Nutrition in Plants – Photosynthesis and Nitrogen Cycle

Nutrition in plants is the process of consuming food by an organism for its body. All organisms need to have the proper nutrition to grow and develop. No matter which kingdom they belong to, nutrition is essential for every organism.

There are two modes of nutrition in plants to choose from. The modes are autotrophic and heterotrophic. The autotrophic plants are capable of making their own food rather than relying on other organisms. They are called autotrophs, and the most common autotrophic process is photosynthesis.

The other mode of nutrition in plants is heterotrophic. The plants depend on other organisms for food in this mode. These plants are heterotrophs because of their dependent nature. Insectivorous plants are an exception as they follow both autotrophic and heterotrophic modes of nutrition.

These modes are according to the features and function of a plant with some emphasis on their structure. These models follow certain processes under them to produce or find food for the plant’s nutrition. We will look at photosynthesis and nitrogen cycle fixation at large. Let’s start with them.

Photosynthesis in Plants

Photosynthesis is the process of producing food by green plants. The plants inhale Co2 from the air through tiny pores present on the leaf surface. These pores are stomata and have guard cells.

There are vessels which run in the stems and leaves are responsible for transporting water and minerals to them. They form a pipeline system to deliver these.

The leaves have chlorophyll which helps in capturing energy from sunlight. The energy from sunlight enables the production of food from carbon dioxide and water. The term of this process is synthesis.

Because this process takes place in the presence of sunlight, it becomes photosynthesis. The main ingredients for the process are – chlorophyll, sunlight, carbon dioxide, and water.

This process can take place in other green parts as well as stems and branches. An example of this is desert plants that have green stems to carry out photosynthesis. This is because they have spine-like leaves for less transpiration.

The Co2 becomes starch by the process of photosynthesis. Other color leaves have chlorophyll too but the majority of other pigments overshadow it. But photosynthesis can take place here as well. The equation for this entire process is – 6CO2 + 6H2O —> C6H12O6 + 6O2.

Chlorophyll Pigments

The co2 fixation is directly connected to chlorophyll. The mesophyll cells in a leaf have chloroplast which enables co2 fixation. The leaves do not have only one pigment but a combination of four pigments – Chlorophyll, chlorophyll b, xanthophylls, and carotenoids.

Photosynthesis Process in Plants

Light Reaction

• It starts with the light reaction which is only possible in the presence of sunlight. This takes place during the day time in the thylakoid membranes of chloroplasts.
• The thylakoid has a sack membrane-like structure called Grana function by light and thus called photosystems.
• These photosystems have pigments and protein which are essential for the light reaction of photosynthesis. There is Photosystem I and Photosystem II.
• The energy from the sunlight converts to ATP and NADPH under the light reaction. They form the basis of dark reaction.
• This conversion is by two electron-transport chains, here they use water and produce oxygen.
• The chemical equation of light reaction – 2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

Dark Reaction

• It is the carbon-fixing reaction. It does not require light for the process.
• Conversion of water and co2 into sugar molecules takes place here.
• This reaction takes place in the stroma in chloroplast where NADPH and ATP are utilized.
• The plants inhale Co2 from the atmosphere through stroma and continue the cycle.
• The ATP and NADPH convert 6 molecules of carbon dioxide into one sugar molecule.
• The chemical equation for the dark reaction – 3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

Factors Affecting Photosynthesis

Carbon dioxide Concentration – The CO 2 concentration in the atmosphere is quite low. The higher concentration of Co2 helps in increasing the rate of photosynthesis. Up to 0.5 % is fine but above that, it can damage in the long run.

Temperature – The dark reaction depends more on temperature as compared to the light reaction. The ideal temperature for photosynthesis is 25-35 degrees. Though it depends on the geographic location as well, tropical plants need higher temperatures for this.

Water – Excess water closes the stomata thus reduces co2 inhalation. It also makes the leaf wilt reducing its metabolic activity.

Light – Higher light intensity increases the rate of photosynthesis. And vice versa for lower light intensity.

Importance of Photosynthesis

• Photosynthesis facilitates the existence of living organisms on earth.
• It ensures the production of food for the primary producers in the food chain.
• It also produces oxygen which is essential for human existence.

Saprotrophic Nutrition and Parasites

Fungi follow a different mode of nutrition altogether. They survive on dead and decaying substances. They release digestive juices on them to convert the substance into solutions. Then they take in all the nutrients from the solution. This nutrition is saprotrophic nutrition.

Plants using it are saprotrophs. Mushroom is an example of this. These are heterotrophic organisms. They live on another organism and benefit from them without any returns. Cuscuta is a parasite.

Symbiosis

Some organisms need to have an interdependent relationship for nutrition and habitat. This is a symbiotic relationship. An example is a Fungi that live in the tree roots. The trees and fungi help each other in certain ways.

The fungi deliver water and nutrition from the soil and the tree in turn gives it food. The soil provides minerals to a Plant. This leads to a decrease in mineral quantity in soil.

