Biology of Cell: Structure and Function — Solved PYQ Notes

UPC 2232013502 · 89 solved questions from 5 years of papers (2022–2026) · 22 repeated questions

Unit-wise weightage

Most repeated questions

1. Illustrate the process of microtubule assembly with the help of suitable diagram. Add a note on their role in cellular mobility. (10) — asked 3× (2022, 2023, 2024, 2025)

2. Explain the various modifications that proteins undergo in the lumen of ER. (7) — asked 3× (2023, 2024, 2025)

3. Discuss the role of secondary messengers in cell signaling. (5) — asked 3× (2022, 2024, 2025)

4. What is oxidative phosphorylation? Describe how the Electron Transport Chain and ATP Synthase in the mitochondria help generate ATP-the energy currency of the cell. (15) — asked 2× (2022, 2024, 2025)

5. Write short notes on ANY THREE of the following : (a) Receptor-mediated endocytosis ; (b) Endosymbiotic Hypothesis ; (c) Cell Cycle checkpoints ; (d) Protein modifications in ER ; (e) Nuclear Pore Complex (15) — asked 2× (2022, 2024, 2025)

6. Write short notes on the followings (any three): (i) Structure of tRNA ; (ii) RNA interference ; (iii) Mismatch Repair ; (iv) Synthesis of rRNA (12) — asked 2× (2022, 2024)

7. Explain cell cycle and discuss its molecular regulation through check points. (12) — asked 2× (2023, 2024)

8. What is Signal Hypothesis? How does vesicular transport take place from ER to Golgi apparatus? (10) — asked 2× (2022, 2024, 2025)

9. Discuss the mechanism of DNA replication in a bacterial cell. (10) — asked 2× (2024, 2025, 2026)

10. Discuss the process of activation of amino acids and formation of initiation complex in prokaryotes. (8) — asked 2× (2022, 2025, 2026)

11. Explain GPCR pathway with any one secondary messenger. (7) — asked 2× (2023, 2024, 2025, 2026)

12. Explain (with illustration) the regulation of Lac operon. (6) — asked 2× (2022, 2024)

13. Give the function of the following : (i) Plasmodesmata ; (ii) Flippase ; (iii) Lamins ; (iv) Liposomes ; (v) Aquaporins (5) — asked 2× (2022, 2024, 2025)

14. Give the contribution of ANY FIVE of the following scientists : (i) Benda ; (ii) Peter Mitchell ; (iii) Blobel & Sabatini ; (iv) Christian de duve ; (v) Tim Hunt, Paul Nurse, Lee Hartwell ; (vi) Lynn Margulis (5) — asked 2× (2022, 2024, 2025)

15. Expand ANY FIVE of the following : (i) GPCR ; (ii) MTOC ; (iii) GERL ; (iv) CFTR ; (v) cAMP ; (vi) ATP (5) — asked 2× (2022, 2024, 2025)

Unit IOverview of Cells and Plasma membrane

Q1Give the function of the following : (i) Plasmodesmata ; (ii) Flippase ; (iii) Lamins ; (iv) Liposomes ; (v) Aquaporins (5)

1Plasmodesmata

• Meaning

  Plasmodesmata are microscopic channels connecting neighbouring plant cells.

• Communication: Allow direct exchange of substances between adjacent cells.

• Transport: Transport water, ions, sugars and small molecules.

• Signalling: Help transfer signalling molecules between plant cells.

• Importance: Maintain coordination between different plant tissues.

2Flippase

• Meaning

  Flippase is a membrane enzyme that moves specific phospholipids between membrane layers.

• Direction: Generally transports phospholipids from the outer leaflet to the inner leaflet.

• Energy: Uses ATP.

• Function: Maintains membrane lipid asymmetry.

• Importance: Supports membrane organisation and cellular signalling.

3Lamins

• Meaning

  Lamins are intermediate filament proteins forming the nuclear lamina.

• Location: Present beneath the inner nuclear membrane.

• Support: Maintain the shape and mechanical stability of the nucleus.

• Organisation: Help organise chromatin and nuclear pores.

• Cell division: Nuclear lamina disassembles and reforms during mitosis.

4Liposomes

• Meaning

  Liposomes are spherical vesicles made of phospholipid bilayers.

• Structure: Contain an aqueous interior surrounded by lipid membranes.

• Drug delivery: Carry medicines into specific cells and tissues.

• Research: Used to study membrane structure and transport.

• Importance: Useful in pharmaceutical and cosmetic applications.

5Aquaporins

• Meaning

  Aquaporins are specialised membrane proteins that form water channels.

• Transport: Allow rapid movement of water across cell membranes.

• Selectivity: Mainly transport water without allowing free passage of ions.

• Location: Found in kidney cells and many other tissues.

• Importance: Help maintain water balance and regulate urine concentration.

Q2With appropriate examples and well-labelled diagrams elaborate the ways in which molecules are passively and actively transported across the plasma membrane. (15)

1Introduction

• Plasma membrane: It is a selectively permeable boundary surrounding the cell.

• Structure: It mainly consists of a phospholipid bilayer and proteins.

• Selective permeability: It allows certain substances to pass while restricting others.

• Transport: Movement of molecules across the membrane occurs through passive and active mechanisms.

• Importance: Transport maintains cellular balance and supplies essential nutrients.

2Passive Transport

• Meaning

  Passive transport is the movement of substances across a membrane without direct expenditure of cellular energy.

• Direction: Substances move down their concentration or electrochemical gradient.

• Energy: It does not directly require ATP.

• Types: The major types are simple diffusion, facilitated diffusion and osmosis.

A. Simple Diffusion

• Meaning

  It is the movement of molecules from higher concentration to lower concentration.

• Membrane: Molecules pass directly through the phospholipid bilayer.

• Suitable molecules: Small, non-polar and lipid-soluble molecules pass easily.

• Examples: Oxygen, carbon dioxide and steroid molecules.

• Rate: It depends on the concentration gradient, membrane area and permeability.

Draw small molecules moving directly through the phospholipid bilayer from high to low concentration.

B. Facilitated Diffusion

• Meaning

  It is passive movement through specific membrane transport proteins.

• Channel proteins: These form pores through which ions or water can pass.

• Carrier proteins: These bind substances and change shape to transport them.

• Direction: Movement occurs down the concentration or electrochemical gradient.

• Examples: Glucose transport through GLUT proteins and ion movement through channels.

• Specificity: Transport proteins recognise particular molecules.

• Saturation: Carrier-mediated transport has a maximum rate when all carriers are occupied.

Draw channel proteins and carrier proteins allowing substances to move down their gradients.

C. Osmosis

• Meaning

  Osmosis is the movement of water through a selectively permeable membrane.

• Direction: Water moves from higher water potential to lower water potential.

• Membrane: The membrane allows water movement while restricting certain solutes.

• Aquaporins: These are membrane proteins that facilitate rapid water movement.

• Example

  Water absorption by intestinal cells and plant roots.

Effect of Different Solutions

Draw three animal cells showing swelling, normal shape and shrinking in different solutions.

3Active Transport

• Meaning

  Active transport moves substances against their concentration or electrochemical gradient.

• Energy: It requires energy, directly or indirectly.

• Direction: Substances move from lower concentration towards higher concentration.

• Proteins: Specific carrier proteins or pumps transport substances.

• Importance: It maintains ion balance and nutrient uptake.

A. Primary Active Transport

• Meaning

  It directly uses energy from ATP hydrolysis.

• Sodium-potassium pump: It transports three sodium ions outside and two potassium ions inside.

• Energy: One ATP molecule is used during each pump cycle.

• Location: It is present in the plasma membrane of animal cells.

• Importance: It maintains resting membrane potential and cell volume.

• Calcium pump: It transports calcium ions out of the cytoplasm.

• Proton pump: It moves hydrogen ions across membranes.

Draw the sodium-potassium pump showing three Na⁺ moving out and two K⁺ moving in per ATP.

B. Secondary Active Transport

• Meaning

  It uses energy stored in an ion gradient rather than directly using ATP.

• Ion gradient: Usually a sodium gradient provides the energy.

• Symport: Two substances move in the same direction.

• Antiport: Two substances move in opposite directions.

• Example

  Sodium-glucose cotransporter moves sodium and glucose into intestinal cells.

• Importance: It helps absorb glucose and other nutrients.

Draw symport and antiport, showing substances moving in the same or opposite directions.

4Vesicular Transport

• Meaning

  Large molecules and particles move through membrane-bound vesicles.

• Endocytosis: The cell takes substances inside through membrane folding.

• Phagocytosis: The cell engulfs large particles, such as bacteria.

