCLE
Uses CO2, ATP, and NADPH to make sugar
Carbon enters as CO2 and leaves as glyceraldehyde-3-phosphate (G3P)
3 molecules of CO2, and 3 turns of the cycle, make 1 G3P
Has three phases: Carbon fixation Reduction Regeneration of the CO2 acceptor
I REALLY LOVE DIAGRAMS
ADAPTATIONS IN ARID CLIMATES
Plants close stomata when weather is hot and dry Reduces access to CO2, builds up O2 Most plants are C3 plants Photorespiration: rubisco bonds with O2 and runs
the Calvin cycle in reverse WASTEFUL, but gets rid of excess O2
C4 PLANTS
Incorporate CO2 into four-carbon compounds in mesophyll
Uses PEP carboxylase, which has a higher affinity for CO2 than rubisco does
Compounds exported to bundle-sheath cells, where they deliver CO2 used for the Calvin cycle
CAM PLANTS
Use crassulacean acid metabolism for carbon fixation
Open their stomata at night, and incorporate CO2 into organic acids
Stomata close during the day