Photosynthesis :
(i) It is an anabolic process.
(ii) Photosynthesis is a constructive process which forms new cell wall substances.
(iii) Dry weight of plant increases.
(iv) Energy is stored in the form of potential energy in glucose molecules. (1 mol of glucose C6H12O6 contains 686 kcal of energy).
(v) Water, CO2 etc. produce glucose which is a complex material.
Chloroplast
Chloroplast is a double membrane organelle. The membrane is unit membrane and is approximately 50Å—70Å thick. It contains proteinaceous liquid matrix which is called stroma. Thylakoids are membrane bond sac like structures embedded within the stroma. Thylakoids are arranged in stacks constitute the grana which are further interconnected by membrane bound channel like structrues called storma lamellae. Inner surface of the thylakoid bears distinct morphological structures called quantasomes. These quantasomes have many chlorophyll molecules and other pigments which are capable of capturing and converting the solar energy into chemical energy, thus acting as a photosynthetic unit. Light reaction occurs within the grana, while dark reaction occurs within the ground substance stroma of the chloroplast.
Photosynthetic Unit Quantasomes
The ‘photosynthetic units’ or ‘quantasomes’ which represents an aggregation of several chlorophyll molecules i.e. the main photosynthetic pigments along with accessory pigments, like carotenoid (carotene, xanthophyll) and biliproteins or phycobillins phycocyanin, phycoerythin) capable of capturing and converting solar energy into chemical energy in the form of ATP, during the light phase.
The main site of photosynthesis are the cells of mesophyll tissue but any green part of the plant body can produce photosynthesis. So to some extent photosynthesis occurs in the green part of a growing stem e.g., Opuntia dilenii (B. Phanimanasha), Photosynthesis also takes place in the thalamus of flower which is a modified stem, in the roots, in the sepals, in the roots of orchids due to the presence of a spongy tissue termed velamen. It also takes place in unicellular organisms (animals) like Euglena, Chrysamoeba, etc.
Photosynthesis in a plant without chlorophyll cannot undergo the process of phytosynthesis. Some bacteria and fungi are the examples.
Bacterial Photosynthesis : This is the photo synthesis performed by the photo synthetic bacteria using light as a source of energy and a chemical compound as a hydrogen donor. Carbohydrate is the food produced through this process.
Some Photosynthetic Bacteria
1. Green sulphur bacteria—This bacteria contains photo-synthetic pigment. Clorobium Chlorobacterium.
2. Purple sulphur bacteria—This bacteria contains photosynthetic pigment. Chromatium.
3. Purple non-sulphur bacteria—Photosynthetic pigment is contained by this bacteria. Rhodospirillum.
Photosynthetic reaction shown by green sulphur bacteria (Chlorobium):
6CO2 + 12H2S
(Hydrogen sulphide)
C6H12 + 12S + 6H2O + energy.
(glucose)
Organelle of phtosynthesis—Chloroplastids. It is double membrane bound organelle present in the plant cell cytoplasm of plant.
Adaptation of leaves for Photosynthesis
1. Leaves have large surface area and are flat so sufficient light is gathered.
2. The midrib and veins of leaves are surrounded by tissues. These impart strength to the leaf and maintian the original shape.
3. A thick cuticle is present on the upper surface and a thin cuticle on the lower surface so that excessive water loss is not there from the large surface area of the leaves.
4. It has stomatal openings through which exchange of gases takes place with the atmosphere.
5. A network of veins supply water to the photosynthetic cells and helps in quick disposal of photosynthetic product.
6. Easy diffusion of gases throughout the whole leaf is allowed by the intercellular air spaces.
7. Maximum number of organelle of photosynthesis are present i.e. chloro-plastid in the mesophyll tissue of leaf.
Carbon dioxide : It goes through the stomata of the leaves. Later it enters the mesophyll tissue of leaves. CO2 also enters a plant body by means of lenticle and cuticle.
In submerged sea plants like Hydrilla, carbondioxide enters the plant body by means of diffusion through the plant surface.
Atmosphere contains about 0.03 to 0.04 percent of CO2. Water is present in the mesophyll tissue of leaf. This water along with carbon dioxide form carbonic acid (CO2 + H2O = H2CO3). In presence of sunlight the carbonic acid again forms CO2 and H2O. This CO2 is used up in photosynthetic mechanism.
Water : Generally plant body absorbs water by the process of osmosis from the soil through the unicellular root hairs.
In photosynthesis, about 1% of the total amount of water is required by the plants. The solar energy is changed into chemical energy by the chlorophyll molecules. The electrons of chlorophyll molecules gets excited by the solar energy and water molecule breaks up into H+ ion and OH– ion by means of light activated chlorophyll. From OH– ions oxygen is formed, later on which is also liberated out. The method of splitting up of water by means of light activated chlorophyll is called photolysis.
H+ ion take up by NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) is converted to NADPH.
Chlorophyll
In the cells of mesophyll tissue of leaves, Chlorophyll is present. Photosyn-thesis occurs within the chloroplasts.
Light : Energy which is needed to carryout the mechanism of photosynthesis by plants is obtained from the sun light.
(i) Cyclic photophosphorylation : It takes place independent of the photolysis mechanism of water and is not accompanied by evolution of oxygen.
(ii) Non-cyclic Photophosphorylation : It is linked to both the formation of reduced pyridine nuclotide and photolysis of water.
NADP+ + ADP + Pi + H2O
NADPH + ATP + ½O2 + H+
Photolysis of water in the chloroplast is shown by the equation given above.
Full names of NADP, ADP, ATP, RuDP PGA, PGAId
NADP = Nicotinamide Adenine Dinucleotide Phosphate.
