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Cell Structure and Function – CBSE NCERT Study Resources

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11th

11th - Biology

Cell Structure and Function

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Overview of the Chapter

This chapter introduces the fundamental concepts of cell structure and function, which form the basis of biology. It covers the discovery of the cell, cell theory, and the structural organization of prokaryotic and eukaryotic cells. The chapter also explains the functions of various cell organelles and their significance in maintaining cellular processes.

Cell Theory

Cell Theory states that:

  • All living organisms are composed of cells.
  • The cell is the basic unit of life.
  • All cells arise from pre-existing cells.

Prokaryotic and Eukaryotic Cells

Cells are broadly classified into two types based on their structure:

  • Prokaryotic Cells: These are primitive cells lacking a well-defined nucleus and membrane-bound organelles. Example: Bacteria.
  • Eukaryotic Cells: These are advanced cells with a true nucleus and membrane-bound organelles. Example: Plant and animal cells.

Cell Organelles and Their Functions

Cell Organelles are specialized structures within a cell that perform specific functions.

  • Nucleus: Controls cellular activities and contains genetic material (DNA).
  • Mitochondria: Powerhouse of the cell, produces ATP through cellular respiration.
  • Endoplasmic Reticulum (ER): Involved in protein synthesis (Rough ER) and lipid metabolism (Smooth ER).
  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion.
  • Lysosomes: Contain digestive enzymes to break down waste materials.
  • Ribosomes: Site of protein synthesis.
  • Plastids (in plant cells): Chloroplasts perform photosynthesis.

Plasma Membrane

Plasma Membrane is a selectively permeable barrier that regulates the movement of substances in and out of the cell.

It is composed of a phospholipid bilayer with embedded proteins, following the fluid mosaic model.

Cytoskeleton

The cytoskeleton provides structural support to the cell and is involved in cell movement. It consists of:

  • Microfilaments
  • Microtubules
  • Intermediate filaments

Differences Between Plant and Animal Cells

Feature Plant Cell Animal Cell
Cell Wall Present Absent
Plastids Present (Chloroplasts) Absent
Vacuoles Large central vacuole Small or absent

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
Define protoplasm.
Answer:

Protoplasm is the living content of a cell surrounded by the plasma membrane.

Question 2:
Which organelle contains digestive enzymes?
Answer:

Lysosomes.

Question 3:
Name the fluid-filled space in chloroplasts.
Answer:

Stroma.

Question 4:
Which structure maintains cell shape in plant cells?
Answer:

Cell wall.

Question 5:
Define plasmolysis.
Answer:

Shrinking of cytoplasm due to water loss.

Question 6:
Name the genetic material in prokaryotes.
Answer:

Nucleoid.

Question 7:
What is the function of peroxisomes?
Answer:

They detoxify harmful substances.

Question 8:
Which organelle is called the suicidal bag?
Answer:

Lysosomes.

Question 9:
What is the role of endoplasmic reticulum?
Answer:

It transports materials within the cell.

Question 10:
Name the site of photosynthesis in plant cells.
Answer:

Chloroplasts.

Question 11:
Name the cell organelle known as the powerhouse of the cell.
Answer:

The mitochondrion is called the powerhouse of the cell because it generates ATP through cellular respiration.

Question 12:
What is the function of ribosomes in a cell?
Answer:

Ribosomes are responsible for protein synthesis. They read mRNA and assemble amino acids into polypeptide chains.

Question 13:
Which cell organelle contains digestive enzymes?
Answer:

The lysosome contains digestive enzymes that break down waste materials and cellular debris.

Question 14:
What is the role of the Golgi apparatus?
Answer:

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

Question 15:
What is the main component of the cell wall in plants?
Answer:

The cellulose is the main component of the cell wall in plant cells, providing structural support.

Question 16:
Name the process by which amoeba engulfs food.
Answer:

Phagocytosis is the process by which amoeba engulfs food particles using pseudopodia.

Question 17:
What is the function of chloroplasts?
Answer:

Chloroplasts perform photosynthesis, converting light energy into chemical energy (glucose).

Question 18:
Which cell structure regulates the entry and exit of materials?
Answer:

The plasma membrane is a selectively permeable barrier that controls material exchange.

Question 19:
What is the significance of microvilli in intestinal cells?
Answer:

Microvilli increase the surface area for efficient absorption of nutrients in the intestine.

Question 20:
Differentiate between prokaryotic and eukaryotic cells.
Answer:
  • Prokaryotic cells lack a nucleus and membrane-bound organelles.
  • Eukaryotic cells have a true nucleus and organelles.
Question 21:
What is the function of centrioles in animal cells?
Answer:

Centrioles help in cell division by organizing spindle fibers during mitosis and meiosis.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
Name the cell organelle known as the powerhouse of the cell and state its primary function.
Answer:

The mitochondrion is called the powerhouse of the cell. Its primary function is to produce ATP (Adenosine Triphosphate) through cellular respiration, providing energy for various cellular activities.

Question 2:
Differentiate between prokaryotic and eukaryotic cells based on nuclear organization.
Answer:

Prokaryotic cells lack a well-defined nucleus; their genetic material lies freely in the cytoplasm as a nucleoid.
Eukaryotic cells have a true nucleus enclosed by a nuclear membrane, separating genetic material from the cytoplasm.