• Pinocytosis: The cell takes in extracellular fluid and dissolved substances.

• Receptor-mediated endocytosis: Specific molecules enter after binding to membrane receptors.

• Exocytosis: Vesicles fuse with the plasma membrane and release substances outside.

• Examples: Hormone secretion, neurotransmitter release and cellular waste removal.

5Comparison Between Passive and Active Transport

6Factors Affecting Membrane Transport

• Concentration gradient: A greater gradient generally increases passive transport.

• Temperature: Higher temperature generally increases molecular movement.

• Molecular size: Smaller molecules usually cross more easily.

• Lipid solubility: Lipid-soluble molecules pass through the bilayer more easily.

• Membrane proteins: Their number and activity influence transport rates.

• Membrane potential: It affects the movement of charged ions.

7Conclusion

Passive transport moves substances down their gradients without direct energy expenditure. Active transport requires energy to move substances against their gradients. Both mechanisms maintain cellular balance and support essential life processes.

Q3How are cells organized into tissues? Explain the various types of "Cell-to-Cell interactions". (8)

1Organisation of Cells into Tissues

• Cell: The basic structural and functional unit of life.

• Tissue: A group of similar or specialised cells performing a common function.

• Cell adhesion: Cells attach to one another through specialised proteins.

• Extracellular matrix (ECM): Non-cellular material supporting and connecting cells.

• Cell junctions: Special structures that connect neighbouring cells.

• Coordination: Cells communicate to perform tissue-specific functions.

• Differentiation: Cells develop specialised structures and functions.

2Levels of Organisation

Cells organise themselves into tissues, organs and organ systems.

3Types of Cell-to-Cell Interactions

A. Tight Junctions

• Structure: Neighbouring plasma membranes are closely sealed.

• Proteins: Mainly formed by claudins and occludins.

• Function: Prevent leakage of substances between cells.

• Example

  Intestinal epithelial lining.

B. Adherens Junctions

• Structure: Connect neighbouring cells through cadherin proteins.

• Proteins: Cadherins connect to actin filaments inside cells.

• Function: Provide mechanical attachment and maintain tissue shape.

• Example

  Epithelial tissues.

C. Desmosomes

• Structure: Strong anchoring junctions between neighbouring cells.

• Proteins: Contain desmoglein and desmocollin.

• Function: Resist mechanical stress.

• Example

  Skin and cardiac muscle.

D. Gap Junctions

• Structure: Form channels between adjacent cells.

• Proteins: Six connexin proteins form one connexon.

• Function: Allow ions and small molecules to pass between cells.

• Example

  Cardiac muscle cells.

E. Cell Adhesion to Extracellular Matrix

• Structure: Cells attach to the extracellular matrix through integrins.

• Proteins: Integrins connect ECM to the cytoskeleton.

• Function: Provide support and transmit signals.

• Example

  Epithelial cells attached to the basement membrane.

4Comparison of Cell Junctions

Q4What are the various models of plasma membrane? (7)

1Introduction

• Plasma membrane: It is a thin, living and selectively permeable boundary surrounding the cell.

• Function: It controls the movement of substances into and out of the cell.

• Membrane models: Scientists proposed different models to explain its structure and organisation.

2Various Models of Plasma Membrane

1. Lipid Bilayer Model (Gorter and Grendel, 1925)

• Scientists: Gorter and Grendel proposed this model in 1925.

• Structure: The membrane consists of two layers of phospholipids.

• Arrangement: Hydrophilic heads face the watery surroundings.

• Hydrophobic tails: They face inward, away from water.

• Evidence: Their experiments with red blood cells supported a lipid bilayer.

2. Sandwich Model (Davson and Danielli, 1935)

• Scientists: Davson and Danielli proposed this model in 1935.

• Structure: A lipid bilayer is covered by protein layers on both surfaces.

• Arrangement: Proteins form continuous layers outside the lipid bilayer.

• Limitation: It could not explain membrane flexibility and the presence of integral proteins.

3. Unit Membrane Model (Robertson, 1959)

• Scientist: J. David Robertson proposed this model in 1959.

• Structure: All biological membranes have a similar three-layered appearance.

• Appearance: It shows two dark layers separated by one light layer.

• Thickness: The membrane is approximately 7.5–10 nm thick.

• Evidence: The model was based on electron microscopy.

• Limitation: It could not explain the dynamic nature of membranes.

4. Fluid Mosaic Model (Singer and Nicolson, 1972)

• Scientists: Singer and Nicolson proposed this model in 1972.

• Structure: The membrane consists of a fluid phospholipid bilayer.

• Proteins: Integral proteins are embedded within the bilayer, while peripheral proteins occur on its surface.

• Fluidity: Lipids and many proteins can move sideways within the membrane.

• Cholesterol: It regulates membrane fluidity and stability in animal cells.

• Carbohydrates: They occur on the outer surface as glycoproteins and glycolipids.

• Importance: This is the widely accepted model of plasma membrane structure.

3Comparison of Models

4Conclusion

The understanding of plasma membrane structure developed through different models. The Fluid Mosaic Model (1972) is widely accepted because it explains membrane flexibility, fluidity and protein arrangement.

Q5Describe the operation of Na⁺-K⁺ pump with help of suitable diagram. (6)

1Introduction

• Na⁺-K⁺ pump: A membrane protein that actively transports sodium and potassium ions.

• Scientific name: Sodium-potassium ATPase.

• Location: Plasma membrane of animal cells.

• Energy: Uses ATP to move ions against their concentration gradients.

• Direction: Pumps three Na⁺ ions outside and two K⁺ ions inside.

2Mechanism of Operation

The pump operates through the following steps:

• Step 1 – Sodium binding: Three Na⁺ ions bind to the pump from inside the cell.

• Step 2 – ATP breakdown: ATP provides energy and phosphorylates the pump.

• Step 3 – Shape change: Phosphorylation changes the pump's shape.

• Step 4 – Sodium release: Three Na⁺ ions are released outside the cell.

• Step 5 – Potassium binding: Two K⁺ ions bind from outside the cell.

• Step 6 – Dephosphorylation: The phosphate group is removed.

• Step 7 – Shape restoration: The pump returns to its original shape.

• Step 8 – Potassium release: Two K⁺ ions are released inside the cell.

3Overall Reaction

\[ 3Na^+_{\text{inside}}+2K^+_{\text{outside}}+ATP \]

\[ \longrightarrow 3Na^+_{\text{outside}}+ 2K^+_{\text{inside}}+ADP+P_i \]

• ATP consumption: One ATP molecule is used per complete pumping cycle.

• Electrogenic transport: More positive charges leave than enter the cell.

4Diagram

5Functions and Importance

• Resting membrane potential: Helps maintain the electrical difference across the membrane.

• Nerve impulses: Maintains ion gradients needed for nerve cell activity.

• Muscle contraction: Supports normal functioning of muscle cells.

• Cell volume: Prevents excessive accumulation of sodium and water.

• Secondary active transport: Provides sodium gradients used by other transport proteins.

• Keywords

  Na⁺-K⁺ ATPaseactive transportATPphosphorylation3 Na⁺2 K⁺membrane potentialelectrogenic pump
• Exam tip

  The most important scoring points are the 3:2 ratio, ATP consumption, phosphorylation and direction of ion movement.

• Conclusion

  The Na⁺-K⁺ pump maintains sodium and potassium gradients, supports membrane potential and helps normal cellular functioning.

Q6What are the major cell-to-cell interactions? (6)

1Meaning

• Cell-to-cell interactions: Communication and physical connections between neighbouring cells.

• Importance: Help cells form tissues, exchange information and coordinate activities.

2Major Types of Cell-to-Cell Interactions

1. Tight Junctions

• Structure: Form sealing connections between adjacent cells.

• Function: Prevent leakage of substances between cells.

• Location: Intestinal epithelial cells and kidney tubules.

2. Adherens Junctions

• Structure: Connect neighbouring cells through cadherin proteins.

• Function: Provide mechanical strength and maintain tissue shape.

• Connection: Linked to actin filaments inside cells.

3. Desmosomes

• Structure: Strong anchoring junctions between neighbouring cells.

• Function: Provide resistance against mechanical stress.

• Connection: Attached to intermediate filaments.

• Location: Skin and heart muscle.

4. Gap Junctions

• Structure: Channels connecting the cytoplasm of neighbouring cells.

• Protein: Made of connexin proteins.

• Function: Allow ions and small molecules to pass directly.

• Importance: Help coordinate electrical and chemical signals.

5. Cell Adhesion Molecules (CAMs)

• Examples: Cadherins, integrins and selectins.

