Photosynthesis and the role of sunlight in photosynthesis

 Photosynthesis

organic food from carbon dioxide and water using energy of sunlight. Co,and water are used as raw materials in the process for synthesis of organic food molecules. Chlorophylls and other photosynthetic pigments capture energy of sunlight and convert it into chemical energy. Photosynthesis acts as energy capturing and storing process. Energy of sunlight is used in the fixation of carbon dioxide to a carbohydrate. This serves as food not only for plants but for the entire life on the planet earth. Therefore all living organisms, directly or indirectly depends on photosynthesis. Autotrophic organisms which are the green plants are able to carry out photosynthesis. Heterotrophic organisms cannot carry out photosynthesis and are unable to use direct energy of sunlight. They, therefore, are dependent for their energy requirement on green plants.

CH,+60 +6HO Overall reaction of photosynthesis is:

(Light) 6CO, +12HQ (chlorophyll)

Photosynthetic Reactants and Products

The water and carbon dioxide are the reactants in photosynthesis while glucose and oxygen are the products.

The Role of Sunlight in Photosynthesis:

Sun is the main source of energy for all living organisms. Light is a kind of energy that travels in the form of electromagnetic waves of different wavelengths. It also acts as beam of particles of different frequencies called photons. There is a wide range of waves for synthesis of organic food

molecules (wavelengths occurring between gamma rays and radio rays). Energy content of photons is inversely proportional to the wavelengths. Short wavelengths are more energetic i.e. have high energy content than

long wave lengths. A portion of the solar radiation is called visible spectrum. Our eyes are sensitive to only a small portion of this solar radiation i.e. visible light that ranges from about 390nm to 760 nm in wavelength. Photosynthetic pigments absorb and utilize a portion of the visible spectrum. Wavelengths shorter than the visible light i.e. ultraviolet

radiation are more energetic and are dangerous to the cells because they can

break organic molecules. Wavelengths longer than visible light i.e. infrared

have low energy content that cannot affect photosynthetic process. Wavelengths of the visible spectrum have the right amount of energy absorbed by photosynthetic pigments for photosynthesis. About forty percent of the total of sunlight that enters our

atmosphere reaches the earth surface. Most of this radiation is within the visible light range. Dangerous higher energy wavelengths are screened out by the ozone layer and upper layers of the atmosphere. Lower energy wavelengths are mostly absorbed or reflected by water vapour 

and gases and are scattered in the atmosphere. Of the total sunlight that strikes the green plants only about a fractionis used in the photosynthesis. This small portion of sunlight sustains all forms of life on earth.There are two types of photosynthetic pigments i.e. chlorophylls and carotenoids. Chlorophylls absorb mostly violet-blue wavelengths (390-430 nm) and red wavelengths (670-700nm). Green wavelengths are mostly reflected therefore chlorophyll appears green. The carotenoids which are called accessory pigments absorb light in the visible spectrum ranging between 500nm and 600 nm in wavelengths.

 Photosynthetic Pigments

Photosynthetic pigments absorb different wavelengths of solar radiation. There are two types of photosynthetic pigments involved in photosynthesis . These are chlorophylls and carotenoids. In eukaryotes

photosynthetic pigments are located in the chloroplasts. A chloroplast consists of three components: An outer most covering (envelop), grana chloroplast is formed by a double membrane structure that encloses the(singular granum) and stroma. The outer most covering (envelop) of the grana and stroma. A granum consists of many flattened fluid-filled membranous sacs or discs called thylakoids which form stacks and resemble a pile of coins. There are many grana which are interconnected by lamellae called intergrana. The grana are visible under the light microscope as grains. Chlorophyll and other photosynthetic pigments (carotenoids) are present within the membranes of the thylakoids. These membranes are the sites of light trapping reaction (light reaction) of photosynthesis,The double membranes envelop of the chloroplast surrounds a large central space called stroma. The stroma contains enzyme (proteins) rich gel-like solution called matrix where light independent reaction (dark reaction) of photosynthesis takes place.

a. Chlorophylls

Chlorophyll is a complex organic compound. It absorbs mainly blue and red portion of sunlight. The green portion is mainly reflected therefore chlorophyll appears green. There are many types of chlorophyll i.e.

Chlorophyll a, b, c, d, e and bacteriochlorophyll

Chlorophyll "a" is the most abundant and most important photosynthetic pigment. It is found in all green plants except bacteria. It

exists in several forms depending on its arrangement in the membrane. Chlorophyll "b" is found in all higher plants and green algae. Chlorophyll "c", d and "e" are found in various groups of algae. Bacterio-chlorophyll is found in bacteria. Chlorophyll molecule is composed of two parts, i.e. head and tail.