Question 3:
List two functions of the Golgi apparatus.
Answer:

Golgi apparatus functions include:

  • Modifying, sorting, and packaging proteins/lipids for secretion
  • Forming lysosomes and other vesicles

Question 4:
What is the significance of ribosomes in protein synthesis?
Answer:

Ribosomes are the sites of protein synthesis. They read mRNA and assemble amino acids into polypeptide chains, essential for cellular functions and structure.

Question 5:
Describe the structure of chromatin in a eukaryotic nucleus.
Answer:

Chromatin consists of DNA wrapped around histone proteins, forming nucleosomes. It condenses into chromosomes during cell division for proper genetic material distribution.

Question 6:
How do centrioles assist in cell division?
Answer:

Centrioles form the spindle apparatus during cell division, helping in chromosome movement by organizing microtubules. They ensure accurate separation of chromosomes.

Question 7:
State the function of vacuoles in plant cells.
Answer:

Vacuoles in plant cells:

  • Store water, nutrients, and waste
  • Maintain turgor pressure for structural support
  • Regulate pH and detoxification

Question 8:
What is the composition of the cell wall in plant cells?
Answer:

The cell wall is primarily made of cellulose, along with hemicellulose, pectin, and lignin. It provides rigidity, protection, and shape to the cell.

Question 9:
Explain the term fluid mosaic model in context of the plasma membrane.
Answer:

The fluid mosaic model describes the plasma membrane as a dynamic structure with:

  • Phospholipid bilayer (fluid)
  • Embedded proteins and cholesterol (mosaic)
It allows flexibility and selective permeability.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain the significance of plasma membrane in a cell.
Answer:

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining its integrity.
It regulates the entry and exit of substances, ensuring homeostasis.
Additionally, it facilitates cell signaling and provides mechanical support by interacting with the cytoskeleton.

Question 2:
Differentiate between prokaryotic and eukaryotic cells based on their nucleus.
Answer:

Prokaryotic cells lack a true nucleus; their genetic material lies freely in the nucleoid region.
Eukaryotic cells have a well-defined nucleus enclosed by a nuclear membrane, separating DNA from the cytoplasm.

Question 3:
Describe the role of lysosomes in cellular waste management.
Answer:

Lysosomes are membrane-bound organelles containing digestive enzymes.
They break down worn-out organelles (autophagy) and foreign particles (phagocytosis).
Their acidic pH optimizes enzyme activity, ensuring efficient waste disposal.

Question 4:
How does the endoplasmic reticulum (ER) contribute to protein synthesis?
Answer:

The rough ER has ribosomes on its surface, where protein synthesis occurs.
Newly formed proteins enter the ER lumen for folding and modification.
The smooth ER aids in lipid synthesis and detoxification, supporting overall cellular function.

Question 5:
Why are mitochondria called the 'powerhouse of the cell'?
Answer:

Mitochondria generate ATP through aerobic respiration, providing energy for cellular activities.
Their double membrane and cristae increase surface area for oxidative phosphorylation.
They contain their own DNA, enabling semi-autonomous replication.

Question 6:
Explain the structure and function of Golgi apparatus.
Answer:

The Golgi apparatus consists of flattened cisternae, vesicles, and vacuoles.
It modifies, sorts, and packages proteins/lipids from the ER for secretion or cellular use.
It also forms lysosomes and adds carbohydrate tags to molecules for identification.

Question 7:
Explain the structure and function of mitochondria with a labeled diagram.
Answer:

Mitochondria are double-membraned organelles known as the powerhouse of the cell due to their role in ATP production.

Structure:

  • Outer membrane: Smooth and permeable.
  • Inner membrane: Folded into cristae to increase surface area.
  • Matrix: Contains enzymes for Krebs cycle and mitochondrial DNA.
Function:
  • Site of aerobic respiration.
  • Generates ATP via oxidative phosphorylation.
  • Plays a role in apoptosis (programmed cell death).
Diagram: (Draw a double-membraned structure with cristae and matrix labeled.)

Question 8:
Differentiate between prokaryotic and eukaryotic cells based on nucleus and membrane-bound organelles.
Answer:

Prokaryotic cells:

  • Lack a true nucleus; genetic material lies in nucleoid.
  • No membrane-bound organelles (e.g., mitochondria, Golgi).
Eukaryotic cells:
  • Have a well-defined nucleus enclosed in a nuclear membrane.
  • Contain membrane-bound organelles like ER, lysosomes.
Key difference: Prokaryotes are simpler, while eukaryotes are complex with compartmentalized functions.

Question 9:
Describe the role of lysosomes in cellular waste management. Why are they called 'suicide bags'?
Answer:

Lysosomes are membrane-bound vesicles containing digestive enzymes.

Role:

  • Break down worn-out organelles, pathogens, and cellular debris via autophagy.
  • Recycle biomolecules for reuse.
Suicide bags: During cell damage, lysosomes release enzymes to digest the cell itself (autolysis), hence the name.

Question 10:
What is the significance of fluid mosaic model in explaining plasma membrane structure?
Answer:

The fluid mosaic model describes the plasma membrane as a dynamic, flexible structure.

Key features:

  • Phospholipid bilayer with embedded proteins (like a mosaic).
  • Proteins float freely, allowing selective permeability.
  • Cholesterol maintains membrane fluidity.
Significance: Explains transport, cell signaling, and membrane repair mechanisms.

