PHOTOSYNTHESIS - National Institute of Open Schooling

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Photosynthesis

MODULE - 2

Forms and Function of Plants and Animals

11 Notes

PHOTOSYNTHESIS

Photosynthesis (Photo = light; synthesis = to join) is the single most important process on earth on which depends the existence of human and almost all other living organisms. It is a process by which green plants, algae and chlorophyll containing bacteria utilize the energy of sunlight to synthesize their own food (organic matters) from simple inorganic molecules. Innumerable number of organic molecules which compose the living world are derived directly or indirectly from the photosynthetic organic matter. The oxidation of organic compounds releases stored energy to be utilized by the organism to carry out essential metabolic processes. It is important to note that photosynthesis is the only natural process which liberates oxygen to be used by all living forms for the process of respiration.

You have studied in lesson 4, that chloroplasts are the organelles that carry out photosynthesis or in other words they act as solar cells producing carbohydrates. In this lesson you will learn how plants carry out photosynthesis.

OBJECTIVES

After completing this lesson, you will be able to : l define photosynthesis; l name the different pigments found in chloroplasts and describe the ultra

structure of chloroplasts with a diagram; l explain the main aspects of the process of photosynthesis; l enumerate the steps involved in the light and dark reactions of photosynthesis; l define the terms absorption spectrum, electron acceptor, photophosphorylation

and action spectrum; l distinguish between, absorption spectrum and action spectrum; light and dark

reactions, cyclic and non-cyclic phosphorylation, C3 and C4 photosynthesis; l list the environmental variables and internal factors affecting photosynthesis; l describe the principle of limiting factor giving suitable graphs.

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11.1 PHOTOSYNTHESIS

Photosynthesis

11.1 Let us look into the significance of the process

Notes

Significance 1. Green plants possess the green pigment, chlorophyll which can capture,

transform, translocate and store energy which is readily available for all forms of life on this planet.

2. Photosynthesis is a process in which light energy is converted into chemical energy.

3. Except green plants no other organism can directly utilise solar energy, hence they are dependent on green pants for their survival.

4. Green plants can prepare organic food from simple inorganic elements (autotrophic) while all other organisms cannot prepare their own food and are called heterotrophic.

5. During photosynthesis, oxygen liberated into the atmosphere makes the environment livable for all other organisms.

6. Simple carbohydrates produced in photosynthesis are transformed into lipids, proteins, nucleic acids and other organic molecules.

7. Plants and plant products are the major food sources of almost all other organisms of the earth.

8. Fossil fuels like coal, gas, oil etc represent the photosynthetic products of the plants belonging to early geological periods.

11.1.1 What is photosynthesis?

Photosynthesis (photo-light; synthesis-to put together) is the process by which green plants, in the presence of light combine water and carbon dioxide to form carbohydrates, oxygen is released as a by product. Current knowledge of photosynthesis has resulted from discoveries made over 300 years of work. Some landmark experiments are given in the box below.

l Joseph Priestly and later Jan Ingenhousz showed that plants have the ability to take up CO2 from the atmosphere and release O2.

l Ingenhousz also discovered that release of O2 by plants was possible only in presence of sunlight and by the green parts of the plant.

l Robert Hill demonstrated that isolated chloroplasts evolve O2 when they are illuminated in the presence of electron acceptor which get reduced. This reaction called Hill reaction accounts for the (break down) use of water (photolysis) as a source of electrons for CO2 fixation and release of O2 as by-product.

Photosynthesis is represented by the following overall chemical equation:

6CO2 + 12H2O

Chlorophyll

Sunlight

C6H12O6 + 6H2O + 6O2

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Photosynthesis

In photosynthesis CO2 is fixed (or reduced) to carbohydrates (glucose C6H12O6). Water is split in the presence of light (called photolysis of water) to release O2. Note that O2 released comes from the water molecule and not from CO2.

MODULE - 2

Forms and Function of Plants and Animals

11.1.2 Where does photosynthesis occur?

Photosynthesis occurs in green parts of the plant, mostly the leaves, sometimes the green stems and floral buds. The leaves contain specialised cells called mesophyll cells which contain the chloroplast the pigment containing organelle. These are the actual sites for photosynthesis.

Notes

Look at the figure 11.1 to recollect what you learnt about the structure of chloroplast in the lesson-4 Cell Structure and Function.

Fig. 11.1 Structure of chloroplast

11.2 PHOTOSYNTHETIC PIGMENTS The thylakoids of the chloroplast contain the pigments which absorb light of different wave length and carry out the photochemical reaction of photosynthesis.

