Macronutrients, Practice Problems and FAQs

We all know that plants are able to synthesise their own food through photosynthesis. They are able to synthesise their own food using sunlight energy in the presence of chlorophyll, utilising carbon dioxide and water. So the plants are called autotrophic.

But do you think only carbohydrates are enough for the proper growth of plants? You all like biriyani. In the kitchen we need many ingredients like spices, vegetables and other things to prepare a delicious dish like biriyani. Our parents will tell us that we need to consume various kinds of food for proper growth and development. 

                                            Fig: Mineral sources

Similarly for proper growth and development, plants also need different minerals like nitrogen, potassium, sodium etc. For example nitrogen is needed to make proteins. If they don’t get enough nitrogen, the plant will not grow properly and may show stunted growth. You know that all enzymes required for the various processes in the body are made up of proteins.

There are other nutrients that also impart the same type of effects on plants. Their deficiency or excess both may cause damage to the plants. These plant nutrients are divided into micronutrients and macronutrients on the basis of their requirement in the plant body. In this article we will take a deep dive into the details of macronutrients.

                                          Fig: Macronutrients

Table of contents

• Introduction to macronutrients
• Essential elements
• Classification of essential elements
• Need of macronutrients
• Macronutrients and their roles
• Practice Problems
• FAQs

Introduction to macronutrients

Plants require water, sunlight, gases, and minerals for their survival. The chemical substances that provide nourishment to the living organisms are termed as nutrients. Different minerals are required in different amounts in the plant body. The process by which plants absorb and utilise mineral elements for their growth and development is called mineral nutrition. 

                                                 Fig: Requirements of a plant

Mineral requirements in plants

Plants are able to produce their own food through the process called photosynthesis. They absorb nutrients through their roots from the soil and are transported through the stem to the different parts that are above ground level. It includes essential elements also. 

Essential elements

Elements which are required by living organisms for growth, development and metabolism are called essential elements. They include carbon, hydrogen, oxygen, nitrogen, sulphur etc.

Classification of essential elements 

Classification of essential elements is done based on their requirement in the plant body and their functions. 

Classification of essential elements based on their requirement 

Essential elements are classified into two types based on their requirement as follows:

• Macronutrients 
• Micronutrients

Macronutrients

They are essential elements which are present in detectable quantities. It is normally present as 1 - 10 mg per gram of the dry matter. They are nine in number and include hydrogen, oxygen, carbon, sulphur, phosphorus, potassium, calcium, nitrogen and magnesium. It is believed that 96% of the dry matter of the plants is formed of hydrogen, carbon and oxygen only. Oxygen is the most abundant element in the plant body, when we consider the fresh weight. 

Micronutrients

They are present in plants only in trace amounts. They are present in less than 0.1 mg per gram of dry matter. They are seven in number. They include iron, copper, molybdenum, zinc, boron, chlorine and manganese. They are involved mainly in the functioning of enzymes. They act as metal activators or cofactors.

                                                          Fig: Types of elements

Classification of essential elements based on their function

Essential elements are classified into four broad categories based on their functions as follows:

Structural elements

These elements form the components of biomolecules and hence they act as structural elements of the cells. Examples include carbon, oxygen and hydrogen.

Energy related components

These elements form the components of the energy related chemical compounds in the cells. Examples include phosphorus in adenosine triphosphate or ATP.

                                     Fig: Phosphorus in ATP

Activators of enzymes

These elements activate or inhibit enzymes. Examples include magnesium which is the activator of ribulose bisphosphate carboxylase-oxygenase (RuBisCO) and phosphoenolpyruvate carboxylase, zinc is the activator of the enzyme alcohol dehydrogenase. 

Maintaining osmotic potential

Some essential elements determine the water potential of a cell. For example, potassium plays an important role in the opening and closing of stomata. 

     GIF: Opening and closing of stomata

Need of macronutrients

Plants require the macronutrients in their body to perform the following major functions:

• They help in the formation of the structural elements of cells.
• They help in the formation of energy related chemical compounds.
• They act as an activator or inhibitor of enzymes.
• They help in changing the osmotic pressure and permeability of the cell membrane.