Question 11:
How do ribosomes differ in prokaryotes and eukaryotes? State their function.
Answer:

Differences:

  • Prokaryotic ribosomes: 70S (50S + 30S subunits).
  • Eukaryotic ribosomes: 80S (60S + 40S subunits).
Function: Ribosomes are sites of protein synthesis (translation), where mRNA is decoded into polypeptide chains.

Question 12:
Explain the importance of cytoskeleton in maintaining cell shape and motility.
Answer:

The cytoskeleton is a network of microfilaments, microtubules, and intermediate filaments.

Functions:

  • Microfilaments (actin): Provide mechanical support and enable cell movement (e.g., muscle contraction).
  • Microtubules: Form spindle fibers during cell division and act as tracks for organelle transport.
  • Intermediate filaments: Anchor organelles and resist stress.
Importance: Maintains cell integrity, facilitates intracellular transport, and aids in cell division.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain the fluid mosaic model of the plasma membrane with reference to its structure and function.
Answer:
Theoretical Framework

The fluid mosaic model describes the plasma membrane as a dynamic structure composed of lipids, proteins, and carbohydrates. Our textbook shows that phospholipids form a bilayer, with hydrophobic tails inward and hydrophilic heads outward.

Evidence Analysis
  • Proteins are embedded (integral) or attached (peripheral), enabling transport and signaling.
  • Cholesterol maintains fluidity, as seen in animal cells.
Critical Evaluation

This model explains selective permeability, e.g., osmosis in red blood cells. Recent studies confirm membrane fluidity affects drug delivery.

Future Implications

Understanding this aids in designing nanocarriers for targeted therapy.

Question 2:
Compare prokaryotic and eukaryotic cells based on their structural organization.
Answer:
Theoretical Framework

Prokaryotes lack membrane-bound organelles, while eukaryotes have a defined nucleus and organelles. We studied examples like E. coli (prokaryote) and human liver cells (eukaryote).

Evidence Analysis
  • Prokaryotes have 70S ribosomes; eukaryotes have 80S.
  • Eukaryotic DNA is linear with histones; prokaryotic is circular.
Critical Evaluation

Prokaryotic simplicity allows rapid reproduction, while eukaryotic complexity enables multicellularity. Current research links organelle defects to diseases like Alzheimer’s.

Future Implications

Studying differences aids antibiotic development targeting prokaryotic features.

Question 3:
Describe the role of lysosomes as cellular 'digestive bags' with two examples.
Answer:
Theoretical Framework

Lysosomes contain hydrolytic enzymes that break down biomolecules. Our textbook shows they maintain cellular cleanliness via autophagy.

Evidence Analysis
  • In white blood cells, lysosomes digest pathogens.
  • During tadpole metamorphosis, lysosomes reabsorb the tail.
Critical Evaluation

Lysosomal dysfunction causes diseases like Tay-Sachs. Recent studies explore enzyme replacement therapy.

Future Implications

Targeting lysosomes could treat metabolic disorders.

Question 4:
Analyze the significance of mitochondria in ATP synthesis with reference to its structure.
Answer:
Theoretical Framework

Mitochondria are the 'powerhouses' due to ATP production via oxidative phosphorylation. We studied their double membrane and cristae increasing surface area.

Evidence Analysis
  • Inner membrane hosts ETC proteins, e.g., cytochrome oxidase.
  • Matrix contains enzymes for Krebs cycle, as seen in muscle cells.
Critical Evaluation

Mutations in mitochondrial DNA cause disorders like MELAS. Current research links mitochondrial efficiency to aging.

Future Implications

Enhancing ATP synthesis could treat energy-deficiency diseases.

Question 5:
Discuss how the endoplasmic reticulum (ER) contributes to protein synthesis and lipid metabolism.
Answer:
Theoretical Framework

The ER is a network of membranes divided into rough (RER) and smooth (SER). RER has ribosomes for protein synthesis, while SER synthesizes lipids.

Evidence Analysis
  • RER produces insulin in pancreatic β-cells.
  • SER detoxifies drugs in liver cells.
Critical Evaluation

ER stress triggers diseases like diabetes. Recent studies show SER’s role in calcium storage.

Future Implications

Modulating ER function could improve metabolic disorders.

Question 6:
Explain the structure and functions of the plasma membrane with reference to the Fluid Mosaic Model. How does it maintain the selective permeability of the cell?
Answer:

The plasma membrane is a dynamic, semi-permeable barrier that surrounds the cell and regulates the movement of substances in and out of it. According to the Fluid Mosaic Model, proposed by Singer and Nicolson, the membrane consists of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.


Structure:
1. Phospholipid Bilayer: Forms the basic framework with hydrophilic heads facing outward and hydrophobic tails inward.
2. Proteins: Integral proteins span the membrane, while peripheral proteins attach superficially.
3. Cholesterol: Provides stability and fluidity.
4. Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) for cell recognition.


Functions:
1. Selective Permeability: Allows only specific molecules (like small, non-polar substances) to pass via diffusion or facilitated transport.
2. Cell Signaling: Receptor proteins detect external signals (e.g., hormones).
3. Transport: Proteins assist in active/passive transport (e.g., ion pumps).
4. Protection: Acts as a barrier against pathogens.


