Phases of Photosynthesis

Photosynthesis occurs in 2 phases, which include 3 main goals:

A. The Light Reactions1. Capturing light energy2. Using the light energy to create chemical

energy (ATP and NADPH (electron carrier in plants))B. The Light Independent Reactions

3. Using the free energy from ATP and reducing power of NADPH to synthesize organic compounds from atmospheric carbon.

Where They Occur

The Light Reactions require chlorophyll and occur on the thylakoid membrane.

The Light Independent reactions (aka The Calvin Cycle) take place in the stroma.

Light Reactions

When light strikes the thylakoid membrane, the photosystems absorb a photon and the energy is passed from pigment to pigment until it reaches the reaction center (P680 or P700).

The light energy excites the electrons in the chlorophyll a molecules.

Excitation of Electrons

Electrons in a chlorophyll molecule are normally at their ground state or lowest potential energy level

Excitation: The photon hits the chlorophyll and the electrons are energized to a higher energy level.

Outside a living organism excited electrons want to return to their ground state and lose potential energy as heat and light (fluoresce).

Non-Cyclic Electron Flow

In a cell the excited electron is captured by a special molecule called the primary electron acceptor (no fluorescing).

The energy of the electrons is used to produce ATP and NADPH in a two-part process called non-cyclic electron flow.

Steps: 1. Photoexcitation

A photon of light strikes the antenna complex of PII and PI

The energy is passed around the antenna pigments until it excites an electron of P680 / P700.

Noncyclic flow begins at PII: A redox reaction

transfers the excited electrons from P680 to the primary electron acceptor (pheophytin)

Therefore: Chlorophyll a is oxidized and the primary acceptor is reduced (redox!)

2. Electron Transport i.

From the primary electron acceptor (pheophytin), the electrons travel by redox through a series of membrane bound electron carriers (cytochrome complexes, aka an ETC)

.

Electron Transport ii.

They power a proton pump called the Q cycle.

Protons are pumped from the stroma into the thylakoid lumen creating a proton gradient.

– (High concentration inside thylakoid and low outside).

Electron Transport iii.

At the end of the ETC the electrons from PII are transferred to PI (to replace the electrons it loses...next)

iv.

At PI: The excited electrons from

P700 pass to an electron acceptor (ferredoxin (Fd)).

From Fd they move through a short ETC to the enzyme NADP reductase.

The enzyme uses two electrons and two protons from the stroma to reduce NADP+ to NADPH.

v.

The PII electrons are also replaced:

A Z protein splits water into oxygen, 2 hydrogen ions and 2 electrons.

The electrons replace the missing electrons from P680.

Oxygen leaves as a waste product

Protons stay in the thylakoid lumen

3. Photophosphorylation

A proton gradient was set up in PII. There is a high concentration of H+ in the lumen.

Protons move through an ATPase along the proton gradient from the thylakoid lumen into the stroma. ATP is generated.

This process is called photophosphorylation since light creates the proton gradient.

Summary of the Light Reactions

The ATP and NADPH go on to the Light Independent reactions...the Calvin Cycle

CYCLIC ELECTRON FLOW

In some cases (low O2, low levels of NADP+, low ATP to NADPH ratio) electrons take a cyclic pathway instead to increase ATP production.

Only PI is used. P700 loses its electron pair to ferredoxin (Fd) but then electrons go through the Q cycle (ETC of PII) and back to P700.

This generates a proton gradient for ATP synthesis, but does not release electrons to generate NADPH.

• Plants cycle back and forth through cyclic and non-cyclic to maintain adequate levels of both ATP and NADPH.