Nutrition in Plants

Nutrition in Plants Synopsis

Synopsis

Nutrition

Nutrition is a process of intake as well as utilization of nutrients by an organism.
Nutrients are the substances required by our body for its growth, repair, work and maintenance.

Autotrophic Nutrition

It is the mode of nutrition in which organisms synthesise their own food from simple inorganic substances such as water and carbon dioxide.
Examples: Green plants and algae are autotrophs.

Heterotrophic Nutrition

It is the mode of nutrition of organisms which cannot synthesise their own food, but they are dependent on other organisms for food.
Examples: yeasts, fungi, bacteria, human beings, tiger, monkey, birds, lion, cow etc.
Heterotrophs are classified into different type basis the nature of food consumed and the mode of feeding.

Photosynthesis

Photosynthesis is a physiological process by which plant cells containing chlorophyll produce food in the form of carbohydrates using carbon dioxide, water and light energy. Oxygen is released as a by-product of this process.

Importance of Photosynthesis

It is the primary source of food for all living organisms on the Earth.
It adds oxygen to the atmosphere to compensate for the oxygen being used in the respiration of organisms and burning of fuels.
It influences the productivity of agricultural crops.

Chloroplast

Chloroplasts are the green plastids present in the green stems, sepals and leaves.
They are the site of photosynthesis.
Each chloroplast is bounded by a double membrane envelope.
Its matrix is filled with a proteinaceous, colloidal matrix called stroma and an elaborate system of membranous lamellae called thylakoids.
The stroma contains enzymes, DNAs, RNAs and 70S ribosomes.
They occur as flattened sacs of two types—grana lamellae and stroma lamellae.
Thylakoid membranes possess photosynthetic pigments–chlorophyll a, chlorophyll b, carotenoids, cytochromes (b and f), ATP synthetase and the enzymes required in the light reactions
of photosynthesis.
Thylakoids are the site of light for photosynthesis.

Pigments Involved in Photosynthesis

Chlorophylls are green photosynthetic pigments present in all photoautotrophs.
These pigments absorb light near both ends of the visible spectrum—the violet–blue and red light—and reflect green light because of which they appear green.
In higher plants, chlorophylls are of five different types:

Chlorophyll a is bluish green, chlorophyll b is yellow green.
Chlorophyll c is found in brown algae, diatoms and dinoflagellates.
Chlorophyll d is found in red algae.
Chlorophyll e is present in Xanthophyceae along with chlorophyll a.

Photosynthetic Units and Photosystems

Green plants have two types of photosynthetic units called photosystems or pigments systems I and II.
Each photosystem contains 250–400 pigment molecules.
The pigments are organised into two light harvesting complexes or antennae within photosystem I and photosystem II.
The pigments help to make photosynthesis more efficient by absorbing the different wavelengths of light.
The single chlorophyll a molecule forms the reaction centre.
The reaction centre is different in PS I and PS II.
In PS I reaction centre chlorophyll a has an absorption peak at 700 nm while in PS II an absorption peak is at 680.
Hence, PS I reaction centre is called P700 and PS II reaction centre is called P680

Electron Transport Chain

The whole scheme of the transfer of electrons, right from PS II uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor and finally downhill to NADP⁺ causing it to be reduced to NADPH + H⁺ is called the Z scheme.

Photolysis of Water

The electrons that were removed from PS II during the electron transport chain are replaced by the photolysis or splitting of water.
During this process, water molecules split into H⁺ and OH^- ions.
The OH^- ions being unstable in nature combine to form water and molecular oxygen along with the release of electrons.

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Light Reaction

The light reaction of photosynthesis occurs only in the presence of sunlight in the thylakoid membranes of grana and stroma lamellae.
Light reaction is also called the photochemical reaction or Hill reaction or photophosphorylation.

Absorption of Light

The various pigments of photosystem II (P₆₈₀) and photosystem I (P₇₀₀) absorb sunlight of different wavelengths.

Excitation of Reaction Centres

The reaction centres receive light energy from their respective photosystems, become activated and their electrons become excited.
The electrons are transferred to primary electron acceptor.
Because of the transfer of electrons, P₆₈₀ and P₇₀₀ become electron-deficient or highly oxidised.

Photosynthetic Electron Transport and Formation of Assimilatory Power

The electrons then move downhill along the electron transport chain from phaeophytin (primary electron acceptor) to plastoquinone (PQ), cytochrome complex (cyt b₆ and cyt f) and plastocyanin to finally reach the reaction centre of PS I (P₇₀₀).
During this transfer, electrons lose energy which is used to generate ATP.
The electrons then move downhill along the electron transport chain from phaeophytin to plastoquinone (PQ), cytochrome complex (cyt b₆ and cyt f) and plastocyanin to finally reach the reaction centre of PS I (P₇₀₀).
During this transfer, electrons lose energy which is used to generate ATP.
From here, the electrons move down to ferredoxin and then to ferredoxin-NADP oxidoreductase, where NADP is reduced to NADPH.
This causes reduction of CO₂ to sugar during the dark reaction of photosynthesis and provides the necessary energy.

