Bio Factsheet
1
Number 153www.curriculum-press.co.uk
The Light Dependent Stage of Photosynthesis
water enters throughroot hairs
lower epidermis
guard cell
stomatal pore
Carbon dioxide enter leaves(through stomatal pore)
CO2
Oxygen given offthrough stomatal pore
O2
wat
er fl
ow b
y tr
ansp
irat
ion
SUNLIGHT
6CO2
+ 6H2O → → → → → C
6H
12O
6 + 6O
2
carbon dioxideenters the leafby diffusionthrough thestomata
water entersthrough roothairs and istransported tothe leaves inthe xylem
glucose - thussunlight energyhas beenconverted intochemicalenergy
oxygen isgiven off
Soil root/root hairs
light
loopof DNA
innermembrane
stroma
lamella
lipidstore
starchgrain
thylakoids
ribosome Structures
Ribosomes in stroma
Starch grain
Grana
Thylakoid membranes
DNA
Functions
Synthesising enzymes – Ribulosebisphosphate carboxylase, for example.
The soluble sugars made in photosynthesiscannot be stored. They can only be stored ifthey are converted into an insoluble form.
Creates large surface area for chlorophyllso lots of light can be absorbed.
Compartmentalization - allowing differentenzyme – driven reactions to occur withinthe chloroplasts at the same time.
Codes for some of the chloroplast proteins.
one granum
The reaction is catalysed by light energy absorbed bychlorophyll contained in chloroplasts in leaves and greenstems.
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) 1994and 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.
Bio Factsheet
2
153 Light Dependent Stage of Photosynthesiswww.curriculum-press.co.uk
LIRs takeplace in thestroma
thylakoidmembrane
stroma a liquidcontaining
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
LightLight
Reaction Centre:central chlorophyll molecules
chlorophyll a molecule
"excited" chlorophyll molecule -containing trapped energy
carotenoid molecule (accessory pigment) - passes trapped energy tochlorophyll molecule
energy passed on to the centralpair in reaction centre
thylakoid membrane
light + water
thylakoidmembrane
stroma
light reactionstake place onthe thylakoidmembrane ATP & NADPH
ATP & NADPH passed to stromatake part in the light independentreaction.
oxygen (to atmosphere)
Photosynthesis can be broken down into two stages:
1. The Light-Dependent Reaction (LDR) which only occurs in thelight.
• Starch grains• Lipids• Enzymes• DNA and RNA• Ribosomes
PhotosystemsOn 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 withchlorophyll 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 nmPSII
680 nm
PSI – is found on the single thylakoids. PSIabsorbs light of wavelength 700 nm mosteffectively.
PSII – is found on the thylakoids which arestacked into grana. PSII absorbs light ofwavelength 680 nm most effectively.
Fig 3a
*
*
phospholipidbilayer
light harvesting unit
Bio Factsheet
3
153 Light Dependent Stage of Photosynthesiswww.curriculum-press.co.uk
Chlorophyll a and b and the caratenoids absorb different parts of the visible spectrumYou 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 thewavelengths absorbed by chlorophyll a, chlorophyll b and the caratenoids. Red (650nm) and blue(460nm) are absorbed strongly. Green (550nm) is poorlyabsorbed 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 stimulatephotosynthesis the most.
