GSEB Class 11 Biology Solutions Chapter 12 Mineral Nutrition

Question 1. “All elements that are present in a plant need not be essential to its survival”. Comment.
Answer: The criteria for an element's essentiality are provided below:

• The element must be absolutely vital for supporting normal growth and reproduction. If the element is absent, plants cannot finish their life cycle or produce seeds.
• The element's requirement must be specific and not replaceable by any other element. In other words, a deficiency in one element cannot be resolved by supplying a different one.
• The element must directly participate in the plant's metabolic processes.

Exam Tip: When commenting on a statement, always define the key terms (like "essential elements" here) and then use those definitions to support your argument.

Question 2. Why is the purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?
Answer: In 1860, Julius Sachs, a well-known German botanist, showed for the first time that plants could grow to maturity in a specific nutrient solution without any soil. This method of growing plants in a nutrient solution is called hydroponics. Since then, many improved methods have been used to determine the necessary mineral nutrients for plants. The main idea of all these methods involves growing plants in a soil-free, clear mineral solution. These methods require purified water and mineral nutrient salts. After many experiments where plant roots were placed in nutrient solutions, and where one element was added, removed, or given in different amounts, a mineral solution suitable for plant growth was found. Through this method, essential elements were identified, and their deficiency symptoms were discovered. Hydroponics has been successfully used as a method for the commercial farming of vegetables like tomatoes, seedless cucumbers, and lettuce. It is important to remember that the nutrient solution must be properly aerated to get the best growth.
In simple words: Purified water and nutrient salts are crucial in hydroponics because even tiny impurities can affect plant growth and skew results, making it hard to identify specific essential minerals. Clean solutions ensure that scientists can accurately study what each mineral does for the plant.

Exam Tip: When discussing experimental techniques like hydroponics, always highlight why specific controls (like purification of water and nutrients) are important for obtaining accurate and reliable results.

Question 3. Explain with examples macronutrients, micronutrients, beneficial nutrients, toxic elements, and essential elements.
Answer:
Macronutrients: These are generally found in large quantities within plant tissues. Macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium. Out of these, carbon, hydrogen, and oxygen are mostly obtained from \( \text{CO}_2 \) and \( \text{H}_2\text{O} \), while plants absorb the others from the soil as mineral nutrition.
Micronutrients: Also known as trace elements, these are required in very tiny amounts. These include iron, manganese, copper, molybdenum, zinc, boron, chlorine, and nickel.
Beneficial nutrients: These include sodium, silicon, cobalt, and selenium. Higher plants need these.
Toxic elements: Any mineral ion concentration in plant tissues that lowers the dry weight of tissues by about 10 percent is considered harmful. The toxicity symptoms are hard to identify. Too many micronutrients can cause toxicity.
Essential elements: These are grouped into 4 categories based on their various functions.
(a) As components of biomolecules and thus structural elements of cells. For example, carbon, hydrogen, oxygen, and nitrogen.
(b) As components of energy-related chemical compounds in plants. For example, magnesium in chlorophyll.
(c) Elements that activate or inhibit enzymes. For example, \( \text{Mg}^{2+} \) activates both ribulose bisphosphate carboxylase oxygenase and phosphoenolpyruvate carboxylase.
(d) Elements that change the osmotic potential of a cell. For example, potassium helps in the opening and closing of stomata.In simple words: Macronutrients are needed in big amounts (like nitrogen), while micronutrients are needed in small amounts (like iron). Beneficial nutrients help plants but aren't always essential (like sodium). Toxic elements harm plants when levels are too high. Essential elements are vital for growth and fit into four main roles: building cells, energy compounds, activating enzymes, or changing cell water balance.

Exam Tip: When explaining nutrient types, always provide clear definitions and at least two examples for each category to illustrate your understanding fully.

