Class 11 : Biology (In English) – Lesson 8. Cell: The Unit of Life
EXPLANATION & SUMMARY
🌿✨ Introduction
🧠 All living organisms, from tiny microbes to giant trees and animals, are built of cells, which are the basic structural and functional units of life.
🧬 Every living being starts as a single cell — either remaining unicellular or dividing repeatedly to form multicellular bodies.
🌱 The cell performs all vital life processes: growth, metabolism, reproduction, and response to stimuli.
The discovery of the cell marked a turning point in biological understanding.
💡 Concept:
Cell theory forms the foundation of biology.
Structure and function of every organism depend on its cells.
🧭 Discovery of the Cell
🔹 Robert Hooke (1665) — observed cork slices under a microscope and coined the term “cell”.
🔹 Anton van Leeuwenhoek — first to observe living cells.
🔹 Schleiden (plants) and Schwann (animals) proposed Cell Theory (1838–39):
➤ All organisms are made of cells.
➤ Cell is the basic unit of structure and function.
🔹 Later, Rudolf Virchow (1855) added: “Omnis cellula-e-cellula” — all cells arise from pre-existing cells.
✏️ Note: The cell theory unified biology by linking all life forms through cellular organisation.
🌱 Characteristics of Cells
🪴 Smallest structural unit of life
🧠 Performs all life activities
⚙️ Has specific organelles for distinct functions
🧬 Stores genetic information (DNA/RNA)
🌿 Shows growth, division, energy transformation
🧫 Types of Cells
1️⃣ 🌿 Prokaryotic Cells
🧪 Simple, primitive; found in bacteria and cyanobacteria
📦 No membrane-bound organelles
🧠 DNA lies in nucleoid region, not enclosed by membrane
⚙️ Functions occur in cytoplasm; ribosomes (70S) small
💧 Cell wall made of peptidoglycan
2️⃣ 🌸 Eukaryotic Cells
🌿 Found in protists, fungi, plants, animals
🧠 Have true nucleus with nuclear membrane
📦 Membrane-bound organelles present (mitochondria, ER, etc.)
🧬 Complex and compartmentalised
🧪 Ribosomes are larger (80S)
💡 Concept:
Prokaryotes = primitive, simple
Eukaryotes = advanced, organised
🌿 Cell Shape and Size
🌱 Shape varies with function: spherical (WBC), spindle (muscle), polygonal (epidermal), elongated (nerve).
📏 Size varies: from 0.1 µm (mycoplasma) to 170 mm (ostrich egg).
✏️ Note: Function influences structure; form follows function.
🍃 Cell Structure — Overview
Eukaryotic cells consist of:
1️⃣ Plasma membrane
2️⃣ Cytoplasm
3️⃣ Nucleus
4️⃣ Cell organelles
🧱 1️⃣ Plasma Membrane
🧠 Structure: Semi-permeable lipid bilayer (phospholipids + proteins)
💡 Described by Fluid Mosaic Model (Singer & Nicolson)
⚙️ Functions:
Selective transport (osmosis, diffusion, active transport)
Maintains internal environment
Communication and recognition
🧪 In plants, covered by cell wall (cellulose) for rigidity.
🧫 In bacteria, wall of peptidoglycan.
🧱 2️⃣ Cell Wall (in Plants)
🌿 Rigid outer covering made of cellulose, hemicellulose, pectin.
⚙️ Functions:
Provides mechanical support
Prevents bursting in hypotonic medium
Allows transport via plasmodesmata (cytoplasmic bridges)
🧪 3️⃣ Cytoplasm
💧 Fluid matrix between nucleus and membrane.
🌿 Contains organelles and cytosol.
⚙️ Site of metabolic reactions.
🧠 4️⃣ Nucleus
🧬 Control centre of cell.
🧫 Surrounded by nuclear envelope with pores.
📦 Contains nucleoplasm, chromatin (DNA + protein), nucleolus.
⚙️ Functions:
Controls cell activities
Stores genetic material
Regulates cell division
🧪 During cell division, chromatin condenses into chromosomes.
