PHOTOSYNTHESIS- FOOD PRODUCING PROCESS

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(Edited)

This name was first proposed by Barnes in 1898. Photosynthesis (photo, light; synthesis, building up) consists in the building up of simple carbohydrates such as sugars, in the green leaf by the chloroplasts in the presences of sunlight (as a source of energy) from carbondioxide and water absorbed from the earth and the soil, respectively. Glucose is formed first all other carbohydrates are derived from it. The process is accompanied by a libration of oxygen. Photosynthesis is the biological cells by which some energy-rich, carbon-containig compounds are produced from carbondioxides and water by the illuminated green cells, librating oxygen as a by-product. It is essentially oxidation-reduction process by which hydrogen is transferred from water to carbondioxide through a carrier substance. The overall reaction may be represented thus

1200px-Photosynthesis_equation.svg.png

Photosynthesis Equation

IMPORTANCE OF PHOTOSYNTHESIS

  1. All life on earth depends on photosynthesis for energy and carbon supply.
  2. Oxygen which is vital for all aerobic forms of life, is produced during photosynthesis.
  3. It is foundation of fossil fuel formation via carbon fixation, a process which takes million of years to develop.
  4. Manufacture of sugar. This is because photosynthesis converts carbon from atmospheric CO2 to the carbon of sugar.

MECHANISM OF PHOTOSYNTHENSIS
Photosynthesis involves two distinct phases, they are:
1.Light dependent reactions and
2.Light independent reactions

1.Light Dependent Reactions
This stage supplies the energy that is utilized during photosynthesis and it occur in the membrane systems within the chloroplasts called thylakoids. Actually the work of light dependent stage occur in two-folds;

a. Provision of hydrogen ion (H+) to reduced CO2. The hydrogen ion is a product of the photochemical splitting of water (H2O).
Equation for the photolysis of water
H2O ---------> (H+) + (OH-)
(Water) (Hydrogen ion) ( Hydroxyl ion)

(OH-)+ (OH-) ------------> H2O + O2
(Oxygen)

b. Production of ATP.
During this stage, light is trapped by the many different pigments in the grana of the chloroplasts. The other non-green pigments pass on their trapped energy to chlorophyll which becomes exited.
When chlorophyll absorbs light they become exited. In this high state, chlorophyll donates high electrons which are used for the two functions above. The energy obtained from light is used to boost electrons to a higher energy state. The energy is thus transferred to chemical energy.

In the first instance, the energy breaks down water into hydrogen (H+) and oxygen (O). The hydrogen is taken up by an organic molecule called Nicotinamide Adenine Di-nucleotide Phosphate (NADP). It is known as a hydrogen carrier. When it takes the hydrogen, it is reduced to NADPH. The oxygen is given off into the atmosphere. This molecule carries the hydrogen into the next phase.

The second function is the production of ATP. This is accompanied when energy is used to bond another phosphate molecule to ADP using light energy, that is, photophosphorylation. The electron flow pathway in phosphorylation may be cyclic or non cyclic. It is cyclic in nature when the electron from the chlorophyll is returned to where the electron was initially donated from; while it is non-cyclic if the electron ends up in NADP. Recall that ATP is the energy carrier of the cells.
It is clear from the above that of the ATP and reduced NADP is simply to carry energy and hydrogen respectively, into the next stage which is light independent reactions.

PHOTOSYSTEMS

The chlorophyll and accessory pigment molecules are located in two type of photosynthesis, known as photosystem I and II (PS I and PSII). These photosystems are visible as particles in the thyaliod membranes, each contains an antenna complex or light harvesting complex, of pigment molecules. The light-harvesting complex contains about 200 to 300 pigment molecules and collects light energy. Different pigments collect light of different wavelength making the process more efficient. All energy is transferred from, molecule to molecule, and finally to a specialized form of chlorophyll-a known as P700 in PS I and P680 in PS II. P stands for pigment, their absorption peak are at wavelength of 700nm and 680nm respectively. The chlorophyll P700 and P680 becomes exited by the energy they absorbed and released high energy electrons.

Chlorophyll -----------> chlorophyll+ + e-
(reduced form) (oxidiced form) (electrons)

2.Light Independent Reactions
This stage used to be known as the dark stage- a misleading name which was coined because the reaction involved did not require light energy. Essentially, it is the stage where CO2 is reduced to form carbohydrate. It takes place in the stroma of the plant chloroplast. It is also an energy driving process; however, the energy is derived from the hydrolysis of ATP produced in light dependent reactions, as well as, the reducing power (NADP.H2). The reactions are control by enzymes. The sequence of the reactions, which are cyclic, was determined by three scientists namely- Calvin, Benson and Basam between 1946 and 1953. Hence the name of the reaction was known as Calvin-Benson cycle.

The first step involves carbondioxide (CO2) fixation where it combines with an organic molecule that has five (5) carbon atoms. This five-carbon molecule which is a CO2 acceptor is called riboluse biphosphate (RuBP). The addition of CO2 to any compound is called carboxylation. The enzyme that controls the rate of the reactions is called carboxylase.
The initial 6-carbon sugar formed from the carboxylation process is unstable; hence, it breaks down into two molecules of a 3-carbon compound called phosphoglycereate. This is the first product of photosynthesis.

The second step is a crucial one which involves the reduction of the phosphoglycerate to form a sugar with 3-carbon atoms called phosphoglyceraldehyde. The energy for this process is obtained from the ATP and the hydrogen from reduced NADPH is used for this reactions. Note that the Addition of hydrogen to any compound is called a reduction reaction in such compound is said to be reduced. The 3 carbon sugar is there after built up to a 6-carbon sugar that is sucrose which is converted to sugar for storage in plants or it may be utilized for cellular respiration. Most ideally 5 out of 6 molecules of the phosphoglyceraldehyde produced are used to regenerate riboluse biphosphte (RuBP) so that the process can continue. The sugar produced during carbon metabolism yields carbon skeleton that can be used for other metabolic reaction like the production of amino acids (which are the building blocks of proteins) and lipids.

References

What is photosynthesis?
Importance of photosynthesis to living organisms
Mechanism of photosynthesis



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Did you forget to include references or what?

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I ommited it.
Just added it now. Thanks for the observation

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Did you forget to include references or what?

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I ommited it.
Just added it now. Thanks for the observation

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