Macronutrients and their roles in plants

Element	Requirements	Major functions
Nitrogen	It is required in the greatest amount by the whole plant. It is absorbed as NO3- (nitrate), NO2- (nitrite), and NH4+(ammonium ion). It is specifically needed by meristematic cells and metabolically active cells. It forms the part of proteins, cytochromes, auxins, cytokinins, DNA, RNA and enzymes. They promote vegetative growth.	Fig: Role of nitrogen
Calcium	It is absorbed in the form of Ca2+ from the soil. It is needed by the meristematic tissue and differentiating parts. It gets accumulated in older leaves. It forms part of the cell wall, middle lamella etc. It acts as the activator of ATPase and amylase enzymes. It is normally required for the formation of mitotic spindles.         Fig: Mitotic spindle formation	Fig: Role of calcium
Sulphur	It is absorbed by plants in the form of SO42- (sulphate). It is present in the amino acids like methionine and cysteine. It is the constituent of coenzyme A and vitamins like thiamine and biotin. Ferredoxins are small proteins containing sulphur and iron that are used in photosynthesis for electron transfer.       Fig: Cysteine	Fig: Role of sulphur
Phosphorus	It is absorbed by plants in the form of (phosphate ions) H2PO4- or HPO42- from the soil. It is present in the cell membrane, proteins, nucleic acids, and is required in the phosphorylation reactions. It forms part of ATP, NAD and NADP. It is required normally for energy transfer reactions in the plant body.	Fig: Role of phosphorus
Potassium	It is absorbed by plants in the form of  K+. It is needed by meristematic tissues, leaves, buds, and root tips in abundant quantities. It helps to maintain an anion-cation balance in the cells. It is involved in protein synthesis, opening and closing of stomata, and activation of enzymes.	Fig: Role of potassium
Magnesium	It is absorbed by plants in the form of  Mg2+. It is present in the centre of the ring structure of chlorophyll. It maintains the structure of the ribosome. It activates enzymes related to respiration, photosynthesis and synthesis of DNA and RNA.            Fig: Structure of chlorophyll	Fig: Role of magnesium
Oxygen	Plants take in oxygen through stomata. They also take oxygen dissolved in the soil through roots.	It forms the part of structural components. It is also required for metabolic reactions.
Carbon	Plants take carbon dioxide through stomata and fix it.	It forms the part of structural components. It is also required for metabolic reactions.
Hydrogen	Plants absorb water with the help of root hairs and utilise the hydrogen from this.	It forms the part of structural components. It is also required for metabolic reactions.

Practice Problems

Q 1. Assertion : Magnesium is important in photosynthesis and carbohydrate metabolism.

Reason : Mg++ is involved in the synthesis of nucleic acids.

A) Both the assertion and the reason are true and the reason is the correct explanation of the assertion
B) Both the assertion and reason are true but the reason is not the correct explanation of the assertion
C) The assertion is true but the reason is false
D) Both the assertion and reason are false

Answer: Magnesium is a constituent of the chlorophyll molecule, without which photosynthesis would not occur. The enzymes commonly involved in carbohydrate metabolism also require magnesium as an activator. Magnesium also acts as an activator for those enzymes involved in the synthesis of nucleic acids (DNA, RNA). Hence option B is correct.

Q 2. Assertion : Iron takes part in the electron transport system of mitochondria.

Reason : Iron has no role in chlorophyll synthesis.

A) Both the assertion and the reason are true and the reason is the correct explanation of the assertion
B) Both the assertion and reason are true but the reason is not the correct explanation of the assertion
C) The assertion is true but the reason is false
D) Both the assertion and reason are false

Answer: Iron is incorporated directly into the cytochromes and into compounds necessary for the electron transport system in mitochondria. Iron is normally required for the synthesis of proteins, chloroplasts and the enzymes involved in the synthesis of chlorophylls. Hence option C is correct.

Q 3. How do macronutrients affect plant growth?

Answer: Macronutrients are essential for plant growth and a good overall state of the plant. The primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K). For example, nitrogen is essential for plant development, since it plays a fundamental role in energy metabolism and protein synthesis.

Q 4. How do plants absorb macronutrients?

Answer: Plants take up essential elements from the soil through their roots and from the air (mainly consisting of nitrogen and oxygen) through their leaves. Nutrient uptake in the soil is taking place mainly by cation exchange, where root hairs pump out hydrogen ions (H+) into the soil with the help of proton pumps.

FAQs

Q 1. What do macronutrients do for the plants? 
Answer: The macronutrients help create new plant cells which organise into the plant tissue. Without these nutrients, growth and survival of the plants will not occur.

Q 2. Which are the three macronutrient elements most commonly used in fertilisers?\
Answer: Nitrogen, phosphorus and potassium, or NPK, are the ‘Big 3’ primary nutrients in commercial fertilisers. These fundamental nutrients normally play the major role in plant nutrition. For example, nitrogen is the most important nutrient and compared to any other element plants absorb more nitrogen.

Q 3. What happens when soil receives too much nitrogen or phosphorus?
Answer: Excess nitrogen will kill plants. Plants tend to be able to tolerate higher amounts of (NO3-) or nitrate than NH4+ (ammonium). However, it can still reach toxic levels. Excess nitrogen causes excessive growth of the plant which reduces stem strength. This can lead to lodging during flowering and grain filling. Excessive amounts of soil phosphorus reduces the ability of the plants to take up required micronutrients like zinc and iron. The leaf will turn yellow while the veins remain green here. 

Q 4. What happens to plants without nitrogen?
Answer: Plants that are deficient in nitrogen have stunted growth, depending on the severity of the deficiency. Leaf growth is inhibited; younger leaves are inhibited in particular. Longitudinal shoot growth is inhibited and it affects the thickness also.

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