Typical Exam Questions1. 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
Rat
e of
Pho
tosy
nthe
sis
Abs
orba
nce/
%
550 700400
Absorption spectrum
Action spectrum
Wavelength (nm)
Wavelength (nm)
violet red
Chlorophyll aChlorophyll bCarotenoids
green
green (550 nm) poorly absorbed - is reflected, henceleaves 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
Bio Factsheet
4
153 Light Dependent Satge of Photosynthesis
Ele
ctro
nC
arri
er(E
C)
PSI
AT
P
PSI
I
light
ene
rgy
abso
rbed
by
chlo
roph
yll
mol
ecul
es
2e−−−− −
2e−−−− −
2e−−−− −
2e−−−− −
light
ene
rgy
NA
DP
NA
DPH
(us
ed i
n L
ight
inde
pend
ent r
eact
ion)
H2O
→→→→ → 2
H+ +
½O
2 +
2e−−−− −
2e−−−− −
give
n of
f thr
ough
stom
atal
por
es
electron energy level
elec
tron
at
high
eren
ergy
lev
el
elec
tron
at
low
er e
nerg
yle
vel
“exc
ited”
elec
tron
sre
leas
ed &
boos
ted
toh
igh
eren
ergy
leve
l
loss
of
elec
tron
s fr
om P
SII
mak
es it
uns
tabl
e. P
SII
stim
ulat
es(s
plitt
ing
/ ph
otol
ysis
of
wat
er)
H2O
→→→→ →
2H+ +
½O
2 +
2e−−−− −
The
ele
ctro
ns th
en p
ass
to P
SII
mak
ing
it st
able
aga
in. T
he H
+ a
re u
sed
(alo
ng w
ith e
lect
rons
fro
m P
SI)
to r
educ
e N
AD
P
ADP + iP
ADP + iP
Ele
ctro
nC
arri
er(E
C)
Ele
ctro
nC
arri
er(E
C)
Ele
ctro
nC
arri
er(E
C)
The
ele
ctro
ns r
elea
sed
from
PSI
I pa
ssth
roug
h a
seri
es o
f ele
ctro
n ca
rrie
rs.
The
ele
ctro
ns lo
se e
nerg
y as
they
pas
sbe
twee
n E
Cs
- thi
s en
ergy
is u
sed
tom
ake
AT
P
ADP + iP
+
AT
P
AT
P
Rou
te A
Rou
te B
Rou
te A
: el
ectr
ons
are
used
as
part
of a
reac
tion
tom
ake
NA
DP
H.
Th
is i
s p
art
of
non
-cyc
lic
phot
opho
spho
ryla
tion
(N
CP
).
Wha
t hap
pens
to th
eel
ectr
ons e
mit
ted
from
PSI
? T
hey
can
go in
two
dire
ctio
ns: R
oute
A o
r B
Rou
te B
: ele
ctro
ns a
re c
ycle
d ba
ck th
roug
h th
e el
ectr
onca
rrie
rs to
gen
erat
e A
TP.
Thi
s pr
oces
s is
cal
led
cycl
icph
otop
hosp
hory
lati
on(C
CP
)It
`s c
alle
d C
PP
bec
ause
:1
.el
ectr
ons
“cyc
le”
: P
SI
→ E
Cs
→ P
SI
→ E
Cs.
....
2.
“pho
to”
- li
ght
is t
he e
nerg
y so
urce
3.
“ph
osp
ho
ryla
tio
n”
- en
erg
y
rele
ased
fr
om
th
eel
ectr
ons
is u
sed
to p
hosp
hory
late
(ad
d a
P)
to A
DP
EC
EC
EC
AT
P
AT
P
AT
PPS
I
EC
Pho
tosy
stem
s in
volv
ed P
SI &
PSI
IP
rodu
cts:
AT
P, r
educ
ed N
AD
P (N
AD
PH
), o
xyge
n2
e−−−− −
Pho
tosy
stem
s in
volv
ed P
SI
Pro
duct
s: A
TP
elec
tron
s pa
ss in
to P
SI
PSI a
bsor
bs li
ght.
Ele
ctro
ns a
gain
bec
ome
exci
ted
& e
mitt
ed. T
hey
pass
to a
n el
ectr
on c
arri
erat
an
even
hig
her e
nerg
yle
vel
7
1
2
2H+
4
3el
ectr
ons
pass
ed t
oan
ele
ctro
n ca
rrie
r
5
6
elec
tron
s co
mbi
ne w
ith H
+ re
leas
ed f
rom
pho
toly
sis
of H
2O a
nd a
re u
sed
to r
educ
ed N
AD
P
wat
erPS
II
EC
EC
EC
AT
PA
TP
AT
PPS
I
EC
H+
+ N
AD
P NA
DP
H
O2
It’s
cal
led
NC
P be
caus
e:1.
“no
n cy
clic
” -
elec
tron
s do
n’t
cycl
e (t
hey
don’
t en
dup
whe
re t
hey
star
ted.
PSII
→ E
Cs
→PS
I →
NA
DP
2.
“pho
to”
- li
ght
is t
he e
nerg
y so
urce
3.
“ph
osp
ho
ryla
tio
n”
- en
erg
y r
elea
sed
fro
m t
he
elec
tron
s is
use
d to
pho
spho
ryla
te (
add
a P
) to
AD
P
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