Biology 12

3 ADP + 3

Calvin cycle 6 ADP + 6

carbon

fixation

reduction

re-formation

of RuBP

RuBP ribulose bisphosphate

PGA 3-phosphoglycerate

PGAP 1,3-bisphosphoglycerate

PGAL glyceraldehyde-3-phosphate

Metabolites of the Calvin Cycle

3

ATP

6 ATP

6 NADPH

6

3 RuBP

6 PGA

5 PGAL

6 PGAL

6 PGAP

3

NADP+

C 3

C 3

C 3

1 PGAL

C 3

C 5

C 3

CO 2

Glucose phosphate and

other organic compounds

These ATP
molecules
were produced
by the photo
reaction.

These ATP and

NADPH molecules

were produced by

the photo reactions.

There is a net gain

of one PGAL.

Pi

Pi

Chapter 3 Cellular Energy • MHR 89

The electrons from photosystem 680 energize
electrons that travel through the electron transport
chain in the thylakoid to pump protons. The
electrons lose energy after they move through the
electron carriers. At photosystem 700, the electrons
are re-energized by light energy. These two electrons
now move along the final carriers of the electron
transport chain to the NADP reductase complex.
Here, two electrons are transferred to NADP+,
which also combines with a hydrogen ion to form
the reduced NADPH, as shown in Figure 3.26.
Both chloroplasts and mitochondria use
chemiosmosis to produce ATP. These organelles
also rely on an electron transport chain to power
proton pumps and move electrons to an electron
acceptor that removes them (such as water or
NADPH). The proton pumps create the hydrogen
ion gradient that both organelles use to make ATP.
Chloroplasts and mitochondria even share the basic
construct of an ATP synthase complex, which is
remarkably similar in both structures. However,
there are some differences in the way that
phosphorylation occurs in the two organelles,
as summarized in Table 3.3.

The Calvin Cycle

In photosynthesis, both the NADPH and the ATP
produced by the photoreactions in the thylakoid

membrane are used during the synthesisreactions

to produce organic molecules from carbon dioxide.

ATP and NADPH molecules are formed on the

thylakoid membrane by means of the ATP synthase

complex and the NADP reductase complex,

respectively (see Figure 3.26). The ATP and NADPH

Oxidative phosphorylation

in mitochondria

Photophosphorylation

in chloroplasts

Electrons in the electron

transport chain are supplied by

the oxidation of food molecules.

food energy → ATP molecules

Inner membrane pumps

hydrogen ions from the matrix

to the intermembrane space.

Intermembrane space serves

as proton reservoir.

ATP synthase resides between

the membrane and the matrix,

producing ATP molecules as

hydrogen ions move back into

the matrix from the

intermembrane space.

Electrons in the electron transport

chain are extracted from water

during photolysis and passed

on to pigment molecules (driven

by captured solar energy) to

donate them to the chain.

light energy → ATP molecules

Thylakoid membrane pumps

hydrogen ions from the stroma

to the thylakoid interior.

Thylakoid interior pools ions.

ATP synthase bridges the

thylakoid membrane and its

interior, producing ATP

molecules as hydrogen ions

move into the stroma.

Table 3.3

Differences in phosphorylation between mitochondria

and chloroplasts

Figure 3.27The Calvin

cycle produces one

molecule of PGAL for

every three molecules of

CO 2 that enter the cycle.

PGAL is used to form

glucose and other

organic compounds.