Light Dependent Reactions

IB Topic 8.2.3: Explain the light dependent reactions

What is light dependent reactions?

Light energy is trapped by the photosynthetic pigment chlorophyll

Chlorophyll molecules do not occur haphazardly in the grana

Rather, they are grouped together in structures called photosystems, held in the thylakoid membranes of the grana

The Structure of PhotosystemsClegg, pg. 278

Photosystems

In each photosystem, several hundred chlorophyll molecules and accessory pigments are arranged

All these pigments harvest light energy They funnel the energy to a single chlorophyll

molecule of the photosystem, known as the reaction center

The different pigments around the reaction centers absorb light at slightly different wavelengths

Two types of photosystems

Two types present in the thylakoid membranes Identified by the wavelength of light that the

chlorophyll of the reaction center absorbs Photosystem 1: activated by light wavelengths

700nm P700

Photosystem 2: activated by light wavelengths 680nm P680

Why two photosystems?

Each have a specific and differing role However, they are grouped together in the

thylakoid membrane, along with specific proteins Enzymes catalyzing

Splitting of water Formation of ATP from ADP and P

Electron carrier molecules

What happens when light reaches the reaction center? Ground-state electrons of the key chlorophyll

molecule are raised to an excited state Said to be photoactivated

As a result, high energy electrons are released (from the chlorophyll molecule)

These electrons bring about the biochemical changes of the light dependent reactions

The spaces vacated by the high energy electrons are continuously filled by non-excited or ground state electrons

Sequence of reactions

First, the excited electrons from photosystem 2 are picked up by plastoquinone and moves away to another position in the membrane

As these excited protons are passed, some of the energy causes the pumping of hydrogen ions (protons) from the chloroplast's matrix into the thylakoid spaces

They accumulated, causing the pH to drop

Sequence of reactions (cont.)

The result? … proton gradient created across the thylakoid membrane

This sustains the synthesis of ATP Called photophosphorylation and

chemiosmosis

Photophosphorylation and chemiosmosis The production of ATP using energy derived

from light is called photophosphorylation Carried out in the thylakoids

Main type is non-cyclic Reduced plastoquinone carries the pair of excited

electrons from the reaction center of P2 Plastoquinone carries the electrons to the start of the

chain of electron carriers and they are passed along As the electrons are passed, energy is released which

is used to pump protons across the thylakoid membrane into the space inside the thylakoids

Next …

Concentration gradient of protons develops (stores potential energy) Photolysis also contributes to the gradient

The protons can travel back across the membrane, down the concentration gradient, by passing through the enzyme ATP synthase

The energy released by the passage of protons is used to make ATP from ADP and P This method of producing ATP is also called chemiosmosis

Occurs in the mitochondrion (cell respiration; next topic)

Next …

When the electron reach the end of the chain of carriers they are passed to plastocyaninNeeded for the next stage of photosynthesis

Completing the light dependent reactions Remaining parts involve photosystem 1

Product: NADPH (needed for light independent reactions) NADPH carries a pair of electrons

Chlorophylls within P1 absorb light energy and pass it to the reaction center This raises the electron’s energy level; photoactivation The excited electron passes along a chain of carriers and is

eventually passed to ferrodoxin The reduced ferrodoxin are used to reduce NADP+ to NADPH +

H+

Next …

The electron that P1 donated is replaced by an electron carried by plastocyanin

P1 and P2 are therefore linked: Electrons excited in P2 plastocyanin P1 Electrons are re-excited with light energy and are eventually

used to reduce NADP+ For reasons later discussed, NADP+ sometimes runs out When this happens, the electrons returns to the ETC which links

the two systems rather than being passed to NADP+ Cyclic photophosphorylation

Animations

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#