11bio8

Cell: Structure and Functions

Explore the fundamental unit of life - the cell, its structure, functions, and the remarkable diversity of cellular organization in living organisms.

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Chapter Overview

Brief Introduction

The cell is the fundamental structural and functional unit of all living organisms. This chapter explores the discovery of cells, cell theory, and the structural organization of prokaryotic and eukaryotic cells. You'll learn about various cell organelles, their structure and functions, and how cells differ in size, shape, and activities.

Learning Objectives

• Understand the cell theory and its historical development
• Differentiate between prokaryotic and eukaryotic cells
• Learn about the structure and function of various cell organelles
• Comprehend the organization of the endomembrane system
• Explore the diversity in cell shapes and sizes
• Understand the structure and function of nucleus and chromosomes

Key Topics Covered

• Cell Theory and its modifications
• Prokaryotic vs Eukaryotic cells
• Cell membrane structure (Fluid Mosaic Model)
• Cell wall and its modifications
• Endomembrane system (ER, Golgi, Lysosomes, Vacuoles)
• Mitochondria and Plastids
• Ribosomes and Cytoskeleton
• Cilia, Flagella and Centrosome
• Nucleus and Chromosomes

Interactive Chapter Index

Cell Theory

Explore the fundamental principles that established cells as the basic unit of life.

Prokaryotic Cells

Learn about the simple yet efficient organization of bacteria and other prokaryotes.

Eukaryotic Cells

Discover the complex compartmentalization of plant and animal cells.

Cell Membrane

Understand the selectively permeable boundary of cells and its fluid mosaic structure.

Cell Organelles

Explore the specialized structures that perform various cellular functions.

Nucleus & Chromosomes

Learn about the control center of the cell and the organization of genetic material.

Full Chapter Notes

8.2 Cell Theory

The cell theory, formulated by Matthias Schleiden and Theodor Schwann in 1838-39, is one of the fundamental principles of biology. It was later modified by Rudolf Virchow in 1855.

Key Points of Cell Theory:

1. All living organisms are composed of cells and products of cells.
2. All cells arise from pre-existing cells (Omnis cellula-e cellula).
3. The cell is the fundamental structural and functional unit of life.

Historical Discoveries:

• Anton Von Leeuwenhoek first saw and described a live cell.
• Robert Brown discovered the nucleus.
• Schleiden (1838) examined plants and concluded all plants are composed of cells.
• Schwann (1839) studied animal cells and reported the presence of plasma membrane.
• Virchow (1855) explained that cells divide and new cells form from pre-existing cells.

Q: What are the main differences between Schleiden and Schwann's original cell theory and Virchow's modified version?

A: The original cell theory by Schleiden and Schwann stated that:

1. All organisms are composed of cells and products of cells
2. Cells are the basic unit of life

Virchow's modification added the crucial third point:

3. All cells arise from pre-existing cells (Omnis cellula-e cellula)

This corrected the earlier belief that cells could arise spontaneously from non-living matter.

Cell Theory - Mind Map

8.4 Prokaryotic Cells

Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller (1-10 μm) and simpler than eukaryotic cells.

Characteristics of Prokaryotic Cells:

• No well-defined nucleus (genetic material is naked)
• No membrane-bound organelles
• Cell wall present (except in mycoplasma)
• Mesosomes (specialized folds of plasma membrane)
• Ribosomes are 70S type
• May have plasmids (small circular DNA)
• May have flagella, pili or fimbriae

Cell Envelope: In bacteria, consists of three layers:

1. Outermost glycocalyx (slime layer or capsule)
2. Cell wall (determines shape and prevents bursting)
3. Plasma membrane (selectively permeable)

Bacteria can be Gram positive or Gram negative based on cell envelope differences.

Figure: Structure of a typical prokaryotic cell

Special Structures:

• Mesosome: Plasma membrane infolding that helps in cell wall formation, DNA replication, respiration
• Flagella: For motility; composed of filament, hook and basal body
• Pili: Elongated tubular structures for attachment
• Fimbriae: Small bristle-like fibers for attachment
• Inclusion bodies: Storage granules (phosphate, glycogen, etc.)

Q: What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

A: Mesosome is a specialized differentiated form of cell membrane in prokaryotes that appears as infoldings of the plasma membrane in the form of vesicles, tubules and lamellae.

