Maharashtra Board 2026 Class 12 Biology Question Paper (Available) - Download PDF with Solutions

Step 1: Understanding the Concept:

In plant physiology, growth rates describe how the number of cells increases over time. Geometric growth specifically refers to a pattern where all daughter cells resulting from a division maintain the ability to divide further.





Step 2: Detailed Explanation:

In geometric growth, both daughter cells derived from the zygote or initial cell continue to divide and redivide. This leads to an exponential increase in cell count, which is characteristic of the early stages of embryo development. In contrast, arithmetic growth involves only one daughter cell continuing to divide while the other differentiates.


Step 3: Final Answer:

The growth pattern where daughter cells divide and redivide repeatedly is called geometric growth. Quick Tip: Geometric growth typically follows a Sigmoid (S-shaped) curve consisting of lag, log (exponential), and stationary phases.

Step 1: Understanding the Concept:

Biotechnology is used to create transgenic plants with enhanced nutritional value by inserting specific genes (transgenes) that regulate biosynthetic pathways.


Step 2: Detailed Explanation:

To increase the level of health-promoting antioxidants known as flavonoids, the gene for the enzyme Chalone isomerase (CHI) is introduced into tomatoes. Other options like Phytoene synthase are used for Golden Rice (pro-vitamin A), and Ferritin is used for iron enrichment.


Step 3: Final Answer:

The correct transgene for flavonoid production in tomatoes is Chalone isomerase. Quick Tip: Flavonoids are known for their antioxidant, anti-inflammatory, and anti-cancer properties.

Step 1: Understanding the Concept:

Cranial capacity (measured in cubic centimeters or cc) is the measure of the volume inside the skull, which serves as an indicator of brain size evolution in human ancestors.





Step 2: Detailed Explanation:

In the evolutionary timeline, the brain size increased progressively:

Homo habilis: 650--800 cc.
Homo erectus: 900 cc.
Neanderthal man: 1400 cc.
Modern Homo sapiens: ~1350--1450 cc.



Step 3: Final Answer:

The cranial capacity of Neanderthal man was 1400 cc. Quick Tip: Neanderthals lived in the Near East and Central Asia between 1,00,000--40,000 years ago and were known to bury their dead.

Step 1: Understanding the Concept:

Detritus is dead organic matter. Organisms that feed on this matter play a critical role in the decomposition process within an ecosystem.


Step 2: Detailed Explanation:

Saprotrophs (also called decomposers, such as fungi and certain bacteria) obtain nutrients by secreting digestive enzymes directly onto dead organic matter (detritus). They break down complex organic molecules into simpler inorganic substances and then absorb them.


Step 3: Final Answer:

The organisms that meet their nutrient requirements by degrading detritus are Saprotrophs. Quick Tip: Saprotrophs are the primary recyclers in an ecosystem, returning minerals to the soil for reuse by plants.

Step 1: Understanding the Concept:

Microorganisms are classified into groups such as bacteria, algae, and fungi based on their biological properties and cellular structure.


Step 2: Detailed Explanation:


Spirulina spp.: A blue-green algae (Cyanobacteria) used as a rich source of Single Cell Protein (SCP).
Aspergillus niger: A fungus used for citric acid production.
Candida utilis: A type of yeast (fungus).
Trichoderma viridi: A soil fungus used as a biocontrol agent.



Step 3: Final Answer:

The algae in the list is Spirulina spp. Quick Tip: Spirulina is highly nutritious, containing 60-70% protein, and can be grown on waste materials like molasses and straw.

Step 1: Understanding the Concept:

Translocation is the process by which organic nutrients (food) are transported through the phloem from the source (leaves) to the sink (roots, fruits, storage organs).





Step 2: Detailed Explanation:

While plants produce glucose through photosynthesis, it is converted into sucrose for transport. Sucrose is used because it is highly soluble in water and chemically non-reactive (non-reducing), making it safe to move through the plant's vascular system without reacting with other components. Starch is used for long-term storage, not transport.


Step 3: Final Answer:

In plants, soluble food is always translocated in the form of sucrose. Quick Tip: The "Pressure Flow Hypothesis" or "Munch Hypothesis" is the most accepted mechanism for the translocation of sugars in phloem.

Step 1: Understanding the Concept:

Invasive species are non-native organisms that, when introduced to a new environment, spread rapidly and cause harm to the local ecosystem, biodiversity, or human health.


Step 2: Detailed Explanation:

Parthenium hysterophorus (commonly known as Carrot Grass or Gajar Ghas) is a notorious invasive species in India. It was accidentally introduced through imported wheat. It competes aggressively with native flora and its pollen causes allergies and respiratory issues in humans.


Step 3: Final Answer:

The invasive plant species is Parthenium. Quick Tip: Other common invasive species in India include Lantana camara and Eichhornia crassipes (Water Hyacinth).

Step 1: Understanding the Concept:

Plasma cells are differentiated B-lymphocytes that act as antibody factories during an immune response. They are specialized for high-volume protein synthesis.




[Image of B-cell differentiation into plasma cell]



Step 2: Detailed Explanation:

Once a B-cell is activated and matures into a plasma cell, its primary function is to secrete soluble antibodies (immunoglobulins) into the blood and lymph. A single mature plasma cell is incredibly efficient, producing approximately 2000 antibody molecules per second.


Step 3: Final Answer:

The rate of antibody production is 2000 molecules per second. Quick Tip: Plasma cells have a very well-developed rough endoplasmic reticulum (RER) to facilitate this massive protein production.

