“Photosynthesis is a process in which energy-poor inorganic oxidized compounds of carbon (CO2) and hydrogen (water) are reduced to energy-rich carbohydrates (sugar-glucose) by using light energy.” Light energy is converted into chemical energy by chlorophyll and photosynthetic pigments. The following processes in photosynthesis take place.
Carbon dioxide (CO2), water and light are reactants in the photosynthesis while glucose and oxygen are the products of photosynthesis. Water is used as reactant in some reactions and released as product in other reactions. So water appears on the both side of the equation. As there is no net yield of H2O, so the equation can be simplified as follows:
Reactions of Photosynthesis
However, it is not a simple process with single step. It is a complex process. It is completed by a series of simple steps or reactions. The reactions of photosynthesis take place in two steps.
1. The Light Dependent Reactions (Light Reactions)
It uses light directly. Light energy is absorbed by chlorophyll and other photosynthetic pigment molecules. These pigments have reducing and assimilating power. They convert light energy into chemical energy. As a result of this energy conversion, NADPH2 (NADPH + H+) and ATP molecules are formed. ATP is a temporary energy storing compound. This stored energy and H+ are used in light independent reactions.
2. The Light Independent Reactions (Dark Reactions)
It does not use light energy directly. The NADPH2 provides electron (and H+) and ATP provides energy in dark reactions. They reduce CO2 to form sugars. This phase of photosynthesis is also called dark reaction as it does not use light energy directly. This phase can take place equally well in light and dark. It needs only NADPH2 and ATP of light reactions.
Water and Photosynthesis
Oxygen released during photosynthesis comes from water. It is an important source of oxygen for atmosphere. Most of the organisms need this oxygen for aerobic respiration and for obtaining energy.
Hypothesis of Van Neil
He made a hypothesis that plants spilt water. Water is a source of hydrogen. It releases oxygen as a by-product. He made his hypothesis on the basis of his experiments on photosynthesis in bacteria. These bacteria make carbohydrates from CO2. But they do not release oxygen.
According to Neil’s hypothesis, water is a source of oxygen, not CO2. This hypothesis was later confirmed in 1940s. The isotope tracer was used in biological research. Water and CO2 containing heavy oxygen isotope 18 were prepared in laboratory. Two groups of green plants were formed.
- One group of plants was given H20 containing 018 with CO2 containing common oxygen O18.
- Second group of plant was given H20 containing common oxygen with C02 containing O18.
It was found that the plants of first group produced 18, but the plants of second group did not produce O18.
Thus water is one of the raw materials of photosynthesis. The other raw material is CO2. Hydrogen is produced by the splitting of water. It reduces NADP+ (Nicotine Amide Dinucleotide phosphate) to NADPH + H+. The NADPH2 has reducing power. It is formed along with ATP during light reactions of the photosynthesis. The NADPH+ reduces the CO2 to form sugar during dark reactions.
Chloroplasts – The Sites of Photosynthesis in plants
All the green parts of a plant have chloroplast. But the leaves are major sites of photosynthesis in most plants. Chloroplasts are present in very large number. About half a million chloroplasts are present per square millimeter of leaf surface. Chloroplasts are present mainly in the cells of mesophyll tissue of the leaf. Each mesophyll has about 20 — 100 chloroplasts.
Structure of Chloroplast
Chloroplast has double membranous envelope. It encloses a dense fluid inside, called Stroma. This stroma contains most of the photosynthetic enzymes. Another system of membranes is suspended in the stroma. These membranes are called thylakoids. The thylakoids are set of interconnected flat disc like sacs.
A fluid filled thylakoid interior space or lumen is present within the thylakoids. It is separated from the stroma by thylakoid membranes. The thylakoid membranes are stacked in columns in some places. “These stacked columns of thylakoids are called grana (singular granum).
Chlorophyll and other photosynthetic pigments are embedded in the thylakoid membranes. Chlorophyll gives green color to plant. Electron acceptors of photosynthetic Electron transport chain are part of these membranes. So thylakoid membranes are involved in ATP synthesis by chemiosmosis.
