Download X. NONCYCLIC FLOW OF ELECTRONS (or LINEAR FLOW) A. Both

X. NONCYCLIC FLOW OF ELECTRONS (or LINEAR FLOW)
A. Both photosystems I and II function and cooperate
B. Electrons pass continuously from water to NADP+
C. Steps in noncyclic flow
1. Light elevates two electrons of P700 in photosystem I to the excited state.
2. These excited electrons DO NOT return to the reaction center, but are stored in
NADPH for use in the Calvin Cycle.
3. Photosystem I lost electrons to NADP. These electron “holes” must be filled.
4. Photosystem II supplies electrons to P700 to fill these “holes”.
5. Two photons of light are absorbed by pigment molecules of photosystem II. This
energy is transferred to P680 reaction center.
6. Excited electrons are trapped by the primary electron acceptor.
7. Electrons are transferred to the same transport chain used in cyclic electron flow.
Electrons lose potential energy until they reach the ground state of P700.
8. These electrons then fill the electron vacancies left in photosystem I when NADP+
was reduced.
9. Chemiosmosis
a. As electrons move from photosystem II to photosystem I, the transport chain
chain pumps H+ across the thylakoid membrane.
b. An ATP synthase uses this to make ATP.
c. This is called NONCYCLIC (OR LINEAR) PHOTOPHOSPHORYLATION
(mechanism is the same as in cyclic photophosphorylation).
TO THIS POINT, NONCYLCLIC ELECTRON FLOW HAS PRODUCED NADPH AND ATP AS WELL AS
RESTORING THE MISSING ELECTRONS TO THE REACTION CENTER OF PHOTOSYSTEM I.
d. The P680 reaction center of photosystem II now has two electron holes to
fill.
e. Oxidized P680 has a great affinity for electrons in water.
f. Water is oxidized and the removal of these electrons splits water into two
two hydrogen ions and an oxygen atom.
g. The oxygen atom immediately combines with a second oxygen atom to
form O2. This is the water splitting step of photosynthesis that releases O2.
THE NET RESULT OF NONCYCLIC ELECTRON FLOW (LINEAR) IS THE PUSHING OF LOW
POTENTIAL ENERGY ELECTRONS FROM WATER TO A STATE OF HIGH POTENTIAL ENERGY IN
NADPH.
FOUR PHOTONS OF LIGHT MUST BE ABSORBED (TWO FOR EACH PHOTOSYSTEM) FOR EACH
PAIR OF ELECTRONS TRANSFERRED BY THE LIGHT REACTIONS.
The two photosystems are connected in a series:
electron current flows from water -------> P680 -------> ETC -------> P700 -------> NADP+
THE LIGHT REACTIONS USUALLY TAKE THE FORM OF NONCYCLIC ELECTRON FLOW.
XI. COMPARISON OF CHEMIOSOMOSIS IN CHLOROPLASTS AND MITOCHONDRIA
A. Electron Transport Chain
1. In mitochondria, the high energy electrons that pass down the transport chain
are extracted by the oxidation of food molecules; thus, mitochondria transfer
chemical energy from food molecules to ATP.
2. In chloroplasts, photosystems capture light energy and use it to drive electrons to the
top of the transport chain; thus, chloroplasts transform light energy into chemical
energy.
B. Spatial Organization
1. Inner membrane of a mitochondrion pumps protons from matrix out to
intermembrane space.
2. The thylakoid membrane pumps protons from the stroma into the thylakoid
space. Thus, ATP forms in the stroma where it drives sugar synthesis during the
Calvin cycle.
XII. THE CALVIN CYCLE
A. STEP ONE- Each molecule of C02 is added to a molecule of ribulose bisphosphate (RUBP).
1. This reaction is catalyzed by the enzyme, rubisco.
2. Rubisco is the most abundant protein in the chloroplast.
3. This reaction produces a six-carbon molecule that immediately splits to form two
molecules of 3-phosphoglycerate(PGA)
4. For every 3 molecules of CO2:
a. 3 molecules of RuBP are formed
b. 6 molecules of 3-phosphoglycerate (PGA)
B. STEP TWO-One molecule of each of ATP and NADPH from the light dependent reactions
are used to convert each molecule of PGA to a 3-carbon sugar phosphate G3P
(Glyceraldehyde-3-phosphate)
C. STEP THREE-G3P undergoes a series of reactions that form several 4-, 5-, and 6-carbon
sugar phosphates.
D. STEP FOUR-The final step uses an ATP molecule from the light reactions to regenerate
RuBP from a 5-carbon sugar phosphate, thus completing the cycle.
