NCERT Solutions for Class 11 Biology Chapter 12: Mineral Nutrition
1. All elements that are present in a plant need not be essential to its survival. Comment.
Ans. Plants tend to absorb different kinds of nutrients from the soil. However not all elements are essential nutrients for a plant as they are not essential for the plant physiology and metabolism. For example, plants growing near radioactive sites tend to accumulate radioactive metals. Similarly, gold and selenium get accumulated in plants growing near mining sites. However, this does not mean that radioactive metals, gold, or selenium are essential nutrients for the survival of these plants.
2. Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?
Ans. In hydroponics, carefully prepared standardized nutrient solutions are used for growing plants. These nutrients are taken up by the roots of the plants. Impure water contains a large number of impurities in the form of soluble minerals dissolved in it. Salts also contain impurities. It is essential to purify the water we use and remove any minerals present in it. Otherwise standard nutrient solution with fixed concentration of minerals will not be possible to maintain for proper growth of the plants. Any minerals if present previously and not removed from the water will cause an error in proper resolution. Hence purification of water and minerals is
required for obtaining maximum or optimum growth of plants.
3. Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
Ans. Macronutrients: These are the elements which are generally present in plant tissues in large amounts (more than 10 millimoles/kg dry matter) and are involved in the synthesis of organic molecule, development of osmotic potential are called macronutrients or macro element. They are 17 in number, e.g., carbon, oxygen, nitrogen, sulphur, potassium, calcium and magnesium etc.
Micronutrients: These are the elements which are required by plants in very small amounts (less than 10 millimoles/ kg dry matter) are called micronutrients, e.g., iron, zinc, manganese, boron, copper, molybdenum, chlorine and nickel. These elements are mostly involved in the functioning of enzymes as activators.
Beneficial nutrients: These are the nutrients which are required by higher plants along with the macro and micro elements are called beneficial nutrients, e.g., sodium, silicon, cobalt and selenium. These nutrients are mostly involved in the functioning of enzymes as activators.
Toxic elements: There is a narrow range of concentration at which the elements are optimum. Any mineral ion concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered as toxic.
Essential elements: Any element required by living organisms to ensure normal growth and maintenance, metabolism and causes deficiency symptoms if not supplied to the plant from medium is called essential element, e.g., C, H, O, N, P, K, S, Mg, Ca, Mn, Cu, Mo, Zn, B, Cl, etc. Potassium play an important role in opening
and closing of stomata, protein synthesis etc. Magnesium is essential for chlorophyll and phosphorus in ATP. Mg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn2+ is an activator of alcohol
dehydrogenase and Mo of nitrogenase during nitrogen metabolism.
4. Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
Ans. Five different deficiency symptoms in plants are:
(a) Chlorosis: It is the loss of chlorophyll leading to yellowing of leaves. This is caused by the deficiencies of N, K, Mg, S and Fe etc.
(b) Necrosis: Killing or death of tissue particularly leaf is called necrosis. This is caused due to deficiency of Ca, Mg, Cu and K, etc.
(c) Whiptail: Degeneration of lamina but not of petiole and mid rib, caused by deficiency of molybdenum.
(d) Die back: It is the killing of shoot apex i.e., stem tip and young leaves. This is caused by the deficiency of K and Cu.
(e) Little leaf disease: Small sized leaves, caused by zinc deficiency.
5. If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?
Ans. Since each element has one or more specific structural or functional role in plants, in the absence of any particular element, plants show certain morphological changes. The parts of the plants that show the deficiency symptoms also depend on the mobility of the element in the plant. For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, bio molecules containing these elements are broken down, making these elements available for mobilising to younger leaves. Another point to be considered is that different plants respond in different ways to the deficiency of the same nutrient. Hence, to identify the nutrient deficient in a plant, all the symptoms developed in its different parts must be studied and compared with the available standard tables.
6. Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
Ans. The occurrence of deficiency symptoms in plant parts depends on the mobility of deficient elements in the plants. These play an important role to determine that deficiency symptoms appear first in younger or older parts of plants. Some elements like, N, and K are highly mobile and move from older parts to younger parts of plants or where they are most needed, so the symptoms of deficiencies of these elements first appear in older parts of the plants. On the other side, some elements like as Ca and S are immobile as these elements are not transported from the older parts of plants, so the symptoms of deficiencies have first appeared in younger parts of the plant.
7. How are the minerals absorbed by the plants?
Ans. Plants absorb minerals in the form of ions from the soil. In order to absorb minerals, they should be dissolved in water. Plants absorb minerals through the roots. The larger the surface area of roots, the more minerals get absorbed.
There are two modes of mineral absorption:
- Passive mineral absorption: It does not require the expenditure of energy and called passive process. It occurs along the higher to lower concentration gradient through diffusion process.
- Active mineral absorption: It is an active process and requires the expenditure of ATP (Adenosine triphosphate). It can occur both along and against the concentration gradient by simple osmosis or through special carrier proteins in the plasma membrane.
There are three main mechanisms of mineral absorption:
- Ion exchange: In this method the ions are adsorbed on the surface of the root can exchange with the ions of the same type charge from the soil solution. For example – Hydrogen ions adsorbed on the surface of root cells can be exchanged with potassium ions in the soil.
- Carrier concept: As per this theory, the plasma membrane is completely impermeable to some ions. The absorption of these ions is facilitated by some special proteins on the plasma membrane called carrier proteins. Here first the ions are combined with carrier protein to form a carrier–ion complex. The carrier-ion complex moves across the plasma membrane. When it reaches to the inner surface of the membrane, it releases the ions into the lumen of the cell. After this it will go back to accept new ions.
- Donnan equilibrium: As per this theory, there are some diffusible ions that are present inside the root cells. These ions do not diffuse outside through the plasma membrane. If a cell has cations as fixed ions, such a cell can absorb anions from the soil in order to maintain electrical potential balance. The absorption of anions causes Donnan equilibrium of anions and cations. By attaining equilibrium the root cells can absorb any mineral in their ionic form.
8. What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N2 -fixation
Ans. The conditions necessary for fixation of atmospheric nitrogen by Rhizobium and their roles in nitrogen fixation are as follows:
- Reducing environment: A reducing environment is necessary for the action of enzyme nitrogenase which is sensitive to free oxygen.
- Presence of nitrogenase enzyme: A Mo-Fe protein — that helps in the conversion of atmospheric free nitrogen into ammonia.
- Presence of leghaemoglobin: The root nodules containing Rhizobium contains leghaemoglobin, which protects nitrogenase from oxygen.
- Source of energy as ATP: Nitrogen fixation is energy requiring process thus ATP is required.
- Source of reducing pow
er NAD(P), H2 or FMNH2-
- Ferredoxin as the electron donor: Ferredoxin makes nitrogen reactive.
- Magnesium ions as cofactors: Magnesium act as a cofactor and helps in the activity of nitrogenase.
9. What are the steps involved in formation of a root nodule?
Ans. Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant.
Principal stages in the nodule formation are summarised as follows:
Rhizobia multiply and colonise the surroundings of roots and get attached to epidermal and root hair cells. The root-hairs curl and the bacteria invade the root-hair. An infection thread is produced carrying the bacteria into the cortex of the root, where they initiate the nodule formation in the cortex of the root. Then the bacteria are released from the thread into the cells which leads to the differentiation of specialised nitrogen fixing cells. The nodule thus formed, establishes a direct vascular connection with the host for exchange of nutrients. These events are depicted in given Figure.
The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenise is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia.
10. Which of the following statements are true? If false, correct them:
(a) Boron deficiency leads to stout axis.
(b) Every mineral element that is present in a cell is needed by the cell.
(c) Nitrogen as a nutrient element, is highly immobile in the plants.
(d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.
Ans. (a) True
All the mineral elements present in a cell are not needed by the cell. For example, plants growing near radioactive mining sites tend to accumulate large amounts of radioactive compounds. These compounds are not essential for plants.
Nitrogen as a nutrient element is highly mobile in plants. It can be mobilised from the old and mature parts of a plant to its younger parts.