Cell: The Unit of Life

Cell

Living vs. Non-living Things

• Living things have cells.
• Non-living things do not have cells.

“Viruses are an exception as they show life-like features only inside a host cell.”

What is a Cell?

• All organisms are made of cells.
  • Unicellular Organisms: One cell, can live independently.
  • Multicellular Organisms: Many cells.
• Cells are the basic structural and functional units of life.
• “A cell is the smallest unit capable of carrying out all vital life processes.”
• Important Discoveries:
  • Anton Von Leeuwenhoek: First saw live cells.
    • “He observed bacteria, protozoa and blood cells using a simple microscope.”
  • Robert Brown: Discovered the nucleus.
  • Microscopes: Helped reveal cell structures.
    • “Improvement in microscopes made it possible to study internal cell structures in detail.”

Cell Theory

• Matthias Schleiden (1838): Found that all plants are made of cells.
• Theodore Schwann (1839): Found that all animals are made of cells; identified the plasma membrane and cell wall.
• Schleiden and Schwann: Formulated the initial cell theory.
• Rudolf Virchow (1855): Added that cells come from pre-existing cells (“Omnis cellula-e cellula”).
• Current Cell Theory:
  • All living organisms are made of cells and their products.
  • All cells come from pre-existing cells.

“Viruses are not included in cell theory because they lack cellular organisation.”

Overview of Cells

Types of Cells

• Plant Cells (Onion Cell)
  • Outer boundary: Cell wall
  • Just inside: Cell membrane
• Animal Cells (Human Cheek Cell)
  • Outer boundary: Cell membrane

Inside the Cell

• Nucleus
  • Membrane-bound structure
  • Contains chromosomes and DNA
  • Eukaryotic cells have a membrane-bound nucleus
  • Prokaryotic cells do not have a membrane-bound nucleus

Cytoplasm

• Semi-fluid matrix inside the cell
• Main area for cellular activities (metabolic reactions).
• Present in both plant and animal cells

Organelles in Eukaryotic Cells

• Membrane-bound Organelles
  • Endoplasmic reticulum (ER)
  • Golgi complex
  • Lysosomes
  • Mitochondria
  • Microbodies
  • Vacuoles

“Prokaryotic cells lack membrane-bound organelles.”

Organelles in All Cells

• Ribosomes
  • Non-membrane bound
  • Found in cytoplasm, chloroplasts (plants), mitochondria, and rough ER
  • “Ribosomes are responsible for protein synthesis.”
• Centrosome (Animal Cells)
  • Non-membrane bound
  • Helps in cell division

Cell Size and Shape

• Size Examples
  • Mycoplasmas: Smallest cells, 0.3 µm
  • Bacteria: 3 to 5 µm
  • Ostrich egg: Largest single cell
  • Human red blood cells: 7.0 µm
  • “Nerve cells are among the longest cells in the human body.”
• Shape Examples
  • Disc-like, polygonal, columnar, cuboid, thread-like, irregular
  • Shape varies with function
  • “Cell shape is closely related to the role the cell performs.”

Prokaryotic Cells

What are Prokaryotic Cells?

• Found in bacteria, blue-green algae, mycoplasma, and PPLO.
• Smaller and multiply faster than eukaryotic cells.
• “They show great diversity in size and structural organisation.”
• Common shapes:
  • Bacillus (rod-like)
  • Coccus (spherical)
  • Vibrio (comma-shaped)
  • Spirillum (spiral)

Cell Structure

• Cell Wall: Surrounds the cell membrane (except in mycoplasma).
  • “It gives definite shape and protects the cell from bursting.”
• Cytoplasm: Fluid matrix inside the cell.
• Nucleus: No well-defined nucleus, genetic material is not enclosed by a membrane.
  • “DNA is present as a single circular chromosome lying freely in the cytoplasm.”
• Plasmids: Small circular DNA outside the genomic DNA, can provide antibiotic resistance.
  • “Plasmids carry extra genetic information that may help in survival under adverse conditions.”