Selective Permeability Mechanism:
The membrane's lipid bilayer blocks large/polar molecules, while channel/carrier proteins selectively transport ions/glucose. Cholesterol maintains fluidity to prevent rigidity or excessive permeability.


Value-Add: The membrane's asymmetry (different composition in inner/outer layers) ensures specialized functions, like immune response (carbohydrate tags) and cytoskeleton attachment (peripheral proteins).

Question 7:
Describe the structure and role of mitochondria in a eukaryotic cell. How does its unique double-membrane organization support its function?
Answer:

Mitochondria are the powerhouses of eukaryotic cells, generating ATP through aerobic respiration. Their structure is optimized for energy production.


Structure:
1. Double membrane: Outer membrane is smooth, while the inner membrane is highly folded into cristae to increase surface area.
2. Matrix: Innermost space containing enzymes for the Krebs cycle, mitochondrial DNA, and ribosomes.


Functions:
1. ATP synthesis: Oxidative phosphorylation occurs on cristae via the electron transport chain.
2. Thermogenesis: Generates heat in brown fat tissue.
3. Apoptosis regulation: Releases cytochrome c to trigger programmed cell death.


Double-Membrane Advantage:
1. The intermembrane space accumulates protons to create a chemiosmotic gradient for ATP synthase.
2. Cristae maximize enzyme binding sites for ETC complexes.
3. The matrix isolates metabolic reactions (e.g., β-oxidation of fats) from cytoplasmic interference.

Question 8:
Describe the structure and function of mitochondria with a labeled diagram. How does its structure support its role as the powerhouse of the cell?
Answer:

The mitochondria is a double-membraned, semi-autonomous organelle found in eukaryotic cells. It is often called the powerhouse of the cell because it generates ATP (adenosine triphosphate), the energy currency of the cell, through cellular respiration.

Structure:
1. Outer membrane: Smooth and permeable to small molecules.
2. Inner membrane: Folded into cristae to increase surface area for ATP production.
3. Intermembrane space: Contains enzymes for ATP synthesis.
4. Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

Function:
The mitochondria performs aerobic respiration, which includes:
1. Glycolysis (in cytoplasm) breaks glucose into pyruvate.
2. Krebs cycle (in matrix) generates electron carriers.
3. Electron transport chain (on cristae) produces ATP via oxidative phosphorylation.

The cristae increase surface area for more ATP synthase enzymes, while the matrix houses enzymes for metabolic reactions. The presence of mitochondrial DNA and ribosomes suggests its prokaryotic origin, supporting the endosymbiotic theory.

Diagram: (A labeled diagram should show all structural parts mentioned above.)

Question 9:
Explain the structure and functions of mitochondria with a well-labelled diagram. How does its structure support its role as the powerhouse of the cell?
Answer:

Mitochondria are double-membraned, rod-shaped organelles found in eukaryotic cells, often referred to as the powerhouse of the cell due to their role in ATP production through aerobic respiration.

Structure:
1. Outer membrane: Smooth and permeable to small molecules.
2. Inner membrane: Folded into cristae, increasing surface area for ATP synthesis.
3. Intermembrane space: Contains enzymes for phosphorylation.
4. Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

Functions:

  • Produces ATP via oxidative phosphorylation.
  • Regulates cellular metabolism and apoptosis (programmed cell death).
  • Stores calcium ions for signaling.

Structural Adaptations:
The cristae increase surface area for electron transport chain proteins.
The matrix contains enzymes for metabolic reactions, ensuring efficient energy production.

Diagram: (A well-labelled diagram should show the outer membrane, inner membrane, cristae, matrix, and intermembrane space.)

Question 10:
Explain the structure and functions of the mitochondria with a labeled diagram. How does its structure support its role as the powerhouse of the cell?
Answer:

The mitochondria are double-membrane-bound organelles found in eukaryotic cells, often referred to as the powerhouse of the cell due to their role in ATP production. Here’s a detailed explanation of their structure and functions:

Structure:
1. Outer Membrane: Smooth and permeable to small molecules.
2. Inner Membrane: Folded into cristae to increase surface area for ATP synthesis.
3. Intermembrane Space: Contains enzymes for phosphorylation.
4. Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

Functions:
1. ATP Production: Through aerobic respiration (Krebs cycle and electron transport chain).
2. Calcium Storage: Regulates cellular calcium levels.
3. Heat Production: Generates heat in brown adipose tissue.
4. Apoptosis: Releases enzymes for programmed cell death.

Diagram: (Draw a labeled diagram showing outer membrane, inner membrane, cristae, matrix, and intermembrane space.)

How Structure Supports Function:
The cristae increase surface area for ATP synthase enzymes, while the matrix houses enzymes for the Krebs cycle, ensuring efficient energy production. The double membrane isolates reactive molecules, preventing cellular damage.

Question 11:
Explain the structure and functions of mitochondria with a well-labeled diagram. How does its structure support its role as the powerhouse of the cell?
Answer:

The mitochondria are double-membraned, rod-shaped organelles found in eukaryotic cells, often referred to as the powerhouse of the cell due to their role in ATP production. Their structure is highly specialized to maximize energy generation.

Structure:
1. Outer membrane: Smooth and porous, allowing the passage of small molecules.
2. Inner membrane: Folded into cristae, increasing surface area for ATP synthesis.
3. Intermembrane space: Contains enzymes for ATP synthesis.
4. Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

Functions:

  • ATP production via aerobic respiration (Krebs cycle and electron transport chain).
  • Regulates cellular metabolism and calcium ion storage.
  • Plays a role in apoptosis (programmed cell death).