• Function: Help cells attach to other cells or the extracellular matrix.

• Importance: Support cell recognition, migration and tissue formation.

3Diagram

4Importance

• Tissue formation: Maintain proper organisation of cells.

• Communication: Transfer signals between neighbouring cells.

• Protection: Prevent unwanted movement of substances.

• Coordination: Help tissues perform specialised functions.

Q7Distinguish between Passive and Facilitated diffusion. (3)

1Meaning

• Passive diffusion: Movement of substances directly through the lipid bilayer.

• Facilitated diffusion: Movement of substances through specific membrane proteins.

• Both processes occur from higher to lower concentration without energy.

2Differences

3Diagram

• Caption: Show direct passage through the bilayer and protein-assisted passage.

• Keywords

  Concentration gradientlipid bilayerchannel proteincarrier proteinATP-independentspecificity
• Conclusion

  Both are energy-independent processes, but facilitated diffusion requires membrane proteins.

Q8Integral and Peripheral Proteins

1Introduction

• Plasma membrane: It is a thin, flexible boundary surrounding the cell.

• Structure: It follows the fluid mosaic model, proposed by Singer and Nicolson (1972).

• Composition: It contains phospholipids, cholesterol, carbohydrates and proteins.

• Membrane proteins: These proteins perform transport, communication, recognition and enzymatic activities.

• Classification: Membrane proteins are mainly classified into integral proteins and peripheral proteins.

• Arrangement: Integral proteins are embedded within the lipid bilayer. Peripheral proteins remain attached to its surface.

2Integral Proteins

• Definition

  Integral proteins are proteins permanently embedded within the phospholipid bilayer.

• Penetration: They may enter one membrane layer or cross the entire membrane.

• Transmembrane proteins: These proteins extend across the complete lipid bilayer.

• Hydrophobic regions: Their non-polar amino acids interact with the hydrophobic fatty acid tails.

• Hydrophilic regions: Their polar portions face the watery environment outside or inside the cell.

• Attachment: They are strongly associated with the membrane through hydrophobic interactions.

• Removal: They generally require detergents or solvents for removal from membranes.

• Structure: Many integral proteins contain alpha-helices or beta-barrel structures.

• Distribution: They occur in the plasma membrane and organelle membranes.

3Types of Integral Proteins

• Transmembrane proteins: These proteins pass completely through the membrane.

• Monotopic proteins: These proteins enter only one side of the lipid bilayer.

• Multipass proteins: These proteins cross the membrane several times.

• Lipid-anchored proteins: These proteins attach to the membrane through covalently linked lipid molecules.

4Functions of Integral Proteins

• Transport: Channel proteins form passages for ions and molecules.

• Carrier proteins: They bind specific substances and transport them across membranes.

• Receptors: They receive chemical signals such as hormones and neurotransmitters.

• Enzymatic activity: Some integral proteins act as enzymes.

• Cell recognition: They help cells identify other cells through specific surface molecules.

• Cell adhesion: They connect neighbouring cells and maintain tissue structure.

• Signal transduction: They transfer external signals into the cell.

• Energy production: Some integral proteins participate in electron transport and ATP production.

5Examples of Integral Proteins

• Aquaporins: These proteins allow rapid movement of water across membranes.

• Sodium-potassium pump: It transports sodium and potassium ions against their concentration gradients.

• Glucose transporters: They facilitate glucose movement across cell membranes.

• Insulin receptor: It receives insulin signals and activates cellular responses.

• Cytochrome proteins: They participate in electron transport during cellular respiration.

6Peripheral Proteins

• Definition

  Peripheral proteins are loosely attached to the inner or outer membrane surface.

• Location: They occur on either side of the phospholipid bilayer.

• Attachment: They attach to phospholipid heads or exposed regions of integral proteins.

• Interactions: Their attachment involves ionic interactions and hydrogen bonds.

• Hydrophobic regions: They generally lack membrane-spanning hydrophobic regions.

• Removal: They can usually be removed using mild salt solutions or changes in pH.

• Penetration: They do not penetrate deeply into the hydrophobic core.

• Association: Some peripheral proteins attach temporarily during cellular activities.

7Functions of Peripheral Proteins

• Structural support: They maintain the shape and stability of the cell.

• Cytoskeleton attachment: They connect the plasma membrane with cytoskeletal elements.

• Enzymatic activity: Some peripheral proteins act as enzymes.

• Cell signalling: They participate in transmitting signals inside cells.

• Cell recognition: They assist in cellular identification and communication.

• Membrane organisation: They help maintain the arrangement of membrane components.

• Mechanical strength: They support the membrane against physical stress.

8Examples of Peripheral Proteins

• Spectrin: It maintains the shape and flexibility of red blood cells.

• Ankyrin: It connects membrane proteins with the cytoskeleton.

• Protein kinase: Certain membrane-associated protein kinases regulate cellular signalling.

• Cytochrome c: It is a peripheral protein associated with the inner mitochondrial membrane.

• Actin-associated proteins: They help maintain cell shape and movement.

9Differences Between Integral and Peripheral Proteins

10Diagram

11Biological Importance

• Cell survival: Both protein types maintain normal membrane functioning.

• Communication: They allow cells to receive and respond to signals.

• Homeostasis: They regulate the internal environment of cells.

• Transport: They control the movement of essential substances.

• Disease connection: Defects in membrane proteins can cause disorders such as cystic fibrosis.

• Keywords

  Fluid mosaic modelSinger and Nicolsonintegral proteinsperipheral proteinstransmembrane proteinshydrophobic interactionsaquaporinsspectrinmembrane transport
• Exam tip

  Draw a labelled membrane diagram and explain attachment, removal methods, functions and examples of both protein types.

• Conclusion

  Integral and peripheral proteins are essential components of biological membranes. They perform transport, signalling, recognition and structural functions. Together, they maintain membrane organisation and normal cellular activities.

Q9Tight junctions and Gap junctions

1Introduction

• Cell junctions: These are specialised structures that connect neighbouring cells or connect cells with the extracellular matrix.

• Location: They are commonly found in epithelial tissues and other animal tissues.

• Purpose: They provide mechanical support, communication and control over the movement of substances between cells.

• Importance: Cell junctions maintain tissue organisation and normal cellular functioning.

• Types: Important cell junctions include tight junctions, gap junctions and anchoring junctions.

• Focus: Tight junctions form barriers, while gap junctions allow direct communication between cells.

2Tight Junctions: Meaning and Definition

• Tight junctions: These are specialised connections between adjacent cells that seal the space between them.

• Alternative name: They are also called zonula occludens.

• Location: They are generally present near the apical surface of epithelial cells.

• Structure: They form continuous, belt-like seals around neighbouring cells.

• Main function: They prevent uncontrolled movement of substances through the space between cells.

• Importance: They maintain the selective permeability of epithelial tissues.

3Structure of Tight Junctions

• Transmembrane proteins: Claudins and occludin are the main proteins involved in tight junction formation.

• Claudins: These proteins form the main sealing strands between neighbouring cells.

• Occludin: It helps regulate junction organisation and barrier properties.

• JAM proteins: Junctional adhesion molecules also contribute to cell adhesion.

• Cytoplasmic proteins: ZO-1, ZO-2 and ZO-3 are important supporting proteins.

• Actin filaments: These proteins connect tight junctions with the cell's actin cytoskeleton.

• Arrangement: Proteins of neighbouring cells interact to create a sealing network.

4Functions of Tight Junctions

• Barrier function: They prevent unwanted substances from passing between adjacent cells.

• Selective permeability: They control the movement of water and ions through the paracellular pathway.

• Cell polarity: They separate the apical and basolateral membrane regions.

• Transport regulation: They help maintain selective absorption and secretion.

• Protection: They prevent harmful substances from entering sensitive tissues.

• Tissue organisation: They maintain the arrangement of epithelial cells.

• Blood-brain barrier: Tight junctions between brain endothelial cells help restrict the entry of many substances into the brain.

5Examples of Tight Junctions

• Intestine: They regulate the passage of substances between intestinal epithelial cells.

• Kidney: They help control the movement of water and ions between renal epithelial cells.

• Blood-brain barrier: They restrict the passage of many substances from blood into brain tissue.

• Urinary bladder: They help prevent leakage of urine through the epithelial lining.

6Gap Junctions: Meaning and Definition

• Gap junctions: These are specialised intercellular connections that allow direct communication between neighbouring animal cells.

• Structure: They contain small channels connecting the cytoplasm of adjacent cells.

• Main function: They allow ions and small molecules to pass directly between cells.

