Cell: The Unit of Life

Cell

Living vs. Non-living Things

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

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.
• Important Discoveries:
  • Anton Von Leeuwenhoek: First saw live cells.
  • Robert Brown: Discovered the nucleus.
  • Microscopes: Helped reveal cell structures.

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.

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
• Present in both plant and animal cells

Organelles in Eukaryotic Cells

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

Organelles in All Cells

• Ribosomes
  • Non-membrane bound
  • Found in cytoplasm, chloroplasts (plants), mitochondria, and rough ER
• 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
• Shape Examples
  • Disc-like, polygonal, columnar, cuboid, thread-like, irregular
  • Shape varies with function

Cells come in many sizes and shapes, and their structures are designed to help them perform specific functions effectively!

Prokaryotic Cells

What are Prokaryotic Cells?

• Found in bacteria, blue-green algae, mycoplasma, and PPLO.
• Smaller and multiply faster than eukaryotic cells.
• Common shapes:
  • Bacillus (rod-like)
  • Coccus (spherical)
  • Vibrio (comma-shaped)
  • Spirillum (spiral)

Cell Structure

• Cell Wall: Surrounds the cell membrane (except in mycoplasma).
• Cytoplasm: Fluid matrix inside the cell.
• Nucleus: No well-defined nucleus, genetic material is not enclosed by a membrane.
• Plasmids: Small circular DNA outside the genomic DNA, can provide antibiotic resistance.

Organelles

• No membrane-bound organelles (like in eukaryotic cells).
• Ribosomes: Only organelle, site of protein synthesis.
• Mesosome: Infoldings of the cell membrane, helps in various functions like respiration and DNA replication.

Cell Envelope and its Modifications

• Cell Envelope: Three-layered structure:
  • Glycocalyx: Outermost layer, can be a slime layer or a capsule.
  • 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 and other functions.
• Chromatophores: Membranous extensions containing pigments (in some prokaryotes like cyanobacteria).

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 for motility.

Ribosomes and Inclusion Bodies

• Ribosomes: Made of two subunits (50S and 30S) forming 70S ribosomes, site of protein synthesis.
• Polyribosomes: Chains of ribosomes on a single mRNA.
• Inclusion Bodies: Reserve materials stored in the cytoplasm, e.g., phosphate granules, glycogen granules, gas vacuoles.

Prokaryotic cells are simple yet efficient, with unique structures like plasmids and mesosomes that help them survive and adapt to various 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.
• Complex locomotory and cytoskeletal structures.
• Genetic material organized into chromosomes.

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.

1. Cell Membrane

Structure of the Cell Membrane

• Studied using electron microscopes and chemical studies.
• Composed mainly of lipids and proteins.
  • Lipids: Phospholipids arranged in a bilayer with polar heads outside and hydrophobic tails inside.
  • Proteins: Integral (within the membrane) and peripheral (on the surface).
  • Contains cholesterol.

Fluid Mosaic Model

• Proposed by Singer and Nicolson in 1972.
• Describes the membrane as a fluid structure with proteins moving within the lipid bilayer.
• Fluidity is essential for cell functions like growth, junction formation, secretion, endocytosis, and cell division.

Transport Across the Membrane

• Selective Permeability: Only certain molecules can pass through.
• 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.
• Active Transport: Requires energy (ATP), moves molecules against the concentration gradient (e.g., Na+/K+ pump).

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.
  • 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
  • Primary Wall: Present in young plant cells, capable of growth.
  • Secondary Wall: Forms on the inner side of the primary wall as the cell matures.
  • Middle Lamella: Layer of calcium pectate that glues neighboring cells together.
  • Plasmodesmata: Channels that connect the cytoplasm of neighboring cells.

The cell wall is a crucial structure in plant and fungi cells, providing protection, shape, and helping cells connect with each other!

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.

a. Endoplasmic Reticulum (ER)

• Structure
  • Network of tubular structures in the cytoplasm.
  • Divides cell space into luminal (inside ER) and extra-luminal (cytoplasm) compartments.
• Types
  • Rough ER (RER)
    • Has ribosomes on its surface.
    • Involved in protein synthesis and secretion.
    • Continuous with the outer nuclear membrane.
  • Smooth ER (SER)
    • Lacks ribosomes, appears smooth.
    • Site of lipid synthesis.
    • In animals, synthesizes steroidal hormones.

b. Golgi Apparatus

• Discovery
  • First observed by Camillo Golgi in 1898.
• Structure
  • Flat, disc-shaped sacs called cisternae.
  • Arranged near the nucleus with two faces: cis (forming) and trans (maturing).
• Function
  • Packages materials for intra-cellular targets or secretion outside the cell.
  • Modifies proteins from the ER.
  • Forms glycoproteins and glycolipids.

c. Lysosomes

• Structure
  • Membrane-bound vesicles formed by the Golgi apparatus.
• Function
  • Rich in hydrolytic enzymes (lipases, proteases, carbohydrases).
  • Digest carbohydrates, proteins, lipids, and nucleic acids.

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.
  • 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 the cell functions smoothly!

4. Mitochondria

• Appearance and Visibility
  • Hard to see under a microscope unless stained.
  • Number varies with cell activity.
• 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.
  • Outer membrane: Continuous boundary.
  • Inner membrane: Has infoldings called cristae to increase surface area.
  • 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, ribosomes (70S), and protein synthesis components.
  • Divides by fission.