Photophosphorylation

The process of ATP formation in chloroplast in the presence of light energy is called photophosphorylation.
After losing their energy during transport, electrons can be used in the following ways:

Electrons from photosystem II are used by photosystem I. This is called non-cyclic photophosphorylation.
Electrons from photosystem II return to the same photosystem. This is called cyclic photophosphorylation.

Biosynthetic Phase/Dark Reaction

The dark reaction is the second phase of photosynthesis which occurs in the stroma of chloroplasts.
The reaction is also known as the biosynthetic phase or Blackman’s reaction or carbon
dioxide fixation.
It uses ATP and NADPH₂ for the fixation and reduction of CO₂ to form carbohydrates.

Types of CO₂ Fixation

Calvin Cycle or C3 Cycle

Each Calvin cycle is completed in three phases—carboxylation, reduction and regeneration.

Thus, for one molecule of glucose, 18 molecules of ATP and 12 molecules of NADPH are required.

Hatch–Slack Cycle or C4 Cycle

The C₄ cycle is found in tropical and subtropical grasses such as maize, sugarcane, pear, millet, all the other monocots and dicots such as Amaranthus and Euphorbia.
C₄ plants are adapted to overcome photorespiration and deliver CO₂ directly to the enzyme RuBisCO.

Anatomy of C4 Plants

C₄ plants show a different type of leaf structure called Kranz anatomy.
There are bundle sheath cells which form several layers around the vascular bundles.
These cells have a thick wall impervious to gaseous exchange and no intercellular spaces.
The chloroplasts in the bundle sheath cells are large and arranged centripetally. They do not have well-defined grana but have starch grains and peripheral reticulum.
The C₄ cycle operates in the bundle sheath cells of the mesophyll.

Biochemical Pathway

C₄ plants carry out double fixation of CO₂—initial and final.
The product of initial fixation in mesophyll cells is transported to bundle sheath cells for final fixation.

C₄ plants require a total of 30 ATP and 12 NADPH molecules to synthesise one molecule of glucose from 6 molecules of CO₂.

Photorespiration

The process of uptake of oxygen and the production of carbon dioxide in light by photosynthesising tissues is known as photorespiration.
It is also called photosynthetic carbon oxidation or the PCO cycle as this process occurs in a series of cyclic reactions.
It occurs in all C₃ plants.
RuBisCO has a greater affinity for CO₂than for O₂. This binding is competitive.
When some oxygen binds to RuBisCO, it decreases CO₂ fixation.
The binding of oxygen and RuBisCO forms one molecule of phosphoglycerate (PGA) and phosphoglycolate.
PGA is used up in the Calvin cycle in the chloroplast, and phosphoglycolate is dephosphorylated into glycolate.
Glycolate is used to form amino acid glycine.
Carbon dioxide is released from mitochondria as a by-product of photorespiration
During photorespiration, neither ATP nor NADPH is synthesised.

Comparison between C₃ Cycle and C₄ Cycle

Factors Affecting Photosynthesis

The rate of photosynthesis is very important in determining the yield of plants including crop plants.
According to Blackman’s law of limiting factors, ‘When a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor’.

This implies that if more than one factor affects a chemical process, then its rate will be determined by a factor which is nearest to its minimal value. It is the factor which directly affects the rate if its quantity is changed.

Light

The quality, intensity and duration of light affect the rate of photosynthesis.
Light between the wavelength of 400 nm and 700 nm is the most effective for photosynthesis and this light is called photosynthetically active radiation (PAR).
Under low light intensity, the rate of photosynthesis is low. As the intensity of light increases, the rate of photosynthesis increases.

Carbon Dioxide

The concentration of CO₂ in natural air varies between 0.03% and 0.04%.
Increase in the concentration of CO₂up to 0.05% increases CO₂fixation.
A slight rise beyond this concentration can have damaging effects over long periods.

Temperature

In the presence of abundant light and carbon dioxide, the rate of photosynthesis increases with a rise in the temperature till it becomes maximum. After that, there is a decrease in the rate of the process.
The rate of photosynthesis is maximum at an optimum temperature of 25–30°C.

Water

An increase in the water content of the leaf results in an increased rate of photosynthesis.
Water indirectly exerts a limiting effect on the rate of photosynthesis.
This is mainly because water helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm.

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Nutrition in Plants