🧵 Ribosomes, Cytoskeleton & Accessory Organelles
Study ribosomes (70S vs 80S), cytoskeleton and centrioles for CUET Agriculture. Microtubules, microfilaments, cilia vs flagella explained.
Ribosomes
Ribosomes are the protein factories of the cell. Unlike most organelles, they are not enclosed by a membrane, making them accessible for rapid protein synthesis.
- Discovered by George Palade (1955) — sometimes called Palade particles.
- Non-membrane-bound organelles; site of protein synthesis (translation).
- Composed of rRNA (~60%) and proteins (~40%). The rRNA plays a catalytic role in forming peptide bonds between amino acids.
Types
| Feature | 70S Ribosomes | 80S Ribosomes |
|---|---|---|
| Subunits | 50S + 30S | 60S + 40S |
| Found in | Prokaryotes, mitochondria, chloroplasts | Eukaryotic cytoplasm |
| Sedimentation coefficient | 70 Svedberg units | 80 Svedberg units |
- Polyribosomes (polysomes) — multiple ribosomes attached to a single mRNA strand; allow simultaneous translation of multiple copies of a protein. This is an efficient way for the cell to produce large quantities of a protein quickly.
- Free ribosomes synthesize cytoplasmic proteins (proteins used within the cell); bound ribosomes (on RER) synthesize secretory and membrane proteins (proteins destined for export or insertion into membranes).
Centrosome and Centrioles
The centrosome is the primary microtubule organizing center (MTOC) of the cell. It plays a critical role during cell division by organizing the spindle apparatus.
Pro Content Locked
Upgrade to Pro to access this lesson and all other premium content.
₹99 charged monthly · Cancel anytime
- All Agriculture & Banking Courses
- AI Lesson Questions (100/day)
- AI Doubt Solver (50/day)
- Glows & Grows Feedback (30/day)
- AI Section Quiz (20/day)
- 22-Language Translation (100/day)
- Recall Questions (20/day)
- AI Quiz (15/day)
- AI Quiz Paper Analysis (100/day)
- AI Step-by-Step Explanations (100/day)
- Spaced Repetition Recall (FSRS)
- AI Tutor
- Immersive Text Questions
- Audio Lessons — Hindi & English
- Mock Tests & Previous Year Papers
- Summary & Mind Maps
- XP, Levels, Leaderboard & Badges
- Generate New Classrooms
- Voice AI Teacher (AgriDots Live)
- AI Revision Assistant
- Knowledge Gap Analysis
- Interactive Revision (LangGraph)
🔒 Secure via Razorpay · Cancel anytime · No hidden fees
Ribosomes
Ribosomes are the protein factories of the cell. Unlike most organelles, they are not enclosed by a membrane, making them accessible for rapid protein synthesis.
- Discovered by George Palade (1955) — sometimes called Palade particles.
- Non-membrane-bound organelles; site of protein synthesis (translation).
- Composed of rRNA (~60%) and proteins (~40%). The rRNA plays a catalytic role in forming peptide bonds between amino acids.
Types
| Feature | 70S Ribosomes | 80S Ribosomes |
|---|---|---|
| Subunits | 50S + 30S | 60S + 40S |
| Found in | Prokaryotes, mitochondria, chloroplasts | Eukaryotic cytoplasm |
| Sedimentation coefficient | 70 Svedberg units | 80 Svedberg units |
- Polyribosomes (polysomes) — multiple ribosomes attached to a single mRNA strand; allow simultaneous translation of multiple copies of a protein. This is an efficient way for the cell to produce large quantities of a protein quickly.
- Free ribosomes synthesize cytoplasmic proteins (proteins used within the cell); bound ribosomes (on RER) synthesize secretory and membrane proteins (proteins destined for export or insertion into membranes).
Centrosome and Centrioles
The centrosome is the primary microtubule organizing center (MTOC) of the cell. It plays a critical role during cell division by organizing the spindle apparatus.
- Found in animal cells (absent in most plant cells, except some lower plants). Despite lacking centrioles, plant cells still form spindle fibers during division — they use other MTOCs.
- Centrosome — contains two centrioles arranged at right angles (perpendicular) to each other, surrounded by amorphous pericentriolar material (PCM). The PCM is where microtubule nucleation actually occurs.
Centriole Structure
- Cylindrical structure made of 9 triplets of microtubules arranged in a cartwheel pattern (9 + 0 arrangement; no central microtubules). Compare this with the 9 + 2 arrangement of cilia and flagella.
