Photosynthesis

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

12H₂0 + 6CO₂ >> C₆H₁₂O₆ + 6O₂ + 6H₂O

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

The innermost membrane of the chloroplast is called the thylakoid membrane.
The thylakoid membrane is folded upon itself forming many disks called grana (singular = granum).
The "cytoplasm" of the chloroplast is called the stroma

The photosynthesis reactions can be broken down into two components:

The light-dependent reactions (the "light" reactions)
The light-independent reactions (the "dark" reactions)

The light-dependent reactions

Goal: To trap sunlight energy and store it as chemical energy to use in all life functions
Why: ATP is a good source of energy, but it does not store well

This is like money, if you had 1 million dollars, would you rather carry around singles or gold?

Where do these reactions occur?: on the membranes of the thylakoids
How is it done?

Pigments - light harvesting molecules form an antenna complex on the thylakoid membranes
Each pigment absorbs a different type of light

Why is this good? It enables the plant to utilize a much wider range of light

In green plants, the primary photosynthetic pigments are Chlorophylls a & b.

How do the light-dependent reactions proceed?

A photon is absorbed by photosystem II (P680)
An electron is raised from a low energy state to a high energy state
the electron then falls down to the low energy state, releasing its energy. However, this energy is not lost, it is picked up by an adjacent pigment molecule where it is used to raise an electron to a higher energy state, etc. etc., until this energy reaches the photosystem (like a bucket brigade or "the wave" at a football game)
At the photosystem, the electron is raised, but instead of falling back down, it is stolen by another, electron defficient molecule in the electron transport chain
Meanwhile the photosystem's stolen electron is replenished by photolysis, or the splitting of H₂O to form H⁺and O₂ (note: the H⁺ is kept in side the thylakoid membrane). The O₂ resulting is the source of all oxygen in our atmosphere
The electron travels down the electron transport system (ETS). Along the way, more H⁺ is pumped into the thylakoid compartment.
The electron eventually reaches photosystem I (P700), where it waits until the electron is excited by another photon
The electron is stolen by another electron acceptor from a second ETS
The final fate of the electron is in converting NADP⁺ to NADPH
The H⁺ is released to generate ATP

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.

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

Goal: to take the recently created NADPH and ATP and store their energy by constructing sugars from CO₂
Where: in the stroma of the chloroplast
Where does the CO₂ come from?

The atmosphere - the leaf opens up its stomates and lets CO₂ in
When this happens, H₂O is inadvertently released
The plant must always balance its carbon intake with water loss

How is CO₂ converted into sugar?

The energy is stored by converting CO₂ into sugars in the Calvin-Benson Cycle

The Calvin-Benson Cycle

A molecule of CO₂ is taken in by the cell and is combined with RuBP (a five carbon sugar, abbreviated as 5C) to form a 6C intermediate sugar via an enzyme called RuBisCo. The 6C then breaks down to form 2 PGA's (phosphoglycerate) - each a 3C
The PGA's undergoes a cyclic pathway, the Calvin-Benson cycle, which will eventually spit out 2 PGAL's (phosphoglyceraldehyde). Two PGAL's can form a sugar phosphate, which can then form a sugar

Photorespiration

One of the biggest faux pas (that's French for big "mistakes") of evolution RuBisCo is not only attracted to CO_2, but it can also use O₂ in the Calvin-Benson Cycle

When O₂ is used in the Calvin-Benson Cycle, no energy is stored - in fact, energy is lost!
The reaction is as follows: O₂ + RUBP - - - > 1 PGA + 1 Phosphogylcolate
- There is very little use for phosphoglycolate in the plant, so the plant must spend energy to convert the phosphoglycolate back to a useful molecule and reclaim the two carbons

Why does photorespiration occur?

When this evolved, the concentration of O₂ was low - this was not a problem
Plants have since evolved ways to reduce the damage caused by O₂ in the Calvin-Benson Cycle
Plants must spend up to 40% of their energy stored in sugars to deal with the damage created by RuBisCo fixing O₂ in the Calvin-Benson Cycle

Rubisco can utilize O₂ as a substrate instead of CO₂

O₂ and CO₂ bind at the same active site
if O₂ and CO₂ are present in equal concentration, CO₂ is fixed 80x faster

BUT the ratio of CO₂/O₂ in water in equilibrium with air at 25^oC=1/24

[CO₂] in air=0.035%
[O₂] in air=21%

Therefore, for every 3 CO₂ incorporated, there is 1 O₂

The plant must then undergo a complex series of reactions to remove the O₂ from the phosphoglycolate

Major efforts have been made to modify the properties of Rubisco to eliminate the oxygenation reaction, especially using molecular genetics

all the results so far indicate that the two reactions cannot be separated.
modifications in Rubisco that reduce the oxygenase activity also reduce the carboxylase activity

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?