Question 4. Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
Answer: Deficiency symptoms observed in plants include:
1. Chlorosis
2. Necrosis
3. Stunted plant growth
4. Premature fall of leaves/buds and
5. Inhibition of cell division
Chlorosis is when chlorophyll is lost, causing leaves to turn yellow. This symptom is caused by a lack of elements like N, K, Mg, S, Fe, Mn, Zn, and Mo.
Similarly, necrosis, which is the death of tissue, especially leaf tissue, happens due to a lack of Ca, Mg, and K. Low levels of N, K, S, and Mo lead to the inhibition of cell division. Some elements, such as N, S, and Mo, can delay flowering if their concentration in plants is low.In simple words: Plants show various signs when they don't get enough minerals. Yellowing leaves (chlorosis) come from lack of nitrogen, potassium, or magnesium. Tissue death (necrosis) happens with low calcium or magnesium. Slow growth (stunted growth) is linked to many deficiencies. Leaves falling early and slow cell division also point to specific mineral shortages.

Exam Tip: When asked to describe deficiency symptoms, it's important to name the symptom, explain what it looks like, and then list the specific mineral deficiencies that cause it for full marks.

Question 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?
Answer: Since each element has one or more specific structural or functional functions in plants, in the absence of any particular element, plants exhibit certain morphological changes. These changes in form indicate certain element deficiencies and are called deficiency symptoms. The deficiency symptoms vary from element to element, and they disappear when the missing mineral nutrient is supplied to the plant. However, if the deprivation continues, it may later cause the plant to die. In this way, by giving different nutrients and observing whether the symptoms vanish, we can experimentally discover the actual deficient mineral element.
In simple words: To find the missing element when symptoms could mean several things, scientists add different nutrients one by one to a plant. Whichever nutrient makes the symptoms disappear is the one the plant was lacking.

Exam Tip: When designing an experiment to identify a single variable (like a deficient nutrient), remember to isolate each potential variable and test it individually to ensure a clear conclusion.

Question 6. Why is it that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
Answer: The deficiency symptoms vary from element to element, and they disappear when the missing mineral nutrient is supplied to the plant. However, if deprivation continues, it may eventually lead to the plant's death. The parts of the plants that show deficiency symptoms also depend on how mobile the element is within the plant. When elements are actively moved within the plant and sent to young developing tissues, the deficiency symptoms tend to show up first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium, and magnesium are first seen in the older, dying leaves. In the older leaves, biomolecules containing these elements are broken down, making these elements available to move to younger leaves. The deficiency symptoms tend to show up first in the young tissues when elements are relatively fixed and are not transported out of the mature organs. For example, elements like sulfur and calcium are part of the cell's structural component and are therefore not easily changed.
In simple words: Deficiency signs show up in young parts if the nutrient cannot move easily from old to new growth (like calcium). They appear in old parts if the nutrient can move and is taken from old leaves to support new growth (like nitrogen). It depends on how mobile the element is inside the plant.

Exam Tip: Remember that the mobility of an element within the plant is the key factor determining whether deficiency symptoms appear first in young or mature leaves. Immobile elements affect new growth, while mobile elements are relocated, affecting older leaves first.

Question 7. How are the minerals absorbed by the plants?
Answer: Plants take in essential elements from the soil through their roots and from the air through their leaves. Nutrient uptake in the soil occurs through cation exchange, where root hairs pump hydrogen ions into the soil using proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles, making the cations available for uptake by the root. In the leaves, stomata open to take in carbon dioxide and release oxygen. The carbon dioxide molecules are utilized as the carbon source during photosynthesis.
In simple words: Plants get minerals from soil through roots by swapping hydrogen ions for mineral ions. They also take in carbon dioxide from the air through tiny openings in leaves called stomata, which they use for making food.

Exam Tip: Focus on the two main absorption sites: roots (cation exchange, proton pumps) and leaves (stomata for gases), and specify the mechanism for each.