FOR THE CALVIN CYCLE TO MAKE ONE MOLECULE OF SUGAR, THE CYCLE MUST MAKE THREE
TURNS WITH ONE MOLECULE OF CO2 BEING FIXED DURING EACH TURN.
ONE MOLECULE OF G3P CAN BE COUNTED AS A NET GAIN. ONLY ONE MOLECULE OF G3P
CAN LEAVE THE CYCLE (6 WERE MADE). THE OTHER 5 G3P ARE RECYCLED TO REGENERATE
THE RuBP.
THE CALVIN CYCLE USES 9 MOLECULES OF ATP, 6 MOLECULES OF NADPH TO MAKE ONE
MOLECULE OF G3P.
G3P is a 3-carbon molecule. Two molecules of this compound can be rapidly converted to glucose (six
carbon molecule).
TO PRODUCE ONE MOLECULE OF GLUCOSE, THE CALVIN CYCLE USES 18 ATPs AND 12 NADPHs.
THE ADP AND NADP+, RESULTING FROM THE CALVIN CYCLE, RETURN TO THE LIGHT
DEPENDENT REACTIONS TO PICK UP MORE ENERGY AND ELECTRONS.
XII. PHOTORESPIRATION
A. A process that uses oxygen, makes CO2, and generally occurs only in light.
B. When the concentration of oxygen is much higher than that of CO2 in the air spaces of a
leaf, the active site of rubisco can accept O2 in the place of CO2.
C. When CO2 binds with rubisco------->3-carbon molecules of PGA form (normal Calvin cycle
production of G3P occurs).
D. In photorespiration O2 binds with rubisco------->1 molecule of PGA + 1 molecule of a 2carbon compound are formed. This 2-carbon compound leaves the chloroplast and is
broken down to release CO2. This slows the rate (by as much as 50%) of photosynthesis
by interfering with the successful performance of the Calvin Cycle by preventing CO2
fixation.
E. Photorespiration is fostered by hot, dry, bright days.
1. Under these conditions, plants close their stomata to prevent dehydration by
reducing water loss through the stomata of the leaf.
2. Photosynthesis then depletes the CO2 levels and increases the O2 levels within the
spongy mesophyll air spaces (this leads to rubisco accepting O2 which causes
photorespiration).
THE LEAF STRUCTURE THAT YOU STUDIED WAS OF A TYPICAL C3 PLANT IN WHICH RUBISCO
ADDs CO2 TO RuBP (5 carbon) TO FORM 2 PGAs (3-CARBON COMPOUNDs).
EXAMPLES OF C3 PLANTS THAT ARE IMPORTANT IN AGRICULTURE INCLUDE RICE, WHEAT,
AND SOYBEANS.
F. Alternative modes of carbon fixation have evolved that minimize photorespiration and
optimize the Calvin cycle-even in hot acrid climates.
1. C4 plants (examples: sugarcane, corn, members of grass family) thrive in hot regions
with intense sunlight where stomata partially close during the day.
a. CO2 is added to a 3-carbon compound PEP (or phosphoenolpyruvate) instead
of RuBP forming a four-carbon (oxaloacetate)
b. C4 plants use the enzyme, PEP carboxylase (instead of rubsico). PEP
carboxylase has a higher attraction for CO2 than rubsico and no affinity for
oxygen.
c. After “fixing” CO2 to a four-carbon compound oxaloacetate and then malate,
this 4-carbon compound is export from mesophyll cells to bundle sheath cells
through plasmodesmata.
d. In bundle sheath cells, the CO2 in released and enters the Calvin Cycle.
In addition to CO2, pyruvate (3C) is released. ATP is used to change
pyruvate into PEP allowing cycle to continue while stomata are closed.
e. In order to change pyruvate into PEP, the bundle sheath cells have PS I
to carry out cyclic electron flow (generating needed ATP).
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2. CAM (Crassulacean acid metabolism) plants (succulent or water-storing
plants)(examples: cacti, pineapples)
a. These plants open stomata during the night and closed them during the
day.
b. During the night, these plants take in CO2 and store it in several organic
molecules. They store the organic molecules containing carbon in vacuoles.
c. During the day, when stomata are closed, the light reactions supply NADPH
and ATP, and the plants converted the stored organic molecules into CO2
to run the Calvin cycle.
NOTICE THAT CAM, C4, AND C3 PLANTS ALL USE THE CALVIN CYCLE TO MAKE SUGAR FROM
CARBON DIOXIDE.