- Each triplet consists of tubules designated A, B, and C.
- Connected by C-A linkers.
Functions
- Form the spindle apparatus during cell division (asters and spindle fibers). The spindle is essential for pulling chromosomes apart.
- Form the basal body of cilia and flagella. A centriole migrates to the cell surface and acts as a template for the growth of a cilium or flagellum.
- Organize microtubules — act as MTOC (Microtubule Organizing Center).
Cilia and Flagella
Cilia and flagella are hair-like projections from the cell surface that enable movement. Despite their different sizes and beating patterns, they share the same fundamental internal structure.
- Both are involved in locomotion and movement of fluids across cell surfaces.
- Both arise from basal bodies (structurally similar to centrioles).
- Covered by the plasma membrane.
| Feature | Cilia | Flagella |
|---|---|---|
| Length | Short (5–10 μm) | Long (up to 150 μm) |
| Number | Many per cell | Few (1–4) per cell |
| Movement | Coordinated, wave-like (rowing) | Undulating, whip-like |
| Examples | Paramecium, tracheal epithelium | Sperm cell, Euglena |
Internal Structure (Axoneme)
The internal structure of both cilia and flagella is called the axoneme, and it has a highly conserved arrangement:
- 9 + 2 arrangement — 9 doublets of microtubules arranged peripherally + 2 central singlet microtubules. This is different from the 9 + 0 arrangement of centrioles/basal bodies.
- Peripheral doublets connected by dynein arms (motor proteins; have ATPase activity). Dynein arms generate the force that causes microtubules to slide past each other, producing the bending motion.
- Central microtubules connected by bridges and enclosed in a central sheath.
- Radial spokes connect peripheral doublets to the central sheath. They help coordinate the bending motion.
- Nexin links connect adjacent peripheral doublets, preventing them from sliding too far apart.
IMPORTANT
Remember the structural formulas: Cilia/Flagella = 9 + 2 (with central microtubules) and Centrioles/Basal bodies = 9 + 0 (no central microtubules). This is a very common exam question.
Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments that gives the cell its shape and enables internal movement. It is not a rigid structure — it is constantly being assembled and disassembled as the cell's needs change.
- Network of protein filaments in the cytoplasm providing structural support, cell shape, and intracellular transport.
- Found in all eukaryotic cells.
Components
| Component | Diameter | Protein | Function |
|---|---|---|---|
| Microtubules | 25 nm | α-tubulin and β-tubulin | Cell shape, chromosome movement (spindle fibers), intracellular transport, form cilia/flagella |
| Microfilaments | 7 nm | Actin | Muscle contraction, cytokinesis (cleavage furrow), cell movement (amoeboid), microvilli |
| Intermediate filaments | 8–12 nm | Keratin, vimentin, desmin, lamin | Mechanical strength, nuclear lamina (lamins), maintain cell shape |
TIP
Size order (thinnest to thickest): Microfilaments (7 nm) → Intermediate filaments (8–12 nm) → Microtubules (25 nm). Mnemonic: "MIM" — Microfilaments are Minimal, Intermediate are In-between, Microtubules are Maximum.
Peroxisomes
Peroxisomes are small, single membrane-bound organelles that specialize in oxidative reactions, particularly those involving hydrogen peroxide.
-
Single membrane-bound organelles containing oxidative enzymes.
-
Contain the enzyme catalase — breaks down H₂O₂ (hydrogen peroxide) into water and oxygen:
- 2H₂O₂ → 2H₂O + O₂
Hydrogen peroxide is a toxic byproduct of many metabolic reactions. Without catalase, it would accumulate and damage the cell.
-
Contain oxidases that produce H₂O₂ during oxidation of substrates. So peroxisomes both produce and break down H₂O₂ — a self-contained detoxification cycle.
-
Important in photorespiration in plants (C2 pathway — glycolate metabolism). Photorespiration involves the coordination of chloroplasts, peroxisomes, and mitochondria.
-
Abundant in liver and kidney cells (detoxification). These organs process many toxic substances, making peroxisomes essential.
Glyoxysomes
Glyoxysomes are a specialized subtype of peroxisomes with a unique and agriculturally important function.
- Specialized type of peroxisome found in plant cells, especially in germinating fatty seeds (e.g., castor, groundnut).
