The reactions of photosynthesis take place inside the chloroplast:

The LIGHT DEPENDENT REACTIONS take place on the surface of the thylakoid membrane, in the PHOTOSYSTEMS--large patches of chlorophyll molecules (and sometimes carotenoid molecules) arranged together with their heads parallel.

**Each photosystem has only one chlorophyll a molecule.

**Each photosystem has hundreds of chlorophyll b & carotenoids.

THE KEY: When chlorophyll absorbs a photon, one of its electrons is boosted to a higher energy state.

Chlorophyll a is the only pigment in the entire photosystem capable of transferring an excited electron to a large protein embedded in the membrane. This protein is called the Primary Electron Acceptor (P.E.A.). Here's how a photosystem harvests light energy.

Chlorophyll b and carotenoids are also busy absorbing photons, but instead of passing their excited electrons to the P.E.A., they pass them to each other and on to chlorophyll a, which can pass all the excited electrons to the P.E.A.

(Envision a constant, incredibly rapid stream of electrons passing from all the photosystem pigment molecules to chlorophyll a, which is constantly handing them off to the P.E.A.)

This phenomenon can be demonstrated if you extract chlorophyll in ether and place it in a glass container and hold it up to a powerful light source.

As the chlorophyll molecules absorb photons from the light source and their electrons are excited, the photons disappear.

However, since the chlorophylls have been untimely ripped from their membranes, there is no P.E.A. to take them to the next step.

Result: the electrons pop back to their stable configurations, "spitting out" the photon, slowed down, as red light.

What you see: the bright green container of chlorophyll, held up to a strong light source, looks like a little container of BLOOD.

(This reaction has nothing to do with the ether! It's just the chlorophylls accepting and then releasing the photon energy!)

This phenomenon is known as FLUORESCENCE.

(ABOVE IS THE SPELLING TIP OF THE DAY: it's not FLOURescence!)

(GRAMMAR TIP OF THE DAY: advice (noun) is not the same as advise (verb).)

********* Photosystems: There are two different types:

Photosystem I - chlorophyll a maximum absorbance at 700nm

(This system participates in Cyclic electron flow AND in non-cyclic electron flow to make ATP)

Photosystem II - chlorophyll a maximum absorbance at 680 nm

(This system participates only in Non-cyclic electron flow to make ATP.)

Photophosphorylation: the process of making ATP (i.e., sticking phosphate groups onto AMP and ADP) with light energy.

During CYCLIC electron flow, the energy of the electron flow is used to pump protons outside the membrane to create a large energy gradient.

During NON-CYCLIC electron flow, the electron flow is used to make both ATP and NADPH.

(It is during non-cyclic electron flow that water is split to provide protons for chemiosmosis (the pumping of protons across the membrane). The waste product: OXYGEN! All the oxygen you're now breathing came from photosynthesis!)

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Here's a summary of a possible model of what actually occurs in the thylakoid membrane during the light dependent reactions.

An Overview in Prose: The light dependent reactions

A. Cyclic photophosphorylation (Photosystem I) - this is the manufacture of ATP with the return of electrons to the Photosystem after they have passed through the electron transport chain.

* As excited electrons are passed from the Primary Electron Acceptor through the proteins of the electron transport chain, the electrons travel "downhill," energetically speaking.

* The Potential Energy given up by the electrons as they pass down the electron transport chain is used by special proteins called PROTON PUMPS to force protons across the thylakoid membrane (into the thylakoid space) against the gradient.

* The pumped protons inside the thylakoid space now hold the potential energy.

* As they re-enter the stroma, they pass through a protein channel enzyme called ATP SYNTHASE. This enzyme uses the Potential Energy (P.E.) from the protons re-entering the stroma to bind a molecule of ADP to a phosphate group, storing that P.E. in the phosphate bond of a new molecule of ATP.

* The process we have just described (the potential energy of protons flowing down a gradient being packaged in the phosphate bonds of ATP) is called CHEMIOSMOSIS. This also happens in the mitochondrion, during the *breakdown* of sugars.

* The end product of cyclic photophosphorylation is ATP.

An Overview in Prose: Non-cyclic photophosphorylation (Photosystems I and II).

In this process, excited electrons from Photosystem II are transferred to Photosystem I and their energy packaged in the bonds of another energy courier, NADPH.

* In the electron transport chain between Photosystem I and Photosystem II, the energy is packaged as ATP.

* In the electron transport chain between Photosystem I and the stroma, the energy is packaged as NADPH.

* A special water-splitting enzyme is located close to P680 of Photosystem II. It's job is to split water via the following reaction:

enzyme

H₂O --------------> 2H⁺ + 1/2O₂ + 2 electrons

(recall that H⁺ is the same as a proton!)

(Note that initially, the split water yields atomic oxygen (O). This is very unstable, and it immediately bonds to other atoms of oxygen to form O₂, which is expired through the stomates.)

The fate of the players:

1. The two electrons are returned to Photosystem II

2. The protons are used to store potential energy (proton pump!)

3. oxygen is the "waste" product.

The products of Non-cyclic photophosphorylation: ATP, NADPH and oxygen (waste product)

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The LIGHT INDEPENDENT REACTIONS:

The chemical energy stored in the high-energy bonds of ATP and NADPH are harvested in the STROMA of the chloroplast, where the Calvin Cycle reactions take place.
An Overview in Prose: The Calvin Cycle (The Light Independent Reactions of Photosynthesis)

What do we have to work with?

ATP from cyclic

ATP & NADPH from non-cyclic

THESE ARE HIGH-ENERGY, VERY BREAKABLE AND HAVE TO BE "USED" QUICKLY!

To store the energy, we need to package it in a more stable form: SUGARS.

The raw material CO₂ is enzymatically converted into sugar (i.e., carbon is fixed from an inorganic form to an organic form) through a series of chemical reactions in the stroma.

** The initial sugar that comes off the Calvin Cycle is called

(G3P) (glyceraldehyde-3-phosphate)

This, G3P is now storing the solar-->electrical-->chemical energy that went from the sun (photon) to the chlorophyll (excited electron) to the ATP and NADPH (chemical bonds).

The Calvin Cycle reactions are ENDERGONIC. They are driven by the EXERGONIC reactions of

ATP --> ADP + phosphate

NADPH --> NADP+ + phosphate

Note that G3P is then available to be converted into more stable sugars such as glucose, sucrose, fructose, etc.

*****THE PLANT HAS NOW STORED THE ENERGY OF THE SUN AS SUGAR!*****

In these reactions, carbon dioxide is linked with the protons to create SUGARS.