Intro to Photosynthesis (chpt 10)

• A. Breakdown orgs into 2 categories based on how they get their nutrients– 1. autotrophs – can make their own

food aka producers• a. photoautotroph – uses light as a source of E to make food• b. chemoautotroph – uses chemicals like sulfur or ammonia to make food (usually bacteria)

• 2. heterotrophs – can’t make their own food aka consumers

II. Where does photosyn take place? chloroplasts!

• A. All green parts of a plant contain chloroplasts which contain chlorophyll – leaves contain chloroplasts in the mesophyll

B. inside photosynthetic plant cell

• 1. stomata – “pores” on underside of leaf that allow CO2 in & let O2 (& water) out

• 2. chlorophyll – green pigment able to absorb light E

• 3. thylakoid – membranous flattened sacks in chloroplast – stacks of thylakoids are called grana

III. pathways of photosyn (in C3 plants – most common)

• A. Equation

6CO2 + 6H2O >>> C6H12O6 + 6O2

B. looks like backwards cell resp.

C. overview of photosynthesis

• 1. 2 parts–a. light reaction• 1) purpose – to make NADPH (another e- carrier) & ATP• 2) occurs in the thylakoid membranes of the chloroplast

• b. dark rxn aka Calvin cycle–1) purpose – to change CO2 into

glucose (or other carbs)

–2) occurs in the stroma of the chloroplast

IV. quick review: light properties• A. light acts as both a particle & a wave

– we’re interested in the wave aspect–1. electromagnetic spectrum

• 2. visible light – ROY G BIVColor Wavelength

red 700nm

orange 630nm

yellow 590nm

green 540nm

blue 490nm

indigo 450nm

violet 380nm

• 3. “pieces” of light = photons–a. Photons have a fixed quantity of

E. The amt of E is related to the wavelength of light.• 1. short wavelength has high E• 2. long wavelength has low E

V. Light rxn of photosynthesis

• A. excitation of chlorophyll–1. Photons are absorbed by atoms

& e- jumps to a higher energy level farther away from the nucleus. The e- quickly loses this E in the form of light/color & falls back down to its original energy level

• 2. photosystems – complex made of chlorophyll A, proteins & other mlcls–a. photosystem I (aka P700) – 1st

discovered, can absorb light in the 700nm range ---- if it absorbs light in that range that means we don’t see that color!

• b. photosystem II (aka p680) – 2nd discovered, absorbs light in the 680nm range• picture p 185?

• note – there are other chlorophylls & pigments, but chlorophyll A is the only one that participates DIRECTLY in photosyn. – the others can funnel light E into chlorophyll A

B. noncyclic electron flow – predominant route of e- flow

• 1. steps of noncyclic flow–a. Light hits P II (P680) & an e- is

excited & captured by the primary e- acceptor. A “hole” now exists in the P680 system–b. water is split & an e- from it is

used to replace the e- lost from P680 ---- O2 is released!!

• c. the excited e- from P680 is passed down to the PI (P700) system along an ETC– like the one in cell resp.

• d. as the e- “falls” down the ETC, ATP is produced for use in the Calvin Cycle

• e. the e- fills a “hole” in the P700 (created when light hit the P700 system, excited an e-, which was then captured by the primary e- acceptor)

• f. the e- from the P700 is then passed along a second ETC where NADP+ is reduced to NADPH --- the NADPH will be used in the Calvin cycle (aka dark rxn)

• 3. cyclic knows to turn on when there is an abundance of NADPH in the chloroplast

• Noncyclic electron flow produces ATP and NADPH in roughly equal quantities. P 187 bottom (Ferrodoxin)

• However, the Calvin cycle consumes more ATP (9) than NADPH (6).

• Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

VI. The Calvin Cycle (aka dark rxn)

• A. general info–1. ATP & NADPH from the light rxn

are used in the Calvin cycle–2. similar to Kreb’s cyc in cell resp..

because there is a regeneration of mlcls–3. overall rxn of the Calvin cycle

(see paper)

• 4. actual output of Calvin cycle is PGAL 3-phosphoglyceraldehyde

• 5. the cycle must occur TWICE to produce 1 molecule of sugar

B. Calvin cycle in 3 parts

1. carbon fixation

• a. CO2 bonds to RuBP via the enzyme rubisco to form a 6-carbon intermediate• b. the 6-C intermediate is

unstable & promptly breaks down into 2 3-C mlcls (for each CO2) this 3-C mlcl is PGA (3-phosphoglycerate)

carbon fixation

2. reduction

• a. ATPs used to add phosphate groups to the carbon chain• b. NADPHs are used to reduce

the carbon chain to G3P• c. CO2 (3 of them) used to

produce 6 mlcls of G3P• d. only 1 G3P can be used – the

other 5 are reused in the cycle

reduction

3. regeneration

• a. carbon skeletons are rearranged to form 3 mlcls of RuBP• b. uses 3 more ATPs

VII. Alternatives to carbon fixation

• A. C3 plants carry out photorespiration – –occurs on hot dry days–occurs in light

–consumes O2

• 1. CO2 enters the leaf through stomata• 2. stomata also function in

transpiration (water loss through evap) so if it’s hot & dry the stomata stay closed to preserve water & thus have a problem taking in CO2

• 3. HOT climate??? can carry out photorespiration

• 4. photorespiration consumes O2• 5. it releases CO2• 6. it makes no ATP• 7. since it uses up CO2 there is

not photosynthetic output – everything decreases (but the plant conserves water)

B. C4 plants us alternative mode of C fixation so their rate of photosyn doesn’t decrease!

• 1. They use an alternate 4-C cmpd that doesn’t break down as easily as RuBP• 2. the enzyme PEP (phosphoenolpyruvate)

is used instead of rubisco. PEP has a higher affinity for CO2 & is able to capture CO2 even at really low amts.

• 3. example: corn, pineapple, & sugar cane

C. CAM plants – succulents/cacti

• 1. stomata open at night to take in CO2• 2. Calvin cycle done at night

when it’s cooler.