Chapter 10

Photosynthesis

Photosynthesis

• main idea: making glucose

• autotroph – self-feeder; -organism which makes its own food

a) phototrophic – uses light b) chemotrophic – uses chemicals from its environment

Photosynthesis overviewA. Photophase (Light reactions) – occur in grana -light dependent

B. Synthetic phase (Dark reactions) – occur in stroma -light independent

Light reactions• Sunlight – form of electromagnetic energy (radiation) that travels in waves• Wavelength range is electromagnetic spectrum, part

of which we see as colors of visible light (ROY G BIV) at 400 – 700 nm• Particles of light are photons (or quanta)• Amount of energy to wavelength is inversely related ex: short wavelength = greater energy of photon violet has 2x greater energy than red

• Pigments – substances that absorb visible light; color seen is most reflected

• blue & red are absorbed by chlorophyll (which reflects green)

• Pigment’s absorption ability is measured quantitatively by spectrophotometer

Splitting of water

• 1st thought that CO2 was split

• 1930’s vanNiel studying photosynthesis in bacteria found these use H2S and

removed H to make sugar, therefore, all phototrophs need an H compound & plants must split water, not CO2

Van Niel’s experiment

• 1950’s experiment conducted using heavy oxygen 18O CO2 + H2O O2

CO2 + H2O O2

Respiration yields 686 kcal of free energy per mole (the exact amount required to reduce CO2 to glucose)

Photooxidation of chlorophyll

• Pigments go from ground state to excited state when photons boost energy to higher

levels (excited state is unstable) so energy is quickly passed or released as heat or light in some cases (fluorescence)

Photosystems

• primary electron acceptor – traps high-energy electrons that have absorbed photons; this energy stored powers ATP & NADPH synthesis

• accessory pigments clustered in antenna complex of several hundred molecules

around a chlorophyll a at the reaction center

Photosystems• Antenna complex, reaction center chlorophyll, &

primary electron acceptor make up a photosystem (light harvesting unit built into the thylakoid membrane)

• 2 kinds:Photosystem I – contains p700 absorbs best at 700 nm (far red)Photosystem II – contains p680 absorbsbest at 680 nm (red)

*p700 & p680 are identical chlorophyll a molecules, but associated with different proteins

ATP formation

• ATP synthetases form ATP when thylakoid membranes pump protons from stroma to thylakoid compartment

Calvin cycle

• Cyclic metabolic pathway in stroma • Uses ATP & NADPH from light reactions

to reduce CO2 to sugar

• CO2 combines with RuBP (5-C sugar) to make a 3-C sugar (G3P) ultimately using ATP & electrons from NADPH

Calvin cycle stepsStep 1: Carbon fixation -enzyme catalyzing RuBP is RuBP carboxylase (rubisco) -6-C intermediate splits into 2 3-PGA

(each 3 CO2 entering, 6 ATP used)Step 2: Reduction

-NADPH2 donates a pair of high energy electrons to form sugar (G3P)

-2 G3P’s can be rapidly converted to one glucose molecule (18 ATP’s &

12 NADPH2)

Calvin cycle

Step 3: Regeneration of CO2 acceptor (RuBP)

-5 molecules of G3P are rearranged into

3 molecules of RuBP

Summary• For 1 G3P, 9 molecules of ATP & 6

molecules of NADPH

• Light reactions regenerate ATP & NADPH

• G3P from the Calvin cycle is the material used to synthesize other organic compounds such as glucose & other carbohydrates

Alternatives for carbon fixation

• photorespiration – -rubisco adds O2 to the Calvin cycle instead of

CO2

-decreases photosynthetic output by siphoning

organic materials from the Calvin cycle

Alternatives for carbon fixationC4 plants:

-preface Calvin cycle by fixing carbon into a 4-C compound (C4) ex: sugar cane & corn -have structural adaptations (bundle sheath cells)

-CO2 is incorporated into organic compounds in mesophyll then they’re exported to bundle

sheath cells (spatial separation) -PEP carboxylase adds CO2 to PEP

(phosphoenolpyruvate)

Alternative for carbon fixation

CAM plants: (Crassulacean acid metabolism) -evolved in succulents as an adaptation to

arid environmentsex: cacti & pineapples

-incorporate carbon into intermediate compounds during the night & when

stomata close during the day, those compounds fuel the Calvin cycle – these steps occur at different times (temporal)