Step 1: Understanding the Concept:

White Blood Cells (WBCs or Leukocytes) are part of the immune system. They vary significantly in size, nucleus shape, and function.




[Image of different types of white blood cells]



Step 2: Detailed Explanation:

Monocytes are the largest type of leukocytes, typically ranging from 12 to 20 \(\mu m\) in diameter. They are agranulocytes with a characteristic kidney-shaped or bean-shaped nucleus. They eventually migrate into tissues to become macrophages.


Step 3: Final Answer:

The largest WBC is the Monocyte. Quick Tip: While Monocytes are the largest in size, Neutrophils are the most numerous WBCs in the human body.

Step 1: Understanding the Concept:

Mortality refers to the death rate in a population. It is measured under two conditions: ideal (theoretical) and actual (ecological).


Step 2: Detailed Explanation:

- Absolute (Potential) Mortality is the death of individuals under ideal conditions.
- Realized (Ecological) Mortality is the actual number of deaths in a population under existing environmental pressures such as predation, disease, and competition. Since environmental pressure is mentioned, the term "realized" is used.


Step 3: Final Answer:

The correct term is realized mortality. Quick Tip: Natality refers to birth rate, while Mortality refers to death rate.

Step 1: Understanding the Concept:

Leydig cells, also known as interstitial cells, are found in the connective tissue surrounding the seminiferous tubules in the testes. They play a critical role in the male reproductive system.





Step 2: Detailed Explanation:

The primary function of Leydig cells is the synthesis and secretion of androgens, mainly testosterone. This process is stimulated by the Luteinizing Hormone (LH) from the pituitary gland. Testosterone is essential for stimulating spermatogenesis and maintaining male secondary sexual characteristics.


Step 3: Final Answer:

The function of Leydig's cells is to produce and secrete the male sex hormone, testosterone. Quick Tip: Leydig cells are located in the interstitial spaces, which is why they are often called interstitial cells of Leydig.

Step 1: Understanding the Concept:

Conservation biology categorizes species based on their risk of extinction. "Endangered" is a specific status assigned by organizations like the IUCN (International Union for Conservation of Nature).


Step 2: Detailed Explanation:

Endangered species are those organisms (plants or animals) that are at a very high risk of becoming extinct in the near future throughout all or a significant portion of their range. This happens because their population is either very small or declining due to habitat loss, poaching, or environmental changes.


Step 3: Final Answer:

Endangered species are species that are in danger of extinction because their numbers have reached a critically low level. Quick Tip: The Red Data Book, maintained by the IUCN, keeps a record of all endangered and threatened species globally.

Step 1: Understanding the Concept:

Sex determination in humans depends on the X and Y chromosomes. While daughters receive an X from their father, sons receive the Y chromosome.


Step 2: Detailed Explanation:

Genes located on the Y chromosome are known as Y-linked or Holandric genes. Since only males possess the Y chromosome, these genes are passed exclusively from the father to his sons. A prominent example is the SRY gene (Sex-determining Region Y), which triggers male development.


Step 3: Final Answer:

The genes inherited directly from father to son are Y-linked genes (e.g., the SRY gene). Quick Tip: Traits controlled by holandric genes, like hypertrichosis (hair on the pinna of the ear), appear only in males.

Step 1: Understanding the Concept:

Energy flow in an ecosystem follows specific efficiency rules: first, the conversion of sunlight by producers, and second, the transfer between trophic levels (Lindeman's 10% Law).




[Image of 10 percent law energy pyramid]



Step 2: Key Formula or Approach:

1. Plants capture only about 1% of the incident solar energy for photosynthesis.

2. Only 10% of energy is transferred from one trophic level to the next.


Step 3: Detailed Explanation:

1. Light Energy received: 5000 J

2. Energy captured by Producers (1% of 5000 J): \[ 5000 \times 0.01 = 50 J \]
3. Energy transferred to Herbivores/Primary Consumers (10% of 50 J): \[ 50 \times 0.10 = 5 J \]
4. Energy transferred to Carnivores/Secondary Consumers (10% of 5 J): \[ 5 \times 0.10 = 0.5 J \]


Step 4: Final Answer:

The amount of energy transferred to the secondary consumers (carnivores) will be 0.5 Joules. Quick Tip: Remember that energy transfer is unidirectional and significantly decreases at each higher trophic level.

Step 1: Understanding the Concept:

Plants require specific mineral nutrients for the synthesis of chlorophyll. When these minerals are deficient, the plant shows visible symptoms.





Step 2: Detailed Explanation:

Chlorosis is the loss or yellowing of chlorophyll in leaves, leading to a pale or yellow appearance. This occurs because the plant cannot produce enough chlorophyll for photosynthesis, usually due to a deficiency of elements like Nitrogen (N), Potassium (K), Magnesium (Mg), Iron (Fe), or Manganese (Mn).


Step 3: Final Answer:

Chlorosis is the yellowing of plant leaves caused by the loss of chlorophyll, typically resulting from mineral deficiencies. Quick Tip: Unlike necrosis (death of tissue), chlorosis involves the fading of green color while the tissue is still alive.

Step 1: Understanding the Concept:

DNA profiling, also known as DNA fingerprinting, is a technique used to identify individuals based on unique patterns in their DNA. It relies on parts of the genome that vary significantly between people.





Step 2: Detailed Explanation:

The key factor in DNA profiling is the presence of VNTRs (Variable Number of Tandem Repeats). These are short sequences of DNA that are repeated multiple times at specific locations in the genome. The number of repeats varies from person to person, serving as a unique genetic "barcode" for identification.


