Cambridge International A Level Biology Revision Guide

flow of electrons in non-cyclic
photophosphorylation
flow of electrons in cyclic
photophosphorylation
Chapter 13: Photosynthesis
chains of electron carriers
2e –
Key
flow of electrons in non-cyclic
photophosphorylation
flow of electrons in cyclic
photophosphorylation
chains of electron carriers
H 2 O
increasing
energy
level
Figure 13.3 The ‘Z scheme’ of electron flow in photophosphorylation.
2e –
1
2O 2
2H +
primary pigment
photosystem II
light
ADP + P i
ATP
primary pigment
photosystem I
light
NADP + 2H +
reduced
NADP
In 1939, Robert Hill showed that isolated chloroplasts (dichlorophenolindophenol), can substitute for the
had ‘reducing power’ and liberated oxygen from 2e water plant’s NADP in this system (Figure 13.4). DCPIP becomes
–
in the presence of an oxidising agent. The ‘reducing
colourless when reduced:
power’ was demonstrated by ADP using + P i
a redox agent that
NADP + 2H +
chloroplasts in light
changed colour on reduction. 2e – This technique can be
oxidised DCPIP
reduced DCPIP
ATP
used to investigate the effect of light intensity or of light
(blue)
(colourless)
H primary pigment
wavelength on 2 O
the rate of photosynthesis of a suspension reduced
1
1
of chloroplasts. Hill used Fe 3+ photosystem I
H
NADP
2
O
2O 2
2 O 2
ions as his acceptor,
but various redox agents, such as the blue dye DCPIP Figure 13.4 shows classroom results of this reaction.
2H +
primary pigment
light
BOX 13.1: Investigating the Hill reaction
photosystem II
Chloroplasts increasing can be isolated from a leafy plant, such as
lettuce energy or spinach, by liquidising the leaves in ice-cold
buffer level and then filtering light or centrifuging the resulting
suspension to remove unwanted debris. Working quickly
and using chilled glassware, small tubes of buffered
chloroplast suspension with added DCPIP solution
are placed in different light intensities or in different
wavelengths of light and the blue colour assessed at
intervals.
The rate of loss of blue colour (as measured in a
colorimeter or by matching the tubes against known
concentrations of DCPIP solution) is a measure of the
effect of the factor being investigated (light intensity or
the wavelength of light) on chloroplast activity.
Colorimeter reading / arbitrary units
blue
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
placed in light
Key
chloroplasts in light
chloroplasts in dark
for five minutes,
then in light
289
Figure 13.4 The Hill reaction. Chloroplasts were
extracted from lettuce and placed in buffer solution
with DCPIP. The colorimeter reading is proportional
to the amount of DCPIP remaining unreduced.
0.2
colourless
0
2 4 6 8 10 12 14 16
Time / minutes