Structural Adaptations:
The cristae increase surface area for ATP synthase enzymes, while the matrix houses enzymes for breaking down pyruvate. The presence of mitochondrial DNA allows semi-autonomous functioning.

Diagram: (A well-labeled diagram should show all structural components mentioned above.)

Value-added note: Mitochondria are inherited maternally in humans, and defects in their function can lead to metabolic disorders like Leigh syndrome.

Question 12:
Describe the structure and functions of the mitochondria with a labeled diagram. How does its structure aid in its function? (5 marks)
Answer:

The mitochondria are double-membraned, rod-shaped organelles known as the powerhouse of the cell due to their role in ATP production. Their structure consists of:

  • Outer membrane: Smooth and permeable to small molecules.
  • Inner membrane: Folded into cristae, increasing surface area for ATP synthesis.
  • Intermembrane space: Contains enzymes for phosphorylation.
  • Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

The cristae provide a large surface area for electron transport chain proteins, while the matrix houses enzymes for cellular respiration. Together, they efficiently generate ATP through oxidative phosphorylation.

Diagram (Labeled): A neat, well-labeled diagram showing all structural components must be included for full marks.

Value-added point: Mitochondria are semi-autonomous due to their own DNA, supporting the endosymbiotic theory.

Question 13:
Describe the structure and functions of mitochondria with a labeled diagram. How is it termed as the powerhouse of the cell?
Answer:

The mitochondria are double-membraned, rod-shaped organelles found in eukaryotic cells. They are often called the powerhouse of the cell because they generate most of the cell's supply of adenosine triphosphate (ATP), which is used as a source of chemical energy.

Structure:
1. Outer membrane: Smooth and porous, allowing the passage of small molecules.
2. Inner membrane: Folded into cristae to increase surface area for ATP production.
3. Intermembrane space: The region between the outer and inner membranes.
4. Matrix: The innermost compartment containing enzymes for the Krebs cycle, mitochondrial DNA, and ribosomes.

Functions:

  • ATP synthesis through oxidative phosphorylation.
  • Regulate cellular metabolism and apoptosis (programmed cell death).
  • Play a role in heat production and calcium storage.

The term powerhouse is justified because mitochondria convert nutrients into energy (ATP) via cellular respiration, fueling various cellular activities.

Diagram: (A labeled diagram should show the outer membrane, inner membrane, cristae, matrix, and intermembrane space.)

Question 14:
Explain the structure and functions of mitochondria with a labeled diagram. How does its structure support its role as the powerhouse of the cell?
Answer:

The mitochondria are double-membraned, rod-shaped organelles found in eukaryotic cells, often referred to as the powerhouse of the cell due to their role in ATP production. Their structure consists of:

  • Outer membrane: Smooth and permeable to small molecules.
  • Inner membrane: Folded into cristae, increasing surface area for ATP synthesis.
  • Intermembrane space: Contains enzymes for ATP synthesis.
  • Matrix: Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle.

The cristae provide a large surface area for electron transport chain proteins, while the matrix houses enzymes for cellular respiration. Together, these features enable mitochondria to efficiently generate ATP through oxidative phosphorylation.

Value-added point: Mitochondria are semi-autonomous as they contain their own DNA and ribosomes, suggesting an evolutionary origin from prokaryotic cells (endosymbiotic theory).

Question 15:
Describe the fluid mosaic model of the plasma membrane. How does this model explain the membrane's selective permeability and dynamic nature? Include a labeled diagram.
Answer:

The fluid mosaic model describes the plasma membrane as a dynamic, flexible structure composed of:

  • Phospholipid bilayer: Forms the basic framework with hydrophilic heads facing outward and hydrophobic tails inward.
  • Proteins: Embedded (integral) or surface-bound (peripheral) for transport, signaling, and adhesion.
  • Cholesterol: Provides stability and fluidity.
  • Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins) for cell recognition.

The model explains selective permeability as proteins regulate molecule passage, while the fluid nature allows membrane components to move laterally, enabling processes like endocytosis and cell signaling.

Application: This dynamic structure is crucial for nutrient uptake, waste removal, and cell communication, making it vital for homeostasis.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
A student observed plasmolysis in onion peel cells under a microscope. Explain the process and its significance in plant cells.
Answer:
Case Deconstruction

Plasmolysis occurs when a plant cell loses water due to osmosis in a hypertonic solution, causing the cell membrane to shrink away from the cell wall.

Theoretical Application
  • Helps study osmotic behavior of cells.
  • Demonstrates the role of turgor pressure in maintaining cell shape.
Critical Evaluation

Our textbook shows plasmolysis is reversible if cells are placed in a hypotonic solution. This confirms the selective permeability of the membrane.

Question 2:
Compare prokaryotic and eukaryotic ribosomes based on structure and function. Provide two examples of organisms for each.
Answer:
Case Deconstruction

Prokaryotic ribosomes (70S) are smaller, while eukaryotic ribosomes (80S) are larger and membrane-bound.

Theoretical Application
  • Prokaryotes: E. coli, Bacillus.
  • Eukaryotes: Amoeba, Human cells.
Critical Evaluation

We studied that ribosomes in both types perform protein synthesis, but antibiotics target prokaryotic ribosomes, showing evolutionary differences.