• Importance: They coordinate the activities of neighbouring cells.

• Nature: Unlike tight junctions, gap junctions do not seal the intercellular space.

7Structure of Gap Junctions

• Connexins: These are the basic protein subunits forming gap junction channels.

• Connexon: Six connexin proteins combine to form one connexon.

• Channel formation: One connexon from each neighbouring cell joins to form a complete channel.

• Intercellular gap: The channel bridges the narrow space between two adjacent plasma membranes.

• Pore: The channel allows ions and small water-soluble molecules to pass.

• Selectivity: Large proteins and nucleic acids generally cannot pass through these channels.

• Regulation: Channel opening and closing can be influenced by calcium concentration and cytoplasmic pH.

8Functions of Gap Junctions

• Cell communication: They allow neighbouring cells to exchange small molecules and ions.

• Electrical coupling: They allow electrical signals to pass between cells.

• Chemical coordination: They help cells coordinate metabolic activities.

• Heart function: They allow electrical impulses to spread between cardiac muscle cells.

• Smooth muscles: They coordinate contractions in certain smooth muscle tissues.

• Embryonic development: They help coordinate communication between developing cells.

• Tissue homeostasis: They support the exchange of signalling molecules and metabolites.

• Calcium signalling: They help transmit certain intracellular signals between neighbouring cells.

9Examples of Gap Junctions

• Cardiac muscle: Gap junctions connect cardiac muscle cells and allow rapid electrical communication.

• Smooth muscle: They coordinate contractions in the uterus and other smooth muscle tissues.

• Liver: They allow the exchange of small metabolites and signalling molecules between hepatocytes.

• Developing embryo: They help neighbouring cells coordinate developmental activities.

10Diagram of Tight and Gap Junctions

Caption: Draw two adjacent cells showing sealing strands in tight junctions and connecting protein channels in gap junctions.

11Difference Between Tight Junctions and Gap Junctions

12Biological Importance of Both Junctions

• Tissue integrity: Both junctions help maintain the organisation of tissues.

• Cell coordination: Tight junctions regulate barrier functions, while gap junctions coordinate cellular activities.

• Organ function: Both are essential for the normal functioning of organs.

• Homeostasis: They help maintain stable conditions within tissues.

• Disease association: Defects in junction proteins can disturb tissue barriers and cell communication.

• Example

  Abnormal gap junction proteins can cause certain inherited disorders, including some forms of deafness.

• Example

  Disrupted tight junctions can increase intestinal permeability and weaken epithelial barriers.

13Conclusion

• Tight junctions seal neighbouring cells and control the movement of substances between them.

• They contain proteins such as claudins and occludin.

• Gap junctions provide direct communication between neighbouring cells.

• They are formed by connexin proteins arranged into connexon channels.

• Both junctions are essential for tissue organisation, communication and normal organ functioning.

Unit IIEndomembrane System

Q10Explain the various modifications that proteins undergo in the lumen of ER. (7)

1Introduction

• Endoplasmic Reticulum (ER): It is a membrane-bound network of tubules and flattened sacs.

• Rough ER: It contains ribosomes and helps synthesise secretory and membrane proteins.

• ER lumen: It is the internal space of the ER where newly synthesised proteins undergo processing.

• Importance: These modifications help proteins attain their correct structure and function.

2Modifications of Proteins in ER Lumen

• 1. Protein folding: Newly synthesised polypeptides fold into their correct three-dimensional structures.

• Molecular chaperones: Proteins such as BiP help newly formed proteins fold correctly.

• 2. Disulfide bond formation: The enzyme protein disulfide isomerase (PDI) helps form disulfide bonds between cysteine residues.

• Importance: These bonds stabilise the protein's three-dimensional structure.

• 3. N-linked glycosylation: An oligosaccharide chain is attached to the nitrogen atom of asparagine residues.

• Enzyme: Oligosaccharyl transferase transfers the sugar chain to the growing protein.

• Importance: Glycosylation helps protein folding, stability and recognition.

• 4. Quality control: The ER checks whether proteins have folded properly.

• Chaperones: Calnexin and calreticulin help in the folding of glycoproteins.

• ER-associated degradation (ERAD): Incorrectly folded proteins are transported back into the cytosol and degraded by proteasomes.

• 5. Proteolytic cleavage: Some proteins undergo removal of specific peptide segments during processing.

• Signal peptide: The signal peptide is generally removed by signal peptidase during protein entry into the ER.

• 6. Assembly of proteins: Different polypeptide chains combine to form functional protein complexes.

• Example

  Some antibody proteins assemble through interactions between heavy and light chains.

3Flowchart of Protein Processing

Draw the sequence of protein entry, folding, modification, quality control and transport from ER to Golgi.

4Transport to Golgi Apparatus

• Vesicle formation: Correctly folded proteins are packed into transport vesicles.

• COPII vesicles: These vesicles carry proteins from the ER towards the Golgi apparatus.

• Further modification: Additional processing and sorting occur inside the Golgi apparatus.

5Conclusion

• The ER lumen provides an environment for protein folding, glycosylation and disulfide bond formation. Its quality-control system ensures that properly folded proteins reach the Golgi apparatus.

• Keywords

  Rough ERprotein foldingBiPPDIdisulfide bondsN-linked glycosylationcalnexinERADCOPII
• Exam tip

  Explain N-linked glycosylation, disulfide bond formation and protein quality control with their respective enzymes and functions.

Q11What is oxidative phosphorylation? Describe how the Electron Transport Chain and ATP Synthase in the mitochondria help generate ATP-the energy currency of the cell. (15)

1Meaning of Oxidative Phosphorylation

• Oxidative phosphorylation: It is the process of producing ATP using energy released during electron transport.

• Location: It occurs on the inner mitochondrial membrane.

• Main components: It involves the electron transport chain and ATP synthase.

• Electron donors: NADH and FADH₂ provide high-energy electrons.

• Oxygen: It acts as the final electron acceptor.

• ATP: It is produced from ADP and inorganic phosphate.

• Importance: It provides most of the ATP produced during aerobic respiration.

2Structure of Mitochondria

• Outer membrane: It surrounds the mitochondrion.

• Inner membrane: It contains the electron transport chain and ATP synthase.

• Intermembrane space: It lies between the two mitochondrial membranes.

• Cristae: These are folds of the inner membrane.

• Matrix: It contains enzymes, mitochondrial DNA and ribosomes.

• Proton gradient: A difference in proton concentration develops across the inner membrane.

Draw the mitochondrion with outer membrane, inner membrane, cristae, matrix and intermembrane space.

3Electron Transport Chain

• Meaning

  The electron transport chain is a series of protein complexes that transfer electrons.

• Location: It is present in the inner mitochondrial membrane.

• Function: It transfers electrons from NADH and FADH₂ to oxygen.

• Energy release: Electron transfer releases energy used to pump protons.

Major Components

• Complex I: NADH dehydrogenase receives electrons from NADH and transfers them to coenzyme Q.

• Complex II: Succinate dehydrogenase transfers electrons from FADH₂ to coenzyme Q.

• Coenzyme Q: It is a mobile electron carrier within the membrane.

• Complex III: Cytochrome bc1 transfers electrons to cytochrome c.

• Cytochrome c: It carries electrons to Complex IV.

• Complex IV: Cytochrome c oxidase transfers electrons to oxygen.

• Oxygen: It combines with electrons and protons to form water.

Proton Pumping

• Complex I: Pumps protons from the matrix into the intermembrane space.

• Complex II: Does not pump protons.

• Complex III: Pumps protons into the intermembrane space.

• Complex IV: Also pumps protons across the inner membrane.

• Gradient: Proton accumulation creates an electrochemical gradient.

4ATP Synthase and ATP Formation

• Meaning

  ATP synthase is an enzyme complex that produces ATP using proton flow.

• Location: It is present in the inner mitochondrial membrane.

• F₀ portion: It forms a channel through which protons move.

• F₁ portion: It contains catalytic sites that synthesise ATP.

• Proton movement: Protons flow back into the matrix through F₀.

• Rotation: Proton flow causes rotation of the enzyme.

• Shape change: The F₁ subunits change shape during rotation.

• ATP formation: ADP combines with inorganic phosphate to produce ATP.

Chemiosmotic Theory

• Proposed by: Peter Mitchell proposed the chemiosmotic theory in 1961.

• Main idea: Electron transport creates a proton gradient across the inner membrane.

• Stored energy: The gradient stores energy as a proton-motive force.

• ATP production: ATP synthase uses this energy to produce ATP.

5Diagram of Oxidative Phosphorylation

Draw the four electron transport complexes, proton pumping, oxygen reduction and ATP synthase.

