Photosynthesis

Word photo means light and synthesis means to prepare.

Photosynthesis: Plants utilize simple inorganic molecules carbon dioxide ($CO_2$) and water ($H_2O$) which react by using light energy in the presence of pigments like Chlorophyll to form glucose and oxygen.

Chlorophyll:

Chlorophyll is the green pigment found in the chloroplast of plant cells. It captures a specific part of visible light only, therefore it is not a reactant but absorbs the energy needed to drive the reaction.

Glucose: The fundamental molecule produced during photosynthesis is simple sugar i.e. Glucose. Glucose is used in most plant metabolism to produce secondary products like starch and other polysaccharides. Plants also use carbohydrates to form fats, proteins, and other chemical like Nucleic Acids. This glucose is also used in respiration as a reactant to produce energy for the metabolism of living organisms.

Different forms of life completely depend on Photosynthesis

Dependence of animals on photosynthesis: Plants are not the only organisms which depend on photosynthesis but animals (Heterotrophs) also depend on phototrophs. These organisms utilize the molecules of phototrophs as food molecules. If an animal is herbivorous it feeds directly on plants. If an animal is carnivorous it depends on those animals which feed on plants. These feeding sequences and relationship are called Food Chains.

Source of oxygen: Photosynthesis is the only process which produces free $O_2$ by splitting water. This $O_2$ is utilized by all living organisms for respiration to produce energy for metabolism. Without $O_2$ living organisms cannot survive. Through Photosynthesis, quantity of $CO_2$ and $O_2$ in nature is maintained by plants. During Photosynthesis plants fix $CO_2$ and release $O_2$ in environment.

$CO_2$ fixation: $CO_2$ has a property to absorb heat of sun. If its quantity increases in environment, there will be increase in an environmental temperature on earth called Global warming. Photosynthesis keeps the quantity of $CO_2$ maintained in environment i.e. indirectly keeping the concentration of $CO_2$ to maintain the temperature of earth.

Chloroplast as light Trapping and storage organelle

Chloroplast: Green part of plants and algae contain special cell which contain special organelle called chloroplast. Chloroplast is a double membrane bounded organelle, have semi-fluid proteins containing medium called Stroma. Another network of membrane is also embedded in it called Thylakoid membrane, somewhere this thylakoid are piled at one another in stack called Grana (Singular - Granum).

The simple looking reaction of photosynthesis is not as simple as it looks. It involves number of chemical reactions which are catalyzed by number of enzymes, either in non cyclic or cyclic ways. Each reaction occurs at different site in chloroplast i.e

Light dependent reactions - Thylakoid membrane: Reaction in which light energy converted into chemical energy and stored in ATP (Adenosine Triphosphate) and $NADPH_2$ (reduced Nicotine amide Adenosine Dinucleotide Phosphate). This conversion occurs at thylakoid membrane where solar energy is captured by pigments located in harvesting complex. This phase of photosynthesis is called light Dependent reaction. It is non Cyclic process coupled with breakdown of $H_2O$ molecules i.e. photolysis, takes place also at thylakoid membrane.

Light independent reactions - Stroma: Reaction in which captured solar energy is transferred to glucose from ATP and $NADPH_2$ . It takes place in stroma, in cyclic manner. During this phase, fixation of atmospheric $CO_2$ also takes place to form organic molecules.

Two phases of Photosynthesis:

Processes of Photosynthesis is mainly divided into phases or reactions:

1. Light Reactions or Light Dependent reactions

2. Dark Reactions or Light Independent reactions

1. Light Reactions or Light Dependent Reactions

What are Light Reactions?: During this phase of photosynthesis, light energy is captured and converted into chemical energy. Some of the light is utilized to split water into oxygen and $H^+$ with $e^-$ (electrons), this splitting of water is called Photolysis. Oxygen which is produced during photolysis is released in the environment where as $H^+$ together with $CO_2$ are used in building Glucose.

Photosystem I and II: In chloroplast, different pigments absorb light of different wavelengths. Among them chlorophyll is the main light capturing molecule which is present in thylakoid membrane. It absorbs violet, blue and red light but reflects green light. Therefore it appears green. In the thylakoid membrane other pigments and electron carrier molecules form highly organized assemblies in a series called photosystems. Each thylakoid contains thousands of copies of two different kind of photosystems called photosystems I and II. Each consists of two major parts

a) A light harvesting complex

b) An electron transport system

Mechanism of light reactions: The conversion of light energy takes places when the chlorophyll of reaction center receives energy. One of the electrons from chlorophyll “a” molecule leaves and jump over the electron transport system. This energized electron moves from one $e^-$ carrier to next. The electron releases energy, when it comes down, this energy drives reactions and produces two energy rich compounds. These are:

i) ATP (Adenosine Triphosphate) ii) $NADPH_2$ (Reduced Nicotinamide Adenosine Dinucleotide Phosphate)

Production of ATP: ADP is the compound which is already present in cell. It combines with phosphate by using energy of photon released from when moving through $e^-$ carriers in photosynthesis.

Production of $NADPH_2$: NADP also present in chloroplast is reduced into $NADPH_2$ by accepting Hydrogen ions ($H^+$), released from splitting of water.

Role of ATP and $NADPH_2$: ATP and $NADPH_2$ both are energy rich compounds which provide energy, Hydrogen ($H^+$) and $e^-$ for the conversion of atmospheric $CO_2$ into carbohydrates in chloroplast during light independent Phase of photosynthesis.

2. Dark Reactions or Light Independent Reactions

What are Dark reactions?: The ATP and $NADPH_2$ synthesized during the light dependent reaction are dissolved in stroma there, and then they provide energy to power the synthesis of Glucose from $CO_2$ and $H_2O$ (i-e $H^+$ and $e^-$ of water). This phase of photosynthesis does not require energy of photon but also takes place in day simultaneously with the light reaction.

This phase of photosynthesis is cyclic phase. It occurs in set of reactions also called Calvin – Benson Cycle due to it's discover or the $C_3$ (three Carbon Containing Compounds formed initially) Cycle. The $C_3$ Cycle requires 3

1. $CO_2$ - normally from air some of it also comes from respiration

2. $CO_2$ Capturing Sugar - a Pentose Sugar

3. Enzymes to catalyze all the reactions

4. Energy from ATP and $NADPH_2$ which come from light dependent reactions.

Limiting Factor

Limiting factors: These are the factors which are capable to affect the output of the reaction or the rate of a biochemical reaction depends on them. For example, Light intensity, Carbon dioxide concentration and temperature can all be limiting factors for the rate of photosynthesis. At low light intensity rate of photosynthesis increase continuously but at high light intensity the rate becomes constant.

Following graph shows the idea of limiting factor.

A- At point Z on graph, light intensity is limiting factor. B- If light intensity is increased to bright light and temperature is moderate, the concentration of $CO_2$ , in air becomes limiting factor. It is clearly observed that the same plant if put into air containing high $CO_2$ then the rate of photosynthesis becomes high.

If there is high light intensity and high $CO_2$ concentration then the temperature becomes the limiting factor but the temperature should not be very high otherwise enzymes become denatured.