But nutrients like nitrogen, potassium, phosphorus, etc are present in manures. The plants utilize a large amount of nitrogen to produce protein. Thus after the harvest season soil is nitrogen deficient. Plants need soluble nitrogen and rhizobium enables this conversion.

As rhizobium is heterotrophic, it lives in the roots to get nutrition. This is again a symbiotic relationship. Most of the leguminous plants use this relationship for a better harvest.

Mineral Nutrition in Plants

The organisms need certain nutrients as part of their basic living need. A few of them are essential for the growth of plants. There are two types of nutrients that are important for plants at large – micronutrients and macronutrients. The division is on the basis of quantity. They are –

• Nitrogen is important for all parts of the plant and especially for meristematic tissue and other cells with metabolic rate. They offer proteins, acids, vitamins, and hormones.
• Phosphorus has certain proteins in the cell membrane. It is mandatory for all phosphorylation reactions.
• The potassium is required more in meristematic tissues, buds, leaves, and root tips. They maintain an anion-cation balance in cells. And is actively participating in protein synthesis, stomata function, enzymes activation, and cell turgidity.
• Calcium is important for all types of tissue as they enable cell wall synthesizing particularly in middle lamella. It facilitates normal cell membrane functioning and releases enzymes for metabolic activities.
• Magnesium releases enzymes for respiration and photosynthesis. They facilitate the synthesis of DNA and RNA as well.
• Other important nutrients are Sodium, Silicon, Cobalt, and Selenium.

Nitrogen Cycle

Nitrogen is an essential element for living organisms. It consists of amino acids, proteins, hormones, and more vitamins. The plants need nitrogen and thus compete with microbes for a limited amount of nitrogen in the soil.

It exists in the form of two atoms combined with a triple covalent bond. Let’s look at the stages of the nitrogen cycle –

Nitrogen Fixation

The nitrogen conversion to ammonia is called nitrogen-fixation. This is the first step nitrogen cycle. Natural lighting and UV radiation enable the conversion of nitrogen into nitrogen oxides. A symbiotic bacteria – Diazotrophs carries out the entire nitrogen fixation process.

There are three types of nitrogen fixation – atmospheric, industrial, and biological. Though atmospheric nitrogen comes from combustions, forest fires, vehicle exhausts, and power stations. Plants can follow any of these for nitrogen fixation.

Nitrification

The conversion of ammonia into nitrate is nitrification. This takes place in the presence of soil. The oxidation of ammonia forms nitrate. Ammonia gas is toxic for plants thus this conversion is essential. The first step is by micrococcus to oxidize ammonia to nitrite.

This nitrite then further becomes refined nitrite with Nitrobacter bacteria. The essential bacteria for this process is chemoautotrophs. The formula for nitrification is – 2NH4 + 3O2 → 2NO2 + 4H+ 2H2O – 2NO2– + O2 → 2NO3

Assimilation

The plant transportation system delivers the soluble nitrate to leaves. They are the primary producers that need this nitrate for producing the plant and animal proteins. This way nitrogen ether the food web of the ecosystem.

Ammonification

It is the process of decomposing dead organisms into ammonia. In leaves, the nitrate becomes ammonia which forms the amino acid group.

This ammonia is useful for other biological processes in the ecosystem. The decomposers are either bacteria or fungi which carry out the process of ammonification in soil.

Denitrification

Some nitrogen from soil re-enters the ecosystem but mostly becomes nitrate by soil bacteria. The nitrate left in the soil becomes nitrogen by the denitrification process. Pseudomonas and Thiobacillus are responsible for this process. These nitrogen fixers are either free-living or symbiotic in nature.

The Nitrogen Cycle Process : Nutrition in plants > Nitrogen Cycle > Nitrification > Denitrification > Atmosphere > Soil

Importance of Nitrogen Cycle

• Firstly, Nitrogen is essential for synthesizing chlorophyll.
• Enables the biochemical process by producing soluble nitrogen.
• Reduces environmental waste by decomposing dead organisms.
• Makes soil more fit for agriculture by providing it minerals and nutrients.
• Lastly, It also acts as an important biomolecule.

Symbiotic Biological Nitrogen Fixation

The legume-bacteria relationship is symbiotic biological nitrogen fixation. Even rhizobium has this relationship with roots in plants like lentils, garden pens, etc. The most common is roots relation with nodules as they are outgrown from it.

The nitrogen fixation nodules are in non-leguminous plants like Alnus. The rhizobium and Frankia live free in soil but fix nitrogen as symbionts.

Conclusion

This article was about different kinds of processes that organisms follow for nutrition in plants. The most important ones are photosynthesis and nitrogen cycle. They are an important part of biology as well as geography. They are essential for the efficient functioning of the ecosystem.

The UPSC Prelims General Study paper can have a question from this topic. This is because it comes under the basic science module. It is also important for UPSC Mains Biology as it is building a basic understanding of the subject.

Aspirants preparing for other competitive exams like SSC, RRB, and more can refer to it as well. All the UPSC candidates must go through this topic before appearing for the exams.