unit 2: photosynthesis Flashcards
overall photosynthesis reaction
6CO2 + 12 H2O -> C6H12O6 + 6H2O + 6O2
where does the oxygen come from in the water produced from photosynthesis?
from CO2
where does the hydrogen come from in the water produced from photosynthesis?
from H2O in the reactants
where does the oxygen come from in the O2 produced from photosynthesis?
from H2O in the reactants
where does the oxygen come from in the glucose produced from photosynthesis?
from CO2
where does the hydrogen come from in the glucose produced from photosynthesis?
from H2O in the reactants
where does the carbon come from in the glucose produced from photosynthesis?
from CO2
describe the flow of energy in photosynthesis
light energy (sun) -> chloroplasts make sugar (organic molecules) -> heterotrophs eat plants and perform cellular respiration (ATP)
granum
stacks of thylakoids
thylakoid
the inner coin like structures in chloroplasts; photosynthesis occurs inside these
stroma
cytoplasm of the chloroplast where the thylakoids lay
lumen
the inner contents of the thylakoids
reactants for light reactions
H2O + light energy + NADP+ + ADP
NADP+ and ADP come from calvin cycle
products for light reactions
O2 + ATP + NADPH
ATP and NADPH go to calvin cycle
reactants for dark reactions
ATP + NADPH + CO2
ATP and NADPH come from the light reactions
products for dark reactions
sugar (CH2O) + NADP+ + ADP
NADP+ and ADP go to light reactions
stomata
pores in the leaves that allow CO2 and O2 out
chloroplasts
in leaves, these contain the photosynthesizing structures
chlorophyll
a green pigment that reflects only green light and absorbs all other wavelengths; alone, chlorophyll cannot perform photosynthesis, but with an arrangement of proteins, photosystems perform photosynthesis in the thylakoid membrane
order of photosystems
II -> I
light reactions: step 1
light energy strikes a pigment molecule in photosystem II, exciting its electrons. when these electrons fall back down, they release energy that is passed to the electrons of the next pigment molecule, causing a chain reaction of excited electrons (not transferring electrons, just energy). eventually this energy is relayed to the P680 pair of chlorophyll A molecules in the reaction center complex to excite their electrons to a higher energy state. these electrons in P680 are supplied by the splitting of water.
light reactions: step 2
one at a time, the electrons from P680 are excited and transferred to the primary electron acceptor of photosystem II. when P680 loses its electrons (oxidized), it becomes P680+, one of the strongest oxidizing agents.
light reactions: step 3
an enzyme catalyzes the splitting of water into 2e- + 2H+ and 1/2 O2, of which the e- replace the ones in P680. H+ are released into the thylakoid space and the oxygen atoms combine to form O2.
light reactions: step 4
photo excited electrons pass from the primary electron acceptor down an electron transport chain from photosystem II to photosystem I. through this chain electrons are passed to more and more electronegative atoms, which releases energy which can do work: establish a gradient of H+ ions across the thylakoid membrane (more inside, lower pH). this gradient will go away when there is no light source.
light reactions: step 5
the potential energy stored in the proton gradient is used to make ATP through photophosphoylation and chemiosmosis.
light reactions: step 6
light energy has been tranferred from the electron transport chain to light-harvesting pigments in PS I, then to the reaction center complex, which houses the P700 pair of chlorophyll A molecules. when these electrons (supplied by PS II), they too are passed to a primary electron acceptor, creating P700+ (another strong oxidizing agent).
light reactions: step 7
photo excited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin. this chain does not create a proton gradient and does not produce ATP>
light reactions: step 8
the enzyme NADP+ reductase catalyze the transfer of electrons from ferredoxin to NADP+ to reduce it to NADPH (takes 2e-). the electrons in NADPH are at a higher energy level than when they started in water, so they are more readily available for reactions in the calvin cycle. this process also removes an H+ from the storm when NADP+ + H+ form NADPH. this NADPH is produced on the side of the membrane facing the stroma; this shuttles the higher energy electrons to other reactions.
linear electron flow
photosynthesis through PS II to PS I
cyclic electron flow
a short circuit: the electrons cycle back from ferredoxin to the cytochrome complex (in the ETC from PS II to PS I) and a plastocyanin molecule (Pc) to the P700 chlorophyll A molecules in the PS I reaction center complex, which again donate their electrons to the primary electron acceptor, then ferredoxin. there is no production of NADPH and no release of oxygen. however, this flow does generate ATP through the ETC.