ADP = Adenosine diphosphate.
ATP = Adenosine triphosphate.
RuDP = Ribulose 1, 5, diphosphate.
PGA = Phosphoglyceric acid.
PGAId = Phosphoglyceraldehyde.
(i) The water, absorbed by the unicellular root hairs, is transported to the mesophyll cells of leaves. Water from these cells diffuses into the cell organelles called chloroplasts.
(ii) The chlorophyll particles present within the chloroplasts are activated by absorbing solar energy of the highly energised invisible photons, as sunlight pass through the leaf surface.
(iii) With the light activated excreted chlorophyll molecules, when water comes in contact it splits up into H+ and OH– ions. This phenomenon of splitting up of H2O molecule into H+ ion and OH– ion by means of light activated chlorophyll is called photolysis of water.
(iv) The extra energy produced by the charged chlorophyll molecules is transferred as chemical energy to another organic compound ADP (Adenosine diphosphate) to perform the dark phase reactions. This step is called photophos-phorylation.
ADP + Pi ® ATP
(v) The H+ ion formed due to photolysis of water, is taken up by an hydrogen acceptor molecule NADP (Nicotinamide Adenine Dinucleotide Phosphate), which is then reduced to NADPH a compound rich in energy.
(vi) The NADPH is next used up in the dark reaction for the reduction of carbon dioxide and finally to glucose.
(vii) The OH– ion formed during photolysis of water is converted to OH radical. It next gives rise to H2O (water) and O2 (oxygen). This oxygen (O2) is liberated in the environment for living beings and the water (H2O) formed is utilized by the cell.
Assimilatory Power of Dark Reaction
Since no light energy is required in the dark reaction, here the energy for the performance of the several steps of reactions initiated by enzymes, comes from the two energy rich compounds produced during light reaction, i.e., ATP—Adenosine tri-phosphate and NADPH2—Reduced nicotinamide adenine dinucleotide phosphate.
The total light reaction is—
4H2O + ADP + Pi + 2NADP ® 2H2O + O2 + ATP + 2NADPH + 2H+ sunlight chlorophyll
Site of dark phase : Dark reaction takes place at Stromta of chloroplastid.
Dark Reaction
It is also called Calvin cycle.
Dark phase reaction is also called as Calvin cycle, after the name of the scientist Melvin Calvin, who investigated the sequence of dark reaction and stated that RuDP the CO2 acceptor is regenerated in this cycle. He was assisted by the co-workers A.A. Benson and J. Bantham.
(i) The process of diffusion makes the carbon dioxide gas enter into the mesophyll cells of the leaves through the stomata and finally diffuses into the chloroplast.
(ii) Carbon dioxide gas is picked up by RuDP (Ribulose 1, 5, diphosphate—a 5—carbon compound) an acceptor molecule to form an unstable six carbon compound (6C), involving carboxylation. This compound breaks down into 2 molecules of Phosphoglyceric acid (PGA). It is a three carbon (3C) compound, and is the first product of photosynthesis.
(iii) Phosphoglyceric acid (PGA) reacts with ATP to form 1-3, diphosphoglyceric acid which in the next step is reduced by NADPH2 to form 3—PGA (3 phosphoglyceraldehyde) by taking hydrogen (H).
(iv) Through different intermediate products in the Calvin cycle 3PGA finally reduced to glucose (C6H12O6). This reduction of atmospheric CO2 to glucose is also called Blackman’s reaction.
The overal dark reaction is—
CO2 + RuDP + NADPH + ATP ® Glucose + NADP + ADP + Pi (carbohydrate)
The Overall reaction for photosyn-thesis is—
6CO2 + 12H2OC6H12O6+
6O2 + 6H2O
Glucose is stored as starch in the plant body.
Photosynthesis is called a redox reaction. Why?
12H2 + 6CO2 + O2
C6H12O6 + 6O2 + 6H2O
As carbon dioxide is reduced to carbohydrate glucose (C6H12O6). Water is oxidised to oxygen simultaneously, Therefore the reaction is called redox reaction.
Source of O2 liberated during Photosynthesis
Source of O2 liberated during photosynthesis is Oxygen (O2) which is of water (H2O) used as component during photosynthesis.
Sources of the different elements of glucose molecule:
(Formed during photosynthesis)
Glucose (C6H12O6) molecule has three elements, such as–carbon, hydrogen and oxygen.
(i) Source of Carbon : Carbon of glucose comes from the component CO2 (Carbon dioxide) taken from air.
(ii) Source of Hydrogen : Hydrogen of glucose comes from H2O (water) which is taken from soil.
(iii) Source of Oxygen : Oxygen of glucose comes from CO2 (Carbon dioxide) taken from air.
Storage of photosynthetic products: Food produced in photosynthesis is stored in storage parts of the plant, such as roots, stems, leaves, fruits and seeds. The stored food moves to different regions of plant when needed.
Transformation of glucose to starch: When glucose saturates the cells of mesophyll tissue, it has to be replaced in order to ensure the continuation of photosynthetic process.
Stored starch : During night starch is stored in roots, stems, leaves etc. Starch formed from glucose in the mesophyll tissues of leaf is converted to soluble glucose during night and transported to different storage regions through phloem vessel.
Fate of glucose during photosynthesis : Glucose is the final product of photosynthesis. If the glucose concentration increases in mesophyll tissue, then glucose is converted to starch with the help of enzyme.
The speed of photosynthesis depends upon the intensity, quality and duration of light. Photolysis of water occurs only in presence of light. Too much of strong light may even stop the process of photosynthesis. The rate of photosynthesis is directly depend on the duration of light.
