Maharashtra Board Class 12 Biology Chapter 7 Plant Growth and Mineral Nutrition Solutions

ℹ️ चित्र व्याख्या (Diagram Explanation): पौधे की वृद्धि और खनिज पोषण से संबंधित कक्षा 12 जीव विज्ञान अध्याय 7 को दर्शाने वाला यह चित्र एक शिक्षिका को छात्रों को समझाते हुए दिखाता है। इसमें इस अध्याय के मुख्य विषय को सुंदर और रंगीन तरीके से प्रस्तुत किया गया है।

Multiple Choice Questions

 

Question 1. Which of the hormone can replace vernalization ?
(a) Auxin
(b) Cytokinin
(c) Gibberellins
(d) Ethylene
Answer: (c) Gibberellins
In simple words: Vernalization is a process where cold treatment promotes flowering. Gibberellins are plant hormones that can mimic this effect, causing plants to flower without needing a cold period.

🎯 Exam Tip: This question tests knowledge of plant hormones and their specific roles, particularly in relation to vernalization. Focus on understanding the functions of each hormone.

 

Question 2. The principle pathway of water translocation in angiosperms is ........................
(a) Sieve cells
(b) Sieve tube elements
(c) Xylem
(d) Xylem and phloem
Answer: (c) Xylem
In simple words: In flowering plants, water is transported from roots to leaves through the xylem, which acts as the main pipeline for water movement.

🎯 Exam Tip: Remember the primary functions of xylem and phloem. Xylem transports water and minerals, while phloem transports food.

 

Question 3. Abscissic acid controls
(a) cell division
(b) leaf fall and dormancy
(c) shoot elongation
(d) cell elongation and wall formation
Answer: (b) leaf fall and dormancy
In simple words: Abscisic acid (ABA) is a plant hormone that primarily helps plants cope with stress by promoting dormancy in buds and seeds, and accelerating leaf drop.

🎯 Exam Tip: Abscisic acid is known as the "stress hormone" due to its role in regulating responses to adverse environmental conditions like drought.

 

Question 4. Which is employed for artificial ripening of banana fruits?
(a) Auxin
(b) Ethylene
(c) Cytokinin
(d) Gibberellin
Answer: (b) Ethylene
In simple words: Ethylene is a gaseous plant hormone widely used to artificially ripen fruits like bananas, as it triggers the natural ripening process.

🎯 Exam Tip: Ethylene is unique among plant hormones for its gaseous nature and its critical role in fruit ripening and senescence.

 

Question 5. Which of the following is required for stimulation of flowering in plants?
(a) Adequate oxygen
(b) Definite photoperiod
(c) Adequate water
(d) Water and minerals
Answer: (b) Definite photoperiod
In simple words: Flowering in many plants is triggered by a specific duration of light and dark periods, known as photoperiod, which is essential for initiating the reproductive phase.

🎯 Exam Tip: Photoperiodism is a crucial factor in flowering, categorizing plants into short-day, long-day, and day-neutral types based on their light requirements.

 

Question 6. For short day plants, the critical period is ........................
(a) light
(b) dark/night
(c) UV rays
(d) Both (a) and (c)
Answer: (b) dark/night
In simple words: Short day plants require a continuous dark period longer than a critical length to flower, meaning the duration of darkness is more critical than the light period.

🎯 Exam Tip: For short-day plants, an uninterrupted dark period is key. If the dark period is broken by a flash of light, flowering may be inhibited.

 

Question 7. Which of the following is NOT day neutral plant?
(a) Tomato
(b) Cotton
(c) Sunflower
(d) Soybean
Answer: (d) Soybean
In simple words: Soybean is a short-day plant, meaning its flowering is influenced by the length of the day, unlike day-neutral plants whose flowering is unaffected by photoperiod.

🎯 Exam Tip: Be familiar with examples of short-day, long-day, and day-neutral plants to correctly answer questions about photoperiodism.

 

Question 8. Essential macro elements are ........................
(a) manufactured during photosynthesis
(b) produced by enzymes
(c) obtained from soil
(d) produced by growth hormones
Answer: (c) obtained from soil
In simple words: Essential macroelements are vital nutrients that plants absorb in large quantities primarily from the soil to support their growth and metabolic functions.

🎯 Exam Tip: Distinguish between macronutrients and micronutrients, remembering that both are obtained externally, mainly from the soil or atmosphere.

