LDR

The Light Dependent Reaction

• When light is absorbed by
• the chlorophyll molecules, a pair of electrons in the molecules gain energy,
• i.e. they become excited. The electrons become so energetic that
• they leave
• the chlorophyll
• molecule and are taken up by a molecule called an electron
• carrier.

Slide 2

• This loss of electrons by the chlorophyll
• molecule causes it to become oxidised/ionised.
• Because this is caused by the absorption of light, it is referred to as photoionisation.

Slide 3

• The electrons are now passed along a series
• of electron carriers in a series of REDOX reactions; each carrier becomes reduced
• when it receives an electron, then reverts to its oxidised state when the
• electron is passed on. The electron carrier molecules form an electron
• transport chain (ETC)
• located in the membranes of the thylakoids.

Slide 4

• Each new carrier is at a slightly lower energy
• level than the previous one so that the electrons lose
• the energy they
• gained from light at each stage.

Slide 5

• During
• Photophosphorylation, this
• energy is ultimately used to combine an inorganic phosphate molecule with ADP
• to make ATP, via an H+ concentration gradient .
• At
• the
• same time a molecule of water is split into hydrogen ions (protons), electrons
• and oxygen in a process known as photolysis.
• The electrons produced by
• photolysis are used to replace those lost from chlorophyll, allowing it to
• continually absorb light energy.

Slide 6

• The protons produced by photolysis react with
• the oxidized coenzyme NADP (nicotinamide adenine dinucleotide
• phosphate), along with the electrons released at the end of the electron
• transfer chain to form reduced NADP.
• The oxygen produced by the photolysis of
• water is either used in respiration or diffuses out of the leaf as a waste
• product of photosynthesis.

Slide 7

• The
• ATP
• and reduced NADP formed by the light-dependent reactions provide the energy and
• the hydrogen necessary to form carbohydrate in the light-independent reactions.
• This energy has been transferred from the photon of light originally absorbed
• by the chlorophyll