Organelles

• No membrane-bound organelles (like in eukaryotic cells).
• Ribosomes: Only organelle present, site of protein synthesis.
  • “Ribosomes are freely suspended in the cytoplasm.”
• Mesosome: Infoldings of the cell membrane, helps in respiration and DNA replication.
  • “They also increase the surface area of the plasma membrane.”

Cell Envelope and its Modifications

• Cell Envelope: Three-layered structure:
  • Glycocalyx: Outermost layer, can be a slime layer or a capsule.
    • “Capsule provides protection and helps in attachment.”
  • Cell Wall: Provides shape and structural support.
  • Plasma Membrane: Selectively permeable, interacts with the environment.
• Mesosome: Extensions of the plasma membrane.
  • Involved in cell wall formation, Respiration, DNA replication and distribution
• Chromatophores: Membranous extensions containing pigments.
  • Found in some prokaryotes like cyanobacteria.
  • “They help in photosynthesis.”

Motility and Surface Structures

• Flagella: Used for movement, composed of filament, hook, and basal body.
• Pili and Fimbriae: Surface structures that help in attachment.
  • Not involved in motility.
  • “They help bacteria attach to surfaces or host tissues.”

Ribosomes and Inclusion Bodies

• Ribosomes: Made of two subunits (50S and 30S) forming 70S ribosomes, site of protein synthesis.
• Polyribosomes: Chains of ribosomes attached to a single mRNA.
• Inclusion Bodies: Reserve materials stored in the cytoplasm.
  • e.g., Phosphate granules, glycogen granules, gas vacuoles.
  • “Gas vacuoles help in buoyancy in aquatic bacteria.”

Prokaryotic cells are simple yet efficient, with unique structures like plasmids, mesosomes, and inclusion bodies that help them survive and adapt to diverse environments.

Eukaryotic Cells

What are Eukaryotic Cells?

• Include protists, plants, animals, and fungi.
• Have compartmentalized cytoplasm with membrane-bound organelles.
• Organized nucleus with a nuclear envelope.
• “DNA is enclosed within a membrane-bound nucleus.”
• Genetic material (DNA) organized into chromosomes.
• Complex locomotory and cytoskeletal structures.

Differences Between Plant and Animal Cells

• Plant Cells: Have cell walls, plastids, and a large central vacuole.
• Animal Cells: Have centrioles, which are absent in most plant cells.

“Plastids are unique to plant cells and are absent in animal cells.”

1. Cell Membrane

Structure of the Cell Membrane

• Studied using electron microscopes and chemical studies.
• Composed mainly of lipids and proteins.
  1. Lipids: Phospholipids arranged in a bilayer with:
     • Polar heads facing outside and
     • Hydrophobic tails facing inside.
  2. Proteins:
     • Integral proteins: Embedded partially or fully within the membrane
     • Peripheral proteins: Present on the surface of the membrane
  3. Contains cholesterol.
     • “Cholesterol helps maintain membrane stability and fluidity.”

Fluid Mosaic Model

• Proposed by Singer and Nicolson in 1972.
• Describes the membrane as a fluid structure with proteins moving within the lipid bilayer.
• “The membrane behaves like protein icebergs floating in a sea of lipids.”
• Fluidity is essential for:
  • Cell growth
  • Junction formation
  • Secretion
  • Endocytosis
  • Cell division

Transport Across the Membrane

1. Selective Permeability: Only certain molecules can pass through the membrane.
2. Passive Transport: No energy required, moves molecules from higher to lower concentration.
   • Simple Diffusion: Movement of neutral solutes along the concentration gradient.
   • Osmosis: Movement of water from higher to lower concentration through a semipermeable membrane.
3. Active Transport: Requires energy (ATP), moves molecules against the concentration gradient (e.g., Na+/K+ pump).
   • “Carrier proteins help move ions across the membrane during active transport.”