6Overall ATP Production

• NADH: Each mitochondrial NADH produces approximately 2.5 ATP.

• FADH₂: Each FADH₂ produces approximately 1.5 ATP.

• Reason: FADH₂ electrons enter through Complex II and bypass Complex I.

• Proton pumping: Approximately 10 protons are pumped per NADH.

• FADH₂: Approximately 6 protons are pumped per FADH₂.

• ATP synthesis: Around four protons are required per ATP when synthesis and transport costs are considered.

7Importance of Oxidative Phosphorylation

• Energy production: It produces large quantities of cellular ATP.

• Metabolism: ATP supports biosynthesis and metabolic reactions.

• Movement: ATP powers muscle contraction and cellular movement.

• Transport: ATP supports active transport across membranes.

• Cell survival: It maintains essential cellular functions.

• Oxygen dependence: Without oxygen, normal mitochondrial electron transport stops.

8Conclusion

Oxidative phosphorylation is the major ATP-producing process in aerobic cells. The electron transport chain creates a proton gradient. ATP synthase uses this gradient to produce ATP efficiently.

Q12What is Signal Hypothesis? How does vesicular transport take place from ER to Golgi apparatus? (10)

1Meaning of Signal Hypothesis

• Definition

  The Signal Hypothesis explains how proteins are directed towards the endoplasmic reticulum.

• Proposed by: Günter Blobel and David Sabatini in 1971.

• Main idea: Proteins contain specific signal sequences that determine their cellular destination.

• Signal peptide: It is a short amino acid sequence, usually present near the N-terminus.

• Function: It directs newly synthesised proteins towards the rough endoplasmic reticulum.

2Mechanism of Signal Hypothesis

• Step 1 – Translation begins: Protein synthesis starts on free ribosomes in the cytoplasm.

• Step 2 – Signal sequence appears: A signal peptide emerges from the ribosome.

• Step 3 – SRP binding: The Signal Recognition Particle (SRP) recognises and binds the signal peptide.

• Step 4 – Translation pauses: SRP temporarily stops protein synthesis.

• Step 5 – ER attachment: The SRP-ribosome complex binds to the SRP receptor on the ER membrane.

• Step 6 – Translocation: The ribosome attaches to the Sec61 translocon.

• Step 7 – Translation resumes: The growing polypeptide enters the ER lumen.

• Step 8 – Signal removal: Signal peptidase may remove the signal peptide.

• Step 9 – Protein processing: The protein folds and undergoes modifications inside the ER.

3Diagram of Signal Hypothesis

Draw the pathway showing SRP recognition, ER attachment and protein translocation.

4Vesicular Transport from ER to Golgi Apparatus

Vesicular transport transfers proteins and lipids between cellular compartments.

A. Protein packaging in ER

• Protein folding: Newly synthesised proteins fold inside the ER.

• Quality control: Misfolded proteins are retained for correction or degradation.

• Sorting: Correctly folded proteins are selected for transport.

• Exit sites: Transport begins at specialised regions of the ER.

B. Formation of COPII-coated vesicles

• COPII: It is a protein coat responsible for forward transport.

• Sar1: The small GTPase Sar1 becomes activated by GTP.

• Coat formation: Sar1 recruits COPII proteins, including Sec23/24 and Sec13/31.

• Cargo selection: Cargo proteins bind to specific cargo receptors.

• Budding: The ER membrane bends and forms transport vesicles.

C. Vesicle movement and fusion

• Transport: Vesicles move towards the Golgi apparatus.

• Tethering: Tethering proteins help vesicles recognise the target membrane.

• Docking: Rab GTPases and tethering factors assist vesicle docking.

• Fusion: SNARE proteins bring the vesicle and target membranes together.

• Release: Vesicle contents enter the Golgi apparatus.

D. Golgi processing

• Cis-Golgi: Receives vesicles from the ER.

• Processing: Proteins undergo glycosylation and other modifications.

• Sorting: Proteins move through medial and trans-Golgi regions.

• Destination: They are sent towards lysosomes, plasma membrane or secretion.

5Diagram of ER-to-Golgi Transport

Draw COPII-coated vesicles carrying cargo from rough ER to cis-Golgi.

• Keywords

  Signal peptideSRPrough ERSec61 transloconCOPIISar1Rab GTPaseSNAREcis-Golgi
• Exam tip

  Explain SRP-mediated targeting first, followed by COPII budding, vesicle docking and SNARE-mediated fusion. Label the diagram clearly.

• Conclusion

  The Signal Hypothesis explains protein targeting to the ER. Vesicular transport then delivers properly folded proteins to the Golgi for modification and sorting.

Q13Give the contribution of ANY FIVE of the following scientists : (i) Benda ; (ii) Peter Mitchell ; (iii) Blobel & Sabatini ; (iv) Christian de duve ; (v) Tim Hunt, Paul Nurse, Lee Hartwell ; (vi) Lynn Margulis (5)

1Benda

• Scientist: Carl Benda was a German scientist.

• Contribution: Coined the term mitochondria in 1898.

• Discovery: Studied these structures in cells using staining techniques.

• Importance: Helped establish mitochondria as important cellular organelles.

2Peter Mitchell

• Scientist: Peter Mitchell was a British biochemist.

• Contribution: Proposed the Chemiosmotic Theory in 1961.

• Explanation: Electron transport creates a proton gradient across the inner mitochondrial membrane.

• Function: Proton movement drives ATP synthesis.

• Recognition: Received the Nobel Prize in Chemistry in 1978.

3Blobel and Sabatini

• Scientists: Günter Blobel and David Sabatini studied protein targeting.

• Contribution: Explained the role of signal sequences in protein transport.

• Signal hypothesis: Proteins contain specific signals that direct them towards the endoplasmic reticulum.

• Function: Helps newly formed proteins reach their correct cellular destinations.

• Recognition: Blobel received the Nobel Prize in Physiology or Medicine in 1999.

4Christian de Duve

• Scientist: Christian de Duve was a Belgian cytologist and biochemist.

• Contribution: Discovered lysosomes in 1955.

• Discovery: Identified lysosomes as enzyme-containing cellular organelles.

• Function: Explained their role in intracellular digestion and waste removal.

• Other contribution: Also contributed to the discovery of peroxisomes.

5Tim Hunt, Paul Nurse and Lee Hartwell

• Scientists: Tim Hunt, Paul Nurse and Lee Hartwell studied cell-cycle regulation.

• Contribution: Discovered important regulators of the cell cycle.

• Lee Hartwell: Identified genes controlling cell-cycle progression, called CDC genes.

• Paul Nurse: Identified CDK1, a key cell-cycle control protein.

• Tim Hunt: Discovered cyclins, proteins regulating cell-cycle progression.

• Recognition: Shared the Nobel Prize in Physiology or Medicine in 2001.

6Lynn Margulis

• Scientist: Lynn Margulis was an American biologist.

• Contribution: Developed the modern Endosymbiotic Theory.

• Explanation: Proposed that mitochondria and chloroplasts originated from free-living bacteria.

• Evidence: These organelles possess circular DNA, 70S ribosomes and divide independently.

• Importance: Explained the evolutionary origin of important eukaryotic organelles.

• Keywords

  Carl BendamitochondriaPeter Mitchellchemiosmotic theorysignal hypothesislysosomescyclinsCDK1endosymbiotic theory
• Exam tip

  Remember each scientist's name, major discovery and its year wherever applicable. These are important scoring points.

• Conclusion

  These scientists made major contributions to cell biology. Their discoveries explained organelle functions, protein transport, energy production and cell-cycle regulation.

Q14Which organelle in the cell is also called the "Suicidal Bag". Enumerate its functions. (5)

1Identification

• Organelle: Lysosome is known as the suicidal bag of the cell.

• Discovery: Lysosomes were discovered by Christian de Duve in 1955.

• Nature: They are small, membrane-bound organelles containing digestive enzymes.

• Structure: They are surrounded by a single membrane.

• Appearance: They are generally spherical vesicles found mainly in animal cells.

2Structure of lysosome

• Membrane: A single membrane separates digestive enzymes from the cytoplasm.

• Enzymes: Lysosomes contain hydrolytic enzymes called acid hydrolases.

• pH: Their internal environment is acidic, around pH 4.5–5.

• Protection: The membrane prevents digestive enzymes from damaging the cytoplasm.

• Origin: Lysosomal enzymes are synthesised in the rough endoplasmic reticulum and processed through the Golgi apparatus.

3Functions of lysosomes

• Intracellular digestion: They digest large molecules such as proteins, lipids and carbohydrates.