5. Plastids

• Functions
  • Found in plant cells and euglenoids.
  • Large and easily observed under a microscope.
  • Contain pigments giving plants specific colors.
• Types of Plastids
  • Chloroplasts
    • Contain chlorophyll and carotenoid pigments.
    • Essential for photosynthesis.
  • Chromoplasts
    • Contain carotene and xanthophyll pigments.
    • Give plants yellow, orange, or red color.
  • Leucoplasts
    • Colorless and store nutrients.
    • 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.
• Structure of Chloroplasts
  • Double membrane-bound.
  • Inner membrane: Less permeable.
  • Stroma: Space inside the inner membrane, contains enzymes for carbohydrate and protein synthesis, circular DNA, and ribosomes.
  • Thylakoids: Flattened sacs in the stroma.
    • Arranged in stacks called grana.
    • Connected by stroma lamellae.
    • Contain chlorophyll pigments.
  • Ribosomes: Smaller (70S) than cytoplasmic ribosomes (80S).

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’s unit, indicating density and size.

7. Cytoskeleton

• Structure and Function
  • Network of protein structures: microtubules, microfilaments, intermediate filaments.
  • Provides mechanical support.
  • Helps in cell movement.
  • Maintains cell shape.

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.
• Internal Structure
  • Covered with plasma membrane.
  • Core (axoneme) has microtubules.
  • 9+2 arrangement: nine peripheral doublets, two central microtubules.
  • Radial spokes and bridges connect tubules.
  • Both cilia and flagella arise from basal bodies.

9. Centrosome and Centrioles

• Structure and Function
  • Centrosome: Contains two centrioles.
  • Centrioles: Cylindrical, perpendicular to each other, cartwheel structure.
  • Made of nine triplets of tubulin protein.
  • Central hub connected to peripheral triplets by radial spokes.
  • Form basal bodies of cilia/flagella.
  • Involved in forming spindle fibers during cell division in animal cells.

10. Nucleus

• Discovery
  • First described by Robert Brown in 1831.
  • Material in the nucleus stained by dyes called chromatin (named by Flemming).
• 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.
  • Nuclear pores allow movement of RNA and proteins between nucleus and cytoplasm.
  • Usually, one nucleus per cell, but some cells have more or lack a nucleus (e.g., mammal erythrocytes).
• Components
  • Nuclear Matrix (Nucleoplasm): Contains nucleolus and chromatin.
  • Nucleolus: Site of ribosomal RNA synthesis, larger and more numerous in protein-synthesizing cells.
  • Chromatin: Contains DNA, histone proteins, non-histone proteins, and RNA.
• Chromosomes
  • Visible during cell division.
  • Contains a primary constriction called the centromere with kinetochores on sides.
  • Holds two chromatids of a chromosome.
  • Classified based on centromere position:
    • Metacentric: Middle centromere, two equal arms.
    • Sub-metacentric: Centromere slightly off-center, one short and one long arm.
    • Acrocentric: Centromere close to the end, one very short and one very long arm.
    • Telocentric: Terminal centromere.
  • Some have non-staining secondary constrictions, forming a satellite.

11. Microbodies

• Description
  • Membrane-bound vesicles with 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 of nutrients.
• Chloroplasts: Contain chlorophyll, crucial for photosynthesis.
• Thylakoids and Grana: Structures in chloroplasts where light energy is captured.
• Ribosomes: Protein factories, differ in size between eukaryotes and prokaryotes.
• Cytoskeleton: Structural support, helps in cell movement.
• Cilia and Flagella: Movement structures, differ in length and function.
• Centrosome and Centrioles: Important for cell division, structural organization.
• Nucleus: Control center with 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 has a cell membrane, nucleus, and cytoplasm.
• Plant cells have a cell wall outside the cell membrane.
• The plasma membrane is selectively permeable and helps transport molecules.
• The endomembrane system includes ER, Golgi complex, lysosomes, and vacuoles.
• All cell organelles have specific functions.
• Centrosome and centriole form the basal body of cilia and flagella for locomotion.
• In animal cells, centrioles also form the spindle apparatus during cell division.
• Nucleus contains nucleoli and chromatin network, controls organelle activities, and plays a role in heredity.
• Endoplasmic reticulum (ER) has tubules or cisternae and is of two types: rough and smooth.
• ER helps in transporting substances and synthesizing proteins, lipoproteins, and glycogen.
• The Golgi body is a membranous organelle with flattened sacs, packing and transporting cell secretions.
• Lysosomes are single-membrane structures with enzymes for digesting macromolecules.
• Ribosomes are involved in protein synthesis and occur freely in the cytoplasm or associated with ER.
• Mitochondria help in oxidative phosphorylation and generating ATP, and have a double membrane with the inner one folding into cristae.
• Plastids are pigment-containing organelles in plant cells.
• Chloroplasts in plant cells trap light energy for photosynthesis; grana are the site of light reactions, and stroma is the site of dark reactions.
• Chloroplasts contain chlorophyll (green), and chromoplasts contain pigments like carotene and xanthophyll (other colors).
• The nucleus is enclosed by a nuclear envelope with nuclear pores, containing nucleoplasm and chromatin material.
• The cell is the structural and functional unit of life.