2 stages of the calvin cycle
- carbon fixation
- reduction
- regeneration of RuBP (CO2 acceptor)
overview of dark reactions (calvin cycle)
CO2 is added to a five carbon molecule, creating a highly unstable 6 carbon molecule (carbon fixation). this six carbon molecule is immediately split in half into two three-carbon molecules. each of these three carbon molecules is then phosphorylated (uses ATP) and reduced, to create a high energy sugar molecule called glyceraldehyde-3-phosphate (G3P; also present in cellular respiration).
role of G3P in plants
in the calvin cycle, 6 G3P are made. 5 go to regenerate the 3 RuBP necessary to keep the cycle going and 1 G3P is used to create glucose and other organic compounds
calvin cycle: step 1 reactants
3 CO2 + 3 RuBP
calvin cycle: step 2 reactants
3 highly unstable 6C molecules
calvin cycle: step 3 reactants
6 ATP + 6 3-phosphoglycerate
calvin cycle: step 4 reactants
6 NADPH + 6 1,3-bisphosphoglycerate
calvin cycle: step 5 (where does G3P go?)
5 G3P go to remake the 3 RuBP necessary to keep the calvin cycle going: 5x3C = 3x5C
1 G3P goes to be used to make sugar and other organic molecules
calvin cycle: step 1 products
3 highly unstable 6 carbon molecules
calvin cycle: step 2 products
6 3-phosphoglycerate
calvin cycle: step 3 products
6 1,3-bisphosphoglycerate
calvin cycle: step 4 products
6 glyceraldehyde-3-phosphate (G3P)
calvin cycle: step 1 overview
carbon dioxide is taken from the atmosphere and added to a sugar, RuBP, to make a highly unstable 6C molecule. this reaction is facilitated by rubisco, which is the most abundant enzyme on earth
rubisco
most abundant enzyme on earth. binds CO2 to RuBP
calvin cycle: step 2 overview
the unstable 6 carbon molecule is almost immediately broken into two 3-phosphoglycerate molecules (building up to what can become a glucose molecule, aka anabolic)
anabolic reaction
reactions that build up larger, more complex molecules to be used by the body; this requires energy
catabolic reaction
the breaking down of large, complex molecules into smaller parts; this releases energy
calvin cycle: step 3 overview
phosphate is added to 3-phosphoglycerate to make 1,3-bisphosphoglycerate (uses 6 ATP, 1 ATP per 3PG)
calvin cycle: step 4 overview
add e- to 1,3-bisphosphoglycerate (oxidize it) from NADPH, creating NADP+ and G3P (the main product of the calvin cycle)
what is the problem of photorespiration?
in hot, dry climates plants will close their stomata so they don’t lose water, which also stops them from taking in CO2 and releasing O2, which reduces their photosynthetic output
what is the role of rubisco in photorespiration? (the problem)
rubisco will add O2 to RuBP in the presence of O2, which takes CO2 out of the plant and does not create ATP, rather uses it, keeping the plant from growing
how do plants avoid photorespiration?
separate rubisco from O2
how do C4 plants avoid photorespiration?
physically separate O2 and rubisco
pepcarboxylase (held in mesophyll cells) binds CO2 into 4C molecules and then bundle sheath cells (which hold rubisco) peel off CO2 to bind with RuBP to make G3P
how do CAM plants avoid photorespiration?
they create a temporal (time based) separation of O2 and rubisco. they tend to open their stomata at night which allows CO2 to come in – uses the same anatomy as C3 plants. however, when the CO2 comes in at night, they store it in an organic acid and peel it off during the day when the calvin cycle can work
