Biological ClassificationClass 11 Biology NCERT Solutions
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Q1Exercises
Discuss how classification systems have undergone several changes over a period of time?
Solution
Classification systems for living organisms have evolved significantly over time as our understanding of biology has deepened. The changes reflect a shift from simple, practical classifications to more complex systems based on scientific criteria and evolutionary relationships.
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Earliest Attempts: The earliest classifications were not scientific but based on practical needs, such as identifying organisms for food, shelter, and clothing. Aristotle made the first scientific attempt, classifying plants into trees, shrubs, and herbs based on simple morphological characters, and animals into those with red blood and those without.
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Two Kingdom System: In Linnaeus' time, a Two Kingdom system was established, dividing all organisms into Kingdom Plantae and Kingdom Animalia. This system was simple and easy to use but had significant drawbacks:
- It did not distinguish between eukaryotes and prokaryotes.
- It failed to separate unicellular and multicellular organisms.
- It grouped photosynthetic organisms (like green algae) with non-photosynthetic organisms (like fungi) under Kingdom Plantae simply because both had cell walls.
- Many organisms, like bacteria and protists, did not fit neatly into either kingdom.
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Five Kingdom System: Due to the inadequacies of the two-kingdom system, a need was felt to include characteristics like cell structure, mode of nutrition, body organisation, and evolutionary relationships (phylogeny). In 1969, R.H. Whittaker proposed the Five Kingdom Classification, which is widely accepted. The five kingdoms are:
- Monera: All prokaryotic organisms.
- Protista: All unicellular eukaryotic organisms.
- Fungi: Heterotrophic, multicellular eukaryotes with chitinous cell walls.
- Plantae: Autotrophic, multicellular eukaryotes with cellulosic cell walls.
- Animalia: Heterotrophic, multicellular eukaryotes without cell walls.
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Three-Domain System: More recently, a three-domain system has been proposed, which further refines classification by dividing Kingdom Monera into two domains (Archaea and Bacteria) based on fundamental differences in their cell wall structure and genetics. The third domain, Eukarya, includes all eukaryotes (Protista, Fungi, Plantae, and Animalia).
These changes show a progression towards a more natural classification system that reflects the evolutionary history and relationships among organisms.
Q2Exercises
State two economically important uses of:
(a)
heterotrophic bacteria
(b)
archaebacteria
Solution
(a) Two economically important uses of heterotrophic bacteria are:
- Curd Production: Bacteria like Lactobacillus are used to convert milk into curd, which is a major dairy product consumed worldwide.
- Antibiotic Production: Many antibiotics, which are crucial for treating bacterial diseases, are produced from heterotrophic bacteria (e.g., Streptomyces).
- Nitrogen Fixation: Bacteria like Rhizobium live in the root nodules of leguminous plants and fix atmospheric nitrogen into usable forms, acting as natural bio-fertilisers and increasing crop yield.
(b) Two economically important uses of archaebacteria are:
- Biogas Production: Methanogens, a type of archaebacteria found in marshy areas and the gut of ruminant animals, are responsible for producing methane (biogas) from animal dung and organic waste. Biogas is used as a fuel.
- Biotechnology: Archaebacteria that live in extreme environments (like thermoacidophiles in hot springs) produce enzymes that can function at high temperatures. These thermostable enzymes are valuable in biotechnological processes and industries.
Q3Exercises
What is the nature of cell-walls in diatoms?
Solution
The cell walls of diatoms are unique and characteristic of the group. They are composed of the following features:
- Composition: The walls are embedded with silica, which makes them hard, rigid, and indestructible.
- Structure: The cell wall consists of two thin, overlapping shells that fit together perfectly, much like a soap box. This structure is called a frustule.
- Persistence: Due to their indestructible nature, the cell walls of diatoms accumulate in their habitat over billions of years. This massive deposit is known as 'diatomaceous earth'.
Q4Exercises
Find out what do the terms 'algal bloom' and 'red-tides' signify.
Solution
Algal Bloom:
The term 'algal bloom' refers to a rapid and excessive growth of algae, particularly cyanobacteria (blue-green algae), in a water body. These blooms often occur in polluted water bodies rich in nutrients like nitrogen and phosphorus. They can form a thick layer on the water surface, which depletes oxygen levels when the algae die and decompose, harming aquatic life.
