C1.3 Photosynthesis

Photolysis

The splitting of water molecules using light energy.

Light Dependent Reactions

The light-dependent reactions of photosynthesis require photosystems to be photoactivated by light.

This releases excited electrons to electron transport chains in the thylakoid membrane.

Includes:

-Non cyclic phosphorylation

-Cyclic phosphorylation

Non Cyclic Phosphorylation

What: Electrons don’t move in a cycle

Process:

1.Photoactivation of photosystem 2: Light energy excites electrons (e^-) which leave chlorophyl and enter the ETC.

2.Photolysis of water: replaces e^- lost through photoactivation.

3.Excited e^- move through the ETC to photosystem 1, producing ATP by chemiosomosis.

4.Photosystem 1 is photoactivated (by light), excited e^- are used to produce NADPH.

Cyclic Phosphorylattion

What: Electrons move in a cycle.

Process:

1.Photoactivation of photosystem 1: Light energy excites e^- which then leave chlorophyll to enter the ETC.

2.The excited e^- move through ETC and return to photosystem 1, producing ATP by chemiosmosis.

The product is ATP, not NADPH, in fact cyclic phosphorylation occurs when there is a limited supply of NADPH.

Phosphorylation + Photophosphorylation

The addition of a phosphate to a molecule.

Photophosphorylation adds a phosphate to ADP using light energy.

The Electron Transport Chain (ETC) role in photosynthesis

Electrons released by photosystem 2 travel along the ETC [within the thylakoid membrane] to reach photosystem 1.

As electrons move along the ETC, energy is created which actively transports protons from the stroma into the thylakoid space, creating a high concentration of protons within the thylakoid space.

Chemiosmosis

Charged protons can’t pass through the thylakoid membrane, so they move through ATP synthase by chemiosmosis.

As protons flow back down the gradient the kinetic energy generated provides the activation energy for ATP synthase to convert ADP and an inorganic phosphate to ATP.

How does reduction occur in Phososysystem

During non cyclic phosphorylation, excited electrons released by the photoactivation of photosystem 1 combine with NADP+ to form NADPH, reducing it as it gains electrons (2) and hydrogen (H⁺).

The NADPH goes on to be used in the light independent reactions, the Calvin cycle of photosynthesis.

What do Thylakoids contribute to photosynthesis

The thylakoid membrane is a system for performing the light dependent reactions.

Photolysis occurs in the thylakoid space, contributing to the thylakoids high concentration of protons.

Protons exit the thylakoid space into the stroma with ATP Synthase.

The reduction of NADP+ occurs in the stroma of the chloroplast.

Rubisco

The enzyme that catalyzes the first reaction in the Calvin cycle.

It is the most abundant enzyme on earth, a high concentration is needed in the stroma, as it works relativley slow and is less effective in low CO2 environments.

What happens with the remaining trios phosphate?

Some of the triose phosphate is converted to glucose → One glucose molecule is produced through six turns of the Calvin cycle.

This can be used to:

-Respired to produce ATP energy

-Converted to starch for storage

=Converted to cellulose for constructing cell walls

All other organic compounds, such as proteins, lipids and nucleic acids  required by plants are produced through photosynthesis.

These compounds are produced using carbon fixation of RuBP in the Calvin cycle. Intermediate compounds within the Calvin cycle are used to produce all other carbon compounds through specific metabolic pathways.

How is light Technically required for photosynthesis

The Calvin cycle requires the light-dependent reactions to produce ATP and NADPH.

If there is not sufficient carbon dioxide for the Calvin cycle, then NADP⁺ will not be regenerated, then non-cyclic photophosphorylation cannot happen, as photosystem 1 will not be able to donate electrons to NADP⁺.