Intro to Photosynthesis (chpt 10) A. Breakdown orgs into 2 categories based on how they get their...
-
Upload
wendy-marilynn-jordan -
Category
Documents
-
view
214 -
download
0
Transcript of Intro to Photosynthesis (chpt 10) A. Breakdown orgs into 2 categories based on how they get their...
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)
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
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!
• 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
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)
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
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.
C. CAM plants – succulents/cacti
• 1. stomata open at night to take in CO2• 2. Calvin cycle done at night
when it’s cooler.