2. Cell Wall

• What is the Cell Wall?
  • Non-living rigid structure outside the plasma membrane.
  • Found in fungi and plants.
• Functions of the Cell Wall
  • Gives shape to the cell.
  • Protects the cell from mechanical damage and infection.
  • “Prevents bursting of the cell due to osmotic pressure.”
  • Helps in cell-to-cell interaction.
  • Acts as a barrier to undesirable macromolecules.
• Composition
  • Algae: Cellulose, galactans, mannans, calcium carbonate.
  • Other Plants: Cellulose, hemicellulose, pectins, proteins.
• Structure of the Cell Wall
  1. Primary Wall: Present in young plant cells, capable of growth.
  2. Secondary Wall: Forms on the inner side of the primary wall as the cell matures.
  3. Middle Lamella: Layer of calcium pectate that glues neighboring cells together.
     • “Softening of fruits occurs due to dissolution of middle lamella.”
  4. Plasmodesmata: Channels that connect the cytoplasm of neighboring cells.
     • “They are protoplasmic connections between adjacent plant cells.”
     • Allow direct cell-to-cell transport and communication.

The cell wall is a crucial structure in plant and fungal cells, providing protection, rigidity, and intercellular connectivity while supporting plant growth and survival.

3. Endomembrane System

• What is the Endomembrane System?
  • A group of membrane-bound organelles with coordinated functions.
  • Includes: Endoplasmic Reticulum (ER), Golgi Complex, Lysosomes, and Vacuoles.
  • Does not include: Mitochondria, Chloroplasts, Peroxisomes.

“Nuclear envelope is also considered part of the endomembrane system because its lumen is continuous with the lumen of ER.”

“The endomembrane system helps in processing, sorting, packaging, and recycling of proteins and lipids.”

a. Endoplasmic Reticulum (ER)

• Structure
  • Network of tubular structures in the cytoplasm.
  • “ER can extend from the nuclear envelope up to the plasma membrane.”
  • “ER consists of cisternae, tubules, and vesicles.”
  • ER divides cell space into:
    • Luminal compartment (inside ER)
    • Extra-luminal compartment (cytoplasm)
• Types
  • Rough ER (RER)
    • Has ribosomes on its surface.
    • Involved in protein synthesis and secretion.
    • Continuous with the outer nuclear membrane.
    • “RER also bears enzymes involved in glycosylation of proteins.”
    • “Enzyme precursors for lysosome formation are synthesised on RER.”
  • Smooth ER (SER)
    • Lacks ribosomes and appears smooth.
    • Site of lipid synthesis.
    • In animals, synthesizes steroidal hormones.
    • “SER helps in detoxification of drugs and xenobiotics.

b. Golgi Apparatus

• Discovery
  • First observed by Camillo Golgi in 1898.
  • “Golgi apparatus was first seen in nerve cells of owl and cat.”
• Structure
  • Flat, disc-shaped sacs called cisternae.
  • Arranged near the nucleus with two faces:
    1. Cis face (forming face)
    2. Trans face (maturing face)
  • “In plant cells, Golgi bodies occur as separate units called dictyosomes.”
  • “Golgi complex also contains vesicles, tubules, and vacuoles.”
• Function
  • Packages materials for intra-cellular targets or secretion outside the cell.
  • Modifies proteins received from ER.
  • Forms glycoproteins and glycolipids.
  • “Proteins move from cis face to trans face during processing.”
  • “Golgi apparatus plays a role in acrosome formation during spermatogenesis.”

c. Lysosomes

• Discovery: “First reported by Christian de Duve in 1955.”
• Structure
  • Membrane-bound vesicles formed by the Golgi apparatus.
• Function
  • Rich in hydrolytic enzymes (lipases, proteases, carbohydrases).
  • Digest carbohydrates, proteins, lipids, and nucleic acids.
  • “These enzymes work optimally at acidic pH.”
  • “Lysosomes are called suicidal bags due to their digestive enzymes.”
  • “In plants and fungi, vacuoles perform lysosomal functions.”
  • “Lysosomes show polymorphism and occur as primary lysosomes, secondary lysosomes, residual bodies, and autophagic vacuoles.”

d. Vacuoles

• Structure
  • Membrane-bound space in the cytoplasm.
  • Contains water, sap, excretory products, and other materials.
  • Bound by a single membrane called the tonoplast.
• Function
  • In Plants
    • Can occupy up to 90% of the cell volume.
    • Tonoplast transports ions and materials into the vacuole.
    • “Maintains osmotic pressure and turgidity of the cell.”
    • “Different types of vacuoles include sap vacuole, food vacuole, contractile vacuole, and air vacuole.”
  • In Amoeba
    • Contractile vacuole helps in osmoregulation and excretion.
  • In Protists
    • Food vacuoles form by engulfing food particles.