Functions of mesosome:

1. Helps in cell wall formation
2. Participates in DNA replication and distribution to daughter cells
3. Assists in respiration and secretion processes
4. Increases surface area of the plasma membrane
5. Increases enzymatic content in the cell

8.5 Eukaryotic Cells

Eukaryotic cells include all protists, plants, animals and fungi. They are characterized by extensive compartmentalization through membrane-bound organelles and a well-defined nucleus.

Key Features:

• True nucleus with nuclear envelope
• Membrane-bound organelles (mitochondria, ER, Golgi, etc.)
• Complex cytoskeleton
• Larger (10-100 μm) than prokaryotic cells
• Linear DNA organized into chromosomes

Plant vs Animal Cells:

• Plant cells have: Cell wall, plastids, large central vacuole, plasmodesmata
• Animal cells have: Centrioles, lysosomes (usually), smaller vacuoles
• Both have: Nucleus, mitochondria, ER, Golgi, plasma membrane, ribosomes, cytoskeleton

Figure: Comparison of plant and animal eukaryotic cells

Q: What are the characteristics of prokaryotic cells?

A: Characteristics of prokaryotic cells:

1. Generally small in size (1-10 μm)
2. No well-defined nucleus (genetic material is naked)
3. No membrane-bound organelles
4. Cell wall present (except in mycoplasma)
5. Mesosomes present (specialized folds of plasma membrane)
6. Ribosomes are 70S type
7. May have plasmids (small circular DNA)
8. May have flagella, pili or fimbriae
9. Simple cytoskeleton (if present)
10. Cell division by binary fission

8.5.1 Cell Membrane

The cell membrane (plasma membrane) is a selectively permeable barrier that separates the cell from its external environment. Its structure was described by Singer and Nicolson (1972) as the Fluid Mosaic Model.

Fluid Mosaic Model Features:

• Composed mainly of phospholipids arranged in a bilayer
• Contains proteins (integral and peripheral), cholesterol and carbohydrates
• Lipids have polar heads (hydrophilic) facing outward and nonpolar tails (hydrophobic) inward
• Proteins float in the lipid bilayer (mosaic pattern)
• Membrane is fluid - lipids and proteins can move laterally

Figure: Fluid mosaic model of plasma membrane

Membrane Transport:

• Passive transport: No energy required (diffusion, osmosis)
• Active transport: Energy (ATP) required to move against concentration gradient (e.g., Na+/K+ pump)
• Polar molecules need carrier proteins to cross membrane

Q: How do neutral solutes move across the plasma membrane? Can polar molecules also move across it in the same way?

A: Neutral solutes move across the plasma membrane by simple diffusion along the concentration gradient, i.e., from higher concentration to lower concentration without any requirement of energy.

Polar molecules cannot move across the membrane in the same way because:

1. The nonpolar lipid bilayer is hydrophobic and acts as a barrier to polar molecules
2. Polar molecules require carrier proteins of the membrane to facilitate their transport
3. Some polar molecules may be transported against their concentration gradient through active transport (requiring ATP)

Cell Organelles

8.5.2 Cell Wall

A non-living rigid structure that forms an outer covering for plant cells and fungi. Provides shape, protection, and prevents bursting.

• Plant cell wall: Cellulose, hemicellulose, pectins, proteins
• Algal cell wall: Cellulose, galactans, mannans, minerals like calcium carbonate
• Middle lamella: Calcium pectate layer that glues neighboring cells together
• Plasmodesmata: Channels through cell walls connecting cytoplasm of adjacent cells

8.5.3 Endomembrane System

Includes ER, Golgi complex, lysosomes and vacuoles. Their functions are coordinated.

Endoplasmic Reticulum (ER)

• Network of membrane-bound tubules and cisternae
• Rough ER: Has ribosomes; protein synthesis and secretion
• Smooth ER: No ribosomes; lipid synthesis, detoxification

Golgi Apparatus

• Stack of flattened membrane sacs (cisternae)
• Packaging and modification of proteins and lipids
• Forms glycoproteins and glycolipids
• Has cis (forming) and trans (maturing) faces

Lysosomes

• Membrane-bound vesicles with digestive enzymes
• Break down macromolecules (proteins, lipids, carbs, nucleic acids)
• Function at acidic pH

Vacuoles

• Membrane-bound space (tonoplast) containing water, sap, wastes
• In plants: Large central vacuole (up to 90% cell volume)
• In Amoeba: Contractile vacuole for osmoregulation
• Food vacuoles in protists

8.5.4 Mitochondria

• "Powerhouse of the cell" - site of aerobic respiration and ATP production
• Double membrane-bound: outer smooth, inner folded into cristae
• Matrix contains 70S ribosomes, circular DNA, enzymes
• Divide by fission