Step 3: Final Answer:

The key factor in DNA profiling is VNTR (Variable Number of Tandem Repeats). Quick Tip: Dr. Alec Jeffreys is considered the father of DNA fingerprinting, while Dr. Lalji Singh is known as the father of DNA fingerprinting in India.

Step 1: Understanding the Concept:

Soil contains water in various forms (hygroscopic, capillary, gravitational, and combined). Not all of this water is available for plants to absorb through their roots.


Step 2: Detailed Explanation:

Combined water is the water that is chemically bound to soil minerals (such as aluminum or iron oxides and silicates) in the form of hydrated oxides. Unlike capillary water, combined water is not available to plants because it is part of the chemical structure of the soil particles and cannot be separated by root suction.


Step 3: Final Answer:

Combined water is the water present in the form of chemical compounds in the soil, which is unavailable for plant absorption. Quick Tip: Capillary water is the only form of soil water that is primarily available for plant roots to absorb.

Step 1: Understanding the Concept:

The anther of a flower consists of several layers. The tapetum is the innermost layer of the microsporangium wall and is in direct contact with the developing pollen grains.





Step 2: Detailed Explanation:

The primary role of the tapetum is to provide nutrition to the developing microspores (pollen grains). Additionally, it secretes:
1. Sporopollenin: A highly resistant organic material that forms the outer layer (exine) of pollen.
2. Pollen-kitt: A sticky substance found on the surface of insect-pollinated pollen grains.
3. Enzymes and hormones necessary for pollen development.


Step 3: Final Answer:

The main role of the tapetum is to provide nourishment to the developing pollen grains and contribute to the formation of the pollen wall. Quick Tip: Cells of the tapetum usually possess dense cytoplasm and generally have more than one nucleus (multinucleate).

Step 1: Understanding the Concept:

For efficient gas exchange (oxygen and carbon dioxide), the respiratory surface where diffusion occurs must have specific physical characteristics to maximize the rate of diffusion according to Fick's Law.





Step 2: Detailed Explanation:

The essential features of a respiratory surface are:

Large Surface Area: To allow the exchange of a sufficient volume of gases to meet metabolic needs.
Thin and Permeable: The surface should be extremely thin (usually one-cell thick) to minimize the diffusion distance.
Moist: Respiratory surfaces must be kept moist because gases can only diffuse across the cell membrane in dissolved form.
Rich Blood Supply: In higher animals, the surface is highly vascularized to maintain a steep concentration gradient for gases.



Step 3: Final Answer:

A respiratory surface must be thin, moist, have a large surface area, and be highly permeable and vascularized for efficient gaseous exchange. Quick Tip: In humans, the total surface area of the alveoli in the lungs is approximately 70 to 100 square meters—roughly the size of a tennis court!

Step 1: Understanding the Concept:

Sex-linkage refers to genes located on sex chromosomes (X and Y). The distinction between complete and incomplete linkage depends on whether the genes are located in homologous or non-homologous regions.


Step 2: Detailed Explanation:

\begin{table[h]
\centering
\begin{tabular{|l|p{6cm|p{6cm|
\hline
Feature & Complete Sex-linkage & Incomplete Sex-linkage
\hline
Location & Genes are located on the non-homologous regions of X or Y chromosomes. & Genes are located on the homologous regions of X and Y chromosomes.
\hline
Crossing Over & Crossing over does not occur in these regions; genes stay together. & Crossing over may occur between the homologous regions.
\hline
Inheritance & Traits are inherited together for generations (e.g., Colour blindness, Haemophilia). & Traits do not always stay together; they show patterns similar to autosomal genes (e.g., Retinitis pigmentosa).
\hline
\end{tabular
\end{table


Step 3: Final Answer:

Complete sex-linkage occurs in non-homologous regions where crossing over is absent, while incomplete sex-linkage occurs in homologous regions where crossing over can take place. Quick Tip: Genes on the non-homologous region of the Y-chromosome are called holandric genes and are passed only from father to son.

Step 1: Understanding the Concept:

Nerve fibers are specialized cells designed for communication. They possess unique physiological properties that allow them to respond to stimuli and transmit messages.


Step 2: Detailed Explanation:

(a) Irritability (Excitability): It is the ability of the nerve fiber to respond to a stimulus and convert it into a nerve impulse (electrical signal). For a response to occur, the stimulus must be of a minimum strength known as the "threshold stimulus."


(b) Conductivity: It is the ability of the nerve fiber to transmit the nerve impulse along its entire length, from the point of initiation toward the axon terminals or synapse. This ensures the message reaches the next neuron or effector organ.


Step 3: Final Answer:

Irritability is the ability to generate an impulse upon stimulation, whereas conductivity is the ability to propagate that impulse along the nerve fiber. Quick Tip: Nerve fibers follow the "All-or-None Law," meaning they either respond completely to a stimulus or do not respond at all.

Step 1: Understanding the Concept:

A nucleosome is the basic unit of DNA packaging in eukaryotes. It consists of a segment of DNA wound around a protein core.





Step 2: Detailed Explanation:

The nucleosome consists of:

Histone Octamer: A core of 8 histone proteins (two each of H2A, H2B, H3, and H4).
DNA: About 146 base pairs of negatively charged DNA wrapped twice around the positively charged octamer.
H1 Histone: A linker histone that seals the DNA as it enters and leaves the nucleosome, helping to stabilize the structure.