⚙️ Cell Organelles
🌿 1. Endoplasmic Reticulum (ER)
🧬 Network of membranes; connects nucleus to cytoplasm
🔹 Rough ER – ribosomes attached; protein synthesis
🔹 Smooth ER – lipid synthesis, detoxification
🌸 2. Golgi Apparatus
📦 Stack of cisternae; modifies, packages materials from ER
⚙️ Forms lysosomes and secretory vesicles
🧫 3. Lysosomes
💣 Membrane sacs with hydrolytic enzymes
⚡ Perform intracellular digestion, remove waste
🧠 Called suicidal bags
⚡ 4. Mitochondria
🧬 Double membrane; inner folded into cristae
🌿 Contain own DNA and ribosomes
⚙️ Site of aerobic respiration → ATP generation
💡 Known as powerhouse of cell
🪴 5. Plastids (Plants only)
🧪 Double membrane organelles
🟢 Chloroplasts – with chlorophyll, perform photosynthesis
🟡 Chromoplasts – coloured pigments
⚪ Leucoplasts – store food
💧 6. Vacuoles
📦 Fluid-filled, membrane-bound sacs
🌿 Plant cells: large central vacuole, stores water & solutes
🐾 Animal cells: small, temporary vacuoles
⚙️ Maintain turgor pressure
⚙️ 7. Ribosomes
🧠 Non-membranous
🧬 Site of protein synthesis
📏 Two subunits: 70S (prokaryote), 80S (eukaryote)
🧬 8. Centrosome and Centrioles (Animals)
🌿 Located near nucleus
🧭 Form spindle fibres during division
⚙️ Help in chromosome movement
🧠 Chromosomes
🌿 Thread-like structures carrying genetic information
🧬 Made of DNA + protein
📈 Visible during cell division
🧪 Types: Autosomes, sex chromosomes
💡 Function: Pass traits from parents to offspring
🌱 Cell Division (Brief Overview)
💡 Purpose: Growth, repair, reproduction
📘 Types:
1️⃣ Mitosis – equational division (growth)
2️⃣ Meiosis – reductional division (gametes)
✏️ Note: Cell division maintains genetic continuity.
🌸 Differences: Plant vs Animal Cells
Feature Plant Cell Animal Cell
Cell wall Present (cellulose) Absent
Plastids Present Absent
Vacuole Large, central Small, temporary
Centrosome Absent Present
Shape Fixed, rectangular Variable
💡 Concept: Structural differences reflect functional needs.
🌿 Specialised Cells
🧠 Different cells perform unique roles:
🌾 RBC – transport oxygen
🪴 Muscle cell – movement
🧬 Neuron – impulse transmission
🌿 Guard cells – regulate stomata
🌍 Importance of Cell Biology
🌱 Explains growth, heredity, and disease
🧠 Forms basis of genetics, physiology, and medicine
⚙️ Understanding cell structure helps in biotechnology and research
🌍 Why This Lesson Matters
🧠 Introduces foundation of life’s organisation
🧬 Builds base for genetics and evolution
🌿 Essential for biomedical sciences
⚡ Crucial for NEET, JEE, and CBSE exams
📝 Quick Recap
🧫 Cell = structural & functional unit
🧠 Prokaryotic vs Eukaryotic
🧱 Plasma membrane – selective barrier
🧬 Nucleus – genetic control
⚙️ Organelles: ER, Golgi, lysosomes, mitochondria, plastids, vacuoles, ribosomes
📈 Division: mitosis & meiosis
🌿 Plant vs Animal → cell wall, plastids, vacuoles
📘 Summary
The cell is the basic building block of all living organisms.
Prokaryotic cells are simple, lacking a true nucleus, while eukaryotic cells are complex with compartmentalisation.
The plasma membrane maintains internal environment, the nucleus controls heredity, and organelles perform specific functions.
Mitochondria produce energy, ER and Golgi handle synthesis and transport, lysosomes manage digestion, and plastids enable photosynthesis.
Cell differences between plants and animals reflect adaptations to lifestyle.
Understanding cell structure and functions is essential to grasp all biological processes — growth, reproduction, and inheritance.
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QUESTIONS FROM TEXTBOOK
🔵 Question 1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
(b) Schleiden and Schwann formulated the cell theory.
(c) Virchow explained that cells are formed from pre-existing cells.
(d) A unicellular organism carries out its life activities within a single cell.
🟢 Answer:
✔️ Option (a) is not correct.
💡 Reason: Robert Brown discovered the nucleus, not the cell.
The cell was discovered by Robert Hooke in 1665.
🔵 Question 2. New cells generate from
(a) bacterial fermentation (b) regeneration of old cells
(c) pre-existing cells (d) abiotic materials
🟢 Answer:
✔️ Option (c) pre-existing cells.