Question 3:
Analyze how the absence of lysosomes would affect cellular function. Support your answer with evidence.
Answer:
Case Deconstruction

Lysosomes contain digestive enzymes for breaking down waste. Their absence would disrupt autophagy and phagocytosis.

Theoretical Application
  • Accumulation of cellular debris.
  • Impaired defense against pathogens.
Critical Evaluation

Our textbook shows genetic disorders like Tay-Sachs disease occur due to dysfunctional lysosomes, proving their critical role.

Question 4:
A scientist claims mitochondria and chloroplasts evolved from prokaryotes. Justify this statement using the endosymbiotic theory.
Answer:
Case Deconstruction

The endosymbiotic theory suggests mitochondria and chloroplasts were free-living prokaryotes engulfed by ancestral eukaryotes.

Theoretical Application
  • Both have their own DNA and ribosomes.
  • Divide independently via binary fission.
Critical Evaluation

We studied their double membranes and circular DNA support this theory, similar to modern cyanobacteria and proteobacteria.

Question 5:
A student observed plasmolysis in onion peel cells under a microscope after adding concentrated salt solution. Explain the process and its significance in plant cells.
Answer:
Case Deconstruction

Plasmolysis occurs when a plant cell loses water due to hypertonic conditions, causing the cell membrane to detach from the cell wall. Our textbook shows this as a clear indicator of osmosis.

Theoretical Application
  • Helps study membrane permeability and water potential.
  • Used in agriculture to test drought resistance.
Critical Evaluation

Reversible plasmolysis confirms cell viability, while irreversible damage indicates cell death. Example: Wilting in plants due to excessive fertilizer.

Question 6:
Mitochondria are called the powerhouse of the cell. Justify this statement with two examples and explain how their structure supports this function.
Answer:
Case Deconstruction

Mitochondria generate ATP via aerobic respiration. Their cristae increase surface area for oxidative phosphorylation.

Theoretical Application
  • Example 1: Muscle cells have abundant mitochondria for energy.
  • Example 2: Liver cells need mitochondria for detoxification.
Critical Evaluation

Their double membrane isolates reactions, while matrix enzymes facilitate the Krebs cycle. Dysfunction leads to fatigue, as seen in mitochondrial diseases.

Question 7:
Compare prokaryotic and eukaryotic ribosomes based on structure and function. How does antibiotic targeting exploit this difference?
Answer:
Case Deconstruction

Prokaryotic ribosomes (70S) are smaller than eukaryotic (80S). Our textbook shows both have subunits but differ in rRNA composition.

Theoretical Application
  • Antibiotics like tetracycline bind 70S ribosomes, blocking bacterial protein synthesis.
  • Eukaryotic ribosomes remain unaffected, reducing side effects.
Critical Evaluation

This specificity is crucial for drug design. Example: Streptomycin disrupts bacterial infections without harming human cells.

Question 8:
Analyze the role of lysosomes as suicide bags with reference to autophagy and apoptosis. Provide one disease linked to lysosomal dysfunction.
Answer:
Case Deconstruction

Lysosomes contain hydrolytic enzymes that digest damaged organelles (autophagy) or entire cells (apoptosis).

Theoretical Application
  • Example: Tadpole tail resorption during metamorphosis.
  • Disease: Tay-Sachs results from undigested lipids due to enzyme deficiency.
Critical Evaluation

Controlled digestion maintains cellular health. Unregulated enzyme release causes necrosis, as seen in arthritis.

Question 9:
Compare prokaryotic and eukaryotic ribosomes based on structure and function. Provide two examples of each cell type.
Answer:
Case Deconstruction

Prokaryotic ribosomes (70S) are smaller and free-floating, while eukaryotic ribosomes (80S) are larger and can be membrane-bound. Our textbook shows E. coli as prokaryotic and human cells as eukaryotic.


Theoretical Application
  • Prokaryotic: Protein synthesis in bacteria (e.g., Streptococcus).
  • Eukaryotic: ER-bound ribosomes secrete proteins (e.g., pancreatic cells).

Critical Evaluation

The size difference affects antibiotic targeting, as drugs like tetracycline specifically inhibit 70S ribosomes without harming human cells.

Question 10:
Analyze how the endomembrane system collaborates in protein synthesis and transport, referencing the Golgi apparatus and rough ER.
Answer:
Case Deconstruction

The rough ER synthesizes proteins, which are then transported to the Golgi apparatus via vesicles for modification. We studied this using pancreatic enzyme secretion as an example.


Theoretical Application
  • Lysosomal enzyme production shows Golgi's role in packaging.
  • Antibody secretion in plasma cells demonstrates coordinated transport.

Critical Evaluation

Disruptions in this system, like in I-cell disease, prove its essential role. The precise vesicle targeting mechanisms remain an active research area.

Question 11:
Critically evaluate why mitochondria are called the 'powerhouse of the cell' with evidence from their structure and ATP production.
Answer:
Case Deconstruction

Mitochondria generate ATP through oxidative phosphorylation in their cristae. Our textbook shows their double membrane structure increases surface area for ATP synthase enzymes.


Theoretical Application
  • Muscle cells contain numerous mitochondria for energy demands.
  • Brown adipose tissue mitochondria produce heat via uncoupling.

