Biology 12

Chapter 3 Cellular Energy • MHR 87

Non-cyclic Electron Pathway

Photosystem 680 absorbs light 680 nm in wavelength.
The shorter the wavelength of light, the higher its
energy. Therefore, photosystem 680 is more powerful
than photosystem 700 because photosystem 680
can capture higher-energy light. In addition to
chlorophyll a, photosystem 680 contains molecules
of chlorophyll b. It also contains molecules of
chlorophyll c, chlorophyll d, and accessory pigments
such as carotenes, xanthophylls, and anthocyanins.
The pigments other than chlorophyll aaid in
absorbing wavelengths of light not absorbed by
chlorophyll a. Green plants, algae, and cyanobacteria
(unlike other bacteria species) use both photosystems
680 and 700 to carry out photosynthesis. In this
case, electrons from photosystem 680 are shunted
to photosystem 700, as shown in Figure 3.25. The
energy from electrons in photosystem 680 is used
to produce ATP molecules. These electrons then
move to photosystem 700 where, after becoming

energized, they are taken up by NADP+

(nicotinamide adenine dinucleotide). After NADP+

accepts two electrons and a hydrogen ion (H+), it

becomes the coenzyme NADPH. The production

of NADPH and ATP are endothermic reactions,

which require an input of energy. The ATP and

NADPH molecules are then used in the synthetic

steps to produce glucose.

After ATP molecules are produced by

photosystem 680, electrons that have passed

through the electron transport system are not

cycled back into photosystem 680. This type of

ATP production is called non-cyclic

photophosphorylation. However, photosystem 680

requires electrons to keep the photosystem

operating. After photosystem 680 transfers an

electron to the electron acceptor, photosystem 680

captures an electron from a Z enzyme. This enzyme

is responsible for splitting water molecules into

hydrogen ions and oxygen molecules and

electron transport syst

em

energy of electron

electron

acceptor

sun

photosystem 680

Z enzyme

electron

acceptor

sun

photosystem 700

synthesis reactions

2

NADPH

ATP

ADP

reaction–centre

chlorophyll a

reaction–centre

chlorophyll a

e−

e−

e−

e−

e−

e−

NADP+

H+

H+

H+

1

2 O^2

H 2 O

CO 2 C 16 H 12 O 6

+

Pi

Figure 3.25The non-cyclic electron pathway. Electrons
from water move from photosystem 680 to photosystem 700
and then to NADP+. The ATP and NADPH molecules that are

produced by these reactions fuel the synthesis reactions

that form glucose.