Photosynthesis 4.1 Light-Dependent Reactions

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Photosynthesis 4.1

Light-Dependent Reactions


Each year, Canada’s

boreal forest convert 12.5

million tonnes of carbon

into energy-rich compounds

for billions of organisms


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



Where photosynthesis

takes place

Thylakoid disks stack to

form grana

Stroma contains catalytic


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


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


Absorb different

combinations of colours

Photosynthetic Pigments

Chlorophyll a and b:

main plant pigment reflects green light


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


Protein based complex

Composed of cluster of pigments that absorb light energy of many wavelengths

Located on the thylakoid membrane


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


The missing electrons from P700 are replaced by

the 2 electrons from P680.

Light Dependent Reaction - Summary




Making ATP by Chemiosmosis


• 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


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