Introduction
Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems. Photosynthesis is carried out by many different organisms, ranging from plants to bacteria. The best known form of photosynthesis is the one carried out by higher plants and algae, as well as by cyanobacteria and their relatives, which are responsible for a major part of photosynthesis in oceans. All these organisms convert CO2 (carbon dioxide) to organic material by reducing this gas to carbohydrates in a rather complex set of reactions. Electrons for this reduction reaction ultimately come from water, which is then converted to oxygen and protons. Energy for this process is provided by light, which is absorbed by pigments (primarily chlorophylls and carotenoids). Chlorophylls absorb blue and red light and carotenoids absorb blue-green light, but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.
Photosynthesis is the synthesis of organic molecules using the energy of light. For the sugar glucose (one of the most abundant products of photosynthesis) the equation is:
PHOTOSYNTHESIS - MILESTONES | |
Stephan Hales - (1727) | He is considered as the father of physiology. He recognized the importance of air and light in the nourishment of plants. |
Joseph Priestly (1772) | Found that vegetation purifies foul air (Bell jar experiments). He called C02 air as phlogiston and 02 as dephlogiston. |
Jan Ingenhousz (1779) | Found that only green parts of plants purify air in the presence of light. |
De Saussure- 1804) | Importance of water in photosynthesis. |
Engelmann (1888) | Plotted the action spectrum of photosynthesis |
Blackman (1905) | Suggested the possibility of a dark reaction along with the light controlled phase of photosynthesis. Proposed the law of limiting factors. |
Van Niel (1931) | Hypothesized all Photosynthetic organisms require a source of hydrogen and in plants this source is water and 02 is evolved by splitting of H2O |
Emerson and Arnold (1932) | Showed the existence of dark and light phase (flash light experiments). |
Robert Hill (1937) | Evolution of 02 by isolated chloroplast under illumination and presence of an electron acceptor (Hill reaction) due to photolysis of water. |
Calvin (1954) | traced the path of carbon in C02 fixation using 14C and Chlorella got Nobel Prize for the same in 1961. |
Emerson | showed the existence of two distinct photochemical processes (due to two photosystems) in light reaction, found red drop and also Emerson Enhancement effect |
Arnon | found Photophosphorylation |
Hatch and Slack (1965) | C4 pathway of C02 fixation in tropical grasses. |
The Chloroplast as site of photosynthesis
The chloroplast is the organelle of photosynthesis. In many ways, the chloroplast resembles the mitochondrion. ü Both are surrounded by a double membrane with an intermembrane space. ü Both have their own DNA. ü Both are involved in energy metabolism. ü Both have membrane reticulations filling their inner space to increase the surface area on which reactions with membrane-bound proteins can take place.
The chloroplast is the organelle of photosynthesis. In many ways, the chloroplast resembles the mitochondrion.
ü Both are surrounded by a double membrane with an intermembrane space.
ü Both have their own DNA.
ü Both are involved in energy metabolism.
ü Both have membrane reticulations filling their inner space to increase the surface area on which reactions with membrane-bound proteins can take place.
Structure of chloroplast
Photosynthetic pigments:.
Chlorophylls: Empirical formulae of chlorophylls are: Carotenoids:
Red Drop:
Mechanism of photosynthesis:
Structure of chloroplast
Chloroplast is a double membraned organelle. The membranes enclose a space filled with proteinaceous fluid called stroma. Stroma contains DNA, ribosomes and enzymes for dark reaction. Membrane bound sac like structures embedded in the stroma forms the thylakoids or lamellae. At certain places thylakoids get arranged in stacks or groups called grana (sing: granum). Adjacent grana are interconnected by thylakoids called frets or stromal thylakoids or intergranal thylakoids. Photosynthetic pigments are located on the thylakoid membrane, in the lipid part. Light phase of photosynthesis takes place in the thylakoid membrane of granum. Pigments required to bring about a photochemical reaction forms a Photosynthetic unit.
Photosynthetic pigments:.
Photosynthetic pigments are of chlorophylls, carotenoids and phycobilins. All these pigments are organised into two distinct light harvesting complexes (LHC) in pigment system I (PSI) and pigment system II (PS II). LHC are with pigments and proteins showing a light harvesting antennae and the reaction centre. The reaction centres are of P7oo(PS I) and P680 (PS II).