 

Question 9. Function of Zinc is ........................
(a) closing of stomata
(b) biosynthesis of 3-LAA
(c) synthesis of chlorophyll
(d) oxidation of carbohydrates
Answer: (b) biosynthesis of 3-LAA
In simple words: Zinc plays a crucial role in plant metabolism, particularly as a cofactor for enzymes involved in the biosynthesis of Indole Acetic Acid (IAA), a vital plant growth hormone.

🎯 Exam Tip: Recall the specific roles of various micronutrients; Zinc is important for auxin synthesis and enzyme activation.

 

Question 10. Necrosis means ........................
(a) yellow spot on the leaves
(b) death of tissue
(c) darkening of green colour in leaves
(d) wilting of leaves
Answer: (b) death of tissue
In simple words: Necrosis refers to the localized death of plant tissue, often caused by nutrient deficiencies, diseases, or environmental stresses, leading to visible damage.

🎯 Exam Tip: Be familiar with common plant deficiency symptoms and their associated terms, such as chlorosis (yellowing) and necrosis (tissue death).

 

Question 11. Conversion of nitrates to nitrogen is called ........................
(a) ammonification
(b) nitrification
(c) nitrogen fixation
(d) denitrification
Answer: (d) denitrification
In simple words: Denitrification is a microbial process where nitrates in the soil are converted back into atmospheric nitrogen gas, effectively removing nitrogen from the ecosystem.

🎯 Exam Tip: Understand the different steps of the nitrogen cycle: nitrogen fixation, ammonification, nitrification, and denitrification, and the organisms involved in each.

 

Question 12. How many molecules of ATP are required to fix one molecule of nitrogen?
(a) 12
(b) 20
(c) 6
(d) 16
Answer: (d) 16
In simple words: Nitrogen fixation is an energy-intensive process, requiring 16 molecules of ATP to convert one molecule of atmospheric nitrogen into ammonia, making nitrogen available for plants.

🎯 Exam Tip: Remember the high energy cost associated with biological nitrogen fixation, highlighting its importance in making atmospheric nitrogen usable for life.

Very Short Answer Questions

 

Question 1. Enlist the phases of growth in plants.
Answer: The three phases of growth are phase of cell division, phase of cell enlargement and phase of cell maturation.
In simple words: Plant growth occurs in three distinct stages: cells first divide to increase number, then enlarge in size, and finally mature to specialize in function.

🎯 Exam Tip: These three phases are fundamental to understanding how plants increase in size and complexity.

 

Question 2. Give full form of IAA.
Answer: Full form is Indole Acetic Acid.
In simple words: IAA stands for Indole Acetic Acid, which is a key plant hormone belonging to the auxin group, essential for cell elongation and growth.

🎯 Exam Tip: Memorize the full forms of common abbreviations related to plant hormones, as they are frequently tested.

 

Question 3. What does it mean by 'open growth'?
Answer: In plants the growth is indeterminate and takes place throughout the life at specific regions having meristems.
In simple words: Open growth means that plants can grow continuously throughout their lifespan because they possess specialized regions called meristems that constantly produce new cells.

🎯 Exam Tip: Open growth, or indeterminate growth, is a characteristic feature of plants, enabling them to adapt and respond to environmental conditions over extended periods.

 

Question 4. Plant stress hormone.
Answer: Abscissic acid.
In simple words: Abscisic acid (ABA) is often called the "stress hormone" because it helps plants respond to adverse conditions like drought, promoting stomatal closure and dormancy.

🎯 Exam Tip: Associate abscisic acid directly with plant responses to stress and dormancy mechanisms.

 

Question 5. What is denitrification?
Answer: Anaerobic bacteria can convert nitrates of soil back into nitrogen gas. That process performed by denitrifying bacteria is denitrification.
In simple words: Denitrification is the biological process where certain anaerobic bacteria convert nitrates in the soil into nitrogen gas, releasing it back into the atmosphere.

🎯 Exam Tip: Denitrification is an important part of the nitrogen cycle that returns nitrogen to the atmosphere, balancing nitrogen fixation.

 

Question 6. Bacteria responsible for conversion of nitrite to nitrate.
Answer: Nitrobacter.
In simple words: Nitrobacter is a specific type of bacterium that plays a key role in the nitrogen cycle by converting nitrite into nitrate, a form readily usable by plants.