Chlorophyll and other pigments absorb light energy. This light energy is converted into chemical energy in the form of ATP and NADPH. These products are used for synthesizing sugar in the stroma of chloroplast.
Chloroplast is absent in photosynthetic prokaryotes. Instead, they have unstacked photosynthetic membranes. These membranes work like thylakoids.
The chloroplast has pigments for absorption of light. Pigments are the substances for the absorption of visible light (380 — 750nm in wave length). Different pigments absorb light of different wave length (colour). The wavelengths that are absorbed by the pigment are disappeared.
An instrument called Spectrophotometer is used for the measurement of relative abilities of different pigments to absorb different lights. “A graph plotting absorption of light of different wave lengths by a pigment is called absorption spectrum of the pigment.”
The thylakoid membranes contain different kinds of pigments. Chlorophyll is the main photosynthetic pigment. There are other accessory pigments present in the chloroplasts. These are:
- Carotenoids: These have yellow and red orange to colour.
- Carotenes: They have red to or colour.
- Xanthophylls: they have yellow colour.
Chlorophyll is a green pigment. It is present in all photosynthetic organisms. It is insoluble in water but it is soluble in organic solvents like carbon tetra chloride (CCl4), alcohol etc.
Types of chlorophyll
There are different kinds of chlorophyll. These are chlorophyll a, b, c, d and e. These pigments are found in eukaryotic photosynthetic plants. The chlorophylls present in photosynthetic bacteria are called bacteriochlorophylls.
Wave length absorbed by chlorophylls
Chlorophylls absorb mainly violet-blue and orange-red wave lengths. Chlorophylls least absorb green and yellow wave lengths. So, these wave lengths are transmitted or reflected. The dark green colour always masks the yellow colour. Hence plant appears green. It remains green unless some other pigment masks it.
Structure of chlorophylls
A chlorophyll molecule has two main parts:
It is flat, square and light absorbing part. Head has a complex porphyrin ring. This ring is made up of four jointed smaller units called pyrrole rings. Each pyrrole ring is composed of carbon and nitrogen atoms. An atom of magnesium is present in the center of porphyrin ring.
This Mg atom coordinates with the nitrogen of each pyrrole ring. That is why the deficiency of magnesium makes the plant yellow. The Haem group of the haemoglobin is also a porphyrin ring. But it contains an iron atom in the place of magnesium in the center.
Hydrophobic hydrocarbon tail
It is long and anchoring part. It is composed of long hydrocarbon chain called Phytol (C20OH39). This Phytol is attached with the one of the pyrrole ring. The chlorophyll molecule is embedded in the hydrophobic core of the thylakoid membrane with this tail.
Carotenoids, Accessory Pigments
Carotenoids are yellow and red to orange pigments. They absorb strongly the blue-violet range of wave lengths. So they absorb different wave lengths than the chlorophylls. In this way the spectrum of absorbed light is increased. Thus more energy is provided for photosynthesis.
Carotenoids and chlorophyll b absorb light and transfer the energy to chlorophyll a. This chlorophyll a then starts the light reaction. So carotenoids and chlorophyll b are called accessory (A) pigments. The order of transfer of energy is as follow:
Some carotenoids protect chlorophyll from intense light. They absorb and dissipate excessive light energy and do not transfer this energy to chlorophylls. Similar carotenoids may also protect human eye.
Light -The driving energy
Light is a form of energy. It is called electromagnetic energy or radiations. Light behaves both as wave and particle. The particles of light are called photons. The radiations from 380 to 750nm wavelength of visible light are most important for life.
The chlorophyll absorbs sunlight energy. It converts this energy into chemical energy during the process of photosynthesis. Chlorophylls do not absorb all the light falling on its surface. The leaf surface absorbs only one percent light energy. The remaining light is reflected or transmitted.