• Heterophagy: They digest materials entering the cell through endocytosis or phagocytosis.

• Autophagy: They break down damaged organelles and recycle their components.

• Autolysis: They can release digestive enzymes and cause self-digestion under certain conditions.

• Defence: They destroy bacteria and other harmful microorganisms engulfed by cells.

• Waste removal: They remove cellular waste and unwanted materials.

• Recycling: They release useful molecules for reuse by the cell.

• Development: They participate in tissue remodelling and programmed cell death-related processes.

4Diagram

5Why is it called the suicidal bag?

• Self-digestion: During severe cell damage, lysosomal membranes may become damaged.

• Enzyme release: Digestive enzymes may enter the cytoplasm.

• Autolysis: These enzymes can contribute to the breakdown of cellular components.

• Importance: This explains the name suicidal bag, although lysosomes normally perform controlled digestion.

• Keywords

  LysosomeChristian de Duve1955acid hydrolasesautophagyheterophagyautolysisintracellular digestion
• Exam tip

  Mention the discovery, single membrane, acidic pH, digestive enzymes and at least six functions.

• Conclusion

  Lysosomes are called suicidal bags because their enzymes can cause cellular self-digestion. They also play essential roles in digestion, defence, recycling and cellular maintenance.

Q15Explain the Endosymbiotic theory. (3)

1Meaning

• Endosymbiotic theory: Explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• It was popularised by Lynn Margulis (1967).

2Explanation

• Ancient host cell: An early larger cell engulfed smaller bacteria.

• Aerobic bacteria: Some bacteria lived inside the host and provided energy.

• Mitochondria: These bacteria gradually evolved into mitochondria.

• Photosynthetic bacteria: Some cyanobacteria-like organisms became chloroplasts.

• Mutual benefit: The host provided protection and nutrients. The bacteria provided useful products.

3Evidence

• Double membrane: Both organelles possess two surrounding membranes.

• Own DNA: They contain circular, bacterial-like DNA.

• Ribosomes: They have 70S ribosomes, similar to bacteria.

• Binary fission: They divide independently inside cells.

4Diagram

• Caption: Draw the evolution of mitochondria and chloroplasts from engulfed bacteria.

• Keywords

  EndosymbiosisLynn Margulismitochondriachloroplastscircular DNA70S ribosomesbinary fission
• Conclusion

  The theory explains how certain bacterial ancestors became permanent organelles.

Q16Write short notes (Any three): (i) Fate of C-skeleton of amino acids ; (ii) Omega Oxidation of Fatty acid ; (iii) Glycogen Metabolism ; (iv) Shuttle system ; (v) Cascade of metabolic events in fasting and starvation (15)

1Fate of Carbon Skeleton of Amino Acids

• Meaning

  After removal of the amino group, the remaining carbon structure is called the carbon skeleton.

• Deamination: Amino acids lose their amino group through deamination.

• Transamination: The amino group is transferred to another molecule, usually alpha-ketoglutarate.

• Glucogenic amino acids: Their carbon skeletons form glucose through gluconeogenesis.

• Ketogenic amino acids: Their carbon skeletons produce ketone bodies or fatty acids.

• Energy production: Carbon skeletons enter different metabolic pathways to produce ATP.

Major Metabolic Entry Points

• Urea formation: The removed nitrogen is converted into urea in the liver.

• Importance: Amino acid breakdown provides energy during fasting and starvation.

2Omega Oxidation of Fatty Acids

• Meaning

  Omega oxidation is an alternative pathway of fatty acid oxidation.

• Location: It mainly occurs in the smooth endoplasmic reticulum.

• Site: Oxidation occurs at the terminal methyl carbon called the omega carbon.

• Enzyme: Cytochrome P450 enzymes help initiate this process.

• First step: The terminal methyl group is converted into an alcohol group.

• Second step: The alcohol is converted into an aldehyde.

• Third step: The aldehyde is converted into a carboxylic acid.

• Product: Dicarboxylic acids are formed.

• Further breakdown: These products may undergo beta oxidation in mitochondria or peroxisomes.

• Importance: Omega oxidation becomes more important when beta oxidation is impaired or overloaded.

Draw the conversion of the terminal methyl group into a carboxyl group.

3Glycogen Metabolism

• Meaning

  Glycogen metabolism involves the synthesis and breakdown of glycogen.

• Glycogen: It is the storage form of glucose in animals.

• Storage: It is mainly stored in the liver and skeletal muscles.

A. Glycogenesis

• Meaning

  Glycogenesis is the formation of glycogen from glucose.

• Glucose phosphorylation: Glucose is converted into glucose-6-phosphate.

• Isomerisation: Glucose-6-phosphate becomes glucose-1-phosphate.

• Activation: Glucose-1-phosphate forms UDP-glucose.

• Glycogen synthase: This enzyme adds glucose units to the growing chain.

• Branching enzyme: It creates branches in glycogen.

B. Glycogenolysis

• Meaning

  Glycogenolysis is the breakdown of glycogen into glucose-containing products.

• Glycogen phosphorylase: It breaks alpha-1,4 glycosidic bonds.

• Debranching enzyme: It removes branch points.

• Glucose-1-phosphate: It is converted into glucose-6-phosphate.

• Liver: Glucose-6-phosphatase releases free glucose into blood.

• Muscles: Glucose-6-phosphate enters glycolysis to provide energy.

Hormonal Regulation

4Shuttle System

• Meaning

  Shuttle systems transfer reducing equivalents from cytoplasmic NADH into mitochondria.

• Need: The inner mitochondrial membrane does not allow NADH to pass directly.

• Purpose: They help transfer electrons for ATP production.

• Types: The two major systems are the malate-aspartate shuttle and glycerol-3-phosphate shuttle.

A. Malate-Aspartate Shuttle

• Location: It is active in liver, heart and kidney cells.

• First step: Cytoplasmic oxaloacetate receives electrons from NADH.

• Formation: Oxaloacetate becomes malate.

• Transport: Malate enters the mitochondrial matrix.

• Conversion: Malate is converted back into oxaloacetate.

• NADH formation: Mitochondrial NADH is produced.

• Result

  Electrons enter the electron transport chain through Complex I.

B. Glycerol-3-Phosphate Shuttle

• Location: It is common in brain and skeletal muscles.

• First step: Cytoplasmic NADH reduces dihydroxyacetone phosphate.

• Formation: Glycerol-3-phosphate is produced.

• Electron transfer: Mitochondrial glycerol-3-phosphate dehydrogenase transfers electrons to FAD.

• Result

  FADH₂ transfers electrons to coenzyme Q.

• ATP yield: It generally produces less ATP than the malate-aspartate shuttle.

Draw both shuttles and show their different entry points into the electron transport chain.

5Cascade of Metabolic Events in Fasting and Starvation

• Meaning

  Fasting and starvation involve hormonal and metabolic changes that maintain blood glucose and energy supply.

• Insulin: Its level decreases when food intake stops.

• Glucagon: Its level increases to promote glucose production.

• Early fasting: Liver glycogenolysis maintains blood glucose.

• Gluconeogenesis: The liver produces glucose from lactate, glycerol and amino acids.

• Lipolysis: Adipose tissue breaks triglycerides into fatty acids and glycerol.

• Beta oxidation: Fatty acids are broken down to produce acetyl-CoA and ATP.

• Ketogenesis: The liver converts excess acetyl-CoA into ketone bodies.

• Prolonged starvation: The brain increasingly uses ketone bodies for energy.

• Protein conservation: Reduced dependence on amino acids helps preserve muscle proteins.

• Late starvation: Severe starvation eventually causes protein breakdown and organ damage.

• Draw the sequence from fasting to hormonal changes and alternative energy production.

• Keywords

  Fastinginsulinglucagonglycogenolysisgluconeogenesislipolysisbeta oxidationketogenesis
• Exam tip

  Explain the hormonal changes and distinguish early fasting from prolonged starvation.

6Conclusion

These metabolic pathways help the body use nutrients efficiently. They maintain energy production during normal conditions and food shortage. Their regulation is essential for cellular survival.

Q17Give an account of the components of Electron Transport Chain and its role in cellular respiration. (7)

1Meaning of Electron Transport Chain

• Electron Transport Chain (ETC): It is a series of electron carriers that transfer electrons through successive oxidation-reduction reactions.

• Location: It is present in the inner mitochondrial membrane in eukaryotic cells.

• Function: It helps produce ATP through oxidative phosphorylation.

• Electron donors: NADH and FADH₂ provide high-energy electrons.

