Photosynthesis

Photosynthesis is an endergonic reaction through which autotrophs convert light energy from the sun into chemical energy in the form of sugars that are usable by cells. It occurs inside the chloroplast in cells and can be broken up into two parts: the light reactions and the Calvin cycle.

If you aren't familiar with the parts of the chloroplast, click here.

As this process takes place inside of the chloroplast, the descriptions will use the parts of the chloroplast in an attempt to orient you as to what is happening and where. Here are the key parts that will be talked about, along with a visual image showing them.

Stroma: the fluid inside the chloroplast

Thyalkoid: the individual disks

Granum: the stacks of disks

Lumen: the fluid-filled center of a granum

The Light Reactions

This is the "photo" part of photosynthesis. The light reactions occur in the thylakoids of chloroplasts. This process will provide the energy needed for the synthesis of sugars in the form of ATP and NADPH.

The light reactions make use of photosystems.

Photosystems are complexes found within the thylakoid membrane. They are made of a reaction-center protein complex, which is surrounded by light-harvesting complexes (which are made of proteins and chlorophyll pigments). The light-harvesting complexes will transfer energy into the reaction center complex.

Step 1

Pigments in Photosystem II (PSII) absorb light energy and transfer it from pigment to pigment. The energy is passed from one chlorophyll to another through electromagnetic energy (UV rays/waves).

Step 2

In the reaction center, chlorophyll a (P680) becomes excited to a higher energy level. It transfers an excited electron to the primary electron acceptor, which is reduced.

Step 3

In order to replace the electron that was lost by P680, water is split, releasing electrons for this purpose. The splitting of water also produces oxygen, which is given off as a byproduct, and hydrogen ions (H+), which go into the thylakoid lumen.

Step 4

The excited electron that has been transferred to the primary electron acceptor begins to move from PS II to PS I. It does so through redox reactions via the electron transport chain (ETC) between the two photosystems.

Step 5

The flow of electrons down the ETC provides energy for ATP synthesis via chemiosmosis. The ATP produced at this step will be used later in the Calvin Cycle for the production of sugars.

Chemiosmosis

Chemiosmosis uses the movement of H+ ions along an electrochemical gradient to power cellular work.

As electrons move down the ETC, they release energy. This energy is used to pump H+ ions across the membrane and from the stroma into the thylakoid lumen. This produces a gradient.

H+ ions will move across the now-present gradient via facilitated diffusion. As they move through ATP synthase, the enzyme catalyzes the phosphorylation of ADP into ATP.

Step 6

Meanwhile, in Photosystem I (PSI), it has been undergoing the same thing that PSII was, simultaneously. Light energy is absorbed and transferred until chlorophyll a (P700) is excited, which loses its electron to the primary electron acceptor. This electron is replaced by the electron that comes over from PSII via the ETC.

Step 7

The primary electron acceptor in PSI will release the electron down a different ETC. The electron will eventually be brought to NADP+ reductase, an enzyme that transfers the electron to NADP+ and reduces it into NADPH (hence the name). NADPH will bring its electron to the Calvin Cycle where it will be used for the reactions involved in created sugars.

What's the difference between P680 and P700?

They are both a special pair of chlorophyll a molecules found in reaction-center complexes. The difference is the wavelength of light they are best at absorbing.

P680 is best at absorbing light at a wavelength of 680nm .

P700 is best at absorbing light at a wavelength of, you guessed it, 700nm.

NADPH

NADPH (nicotinamide adenine dinucleotide phosphate) is a cofactor and an electron donor used in redox reactions. It is found and used by all forms of cellular life. In photosynthesis, it carries excited electrons from the light reaction to the Calvin Cycle, where they are used to supply energy to create sugars.

The Calvin Cycle

The Calvin Cycle is the "synthesis" part of photosynthesis. It occurs in the stroma of chloroplast and forms sugar using ATP and NADPH that were generated during the light reactions.

It has three major phases:

Carbon Fixation: CO2 is captured and incorporated into organic molecules by the enzyme, rubisco, to create 3PGA (3-phosphoglycerate).
Reduction: 3PGA is phosphorylated by ATP and is reduced (by receiving electrons from NADPH). This produces G3P (glyceraldehyde 3-phosphate). Some G3P is a product of the cycle and will go on to form glucose. Other G3P will continue on in the cycle.
Regeneration: The G3P that continues in the cycle will regenerate RuBP (ribulose bisphsophate - the organic molecule that rubisco incorporates with CO2) through the use of ATP. This will allow the cycle to repeat again.

One "spin" of the Calvin Cycle produces two 3PGA, which is a three-carbon molecule that is then converted into G3P. Five out of every six G3P that are produced are used in regeneration to produce the five-carbon molecule RuBP. Since the cell needs two G3P in order to produce glucose, the six-carbon molecule, the Calvin Cycle must occur six times per glucose molecule.