Step 3: Final Answer:

The nucleosome resembles a "bead on a string," where the octamer is the bead, and DNA is the string wrapped around it, secured by the H1 histone. Quick Tip: Histones are rich in basic amino acids like Lysine and Arginine, which gives them the positive charge needed to bind to the negatively charged DNA.

Step 1: Understanding the Concept:

Micropropagation is a tissue culture technique used to produce a large number of plants from a small piece of plant tissue (explant) in a short period.





Step 2: Detailed Explanation:

The four major advantages are:

Rapid Multiplication: Large numbers of plants can be produced in a very short time from a small amount of starting material.
Disease-Free Plants: Healthy plants can be recovered from diseased ones by using meristem culture (since meristems are usually virus-free).
Soma-clones: The resulting plants are genetically identical to the parent plant, ensuring the preservation of desirable traits.
Year-round Production: Plants can be grown in controlled laboratory conditions regardless of the external weather or season.



Step 3: Final Answer:

Micropropagation allows for rapid production of genetically identical, disease-free plants throughout the year. Quick Tip: Commercial crops like banana, tomato, and apple are frequently produced using micropropagation.

Step 1: Understanding the Concept:

The conversion of rock into soil is a biological weathering process known as pedogenesis. Certain pioneer organisms are capable of breaking down hard rock surfaces to create a layer suitable for other plants.





Step 2: Detailed Explanation:

The primary organisms involved are Lichens and Mosses:

Lichens: These are the pioneer species. They secrete organic acids (like oxalic acid) that chemically weather the rock, breaking it down into smaller particles.
Mosses: Once lichens create a tiny bit of soil, mosses grow in the crevices. They hold water and further cause mechanical weathering through their root-like structures (rhizoids), eventually creating enough soil for higher plants.



Step 3: Final Answer:

Lichens and mosses convert rock into soil by secreting acids that chemically break down the rock and by physically prying apart rock particles. Quick Tip: Lichens are a symbiotic association between an alga and a fungus, where the fungus provides the acids for rock disintegration.

Step 1: Understanding the Concept:

Noise pollution is defined as unwanted or excessive sound that can have deleterious effects on human health and environmental quality.


Step 2: Detailed Explanation:

Noise pollution affects humans both physiologically and psychologically:

Hearing Loss: Constant exposure to high decibels can cause temporary or permanent damage to the hair cells of the cochlea, leading to deafness.
Psychological Issues: It causes irritability, anxiety, stress, and lack of concentration.
Sleep Disturbance: Even low-level noise can interfere with sleep cycles, leading to fatigue and reduced productivity.
Physiological Changes: Noise can trigger the "fight or flight" response, causing increased heart rate, high blood pressure (hypertension), and digestive disorders.



Step 3: Final Answer:

Harmful effects include permanent hearing loss, increased blood pressure, chronic stress, and severe sleep disturbances. Quick Tip: According to the WHO, noise levels above 75 dB for long durations can cause hearing impairment, while 120 dB causes physical pain.

Step 1: Understanding the Concept:

Homo erectus (meaning "upright man") is a pivotal species in human evolution, showing significant transitions from ape-like features to modern human skeletal structures.





Step 2: Detailed Explanation:

Key skeletal features include:

Cranial Capacity: Their brain size was approximately 900 cc, larger than Homo habilis but smaller than modern humans.
Skull Shape: The skull was low and long, with prominent brow ridges (supraorbital ridges) and a receding forehead.
Post-cranial Skeleton: They possessed a fully upright posture and a skeleton very similar to modern humans, adapted for efficient long-distance walking and running.
Jaw and Teeth: They had large jaws and teeth, but smaller than those of Australopithecus, with no prominent chin.



Step 3: Final Answer:

Homo erectus featured a 900 cc brain, prominent brow ridges, and a fully bipedal, modern human-like body structure. Quick Tip: Homo erectus is credited with being the first human ancestor to use fire and create sophisticated stone tools (Acheulean hand axes).

Step 1: Understanding the Concept:

Gene transfer technology (Genetic Engineering) allows for the direct manipulation of an animal's genome to achieve specific production or health goals much faster than traditional breeding.


Step 2: Detailed Explanation:

The main objectives are:

Increased Productivity: Enhancing the yield of milk, meat, or wool (e.g., increasing milk protein content).
Disease Resistance: Introducing genes that make animals resistant to common viral, bacterial, or parasitic infections.
Faster Growth Rates: Modifying growth hormone genes to ensure animals reach market weight more quickly.
Nutritional Enhancement: Altering the composition of animal products to be healthier for human consumption (e.g., lower fat content).



Step 3: Final Answer:

Objectives include improving yield, ensuring disease resistance, accelerating growth, and enhancing the nutritional quality of animal products. Quick Tip: Transgenic "Rosie," the first transgenic cow, produced human protein-enriched milk containing alpha-lactalbumin.

Step 1: Understanding the Concept:

Water absorption in plants occurs primarily through roots. The efficiency of this process is influenced by several internal and environmental conditions.





Step 2: Detailed Explanation:

Four major factors affecting water absorption are:

Availability of Soil Water: Absorption is highest when soil water is in the form of capillary water. Excess or lack of water inhibits the process.
Soil Temperature: Low temperatures increase the viscosity of water and decrease root permeability, thereby reducing absorption.
Concentration of Soil Solution: If the soil solution is too concentrated (hypertonic), water cannot enter the roots via osmosis.
Soil Aeration: Roots require oxygen for respiration to provide energy for active absorption. Poorly aerated (waterlogged) soils reduce water uptake.



