Biology

(a) Ecosystem [2]

An ecosystem is a self-regulating, functional unit of nature in which living organisms (biotic components) interact with one another and with their non-living surroundings (abiotic components) such as air, water, soil, sunlight and temperature, exchanging energy and matter in a continuous cycle. The term was coined by A. G. Tansley (1935). Examples: pond, forest, grassland, desert, ocean.

(b) Forest ecosystem [8]

Definition: A forest ecosystem is a natural terrestrial ecosystem dominated by trees and other woody vegetation, in which producers, consumers and decomposers interact with the physical environment to form a stable, self-sustaining system.

Components:

1. Abiotic components

• Climatic: sunlight (light intensity and duration), temperature, rainfall, humidity, wind.
• Edaphic (soil): soil type, soil moisture, soil pH, minerals and humus.
• Inorganic substances: carbon, nitrogen, oxygen, phosphorus, calcium, potassium, etc., present in soil, water and air.
• Organic substances: carbohydrates, proteins, lipids, humic acids formed from dead remains.

2. Biotic components

• Producers (autotrophs): green plants — canopy trees (Sal, Sisau, Khair, Champ), shrubs, herbs, mosses and ferns. They trap solar energy and prepare food by photosynthesis.
• Consumers (heterotrophs):
  • Primary consumers (herbivores): deer, wild boar, monkey, squirrel, insects, birds.
  • Secondary consumers (carnivores): fox, jackal, snake, wild cat, owl.
  • Tertiary consumers (top carnivores): tiger, leopard, hawk, vulture.
  • Decomposers (saprophytes): bacteria, fungi, actinomycetes that break down dead leaves, wood and animal remains into simple inorganic substances, which return to the soil.
• Scavengers: crows, vultures, ants that feed on dead bodies.

Food chain and energy flow (trophic levels):

$\text{Sun} \rightarrow \text{Producers} \rightarrow \text{Herbivores} \rightarrow \text{Carnivores} \rightarrow \text{Top carnivores} \rightarrow \text{Decomposers}$

Example: Grass/leaves → grasshopper → frog → snake → hawk.

Energy flows in one direction only (unidirectional) and follows the 10 % law — only about 10 % of energy is transferred from one trophic level to the next; the rest is lost as heat in respiration. Nutrients, however, are recycled through the soil by decomposers, so matter cycles while energy flows.

Diagram: Forest ecosystem

                    SUNLIGHT
                       |
        +--------------+--------------+
        |                             |
   [Canopy trees]              [Shrubs, herbs]
        |  (Producers)                |
        |                             |
        +-------------+---------------+
                      |
              Primary consumers
        (deer, monkey, squirrel, insects)
                      |
              Secondary consumers
              (fox, jackal, snake, owl)
                      |
              Tertiary consumers
                (tiger, leopard, hawk)
                      |
        Dead leaves, wood, animal remains
                      |
                 Decomposers
          (bacteria, fungi, actinomycetes)
                      |
        Simple inorganic nutrients -> Soil
                      |
        (absorbed again by producers)

Significance for natural balance

1. Oxygen and carbon dioxide balance: Forests absorb CO₂ and release O₂ during photosynthesis, thus maintaining the atmospheric gas balance and reducing global warming.
2. Water cycle and rainfall: Forests cause transpiration and help in cloud formation and rainfall; they recharge groundwater and maintain springs and streams.
3. Soil conservation: Roots bind the soil, preventing erosion, landslides and floods; leaf litter adds humus and keeps the soil fertile.
4. Habitat and biodiversity: Forests provide food, shelter and breeding places to countless plants and animals, thus conserving biodiversity.
5. Energy flow and food chain: They form the base of the food chain and transfer solar energy to all trophic levels, keeping the ecosystem functioning.
6. Nutrient cycling: Decomposers recycle nitrogen, carbon, phosphorus and other nutrients from dead matter back to the soil.
7. Climate regulation: Forests moderate temperature, check wind speed and maintain humidity, giving a balanced local climate.
8. Control of pollution and natural disasters: They absorb dust, noise and harmful gases and check desertification and floods.
9. Economic and social value: Timber, fuelwood, fodder, medicinal herbs, tourism and employment support human livelihood.

Conclusion: The forest ecosystem is the most important natural ecosystem; its protection through afforestation, controlled grazing, prevention of deforestation and community forest management is essential for maintaining ecological balance and sustainable development.

Major Distinguishing Factors of the Phylum Chordata

Chordates are characterised by certain fundamental features which are present at least in some stage of their life cycle. These fundamental or distinguishing features are:

1. Notochord: A solid, flexible rod of cells called notochord is present at least in the embryonic stage. It lies dorsal to the alimentary canal and provides support to the body. In vertebrates, it is replaced by the vertebral column in the adult.

2. Dorsal hollow nerve cord: The nerve cord is dorsal in position, hollow and single. It lies above the notochord. In vertebrates, it becomes the brain and spinal cord.

3. Pharyngeal gill slits: Paired gill slits are present in the wall of the pharynx at least in the embryonic stage. They are used for respiration in aquatic forms and are modified in terrestrial forms.