💡 According to Rudolf Virchow, “Omnis cellula e cellula” — all cells arise from pre-existing cells.
🔵 Question 3. Match the following:
Column I Column II
(a) Cristae (ii) Infoldings in mitochondria
(b) Cisternae (iii) Disc-shaped sacs in Golgi apparatus
(c) Thylakoids (i) Flat membranous sacs in stroma
🟢 Answer:
✔️ Correct matching:
(a) → (ii), (b) → (iii), (c) → (i)
💡 Explanation:
Cristae: Folds of inner mitochondrial membrane.
Cisternae: Flattened sacs of Golgi apparatus.
Thylakoids: Flattened sacs in chloroplast stroma.
🔵 Question 4. Which of the following is correct?
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well-defined cell wall.
(c) In prokaryotes, there are no membrane-bound organelles.
(d) Cells are formed de novo from abiotic materials.
🟢 Answer:
✔️ Option (c) is correct.
💡 Prokaryotic cells lack membrane-bound organelles like nucleus, mitochondria, etc.
🔵 Question 5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
🟢 Answer:
🧫 Mesosomes are infoldings of the plasma membrane in prokaryotes (like bacteria).
🌿 Functions:
➡️ Help in cell wall formation during division.
➡️ Assist in DNA replication and distribution.
➡️ Help in respiration (contain enzymes).
➡️ Aid in enzyme secretion and cell compartmentalisation.
🔵 Question 6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?
🟢 Answer:
🌸 Neutral solutes move by simple diffusion — from high concentration to low concentration across the membrane.
⚗️ Polar molecules cannot pass freely due to the lipid bilayer barrier. They are transported by:
Facilitated diffusion via carrier proteins (no energy used).
Active transport using energy (ATP) through pumps.
💡 Examples: Glucose via facilitated diffusion, Na⁺ via active transport.
🔵 Question 7. Name two cell-organelles that are double membrane-bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.
🟢 Answer:
🧬 Organelles: Mitochondria and Chloroplast
🌿 Common characteristics:
Double membrane.
Contain DNA and ribosomes.
Capable of self-replication.
🧪 Mitochondria:
Inner membrane folded into cristae
Site of cellular respiration
Produces ATP (powerhouse of cell)
🌱 Chloroplast:
Present in plant cells, contains thylakoids arranged as grana
Site of photosynthesis
✏️ Diagram descriptions:
Mitochondrion: outer & inner membrane, cristae, matrix.
Chloroplast: outer & inner membrane, grana, stroma.
🔵 Question 8. What are the characteristics of prokaryotic cells?
🟢 Answer:
🧫 Prokaryotic cells (e.g. bacteria):
No true nucleus — nucleoid present.
No membrane-bound organelles.
Cell wall made of peptidoglycan.
Ribosomes are 70S type.
Reproduction: binary fission.
Genetic material: circular DNA.
✔️ Simple organisation, performs all vital functions.
🔵 Question 9. Multicellular organisms have division of labour. Explain.
🟢 Answer:
🌿 In multicellular organisms, cells are specialised for specific functions.
➡️ Example:
Nerve cells — conduction
Muscle cells — movement
RBCs — oxygen transport
💡 Division of labour improves efficiency and survival.
🔵 Question 10. Cell is the basic unit of life. Discuss in brief.
🟢 Answer:
🧬 The cell is the structural and functional unit of all living organisms.
➡️ All organisms are made of cells.
➡️ All functions — growth, metabolism, reproduction — occur in cells.
➡️ Cells arise from pre-existing cells.
✔️ Thus, cell = fundamental unit of life.
🔵 Question 11. What are nuclear pores? State their function.
🟢 Answer:
🧠 Nuclear pores are openings in the nuclear envelope.
🌿 Functions:
Allow exchange of materials (RNA, proteins) between nucleus and cytoplasm.
Maintain communication between nucleoplasm and cytoplasm.
🔵 Question 12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.
🟢 Answer:
🧫 Lysosomes:
Contain hydrolytic enzymes.
Function: intracellular digestion, destruction of worn-out organelles.
🌿 Vacuoles:
Contain cell sap (water, ions, sugars).
Function: storage, osmoregulation, turgidity.
✔️ Both are endomembrane structures but have different roles.
🔵 Question 13. Describe the structure of the following with the help of labelled diagrams:
(i) Nucleus (ii) Centrosome
🟢 Answer:
(i) 🧠 Nucleus:
Surrounded by double membrane with nuclear pores.