Figure: Structure of mitochondrion

8.5.5 Plastids

• Found in plants and euglenoids
• Types:
  • Chloroplasts: Contain chlorophyll for photosynthesis
  • Chromoplasts: Contain carotenoids (yellow, orange, red pigments)
  • Leucoplasts: Colorless; store nutrients (amyloplasts-starch, elaioplasts-fats, aleuroplasts-proteins)
• Double membrane-bound; inner membrane forms thylakoids (stacked as grana)
• Stroma contains 70S ribosomes, circular DNA, enzymes for carbohydrate synthesis

Figure: Structure of chloroplast

8.5.6 Ribosomes

• Granular structures made of RNA and proteins (no membrane)
• Site of protein synthesis
• Eukaryotic ribosomes: 80S (60S + 40S subunits)
• Prokaryotic ribosomes: 70S (50S + 30S subunits)
• Found free in cytoplasm or attached to ER

8.5.7 Cytoskeleton

• Network of protein filaments (microtubules, microfilaments, intermediate filaments)
• Functions: Mechanical support, motility, cell shape maintenance

8.5.8 Cilia and Flagella

• Hair-like projections for movement (flagella longer than cilia)
• Core called axoneme with 9+2 arrangement of microtubules
• Emerges from basal body (similar to centriole)

8.5.9 Centrosome and Centrioles

• Centrosome contains two centrioles (animal cells)
• Centrioles have 9 triplet microtubules arranged radially
• Form basal bodies of cilia/flagella and spindle fibers during cell division

Q: Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions.

A: Two double membrane-bound organelles are:

1. Mitochondria:

• Characteristics:
  • Sausage-shaped or cylindrical
  • Double membrane-bound (outer and inner membrane)
  • Inner membrane forms folds called cristae
  • Matrix contains 70S ribosomes, circular DNA, and enzymes
• Functions:
  • Site of aerobic respiration
  • Produces cellular energy in the form of ATP ("powerhouse of the cell")
  • Contains enzymes for Krebs cycle and electron transport chain

2. Chloroplast (a type of plastid):

• Characteristics:
  • Lens-shaped, oval or discoid
  • Double membrane-bound
  • Inner membrane forms flattened sacs called thylakoids (stacked as grana)
  • Stroma contains 70S ribosomes, circular DNA, and enzymes
  • Contains chlorophyll and carotenoid pigments
• Functions:
  • Site of photosynthesis
  • Traps light energy and converts it to chemical energy
  • Synthesizes carbohydrates

8.5.10 Nucleus

The nucleus is the control center of the cell, first described by Robert Brown (1831). It contains the cell's genetic material.

Structure of Nucleus:

• Nuclear envelope: Double membrane with nuclear pores
• Chromatin: DNA-protein fibers (condense to form chromosomes during division)
• Nucleolus: Spherical structure for rRNA synthesis
• Nuclear matrix: Proteinaceous network in nucleoplasm

Figure: Structure of nucleus

8.5.11 Chromosomes

• Visible during cell division (condensed chromatin)
• Composed of DNA, histones, non-histone proteins and RNA
• Have primary constriction (centromere) with kinetochores
• Types based on centromere position:
  • Metacentric: Middle centromere
  • Sub-metacentric: Slightly off-center
  • Acrocentric: Near end
  • Telocentric: Terminal centromere
• Some have secondary constrictions forming satellite

Figure: Structure of chromosome showing centromere and kinetochore

Q: What is a centromere? How does the position of centromere form the basis of classification of chromosomes?

A: Centromere is the primary constriction on the chromosome where the chromatids are held together. It is the site of attachment of spindle fibers during cell division through kinetochores.

Classification of chromosomes based on centromere position:

1. Metacentric: Centromere is at the middle, forming two equal arms of the chromosome.
2. Sub-metacentric: Centromere is slightly away from the middle, resulting in one shorter arm and one longer arm.
3. Acrocentric: Centromere is situated close to its end, forming one extremely short arm and one very long arm.
4. Telocentric: Centromere is at the terminal end of the chromosome (not found in humans).

This classification helps in identifying chromosomes and studying their behavior during cell division.

