Ribosomes CBSE Class 11: MCQs, Case Study & Assertion Reason Questions
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| Structure and function of ribosomes showing 70S and 80S subunits involved in protein synthesis. |
Ribosomes
1. What are Ribosomes?
- Ribosomes are tiny granular (grain-like) structures present inside cells.
- They were first observed under an electron microscope.
- They were discovered by in 1953.
- Under the electron microscope, they appear as dense particles.
2. Composition of Ribosomes
- Ribosomes are made up of:
- Ribosomal RNA (rRNA)
- Proteins
- Therefore, ribosomes are ribonucleoprotein particles.
3. Membrane Around Ribosomes
- Ribosomes are not surrounded by any membrane.
- Hence, they are called non-membranous cell organelles.
4. Function of Ribosomes
- Ribosomes are the site of protein synthesis.
- They help in translating genetic information into proteins.
- Therefore, ribosomes are known as the "Protein Factories of the Cell."
Types of Ribosomes
5. Ribosomes in Eukaryotes
- Eukaryotic cells contain 80S ribosomes.
- Examples:
- Plant cells
- Animal cells
- Fungi
- Protists
6. Ribosomes in Prokaryotes
- Prokaryotic cells contain 70S ribosomes.
- Examples:
- Bacteria
- Cyanobacteria
Ribosomal Subunits
7. Structure of 80S Ribosome
- An 80S ribosome has two subunits:
- Large subunit = 60S
- Small subunit = 40S
- Therefore:
- 80S = 60S + 40S
8. Structure of 70S Ribosome
- A 70S ribosome also has two subunits:
- Large subunit = 50S
- Small subunit = 30S
- Therefore:
- 70S = 50S + 30S
Meaning of "S"
9. What is S?
- "S" stands for Svedberg Unit.
- It is named after .
10. What Does Svedberg Unit Measure?
- It measures the sedimentation coefficient.
- Sedimentation means settling down during centrifugation.
- It indirectly indicates:
- Size of the particle
- Shape of the particle
- Density of the particle
11. Important NEET Point
- Svedberg units are not additive.
- Therefore:
- 60S + 40S = 80S (not 100S)
- 50S + 30S = 70S (not 80S)
Reason: Sedimentation coefficient depends on size, shape, and density together.
Quick NEET Facts
✅ Ribosomes were discovered by George Palade (1953).
✅ Ribosomes are made of rRNA + proteins.
✅ Ribosomes are non-membranous.
✅ Ribosomes are the site of protein synthesis.
✅ Eukaryotic ribosomes = 80S (60S + 40S).
✅ Prokaryotic ribosomes = 70S (50S + 30S).
✅ "S" = Svedberg Unit (sedimentation coefficient).
✅ Svedberg units are not additive.
One-Line NEET Revision
Ribosomes are non-membranous ribonucleoprotein particles discovered by George Palade in 1953; they are the site of protein synthesis and occur as 80S (60S + 40S) in eukaryotes and 70S (50S + 30S) in prokaryotes.
RIBOSOMES
│
├── Discovery
│ ├── Observed under electron microscope
│ ├── Dense granular particles
│ └── Discovered by George Palade (1953)
│
├── Composition
│ ├── Ribosomal RNA (rRNA)
│ └── Proteins
│
├── Nature
│ ├── Non-membranous organelle
│ └── Ribonucleoprotein particle
│
├── Function
│ ├── Site of protein synthesis
│ └── Called "Protein Factory of Cell"
│
├── Types of Ribosomes
│ │
│ ├── Prokaryotic Ribosome
│ │ ├── 70S
│ │ ├── Large Subunit = 50S
│ │ └── Small Subunit = 30S
│ │
│ └── Eukaryotic Ribosome
│ ├── 80S
│ ├── Large Subunit = 60S
│ └── Small Subunit = 40S
│
├── Svedberg Unit (S)
│ ├── S = Sedimentation Coefficient
│ ├── Measures rate of sedimentation
│ ├── Indicates size and density
│ └── Not additive
│
├── Important Facts
│ ├── 60S + 40S = 80S
│ ├── 50S + 30S = 70S
│ ├── S values are not added mathematically
│ └── Depends on size, shape and density
│
└── NEET Revision
├── Discovered by George Palade
├── Made of rRNA + proteins
├── Non-membranous
├── Protein synthesis site
├── Eukaryotes → 80S (60S + 40S)
└── Prokaryotes → 70S (50S + 30S)
RIBOSOMES QUESTION BANK (CBSE CLASS 11)