The endomembrane system in eukaryotic cells includes various organelles that work together to manage and distribute cellular materials, ensuring smooth cellular functioning.

4. Mitochondria

• Appearance and Visibility
  • Hard to see under a microscope unless stained.
  • Number varies according to metabolic activity of the cell.
• Shape and Size
  • Sausage-shaped or cylindrical.
  • Diameter: 0.2-1.0 µm (average 0.5 µm).
  • Length: 1.0-4.1 µm.
• Structure
  • Double membrane-bound organelle.
  • Outer membrane: Forms continuous boundary.
  • Inner membrane: Forms infoldings called cristae.
    • “Cristae increase the surface area for respiration.”
    • “Inner membrane contains enzymes of electron transport chain.”
    • “Mushroom-shaped oxysomes (F₁ particles) are present on cristae.”
  • Inner compartment: Filled with matrix (dense substance).
• Function
  • Site of aerobic respiration.
  • Produces ATP (cellular energy).
  • Known as the “powerhouse” of the cell.
• Components
  • Contains circular DNA, RNA, 70S ribosomes, and enzymes for protein synthesis.
  • Divides by fission.

“Mitochondria are semi-autonomous as they possess their own DNA and ribosomes.”

5. Plastids

• “Plastids were first introduced as a term by E. Haeckel in 1866.”
• Functions
  • Found in plant cells and euglenoids.
  • Large and easily observed under a microscope.
  • Contain pigments giving plants specific colors.
• Types of Plastids
  1. Chloroplasts
     • Contain chlorophyll and carotenoid pigments .
     • Essential for photosynthesis.
     • “In algae, chloroplasts may be called chromatophores, such as rhodoplasts in red algae and phaeoplasts in brown algae.”
  2. Chromoplasts
     • Contain carotene and xanthophyll pigments.
       • Give plants yellow, orange, or red color.
     • “Provide colour to flowers and fruits, helping in pollination and seed dispersal.”
     • “They are also sites of synthesis of membrane lipids.”
  3. Leucoplasts
     • Colourless plastids meant for storage.
     • Generally found in non-green cells.
     • 3 Types:
       • Amyloplasts: Store starch (e.g., in potatoes).
       • Elaioplasts: Store oils and fats.
       • Aleuroplasts: Store proteins.
• Chloroplasts
  • Found mainly in mesophyll cells of leaves.
  • Shapes: Lens-shaped, oval, spherical, discoid, or ribbon-like.
  • Size: 5-10 µm length, 2-4 µm width.
  • Number: 1 per cell in Chlamydomonas (green alga) to 20-40 per cell in mesophyll cells.
• Structure of Chloroplasts
  • Double membrane-bound organelles.
  • Inner membrane: Less permeable than the outer membrane.
  • Stroma:
    • Fluid-filled space inside the inner membrane.
    • Contains enzymes for carbohydrate and protein synthesis.
    • Contains circular DNA and ribosomes.
  • Thylakoids:
    • Flattened membranous sacs present in the stroma.
    • Arranged in stacks called grana.
    • Grana are connected by stroma lamellae.
    • Contain chlorophyll pigments.
  • Ribosomes: Smaller (70S) compared to cytoplasmic ribosomes (80S).

“Like mitochondria, chloroplasts are semi-autonomous organelles having their own DNA, RNA and ribosomes.”

Chloroplasts and Mitochondria as Endosymbionts

Both are semi-autonomous organelles.
Both are double membrane-bound.
Both contain circular DNA, RNA and 70S ribosomes.
Both are energy-transducing organelles.
Both are found only in eukaryotic cells.

6. Ribosomes

• Discovery and Composition
  • Discovered by George Palade in 1953.
  • Made of RNA and proteins.
  • Not surrounded by a membrane.
• Types and Subunits
  • Eukaryotic ribosomes: 80S (60S + 40S subunits).
  • Prokaryotic ribosomes: 70S (50S + 30S subunits).
  • ‘S’ stands for Svedberg unit, indicating sedimentation coefficient (size and density).