- Contain enzymes of the glyoxylate cycle — convert stored fats into carbohydrates (sucrose) during seed germination. This is crucial because the germinating seedling needs sugars for energy before it can photosynthesize, and the seed's primary energy reserve is stored fat.
- Key enzymes: isocitrate lyase and malate synthase. These two enzymes are unique to the glyoxylate cycle and are not found in animals (which is why animals cannot convert fats to sugars).
NOTE
The glyoxylate cycle is essentially a modified Krebs cycle that bypasses the two decarboxylation steps, allowing the net conversion of acetyl-CoA (from fat breakdown) into oxaloacetate (which can be converted to glucose).
Key Points to Remember
- Ribosomes discovered by George Palade (1955); rRNA ~60%, proteins ~40%
- 70S in prokaryotes/mitochondria/chloroplasts; 80S in eukaryotic cytoplasm
- Free ribosomes → cytoplasmic proteins; Bound ribosomes (on RER) → secretory/membrane proteins
- Centrioles = 9 triplet arrangement (9 + 0); found in animal cells
- Cilia/Flagella = 9 + 2 arrangement; dynein arms provide movement force
- Cytoskeleton sizes: Microfilaments (7 nm, actin) → Intermediate (8-12 nm) → Microtubules (25 nm, tubulin)
- Peroxisomes contain catalase (breaks down H₂O₂); abundant in liver & kidney
- Glyoxysomes in germinating fatty seeds (castor, groundnut); convert fats → carbohydrates via glyoxylate cycle; unique enzymes: isocitrate lyase + malate synthase
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Ribosomes — Discovery | Discovered by George Palade (1955); also called Palade particles |
| Ribosomes — Nature | Non-membrane-bound; site of protein synthesis (translation) |
| Ribosomes — Composition | rRNA (~60%) + proteins (~40%); rRNA plays catalytic role |
| 70S Ribosomes | Subunits: 50S + 30S; found in prokaryotes, mitochondria, chloroplasts |
| 80S Ribosomes | Subunits: 60S + 40S; found in eukaryotic cytoplasm |
| Polyribosomes (Polysomes) | Multiple ribosomes on one mRNA; allow simultaneous translation of multiple protein copies |
| Free vs Bound Ribosomes | Free → cytoplasmic proteins; Bound (on RER) → secretory and membrane proteins |
| Centrosome | Primary MTOC (microtubule organizing center); found in animal cells (absent in most plants) |
| Centrioles — Structure | 9 triplets of microtubules in cartwheel pattern; 9 + 0 arrangement (no central microtubules) |
| Centrioles — Functions | Form spindle apparatus during division; form basal body of cilia/flagella; organize microtubules |
| Cilia | Short (5–10 μm); many per cell; coordinated wave-like movement; e.g., Paramecium, tracheal epithelium |
| Flagella | Long (up to 150 μm); few (1–4) per cell; undulating whip-like movement; e.g., sperm, Euglena |
| Axoneme — Structure | 9 + 2 arrangement — 9 peripheral doublets + 2 central singlets |
| Dynein Arms | Motor proteins on peripheral doublets; have ATPase activity; generate bending force |
| Centriole vs Cilia/Flagella | Centriole/basal body = 9 + 0; Cilia/flagella = 9 + 2 |
| Microtubules | Diameter 25 nm; protein = α- and β-tubulin; cell shape, spindle fibers, cilia/flagella, intracellular transport |
| Microfilaments | Diameter 7 nm; protein = actin; muscle contraction, cytokinesis (cleavage furrow), amoeboid movement, microvilli |
| Intermediate Filaments | Diameter 8–12 nm; proteins = keratin, vimentin, desmin, lamin; mechanical strength, nuclear lamina |
| Cytoskeleton Size Order | Microfilaments (7 nm) < Intermediate (8–12 nm) < Microtubules (25 nm) |
| Peroxisomes | Single membrane-bound; contain catalase (breaks down H₂O₂ → H₂O + O₂); abundant in liver and kidney |
| Peroxisomes — Plant Role | Important in photorespiration (C2 pathway / glycolate metabolism) |
| Glyoxysomes | Specialized peroxisomes in germinating fatty seeds (castor, groundnut) |
| Glyoxysomes — Function | Convert fats → carbohydrates (sucrose) via glyoxylate cycle |
| Glyoxylate Cycle — Key Enzymes | Isocitrate lyase and malate synthase (not found in animals) |
Lesson Doubts
Ask questions, get expert answers