4. Post-anal tail: The tail is present behind the anus and helps in locomotion, especially in aquatic forms. It is reduced or absent in many adult terrestrial chordates.

5. Ventral heart: The heart is ventral in position and pumps blood through a closed circulatory system.

6. Endostyle or thyroid gland: The endostyle is present in the floor of the pharynx, which in vertebrates becomes the thyroid gland.

7. Bilateral symmetry, triploblastic and coelomate: Chordates are bilaterally symmetrical, triploblastic animals with a true coelom (enterocoelic in origin).

8. Metameric segmentation: The body shows metameric segmentation, at least in the embryonic stage (e.g., somites in vertebrates).

9. Closed circulatory system: Blood circulates through a closed system of vessels, and the blood contains haemoglobin (in most forms).

10. Ventral muscular heart and hepatic portal system: A muscular heart lies ventral to the gut, and a hepatic portal system is present.

These fundamental features are common to all chordates, but they show wide diversity in structure and function in different classes.

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Critical Comparison of Adaptations in Pisces, Aves and Mammalia

1. Locomotion

2. Respiration

3. Reproduction

Critical Comparison

• Locomotion: Pisces are adapted for aquatic life (swimming), Aves for aerial life (flight), and Mammalia for diverse habitats (walking, running, climbing, swimming, flying). Thus, mammals show the greatest diversity in locomotion.
• Respiration: Pisces respire through gills (aquatic), while Aves and Mammalia respire through lungs (aerial). Mammals have an additional advantage of the diaphragm for efficient breathing.
• Reproduction: Pisces show mostly external fertilisation and oviparity; Aves show internal fertilisation and oviparity with parental care; Mammalia show internal fertilisation and viviparity with mammary glands for nourishing young ones. Viviparity and mammary glands give mammals a reproductive advantage over the other two classes.

Thus, though all three classes belong to the same phylum Chordata and share fundamental chordate features, they show remarkable adaptive diversity in locomotion, respiration and reproduction according to their habitats and modes of life.

a) Differences between DNA and RNA

Why DNA is a better genetic material than RNA

1. Stability: The deoxyribose sugar in DNA lacks the 2′–OH group present in ribose, so DNA is chemically more stable and less reactive than RNA. RNA is easily hydrolysed, especially in alkaline conditions.
2. Double-strandedness: DNA is double-stranded and its two strands are complementary. This allows accurate replication and repair of damage, so errors are less likely to be inherited. RNA being single-stranded cannot repair itself in this way.
3. Thymine instead of uracil: DNA contains thymine, which is more stable than uracil. Cytosine in DNA can spontaneously change to uracil, but this is recognised as damage and repaired; in RNA, uracil is normal, so such changes cannot be detected.
4. Rate of mutation: RNA mutates faster (RNA viruses show high mutation rates), while DNA mutates at a much lower rate. A slow rate of mutation is essential for the stability of the genetic information of a species.
5. Better expression and transmission: DNA can carry information for many generations without change, and its base sequence can be transcribed into RNA when needed. Thus DNA serves as a more reliable store of hereditary information.

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b) Mutation

Definition: A mutation is a sudden, heritable change in the base sequence of DNA or in the structure or number of chromosomes. It may occur in somatic cells or in germ cells; only germinal mutations are transmitted to the offspring.

Causes of mutation

1. Physical mutagens:
   • Ionising radiations such as X-rays, gamma rays and alpha particles break the DNA strands.
   • Non-ionising radiation such as ultraviolet light causes formation of thymine dimers.
2. Chemical mutagens:
   • Base analogues (e.g. 5-bromouracil) get incorporated in place of normal bases.
   • Deaminating agents (e.g. nitrous acid) change cytosine to uracil.
   • Alkylating agents (e.g. ethyl methane sulphonate) and intercalating agents (e.g. acridine orange) alter or insert bases.
3. Biological causes: Some viruses and transposons (jumping genes) insert themselves into DNA and disturb gene function.
4. Spontaneous causes: Errors during DNA replication, tautomeric shifts of bases, and depurination or deamination occurring naturally.

Effects of genetic mutation

1. Harmful (deleterious) effects: Most mutations are harmful. Examples: sickle-cell anaemia (point mutation in the β-globin gene), haemophilia, colour blindness, phenylketonuria, and chromosomal disorders such as Down's syndrome (trisomy of chromosome 21).
2. Beneficial effects: A few mutations are useful and provide variation for natural selection. Example: the mutation causing sickle-cell trait gives resistance to malaria in heterozygous persons; mutation in bacteria gives antibiotic resistance.
3. Neutral effects: Some mutations occur in non-coding regions and produce no observable change in the phenotype.
4. Source of variation and evolution: Mutations produce new alleles, which are the raw material of evolution and the basis of genetic diversity in populations.
5. Lethal effects: Certain mutations are so severe that they cause death of the organism, e.g. large deletions in essential genes.
6. Change in protein structure and function: A base change may alter an amino acid (missense), stop protein synthesis early (nonsense), or shift the reading frame (frameshift), producing abnormal or non-functional proteins.