PHOTOSYNTHSIS:

• uses CO2 as a carbon source and light as an energy source• directly and indirectly supplies energy to most living organisms

• synthesizes energy rich organic molecules from energy poor molecules like H2O and CO2

LEAVESLEAVES are the major organs of photosyn-thesis in plants.

CHLOROPLASTS:CHLOROPLASTS:* plastid containing chlorophyll

PHOTOSYNTHESIS:6 CO2 + 12 H2O + light C6H12O6 + 6 O2 + 6

H2O

LIGHT REACTIONS:Convert light energy to chemical bond energy in ATP and NADPH• occurs in thylakoid

membrane• reduce NADP+ to NADPHNicotinamide adenine dinucleotide phosphate (coenzyme)• give off O2 as a by product

of splitting H2O• generate ATP

LIGHT: Sun radiates a full spectrum of electromagnetic energy• atmosphere screens out

most, other than visible light

I.R. U.V.

• this visible range of light drives photosynthesis

Blue and red wavelengths are most effectively absorbed by chlorophyll.

Green is useless b/c green light is reflected back.

Chlorophyll a is the primary plant pigment

•Does use accessory pigments to help increase the absorption spectra

•Chlorophyll b

• Carotenoids

Chlorophyll = pigment with a specific absorption spectrum•Can be determined with

a spectrophotometerWhen a pigment absorbs photons of light, the e- jump from ground state to excited state* This is unstable so they

fall back releasing E.Thylakoids trap released energy.

Pigments are clustered together.Only one chlorophyll molecule can initiate the transfer of excited e- to start light reactions = reaction centerOther pigment molecules act as a light gathering antennae.

PHOTOSYSTEM I: reaction center is P700

Absorbs best at 700 nanometers

CYCLIC ELECTRON FLOW:•Uses only Photosystem I• Generates ATP w/o producing NADPH or making O2• cyclic b/c excited e- leave chlorophyll a at the reaction center and return to the reaction center

• absorption of two photons of light (e- travel in pairs) a series of REDOX reactionsOccurs in the thylakoid

• at each step in the ETC, e- lose potential E until they return to ground state

PHOTOSYSTEM II: reaction center is P680

Absorbs best at 680 nanometersNONCYCLIC ELECTRON

FLOW:• Uses both photosytem I and II to transform light energy into chemical energy stored in NADPH and ATP

• occurs in thylakoid• passes e- from H2O to NADP+• produces ATP by non-cyclic photophosphorylation• produces NADPH• produces O2

• Light excites e- from P700• e- do not return to the

reaction center, but are stored as high energy in NADPH• oxidized chlorophyll

becomes an oxidizing agent and its “e- holes” must be filled•Photosystem II supplies e- to P700 to fill these “holes”

• e- ejected from the P680 are trapped by the photosystem II primary e- acceptor• e- are transferred to the primary e- acceptor by the same path used in cyclic e-flow• as the e- move down the ETC, they lose PE until they reach ground state at P700• e- fill vacancies left in PS I when NADP+ is reduced

HOW WATER IS SPLIT:

As e- flow from P680 to P700, e- are restored to P700, but this leaves P680 reaction center missing e-.

• a water splitting enzyme extracts e- from water and passes them to oxidized P680P680 has a great affinity for

e-

PHOTOLYSIS

• as water is oxidized, the removal of e- splits water into two hydrogen ions and an oxygen atom• the oxygen atom combines with a second oxygen atom to form O2

Released as a by-product

DARK REACTIONS: Carbon Fixation1. Each molecule of CO2 is

attached to a 5-C sugar = ribulose biphosphate (RuBP)• reaction is catalyzed by

the enzyme RuBP carboxylase RUBISC

O• product is an unstable 6-C intermediate that becomes two molecules of 3-phosphoglycerate

CALVIN BENSON Cycle

* For every 3 molecules of CO2 that enter the Calvin cycle (via RUBISCO), 3 molecules of RuBP are carboxylated6 molecules of

3-phophoglycerate are formed

2. Each molecule of 3-phospho-glycerate is phosphorylated by an enzyme that transfers a phosphate group from ATP.produces 1,3-

diphosphoglycerate* For every 3 molecules of CO2 entering the cycle, 6 molecules of ATP must be used to make 6 molecules of 1,3 diphospho-glycerate

3. 1,3-diphosphoglycerate has unstable phosphate bonds so it is ready for the addition of high energy e- donated from NADPH• 1,3-diphosphoglycerate is reduced to glyceraldehyde phosphate (by hydrolysis of the phosphate bond)

• e- from NADPH reduce the carboxyl group of 3-phospho-glycerate to the carbonyl group of glyceraldehyde phosphatestores more

PE• glyceraldehyde phosphate is a 3-C sugar (same sugar is produced from the splitting of glucose during glycolysis

So far in the Calvin cycle:• 3 molecules of CO2 have produced 6 molecules of glyceraldehyde phosphate

* Only 1 molecule of 3C sugar can be counted as a net gain of carbohydrate.

4. A complex series of reactions rearrange the carbon skeletons of 5 molecules of glyceraldehyde phosphate into 3 molecules of RuBP.• reactions require 3

molecules of ATP• RuBP is regenerated and can receive CO2 to begin cycle again

PHOTORESPIRATION:Process that uses O2, makes CO2, uses no ATP, and decreases photosynthesis.• active site on RUBISCO

can accept O2 as well as CO2• produces no ATP• decreases photosynthesis by reducing organic molecules needed in Calvin cycle

* When O2 concentration in the leaf is higher than the CO2 concentration, RUBISCO accepts O2 and transfers it to RuBP.• fostered on dry, hot

days when plants close stomates to prevent dehydration due to water loss from the leaf

If photorespiration can be reduced, some plants would increase crop yields b/c plants would not lose valuable CO2.May be a result from earlier

times when atmosphere contained less O2 and more CO2 RUBISCO’s active site

developed an inability to distinguish between O2 and CO2

C4 PLANTS: plants that incorporate CO2 into 4-C compounds (instead of 3-C 3-phosphoglycerate = C3 plant)

ex. corn, sugar cane

Allows plants to fix CO2 under conditions that favor photo-respiration.

1. CO2 is added to phosphophenol pyruvate (PEP) to form oxaloacetate (4C)• PEP carboxylase adds CO2

to PEP (compared to RUBISCO it has a much better affinity for CO2 and NO affinity for O2)

2. After CO2 has been fixed by mesophyll cells, they convert oxaloacetate to another 4-C compound (usually malate)

3. Mesophyll cells export malate to bundle sheath cells through plasmodesmata.• in the bundle sheath cells, malate releases CO2 which joins with RuBP by RUBISCO and goes through Calvin cycle• mesophyll cells pump CO2 into bundle sheath cells, preventing photorespiration

CAM PLANTS: Crassulacean Acid Metabolismex. succulent plants adapted to arid conditions (cacti)• usually open stomates at night and close them during the day (opposite of most plants)

Helps them to conserve water but prevents CO2 from entering leaves

• when stomates are open at night CO2 is taken up and incorporated into a variety of organic acids = crassulacean acid• organic acids are stored in vacuoles in the mesophyll until morning, when stomates close• during the day, the light reactions supply ATP and NADPH