Question 8. What are the conditions necessary for the fixation of atmospheric nitrogen by Rhizobium? What is their role in \( \text{NO}_2 \) fixation?
Answer: Many types of symbiotic biological nitrogen-fixing associations are known. The most prominent among them is the legume-bacteria relationship. Rod-shaped Rhizobium species have this kind of relationship with the roots of various legumes, such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc. The most common association on roots forms nodules. These nodules are small outgrowths on the roots. The microbe, Frankia, also produces nitrogen-fixing nodules on the roots of non-leguminous plants. Both Rhizobium and Frankia are free-living in the soil, but as symbionts, they can fix atmospheric nitrogen.
In simple words: Rhizobium needs a symbiotic relationship with legume roots to fix atmospheric nitrogen into a usable form for plants. They form root nodules, which create the right environment for this process. Their role is to convert atmospheric nitrogen into compounds like ammonia, which plants can then use.

Exam Tip: When discussing nitrogen fixation, always mention the symbiotic relationship (Rhizobium-legumes), the formation of nodules, and the final product (ammonia) as these are key points.

Question 9. What are the steps involved in the formation of a root nodule?
Answer: Nodule formation involves a series of multiple interactions between Rhizobium and the host plant's roots. The main stages in nodule formation are summarized as follows:
Rhizobia multiply and colonize the areas around roots, attaching to epidermal and root hair cells. The root hairs curl, and the bacteria enter the root hair. An infection thread is formed, carrying the bacteria into the root's cortex, where they start nodule formation. Then, the bacteria are released from the thread into the cells, leading to the differentiation of specialized nitrogen-fixing cells. The nodule thus created establishes a direct vascular connection with the host for nutrient exchange.
The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenase is a Mo-Fe protein that speeds up the conversion of atmospheric nitrogen to ammonia, which is the first stable product of nitrogen fixation. The reaction is as follows:
\( \text{N}_2 + 8\text{e}^- + 8\text{H}^+ + 16\text{ATP} \implies 2\text{NH}_3 + \text{H}_2 + 16\text{ADP} + 16\text{Pi} \)
The enzyme nitrogenase is very sensitive to molecular oxygen; it needs anaerobic conditions. The nodules have adaptations that ensure the enzyme is protected from oxygen. To protect these enzymes, the nodule contains an oxygen scavenger called leghaemoglobin (Lb). It is interesting to note that these microbes live as aerobes under free-living conditions (where nitrogenase does not work), but during nitrogen-fixing events, they become anaerobic (thereby protecting the nitrogenase enzyme).
In simple words: Root nodule formation starts with Rhizobia gathering around roots, curling root hairs, and entering the root. They form an "infection thread" that moves them into the root's inner part, where they trigger specialized nitrogen-fixing cells to grow into a nodule. This nodule then helps the plant fix nitrogen from the air.

Exam Tip: When detailing a biological process, breaking it down into sequential steps makes the explanation clearer and easier to follow for the examiner.

Question 10. Which of the following statements are true? If false, correct them:
(a) Boron deficiency leads to the stout axis.
Answer: (a) False. Boron deficiency causes the death of root and stem tips, stunting of stem and root growth, and chlorosis. It does not lead to a stout axis.
(b) Every mineral element that is present in a cell is needed by the cell.
Answer: (b) False. Not all mineral elements present in a cell are needed by the cell. For example, plants growing near radioactive mining sites tend to gather large amounts of radioactive compounds, which are not essential.
(c) Nitrogen as a nutrient element, is highly immobile in plants.
Answer: (c) False. Nitrogen, as a nutrient element, is highly mobile in plants. It can be moved from the old and mature parts of a plant to its younger parts.
(d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.
Answer: (d) True.
In simple words: (a) False, boron deficiency causes stunted growth and tip death, not a stout axis. (b) False, not every element found in a cell is necessary for it. (c) False, nitrogen moves easily within plants from older to younger parts. (d) True, it's easy to show micronutrients are essential because plants need them in very small amounts.

Exam Tip: For true/false questions, always provide a clear "True" or "False" statement and, if false, offer a concise correction explaining why the original statement is incorrect.