Chapter Summary

• Cell is the fundamental structural and functional unit of life
• Cell theory states all organisms are made of cells and cells arise from pre-existing cells
• Prokaryotic cells lack membrane-bound organelles and nucleus
• Eukaryotic cells have true nucleus and membrane-bound organelles
• Plasma membrane is selectively permeable with fluid mosaic structure
• Endomembrane system includes ER, Golgi, lysosomes and vacuoles
• Mitochondria are sites of aerobic respiration (ATP production)
• Plastids (in plants) include chloroplasts (photosynthesis), chromoplasts and leucoplasts
• Nucleus contains chromatin (DNA+proteins) which condenses to form chromosomes during division

NCERT Solutions

Question 1: Which of the following is not correct?

(a) Robert Brown discovered the cell.

Correct Answer: (a) Robert Brown discovered the cell.

Explanation: Robert Brown discovered the nucleus in 1831. The cell was discovered by Robert Hooke in 1665 when he observed cork cells under a microscope. The other statements are correct:

• (b) Schleiden and Schwann formulated the cell theory in 1838-39
• (c) Virchow explained that cells are formed from pre-existing cells in 1855
• (d) A unicellular organism carries out its life activities within a single cell

Question 2: New cells generate from

(a) bacterial fermentation (b) regeneration of old cells (c) pre-existing cells (d) abiotic materials

Correct Answer: (c) pre-existing cells

Explanation: According to Virchow's modification of cell theory (Omnis cellula-e cellula), new cells are formed from pre-existing cells. This disproved the earlier theory of spontaneous generation which suggested cells could arise from non-living matter.

Question 3: Match the following

Column I      Column II

(a) Cristae      (i) Flat membranous sacs in stroma

(b) Cisternae      (ii) Infoldings in mitochondria

(c) Thylakoids      (iii) Disc-shaped sacs in Golgi apparatus

Correct Matching:

• (a) Cristae - (ii) Infoldings in mitochondria
• (b) Cisternae - (iii) Disc-shaped sacs in Golgi apparatus
• (c) Thylakoids - (i) Flat membranous sacs in stroma

Question 4: Which of the following is correct:

(a) Cells of all living organisms have a nucleus.

(b) Both animal and plant cells have a well defined cell wall.

(c) In prokaryotes, there are no membrane bound organelles.

(d) Cells are formed de novo from abiotic materials.

Correct Answer: (c) In prokaryotes, there are no membrane bound organelles.

Explanation: The other statements are incorrect because:

• (a) Prokaryotic cells don't have a well-defined nucleus
• (b) Animal cells lack cell walls (only plant cells have them)
• (d) Cells arise from pre-existing cells, not from abiotic materials

Question 11: What are nuclear pores? State their function.

Nuclear pores are openings in the nuclear envelope formed by the fusion of its two membranes. They are present at intervals throughout the nuclear envelope.

Functions of nuclear pores:

1. Allow selective movement of molecules between nucleus and cytoplasm
2. Facilitate transport of RNA and proteins in both directions
3. Enable communication between nucleus and cytoplasm
4. Regulate the passage of macromolecules like mRNA and ribosomal subunits

Question 12: Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

Similarity: Both lysosomes and vacuoles are membrane-bound organelles that are part of the endomembrane system.

Differences in function:

Lysosomes	Vacuoles
Contain digestive enzymes (hydrolases)	Contain water, sap, excretory products, etc.
Break down macromolecules (intracellular digestion)	Store materials (nutrients, wastes, pigments)
Destroy old organelles (autophagy)	Maintain turgor pressure in plant cells
Destroy pathogens (phagocytosis)	In protists: contractile vacuoles for osmoregulation
Cause cell death when ruptured (autolysis)	Food vacuoles in protists for digestion

Question 13: Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome

(i) Nucleus:

• Double membrane-bound structure (nuclear envelope) with nuclear pores
• Outer membrane continuous with endoplasmic reticulum
• Contains nucleoplasm (nuclear matrix)
• Chromatin material (DNA + proteins) present in nucleoplasm
• One or more spherical nucleoli (site of rRNA synthesis)

Diagram: (Refer to nucleus diagram in chapter content)

(ii) Centrosome:

• Found in animal cells near the nucleus
• Contains two cylindrical centrioles arranged perpendicular to each other
• Each centriole has 9 triplet microtubules arranged radially (9+0 pattern)
• Surrounded by amorphous pericentriolar material
• Forms basal bodies of cilia/flagella
• Organizes spindle fibers during cell division

Diagram: (Refer to centrosome structure in chapter content)

Practice Questions

1. Which of the following is not a part of the endomembrane system?

a) Endoplasmic reticulum b) Golgi apparatus c) Mitochondria d) Lysosomes

Correct Answer: c) Mitochondria

Explanation: The endomembrane system includes organelles whose functions are coordinated: endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. Mitochondria are not part of this system as their functions are not coordinated with these organelles.

