Photosynthesis


Learning Goals for Photosyntheis - After these lectures, students will be able to:


Photosynthesis is simply the process by which organisms convert solar energy to chemical energy

12H20 + 6CO2    arrow  C6H12O6 + 6O2 + 6H2O

or, in a more balanced form:

6H20 + 6CO2   arrow  C6H12O6 + 6O2

This is an energy requiring reaction - the energy source is sunlight

Plants produce sugars as a source of food. However, they produce way more than they need to survive. This is good because all other life on earth must survive on the food energy obtained by this excess


All photosynthesis occurs in the chloroplast, so let's review the anatomy of a chloroplast

Electron Micrograph of a Chloroplast


The photosynthesis reactions can be broken down into two components:


The light-dependent reactions


 

How do the light-dependent reactions proceed?


NADPH, NADH - Electron Carrier Molecules

NADP+ and NAD+ are related molecules that serve as electron carriers


Non-Cyclic Photophosphorylation - A More Detailed Look

The form of photosynthesis with which we are most familiar is non-cyclic photophosphorylation. It consists of two sets of pigments to excite. They are called PS1, or photosystem 1, and PS2, or photosystem 2. PS1 is better excited by light at about 700 nm, and is thus sometimes called P- 700. PS2 cannot use photons of wavelength longer than 680 nm, and is thus sometimes called P- 680.

Energy enters the system when PS2 becomes excited by light. Electrons are shed by the excited PS2 (oxidation), which grabs electrons from water, producing a molecule of oxygen gas for every two waters split. PS2 thus returns it to its unexcited state (reduction) . The electrons are passed through a chain of oxidation-reduction reactions. Each arrow in the diagram above actually represents a reaction like this one:

Each element in the pathway is reduced by the electrons, and turns right around to reduce its neighbor in the pathway by giving it the electrons, thus becoming reoxidized and ready for the next electrons to pass through the photosystem

H+ Pumping.

Electrons leaving photosystem II are transferred transferred through a series of molecules and ultimately end up transferred to to a molecule called plastoquinone (PQ).  The PQ reacts with two hydrogen ions from the stroma and the two electrons from photosystem II to form PQH2.

2H+stroma +  2e- + PQ  arrow PQH2

The PQH2 diffuses across the thylakoid membrane, passes the two electrons to the next electron carrier and releases the two hydrogen ions into the lumen.

PQH2 arrow PQ + 2H+lumen +  2e-

PQ can then diffuse back across the membrane to repeat the process. The net result of the Q cycle is to move two hydrogen ions from the stroma to the lumen.

Electron Recovery

Part of Photosystem II has the ability to split water and release oxygen.  PSII is the only known biological molecule capable of oxidizing water.  The electrons produced by the oxidation of water supply a steady source of electrons for Photosystem II.

2H2O arrow 4H+ + 2e- + O2


Cyclic Photophosphorylation

Sometimes an organism has all the reductive power (NADPH) that it needs to synthesize new carbon skeletons, but still needs ATP to power other activities in the chloroplast. Many bacteria can shut off PS2, allowing the production of ATP in the absence of glucose . A proton gradient is generated across the membrane using the mechanisms of photosynthesis. This type of energy generation is called cyclic photophosphorylation.

This may seem counter-intuitive. It appeared from noncyclic phtotphosphorylation that PS1 was responsible for NADPH production, while in cyclic photophosphorylation it is important for ATP production. This apparent dichotomy can be resolved when we understand what makes PS1 both a good candidate for noncyclic photophosphorylation and for NADPH production. PS1 is very good at transferring an electron, whether it be to NADP or to ferredoxin (fd). It is a powerful reductant. PS2, on the other hand, is better at grabbing electrons from water to transfer them to quinone (Q). It is a good oxidant.

As you can see, the electron transferred is not derived from water, but from PS1 itself. It therefore must be recycled to PS1.


The light-independent reactions


The Calvin-Benson Cycle


Photorespiration

One of the biggest faux pas (that's French for big "mistakes") of evolution RuBisCo is not only attracted to CO2, but it can also use O2 in the Calvin-Benson Cycle Why does photorespiration occur? Major efforts have been made to modify the properties of Rubisco to eliminate the oxygenation reaction, especially using molecular genetics

Nature, however, has worked out a system to avoid photorespiration, it is called the C4 photosynthetic pathway. We will discuss this next lecture - can'tcha wait?

Photorespiration image taken from answers.com. Used without permission.