Contains nucleoplasm, chromatin, nucleolus.
Controls cell activities.
✏️ Diagram description: Circular body, double membrane, nucleolus, chromatin threads.
(ii) ⚙️ Centrosome:
Present near nucleus in animal cells.
Consists of two centrioles perpendicular to each other.
Helps in spindle formation during cell division.
✏️ Diagram description: Two cylindrical centrioles arranged at right angles.
🔵 Question 14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes? Support your answer with a diagram showing the position of centromere on different types of chromosomes.
🟢 Answer:
🧬 Centromere:
Constricted region on chromosome dividing it into arms.
Helps attach chromosome to spindle during division.
🌿 Based on centromere position:
Metacentric: Centromere in middle; equal arms.
Sub-metacentric: Slightly off-centre; unequal arms.
Acrocentric: Near one end; one very short arm.
Telocentric: At terminal end; one arm.
✏️ Diagram description: Shows all four types with centromere positions.
✔️ Function: Segregation during cell division.
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OTHER IMPORTANT QUESTIONS FOR EXAMS
(CBSE MODEL QUESTIONS PAPER)
ESPECIALLY MADE FROM THIS LESSON ONLY
🔴 Question 1:
The cell was discovered by:
🔴1️⃣ Robert Brown
🟢2️⃣ Robert Hooke
🟡3️⃣ Leeuwenhoek
🔵4️⃣ Schleiden
🟢 Answer: 2️⃣ Robert Hooke
🔴 Question 2:
Smallest living unit of life is:
🔴1️⃣ Atom
🟢2️⃣ Molecule
🟡3️⃣ Cell
🔵4️⃣ Organelle
🟢 Answer: 3️⃣ Cell
🔴 Question 3:
Prokaryotic cells lack:
🔴1️⃣ DNA
🟢2️⃣ Plasma membrane
🟡3️⃣ Nucleus
🔵4️⃣ Ribosomes
🟢 Answer: 3️⃣ Nucleus
🔴 Question 4:
Which of the following is a prokaryote?
🔴1️⃣ Fungi
🟢2️⃣ Algae
🟡3️⃣ Bacteria
🔵4️⃣ Protozoa
🟢 Answer: 3️⃣ Bacteria
🔴 Question 5:
The functional unit of endoplasmic reticulum is:
🔴1️⃣ Cisternae
🟢2️⃣ Ribosome
🟡3️⃣ Golgi body
🔵4️⃣ Lysosome
🟢 Answer: 1️⃣ Cisternae
🔴 Question 6:
Which cell organelle is called powerhouse of cell?
🔴1️⃣ Lysosome
🟢2️⃣ Golgi apparatus
🟡3️⃣ Mitochondria
🔵4️⃣ Ribosome
🟢 Answer: 3️⃣ Mitochondria
🔴 Question 7:
Lysosomes are formed from:
🔴1️⃣ Ribosomes
🟢2️⃣ Endoplasmic reticulum
🟡3️⃣ Golgi apparatus
🔵4️⃣ Mitochondria
🟢 Answer: 3️⃣ Golgi apparatus
🔴 Question 8:
Which organelle is responsible for photosynthesis?
🔴1️⃣ Mitochondria
🟢2️⃣ Chloroplast
🟡3️⃣ Ribosome
🔵4️⃣ Golgi body
🟢 Answer: 2️⃣ Chloroplast
🔴 Question 9:
Ribosomes are sites of:
🔴1️⃣ Respiration
🟢2️⃣ Photosynthesis
🟡3️⃣ Protein synthesis
🔵4️⃣ Lipid synthesis
🟢 Answer: 3️⃣ Protein synthesis
🔴 Question 10:
Cell theory was proposed by:
🔴1️⃣ Schleiden and Schwann
🟢2️⃣ Hooke and Leeuwenhoek
🟡3️⃣ Virchow and Pasteur
🔵4️⃣ Brown and Schwann
🟢 Answer: 1️⃣ Schleiden and Schwann
🔴 Question 11:
Name the two main types of cells.
🟢 Answer:
1️⃣ Prokaryotic cells: Without membrane-bound organelles (e.g. Bacteria).
2️⃣ Eukaryotic cells: With true nucleus and organelles (e.g. Plant, Animal).
🔴 Question 12:
Define cell theory.
🟢 Answer:
Proposed by Schleiden and Schwann.
States that:
1️⃣ All living organisms are composed of cells.
2️⃣ Cell is the structural and functional unit of life.