2Components of Electron Transport Chain

• Complex I – NADH dehydrogenase: It receives electrons from NADH and transfers them to coenzyme Q. It pumps protons into the intermembrane space.

• Complex II – Succinate dehydrogenase: It receives electrons from FADH₂ and transfers them to coenzyme Q. It does not pump protons.

• Coenzyme Q: Also called ubiquinone, it is a mobile carrier that transfers electrons from Complex I and II to Complex III.

• Complex III – Cytochrome bc₁ complex: It transfers electrons from coenzyme Q to cytochrome c. It pumps protons across the membrane.

• Cytochrome c: It is a small mobile protein that carries electrons from Complex III to Complex IV.

• Complex IV – Cytochrome c oxidase: It transfers electrons to oxygen, the final electron acceptor. Oxygen combines with electrons and protons to form water.

• Complex V – ATP synthase: It uses the proton gradient to produce ATP from ADP and inorganic phosphate.

3Role in Cellular Respiration

• Electron transfer: Electrons move through the complexes and release energy.

• Proton pumping: Complexes I, III and IV pump protons into the intermembrane space.

• Proton gradient: This creates an electrochemical gradient across the inner mitochondrial membrane.

• Chemiosmosis: Protons move back into the mitochondrial matrix through ATP synthase.

• ATP production: ATP synthase uses this energy to convert ADP and inorganic phosphate into ATP.

• Oxygen requirement: Oxygen accepts electrons at Complex IV and forms water.

• ATP yield: Approximately 2.5 ATP per NADH and 1.5 ATP per FADH₂ are produced.

4Flowchart

Draw the electron flow through ETC complexes and show proton movement through ATP synthase.

5Conclusion

• The ETC is essential for aerobic respiration. It creates a proton gradient that drives ATP synthesis through oxidative phosphorylation.

• Keywords

  NADHFADH₂Complex I–IVcoenzyme Qcytochrome coxygenproton gradientATP synthase
• Exam tip

  Remember that Complexes I, III and IV pump protons, while Complex II does not. Oxygen is the final electron acceptor.

Q18Write an account on the structure and function of mitochondrial respiratory chain. (6)

1Introduction

• Mitochondrial respiratory chain: A series of electron carriers present in the inner mitochondrial membrane.

• Main function: Transfers electrons and produces energy in the form of ATP.

• Other name: Electron Transport Chain (ETC).

• Process: Forms the final stage of aerobic respiration.

2Structure

The respiratory chain consists of four major protein complexes and two mobile electron carriers.

• Complex I (NADH dehydrogenase): Receives electrons from NADH and transfers them to ubiquinone.

• Complex II (Succinate dehydrogenase): Receives electrons from FADH₂ and transfers them to ubiquinone.

• Coenzyme Q: Mobile carrier that transfers electrons to Complex III.

• Complex III (Cytochrome bc₁): Transfers electrons to cytochrome c.

• Cytochrome c: Mobile protein carrier that transfers electrons to Complex IV.

• Complex IV (Cytochrome c oxidase): Transfers electrons to oxygen, forming water.

• ATP synthase (Complex V): Uses the proton gradient to produce ATP.

3Mechanism of Electron Transport

• Electron donation: NADH and FADH₂ donate high-energy electrons.

• Electron movement: Electrons pass through successive respiratory complexes.

• Proton pumping: Complexes I, III and IV pump protons into the intermembrane space.

• Proton gradient: Creates an electrochemical difference across the inner membrane.

• ATP production: Protons return through ATP synthase, producing ATP.

• Final acceptor: Oxygen accepts electrons and combines with protons to form water.

4Diagram

5ATP Yield

• NADH: Approximately 2.5 ATP molecules are produced per NADH.

• FADH₂: Approximately 1.5 ATP molecules are produced per FADH₂.

• Importance: Provides most ATP during aerobic respiration.

• Keywords

  ETCComplex IComplex IICoenzyme Qcytochrome cComplex IVproton gradientATP synthase
• Exam tip

  Remember that oxygen is the final electron acceptor and Complexes I, III and IV pump protons.

• Conclusion

  The mitochondrial respiratory chain transfers electrons, creates a proton gradient and drives ATP synthesis through oxidative phosphorylation.

Q19Why is Golgi apparatus termed as the "Post Office of the Cell"? Discuss with suitable diagram. (6)

1Introduction

• Golgi apparatus: A membrane-bound organelle involved in modifying, sorting and packaging cellular materials.

• Discovery: Described by Camillo Golgi in 1898.

• Structure: Consists of flattened membrane-bound sacs called cisternae.

• Post office: Sorts and sends proteins and lipids to their correct destinations.

2Structure of Golgi Apparatus

• Cis face: Receiving side that accepts vesicles from the endoplasmic reticulum.

• Cisternae: Flattened sacs where materials undergo modification.

• Medial region: Contains enzymes for processing proteins and lipids.

• Trans face: Shipping side from which materials leave.

• Golgi vesicles: Small membrane sacs that carry packaged substances.

3Functions

• Modification: Adds carbohydrate groups to proteins through glycosylation.

• Sorting: Separates molecules according to their destinations.

• Packaging: Encloses materials inside transport vesicles.

• Secretion: Sends proteins outside the cell through exocytosis.

• Lysosome formation: Produces vesicles containing digestive enzymes.

• Lipid processing: Modifies and transports certain lipids.

• Membrane renewal: Supplies membrane components to different cell regions.

4Diagram

5Why Is It Called the Post Office?

• Receiving: Accepts proteins and lipids from the ER.

• Processing: Modifies and prepares materials.

• Addressing: Adds molecular signals for correct destinations.

• Dispatching: Sends materials through transport vesicles.

• Delivery: Supplies materials to the membrane, lysosomes or outside the cell.

• Keywords

  Golgi apparatusCamillo Golgicis facetrans facecisternaeglycosylationsortingpackaging
• Exam tip

  Clearly label cis face, cisternae, trans face and vesicles in the diagram.

• Conclusion

  Golgi apparatus is called the post office because it modifies, sorts, packages and delivers cellular products to their destinations.

Q20Discuss how peroxisomes are involved in detoxification process. (4)

1Meaning

• Peroxisomes: These are small, single-membrane-bound organelles found in eukaryotic cells.

• They contain oxidative enzymes such as oxidases and catalase.

• They help break down toxic substances and protect cells from harmful chemicals.

2Role in detoxification

• Oxidation: Peroxisomal oxidases break down fatty acids and harmful substances.

• Hydrogen peroxide formation: These reactions produce hydrogen peroxide (H₂O₂) as a by-product.

• Toxicity: Hydrogen peroxide is harmful because it can damage cellular components.

• Catalase activity: The enzyme catalase converts hydrogen peroxide into water and oxygen.

• Alcohol detoxification: In liver cells, peroxisomes help break down alcohol and other toxic compounds.

• Fatty acid breakdown: They perform β-oxidation of very-long-chain fatty acids.

• Cell protection: These activities reduce oxidative damage and maintain cellular health.

3Chemical reaction

The catalase enzyme breaks down hydrogen peroxide through the following reaction:

\[ 2H_2O_2 \xrightarrow{\text{Catalase}} 2H_2O+O_2 \]

• Reactant: Hydrogen peroxide.

• Products: Water and oxygen.

• Importance: This reaction prevents the accumulation of harmful hydrogen peroxide.

4Flowchart of detoxification

Q21What are the polymorphic forms of Lysosomes? (4)

1Meaning

• Lysosomes: These are membrane-bound cell organelles containing digestive enzymes.

• They perform intracellular digestion and remove unwanted cellular materials.

• Lysosomes show different forms depending on their stage and digestive activity.

2Polymorphic forms of lysosomes

• 1. Primary lysosomes: Newly formed lysosomes containing digestive enzymes. They have not yet participated in digestion.

• 2. Secondary lysosomes: Formed when primary lysosomes fuse with food-containing vesicles. Active digestion takes place inside them.

• 3. Autophagic vacuoles: Formed when lysosomes digest damaged organelles or old cell components. This process is called autophagy.

• 4. Residual bodies: Contain undigested materials after digestion. They may remain inside the cell or release waste through exocytosis.

3Diagram

Q22Write exact location and function of the following (any three): (i) Mannose-6-phosphate ; (ii) Cytochrome p450 ; (iii) SDH ; (iv) Hydrolases (3)

1Mannose-6-phosphate

• Location: Present on the oligosaccharide chains of lysosomal enzymes.

• Formation: Added to enzymes in the cis-Golgi network.

• Function: Acts as a targeting marker for lysosomal enzymes.

• Role: Helps transport enzymes from the Golgi apparatus to lysosomes through mannose-6-phosphate receptors.