“The Plot (graph) showing absorption of light of different wave lengths by a compound is called absorption spectrum.” It indicates that the absorption is maximum in the blue and red part of the spectrum.
These two absorption peaks of blue and red have wave length of 430 and 670nm respectively. The absorption peaks of the carotenoids are of different forms of chlorophylls.
“The plot (graph) showing relative effectiveness of different wavelengths (colour) of light n photosynthesis is called action spectrum.” The first action spectrum was obtained by German biologist, TW. Engelmann in 1883. He worked on spirogyra. Different pigments absorb different wavelengths.
The effectiveness of different wave lengths is different in different pigments. Action spectrum can be measured by a simple method. The plant is illuminated with light of different wavelengths (colour). The relative amount of CO2 consumed or oxygen released during photosynthesis for each wave length is estimated. Action spectrum shows that the chlorophyll is the photosynthetic pigment.
The action spectrum of photosynthesis is similar to the absorption spectrum of chlorophylls. There are same peaks and valley for the absorption of light and consumption of CO2.
However, the action spectrum of photosynthesis does not exactly parallel to the absorption spectrum of chlorophyll. The peaks of the action spectrum of photosynthesis are comparatively broader than the absorption spectrum of chlorophylls. Similarly the valleys of the action spectrum are narrow and not deep. It indicates that the photosynthesis in the most absorbed range (in blue and red) is more than the absorption of these wave lengths by chlorophylls. Similarly photosynthesis in 500 — 600 nm (green light) is more than the absorption of green light by the chlorophylls.
This difference occurs due to accessory pigments like carotenoids. These accessory pigments absorb light in these zones and transfer some of the absorbed light to chlorophyll. This chlorophyll then converts this light energy to chemical energy. When equal intensities of light are given, there is more photosynthesis in the red part than in the blue part of the spectrum.
Role of Carbon Dioxide: A Photosynthetic Reactant
The reduction of CO2 takes place during light — independent reaction of photosynthesis and sugar is formed. It uses the products of light dependent reactions, ATP and NADPH. It is clear that photosynthesis does not occur in the absence of CO2.
About 10 % of total photosynthesis is carried out by terrestrial plants. The remaining 90% photosynthesis occurs in oceans, lakes and ponds. Aquatic photosynthetic organisms use dissolved CO2, bicarbonates and soluble carbonates as carbon source. These substances are present in water. Air contains about 0.03 to 0.04% CO2. The terrestrial plants use this atmospheric CO2 during their photosynthesis.
CO2 enters the leaves through stomata. This CO2 is dissolved in the water. The cell walls of the mesophyll cells absorb this dissolved CO2. Stomata are present in large number in a leaf. Their number is proportional to the amount of gas diffusing into leaf. Stomata cover only 1-2 % of the total leaf surface. But they allow much more gas to diffuse.
The entry of CO2 depends on the opening of the stomata. Stomata have guard cells. These guard cells have peculiar structure. Its shape can be changed. So guard cells regulate the opening and closing of stomata.
Stomata are adjustable pores. They open during the day when CO2 is required for photosynthesis. They partially close at night when photosynthesis stops.
There is a noticable Difference between Photosynthesis and Respiration.Photosynthesis occurs during day time, whereas respiration occurs both at day and night. The leaves (and other actively metabolizing tissues) respire during night. They utilize oxygen and release CO2.
There is low intensity of light at dawn and dusk. Thus the rate of photosynthesis and the rate of respiration become equal during that short period of time. So the amount of oxygen released from photosynthesis is equal to the amount used by cellular respiration.
Similarly, the amount of carbon dioxide released by respiration is used by the photosynthesizing cells. “So there is no net gas exchange between leaves and atmosphere at this moment. This is called compensation point.” The rate of photosynthesis increases with the increase of intensity of light. So, more CO2 is required. This CO2 can only be supplied by respiration.
Similarly, photosynthesis produces more oxygen than the need of the respiring cells. So there is net release of oxygen and uptake of carbon dioxide.