“Ribosomes are the sites of protein synthesis and are called protein factories.”

7. Cytoskeleton

• Structure and Function
  • Network of proteinaceous structures present in cytoplasm.

• Types:
  • Microfilaments
  • Microtubules
  • Intermediate filaments

• Functions:
  • Provides mechanical support.
  • Maintains cell shape.
  • Helps in intracellular transport.
  • Assists in cell movement and organelle distribution.

8. Cilia and Flagella

• Appearance and Function
  • Hair-like outgrowths of the cell membrane.
  • Cilia: Short, work like oars, move the cell or surrounding fluid.
  • Flagella: Longer, move the cell.
  • “Prokaryotic flagella are structurally different from eukaryotic flagella.”
• Internal Structure
  • Covered with plasma membrane.
  • Core called axoneme and has microtubules.
  • Axoneme shows 9 + 2 arrangement of microtubules:
    • 9 peripheral microtubule doublets.
    • 2 central microtubules.
  • Radial spokes connect 9 peripheral doublets to 2 central pair.
  • Bridges connect adjacent 9 peripheral doublets.
  • “Both cilia and flagella arise from centriole-like basal bodies.”

Microtubules are made of tubulin protein.
Dynein protein present in arms of microtubule help in movement.
Nexin protein forms interdoublet links.

9. Centrosome and Centrioles

• Structure and Function
• Centrosome: An organelle usually containing two centrioles.
  • Present in nearly all animal cells and in motile plant cells (e.g., algal zoospores, fern sperm).
  • Surrounded by amorphous pericentriolar material.
• Centrioles:
  • Cylindrical structures arranged perpendicular to each other.
  • Each centriole shows a cartwheel organisation.
  • Structure:
    • Made of nine evenly spaced peripheral fibrils of tubulin protein.
    • Each peripheral fibril is a triplet of microtubules.
    • Adjacent triplets are interconnected.
    • Central proteinaceous hub present.
    • Hub is connected to peripheral triplets by radial spokes
  • Functions:
    • Form basal bodies of cilia and flagella.
    • Help in the formation of spindle fibres during cell division in animal cells.

10. Nucleus

• Discovery
  • First described by Robert Brown in 1831.
  • The nuclear material stained by basic dyes was named chromatin by Flemming.
• General Features:
  • Largest cell organelle.
  • Double membrane-bound.
  • Carries all genetic information of the cell.
• Structure
  • Contains chromatin, nuclear matrix, and nucleoli (one or more).
  • Nuclear envelope: Double membrane with a space (10-50 nm) called perinuclear space.
    • Outer membrane is continuous with the endoplasmic reticulum and has ribosomes.
    • Interrupted by nuclear pores formed by fusion of inner and outer membranes.
  • Nuclear pores allow bidirectional movement of RNA and proteins between nucleus and cytoplasm.
  • Number of Nuclei:
    • Usually one nucleus per cell.
    • Some cells are binucleate (e.g., Paramecium).
    • Some are multinucleate (certain fungi and algae).
    • Some mature cells lack nucleus (e.g., mammalian erythrocytes, sieve tube cells).
• Components
  • Nuclear Matrix (Nucleoplasm): Semi-fluid substance containing nucleolus and chromatin.
  • Nucleolus:
    • Spherical, non-membrane bound structure.
    • Site of ribosomal RNA (rRNA) synthesis.
    • Larger and more numerous in cells actively synthesising proteins.
  • Chromatin:
    • Thread-like nucleoprotein fibres present in interphase nucleus.
    • Composed of DNA, histone proteins, non-histone proteins and RNA.
  • Chromatin Types:
    1. Euchromatin:
       • Lightly stained, loosely packed.
       • Genetically active.
       • Site of transcription.
    2. Heterochromatin:
       • Darkly stained, highly condensed.
       • Genetically inactive.
• Chromosomes
  • Visible during cell division.
  • Formed by condensation of chromatin.
  • Structure:
    • Each chromosome has a primary constriction called centromere.
    • Kinetochores are disc-shaped structures present on either side of centromere.
    • Centromere holds two chromatids together.
• In some chromosomes, non-staining secondary constrictions occur at fixed positions.
  • These form small fragments called satellites.
• Classification of Chromosomes based on Centromere Position:
  1. Metacentric: Centromere in the middle, Two equal arms.
  2. Sub-metacentric: Centromere slightly away from the middle, one short and one long arm.
  3. Acrocentric: Centromere close to one end, one very short and one very long arm.
  4. Telocentric: Centromere terminal at the end of chromosome.