Red Tides:
The term 'red tide' refers to a specific type of harmful algal bloom caused by the rapid multiplication of certain photosynthetic dinoflagellates, such as Gonyaulax. These organisms contain red pigments, and their high concentration makes the sea appear red. Red tides are dangerous because these dinoflagellates release toxins that can kill other marine animals like fish and shellfish, and can be harmful to humans who consume the contaminated seafood.
Q5Exercises
How are viroids different from viruses?
Solution
Viroids and viruses are both infectious agents, but they differ in several key aspects:
| Feature | Viruses | Viroids |
|---|---|---|
| Structure | Composed of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. | Consists only of a free, single-stranded RNA molecule; it lacks a protein coat. |
| Genetic Material | Can be DNA (single or double-stranded) or RNA (single or double-stranded). | Is always a low molecular weight, single-stranded RNA. |
| Size | Larger than viroids. | Smaller than viruses. |
| Discovery | Discovered by Dmitri Ivanowsky in 1892. | Discovered by T.O. Diener in 1971. |
| Example Disease | Cause diseases like influenza, AIDS, and mosaic disease of tobacco. | Cause diseases primarily in plants, such as potato spindle tuber disease. |
Q6Exercises
Describe briefly the four major groups of Protozoa.
Solution
Protozoans are heterotrophic, single-celled eukaryotes, considered to be primitive relatives of animals. They are classified into four major groups based on their locomotory structures:
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Amoeboid Protozoans:
- Habitat: Live in freshwater, seawater, or moist soil.
- Locomotion: They move and capture prey using pseudopodia (false feet), which are extensions of their cytoplasm.
- Examples: Amoeba is a free-living example. Marine forms often have silica shells. Entamoeba is a parasitic form that causes amoebic dysentery.
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Flagellated Protozoans:
- Habitat: They can be either free-living or parasitic.
- Locomotion: They possess one or more flagella for movement.
- Examples: Parasitic forms are known to cause diseases. Trypanosoma causes sleeping sickness.
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Ciliated Protozoans:
- Habitat: These are aquatic organisms.
- Locomotion: They are very active due to the presence of thousands of cilia covering their body. The coordinated movement of these cilia helps in locomotion and also steers food-laden water into a cavity called the gullet.
- Examples: Paramoecium is a common example.
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Sporozoans:
- Characteristics: This group includes diverse organisms that are exclusively parasitic. Their defining feature is an infectious, spore-like stage in their life cycle. They lack specialized locomotory structures.
- Examples: The most notorious sporozoan is Plasmodium, the malarial parasite, which has a devastating effect on the human population.
Q7Exercises
Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Solution
Yes, while the vast majority of plants are autotrophic (producing their own food through photosynthesis), some plants are partially heterotrophic. They supplement their nutrition by obtaining nutrients from other sources. These can be categorized as:
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Insectivorous Plants: These plants grow in nutrient-deficient soil (especially nitrogen-deficient soil). They trap and digest insects and other small animals to obtain nitrogen and other essential nutrients. However, they still perform photosynthesis. Examples include:
- Bladderwort
- Venus fly trap
- Pitcher plant
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Parasitic Plants: These plants derive some or all of their nutritional requirements from another living plant (the host). Cuscuta (dodder) is an example of a total parasite that has lost its chlorophyll and cannot photosynthesize, depending entirely on its host for food.
Q8Exercises
What do the terms phycobiont and mycobiont signify?
Solution
The terms phycobiont and mycobiont refer to the two components of a lichen, which is a symbiotic association between algae and fungi.
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Phycobiont: This is the algal component of the lichen. The phycobiont is autotrophic, meaning it performs photosynthesis and prepares food for both itself and its fungal partner.
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Mycobiont: This is the fungal component of the lichen. The mycobiont is heterotrophic and provides the structural framework, shelter, and protection for the alga. It also absorbs mineral nutrients and water from the substrate for its partner.
This mutually beneficial relationship allows lichens to colonize harsh environments where neither organism could survive alone.