The role of the pigments is to absorb light energy, thereby converting it to chemical energy. These pigments are located on the thylakoid membranes and the chloroplasts are usually so arranged within the cells that the membranes are at right angles to the light source for maximum absorption. The photosynthetic pigments of higher plants fall into two classes the chlorophyll and carotenoids.

The photosynthetic pigment chlorophyll is the principle pigment involved in photosynthesis. It is a large molecule and absorbs light in the violet blue and in the red region of the visible spectrum and reflects green light and thus leaves appear green in colour. Carotenoids (carotene and xanthophyll) absorb light in the regions of the spectrum not absorbed by the chlorophyll.

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Notes

Photosynthesis

Chlorophyll a (a special type of chlorophyll) is the main pigment that traps solar energy and converts it into electrical and chemical energy. Thus it is called the reaction centre.

All other pigment such as chlorophyll b and carotenoids are called accessory pigments since they pass on the absorbed energy to chlorophyll a (Chl.a) molecule. These pigments, that is the reaction centres (Chl. a) and the accessory pigments (harvesting centre) are packed into functional clusters called photosystems. Photosystems are of two types PSI and PSII.

About 250-400 pigment molecules constitute a single photosystem. Two different photosystems contain different forms of chlorophyll a in their reaction centres. In photosystem I (PSI), chlorophyll a with maximum absorption at 700 nm (P700) and in photosystem II (PSII), chlorophyll a with peak absorption at 680 nm (P680), act as reaction centres. (P stands for pigment). The primary function of the two photosystems, which interact with each other is to trap the solar energy and convert into the chemical energy (ATP). The differences between them are given in the following table 11.1.

Table 11.1 Difference between Photosystem I and Photosystem II

Photosystem I

l PS I has a reaction centre of chlorophyll `a' molecule with maximum light absorption at 700 nm wavelength. This reaction centre is also referred to as P700.

l Primary electron acceptor is an iron protein (Fe-Sprotein)

l A set of electron carriers the plastocyanin, ferredoxin and cytochrome

Photosystem II

l PS II has a reaction centre of chlorophyll `a' molecule with maximum light absorption at 680 nm. This reaction centre is also referred to to as P680.

l Primary electron acceptor in a colourless less chlorophyll a that lacks magnesium (Mg) and is known as phaeophyll a.

l A set of electron carriers the phaeophytin plastoquinone, cytochromes.

11.3 ROLE OF SUNLIGHT IN PHOTOSYNTHESIS

Light consists of small particles or packages of energy called "photons". A single photon is also called quantum. What does the chlorophyll do? It absorbs light energy.

Chlorophyll molecules absorb light energy and get into an excited state and lose an electron to the outer orbit. No substance can remain in an excited state for long, so the energised and excited chlorophyll molecule comes down to a low energy state known as ground state and release the extra amount of energy. This energy can be lost as heat, or as light (flourescence) or can do some work. If photosynthesis, it works by splitting water moelcule to produce H+ and OH? ions.

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Photosynthesis

Carotene is orange-yellow pigment present along with chlorophylls in the thylakoid membrane. A carotene molecule breaks down into the vitamin A molecules. It is this pigment which gives carrot its colour.

MODULE - 2

Forms and Function of Plants and Animals

Absorption and Action Spectra

For investigating a process such as photosynthesis that is activated by light, it is important to establish the action spectrum for the process and to use this to identify the pigments involved. An action spectrum is a graph showing the effectiveness of different wavelengths (VIBGYOR) of light in stimulating the process of photosynthesis, where the response could be measured in terms of oxygen produced at different wavelengths of light. An absorption spectrum is a graph of the relative absorbance of different wavelengths of light by a pigment. An action spectrum for photosynthesis is shown in Fig. 11.2 together with an absorption spectrum for the combined photosynthetic pigments. Note the close similarity, which indicates that the pigments, chlorophyll in particular, are responsible for absorption of light in photosynthesis.

All wavelengths of light are not equally effective in photosynthesis i.e. the rate of photosynthesis is more in some and less in others.

Notes

Fig. 11.2 Spectra of electromagnetic radiation B. Action Spectrum

Photosynthesis is very little in green and yellow light, because these rays are reflected back from the leaf. Photosynthesis is maximum in blue and red light.

INTEXT QUESTIONS 11.1

1. (i) Define photosynthesis ..................................................................................................................

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