What is PHOTOSYNTHESIS

Photosynthesis is the most common phenomenon that occurs in the environment. In the process of photosynthesis, 6 carbon dioxide molecules, 12 water molecules, plus sunlight react and form products like glucose, plus 6 water molecules, plus 6 oxygen molecules.

We can say that photosynthesis is the conversion of light energy into a usable form of chemical energy.

The process of photosynthesis can be divided into:

Light reaction
Dark reaction

Light reaction.

Light reactions take place in the presence of light. The light reaction has two systems known:

Photosystem 1
Photosystem 2

Now, what is a photosystem?

A photosystem is a combining unit of reaction center, antenna, primary acceptor, and some enzymes that helps in reaction. All of these units collectively work and form a system that is called a photosystem.

These divisions are on the base of their discovery. Photosystem 1 was discovered earlier while Photosystem 2 discover later. The location of both photosystems in the cell is thylakoid.

If the light’s wavelength is less than 680 nanometers, then both photosystem one and photosystem two will be active.

Photosystem two.

The reaction center of photosystem two is chlorophyll a, which can absorb light at about 680 nanometer’s wavelength.

An antenna is a group of pigments like chlorophyll b, carotenoid, and xanthophyll.

Now, what are pigments?

The pigments are chemicals that release electrons under specific energy or release electrons after absorbing a specific wavelength of light.
They have colors. Chlorophyll a is brown or green, chlorophyll b is green or yellow, catenoid is yellow or orange, and xanthophyll is orange in color.

Light falls on the antenna of photosystem two, with a wavelength of fewer than 680 nanometers.
Suppose, the wavelength of light is about 640 nanometers.
Antenna receives this light energy and excites the electrons of pigments, but only those pigments can excite its electrons which can absorb this specific wavelength of light.
Now, these excited electrons give their energy to other pigments but can transmit their energy to only those that can absorb light of more than 640 nanometers wavelength.

You can understand this by supposing an example.

Suppose a pigment which we call pigment x This pigment x can absorb light only 500 nanometers wavelength.

Suppose another pigment named, pigment y and pigment z.

Pigment y can absorb the light with 520 nanometers wavelength and pigment z can absorb light with 550 nanometers wavelength.
Now, pigment x can give its energy to both pigment y and pigment z but pigment y and pigment z cannot give their energy to pigment x.

This light energy excited the electrons of pigments, which are traveling toward the reaction center.

The reaction center takes all the energy from excited electrons. The reaction center is chlorophyll a, which can absorb the light of 680 nanometers wavelength. It can be called P680. This reaction center also has some enzymes with it.
The reaction center, after taking all the energies from the electrons of pigments, gives its electron to the primary acceptor. This primary acceptor is called pheophytin.
This primary acceptor moves this electron towards electron carriers.
These electron carriers make an electron transport chain.
Through, this electron transport chain, electrons move towards photosystem one.
The reaction center of photosystem 1 is P700.
This means that the reaction center of photosystem one can absorb the light with 700 wavelengths.
So we can say that photosystem one has its excited electrons plus electrons from photosystem two.

The names of these electron careers are:

Plasto quinine (Pq)
Cytochrome b6f (c y t b6f)
Plasto cyanine (Pc)

While moving in the electron transport chain, ATP is formed with the help of an ADP molecule.

Now, what are ATP and ADP?

ATP is termed adenosine tri-phosphate, while ADP is adenosine di-phosphate. One ATP molecule has three phosphates while one ADP molecule has two phosphates with it.

In photosystem one, the electrons are moved toward the primary acceptor of photosystem one.

Then electrons are transferred from the primary acceptor to ferredoxin (fd).
Finally, the electrons move towards NADP, which forms NADPH.
NADP is present in the form of NADP+, which means deficient of one electron.
This NADP+ takes two electrons and two hydrogen ions to form NADPH+ and one hydrogen ion.
This conversion takes place in the presence of an enzyme called NADP reductase.
As we discussed earlier, an electron is transferred from photosystem two to photosystem one. It means that photosystem two is electron deficient.
To overcome this deficiency, photosystem 2 breaks water molecules and this breakdown of the water molecule is called photolysis.
In the process of photolysis, half an oxygen molecule is released which moves out from the cell.
Hydrogen ions or protons and electrons are also produced in the process of photolysis, which is used by the cell.

Points to be remembered:

Electrons move from lower wavelength pigments to higher wavelength pigments. For this reason, the electron moves from Photosystem 2 to Photosystem 1, because the reaction center of Photosystem 2, can absorb the wavelength of light of 680 nanometers, while the reaction center of Photosystem 1, can absorb the light of the wavelength of 700 nanometers.
Photosystem 2 has high energy as compared to Photosystem 1.
Photosynthesis is a downhill movement of electrons because energy is reciprocal to wavelength and the electron moves from lower to higher wavelength.
It means the electron moves from a higher energy area to a lower energy area.
Photosystem 2 will be active only if the light’s wavelength is less than 680 or equal to 680 nanometers.
If the coming light’s wavelength is more than 680 nanometers, then photosystem 2 will be inactive and just photosystem 1 will work.
The overall process of photosynthesis is non-cyclic (when both PS2 and PS1 work collectively).
But when only photosystem 1 is working, then the flow of electrons is cyclic.
In cyclic photosynthesis, photosystem 1 gives its electron to its primary acceptor, then the electron moves from the primary acceptor to the electron carrier which is plasto quinine (pq).
From plasto quinine, it transfers to cytochrome, where it forms an ATP molecule.
Cytochrome transfers this electron to plasto cyanine, and through plasto cyanine, it moves back to photosystem 1.
In this way, the cyclic process is complete, and only ATP is formed in the cyclic process.
In the non-cyclic process of photosynthesis, ATP, NADPH, and oxygen are released.
These ATP and NADPH are moved towards the dark reaction of photosynthesis and oxygen released.

Dr. Malik | The doctor of knowledge

PHOTOSYNTHESIS | Light reaction of photosynthesis | Dr MALIK

What is PHOTOSYNTHESIS