CHAPTER 13  PHOTOSYNTHESIS

1. Introduction

Nearly all of the members (with some exceptions) categorized under the kingdom Plantae are autotrophic in nature. 

Being autotrophic gives the plants the ability to synthesize their own food for their own nutrition and well being as well as providing a source of nutrition for heterotrophic organisms which are incapable of synthesizing their own food and hence are dependent on autotrophs. 

Plants fix gaseous carbon dioxide from the atmosphere and water transported from the roots in order to synthesize their food in the form of complex, organic substances, majorly sugars and starches while releasing free, gaseous oxygen

which returns to the environment and is used by other organisms apart from the plant itself during the respiration process. 

This process,which requires the presence of electromagnetic radiations (light) or solar energy (sun light) is termed as Photosynthesis. The light is captured by specialized organs in the leaves of plants termed as chloroplast which are abundant in light capturing, mostly green colored pigments termed as chlorophyll and the presence of these chlorophyll

Pigments imparts green color to the leaves. 

Photosynthesis may appear like a single step process but is in fact a series or cascade of mechanisms that is constantly monitored, heavily regulated and under huge

influence by a variety of factors.

Although higher plants and algae perform a majority of the photosynthetic activity, there are certain protists

and bacteria such as cyanobacteria (blue green algae) which possess a light harvesting pigment termed as

bacteriochlorophyll (ancient, archaic origin) and are also autotrophic in nature. 

However, their benefits to humans are very limited and hence they are not as well researched as plants.

2. Photosynthesis

‘Photosynthesis is defined as the photo-biochemical/physicochemical mechanism, incorporating

anabolic, reductive and endergonic processes carried out by green plants, in which complex, energy-rich organic compounds (carbohydrates such as sugars, starches) are synthesized from simple inorganic raw materials composed of water (H2O) and carbon dioxide (CO2) in presence

of electromagnetic radiations (light or solar energy) and light-capturing pigments (chlorophyll, carotenoids) with release of oxygen (O2) as a byproduct.’

This mode of nutrition using photosynthesis process is termed as photoautotrophism.

The basic mode of photosynthesis occurs in the following steps:

1. Absorption and retention of electromagnetic radiations (composed of photons and waves) in the form

of light energy from the sun.

2. Conversion of the dynamic, solar light energy into stable, chemical potential (energy) which is stored.

15.3

Hence, the overall reaction of photosynthesis is shown as

6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2 ↑

Nearly 90% of the total photosynthesis seen on Earth is carried out by marine plants composed of phytoplankton and algae while only 10% is carried out by terrestrial plants.

Approximate annual fixation of carbon in the form of CO2 through photosynthesis is around 258 billion tons.

In photosynthesis, light energy (dynamic) is converted into chemical energy (stable and storage). 

Only about 0.2% of the total light energy falling on the earth is utilized by photosynthetic organisms.

First organism capable of photosynthesis were bacteria but first true and oxygenic photosynthesis was discovered in cyanobacteria/blue-green algae (BGA).

2.1 Historical Landmarks in Studies of Photosynthesis

A record of the early studies and experiments conducted which served as landmarks in the field of photosynthesis are listed below

1. The first scientist to establish the fact that gaseous constituents of air and components of light, both

contributes towards the building up of the plant body and photosynthetic nourishment occurring in plants plant was Stephan Hales (1927).

2. He is also considered to have discovered the process of photosynthesis and is called as ‘Father of Plant Physiology’.

3. Joseph Priestley (1770) demonstrated that purification of the respired air released by animals was carried out by plants. In a series of experiments involving bell jar, candle, mint (pudina) plant and rat the essential role of air in the growth, development, and survival of green plants was demonstrated.

Priestley noted that candle burning on its own in a closed space such as a bell jar, gets extinguished

very quickly. Burning of the candle results in the generation of impure air which was labeled as

phlogiston. 

When a live mouse was placed along with the candle in the closed bell jar, it resulted in the death of the mouse due to suffocation. 

However, on placing a live mint plant in the closed bell jar containing the burning candle and live mouse both resulted in both the plant and mouse surviving while

the burning of candle continues. 

The final conclusion achieved was that the candle which requires air continued burning and/or the animal that breathes air remained alive due to the presence of plants. 

The hypothesis proposed by Priestley was that the restoration of the phlogiston to its pure form, which was

termed as dephlogiston was achieved by plants. This pure air or dephlogiston was whatever breathing animals and burning candles remove. 

4. Oxygen is an essential gas for survival was also established by Priestley (1774).

5. Jan Ingenhousz stated that green leaves give out dephlogisticated air (air rich in oxygen) in presence

of sunlight while in the absence of sunlight (in the dark) they give out phlogisticated air (air rich in CO2) and make the air ‘impure’.

