All green plants are able to manufacture their own food through the process of photosynthesis.
Green plants differ from other living organisms in their mode of nutrition because of their ability to utilise light energy during the process of food manufacturing (photosynthesis). Photosynthesis is therefore the process by which green plants (autotrophs) synthesise food (organic compounds) from carbon dioxide, and water, using energy from the sunlight absorbed by chlorophyll, while oxygen is liberated as a by-product. This process can be represented as follows:
Mechanism of photosynthesis
Photosynthesis consists of a series of complex reactions which can be divided into two groups, namely the light reaction and the dark reaction.
All green plants have chloroplast which contains the green pigment called chlorophyll, which absorbs light energy from sunlight. This energy is used during the process of photolysis to split water molecules (H2O) into hydrogen (H+) and hydroxyl ion (OH-).
The oxygen that is given off in photosynthesis comes from water and not from carbon dioxide as the above equation shows. This process of splitting water molecules by light is known as photolysis.
While the hydroxyl component is split into water and oxygen (light reaction), the hydrogen components undergo a series of reaction in which they reduce carbon dioxide to form sugar. Sugars are built up from the hydrogen components.
They are called dark reactions not because they take place in the dark, but because light is not directly necessary for the reaction. The formation of sugars takes place through a series of enzyme controlled reactions.
In this equation, (CH2O) is the carbon skeleton from which simple sugars and other organic food compound (proteins, lipids) are formed.
Materials and conditions necessary for photosynthesis
Chlorophyll, light, water, carbondioxide and suitable temperature are the conditions necessary for photosynthesis to take place. The material consists of carbon, oxygen, magnesium, hydrogen and nitrogen. This pigment traps light energy from the sun, converting it into chemical energy. The various pigments present in chlorophyll are chlorophyll a (blue-green), chlorophyll b (yellow-green), xanthophyll (yellow), carotene (orange), and phaeophytin (grey).
Carbon dioxide: This gas from the atmosphere diffuses through the stomata into the intercellular airspace of the leaf. It gets into the spongy mesophyll and then dissolves in the water in mesophyll cells. From here, the carbon dioxide diffuses to the chloroplasts and is used by plants during photosynthesis. Water is absorbed from the soil into the root hairs of plants through the process known as osmosis. It gets to the root xylem and it is then conducted to the stem xylem and to the leaf veins. This water ends up in the chloroplast of the mesophyll cells.
Suitable temperature: Temperature variation affects the rate of photosynthesis. For photosynthesis to occur, the optimum temperature is about 30 degrees C. A higher temperature (above 40 degrees C) retards the process of photosynthesis by damaging the chlorophyll, proteins and other substances in the protoplasm. At a lower temperature, the rate of photosynthesis is reduced.
Significance of photosynthesis
(i) Green plants use the process of photosynthesis to trap light energy from the sun and this is used in manufacturing food substances used for growth.
(ii) All other living organisms depend directly or indirectly on photosynthesis, e.g., green plants or autotrophs are primary producers in any ecosystem.
(iii) The oxygen released by plants during photosynthesis sustains life and other processes like combustion.
(iv) During the process of photosynthesis, oxygen is released to the atmosphere and carbon dioxide is removed from it. This oxygen is used for respiratory processes in animals and plants. Photosynthesis helps to maintain a fairly constant concentration of the oxygen and carbon dioxide in the atmosphere.
(v) Cellular respiration occurs only in the presence of sugar which is the first product of photosynthesis.
The first product of photosynthesis is glucose which is a simple sugar. This can be converted to starch for storage. The test for starch is usually to find out whether a plant has photosynthesised or not. Part of the synthesised food is converted into soluble forms and carried in the phloem vessels to different parts of the plant, where it is required or it can be stored when it is not needed.
This process of transporting food from one part of the plant to another is called translocation.
Test for starch
(i) Remove a healthy green leaf from a plant.
(ii) Boil the leaf in water for about five minutes to kill the protoplasm, inactivate the enzymes present, render the cell walls more permeable and burst the starch grains.
(iii) Transfer the leaf into a test-tube placed in a water bath in order to remove the chlorophyll from the leaves.
(iv) Dip the leaf inside warm water to soften it so that iodine can easily penetrate the leaf.
(v) Place the leaf in iodine solution for about five minutes, then wash with cold water.
(vi) The blue-black colour indicates the presence of starch.
To show that light is necessary for photosynthesis
Two potted plants, beaker, iodine solution, boiling tube, forceps, Bunsen burner, alcohol and petri dish.
(i) Place the two well watered potted plants (same age and species) in an airy dark cupboard for two days to distarch the leaves.
