LECTURE

PHOTOSYNTHESIS

Photosynthesis is the process by which plants, some bacteria and some protistans use the energy

from sunlight to produce sugar, which cellular respiration converts into ATP, the fuel used by all

living things. It is also the synthesis of carbohydrates by green organ of a plant in the presence of

sunlight from carbon-dioxide and water taken up from air and soil respectively, oxygen being a

byproduct.

The overall reaction for this process is:

6CO2 + 6H2O → C6H12O6 + O6

These reactions take place in the presence of light and chlorophyll

In photosynthesis, oxygen may or may not be evolve as a byproduct. If oxygen is evolved, it is

called oxygenic photosynthesis (distributed in all eukaryotes and cyanobacteria) and if oxygen is

not evolved, it is called anoxygenic photosynthesis (common in photosynthetic bacteria).

Importance of photosynthesis

- It is the monopoly of plants. This process purifies the air as well as supplies food to

entire universe (herbivores and carnivores) directly or indirectly.

- Simple carbohydrates produced in this process are further converted into lipids,

nucleic acids, proteins and other organic molecules.

- Photosynthesis provides raw material for respiration.

Plants are the only photosynthetic organisms to have leaves, although, not all plants have leaves.

A leaf may be viewed as a solar collector crammed full of photosynthetic cells. Photosynthesis

does take place in the green leaves of plants but it does so also in other green parts of the plants.

The mesophyll cells in the leaves, have a large number of chloroplasts. Usually the chloroplasts

align themselves along the walls of the mesophyll cells, such that they get the optimum quantity

of the incident light.

The raw materials of photosynthesis, water and carbon-dioxide, enter the cells of the leaf and the

products of photosynthesis, sugar and oxygen leave the leaf.

Raw materials for photosynthesis

- Photosynthetic pigments

- Light

- Carbon-dioxide

- H-donor

The nature of light

Light is a source of energy for entire universe, a visible part of broad electromagnetic radiations

emitted by sun. White light is separated into different colours of light by passing it through a

prism. The order of colours is determined by the wavelength of light. Light could behave as a

wave and also a particle. Light energy extends from 390 nm to 760 nm. All electromagnetic

radiations including light are transverse waves having different wavelengths. Light comprises of

seven colours i.e. wavelengths and each has a specific wavelength viz violet (390-430 nm),
indigo (430-460 nm), blue (460-500 nm), green (500-560 nm), yellow (560-600 nm), orange

(600-650 nm), red (650-760 nm). All the seven colours can be recalled using a single a word

‘VIBGYOR’, which consists of initial letter of all colours. Red light of above 700 nm is called

far red (700-760 nm), Electromagnetic radiation is emitted by matter in discontinuous units

called photons. Therefore, light is made up of photons. Energy of photon is called quantum.

Quantum energy is measured in units called Einsteins, each Einsteins unit contain 6.02 × 1023

photons.

The absorption of light at different wavelengths by its objects is called absorption spectrum

while the degree of physiological response caused by different wavelength of light is called

action spectrum.

The normal state of an atom or molecule is called ground state. When a photon of light collides

with the capturing molecule/chlorophyll molecule, an electron of the outer most orbit of

chlorophyll has increased vibrational energy which pushes an electron into high energy orbit

causing activation of chlorophyll or excitation of chlorophyll. This state is known as excited

singlet state.

Pigments

Pigments are substances that have the ability to absorb light, at specific wavelengths.

Photosynthetic pigments

These are coloured complex organic molecules in a biological system which absorb radiant

energy in the visible range of electromagnetic spectrum (light) and convert it into chemical form

of energy.
On the basis of solubility, pigments can be divided into two:

- Water soluble pigments

- Organic solvent soluble pigments

Water soluble pigments are water soluble and include phycobilins. Phycobilins are the

chromatophore moiety of the bili protein. They are protein linked pigments and are heat

sensitive. Phycobilins are chlorophyll like but they differ from the later in absence of phytol tail

and Mg2+ in open tetra pyrrole ring. Phycobilins are of three types:

Phycocyanin: is blue in colour and absorbs red and green light. It is distributed in blue-green

algae.

