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