11. Microbodies

• Description
  • Small, membrane-bound vesicles.
  • Contain specific enzymes.
  • Present in both plant and animal cells.

Key Points to Remember

• Mitochondria: Powerhouse of the cell, produces ATP.
• Plastids: Found in plants, involved in photosynthesis and storage.
• Chloroplasts: Contain chlorophyll; site of photosynthesis.
• Thylakoids and Grana: Structures in chloroplasts where light energy is captured during photosynthesis.
• Ribosomes: Protein factories; 70S in prokaryotes, 80S in eukaryotes.
• Cytoskeleton: Maintains shape; aids in movement and transport.
• Cilia and Flagella: Locomotory structures with 9+2 arrangement.
• Centrosome and Centrioles: Spindle formation and basal bodies.
• Nucleus: Control centre containing chromatin and nucleoli.
• Chromosomes: DNA carriers, visible during cell division, classified by centromere position.
• Microbodies: Enzyme-containing vesicles in all eukaryotic cells.

Chapter Summary:

• All organisms are made of cells or aggregates of cells.
• Cells vary in shape, size, and functions.
• Based on presence or absence of a membrane-bound nucleus and other organelles, cells are classified as eukaryotic or prokaryotic.
• A typical eukaryotic cell consists of a plasma membrane, cytoplasm and a well-defined nucleus.
• Plant cells possess an additional rigid cell wall outside the plasma membrane.
• The plasma membrane is selectively permeable and regulates the movement of substances into and out of the cell.
• Transport across the membrane occurs by passive and active mechanisms.
• The endomembrane system includes ER, Golgi complex, lysosomes, and vacuoles.
  • All functions in a coordinated manner.
• Each cell organelle performs a specific function essential for cell survival.
• Centrosome and centrioles are present mainly in animal cells.
• Centrioles form the basal bodies of cilia and flagella for locomotion.
• Centrioles also help in spindle apparatus formation during cell division.
• In animal cells, centrioles also form the spindle apparatus during cell division.
• Nucleus contains nucleoli and a chromatin network.
  • It controls cellular activities and stores genetic information responsible for heredity.
• Endoplasmic reticulum (ER) is a network of membranous tubules and cisternae and occurs in two forms:
  • Rough ER and Smooth ER.
  • Rough ER is involved in protein synthesis and secretion, while smooth ER synthesizes lipids and helps in detoxification of drugs and poisons.
• ER helps in transporting substances and synthesizing proteins, lipoproteins, and glycogen.
• The Golgi apparatus is a membranous organelle composed of flattened sacs.
  • It modifies, packages and transports proteins and lipids synthesized in the ER.
• Lysosomes are single membrane-bound vesicles rich in hydrolytic enzymes.
  • They digest macromolecules and are involved in intracellular digestion.
• Ribosomes are non-membrane bound organelles involved in protein synthesis.
  • They may occur freely in the cytoplasm or attached to the rough endoplasmic reticulum.
• Mitochondria are double membrane-bound organelles involved in aerobic respiration and ATP production.
  • The inner membrane forms cristae which increase the surface area for oxidative phosphorylation.
• Plastids are pigment-containing organelles found in plant cells.
  • Chloroplasts trap light energy for photosynthesis.
  • Grana are the sites of light reactions, while the stroma is the site of dark reactions.
  • Chromoplasts contain carotenoid pigments such as carotene and xanthophyll, imparting different colours to plant parts.
• The nucleus is enclosed by a double-layered nuclear envelope with nuclear pores.
  • It contains nucleoplasm and chromatin material and acts as the control centre of the cell.
• Thus, the cell is the basic structural and functional unit of life.