3️⃣ All cells arise from pre-existing cells (Virchow).
🔴 Question 13:
Write the main differences between prokaryotic and eukaryotic cells.
🟢 Answer:
Feature Prokaryotic Cell Eukaryotic Cell
Nucleus Absent (nucleoid) Present with nuclear membrane
Organelles No membrane-bound organelles Membrane-bound organelles present
Example Bacteria, Cyanobacteria Plants, Animals, Fungi
Size Small (1–10 µm) Large (10–100 µm)
💡 Eukaryotes show compartmentalisation and complexity.
🔴 Question 14:
Describe the structure and functions of plasma membrane.
🟢 Answer:
Structure:
1️⃣ Composed of lipid bilayer (phospholipids + proteins).
2️⃣ Fluid Mosaic Model proposed by Singer and Nicolson.
3️⃣ Selectively permeable membrane.
Functions:
✔️ Regulates entry and exit of substances.
✔️ Maintains cell shape.
✔️ Helps in communication and recognition.
🔴 Question 15:
What are cell organelles? Name the membrane-bound and non-membrane-bound organelles.
🟢 Answer:
Definition: Structures in cytoplasm performing specific functions.
Membrane-bound: Nucleus, ER, Golgi body, Lysosomes, Mitochondria, Plastids.
Non-membrane-bound: Ribosomes, Centrioles.
Function: Division of labour within cell.
🔴 Question 16:
Describe the structure and function of nucleus.
🟢 Answer:
Structure:
1️⃣ Double membrane nuclear envelope with pores.
2️⃣ Contains nucleoplasm, chromatin, and nucleolus.
Functions:
✔️ Controls cellular activities.
✔️ Contains genetic material (DNA).
✔️ Regulates cell division and heredity.
🔴 Question 17:
What are ribosomes? Mention their types and functions.
🟢 Answer:
Definition: Small, non-membranous organelles made of RNA & protein.
Types:
1️⃣ 70S: Found in prokaryotes.
2️⃣ 80S: Found in eukaryotes.
Function: Protein synthesis (called protein factories).
🔴 Question 18:
Describe the structure and functions of mitochondria.
🟢 Answer:
Structure:
1️⃣ Double membrane — outer smooth, inner folded (cristae).
2️⃣ Matrix with DNA, RNA, enzymes.
Functions:
✔️ Powerhouse of cell — produces ATP.
✔️ Site of respiration.
✔️ Self-replicating due to DNA.
🔴 Question 19:
What is endoplasmic reticulum (ER)? Name its types and functions.
🟢 Answer:
Definition: Network of membranes forming channels in cytoplasm.
Types:
1️⃣ Rough ER (RER): With ribosomes; protein synthesis.
2️⃣ Smooth ER (SER): No ribosomes; lipid synthesis, detoxification.
Functions: Transport and compartmentalisation.
🔴 Question 20:
Explain the structure and function of Golgi apparatus.
🟢 Answer:
Structure:
• Stack of membrane-bound sacs called cisternae.
• Has cis (forming) and trans (maturing) faces.
Functions:
✔️ Modifies and packages proteins.
✔️ Forms lysosomes.
✔️ Secretes materials outside cell.
🔴 Question 21:
Describe the structure and function of lysosomes.
🟢 Answer:
Structure:
• Single membrane-bound vesicles formed from Golgi apparatus.
• Contain hydrolytic enzymes.
Function:
✔️ Intracellular digestion.
✔️ Destroy damaged organelles (suicidal bags).
✔️ Defence against pathogens.
🔴 Question 22:
What are plastids? Write their types and functions.
🟢 Answer:
Definition: Double-membraned organelles found in plant cells 🌿.
Types:
1️⃣ Chloroplasts: Green; contain chlorophyll; photosynthesis.
2️⃣ Chromoplasts: Coloured pigments; attract pollinators 🌸.
3️⃣ Leucoplasts: Colourless; store food (amyloplast, elaioplast, aleuroplast).
Function: Photosynthesis and storage.
🔴 Question 23:
Explain the structure of a prokaryotic cell (bacterium 🦠) with labelled parts.
🟢 Answer:
Shape: Spherical (coccus), rod-shaped (bacillus), spiral, or comma-shaped.
Main parts:
1️⃣ Cell envelope:
• Glycocalyx (capsule/slime layer) → protection.
• Cell wall → rigidity.
• Plasma membrane → selectively permeable.
