photosynthesis

1. Bio Factsheet
1
Number 153
www.curriculum-press.co.uk
The Light Dependent Stage of Photosynthesis
water enters through
root hairs
lower epidermis
guard cell
stomatal pore
Carbon dioxide enter leaves
(throughstomatalpore)
CO2
Oxygen given off
throughstomatalpore
O2
water
flow
by
transpiration
SUNLIGHT
6CO2
+ 6H2
O →
→
→
→
→ C6
H12
O6
+ 6O2
carbon dioxide
enters the leaf
by diffusion
through the
stomata
water enters
through root
hairs and is
transported to
the leaves in
the xylem
glucose - thus
sunlight energy
has been
converted into
chemical
energy
oxygen is
given off
Soil
root/root hairs
light
loop
of DNA
inner
membrane
stroma
lamella
lipid
store
starch
grain
thylakoids
ribosome Structures
Ribosomes in stroma
Starch grain
Grana
Thylakoid membranes
DNA
Functions
Synthesising enzymes – Ribulose
bisphosphate carboxylase, for example.
The soluble sugars made in photosynthesis
cannot be stored. They can only be stored if
they are converted into an insoluble form.
Creates large surface area for chlorophyll
so lots of light can be absorbed.
Compartmentalization - allowing different
enzyme – driven reactions to occur within
the chloroplasts at the same time.
Codes for some of the chloroplast proteins.
one granum
The reaction is catalysed by light energy absorbed by
chlorophyll contained in chloroplasts in leaves and green
stems.
Photosynthesis occurs in the chloroplast and you’ve got to know your basic chloroplast structure (Fig 2).
Fig 2
Fig 1 summarises what you learned at GCSE.
This Factsheet focusses on te first stage of photosynthesis: the light dependent stage. It updates Factsheet 2 (The essential guide to photosynthesis) 1994
and reviews the exam questions which have appeared since September 2000 (all speccification)
Fig 1
Exam Hint: In the exam you won't be asked to draw the chloroplast but could be asked to label or describe the functions of the parts.

2. Bio Factsheet
2
153LightDependentStageofPhotosynthesis
www.curriculum-press.co.uk
LIRs take
place in the
stroma
thylakoid
membrane
stroma a liquid
containing
ATP + NADPH (from LDR) + CO2
(from atmosphere) → sugars
sugar
2. The Light- Independent reaction (LIR), which as the name implies,
doesn’t need light. So it can, in theory, occur day and night.
Fig 3 b. Photosystem unit
Light
Light
Reaction Centre:
central chlorophyll molecules
chlorophyll a molecule
"excited" chlorophyll molecule -
containing trapped energy
carotenoid molecule (accessory pigment)
- passes trapped energy to
chlorophyll molecule
energy passed on to the central
pair in reaction centre
thylakoid membrane
light + water
thylakoid
membrane
stroma
light reactions
take place on
the thylakoid
membrane ATP & NADPH
ATP & NADPH passed to stroma
take part in the light independent
reaction.
oxygen (to atmosphere)
Photosynthesis can be broken down into two stages:
1. The Light-Dependent Reaction (LDR) which only occurs in the
light.
• Starch grains
• Lipids
• Enzymes
• DNA and RNA
• Ribosomes
Photosystems
On the thylakoid membranes chlorophyll molecules are arranged into clusters called photoystems I and II (PSI and PSII) (Fig3a)
Both PSI and PSII consist of primary pigments (forms of chlorophyll a molecules) and accessory pigments (other forms of chlorophyll a, along with
chlorophyll b and caratenoids) (Fig 3 b). The role of the accsesory pigments is to capture light energy and pass it to a chlorophyll a - the primary pigment.
PSI
700 nm
PSII
680 nm
PSI – is found on the single thylakoids. PSI
absorbs light of wavelength 700 nm most
effectively.
PSII – is found on the thylakoids which are
stacked into grana. PSII absorbs light of
wavelength 680 nm most effectively.
Fig 3a
*
*
phospholipid
bilayer
light harvesting unit