(ii) Expose one of the plants to sunlight for 5 to 10 hours and leave the other in the dark.
(iii) Test the leaves from each of the plants for starch.
The leaf from the exposed plants shows blue-black colouration, while the leaf from the unexposed plant takes up the iodine colour.
Since the leaf exposed to light shows the presence of starch, this confirms that light is necessary for photosynthesis to take place. To show that carbon-dioxide is necessary for photosynthesis
(i) Keep two potted plants of the same species and age in the dark for about 48 hours to distarch the leaves.
(ii) Set up the apparatus shown in the below figures and leave the two plants A and B in the sun for one day.
(iii) Place potassium hydroxide in A to remove carbon dioxide while sodium hydrogen carbonate is placed in B to supply carbon (IV) oxide.
(iv) Tests for starch in A show the absence of starch while the tests for starch in B show the presence of starch.
To show that chlorophyll is necessary for photosynthesis
(i) Expose potted variegated leaves (Croton or ice plant) to sunlight for about 3 hours.
(ii) Get the variegated leaf from the plant.
(iii) Draw the leaf, mapping out the green and white patches, then test for starch.
(iv) The green portions of the variegated leaf changed into blue-black, confirming the presence of starch, while the non-green portions did not change. This confirms that photosynthesis did not take place in the non-green portions of the leaf, due to lack of chlorophyll.
Mineral requirement of plants
Plants utilise glucose and other carbohydrates from carbon dioxide and water. They also need mineral salts for the formation of proteins required for their healthy growth. Mineral salts cannot be manufactured by plants, but they are obtained from the soil.
Apart from carbon, hydrogen and oxygen, there are seven essential (major) elements required by plants. These are nitrogen, phosphorus, potassium, calcium, magnesium, sulphur and iron. Other non-essential (minor or trace) elements needed by plants are copper, boron, zinc, molybdenum, manganese, cobalt and chlorine. If an element is missing from the soil, the plants will not thrive well and therefore, suffer from deficiency disease (see Tables 1 and 2).
Nitrogen is one of the essential or macro elements needed by plants. It is one of the crucial elements in life because it forms a component of protein found in living things. The nitrogen percentage in the atmosphere is about 79%. This atmospheric nitrogen is not biologically active and cannot be used in this form by living organisms. All the nitrogen required by plants for their metabolic activities are obtained from nitrogenous compounds present in the soil in form of nitrates and ammonium compounds.
When rain falls, some nitrogenous compounds are washed off from the soil by the process of leaching action. The various changes through which nitrogen passes in the process of being utilised by animals and plants and returned to environment is known as the nitrogen cycle. It shows how nitrogen gets into the chemistry of life processes.
Conversion of atmospheric nitrogen to soil nitrogen
(i) Thunderstorms: During this process, nitrogen combines with oxygen to form nitrous and nitric oxides which are gaseous oxides of nitrogen. These dissolve in rain water, forming nitrous and nitric acids respectively. Inside the soil, these acids react with mineral salts to form nitrates, which can easily be absorbed by the root hair of the plants.
(ii) Nitrogen fixation: There is symbiotic and non-symbiotic biological fixation.
a. Symbiotic nitrogen fixation: This is carried out by symbiotic nitrogen fixing bacteria, like rhizobium and bacillus species present in the root nodules of leguminous plants. These bacteria can convert the free atmospheric nitrogen into nitrates which are utilised by plants. The plants on the other hand provide the bacteria with energy and shelter.
b. Non-symbiotic nitrogen fixation: The bacteria responsible for this activity are the Azofobacter and Ciostridium species which live freely in the soil and convert free nitrogen in the soil into nitrates.
(iii) Putrefaction: This can be brought about through the activities of saprophytic bacteria and fungi. They cause the decay of dead remains of plants and animals. During the process of putrefaction, ammonia, carbon dioxide and water are released into the atmosphere. The carbon dioxide is given back to the atmosphere, while ammonia is converted to nitrates by other micro-organisms.
(iv) Nitrification: Nitrifying bacteria, e.g., nitrosomonas and nitrobacter are responsible for converting ammonia to nitrates. Nitrosomonas converts ammonia to nitrites and nitrites are converted to nitrates by nitrobacter.
(v) De-nitrification: Denitrifying bacteria reduce nitrates in the soil to gaseous nitrogen which escapes into the air. This becomes atmospheric nitrogen. The amount of nitrogen in the atmosphere is maintained by a balance between the processes, which withdraw nitrogen from it (nitrogen fixation) and those which add nitrogen to it (de-nitrification).