2️⃣ Cytoplasm: No membrane-bound organelles; contains ribosomes (70S).
3️⃣ Nucleoid: Single circular DNA molecule; no nuclear membrane.
4️⃣ Inclusions: Storage granules (phosphate, glycogen).
5️⃣ Flagella (for movement), pili and fimbriae (attachment).
Feature: Simple structure, fast reproduction, primitive nucleus.
🔴 Question 24:
Describe the structure of a eukaryotic cell.
🟢 Answer:
Shape: Varies — spherical, cuboidal, polygonal.
Main components:
1️⃣ Plasma membrane: Lipid bilayer; selectively permeable.
2️⃣ Cytoplasm: Contains organelles.
3️⃣ Nucleus: True nucleus with double membrane & DNA.
4️⃣ Cell organelles:
• Mitochondria — respiration.
• ER — transport & synthesis.
• Golgi apparatus — packaging.
• Lysosomes — digestion.
• Ribosomes — protein synthesis.
• Plastids (plants 🌿) — photosynthesis/storage.
• Centrioles (animals 🧫) — cell division.
Feature: High organisation, compartmentalisation, complex nucleus.
🔴 Question 25:
Explain the Fluid Mosaic Model of plasma membrane.
🟢 Answer:
Proposed by: Singer and Nicolson (1972).
Structure:
1️⃣ Lipid bilayer — phospholipids with hydrophilic heads and hydrophobic tails.
2️⃣ Proteins —
• Integral proteins (span membrane)
• Peripheral proteins (surface).
3️⃣ Cholesterol — provides stability.
Features:
✔️ Fluid nature → lipids & proteins move laterally.
✔️ Mosaic → irregular arrangement of proteins.
Function:
• Selective transport.
• Cell recognition.
• Flexibility & stability.
🔴 Question 26:
Describe the structure of mitochondria and its functions.
🟢 Answer:
Structure:
1️⃣ Double membrane: Outer smooth, inner folded (forms cristae).
2️⃣ Matrix: Contains DNA, RNA, ribosomes, enzymes.
3️⃣ Cristae: Increase surface area for enzyme activity.
Functions:
✔️ Site of aerobic respiration.
✔️ Produces ATP — “powerhouse of cell”.
✔️ Can replicate independently.
✔️ Stores enzymes for Krebs cycle and ETC.
🔴 Question 27:
Explain the structure of chloroplast 🌿 with labelled parts.
🟢 Answer:
Structure:
1️⃣ Double membrane-bound organelle.
2️⃣ Stroma: Fluid containing DNA, RNA, ribosomes, enzymes.
3️⃣ Thylakoids: Flattened sacs arranged in stacks (grana).
4️⃣ Grana connected by stromal lamellae.
5️⃣ Chlorophyll pigments in thylakoid membranes capture light.
Function:
✔️ Site of photosynthesis.
✔️ Light reactions in grana; dark reactions in stroma.
✔️ Synthesises glucose and oxygen.
🔴 Question 28:
Describe the structure and functions of Golgi apparatus.
🟢 Answer:
Structure:
• Series of flattened membrane sacs (cisternae) arranged parallel.
• Has cis face (receiving) and trans face (shipping).
Functions:
1️⃣ Modification and packaging of materials.
2️⃣ Formation of lysosomes.
3️⃣ Secretion of glycoproteins and mucilage.
4️⃣ Involved in cell wall formation in plants 🌿.
🔴 Question 29:
Describe the structure and functions of nucleus.
🟢 Answer:
Structure:
1️⃣ Nuclear envelope — double membrane with pores.
2️⃣ Nucleoplasm — fluid with enzymes & nucleotides.
3️⃣ Chromatin:
• Euchromatin — active DNA.
• Heterochromatin — inactive DNA.
4️⃣ Nucleolus — synthesises rRNA.
Functions:
✔️ Controls cell activities.
✔️ Contains hereditary material (DNA).
✔️ Directs protein synthesis.
✔️ Regulates cell division.
🔴 Question 30:
Write short notes on:
(a) Centrosome and Centrioles
(b) Cytoskeleton
🟢 Answer:
(a) Centrosome and Centrioles:
• Present in animal cells near nucleus.
• Contains two centrioles arranged perpendicular.
• Each centriole = 9 triplets of microtubules.
• Function: Spindle formation during cell division.
(b) Cytoskeleton:
• Network of microtubules, microfilaments, and intermediate filaments.
• Provides shape, support, and movement of organelles.
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