Nutrition in plants
Green plants are autotrophic. They synthesize their own food by the process of photosynthesis.Fig: Green plants, autotrophsAutotrophic plants are able to produce food so they are known as producers.
Photosynthesis: Photosynthesis is a process by which green plants synthesize organic food (carbohydrate) from carbon dioxide and water using solar energy by chlorophyll pigments.
The sugar produced is stored in the form of starch in plants.
Importance of photosynthesis:
Photosynthesis is an anabolic process in nature for providing food supply to the living organisms.
It purifies the atmospheric air, by consuming CO2 and evolving oxygen.
The overall equation of photosynthesis is:
6CO2 + 12 H2O C6H12O6 + 6H2O + 6O2
Requirements for photosynthesis:
(2) Photosynthetic pigment
(3) Carbon dioxide
CO2 and water work as raw materials which are obtained from the atmosphere and the soil respectively.Fig: Photosynthesis1. Sunlight:
Sun is a natural source of light for photosynthesis. Sunlight is an electromagnetic spectrum.
Photosynthetic pigments absorb only visible/white light from electromagnetic spectrum. White light (380 nm to 760 nm) is composed of wavelength of seven different colours viz. violet, indigo, blue, green, yellow, orange and red (VIBGYOR).
2. Photosynthetic pigment:
These are chlorophyll and carotenoids (carotenes and xanthophylls) and phycobilins. These pigments absorb only visible light. Chlorophyll-a and b absorb only blue and red light and reflect green light.
3. Carbon dioxide:
All the plants need carbon dioxide to form carbohydrates. The carbon dioxide is obtained by the plants from the atmospheric air. In the terrestrial plants, the CO2 enters into the cells of leaves through tiny pores called stomata which always remain present on the surface of leaves.
These are tiny pores or microscopic aperture guarded by two kidney shaped or bean shaped guard cells.
(i) Massive amount of a gaseous exchange take place in the leaves through stomata. Exchange of gasses also occurs across the surface of stem, root and leaves.
(ii) Large amount of water is lost through stomata.
These are kidney shaped cells which cover single stomata. They contain chloroplast also.
They regulate the opening and closing of the stomata and also perform photosynthesis.
Opening and Closing of Stomata:
When the guard cells swell due to the entry of water, the stomata gets opened. But when the guard cells shrink due to the loss of water, the stomata gets closed.
Fig: Opening and closing of stomata
Desert plants take up CO2 at night and prepare an intermediate which is acted upon by the energy absorbed by the chlorophyll during the day and form glucose.
In aquatic plants, CO2 is obtained from the water where it remains present in dissolved form. Such plants absorb carbon dioxide in solution form, all over their surface from the surrounding water.
Water is always needed by the plants for its use during photosynthesis.
Inside the chloroplasts of the leaves, the water molecules split into hydrogen and oxygen with the help of light energy of solar light.
Some mineral salts like N, P, K, Fe, Mg required by the plants are also transported to different parts of the plant along with the water.
Nitrogen is an essential element used in the synthesis of proteins and other compounds (chlorophyll, DNA and RNA).
Nitrogen is taken up in the form of inorganic nitrates or nitrites which have been prepared by symbiotic bacteria from atmospheric N2.
Site of Photosynthesis:
Green plastid (Chloroplast or Kitchen of the cell).
When we observe the cross section of a leaf under microscope, we can see the mesophyll cells full of green dots. These green dots are chloroplasts containing chlorophylls.
They are green coloured plastids. Their green colour is due to the presence of green pigments the chlorophylls. Each developed chloroplast has two distinct areas - grana and stroma.
The light reaction of photosynthesis takes place in this part of chloroplast. In a granum, large number of lamellae remain arranged like a stack of coins. These lamellae are called as thylakoids, which contain chlorophyll pigments.
It forms the matrix of the chloroplast. The dark reactions of photosynthesis take place in stroma.
Mechanism of photosynthesis
During photosynthesis following events occur:-
(i) Absorption of light energy by chlorophyll.
(ii) Conversion of light energy into chemical energy and splitting of water molecule into hydrogen & O2
(iii) Reduction of CO2 to carbohydrates.
All these events can be categorised into two main phases:-
(A) Light phase (B) Dark phase
(A) Light reaction:
This step occurs inside thylakoid membranes of chloroplasts. In this step firstly absorption of solar energy by chlorophyll molecules takes place.
During light reaction, light energy breaks up water molecules into hydrogen and oxygen and this process is called photolysis of water.
Light energy converts into chemical energy in the form of ATP and NADPH2 (Nicotinamide adenine dinucleotide phosphate).
Oxygen is released as a by product of light reaction occuring during photosynthesis.
(B) Dark reaction:
In this step synthesis of carbohydrates from carbon dioxide takes place. It is not light dependent hence it is called as dark reaction.
This reaction occurs inside stroma of chloroplasts where light energy is not captured.
During this reaction, the chemical energy formed during light reaction (ATP and NADPH2) is utilised for the fixation and conversion of carbon dioxide into a simple carbohydrate, that is glucose.
The glucose molecules thus formed are further converted by the cellular machinery into various chemicals required by the plants.
Difference between light and dark reactions
|S .No.||Features||Light reaction||Dark reaction|
|1||Requirement of light||Required||Not Required|
|2||Takes place inside||Thylakoid membranes of the chloroplast||The stroma region of chloroplast|
|3||ATP and NADPH2||ATP and NADPH2 are produced by the conversion of light energy into chemical energy||ATP and NADPH2 formed during light reaction are used for the fixation of CO2 into carbohydrate|
|4||Sugar formation||No sugar formation takes place||Sugar formation takes place|
|5||Release of oxygen||Oxygen is released||No Oxygen is released|
Factors affecting photosynthesis:
The source of light for planet earth is sun, although some marine algae also utilise the light of moon.
Out of the total solar energy, only 2% solar energy is used in photosynthesis.
The quality and intensity of light also affects photosynthesis.
Quality- Red and blue lights are most effective in photosynthesis.
But the rate of photosynthesis is maximum in red light.
There is no photosynthesis in presence of green light because green parts of plants reflect whole of the green light.
Intensity- The increase in intensity of light increases photosynthesis.
Intensity of sunlight ∝ Rate of photosynthesis
At very high light intensity, the photosynthesis is decreased due to photo-oxidation of the constituents (solarization).
Water is an essential raw material in photosynthesis. Only 1% of the absorbed water is utilised in photosynthesis.
Less availability of water reduces the rate of photosynthesis. Under water deficient conditions the stomatal aperture remains closed to reduce the loss of water by transpiration. As a consequence, the entry of CO2 is also stopped into the leaves.
The rate of photosynthesis increases by increase in temperature upto 40°C. Above this temperature, there is a decrease in the photosynthesis. Similarly, low temperature also inhibits photosynthesis.
The temperature affects photosynthesis by affecting the activity of enzymes. We know that the dark reaction of photosynthesis involves several enzymes. These enzymes function at a specific range of temperature.
Low temperature lowers the activity of enzymes and high temperature causes inactivation of enzymes.
Atmosphere is the main source of CO2 for terrestrial plants.
In atmosphere CO2 is present at the tune of 0.03%.
The rate of photosynthesis increases by increasing the concentration of CO2. But after a certain limit, the excess concentration of CO2 proves to be toxic to the cells.
Over concentration of oxygen stops photosynthesis.
It increases the rate of respiration manifold and disturbs the excited condition of the chloroplast. In these conditions photosynthesis is not possible.