Step 3: Final Answer:

The four factors are soil water availability, soil temperature, concentration of soil solution, and soil aeration. Quick Tip: Optimal water absorption usually occurs at soil temperatures between 20°C and 35°C.

Step 1: Understanding the Concept:

Alzheimer's disease is a progressive neurological disorder that causes the brain to shrink (atrophy) and brain cells to die. It is the most common cause of dementia—a continuous decline in thinking, behavioral, and social skills.





Step 2: Detailed Explanation:

The disease is characterized by the accumulation of amyloid-beta plaques and neurofibrillary tangles (tau protein) in the brain, which disrupt cell communication. Symptoms typically begin with mild memory loss and progress to severe cognitive impairment, loss of speech, and inability to carry out daily tasks. It is generally observed in the elderly (above 65 years).


Step 3: Final Answer:

Alzheimer's is a chronic neurodegenerative disease that leads to progressive memory loss and cognitive decline due to brain cell death and protein buildup. Quick Tip: There is currently no cure for Alzheimer's, but treatments and lifestyle changes can help manage symptoms and improve the quality of life.

Step 1: Understanding the Concept:

Biological nitrogen fixation is a highly energy-intensive process mediated by the enzyme nitrogenase. It converts atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)).


Step 2: Key Formula or Approach:

The standard equation for nitrogen fixation is: \[ N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i \]
From this, we see that the production of \(2\) molecules of \(NH_3\) requires \(16\) ATP.
Therefore, \(1\) molecule of \(NH_3\) requires \(8\) ATP.


Step 3: Detailed Explanation:

Calculation:
Number of \(NH_3\) molecules = \(1000\).
ATP per molecule = \(8\).
Total ATP required = \(1000 \times 8 = 8000\) ATP.

Specialized Cell:
In many cyanobacteria (like Nostoc and Anabaena), nitrogen fixation occurs in specialized, thick-walled cells called Heterocysts, which provide the anaerobic environment necessary for the nitrogenase enzyme to function.





Step 4: Final Answer:

Total ATP required is 8000 ATP. The specialized cell in cyanobacteria is the Heterocyst. Quick Tip: The nitrogenase enzyme is highly sensitive to oxygen; hence, heterocysts lack Photosystem II to prevent oxygen evolution.

Step 1: Understanding the Concept:

Different organisms have evolved specific respiratory organs based on their habitat (aquatic vs. terrestrial) and their level of organization.


Step 2: Detailed Explanation:

Based on biological classification and respiratory physiology:

Limulus (King Crab): Uses Book gills (f).
Protists: Use their general Plasma membrane (e) for diffusion.
Tadpoles of frog: Being aquatic, they use External gills (d).
Fishes: Use Internal gills (b) protected by an operculum.
Reptiles: Being terrestrial, they use Lungs (a).
Spiders: Use Book lungs (c).



Step 3: Final Answer:

The correct matching is: (1)-(f), (2)-(e), (3)-(d), (4)-(b), (5)-(a), (6)-(c). Quick Tip: "Book" organs (book lungs/gills) are named so because their structure resembles the folded pages of a book, maximizing surface area.

Step 1: Understanding the Concept:

Criss-cross inheritance is a type of sex-linked inheritance where a trait is passed from a father to his grandson through his carrier daughter. Bleeder's disease (Haemophilia) is an X-linked recessive disorder.


Step 2: Detailed Explanation:

In Haemophilia, the gene is located on the X-chromosome. A male (\(X^hY\)) is affected, while a female (\(X^HX^h\)) is a carrier.
Example Chart (Affected Father \(\times\) Normal Mother):

Parents: \(X^hY\) (Affected Father) \(\times\) \(X^HX^H\) (Normal Mother)
\(F_1\) Generation: \(X^HX^h\) (Carrier Daughters) and \(X^HY\) (Normal Sons).
The trait has passed from father to daughter (who is a carrier).

Second Step (Carrier Daughter \(\times\) Normal Man):

\(F_2\) Generation: \(50%\) of grandsons will be affected (\(X^hY\)).

This "Father \(\rightarrow\) Daughter \(\rightarrow\) Grandson" flow is criss-cross inheritance.





Step 3: Final Answer:

Criss-cross inheritance shows how X-linked recessive traits like Haemophilia skip a generation in terms of expression, moving from affected males to carrier females to affected grandsons. Quick Tip: Haemophilia is called "Bleeder's disease" because even a minor cut can lead to continuous bleeding due to the lack of clotting factors.

Step 1: Understanding the Concept:

Organisms use various strategies to survive unfavorable environmental conditions. Hibernation and aestivation are states of dormancy, while adaptations are specific traits for survival.


Step 2: Detailed Explanation:

(a) Distinction:
\begin{table[h]
\centering
\begin{tabular{|l|l|l|
\hline
Feature & Hibernation & Aestivation
\hline
Common Name & Winter sleep. & Summer sleep.
\hline
Reason & Avoid extreme cold. & Avoid extreme heat and desiccation.
\hline
Example & Polar bears, Frogs. & Snails, certain Fish.
\hline
\end{tabular
\end{table

(b) Behavioural adaptations of desert animals:
1. Nocturnality: Many animals (like desert rodents) remain in burrows during the day and are active only at night to avoid high temperatures.
2. Burrowing: Animals dig deep into the sand where the temperature is significantly lower than at the surface.


Step 3: Final Answer:

Hibernation is winter dormancy, while aestivation is summer dormancy. Desert animals adapt by being nocturnal and living in burrows. Quick Tip: Adaptations can be physiological (concentrated urine), morphological (thick fur), or behavioural (seeking shade).

