Biological Classification — NCERT Solutions

Given/Concept: Most plants are autotrophic (prepare their own food by photosynthesis), but some have evolved to supplement their nutrition by heterotrophic means.

Yes, some plants are partially heterotrophic. These include:

1. Insectivorous (Carnivorous) Plants:

• These plants grow in nitrogen-deficient soils and supplement their nitrogen requirement by trapping and digesting insects.
• They are partially heterotrophic because they are photosynthetic but obtain minerals/nitrogen from animals.
• Examples:
• Drosera (Sundew) — has sticky glandular hairs to trap insects.
• Nepenthes (Pitcher plant) — has a pitcher-shaped leaf to trap and digest insects.
• Utricularia (Bladderwort) — aquatic plant with bladder-like traps.
• Venus flytrap (Dionaea muscipula) — has hinged leaves that snap shut on insects.

2. Parasitic Plants:

• Cuscuta (Dodder plant) — a total stem parasite that has lost chlorophyll and depends entirely on the host for nutrition. (This is actually fully heterotrophic.)
• Viscum (Mistletoe) — a partial parasite; it has chlorophyll and can photosynthesize but absorbs water and minerals from the host plant.

Conclusion: Insectivorous plants like Drosera and Nepenthes are the best examples of partially heterotrophic plants — they photosynthesize but also digest insects to obtain nitrogen and other nutrients.

Given/Concept: These terms relate to the two components of lichens, which are symbiotic associations.

Phycobiont:

• The term 'phycobiont' refers to the algal component of a lichen.
• The word is derived from phyco = algae and bion = living organism.
• The phycobiont is autotrophic — it contains chlorophyll and prepares food (carbohydrates) by photosynthesis for both itself and the fungal partner.
• Example: Green algae (Trebouxia) or Cyanobacteria (Nostoc).

Mycobiont:

• The term 'mycobiont' refers to the fungal component of a lichen.
• The word is derived from myco = fungus and bion = living organism.
• The mycobiont is heterotrophic — it cannot prepare its own food.
• The fungus provides shelter, absorbs water and mineral nutrients from the substrate for both partners.
• It forms the structural body (thallus) of the lichen.

Symbiotic Relationship:
$$\text{Phycobiont (Algae)} + \text{Mycobiont (Fungi)} \rightarrow \text{Lichen (Mutualistic association)}$$

This is a mutualistic symbiosis — both partners benefit from the association. Lichens are also excellent pollution indicators as they cannot survive in polluted environments.

Given/Concept: Kingdom Fungi is divided into four classes — Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.

(i) Mode of Nutrition:

(ii) Mode of Reproduction:

Note: Deuteromycetes are called Imperfect Fungi because only the asexual phase is known. When the sexual stage is discovered, they are reclassified into other groups.

Given/Concept: Euglenoids are a group of unicellular protists belonging to Kingdom Protista.

Characteristic Features of Euglenoids:

1. Habitat: They are mostly found in fresh water (stagnant water rich in organic matter).

2. Cell structure:

• They are unicellular eukaryotes.
• Instead of a cell wall, they have a protein-rich layer called pellicle which makes their body flexible.

3. Locomotion:

• They possess two flagella — one short and one long — for locomotion.
• The longer flagellum emerges from a flask-shaped structure called the gullet at the anterior end.

4. Mode of Nutrition:

• They are mixotrophic — they can be both autotrophic (in the presence of sunlight, they photosynthesise using chloroplasts) and heterotrophic (in the absence of sunlight, they feed on organic matter like animals).
• This dual mode of nutrition makes them a connecting link between plants and animals.

5. Photosynthetic pigments:

• They contain chlorophyll a and b (same as in higher plants).
• They have pyrenoids for starch synthesis.

6. Eyespot (Stigma):

• They have a photosensitive organelle called eyespot (stigma) which helps them detect light.

7. Reproduction:

• Reproduction is primarily by asexual means — longitudinal binary fission.
• Sexual reproduction is not well documented.

8. Reserve food: Stored as paramylon (a carbohydrate).

Example: Euglena

Significance: Euglenoids are considered a connecting link between plants and animals due to their mixotrophic nature.

Given/Concept: Viruses are non-cellular, obligate intracellular parasites discovered by D.J. Ivanowsky (1892) and characterised as nucleoprotein particles.

Structure of Viruses:

1. Size: Viruses are extremely small, ranging from 20 nm to 300 nm in diameter. They are smaller than bacteria and can only be seen under an electron microscope.

2. Basic Structure:

• Viruses are nucleoprotein particles consisting of:
• A nucleic acid core (genetic material)
• A protein coat called capsid surrounding the nucleic acid
• The capsid is made up of protein subunits called capsomeres.

3. Envelope:

• Some viruses (e.g., HIV, Influenza virus) have an additional lipoprotein envelope surrounding the capsid — these are called enveloped viruses.
• Viruses without an envelope are called naked viruses.

4. Shape: Viruses may be:

• Helical (rod-shaped) — e.g., Tobacco Mosaic Virus (TMV)
• Icosahedral (polyhedral) — e.g., Adenovirus
• Complex — e.g., Bacteriophage (T4 phage)

Nature of Genetic Material:

1. Viruses contain either DNA or RNA as their genetic material, never both simultaneously.

2. Plant viruses generally contain single-stranded RNA as genetic material.

• Example: Tobacco Mosaic Virus (TMV) — ssRNA

3. Animal viruses may contain:

• Double-stranded DNA (e.g., Herpes virus, Adenovirus)
• Single-stranded RNA (e.g., HIV, Influenza virus)
• Double-stranded RNA (e.g., Reovirus)

4. Bacteriophages (viruses that infect bacteria) generally contain double-stranded DNA.

• Example: T4 bacteriophage

5. Viruses with RNA as genetic material are called retroviruses if they use reverse transcriptase to make DNA from RNA (e.g., HIV).

Summary Table:

Four Common Viral Diseases:

(Other examples: AIDS — HIV; Dengue — Dengue virus; Rabies — Rabies virus)

Given/Concept: Viruses exhibit characteristics of both living and non-living entities, making their classification controversial.

This is an activity-based question. Below is a structured framework for the class discussion:

Arguments that Viruses are NON-LIVING:

1. They can be crystallised (like chemicals) and stored for long periods — e.g., TMV was crystallised by W.M. Stanley in 1935.
2. They have no cellular structure — they lack a cell membrane, cytoplasm, and organelles.
3. They do not carry out metabolism (no respiration, no energy production) outside a host cell.
4. They cannot grow or divide independently.
5. They do not respond to stimuli on their own.
6. They are essentially inert chemical molecules (nucleoprotein) outside a host.

Arguments that Viruses are LIVING:

1. They possess genetic material (DNA or RNA) — the fundamental criterion of life.
2. They can reproduce (replicate) inside a host cell.
3. They show mutations — genetic changes occur, leading to new strains (e.g., new COVID-19 variants).
4. They show host specificity — they can recognise and attach to specific host cells.
5. They are subject to natural selection and evolution.
6. They can cause diseases — indicating biological interaction with living systems.

Conclusion:

Viruses are considered to be on the borderline between living and non-living. They are often described as 'living chemicals' or obligate intracellular parasites. Outside a host cell, they behave as non-living matter; inside a host cell, they exhibit all properties of living organisms.

$$\boxed{\text{Viruses are neither completely living nor completely non-living — they are at the boundary of life.}}$$

Note to students: During the class discussion, students should be divided into two groups — one arguing for 'living' and one for 'non-living' — and present their points with examples. The teacher should then summarise the dual nature of viruses.