Critical Evaluation

While vital for energy, mitochondria also regulate apoptosis. Recent studies link mitochondrial dysfunction to neurodegenerative diseases like Parkinson's.

Question 12:
A student observed plasmolysis in onion peel cells under a microscope after adding a concentrated salt solution. Explain the osmotic behavior of cells and predict what happens if the cells are placed in distilled water.
Answer:
Case Deconstruction

Plasmolysis occurs when water exits the cell due to hypertonic external solution, shrinking the protoplasm away from the cell wall.

Theoretical Application
  • In distilled water, the cell becomes hypotonic, causing endosmosis and restoring turgidity.
  • Example: Red blood cells lyse in pure water due to excessive inflow.
Critical Evaluation

Our textbook shows plant cells resist bursting due to rigid walls, unlike animal cells. This demonstrates selective permeability of plasma membranes.

Question 13:
Mitochondria isolated from muscle cells show higher cristae density than those from skin cells. Analyze this observation linking structure to function.
Answer:
Case Deconstruction

Cristae increase surface area for ATP production via oxidative phosphorylation.

Theoretical Application
  • Muscle cells demand more energy for contraction, requiring enhanced cristae.
  • Example: Liver mitochondria also have dense cristae for detoxification.
Critical Evaluation

We studied that aerobic respiration efficiency correlates with cristae morphology, proving evolutionary adaptation in high-energy tissues.

Question 14:
When lysosomal enzymes leak into cytoplasm, cells undergo autolysis. Discuss the compartmentalization advantage in eukaryotic cells with two examples.
Answer:
Case Deconstruction

Lysosomes contain hydrolytic enzymes that digest cellular waste at pH 5.

Theoretical Application
  • Compartmentalization prevents enzyme mixing, e.g., ribosomes in cytoplasm vs. DNA in nucleus.
  • Example: Peroxisomes isolate toxic H2O2 degradation.
Critical Evaluation

Our textbook shows membrane-bound organelles optimize metabolic pathways, as seen in endomembrane system coordination.

Question 15:
Researchers found gap junctions absent in plant cells but present in animal tissues like heart muscles. Compare intercellular communication mechanisms in both cell types.
Answer:
Case Deconstruction

Gap junctions allow direct cytoplasmic connection in animals via connexin proteins.

Theoretical Application
  • Plants use plasmodesmata for symplastic transport through cell walls.
  • Example: Cardiac muscle synchronizes contractions via gap junctions.
Critical Evaluation

We studied that structural differences reflect functional needs: rapid signaling in animals vs. solute sharing in plants.

Question 16:
A student observed a cell under a microscope and noticed that it lacked a well-defined nucleus and membrane-bound organelles. However, the cell showed the presence of ribosomes and a single circular DNA molecule.

Based on the observation, answer the following:

  • Identify the type of cell observed.
  • Explain any two features of such cells that distinguish them from other cell types.
Answer:

The observed cell is a prokaryotic cell, as it lacks a well-defined nucleus and membrane-bound organelles but contains ribosomes and a single circular DNA molecule.

Two distinguishing features of prokaryotic cells are:

  • Absence of a true nucleus: Instead of a membrane-bound nucleus, prokaryotic cells have a nucleoid region where the genetic material (a single circular DNA molecule) is located.
  • Lack of membrane-bound organelles: Prokaryotic cells do not contain organelles like mitochondria, Golgi apparatus, or endoplasmic reticulum. However, they possess ribosomes for protein synthesis.

Additionally, prokaryotic cells often have a cell wall made of peptidoglycan, which provides structural support and protection.

Question 17:
A group of students conducted an experiment to study the effect of hypertonic solution on onion peel cells. They observed that the cells shrank and the cytoplasm detached from the cell wall.

Based on the experiment, answer the following:

  • Name the process responsible for the observed changes.
  • Explain why the cytoplasm detached from the cell wall in this scenario.
Answer:

The process responsible for the observed changes is plasmolysis.

Explanation for cytoplasm detachment:
When onion peel cells are placed in a hypertonic solution, the water concentration outside the cell is lower than inside. Due to osmosis, water moves out of the cell, causing the cytoplasm to shrink.
The cell wall is rigid and does not contract, while the cell membrane (along with the cytoplasm) pulls away from the wall, leading to detachment. This phenomenon is called plasmolysis.

Plasmolysis is reversible if the cell is placed back in a hypotonic or isotonic solution, demonstrating the dynamic nature of osmotic balance in plant cells.

Question 18:
A student observed a cell under a microscope and noticed that it lacked a well-defined nucleus and membrane-bound organelles. However, the cell had a cell wall and ribosomes.

a) Identify the type of cell observed.

b) Justify your answer with two reasons.

c) Name one organism that possesses such cells.

Answer:

a) The cell observed is a prokaryotic cell.

b) Justification:

  • Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles, which matches the observation.
  • The presence of a cell wall and ribosomes is characteristic of prokaryotic cells, as they have a simple structure.

c) An example of an organism with prokaryotic cells is Escherichia coli (E. coli), a common bacterium.

Question 19:
A group of students conducted an experiment to study the effect of hypertonic solution on onion peel cells. They observed the cells under a microscope before and after adding the solution.

a) What change would they observe in the cells after adding the hypertonic solution?

b) Explain the phenomenon responsible for this change.

c) How would the observation differ if a hypotonic solution was used instead?

