NCERT Class 11 Biology: Viruses, Viroids, Prions and Lichens Explained Simply
- Dr.Sanjaykumar Pawar
Viruses
- Viruses are acellular (non-cellular)
Comparison of Viruses, Viroids, Prions, and Lichens showing structure, composition, and biological roles in Class 11 Biology.
organisms. - They are not considered fully living because they do not have a cellular structure.
- Outside a host cell, viruses remain inactive and crystalline (non-living form).
- Inside a living cell, they become active and use host machinery to reproduce.
- They are called obligate intracellular parasites (can live and reproduce only inside host cells).
- They kill or damage host cells during reproduction.
- The word virus means “poisonous fluid”.
Discovery of Viruses (Important for exams)
- Dmitri Ivanovsky (1892) discovered microbes causing tobacco mosaic disease.
- These agents were smaller than bacteria and passed through bacteria-proof filters.
- M.W. Beijerinck (1898) named them “virus” and called them “contagium vivum fluidum” (infectious living fluid).
- W.M. Stanley (1935) showed viruses can be crystallized and are made of proteins.
Structure of Viruses
- Viruses are made of nucleoprotein = nucleic acid + protein coat.
- Genetic material may be DNA or RNA (never both).
- Protein coat is called capsid.
- Capsid is made of small units called capsomeres.
- Capsid protects the genetic material.
- Viral shapes may be helical or polyhedral.
Types of Viral Genetic Material
- Plant viruses → usually single-stranded RNA
- Animal viruses → RNA or DNA (single or double-stranded)
- Bacteriophages (bacteria-infecting viruses) → usually double-stranded DNA
Diseases Caused by Viruses
- Humans:
- Common cold
- Influenza (flu)
- Mumps
- Smallpox
- Herpes
- AIDS
- Plants:
- Mosaic disease
- Leaf curling and rolling
- Yellowing of leaves
- Vein clearing
- Dwarfing and stunted growth
Viroids
- Discovered by T.O. Diener (1971).
- They are smaller than viruses.
- Viroids are made of only free RNA (no protein coat).
- RNA is of low molecular weight.
- They cause plant diseases like potato spindle tuber disease.
- Viroids are considered simplest infectious agents.
Prions
- Prions are infectious agents made only of abnormal proteins.
- They contain no DNA or RNA.
- They cause slow and fatal neurological diseases.
- Prions are similar in size to viruses but structurally different.
- Important diseases caused by prions:
- Mad cow disease (BSE) in cattle
- Creutzfeldt–Jacob disease (CJD) in humans
Lichens
- Lichens are symbiotic associations between algae and fungi.
- This is a mutually beneficial relationship.
Components of Lichen
- Algae = phycobiont (photosynthetic partner)
- Fungi = mycobiont (non-photosynthetic partner)
Functions
- Algae → prepare food by photosynthesis
- Fungi → provide protection, water, and minerals
Important Point
- Lichens look like a single organism, but they are actually two different organisms living together.