Chlorophylls:
Many types of chlorophyll are known and these are represented as Chl.a, Chl.b, Chl.c, and Chl.d. Bacteriochlorophyll and bacterioviridin are seen in bacteria. Higher plants mainly have Chl.a and Chl.b. Chlorophyll a is the primary Photosynthetic pigment or universal Photosynthetic pigment found in all oxygenic photoautotrophs. It is absent in anoxygenic photoautotrophic bacteria. Chl.b is present along with Chl.a in plants.
Structure of chlorophyll pigment:
It consists of a Mg - porphyrin head which is hydrophilic and a phytol tail (20 carbon) which is lipophilic. Porphyrin consists of nitrogen containg 4 pyrrole rings which are attached to the central Mg. These rings have side groups attached to it. Chl.a and Chl.b differ in the side group attached to the 2nd pyrrole ring at 3rd carbon. Chlorophyll a has methyl (-CH3) group while b has aldehyde (CHO) group in one of the pyrrole ring.
Empirical formulae of chlorophylls are:
Chl.a - C55H72O5N4Mg and Chl.b - C55H70O6N4Mg
Carotenoids:
These are carotenes and carotenoids seen around the chlorophylls.
Absorbs blue-violet region of light energy for photosynthesis.
Function as accessory pigments transferring light energy to Chlorophyll a and also protects chlorophyll from photooxidation at high light intensities.
ü It is of two types - carotene and xanthophylls, the former is orange and the later as yellow on isolation.
Carotene (C40H56)
Carotene is orange in colour.
This was extracted from carrot for the first time, hence the name.
β- carotene is the most common carotene.
Lycopene is a carotene found in tomatoes and red pepper
Present in all higher plants and some lower forms. Xanthophyll (C40H56O2) (carotenol)
Yellow coloured carotenoid pigments are called Xanthophyll.
It is the oxidized form of carotene.
This pigment is more abundant than carotenes.
Leutin, a common xanthophylls give yellow colour to leaves during autumn.
Absorption and Action Spectra:
A graphic representation of the rate of absorption of different wave lengths by the pigment gives the absorption spectrum of that pigment. Chl. a and b shows good rate of absorption at blue and red regions. A graphic representation of rate of action (photosynthesis) against the different wavelengths gives action spectrum of photosynthesis. Maximum rate of photosynthesis occurs at red wavelength along with blue wave length.
Red Drop:
Action spectrum (effect of different wave lengths of light on quantum yield) of photosynthesis shows that quantum yield is slightly dropped in the region 440 - 520 nm, remains constant, and maximum in region 600 - 680 nm but there is sudden fall or drop in quantum yield above 680 nm (far red light) is called red drop. This phenomenon was discovered by Emerson and Lewis (1943).
Emerson's Enhancement Effect:
Emerson et al. (1957) discovered that far red light (above 680 nm), which is photosynthetically
inefficient can be made efficient by supplementing it with a beam of shorter wave length (red
beam below 680 nm). Further the quantum yield in combined beam (far red + red) is more than the sum total of quantum yields in two separate beams. This enhancement in quantum yield by supplementing far red light with red light is called Emerson Enhancement Effect. Mechanism of photosynthesis:
Photosynthesis occurs in two steps as Light phase occurs in sunlight and dark phase follows this immediately.
Light Reaction:
During light phase the LHC capture solar energy and convert it into the assimilatory powers ATP and NADPH. In a LHC only a particular chlorophyll molecule of 'a' called P7o0 or P680 can emit electron on getting excited and other pigments transfer the absorbed energy to this molecule called photocentre or reaction center and all other pigments act as accessory pigments only. So the Photosynthetic unit consists of two parts reaction centre (Single chlorophyll a molecule) and light harvesting molecules (accessory pigments). There are two pigments or photosystems depending on the reaction center. They are PSI with P7o0 as the photo center and PSII withP680 as the photo center. When the ejected electron with high potential energy moves down the electron transport chain, ATP is formed by a process called Photophosphorylation.
