Chapter 7

Definition

“The process in which green parts of the plants, especially leaves manufacture their own food material utilizing raw materials like CO₂ and H₂O and sunligh as the source of energy is called Photosynthesis.”

The food prepared is glucose, which finally changes into starch for storage.

Explanation

All green plants contain chlorophyll and are capable to prepare their own food. They absorb water from the soil, CO2 from air and combine them chemically to produce Glucose (C6H12O6). For reaction, Energy is obtained from sunlight specially ultra violet rays by means of chlorophyll which acts as a Photo-chemica cell. The whole process can be shown as follows;

In this process oxygen is obtained as By-Product which is very vital for all living beings.

Mechanism of Photosynthesis

The above equation is just an abbreviation of Photosynthesis. In reality, it is a complex chain of oxidation and reduction reactions governed by Enzymes.

Chemical reactions of photosynthesis are of two types i.e.

1.        Light Reactions

2.        Dark Reactions

1.   Light Reactions

These are light dependent reaction in which sunlight is captured and converted into potential chemical energy in the form of ATP. For this reason they are also called Photo Chemical Reaction.

In light reactions three major and important works are performed;

·        Light energy is converted into ATP (Phosphorylation).

·        Water (H₂O) is splitted into H⁺ and OH^- ions (Photolysis).

·        Reducing power (NADPH₂) is obtained.

All of these steps are dependent upon chlorophyll which is of two types:

(a) Chlorophyll A or Pigment System I

It emits electrons (e^-) in light of 680 Hertz frequency or more i.e. Ultra violet light.

(b) Chlorophyll B or Pigment System II

It accepts electrons and hydrolysis water in light of less than 680 hertz frequency.

Phosphorylation

Synthesis of ATP is called Phosphorylation. ATP is a high energy substance from which 23000 calories are liberated or obtained when phosphate bonds are break. It is manufactured by two methods.

(i)                Cyclic Phophorylation

(ii)             Non cyclic Phosphorylation

(i) Cyclic Phosphorylation

In this method, Electrons emitted by “Chlorophyll A are transferred to Plastoguinone and to cytochrome oxidase system. From here they are transferred back to to chlorophyll A after releasing energy required for the synthesis of ATP.

(ii) Non Cyclic Phosphorylation

In this method, chlorophyll A emit electrons utilizing solar energy. The electrons are accepted by an acceptor Ferridoxin which transfers them to a reducing substance NADP.

Chlorophyll B in the presence of sunlight splits water molecules into H⁺ and OH^- ions. H⁺ ions are accepted by NADP, which changes to NADPH₂ for reduction reactions.

OH^- ions combine together to form H₂O and O₂ gas with the liberation of electrons, these electrons are accepted by Plastoguinone acceptor from where they are transferred to Cytochrome Oxidase system of enzymes. Here electrons are transferred through different energy levels and the liberated energy of electrons is utilized in the building up of ATP. Finally electrons are transferred to chlorophyll A. As these electrons are obtained from OH^- ions of water, therefore it is called Non Cyclic Phosphorylation.

2.   Dark Reactions

These reactions are independent of solar energy but depend upon chemical energy i.e. ATP synthesized in the light reactions. Details are as follows:

The reactions start with the oxidation of sugar Ribulose 5 phosphate into Ribulose 1, 5-Di phosphate by means of ATP. It is a 5-carbon compound which combines with atmospheric CO₂ to yield an unstable 6 carbon intermediate compound. This compound splits into two molecules of 3 carbon molecules namely 3-phophoglyceric acid. Each molecule of 3-PGA oxidezes into 1, 3- Diphopho Glyceric Acid at the expence of ATP molecules. Each molecules of 1, 3-Diphospoho Glyceric Acid reduces by means of NADPH₂ into 3 phophoglyceraldehyde.

3-phosphoglyceraldehyde is utilized in 4 different ways, one way is the formation of Ribulose, 5-Phophate while the other way of its utilization is formation of 6-P Fructose 6-Phosphate yields after certain steps the major stable product of photosynthesis i.e. Glucose.

Glucose (C₆H₁₂O₆) is the final product of photosynthesis which is stored in the storage tissues in the form of starch (C₆H₁₀O₅)_n.

Significance

·        Plants prepare food by this method.

·        Atmospheric CO₂ is utilized and decreases i.e. helps to control pollution.

·        Water is utilized by the plant.

·        Oxygen is liberated which is essential for life of all living beings.

= Respiration =

Definition

“The process by which living beings obtain energy by the oxidation of food stuff is called Respiration.”

or

“Oxidation of nutrients, especially Glucose by which energy is released along with CO₂ in living biengs is called Respiration.”

Explanation

All living beings require energy for their activities. To get energy they oxidize glucose (generally) into CO₂ and H₂O with liberation of heat energy. This whole process is governed by Enzymes, it is catabolic process. The potential chemical energy of glucose is converted into heat energy in a step-wise manner.

