The Light Dependent Stage of Photosynthesis.pdf

photosynthesis

1. BioFactsheet
1
Number 153www.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 (through stomatal pore)
CO
2
Oxygen given off through stomatal pore
O
2
water flow by transpiration
SUNLIGHT
6CO
2
+ 6H
2
O → → → → → C
6
H
12
O
6
+ 6O
2
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
153 Light Dependent Stage of Photosynthesis
www.curriculum-press.co.uk
LIRs take
place in the
stroma
thylakoid membrane
stroma a liquid containing
ATP + NADPH (from LDR) + CO
2
(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 nmPSII
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
153 Light Dependent Stage of Photosynthesis
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 700400
Absorption spectrum
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
153 Light Dependent Satge of Photosynthesis
Electron
Carrier
(EC)
PSI
AT P
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
ADP + iP
ADP + iP
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
ADP + iP
+
AT P
AT P
Route ARoute 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
AT P
AT P
AT P
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
AT P
AT P
AT P
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