Photosynthesis 4.1 Light-Dependent Reactions
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Photosynthesis 4.1
Light-Dependent Reactions
Photosynthesis
Each year, Canada’s
boreal forest convert 12.5
million tonnes of carbon
into energy-rich compounds
for billions of organisms
Photosynthesis
A series of metabolic pathways
2 main steps:
1. Light-dependent reactions:
Energy ATP and NADPH
2. Light-independent reactions
CO2 + ATP + NADPH sugar
Light-Dependent Reactions
Leaf Structure
Water and carbon dioxide
used to make glucose
Water enters through roots
and is transported to leaves
Carbon dioxide enters
through stomata in the
leaves
Chloroplast
Where photosynthesis
takes place
Thylakoid disks stack to
form grana
Stroma contains catalytic
enzymes
Light Energy
Light is absorbed as photons
Each photon carries a specific
amount of energy
With the right amount of
energy, electrons can jump up
to an upper energy level
Photon
Each wavelength is associated with a certain amount of energy in its photons
Longer wavelength
less energy
An atom in a plant can only absorb photons that have an amount of energy that is exactly equal to the difference between two energy levels
Leaf colours - Pigments
Photosynthetic Pigments
Absorb certain
wavelengths of visible
light and pass on to other
compounds
Absorb different
combinations of colours
Photosynthetic Pigments
Chlorophyll a and b:
main plant pigment reflects green light
Carotenoids:
accessory plant pigments reflect yellow,
orange, and red light
Absorbance spectrum:
a graph that shows the relative amounts
of light of different wavelengths that a
compound absorbs
Photosystems
Protein based complex
Composed of cluster of pigments that absorb light energy of many wavelengths
Located on the thylakoid membrane
Photosystem
Antenna complex:
• Pigments that capture photon (chlorophyll b, carotenoids, etc)
• transfer energy to reaction centre
Reaction centre
• Made up of a pair of chlorophyll a molecules and proteins
• With energy received, 2 electrons are ‘excited’, jump up in energy level and captured by an electron carrier
Photosystems I and II
Chloroplasts in plants and algae use two photosystems that work together to convert light energy into chemical energy.
1. Photosystem I (P700) :
• reaction center that absorbs wavelengths of 700 nm
2. Photosystem II (P680) :
• reaction center that absorbs wavelengths of 680 nm
Light-Dependent Reactions – Step 1
P680 molecule absorbs a photon in its antenna complex
Transfers energy to reaction center
an electron is ‘excited’ (this can occur 200 times a second)
electron acceptor takes the electrons (P680 P680+)
P680+ has a strong attraction for electrons and pulls them
from water.
TWO H2O molecules are split and transfer FOUR electrons
to the reaction center of P680 to replenish it.
Oxygen gas is formed and released
FOUR H+ remain in the thylakoid space.
Light-Dependent Reactions – Step 2
Electron carrier transfers electrons to a series of increasingly electronegative complexes
(electron transport system – a series of REDOX reactions)
Energy released by the redox reactions is used by b6-f complex to pump a H+ from the stroma into the thylakoid space
creates an electrochemical gradient.
Light-Dependent Reactions – Step 3
P700 absorbs photons in the antenna complex.
Energy is transferred until it reaches the reaction
center and excites another 2 electrons.
An electron carrier captures the 2 excited electrons
and carries it to the enzyme NADP reductase.
NADP+ undergoes a REDOX reaction with the NADP
reductase and captures the electrons to become
NADPH.
The missing electrons from P700 are replaced by
the 2 electrons from P680.
Light Dependent Reaction - Summary
Electron
Transport
System
Making ATP by Chemiosmosis
Photophosphorylation:
• using photons to drive the phosphorylation of ADP to ATP through chemiosmosis
Similar to aerobic respiration
B6-f complex releases a large amount of H+ into the thylakoid space
electrochemical gradient
Thylakoid membrane impermeable to H+
H+ must pass through ATP synthase
Oxidative phosphorylation to produce ATP
Noncyclic Photophosphorylation
Z scheme
Unidirectional
Generates 1 NADPH and 1 ATP
Ratio not sufficient for
light-independent reactions
Light independent reaction
requires 3 ATP : 2 NADPH
Cyclic Photophosphorylation
Only Photosystem I is used
Electrons return to P700 through the b6-f complex
Produces H+ gradient
produces more ATP
No NADPH or O2 produced
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