🎯 Exam Tip: Recall the bacteria involved in the nitrification process: Nitrosomonas converts ammonia to nitrite, and Nitrobacter converts nitrite to nitrate.

 

Question 7. What is the role of gibberellins in rosette plants?
Answer: In rosette plants like beet and cabbage, bolting, i.e. elongation of internodes before flowering is observed due to effect of gibberellins.
In simple words: In rosette plants, gibberellins promote "bolting," which is the rapid elongation of stems (internodes) before flowering, lifting the flowers above the compact leaf cluster.

🎯 Exam Tip: Gibberellins are known for their role in stem elongation, seed germination, and breaking dormancy, and their effect on bolting in rosette plants is a classic example.

 

Question 8. Vernalization
Answer: The response of plant to the influence of low temperature on flowering in plants is called vernalization.
In simple words: Vernalization is a process where plants are exposed to a period of cold temperatures to promote or accelerate flowering.

🎯 Exam Tip: Vernalization is important for many temperate zone plants to ensure flowering occurs at an optimal time in their life cycle.

 

Question 9. Photoperiodism
Answer: The response of plant to the influence of light for initiation of flowering is known as photoperiodism.
In simple words: Photoperiodism is a plant's physiological response to the relative lengths of day and night, primarily influencing the timing of flowering.

🎯 Exam Tip: Photoperiodism helps plants synchronize their flowering with favorable seasonal conditions.

 

Question 10. What is grand period of growth?
Answer: There are three phases of growth and the total time required for all phases to occur is called grand period of growth.
In simple words: The grand period of growth is the entire time span from the initial phase of cell division, through enlargement, to final maturation of cells in a plant organ.

🎯 Exam Tip: The grand period encompasses the lag, log (exponential), and stationary phases of the typical S-shaped growth curve.

Short Answer Questions

 

Question 1.
(i) Differentiation
Answer:
1. It is a process of maturation of cells derived from apical meristems.
2. Differentiation is a permanent change in structure and function of cells that leads to its maturation.
3. Cell undergoes major anatomical and physiological change during differentiation process.
4. In hydrophytic plants parenchyma cells develop large schizogenous cavities which help them in aeration, buoyancy and mechanical support.
In simple words: Differentiation is the process where newly formed cells from meristems undergo permanent changes in their structure and function to become specialized cell types.

🎯 Exam Tip: Differentiation is crucial for forming various tissues and organs, enabling the plant to perform diverse functions.

 

(ii) Redifferentiation
Answer:
1. It is a process in which cells produced by de-differentiation lose their capacity of division and become mature.
2. The cells mature to perform specific function.
3. Interfascicular cambium is formed by process of dedifferentiation loses its capacity to divide.
4. Secondary xylem and secondary phloem is formed form this cambium in vascular cylinder.
In simple words: Redifferentiation is when cells that have already dedifferentiated (regained division capacity) mature again and become specialized to perform a specific function, thus losing their division capacity once more.

🎯 Exam Tip: Understand the sequence: Differentiation -> Dedifferentiation (cells become meristematic again) -> Redifferentiation (dedifferentiated cells specialize again).

 

Question 2. Arithmetic growth and Geometric growth
Answer:

Arithmetic growth	Geometric growth
1. In arithmetic growth only one daughter cell continues to divide, while the other undergoes differentiation and maturation.	1. In geometric growth both the daughter cells continue to divide and redivide again and again.
2. Rate of growth is constant.	2 Rate growth is initially slow but later on rapid rate.
3. Linear curve is obtained.	3. Exponential curve is obtained.
4. Mathematical expression is \(L_t = L_o + rt\) where \(L_t\) = length of time 't' \(L_o\) = Length at time zero \(r_t\) = growth rate, \(t\) = time of growth	4. Mathematical expression is \(W_t = W_o e^{rt}\) where, \(W_t\) = final size, \(W_o\) = initial size, \(r\) = growth rate, \(t\) \(E\) = base of natural logarithm
5. e.g. Elongation of root	5. e.g. Divisions of zygote during embryo development.

In simple words: Arithmetic growth is a steady increase where only one daughter cell continues to divide, while geometric growth involves both daughter cells dividing repeatedly, leading to an exponential increase.

🎯 Exam Tip: Differentiate between arithmetic and geometric growth by their characteristic curves (linear vs. S-shaped) and how their cell division patterns differ.

