Biology 12

+

photosystem II

photosystem I

light

antenna

complex

antenna

complex

thylakoid space

stroma

light

cytochrome

complex

NADP

reductase

NADPH

ATP

2

synthesis

reactions

thylakoid

+ membrane

ATP synthase

proton

pump

Z enzyme

e−

e−

e−

NADP+

H+

H+

H+

H+

H+

H+

H+

H+

H+

ADP+

1

2 O^2

H 2 O

Pi

88 MHR • Unit 1 Metabolic Processes

channelling electrons to the electron acceptor

(see Figure 3.25). This process is called photolysis

because light energy is required to split bonds

within the water molecule. All of the oxygen that

we breathe, and all the oxygen in Earth’s

atmosphere, has been generated through the

photolysis stage of photosynthesis.

In addition to passing electrons from water to

chlorophyll molecules, the Z enzyme that performs

photolysis also donates a hydrogen ion from the

same water molecule to the reaction-centre of

photosystem 680. This hydrogen ion joins the

electron in its journey along the electron transport

chain. The electron–hydrogen ion combination

supplies energy to an electron transport chain

comprised of cytochrome enzymes. This chain of

enzymes in turn drives a proton pump, similar to

the one you learned about in chemiosmosis in the

mitochondrion. The photosynthetic proton pump,

like proton pumps in the electron transport chain

of the mitochondrion, moves H+ions out of the

stroma, into a membrane-enclosed space, as

illustrated by Figure 3.26. Just as the inner membrane

of the mitochondrion contains an ATP synthase

complex that opens to the matrix, the thylakoid

membrane of the chloroplast contains an ATP

synthase complex where H+ions flow through to

the stroma and energize the phosphorylation of

ADP. This process is called photophosphorylation.

The thylakoid space serves as a reservoir for

hydrogen ions. Every time the Z enzyme splits

water to form two hydrogen ions, the thylakoid

space receives them. Whenever photosystem 680

donates an electron to the electron transport system,

giving up energy along the way to drive the proton

pump, hydrogen ions move in from the stroma.

A hydrogen ion gradient is formed when the

thylakoid space contains more hydrogen ions than

the stroma. The movement of hydrogen ions across

the thylakoid membrane releases energy that is

used in ATP synthesis. This gradient forces the

hydrogen ions through the ATP synthase complex

that resides on the membrane of the thylakoid

body. This movement of hydrogen ions provides

the energy required to join ADP and Piin the

chemiosmotic synthesis of ATP.

To learn how the intensity and wavelength of light can

affect ATP/NADPH production in chloroplasts, go to your

Electronic Learning Partner now.

ELECTRONIC LEARNING PARTNER

Figure 3.26Within the thylakoid

membrane, enzyme complexes

pump hydrogen ions from the

stroma into the thylakoid space.

This process forms a hydrogen

ion gradient. As hydrogen ions

flow down the gradient and back

into the stroma through the ATP

synthase complex, ATP molecules

are formed. As you can see in

this diagram, photosystem 680

is also called photosystem II.

Photosystem 700 is also called

photosystem I.