A. Multiple Choice Questions (MCQs)
1. Ribosomes were discovered by:
a) Robert Hooke
b) George Palade
c) Schleiden
d) Virchow
Answer: (b) George Palade
2. Ribosomes are composed of:
a) DNA and proteins
b) RNA and lipids
c) RNA and proteins
d) DNA and RNA
Answer: (c) RNA and proteins
3. Ribosomes are:
a) Membranous organelles
b) Double membranous organelles
c) Non-membranous organelles
d) Vacuoles
Answer: (c) Non-membranous organelles
4. The ribosomes present in prokaryotes are:
a) 80S
b) 90S
c) 70S
d) 60S
Answer: (c) 70S
5. The large subunit of 80S ribosome is:
a) 50S
b) 30S
c) 60S
d) 40S
Answer: (c) 60S
6. The site of protein synthesis is:
a) Nucleus
b) Mitochondria
c) Ribosome
d) Golgi body
Answer: (c) Ribosome
B. Very Short Answer Questions
1. Who discovered ribosomes?
Answer: George Palade.
2. What are ribosomes made of?
Answer: rRNA and proteins.
3. Are ribosomes membrane-bound?
Answer: No.
4. What is the function of ribosomes?
Answer: Protein synthesis.
5. What does S stand for in 70S ribosome?
Answer: Svedberg Unit.
C. Short Answer Questions
1. Why are ribosomes called protein factories?
Answer: Ribosomes are the sites where proteins are synthesized. They join amino acids according to genetic instructions to form proteins. Hence they are called protein factories of the cell.
2. Differentiate between 70S and 80S ribosomes.
| 70S Ribosome | 80S Ribosome |
|---|---|
| Found in prokaryotes | Found in eukaryotes |
| 50S + 30S subunits | 60S + 40S subunits |
| Smaller | Larger |
3. What is Svedberg Unit?
Answer: Svedberg Unit (S) is the sedimentation coefficient that indicates the rate of settling of particles during centrifugation.
D. Long Answer Questions
1. Describe the structure and composition of ribosomes.
• Ribosomes are granular structures discovered by George Palade in 1953.
• They are made of rRNA and proteins.
• Ribosomes are non-membranous organelles.
• Each ribosome consists of a large and a small subunit.
• Eukaryotic ribosomes are 80S (60S + 40S).
• Prokaryotic ribosomes are 70S (50S + 30S).
• Ribosomes are the sites of protein synthesis.
2. Explain the significance of ribosomes.
• Ribosomes synthesize proteins.
• They read information carried by mRNA.
• They join amino acids to form proteins.
• Proteins are essential for growth, repair and metabolism.
• Therefore ribosomes are called protein factories.
E. Fill in the Blanks
1. Ribosomes were discovered by George Palade.
2. Ribosomes are composed of rRNA and proteins.
3. Prokaryotic ribosomes are 70S.
4. Eukaryotic ribosomes are 80S.
5. Ribosomes are the site of protein synthesis.
6. S stands for Svedberg Unit.
F. Assertion and Reason
Assertion (A): Ribosomes are called protein factories.
Reason (R): Ribosomes are the site of protein synthesis.
Answer: Both A and R are true and R is the correct explanation of A.
Assertion (A): Ribosomes are membrane-bound organelles.
Reason (R): Ribosomes contain rRNA and proteins.
Answer: A is false but R is true.
G. Statement Based Questions
Identify the correct statements:
(i) Ribosomes are non-membranous.
(ii) Ribosomes contain DNA and lipids.
(iii) Ribosomes synthesize proteins.
(iv) George Palade discovered ribosomes.
a) i, iii and iv
b) ii and iii
c) i and ii
d) All of these
Answer: a) i, iii and iv
H. Match the Columns
| Column A | Column B |
|---|---|
| 1. George Palade | a. Protein synthesis |
| 2. Ribosome | b. 50S + 30S |
| 3. 70S Ribosome | c. Discovery of ribosome |
| 4. 80S Ribosome | d. 60S + 40S |
Answers:
1-c
2-a
3-b
4-d
I. Case Study Questions
A scientist observed small dense particles inside a cell using an electron microscope. These particles were made of RNA and proteins and were involved in protein synthesis.
1. Name the organelle.
Answer: Ribosome
2. Who discovered it?
Answer: George Palade
3. What is its major function?
Answer: Protein synthesis
4. Which ribosome type is found in bacteria?
Answer: 70S Ribosome
5. What are ribosomes made of?
Answer: rRNA and proteins
J. HOTS Questions
1. Why is 60S + 40S equal to 80S and not 100S?
Because S denotes Svedberg Unit, which depends on size, shape and density. Therefore S values are not additive.
2. Why would a cell die if ribosomes stop functioning?
Without ribosomes proteins cannot be synthesized. Since proteins are essential for all cellular activities, the cell would eventually die.
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