Answer:

a) After adding the hypertonic solution, the students would observe shrinkage or plasmolysis of the onion peel cells, where the cell membrane detaches from the cell wall.

b) The phenomenon responsible is osmosis. In a hypertonic solution, water moves out of the cell due to the higher solute concentration outside, causing the cell to lose water and shrink.

c) If a hypotonic solution was used, the cells would swell or become turgid due to water entering the cell. In extreme cases, the cell might burst (lysis) if it lacks a cell wall, but onion cells have a rigid cell wall that prevents bursting.

Question 20:
A student observed a cell under a microscope and noticed that it lacked a well-defined nucleus and membrane-bound organelles. However, the cell had a cell wall and ribosomes.

(i) Identify the type of cell observed.
(ii) Justify your answer by explaining two key features of this cell type.
(iii) How does the absence of a nucleus affect the functioning of such cells?

Answer:

(i) The cell observed is a prokaryotic cell, likely a bacterial cell.

(ii) Two key features justifying this identification are:

  • Lack of a well-defined nucleus: Prokaryotic cells have genetic material (DNA) scattered in the cytoplasm without a nuclear membrane.
  • Absence of membrane-bound organelles: They only contain non-membrane-bound structures like ribosomes.

(iii) The absence of a nucleus means the DNA is freely floating in the cytoplasm, allowing faster transcription and translation. However, it also limits the complexity of genetic regulation compared to eukaryotic cells.

Question 21:
A plant cell was placed in a hypertonic solution, and its cytoplasm shrank away from the cell wall.

(i) Name the phenomenon observed.
(ii) Explain why the cytoplasm shrank.
(iii) What would happen if the same cell is placed in a hypotonic solution?

Answer:

(i) The phenomenon observed is plasmolysis.

(ii) The cytoplasm shrank because:

  • The hypertonic solution had a higher solute concentration than the cell's cytoplasm.
  • Water moved out of the cell due to osmosis, causing the cytoplasm to detach from the cell wall.

(iii) If placed in a hypotonic solution, water would enter the cell by osmosis, causing the cell to become turgid. The cell wall prevents bursting, maintaining structural support.

Question 22:
A student observed a cell under a microscope and noticed that it lacked a well-defined nucleus and membrane-bound organelles. However, it had a cell wall and ribosomes.

(a) Identify the type of cell observed.
(b) Explain two features that distinguish this cell from a eukaryotic cell.
Answer:

(a) The cell observed is a prokaryotic cell, as it lacks a well-defined nucleus and membrane-bound organelles.

(b) Two distinguishing features are:

  • Nuclear membrane: Prokaryotic cells lack a true nucleus with a nuclear membrane, while eukaryotic cells have a well-defined nucleus enclosed by a nuclear envelope.
  • Membrane-bound organelles: Prokaryotic cells do not possess organelles like mitochondria, Golgi apparatus, or endoplasmic reticulum, which are present in eukaryotic cells.

Additionally, prokaryotic cells have a simpler structure with a single circular DNA molecule, while eukaryotic cells have linear DNA organized into chromosomes.

Question 23:
A plant cell was placed in a hypertonic solution, and its shape changed significantly.

(a) What is this phenomenon called?
(b) Describe the changes observed in the cell and explain why they occurred.
Answer:

(a) This phenomenon is called plasmolysis.

(b) The changes observed include:

  • The cell membrane shrinks away from the cell wall due to water loss.
  • The cytoplasm contracts, leading to a shrunken appearance.

This occurs because the hypertonic solution has a higher solute concentration than the cell's cytoplasm. Water moves out of the cell by osmosis, causing the cell to lose turgor pressure and shrink. Plasmolysis is reversible if the cell is placed back in a hypotonic or isotonic solution.

Question 24:
A student observed a cell under a microscope and noticed that it lacked a cell wall and chloroplasts but contained a nucleus and mitochondria. Based on these observations, answer the following:
a) Identify the type of cell and justify your answer.
b) Explain the role of mitochondria in this cell.
Answer:

a) The observed cell is an animal cell. This is because:

  • Animal cells lack a cell wall, which is present in plant cells.
  • They also do not contain chloroplasts, which are responsible for photosynthesis in plant cells.
  • The presence of a nucleus and mitochondria confirms it is a eukaryotic cell, typical of animal cells.

b) Mitochondria are the powerhouses of the cell. Their roles include:

  • Producing ATP (adenosine triphosphate) through cellular respiration, which provides energy for cellular activities.
  • They contain their own DNA and ribosomes, enabling them to replicate independently.
  • Mitochondria also play a role in apoptosis (programmed cell death) and regulate metabolic pathways.

Question 25:
A plant cell was placed in a hypertonic solution, and its plasma membrane shrank away from the cell wall. Analyze this observation and answer:
a) What is this phenomenon called?
b) How does the cell wall protect the cell in such conditions?
Answer:

a) This phenomenon is called plasmolysis. It occurs when a plant cell loses water due to osmosis in a hypertonic solution, causing the plasma membrane to detach from the cell wall.

b) The cell wall provides structural support and protection:

  • It is rigid and prevents the cell from bursting or collapsing completely under osmotic stress.
  • The cell wall maintains the cell's shape even when the protoplasm shrinks during plasmolysis.
  • It also allows the cell to regain its original structure when placed back in an isotonic or hypotonic solution, a process called deplasmolysis.

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