Q9Exercises
Give a comparative account of the classes of Kingdom Fungi under the following:
(i)
mode of nutrition
(ii)
mode of reproduction
Solution
Here is a comparative account of the four classes of Kingdom Fungi:
| Class | (i) Mode of Nutrition | (ii) Mode of Reproduction |
|---|---|---|
| Phycomycetes | Found in aquatic habitats, on decaying wood, in moist places, or as obligate parasites on plants. | Asexual: By motile zoospores or non-motile aplanospores, produced endogenously in a sporangium. |
| Sexual: By fusion of gametes (isogamous, anisogamous, or oogamous) leading to the formation of a zygospore. | ||
| Ascomycetes | Saprophytic, decomposers, parasitic, or coprophilous (growing on dung). | Asexual: By non-motile spores called conidia, produced exogenously on conidiophores. |
| Sexual: By sexual spores called ascospores, produced endogenously in a sac-like structure called an ascus. Asci are arranged in fruiting bodies called ascocarps. | ||
| Basidiomycetes | Grow as decomposers in soil and on wood (logs, stumps), or as parasites (rusts and smuts). | Asexual: Asexual spores are generally not found. Vegetative reproduction by fragmentation is common. |
| Sexual: No sex organs. Plasmogamy occurs by fusion of two somatic cells, leading to a dikaryotic basidium. Karyogamy and meiosis in the basidium produce four basidiospores exogenously. | ||
| Deuteromycetes | Saprophytes or parasites. A large number are decomposers of litter, helping in mineral cycling. | Asexual: Reproduce only by asexual spores known as conidia. |
| Sexual: The sexual stage (perfect stage) is either absent or has not yet been discovered. For this reason, they are called 'imperfect fungi'. |
Q10Exercises
What are the characteristic features of Euglenoids?
Solution
Euglenoids are a group of unicellular protists with the following characteristic features:
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Habitat: They are primarily found in freshwater, particularly in stagnant water bodies.
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Cell Wall: They lack a rigid cell wall. Instead, they have a flexible, protein-rich outer layer called a pellicle, which allows them to change their shape.
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Locomotion: They possess two flagella, typically one short and one long, which help in movement.
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Mode of Nutrition: Euglenoids exhibit a mixotrophic mode of nutrition.
- In the presence of sunlight, they are photosynthetic (autotrophic) like plants.
- When deprived of sunlight, they behave as heterotrophs, capturing and ingesting other smaller organisms (predation).
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Photosynthetic Pigments: The pigments found in euglenoids (chlorophyll a and b) are identical to those present in higher plants.
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Example: A common example is Euglena.
Q11Exercises
Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Solution
Structure of Viruses:
Viruses are non-cellular, infectious agents that can only replicate inside the living cells of other organisms. They have an inert, crystalline structure outside a host cell. A virus particle consists of two main components:
- Genetic Material: The core of the virus contains the genetic material, which carries the information for replication.
- Capsid: The genetic material is protected by an outer protein coat called the capsid. The capsid is made of small protein subunits called capsomeres, which are arranged in specific geometric forms, such as helical or polyhedral.
Nature of Genetic Material:
A key feature of viruses is that they contain only one type of nucleic acid, either DNA or RNA, but never both.
- Plant Viruses: Generally have single-stranded RNA (ssRNA).
- Animal Viruses: Can have either single or double-stranded RNA (ssRNA or dsRNA), or double-stranded DNA (dsDNA).
- Bacteriophages (viruses that infect bacteria): Usually have double-stranded DNA (dsDNA).
Four Common Viral Diseases:
- Influenza (Flu)
- Common Cold
- Mumps
- AIDS (Acquired Immunodeficiency Syndrome)
Q12Exercises
Organise a discussion in your class on the topic - Are viruses living or nonliving?
Solution
Viruses are fascinating entities that exist on the borderline between living and non-living things. A discussion on this topic would involve arguments for both perspectives.
Arguments for Viruses being Non-living:
- Acellular Structure: They lack a cellular structure, which is the basic unit of all known living organisms. They do not have a cytoplasm, nucleus, or other organelles.
- Inert Outside Host: Outside a living host cell, viruses are inert, crystalline particles with no metabolic activity. They cannot grow, respire, or reproduce on their own.
- Can be Crystallised: W.M. Stanley showed that viruses could be crystallised, a property typically associated with non-living chemical compounds rather than living organisms.
Arguments for Viruses being Living:
- Presence of Genetic Material: They possess genetic material (either DNA or RNA), which is a fundamental characteristic of life. This genetic material can mutate and evolve.
- Replication: Once inside a host cell, they take over the host's cellular machinery to replicate themselves, producing numerous copies. This ability to reproduce is a key feature of life.
- Host Specificity: They are obligate parasites and often show specificity for their hosts, indicating a complex biological interaction.
Conclusion:
Viruses cannot be definitively classified as either living or non-living. They exhibit characteristics of both. They are non-living when outside a host but behave like living organisms once they infect a cell. Therefore, they are often described as "organisms at the edge of life," representing a unique link between living and non-living matter.