VIRUSES, VIROIDS, PRIONS AND LICHENS
│
├── General Note
│ ├── Not included in Whittaker’s five-kingdom classification
│ └── Include acellular and symbiotic forms
│
├── VIRUSES
│ ├── Nature
│ │ ├── Non-cellular organisms
│ │ ├── Inert outside host cell
│ │ ├── Obligate intracellular parasites
│ │ └── Show living + non-living characteristics
│ │
│ ├── Discovery
│ │ ├── Ivanowsky (1892) → Tobacco mosaic disease
│ │ ├── Beijerinck (1898) → Contagium vivum fluidum
│ │ └── Stanley (1935) → Crystallisation of viruses
│ │
│ ├── Structure
│ │ ├── Nucleoprotein
│ │ ├── Genetic material
│ │ │ ├── Either DNA or RNA (never both)
│ │ │ ├── Plants → usually ssRNA
│ │ │ ├── Animals → RNA or DNA
│ │ │ └── Bacteriophages → dsDNA
│ │ ├── Protein coat (capsid)
│ │ │ └── Made of capsomeres
│ │ └── Shapes
│ │ ├── Helical
│ │ └── Polyhedral
│ │
│ ├── Replication
│ │ ├── Inside host cell only
│ │ ├── Hijack host machinery
│ │ └── Host cell often destroyed
│ │
│ └── Diseases
│ ├── Mumps
│ ├── Smallpox
│ ├── Herpes
│ ├── Influenza
│ ├── AIDS
│ └── Plant diseases
│ ├── Mosaic formation
│ ├── Leaf rolling/curling
│ ├── Yellowing
│ └── Stunted growth
│
├── VIROIDS
│ ├── Discovered: T.O. Diener (1971)
│ ├── Nature
│ │ ├── Infectious RNA only
│ │ ├── No protein coat
│ │ └── Smaller than viruses
│ ├── Genetic material
│ │ └── Low molecular weight RNA
│ └── Disease
│ └── Potato spindle tuber disease
│
├── PRIONS
│ ├── Nature
│ │ ├── Infectious protein particles
│ │ ├── Abnormally folded proteins
│ │ └── Similar size to viruses
│ └── Diseases
│ ├── Bovine spongiform encephalopathy (BSE / mad cow disease)
│ └── Creutzfeldt–Jacob disease (CJD) in humans
│
└── LICHENS
├── Nature
│ ├── Symbiotic association
│ ├── Algae + Fungi
│ └── Appear as single organism
│
├── Components
│ ├── Phycobiont (algal partner)
│ │ └── Autotrophic (food producer)
│ └── Mycobiont (fungal partner)
│ └── Heterotrophic (supporting role)
│
├── Mutual Benefits
│ ├── Algae → food production
│ └── Fungi → protection, water & minerals
│
└── Ecological Role
└── Indicator of clean environment (implied ecological importance)
complete CBSE Class 11 exam practice set from the topic:
Viruses, Viroids, Prions and Lichens (NCERT-based, NEET + CBSE level)
🟢 VERY SHORT ANSWER QUESTIONS (1 MARK)
Q1. What are viruses made of?
Ans: Nucleic acid and protein coat (capsid).
Q2. Name the infectious RNA particle without protein coat.
Ans: Viroid.
Q3. What is the proteinaceous infectious agent called?
Ans: Prion.
Q4. What is the algal component of lichen called?
Ans: Phycobiont.
Q5. What is the fungal component of lichen called?
Ans: Mycobiont.
Q6. Are viruses cellular or acellular?
Ans: Acellular.
Q7. Who discovered viroids?
Ans: T.O. Diener.
Q8. Name one bacteriophage.
Ans: T4 phage.
🟡 SHORT ANSWER QUESTIONS (2–3 MARKS)
Q1. Why are viruses considered non-living outside the host cell?
Ans:
Viruses do not show metabolic activity outside a host cell. They remain inert and crystalline and cannot reproduce without host machinery.
Q2. What is a capsid?
Ans:
Capsid is the protein coat of a virus made of subunits called capsomeres that protects viral genetic material.
Q3. Differentiate between viroids and viruses.
Ans:
- Viruses contain nucleic acid + protein coat
- Viroids contain only RNA and no protein coat
Q4. What are prions? Name a disease caused by them.
Ans:
Prions are infectious protein particles causing neurological diseases. Example: Mad cow disease (BSE).
Q5. What is symbiosis in lichens?
Ans:
Lichens are mutual association between algae and fungi where algae prepare food and fungi provide shelter and nutrients.
Q6. Why are viruses called obligate parasites?
Ans:
Because they can reproduce only inside living host cells.
🔵 LONG ANSWER QUESTIONS (5 MARKS)
Q1. Explain structure, nature and replication of viruses.