6. Jean Senebier discovered that green plants utilize carbon dioxide. This is regarded as a very important

discovery in the field of photosynthesis.

7.N.T. de Saussure showed that during photosynthesis, the volume of carbon dioxide consumed by plants is equal to the volume of oxygen liberated by them. Saussure also proved that water is necessary for photosynthesis.

8. Pelletier and Caventou denoted the green colored substance in the leaves as ‘chlorophyll’.

9. F.F. Blackman noted that photosynthesis is carried out in two steps: a photochemical reaction which requires the presence of light and a reaction for which light is not necessary (light and dark reactions).

Blackman also put forth the law of limiting factor.

10. Warburg performed the Flashing experiment 1919.

11. Emerson and Arnold showed that the light reaction of photosynthesis has two distinct photochemical processes.

12. Robert Hill using Stellaria showed that isolated chloroplasts in the presence of sunlight, water and

a suitable hydrogen acceptor, release oxygen, even if carbon dioxide is absent. This experiment is considered to be equivalent to the light reaction. 

Hill used certain chemicals termed as ‘Hill’s reagents’.

They are redox dyes which change colors when reduced. The common electron acceptors are ferricyanide, benzoquinone and dichlorophenol indophenol (DCPlP), while NADP+ is a natural H+ acceptor in photosynthesis.

13. Van Niel carried out experiments using green and purple sulfur bacteria. 

He showed that hydrogen released from suitable oxidizable compounds reduces CO2 to carbohydrates and also put forth that water is the source of oxygen in photosynthesis.

6CO2 + 12H2S → C6H12O6 + 12S + 6H2O

14. Mayer stated that green plants convert solar energy into chemical (potential) energy in the form of

organic substances.

15. Ruben, Hassid, and Kamen carried out experiments using radioactive oxygen (O18) and proved that the source of oxygen in photosynthesis is water.

16. Julius von Sachs (1854) discovered that the green parts in plants is where glucose is synthesized and

that glucose is usually stored as starch.

17. T.W Engelmann worked using on Cladophora and Spirogyra. 

He noted that when light is split using a prism and used to illuminate the algae, the organisms aggregate in the blue and red regions.

18. Melvin Calvin along with his co-workers used radioactive carbon to study the various reactions involved

in the conversion of CO2 to carbohydrates. They elucidated a biochemical pathway called the C3 or Calvin’s cycle. 

Calvin won a Noble Prize in 1961.

19. MD Hatch and C.R Slack elucidated another biochemical pathway for CO2 fixation which is carried out in tropical plants. The first compound in the pathway is 4 carbon compound and hence the pathway is called the for C4 cycle.

20. Huber et. al. worked on understanding the 3-D structure of the reaction center of the bacteria

Rhodopseudomonas viridis. They also won a Nobel Prize.

In the presence of monochromatic light longer than 680 nm wavelength, the quantum yield of photosynthesis suddenly drops down, this phenomenon is called a red drop.

When non-monochromatic light of wavelengths shorter and greater than 680 nm (combined light) is provided the, photosynthetic, activity increases, this is called as

Emerson effect or enhancement effect.

Pigments Participating in Photosynthesis

There are three main types of photosynthetic pigments–Chlorophylls, Carotenoids, and Phycobilins.

Chlorophylls

Chlorophyll – Chloros in Greek means green while phyllon means leaf.

They are the photosynthetic pigments found in higher plants and many other photosynthetic organisms.

They are the main pigments concerned with harvesting solar energy.

They are specialized lipid molecules embedded in the thylakoid membrane of the chloroplasts.

Arnoff and Allen (1966) recognized 9 types of chlorophylls.

Some of them are (1) Chlorophyll-a; (2) Chlorophyll-b, (3) Chlorophyll-c etc.

Chlorophyll-a and chlorophyll-b are the two main types of chlorophylls found in plants.

Generally, light energy absorbed by other photosynthetic pigment is transferred to chlorophyll-a.

Chlorophyll-a:

Empirical formula of chlorophyll-a is C55H72O5N4Mg

Chlorophyll-a molecule has a porphyrin (a tetrapyrrole closed ring derivative) head and a phytol (C20H39OH) tail.

A vinyl group is present at the second carbon position in the tetrapyrrole ring.

A methyl group is present at the third carbon position of the tetrapyrrole ring.

An Mg atom in nonionic form is held within the head with two covalent and two coordinate bonds.

Chlorophyll-a absorbs violet-blue and red lights with absorption maxima at 430 nm and 662 nm.

Except for bacteria, it is found in all photosynthetic organisms.

Chlorophyll-b:

The empirical formula of chlorophyll-b is C55H70O6N4Mg.

It has a formyl (CHO) group at the third carbon position of the tetrapyrrole ring. Otherwise, it is similar to chlorophyll-a.

It absorbs blue and orange wavelengths with the absorption maxima at 430 nm and 644 nm.