The head contains a central magnesium atom to which are attached four N- rings called Pyrrole rings. The four rings (tetra Pyrrole ring) are collectively called porphyrin. The head is hydrophilic and lies on the surface of the thylakoid membrane.Long hydrocarbon chain called phytol side chain (tail) is attached to one of the Pyrrole rings. It is hydrophobic. It lies

embedded in the thylakoid membrane.

Chlorophyll "a" and "b" differ from each other in only one of the functional groups bonded to the porphyrin. Chlorophyll "a" has methy group (-CH) while Chlorophyll "b" ha

carbonyl group (-CHO).The empirical

formulae of chlorophyll "a" and "b" are

Chlorophyll "a" ( C55H72O5N4Mg)

Chlorophyll "b" ( CssH700N4Mg)

b. Carotenoids

They include carotenes and xanthophylls. They are yellow, orange, red or brown pigments. Carotenoids play two important roles in plants. They absorb light and transfer light energy to chlorophyll "a". Therefore, they are called accessory pigments. Carotenoids protect chlorophyll from intense light and from oxidation by oxygen produced in photosynthesis.

a.Absorption Spectra of Chlorophylls and Carotenoids

Absorption spectrum is the amount of light absorbed at different wavelengths from the visible spectrum of light. Photosynthetic pigments absorb light only in the visible part of light spectrum. The most important

Chiefly absorb light in violet blue (400 nm - 460 nm) and red parts (630nm

660 nm) pigments in photosynthesis are chlorophyll "a" and chlorophyll "b". They chiefly absorb light in violet blue (400nm-460nm) and red parts (630nm-660nm)of the spectrum. The absorption spectra of both of these chlorophylls are somewhat different from each other. This clear from the different peaks..

The carotenoids absorb light between 430-470 nm of light spectrum and transfer it to chlorophyll 'a' molecule. To measure the absorption of a pigment, a pure solution of the extracted pigment is obtained. It is then exposed to different wavelengths of light inside spectrophotometer. It is an instrument that measures the amount of light that passes through the solution. The amount of light can be calculated from the amount projected on the solution and the amount of light received at the other end after passing through the spectrophotometer. This gives the measurement of the absorption spectrum of a particular pigment. Chlorophyll "a" and "b".  different absorption spectra as shown above.

 b. Action Spectrum

Action spectrum is a measure of effectiveness of light of various
wavelengths in driving photosynthesis. Whole amount of energy absorbed
by the pigments is not stored
organic compounds. Some of it is released
as heat and the rest is stored in organic compounds as chemical energy.
Action spectrum of a particular pigment can be calculated by
measuring the rate of photosynthesis at each type of wavelength of light. As
photosynthesis produces oxygen, the rate of production of oxygen can be
used as a measure of the rate of photosynthesis. This gives an action
spectrum of photosynthetic pigments for different wavelengths. Red and blue turn out to be the moste ffective wavelengths inp hotosynthesis. The action Spectrum is somewhat different from absorption spectrum of chlorophyll

Role of co, as one of the raw materials of photosynthesis

The carbon of CO is fixed in organic compounds in photosynthesis. Carbon is most important component of organic compounds. Carbon- carbon chain forms the back bone of the long hydro-carbon molecule and makes a stable and symmetric compound. Carbon makes bonds with

oxygen linking the monosaccharide of carbohydrates. It makes bonds with

nitrogen linking amino acids of protein molecules, Carbon dioxide is used as one of the raw material for photosynthesis. In the absence of carbon dioxide the process of photosynthesis does not occur.

 Role of water in photosynthesis

Water is one of the raw materials used in photosynthesis. Water molecule is broken down into hydrogen and oxygen. Hydrogen combines with carbon dioxide forming organic food molecule. Oxygen is released

into the air and is the source of atmospheric oxygen. Earlier it was thought that the oxygen released in the process of

photosynthesis comes from CO. In 1930. Van Neil hypothesized that plants split water to release oxygen as a by-product. The idea of Neil was supported by Hill. In 1937 he observed that when isolated chloroplasts were given light in complete absence of CO, and some hydrogen acceptor was present oxygen is released. Other scientists later confirmed Neil's hypothesis when first use of an isotopic tracer (O") in biological research

was made. Water and carbon dioxide containing heavy-oxygen isotope O were prepared in the laboratory. Experimental green plants in one group were supplied with HO containing and with co, containing only common oxygen O Plants in

the second group were supplied with water containing common oxygen 0 but with Cocontaining o". It was found that plants of first group produced but the plants of second group did not.