 

Question 3. Enlist the role and deficiency symptoms of: (a) nitrogen (b) phosphorus (c) potassium.
Answer:
(a) Nitrogen:
Role: Constituent of proteins as amino acids, nucleic acids, vitamins, hormones, coenzymes, ATP and chlorophyll molecule.
Deficiency symptoms : stunted growth and chlorosis.
(b) Phosphorus:
Role: Constituent of cell membrane, certain proteins, nucleic acids and nucleotides, required for all phosphorylation reactions.
Deficiency symptoms : Poor growth, leaves dull green
(c) Potassium :
Role: Determination of anion - cation balance in cell, necessary for protein synthesis, involved in formation of cell membrane, opening and closing of stomata, activates enzymes, helps in maintenance of turgidity of cells.
Deficiency symptom : Yellow edges in leaves, premature death.
In simple words: Nitrogen, phosphorus, and potassium are essential macronutrients; nitrogen is key for proteins and chlorophyll, phosphorus for energy and membranes, and potassium for water balance and enzyme activation, with each having distinct deficiency symptoms.

🎯 Exam Tip: Memorize the specific roles and visual deficiency symptoms of the primary macronutrients (N, P, K) as they are frequently assessed.

 

Question 4. What is short day plant? Give any two examples.
Answer: The plants which flower when the day length or light period is shorter than the critical photoperiod are called short day plants or SDP
SDPs usually flower during winter and late summer.
Examples - Dahlia, Aster, Tobacco, Chrysanthemum, Soybean (Glycine max) and Cocklebur (Xanthium).
In simple words: Short-day plants flower when the duration of light is shorter than a critical period, meaning they require long, uninterrupted nights to initiate flowering.

🎯 Exam Tip: For short-day plants, the critical factor is the length of the dark period; a long, continuous dark phase triggers flowering.

 

Question 5. What is vernalization? Give its significance.
Answer: A low temperature or chilling treatment that induces early flowering in plants is known as vernalization.
Significance:
1. Due to chilling treatment crops can be produced earlier.
2. Crops can be grown in areas where they do not grow naturally.
In simple words: Vernalization is a cold treatment applied to plants that promotes early flowering, enabling earlier crop production and extending the cultivation range of certain plants.

🎯 Exam Tip: Vernalization allows plants to synchronize their flowering with favorable growing seasons, preventing premature development.

Long Answer Questions

 

Question 1. Explain sigmoid growth curve with the help of diagram.
Answer:
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र पौधों में 'सिग्मॉइड ग्रोथ कर्व' (S-आकार का वृद्धि वक्र) को दर्शाता है, जिसमें वृद्धि को समय के साथ अंग के आकार या वजन के रूप में दिखाया गया है। वक्र में तीन मुख्य चरण हैं: शुरुआती धीमी गति वाला 'लैग फेज', तेजी से बढ़ने वाला 'एक्सपोनेंशियल फेज', और अंत में स्थिर हो जाने वाला 'स्टेशनरी फेज' जहाँ वृद्धि दर कम हो जाती है।
1. When growth occurs in plants three distinct phases of growth are noticed.
2. Phase of cell formation is first phase where meristematic cells divide and new cells added.
3. In phase of cell enlargement newly formed cells elongate and with turgidity there is cell enlargement.
4. In phase of cell maturation cells get differentiated.
5. When we compare the growth rate it differs in these three phases.
6. In first phase or lag phase it is slow, while in log phase or exponential phase, growth rate accelerates and it reaches maximum.
7. In stationary phase of maturation growth rate slows down and comes to steady state.
8. When this changing rate of growth is plotted against time duration in a graph a sigmoid or S-shaped growth curve is obtained.
In simple words: The sigmoid growth curve illustrates plant growth over time, starting with a slow lag phase, accelerating rapidly in the exponential phase, and then stabilizing in the stationary phase as growth rate declines, forming an S-shape.

🎯 Exam Tip: Understand the characteristics of each phase (lag, log/exponential, stationary) and how they contribute to the overall S-shaped growth pattern.