Ans:
Viruses are acellular organisms made of nucleic acid (DNA or RNA) and protein coat called capsid. Capsid is made of capsomeres. Outside host they are inactive. Inside host, they inject genetic material and use host machinery to replicate, producing new viruses. They are obligate intracellular parasites.
Q2. Describe viroids and prions with examples.
Ans:
Viroids are infectious RNA molecules without protein coat causing plant diseases like potato spindle tuber disease.
Prions are infectious proteins causing diseases like Creutzfeldt-Jacob disease in humans and mad cow disease in cattle.
Q3. Explain lichens and their importance.
Ans:
Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont). Algae perform photosynthesis while fungi provide water and minerals. They indicate air pollution and are used as bioindicators.
Q4. Why are viruses difficult to classify as living or non-living?
Ans:
Viruses are non-living outside host (no metabolism, inert crystals) but show living properties inside host (reproduction, mutation). Hence they show characteristics of both living and non-living organisms.
🟠 MCQs (MULTIPLE CHOICE QUESTIONS)
Q1. Virus is made of:
A. Cells only
B. Protein only
C. DNA/RNA + protein coat
D. Lipids only
Ans: C
Q2. Viroids contain:
A. DNA + protein
B. RNA only
C. Protein only
D. Lipid + DNA
Ans: B
Q3. Prions are:
A. Bacteria
B. Virus
C. Infectious proteins
D. Fungi
Ans: C
Q4. Capsid is made of:
A. Lipids
B. Capsomeres
C. Carbohydrates
D. RNA only
Ans: B
Q5. Lichen is association between:
A. Algae + bacteria
B. Fungi + algae
C. Fungi + virus
D. Algae + protozoa
Ans: B
Q6. Disease caused by prion:
A. Tuberculosis
B. Malaria
C. Mad cow disease
D. Cholera
Ans: C
Q7. Viroids were discovered by:
A. Ivanovsky
B. Beijerinck
C. T.O. Diener
D. Pasteur
Ans: C
🔷 ASSERTION AND REASON QUESTIONS
Q1.
Assertion (A): Viruses are considered living organisms.
Reason (R): They can reproduce independently.
Ans:
A is false, R is false
Q2.
A: Viroids lack protein coat.
R: They contain only RNA.
Ans:
Both A and R are true, and R is correct explanation.
Q3.
A: Prions cause neurological diseases.
R: They are made of nucleic acids.
Ans:
A is true, R is false
Q4.
A: Lichens are good air pollution indicators.
R: They are very sensitive to sulfur dioxide.
Ans:
Both true, R explains A.
🟣 FILL IN THE BLANKS
Q1. Viruses are ______ outside the host cell.
Ans: Inert
Q2. Capsid is made of small units called ______.
Ans: Capsomeres
Q3. Viroids consist of only ______.
Ans: RNA
Q4. Fungal partner in lichen is called ______.
Ans: Mycobiont
Q5. Algal partner in lichen is called ______.
Ans: Phycobiont
🟤 MATCH THE FOLLOWING
| Column A | Column B |
|---|---|
| Virus | Protein + nucleic acid |
| Viroid | RNA only |
| Prion | Infectious protein |
| Lichen | Algae + fungi |
| Capsid | Protein coat |
Answers:
Virus → Protein + nucleic acid
Viroid → RNA only
Prion → Infectious protein
Lichen → Algae + fungi
Capsid → Protein coat
🟠 CASE STUDY QUESTION
Case:
A scientist studies an infectious agent that contains only RNA and no protein coat. It causes disease in potato plants.
Questions:
Q1. Identify the agent.
Ans: Viroid
Q2. Name the disease caused.
Ans: Potato spindle tuber disease
Q3. Does it have a protein coat?
Ans: No
Q4. Is it larger or smaller than viruses?
Ans: Smaller than viruses
Q5. Who discovered it?
Ans: T.O. Diener
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