2Cytochrome P450

• Location: Mainly present in the smooth endoplasmic reticulum (SER) and also in mitochondria.

• Function: Helps in detoxification of drugs and harmful chemicals.

• Role: Participates in oxidation reactions and metabolism of various substances.

3SDH (Succinate Dehydrogenase)

• Location: Inner mitochondrial membrane.

• Function: Catalyses conversion of succinate into fumarate.

• Role: Participates in the Krebs cycle and electron transport chain (Complex II).

4Hydrolases

• Location: Mainly present inside lysosomes.

• Function: Break down proteins, lipids, carbohydrates and nucleic acids.

• Role: Perform intracellular digestion using hydrolysis reactions.

Q23State the contributions of (any three): (i) Hugh Davson and James Danielli ; (ii) Camillo Golgi ; (iii) David Sabatini and Gunter Blobel ; (iv) Christian de Duve (3)

1Hugh Davson and James Danielli

• Contribution: Proposed the Davson–Danielli model (1935) of the plasma membrane.

• Model: Suggested a phospholipid bilayer covered by protein layers on both sides.

• Importance: Provided an early explanation of membrane structure.

2Camillo Golgi

• Contribution: Discovered the Golgi apparatus in 1898.

• Method

  Used silver staining to observe this cell organelle.

• Function identified: Helped establish its role in modification, packaging and secretion of cellular materials.

3David Sabatini and Gunter Blobel

• Contribution: Proposed the Signal Hypothesis (1971).

• Concept: Identified signal sequences that direct newly synthesised proteins towards the ER.

• Importance: Explained how proteins enter the secretory pathway.

4Christian de Duve

• Contribution: Discovered lysosomes in 1955 and later identified peroxisomes.

• Method

  Used cell fractionation and biochemical studies.

• Importance: Established lysosomes as organelles containing digestive enzymes.

Q24State the contributions of (any three): (i) Gorter and Grendel ; (ii) Benda ; (iii) Peter Mitchell ; (iv) Tim Hunt, Paul Nurse, L H Hartwell (3)

1Gorter and Grendel (1925)

• Contribution: Proposed the lipid bilayer model of the plasma membrane.

• Experiment: Extracted lipids from red blood cells and spread them on water.

• Finding: The lipid layer covered approximately twice the red blood cell surface area.

• Conclusion

  The cell membrane consists of a phospholipid bilayer.

2Benda (1898)

• Contribution: Coined the term mitochondria.

• Discovery: Described these structures in cells.

• Importance: His terminology became widely accepted in cell biology.

3Peter Mitchell (1961)

• Contribution: Proposed the Chemiosmotic theory.

• Explanation: ATP synthesis occurs through a proton gradient across membranes.

• Mechanism: Protons flow through ATP synthase, producing ATP.

• Importance: Explained ATP production during cellular respiration.

4Tim Hunt, Paul Nurse and L. H. Hartwell

• Tim Hunt (1982): Discovered cyclins, proteins controlling the cell cycle.

• Paul Nurse: Identified the role of CDK1 in cell-cycle regulation.

• L. H. Hartwell: Discovered important cell-cycle genes and checkpoints in yeast.

• Importance: Their discoveries explained how cells control cell division.

Q25Distinguish between co-translational and post-translational transport of proteins. (3)

1Meaning

• Co-translational transport: Protein enters the endoplasmic reticulum (ER) while it is being synthesised.

• Post-translational transport: Protein enters an organelle after its synthesis is completed.

2Differences

3Mechanism

• Co-translational: SRP recognises the signal peptide and directs the ribosome towards ER.

• Post-translational: Completed proteins are guided to their target organelles by signal sequences.

4Flowchart

• Caption: Show both pathways and their different transport timings.

• Keywords

  ERrough ERSRPsignal peptideribosomesecretory proteinstargeting sequence
• Conclusion

  Co-translational transport occurs during synthesis, while post-translational transport occurs afterwards.

Q26Expand the following: (i) SRP ; (ii) MPF (2)

1SRP

• Full form: Signal Recognition Particle.

• Meaning

  It is a complex made of RNA and proteins.

• Function: It recognises the signal sequence on newly forming proteins.

• Role: It directs ribosomes towards the rough endoplasmic reticulum (RER) for protein synthesis.

2MPF

• Full form: Maturation Promoting Factor (also called M-phase Promoting Factor).

• Meaning

  It is a protein complex made of Cyclin B and CDK1.

• Function: It helps the cell enter the M phase of the cell cycle.

• Role: It promotes chromosome condensation and mitotic spindle formation.

Q27Why are Lysosomes called "suicidal bags"? (2)

1Meaning

• Lysosomes: These are membrane-bound cell organelles containing digestive enzymes.

• Enzymes: They contain hydrolytic enzymes that break down proteins, fats, carbohydrates and other materials.

2Reason for the name

• Autolysis: If the lysosomal membrane breaks, digestive enzymes are released into the cytoplasm.

• Self-digestion: These enzymes may digest the cell's own components.

• Cell death: Extensive self-digestion can cause cell damage and death.

• Function: Lysosomes also remove worn-out organelles and cellular waste.

Q28(i) Define the following terms (Any four): (a) Ubiquinone ; (b) Chemiosmosis ; (c) Ketogenesis ; (d) Substrate level phosphorylation ; (e) Catabolism ; (f) Uridine Diphosphate Glucose (UDPG) ; (ii) Differentiate between (Any two): (a) Phosphofructokinase I and Phosphofructokinase II ; (b) Glycogen phosphorylase and Glycogen synthase ; (c) Acyl CoA and Acetyl CoA ; (d) SGOT and SGPT ; (e) Phosphoenolpyruvate carboxykinase and Pyruvate kinase ; (iii) Write the steps to bring about the following conversions with Structural formula (Any two): (a) Pyruvate to Acetyl CoA ; (b) Succinyl CoA to Succinate ; (c) Fatty acid to Fatty acyl CoA ; (d) Aspartate to Glutamate ; (iv) Give reasons for the following (Any Three): (a) Elevated level of glucose and acetone in untreated diabetes mellitus. ; (b) Strenuous exercise leads to an increase in formation of lactate. ; (c) Upon entering a cell glucose is phosphorylated. Give two reasons why this reaction is required. ; (d) Role of biotin in Fatty acid oxidation

1Definitions

(a) Ubiquinone

• Meaning

  Ubiquinone is also called Coenzyme Q (CoQ).

• Structure: It is a lipid-soluble electron carrier present in the inner mitochondrial membrane.

• Function: It transfers electrons from Complex I and Complex II to Complex III.

• Importance: It helps establish the proton gradient required for ATP production.

(b) Chemiosmosis

• Meaning

  Chemiosmosis is the movement of protons across a membrane through a specialised protein.

• Process: The electron transport chain pumps protons into the mitochondrial intermembrane space.

• ATP formation: Protons return through ATP synthase, which uses their energy to produce ATP.

• Importance: It is the main mechanism of ATP formation during oxidative phosphorylation.

(c) Ketogenesis

• Meaning

  Ketogenesis is the formation of ketone bodies from acetyl-CoA.

• Location: It occurs mainly in the mitochondrial matrix of liver cells.

• Products: The three ketone bodies are acetoacetate, beta-hydroxybutyrate and acetone.

• Importance: Ketone bodies provide energy to extrahepatic tissues during prolonged fasting.

(d) Substrate-level phosphorylation

• Meaning

  It is the direct formation of ATP or GTP by transferring a phosphate group.

• Process: A high-energy phosphate group moves from a metabolic intermediate to ADP or GDP.

• Examples: It occurs during glycolysis and the Krebs cycle.

• Importance: It produces ATP without requiring the electron transport chain.

(e) Catabolism

• Meaning

  Catabolism is the breakdown of complex molecules into simpler substances.

• Energy: It releases energy, which is commonly captured in the form of ATP.

• Examples: Glycolysis, beta-oxidation and cellular respiration.

• Importance: It supplies energy and metabolic intermediates to cells.

(f) Uridine Diphosphate Glucose (UDPG)

• Meaning

  UDPG is an activated form of glucose.

• Formation: It is formed from glucose-1-phosphate and UTP.

• Reaction: Glucose-1-phosphate + UTP → UDP-glucose + PPi.

• Function: It donates glucose units during glycogen synthesis.

• Importance: It also participates in the synthesis of glycoproteins and other carbohydrates.

2Differences between important enzymes and molecules

(a) Phosphofructokinase I and Phosphofructokinase II

(b) Glycogen phosphorylase and Glycogen synthase