3. Bio Factsheet
3
153LightDependentStageofPhotosynthesis
www.curriculum-press.co.uk
Chlorophyll a and b and the caratenoids absorb different parts of the visible spectrum
You need to know about two important graphs :
1. The absorption spectrum is a graph that shows which wavelengths of light are absorbed by a pigment. The absorption spectrum below shows the
wavelengths absorbed by chlorophyll a, chlorophyll b and the caratenoids. Red (650nm) and blue(460nm) are absorbed strongly. Green (550nm) is poorly
absorbed most of it is in fact reflected – which is why leaves appear green.
2. The action spectrum is a graph which shows the wavelengths of light that are actually used in photosynthesis.
As you might expect, the absorption and action spectrums look similar ie the wavelengths that are absorbed most strongly are the ones that stimulate
photosynthesis the most.
Typical Exam Questions
1. What is the advantage of having more than one type of pigment?
Answer : because each type absorbs a different part of the visible spectrum. So, having several pigments means that more light can be absorbed.
The job of the accessory pigments is to absorb light energy and pass it to the primary pigment molecules.
2. Define absorbtion spectrum/action spectrum
Fig 4. Absorption/action spectrum
Rate
of
Photosynthesis
Absorbance/
%
550 700
400
Absorptionspectrum
Action spectrum
Wavelength (nm)
Wavelength (nm)
violet red
Chlorophyll a
Chlorophyll b
Carotenoids
green
green (550 nm) poorly absorbed - is reflected, hence
leaves appear green
blue
red
red and blue stimulate photosynthesis the most
blue green red
Fig 5. Overleaf describes what happens in the light dependent stage. Start at 1 and follow the numbers round.
0
100
low
high

4. Bio Factsheet
4
153LightDependentSatgeofPhotosynthesis
Electron
Carrier
(EC)
PSI
ATP
PSII
light
energy
absorbed
by
chlorophyll
molecules
2e
−
−
−
−
−
2e
−
−
−
−
−
2e
−
−
−
−
−
2e
−
−
−
−
−
light
energy
NADP
NADPH
(used
in
Light
independent
reaction)
H
2
O
→
→
→
→
→
2H
+
+
½O
2
+
2e
−
−
−
−
−
2e
−
−
−
−
−
given
off
through
stomatal
pores
electron
energy
level
electron
at
higher
energy
level
electron
at
lower
energy
level
“excited”
electrons
released
&
boosted
to
higher
energy
level
loss
of
electrons
from
PSII
makes
it
unstable.
PSII
stimulates
(splitting
/
photolysis
of
water)
H
2
O
→
→
→
→
→
2H
+
+
½O
2
+
2e
−
−
−
−
−
The
electrons
then
pass
to
PSII
making
it
stable
again.
The
H
+
are
used
(along
with
electrons
from
PSI)
to
reduce
NADP
A
D
P
+
i
P
A
D
P
+
i
P
Electron
Carrier
(EC)
Electron
Carrier
(EC)
Electron
Carrier
(EC)
The
electrons
released
from
PSII
pass
through
a
series
of
electron
carriers.
The
electrons
lose
energy
as
they
pass
between
ECs
-
this
energy
is
used
to
make
ATP
A
D
P
+
i
P
+
ATP
ATP
Route
A
Route
B
Route
A
:
electrons
are
used
as
part
of
a
reaction
to
make
NADPH.
This
is
part
of
non-cyclic
photophosphorylation
(NCP).
What
happens
to
the
electrons
emitted
from
PSI?
They
can
go
in
two
directions:
Route
A
or
B
Route
B:
electrons
are
cycled
back
through
the
electron
carriers
to
generate
ATP.
This
process
is
called
cyclic
photophosphorylation(CCP)
It`s
called
CPP
because:
1.
electrons
“cycle”
:
PSI
→
ECs
→
PSI
→
ECs.....
2.
“photo”
-
light
is
the
energy
source
3.
“phosphorylation”
-
energy
released
from
the
electrons
is
used
to
phosphorylate
(add
a
P)
to
ADP
EC
EC
EC
ATP
ATP
ATP
PSI
EC
Photosystems
involved
PSI
&
PSII
Products:
ATP,
reduced
NADP
(NADPH),
oxygen
2e
−
−
−
−
−
Photosystems
involved
PSI
Products:
ATP
electrons
pass
into
PSI
PSI
absorbs
light.
Electrons
again
become
excited
&
emitted.
They
pass
to
an
electron
carrier
at
an
even
higher
energy
level
7
1
2
2H
+
4
3
electrons
passed
to
an
electron
carrier
5
6
electrons
combine
with
H
+
released
from
photolysis
of
H
2
O
and
are
used
to
reduced
NADP
water
PSII
EC
EC
EC
ATP
ATP
ATP
PSI
EC
H
+
+
NADP
NADPH
O
2
It’s
called
NCP
because:
1.
“non
cyclic”
-
electrons
don’t
cycle
(they
don’t
end
up
where
they
started.
PSII
→
ECs
→PSI
→
NADP
2.
“photo”
-
light
is
the
energy
source
3.
“phosphorylation”
-
energy
released
from
the
electrons
is
used
to
phosphorylate
(add
a
P)
to
ADP