Types of Respiration

The initial step of respiration takes place in the cytoplasm known as Glycolysis. In glycolysis, Glucose is converted into Pyruvic Acid which further oxidizes in Mitochondria. Oxidation of Pyruvic Acid is of two types:

1.        Aerobic Respiration (Dependent on Oxygen)

2.        Anaerobic Respiration (Independent of Oxygen)

1.   Aerobic Respiration

In this oxidation pathway complete oxidation of Pyruvic acid takes place by means of oxygen and CO₂ and H₂O are formed with liberation of 673 K.cal/Mole energy. Whole process may be summarized as follows:

2. Anaerobic Respiration

In this method, Pyruvic acid oxidizes in the absence of oxygen incompletely by which CO2 is produced along with Ethyl alcohol (C2H5OH) in plants and Lactic acid in animals. Only a small amount of energy (23 k.cal/mole) is released in this process. It may be summarized as follows:

Details and Mechanics of Aerobic Respiration

1.   Glycolysis

In glycolysis, Each molecule of glucose yeilds two molecules of Pyruvic acid. These reactions take place in cytoplasm by means of enzymes.

First of all, glucose molecule is phosphorated by ATP by the enzyme Glucokinase. Thus Glucose, 6-Phosphate is obtained.

 

G, 6-P is converted into Fructose, 6-P by the enzyme Isomerase, further oxidation of F, 6-P takes place by ATP under influence of enzyme Phospho Hexokinase.

 

Fructose 1, 6-Di P molecule is split into two molecules, i.e. 3-P Glyceraldehyde and Dihydroxy Acetone –P by the enzyme Aldolase.

Both of these compounds are intermediate and therefore we can say that from each molecule of F 1, 6-Di P, Two molecules of 3-P Glyceraldehyde are obtained.

3-P Glyceraldehyde molecules are further phosphorylated by ATP into 1, 3- Di P Glyceraldehyde by the enzyme Dchydrogenase.

 

1, 3 Di P Glyceraldehyde is reduced to 3-P Glyceric Acid by the enzyme Phophokinase. One ATP molecule is generated during the course.

3P Glyceric Acid is converted into 2- P Glyceric Acid by the enzyme Mutase.

Enzyme Enolase removes one H₂O molecule from 2-P Glyceric Acid to convert it into Enol Pyruvic Acid.

 

From Phospho enol Pyruvic acid, enzyme Triose Kinase shifts P radical to ADP by which one ATP is genrated along with the formation of Pyruvic Acid.

 

Now Pyruvic enters into Aerobic phase of reaction taking place in the mitochondria.

2.   Aerobic Respiration

In aerobic Respiration, Pyruvic acid oxidizes with O₂ by which large amount of energy is liberated. Details are as follows.

Enzyme Pyruvic Dehydrogenase acts on Pyruvic Acid and removes two H atoms along with one molecule of CO₂ by which Acetyl Radical is formed. H atoms are accepted by NADP.

 

Acetyle Radical combines with Co Enzyme A-SH by which Acetyle Co-Enzyme is formed.

 

Kreb’s Citric Acid Cycle

Acetyle Co enzyme now enters into Mitochondria where complete oxidation of this compound takes place. Details are as follows:

·        Acetyle Co enzyme A combines with Oxalo Acetic Acid of Mitochondria by which Citric Acid is formed.

·        Citric Acid is first converted into Cis Aconitic Acid and then into Iso Citric Acid by the enzyme Aconitase.

·        Iso citric acid oxidizes into Oxalo Succinic Acid due to removal of H⁺ ions by the enzyme dehyrogenase and NADP.

·        Emnzyme Decarbozylase removes one molecule of CO2 from Oxalo Succineic Acid which changes to L-Keto Glularic Acid.

·        L-Keto Glutaric Acid oxidizes by the removal of H⁺ ions under influence of NADP and enzyme dehydrogenase into Succinyle Co enzyme A due to combination of Co enzyme A.

·        Succinyle Co Enzyme A gives rise to Succinic Acid by the enzyme succinic synthetase.

·        Succinic Acid oxidizes to Fumeric Acid by the enzyme dehyrogenase with the help of FAD.

·        Frmaric Acid combines with one molecule of H₂O and converts into Malic Acid by the enzyme Hydratase.

·        Malic Acid oxidizes to Oxalo Acetic Acid by means of NADP and dehydrogenase Enzyme.

Oxalo Acetic Acid again enters into the cycle and whole cycle is repeated.

 

Estimation of Energy From Cycle

All H+ ions removed by NADP and FAD are transfer into Cytochrome Oxidase Electtron Career System where they are combined with atmospheric O₂ to form H₂O. Energy thus released in this process is utilized in the synthsis of ATP. From each molecule of NADPH⁺H^- 3 ATP are generated while from FADH⁺H^- 2 ATP are obtained. On total estimation, each molecule of Glucose (C₆H₁₂O₆) yields 36 ATPs.