 

Question 2. Describe the types of plants on the basis of photoperiod required, with the help of suitable examples.
Answer:
1. Effect of light duration on flowering of plants is known as photoperiodism.
2. Depending on photoperiodic response, plants are categorised into three types - Short day plants, long day plants and day neutral plants.
1. Short day plants : Plants that flower under short day length conditions are callec short day plants. Plants such as Dahlia, Xanthium, Soybean, Aster, Tobacco and Chrysanthemum are short day plants or SDR. Short day plants require a long uninterrupted dark period for flowering. Therefore, they are also called long night plants.
2. Long day plants : Plants that flower only when they are exposed to light period longer than their critical photoperiod are called long day plants or LDP Long day plants require a short dark or night period for flowering. Hence, they are also called short night plants. Plants such as radish, spinach, wheat, poppy, cabbage, pea, sugar beet, etc. are long day plants.
3. Day neutral plants : Plants in which the flowering is not affected by the day length period are called day neutral plants or DNP or photoneutral plants. Plants such as cucumber, sunflower, cotton, balsam, maize, tomato, etc. are day neutral plants.
In simple words: Plants are categorized as short-day, long-day, or day-neutral based on their flowering response to photoperiod; short-day plants need long nights, long-day plants need short nights, and day-neutral plants flower regardless of day length.

🎯 Exam Tip: For each category, remember the critical photoperiod requirement (or lack thereof) and at least two to three common examples.

 

Question 3. Explain biological nitrogen fixation with example.
Answer:
1. Conversion of atmospheric nitrogen into nitrogenous salts to make it available to plants for its update is described as nitrogen fixation.
2. When living organisms are involved in nitrogen fixation process it is known as biological nitrogen fixation.
3. The process is mainly carried out by prokaryotic organisms, i.e. different kinds of bacteria present in soil.
4. The nitrogen fixing organisms are known as diazotrophs or nitrogen fixers and about 70% nitrogen is fixed by them.
5. The nitrogen fixers are either free living bacteria or symbiotic associated with other higher plants e.g. Rhizobium.
6. The cyanobacteria have specialized cells heterocysts which help in process of nitrogen fixation.
7. Nitrogen fixation is high energy requiring process and 16 ATP molecules are needed for fixation of one molecule of nitrogen to ammonia.
8. Soil bacteria like Nitrosomonas, Nitrosocyccus convert ammonia to nitrate and the Nitrobacter convert nitrite to nitrate. This is known as nitrification, biological oxidation.
9. These bacteria are chemoautotrophic and utilize these processes for their metabolism.
10. Fabaceae plants like pea, bean have root nodules which harbour symbiotic bacterium Rhizobium which fixes nitrogen. It is host specific, soil bacterium, Nitrogen is made available to host plant.
In simple words: Biological nitrogen fixation is the conversion of atmospheric nitrogen into plant-usable nitrogenous compounds by living organisms, primarily bacteria like Rhizobium in symbiotic association with legumes, and also by free-living bacteria and cyanobacteria.

🎯 Exam Tip: Focus on the role of microorganisms, the energy requirement, and examples of symbiotic relationships in biological nitrogen fixation.

 

Question 4. Write on macro and micro nutrients required for plant growth.
Answer:
1. Plants absorb mineral nutrients from their surroundings.
2. For a proper growth of plants about 35 to 40 different elements are required.
3. Plants absorb these nutrients in ionic or dissolved form from soil with their root system e.g. Phosphorus as \(PO_4\), Sulphur as \(SO_4^{2-}\) etc.
4. Based on their requirement in quantity, they are classified as major nutrients or macronutrients and those needed in small amounts Eire minor or micronutrients.
5. Macroelements are required in large amounts, as they play nutritive and structural roles e.g. C, H, O, R Mg, N, K, S and Ca. - Ca pectate cell wall component, Mg component of chlorophyll.
6. C, H, O are non-mineral major elements obtained from air and water e.g. \(CO_2\) is source of carbon, Hydrogen from water.
7. Microelements are required in traces as they mainly have catalytic role as co-factors or activators of enzymes.
8. Microelements may be needed for certain activity in life cycle of plant e.g. B for pollen germination, Si has protective role during stress conditions and fungal attacks, Al enhances availability of phosphorus.
9. The important micronutrients for plant growth are Mn, B, Cu, Zn, CI.
In simple words: Macronutrients like N, P, K, C, H, O are needed in large quantities for structural and metabolic functions, while micronutrients such as Mn, B, Cu, Zn are required in trace amounts, mainly acting as enzyme co-factors.

🎯 Exam Tip: Distinguish between macro- and micronutrients based on the quantity required and their general functions (structural vs. catalytic/regulatory).