Step 1: Understanding the Concept:

These terms represent specific phenomena in the reproductive biology of plants, relating to fertilization, fruit development, and seed formation.


Step 2: Detailed Explanation:

(a) Siphonogamy: It is a condition in plants where the male gametes are carried to the egg cell through a pollen tube. This is a characteristic feature of seed plants (Gymnosperms and Angiosperms), allowing fertilization to occur without the need for external water.


(b) Parthenocarpy: It is the process of fruit development without fertilization. This results in seedless fruits. It can occur naturally or can be induced by plant hormones like auxins and gibberellins (e.g., Banana, Grapes).


(c) Polyembryony: It is the occurrence of more than one embryo in a single seed. It can occur due to the presence of multiple embryo sacs or the fertilization of multiple eggs (e.g., Citrus fruits, Mango).


Step 3: Final Answer:

Siphonogamy is fertilization via a pollen tube, parthenocarpy is fruit development without fertilization, and polyembryony is the presence of multiple embryos in a seed. Quick Tip: Parthenocarpic fruits are commercially valuable because they are seedless and often have a longer shelf life.

Step 1: Understanding the Concept:

Transpiration is the loss of water in the form of vapor from the aerial parts of the plant. Stomatal movement (opening and closing) regulates this process and is driven by turgor changes in the guard cells.




[Image of stomata opening and closing mechanism]


Step 2: Detailed Explanation:

Starch-Sugar Interconversion Theory (Lloyd):
1. Opening (Day): During the day, photosynthesis occurs in guard cells, decreasing \(CO_2\) concentration and increasing pH. This activates the enzyme phosphorylase, which converts insoluble starch into soluble sugar (glucose-1-phosphate). The osmotic pressure of guard cells increases, water enters from neighboring cells (endosmosis), and guard cells become turgid, causing the stoma to open.


2. Closing (Night): In the dark, \(CO_2\) accumulates due to respiration, lowering the pH. This promotes the conversion of sugar back into starch. Starch is insoluble, so the osmotic pressure drops. Water leaves the guard cells (exosmosis), they become flaccid, and the stoma closes.


Step 3: Final Answer:

Transpiration is the loss of water vapor from plants. According to the starch-sugar theory, the conversion of starch to sugar increases guard cell turgidity to open stomata, while the reverse closes them. Quick Tip: While the starch-sugar theory was historically important, the modern "K+ Ion Transport Theory" is currently the most widely accepted explanation for stomatal movement.

Step 1: Understanding the Concept:

The genetic code is the set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins by living cells.




Step 2: Detailed Explanation:

The six main characteristics are:

Triplet Nature: Each codon consists of three nitrogenous bases (e.g., AUG).
Universality: The same code applies to almost all organisms, from bacteria to humans.
Non-overlapping: The code is read sequentially, three bases at a time; no base is shared between adjacent codons.
Comma-less: There are no pauses or "punctuation" between codons during translation.
Degeneracy: One amino acid can be coded by more than one codon (except Tryptophan and Methionine).
Unambiguous: One specific codon always codes for only one specific amino acid.



Step 3: Final Answer:

The genetic code is triplet, universal, non-overlapping, comma-less, degenerate, and unambiguous. Quick Tip: The codon AUG has a dual function: it codes for the amino acid Methionine and also acts as the "Start Codon" for protein synthesis.

Step 1: Understanding the Concept:

The body's organs originate from three primary germ layers: ectoderm, mesoderm, and endoderm. The pancreas contains endocrine tissues known as Islets of Langerhans, which secrete vital hormones.


Step 2: Detailed Explanation:

(a) Germ Layer Origins:
- Adrenal Cortex: Developed from the mesoderm.
- Pancreas: Developed from the endoderm.


(b) Role of Pancreatic Cells:
- Alpha (\(\alpha\)) cells: These cells secrete the hormone glucagon. Glucagon increases blood glucose levels by stimulating the liver to convert glycogen into glucose (glycogenolysis).
- Delta (\(\delta\)) cells: These cells secrete somatostatin. Somatostatin acts as an inhibitor, regulating and suppressing the secretion of both insulin and glucagon to maintain metabolic balance.


Step 3: Final Answer:

Adrenal cortex is mesodermal, while the pancreas is endodermal. Alpha cells produce glucagon (increases sugar), and delta cells produce somatostatin (inhibits insulin/glucagon). Quick Tip: The Islets of Langerhans also contain Beta (\(\beta\)) cells, which are the most numerous and secrete insulin to lower blood sugar levels.

Step 1: Understanding the Concept:

Addiction is a complex condition characterized by compulsive drug use despite harmful consequences. Drugs affect the central nervous system and cardiovascular system in various ways.


Step 2: Detailed Explanation:

Definition: Addiction is a psychological and sometimes physical attachment to certain effects—such as euphoria or a temporary feeling of well-being—associated with drugs and alcohol, leading to a state where the individual cannot function normally without the substance.


Physiological Effects:
- (a) Cocaine: It interferes with the transport of the neurotransmitter dopamine. It has a potent stimulating action on the central nervous system, producing a sense of euphoria and increased energy. Excessive dosage causes hallucinations and severe damage to the cardiovascular system.
- (b) Cannabinoids: These interact with cannabinoid receptors in the brain. Their primary physiological effect is on the cardiovascular system of the body, leading to increased heart rate and blood pressure, alongside altered sensory perception.


