5 Steps to a 5 AP Biology, 2014-2015 Edition

Photosynthesis ❮ 79

brought back into the thylakoid compartment, creating an H+gradient similar to what we

saw in oxidative phosphorylation. During the light-dependent reactions, when hydrogen

ions are taken from water during photolysis, the proton gradient grows larger, causing some

protons to leave, leading to the formation of ATP.

You’ll notice that this process in plants is a bit different from oxidative phosphoryla-

tion of the mitochondria, where the proton gradient is created by pumping protons from

the matrix outto the intermembrane space. In the mitochondria, the ATP is produced

when the protons move back in.But in plants, photophosphorylation creates the gradient

by pumping protons in from the stroma to the thylakoid compartment, and the ATP is

produced as the protons move back out.The opposing reactions produce the same happy

result—more ATP for the cells.

Light-Independent Reactions (Calvin Cycle)

After the light reactions have produced the necessary ATP and NADPH, the synthesis

phase of photosynthesis is ready to proceed. The inputs into the Calvin cycle are NADPH

(which provides hydrogen and electrons), ATP (which provides energy), and CO 2. From

here on, just so we don’t drive you insaneswitching from term to term, we are going to call

the dark reactions of photosynthesis the Calvin cycle(Figure 8.4). The Calvin cycle occurs

Figure 8.4 The Calvin cycle. (FromBiology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998,
2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies.)

Metabolites of the Calvin Cycle

RuBP ribulose 1,5-bisphosphate

PGA 3-phosphoglycerate

PGAL glyceraldehyde-3-phosphate

PGAP 1,3-bisphosphoglycerate

These ATP and NADPH

molecules were

produced by the light

reactions.

H 2 O

O 2 CH 2 O

carbohydrate

CO 2

ADP+ PNADP+

Solar energy

reactionsLight NADPH Calvincycle

ATP

Thylakoid

membrane Stroma

3 CO 2

Intermediate

3 C 6

3 RuBP

C 5

5 PGAL

C 3

6 PGAL

C 3

6 PGAP

C 3

6 PGA

C 3

COfixation (^2) 6 ATP
3 ADP + 3 P
6 ADP + 6 P
Regeneration
of RuBP
These ATP molecules
were produced by the
light reactions.
CO 2
reduction
6 NADPH
6 NADP+
Net gain of one PGAL
Other organic molecules Glucose
x 2
Calvin cycle
ATP^3
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