 Mechanism of Photosynthesis

The Mechanism of photosynthesis consists of two distinct steps, on

that requires light is called light reaction and the other that does not require

light called dark reaction.

 a. Light Reaction (Light dependent reaction)

Light reaction takes place in the granum of chloroplast. It is initiated when photosynthetic pigments capture light energy. Photosynthetic pigments are organized into clusters called photosystems. There are two photosystems i.e. photosystem I (PSI) and photosystem II (PS IT). Each

photosystem consists of several hundred pigment molecules including chlorophyll a, chlorophyll b, carotenoids and electron acceptors. There are two parts of each photosystems i.e. antenna complex and

reaction center. The antenna complex has many molecules of chlorophyll b and carotenoids, all absorb energy and transfer it to the reaction center: Reaction center has one or more molecules of chlorophyll a molecules along with primary electron acceptor and electron carriers. Photosystem I absorbs light of 700 nm and is called P700 whereas photosystem II absorbs light of 680 nmand is called P680. The primary 

electron accepter traps the electrons from the reaction center and the passes them on to the series of electron carriers. There are two possible paths ways of electrons in the light reaction of photosynthesis. They are called non-cyclic electron transport and cyclic electron transport. 

Non cyclic electron transport of light reaction. 

When sunlight strikes the photosystem II (P 680 ) absorbed by the chlorophyll molecules. The activated chlorophyll losses its two electrons and the positively charged chlorophyll molecule is left in the photosystem with a gap of two electrons. The high energy electrons instead of falling back into the photosystem are captured by an electron acceptor called Plastoquinone (PQ). From plastoquinone the electrons pass along a series of electrons acceptor molecules from one to another in oxidation reduction process. These electron acceptors include cytochrome 'b', cytochrome 'f and plastocyanin. This transfer of electrons constitutes electron transport chain. Each molecule in the electron transport chain is alternately reduced when it gains electron and is oxidized when it losses electrons. When electrons are passed through electron transport chain, they lose energy. Some of the energy lost by electrons between cytochrome 'b' and cytochrome 'f is used in making ATP from ADP and inorganic

phosphate. The process of formation of ATP from ADP and inorganic phosphate using energy from sunlight is called photophosphorylation called photosystem I (P700). At the same time light falls on photosystem I. The electrons from plastocyanin are received by another photosystem and activates its two electrons. Activatedelectrons are received by

Ferredoxin reducing substance (FRS); electron accepter of PSI. From FRS electrons are passed to oxidized NADP (Nicotinamide

adenine dinucleotide phosphate). The reduced NADP receives hydrogen from water and is converted into NADPH

When photosystem II absorbs light, water molecule splits into OH and H. The OH ions react to form some water again and release oxygen and electrons 40 4HO2e 4 он) IHO -

Electrons from water molecules are accepted by positively charged chlorophyll molecule of Photosystem II, filling the gap produced by the two energized electrons. The electron deficiency of photosystem has been filled by electrons coming from photosystem IL. This transport of electrons is called Non eyelie electron transport because electrons don not move in a cycle. It involves both the photosystems and follows rigzag path.

In non-cyclic electron transport system electrons do not move in a cycle. Both the photosystems are involved and ATP and NADPH are produced which are used in dark reaction. Therefore it is also called Z

Scheme (Zigzag Scheme) of light reaction. The ATP synthesis during this non cyclic electron flow is called Non-cyclic Photophosphorylation

Cycle electron transport

In contrast to non-cyclic electron transport, the cyclic electron transport involves only photosystem. It occurs in mare conditions if the activity of photosystem It is blocked. When P700 form of chlorophyll molecule in photosystem I absorbs light, it is activated

This activated chlorophyll loses electrons, which are captured by ferrodoxin

reducing substance (FRS). From FRS the electrons fall back to P 700 chlorophyll molecule through a series of electron carriers. ATP molecules are produced during cyclic electron flow. The electrons which are ejected from P 700 molecules are cycled back in the above electron transport therefore the process is called cyclic electron transport. ATP synthesis during this cyclic electron flow is called cyclic photophosphorylation.

Water and energy of sunlight are used in light reaction. The products of light reactions are ATP and NADPH. Both of these are transported from grana to stroma for use in dark reaction.