Step 3: Final Answer:

Addiction is a state of dependency on a substance. Cocaine acts as a stimulant and interferes with dopamine, while cannabinoids primarily affect the cardiovascular system and brain receptors. Quick Tip: Cannabinoids are obtained from the inflorescences of the plant Cannabis sativa (Hemp plant).

Step 1: Understanding the Concept:

Infertility is the inability to conceive even after unprotected sexual cohabitation. To assist such couples, Assisted Reproductive Technologies (ART) like ZIFT and GIFT are utilized to facilitate fertilization or pregnancy.


Step 2: Detailed Explanation:

Infertility: It is defined as the inability to produce children despite two years of unprotected sexual cohabitation. It can be caused by physical, congenital, immunological, or even psychological factors.

ZIFT (Zygote Intra-Fallopian Transfer): In this technique, the zygote (up to 8 blastomeres stage) is formed through in vitro fertilization (IVF) in a laboratory and then transferred directly into the Fallopian tube of the female.

GIFT (Gamete Intra-Fallopian Transfer): This involves the transfer of an ovum collected from a donor into the Fallopian tube of another female who cannot produce one but can provide a suitable environment for fertilization and further development.


Step 3: Final Answer:

Infertility is the inability to conceive naturally. ZIFT involves transferring a laboratory-created zygote into the Fallopian tube, while GIFT involves transferring a donor ovum into the Fallopian tube. Quick Tip: ZIFT is a type of IVF (In-Vitro Fertilization) procedure, whereas GIFT is an In-Vivo fertilization method because fertilization happens inside the body.

Step 1: Understanding the Concept:

The Urey-Miller experiment provided the first experimental evidence for the "chemical evolution" theory, showing that organic molecules could form from inorganic precursors under conditions simulating primitive Earth.





Step 2: Detailed Explanation:

The setup consisted of:

Spark Chamber: Contained gases like \(CH_4, NH_3, H_2\) and water vapor (\(H_2O\)) to simulate the primitive atmosphere.
Electrodes: Provided electric discharge (sparks) to simulate lightning.
Condenser: Cooled the gases into liquid form, simulating rain.
Boiling Flask: Provided the necessary water vapor and simulated the primitive ocean.



Step 3: Final Answer:

The experiment successfully produced amino acids (like glycine and alanine) after running the circulation for several days. Quick Tip: The ratio of gases used in the experiment (\(CH_4 : NH_3 : H_2\)) was \(2:1:2\) at a temperature of approximately 800°C.

Step 1: Understanding the Concept:

Plant growth is regulated by chemical messengers called Plant Growth Regulators (PGRs). The rate of growth varies throughout the lifespan of an organ or plant.





Step 2: Detailed Explanation:

(a) Grand Period of Growth: It is the total time during which the maximum growth of an organ or the whole plant occurs. It corresponds to the log phase (exponential phase) of the sigmoid growth curve, where growth is extremely rapid after the initial slow lag phase.

(b) Examples of PGRs:
1. Growth Promoters: These stimulate cell division and enlargement. Examples: Auxins and Gibberellins.
2. Growth Inhibitors: These promote dormancy and abscission. Examples: Abscisic Acid (ABA) and Ethylene (though Ethylene has both roles, it is largely inhibitory).


Step 3: Final Answer:

The grand period of growth is the phase of maximum growth rate. Promoters include Auxins and Gibberellins; inhibitors include Abscisic acid and Ethylene. Quick Tip: The sigmoid curve represents a universal pattern of growth for most living organisms in a finite environment.

Step 1: Understanding the Concept:

Biofertilizers are eco-friendly alternatives to chemical fertilizers. They utilize living microorganisms to improve the nutrient availability in the soil, specifically nitrogen.


Step 2: Detailed Explanation:

(a) Biofertilizer: These are preparations containing living or latent cells of efficient strains of microorganisms (bacteria, algae, or fungi) that help increase the nutrient content of the soil by biological nitrogen fixation, phosphorus solubilization, or promoting plant growth through synthesis of growth-promoting substances.





(b) T.S. of Root Nodules:
A transverse section (T.S.) of a mature root nodule (commonly found in legumes like Pea or Soybean) shows several zones:
1. Epidermis/Cortex: The outer protective layers.
2. Vascular Bundles: To transport nutrients and fixed nitrogen.
3. Infected Zone (Bacteroid zone): The central part of the nodule containing the nitrogen-fixing bacteria (Rhizobium). This area often appears pink due to the presence of leghaemoglobin.


Step 3: Final Answer:

Biofertilizers use microbes to enrich soil nutrients. Root nodules are specialized structures on legume roots where Rhizobium bacteria live symbiotically. Quick Tip: Leghaemoglobin is an oxygen scavenger that protects the nitrogenase enzyme from oxygen, ensuring efficient nitrogen fixation.

Step 1: Understanding the Concept:

A human sperm (spermatozoon) is a microscopic, elongated, and motile male gamete. It is composed of four distinct regions: head, neck, middle piece, and tail, all enclosed by a plasma membrane.





Step 2: Detailed Explanation:


Head: It contains an elongated haploid nucleus. The anterior portion is covered by a cap-like structure called the acrosome, which contains enzymes (like hyaluronidase) that help the sperm penetrate the ovum.
Neck: A very short region containing two centrioles: the proximal centriole (plays a role in first cleavage of zygote) and the distal centriole (gives rise to the axial filament).
Middle Piece: Contains numerous mitochondria coiled around the axial filament (Nebenkern). These mitochondria produce energy (ATP) for the movement of the tail.
Tail: The longest part of the sperm, consisting of an axial filament. Its lashing movements facilitate sperm motility essential for fertilization.