Phycoerythrin: is red in colour, absorbs blue and green light. It is distributed in red algae

Allophycocyanin: is also blue in colour, absorbs red and green light. It is distributed in blue-

green algae and red algae.

Organic solvent soluble pigments include different types of chlorophylls and carotenoids

(C40H56)

Chlorophyll and accessory pigments

A chromatographic separation of the leaf pigments shows that the colour that we see in leaves is

not due to a single pigment but due to four pigments: Chlorophyll a (bright or blue green in the

chromatogram), chlorophyll b (yellow green), xanthophylls (yellow) and carotenoids (yellow to

yellow-orange). The colour of these pigments comes from the wavelengths of light reflected.

Chlorophyll, the green pigment common to all photosynthetic cells absorb all wavelengths of

visible light except green, which it reflects to be detected by the eyes. Though chlorophyll is the
major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b,

xanthophylls and carotenoids, which are called accessory pigments, also absorb light and

transfer the energy to chlorophyll a. Indeed, they not only enable a wider range of wavelength of

incoming light to be utilized for photosynthesis but also protect chlorophyll a from photo-

oxidation.

The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of

light at generating electrons. Three things would occur if a pigment absorbs light energy:

- Energy will be dissipated as heat

- Energy may be emitted immediately as a longer wavelength in a phenomenon known

as fluorescence

- Energy may trigger a chemical reaction as in photosynthesis

Carotenoids are of two kinds: carotene and xanthophyll (carotenol).

Carotenes are orange in colour and represented by the empirical formula C 40H56. They are

unsaturated hydrocarbons made up of carbon and hydrogen only. The most abundant carotenes

are α and β carotene.

carotene. The common xanthophylls are leutin, zeaxanthin, neoxanthin, violaxanthin.

Quantasomes are photosynthetic pigment units. Three types of quantasome packing are found in

thylakoid membranes, they are:

- Random

- Linear

- Paracrystalline

Random is the most common out of the three. A quantasome has both photosystem I (PSI) and

(PSII). Each photosystem comprises of about 300 primary chlorophyll complexes called

antenna chlorophyll and reaction center. Antenna chlorophyll along with carotenoids are called

antenna molecules. Antenna molecules gather light and transfer it to reaction center by inductive

resonance transfer technique.

Reaction centres are made up of protein and chlorophyll molecules of special type. In the

reaction center, light energy is converted into chemical energy. Reaction centres are of following

three types:

i. P680

ii. P700

iii. P890

Photosystem II are located in grana stacks while PSI to stroma lamella and grana lamella.

PSII is inactive in far-red light. It carries out evolution of molecular oxygen (photo-oxidation of

water).
PSI is responsible for reduction of ferredoxin and NADP +. It is associated with cyclic

photophosphorylation. Both photosystems are active in red region of electromagnetic spectrum.

One is active in short-red (660-680 nm) while the other in far-red light (680-700 nm).

S/ PSI PSII

1 It is located on the outer surface of thylakoid It is located on the inner surface of the thylakoid

membrane towards stroma membrane

2 PSI is present in unstacked thylakoid It is predominantly present in stacked thylakoid

membrane and causes light induced reduction membrane

of NADP+.

3. Here molecular oxygen is not evolved Molecular oxygen is evolved and photolysis of

water takes place

4. It is involved in both cyclic and non-cyclic It is involved in non-cyclic photophosphorylation

photophosphorylation

5 Strong reductant is produced which reduces PSII donates electrons to PSI when NADP+.is

NADP+.to NADPH + H+ reduced

6 Reaction center is made up of P700, a special Reaction center is made up of P680, a special type

chlorophyll a molecule of chlorophyll a molecule

7 Chla/Chlb ratio is high (5) Chla/Chlb ratio is low (2-2.5)

The two pigment systems are connected by electron transport system, comprising PQ

(plastoquinone), PC (plastocyanin) and cytochrome b6/f complex. PQ is derivative of

benzoquinone, PC is copper containing protein, cytochromes are proteins coupled with iron

chelated porphyrin group.