2. The fluid mosaic model of plasma membrane was proposed by:

a) Schleiden and Schwann b) Robert Brown c) Singer and Nicolson d) Camillo Golgi

Correct Answer: c) Singer and Nicolson

Explanation: Singer and Nicolson proposed the fluid mosaic model in 1972. Schleiden and Schwann formulated the cell theory, Robert Brown discovered the nucleus, and Camillo Golgi discovered the Golgi apparatus.

3. Which of the following is not a function of the Golgi apparatus?

a) Packaging of materials b) Formation of glycoproteins c) Synthesis of lipids d) Modification of proteins

Correct Answer: c) Synthesis of lipids

Explanation: Lipid synthesis occurs in the smooth endoplasmic reticulum. The Golgi apparatus performs packaging of materials for secretion, formation of glycoproteins and glycolipids, and modification of proteins synthesized in the ER.

1. Differentiate between rough ER and smooth ER.

Differences between Rough ER and Smooth ER:

Rough ER	Smooth ER
Has ribosomes attached to its surface	No ribosomes on surface
Mainly involved in protein synthesis and secretion	Mainly involved in lipid synthesis
Abundant in cells actively synthesizing proteins (e.g., pancreatic cells)	Abundant in cells synthesizing lipids (e.g., adipocytes, liver cells)
Continuous with nuclear membrane	Not necessarily continuous with nuclear membrane

2. What are the functions of lysosomes?

Functions of lysosomes:

1. Intracellular digestion: Break down macromolecules (proteins, lipids, carbohydrates, nucleic acids) using hydrolytic enzymes
2. Autophagy: Digest old or non-functional organelles
3. Phagocytosis: Digest foreign particles or pathogens that enter the cell
4. Autolysis: Cause cell death when lysosomal enzymes are released (e.g., during metamorphosis)
5. Bone remodeling: Osteoclasts use lysosomal enzymes to break down bone tissue
6. Sperm penetration: Acrosome (modified lysosome) helps sperm penetrate egg

1. Describe the structure and functions of mitochondria with a labeled diagram.

Structure of Mitochondria:

1. Double membrane-bound: Outer membrane is smooth while inner membrane is folded into cristae
2. Two compartments:
   • Outer compartment (perimitochondrial space)
   • Inner compartment (matrix)
3. Matrix contains:
   • 70S ribosomes
   • Single circular DNA molecule
   • RNA molecules
   • Enzymes for Krebs cycle
4. Cristae: Increase surface area for oxidative phosphorylation

Functions of Mitochondria:

1. Powerhouse of the cell: Site of aerobic respiration and ATP production
2. Oxidative phosphorylation: Electron transport chain on cristae produces ATP
3. Krebs cycle: Occurs in matrix
4. Thermogenesis: Produce heat in brown adipose tissue
5. Calcium storage: Regulate calcium ion concentration in cytosol
6. Apoptosis: Release cytochrome c to initiate programmed cell death

Diagram: (Refer to mitochondria diagram in chapter content)

2. Explain the structure of chloroplast with a labeled diagram. How is it different from mitochondria?

Structure of Chloroplast:

1. Double membrane-bound: Outer and inner membrane (less permeable)
2. Stroma: Fluid-filled space containing:
   • 70S ribosomes
   • Circular DNA
   • Enzymes for dark reactions of photosynthesis
3. Thylakoids: Flattened membranous sacs containing chlorophyll
4. Grana: Stacks of thylakoids (site of light reactions)
5. Stroma lamellae: Tubular connections between grana

Differences between Chloroplast and Mitochondria:

Chloroplast	Mitochondria
Found only in plant cells and photosynthetic protists	Found in all eukaryotic cells
Contains chlorophyll and other photosynthetic pigments	Lacks photosynthetic pigments
Inner membrane forms thylakoids and grana	Inner membrane forms cristae
Site of photosynthesis (light and dark reactions)	Site of aerobic respiration and ATP production
Produces carbohydrates and oxygen	Produces ATP, CO2 and water

Diagram: (Refer to chloroplast diagram in chapter content)

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What is the cell theory?

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The cell theory states:

1. All living organisms are composed of cells and products of cells
2. All cells arise from pre-existing cells (Omnis cellula-e cellula)
3. The cell is the fundamental structural and functional unit of life

Formulated by Schleiden & Schwann (1838-39), modified by Virchow (1855)

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