Step 3: Final Answer:

The human sperm is a highly specialized cell consisting of a head (with acrosome), a neck, a middle piece (powerhouse), and a tail for locomotion. Quick Tip: A human male ejaculates about 200 to 300 million sperm during coitus, of which at least 60% must have normal shape and size for fertility.

Step 1: Understanding the Concept:

The cardiac cycle refers to the sequence of electrical and mechanical events that occur from the beginning of one heartbeat to the beginning of the next. An Electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during this cycle.


Step 2: Detailed Explanation:

Cardiac Cycle: It consists of a period of relaxation (diastole) and a period of contraction (systole). In a healthy human with a heart rate of 72 beats/min, the duration of one cardiac cycle is approximately 0.8 seconds.

ECG Components:

P-wave: Represents the electrical excitation or atrial depolarization, which leads to the contraction of both atria.
QRS complex: Represents the ventricular depolarization, which initiates the ventricular contraction (systole).
T-wave: Represents the return of ventricles from excited to normal state (ventricular repolarization).



Step 3: Final Answer:

The cardiac cycle is the complete set of events in one heartbeat. In an ECG, the P-wave represents atrial depolarization and the QRS complex represents ventricular depolarization. Quick Tip: The number of QRS complexes that occur in a given time period can be used to determine the heart rate of an individual.

Step 1: Understanding the Concept:

The adrenal cortex secretes corticoid hormones, including glucocorticoids (like cortisol) and mineralocorticoids (like aldosterone). An imbalance in these hormones—whether too little (hyposecretion) or too much (hypersecretion)—leads to specific metabolic and physical disorders.


Step 2: Detailed Explanation:

A. Hyposecretion (Addison's Disease):
This is caused by the deficiency of adrenal cortex hormones.
Symptoms:

Skin Pigmentation: Bronzing of the skin, especially in areas exposed to the sun and pressure points.
Low Blood Pressure: Hypotension due to loss of sodium and water (low aldosterone).
Fatigue: Extreme muscle weakness and weight loss.


B. Hypersecretion (Cushing's Syndrome):
This is caused by excess production of cortisol.
Symptoms:

Moon Face: Accumulation of fat in the face, making it appear round.
Buffalo Hump: Unusual fat deposition between the shoulders.
Hyperglycemia: High blood sugar levels and potential development of "adrenal diabetes."



Step 3: Final Answer:

Hyposecretion causes Addison's disease (pigmentation, low BP, weakness), while hypersecretion causes Cushing's syndrome (moon face, fat deposition, high blood sugar). Quick Tip: Cortisol is often called the "stress hormone" because it helps the body respond to long-term stress by mobilizing energy reserves.

Step 1: Understanding the Concept:

Endosperm is the nutritive tissue for the developing embryo. Based on the mode of development and the timing of cytokinesis (cell wall formation), it is classified into different types.


Step 2: Detailed Explanation:

1. Nuclear Endosperm:
This is the most common type. In this development, the Primary Endosperm Nucleus (PEN) undergoes successive free nuclear divisions without immediate cell wall formation. This results in a large number of free nuclei in the embryo sac. A large central vacuole usually forms, pushing nuclei to the periphery.
Example: Coconut water (liquid nuclear endosperm), Wheat, Maize.

2. Helobial Endosperm:
This is an intermediate type, occurring mostly in Monocots (Order Helobiales). The first division of the PEN is followed by a transverse cell wall, dividing the embryo sac into a large micropylar chamber and a small chalazal chamber. Further divisions in both chambers (usually more frequent in the micropylar) remain free nuclear for a while before turning cellular.
Example: Asphodelus, Vallisneria.


Step 3: Final Answer:

Nuclear endosperm involves free nuclear division without walls (e.g., Coconut water), whereas Helobial endosperm is a hybrid type involving an initial wall formation followed by free nuclear divisions (e.g., Asphodelus). Quick Tip: In a coconut, the "water" is free-nuclear endosperm, while the white "kernel" is the cellular endosperm formed at a later stage.

Step 1: Understanding the Concept:

Gene therapy is a technique that involves the introduction, removal, or alteration of genetic material within a person's cells to treat or prevent disease. It is classified based on the type of cells being modified: germ cells or somatic cells.


Step 2: Detailed Explanation:

(a) Germ Line Gene Therapy:
In this method, functional genes are introduced into the germ cells (sperms, eggs, or early embryos).

Why it is not encouraged: It is not encouraged due to significant ethical and social reasons. Since the genetic changes are heritable, they are passed on to all future generations. There are concerns regarding unpredictable long-term effects on the human gene pool and the potential for "designer babies."


(b) Somatic Cell Gene Therapy:
This involves the insertion of a functional gene into the somatic cells (non-reproductive body cells like bone marrow, skin, or liver cells) of a patient.

Unlike germ line therapy, the genetic changes are limited to the individual patient and are not passed on to their offspring.
Acquired Disorders: This therapy is used for disorders like Cancer and AIDS. It is also extensively researched for hereditary diseases like SCID and Cystic Fibrosis.



Step 3: Final Answer:

Germ line therapy modifies heritable reproductive cells but is discouraged for ethical reasons. Somatic cell therapy modifies non-heritable body cells and is used for diseases like Cancer and AIDS. Quick Tip: The first clinical gene therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) deficiency, a form of SCID.