Chapter 5: The Unit of Life

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Introduction

Overview

• Biology studies living organisms, revealing diversity and unity through cell theory.

• Cell theory shows all life forms share cellular organization essential for physiological and behavioral functions.

• Organisms range from unicellular (single cell) to multicellular (many cells) forms.

Memory Aids

• Cell theory unites diverse life forms.

• Unicellular = one cell; multicellular = many cells.

Quick Checkpoints

• What does cell theory explain about life?

• Difference between unicellular and multicellular organisms?

What is a Cell?

Definition and Historical Discoveries

• Cells are the smallest units capable of independent life functions; basic structural and functional units of life.

• Robert Hooke (1635-1703) first observed cell walls in cork; actual living cells were not seen.

• Anton von Leeuwenhoek first observed and sketched living cells like bacteria and protozoa.

• Robert Brown (1831) discovered the nucleus in plant cells.

• Microscope improvements revealed cell structures; Leeuwenhoek is father of microscopy.

• Electron microscope invented by Max Knoll and Ernst Ruska (1931) enhanced cell study.

Memory Aids

• Cell = smallest life unit.

• Hooke = cell walls; Leeuwenhoek = living cells; Brown = nucleus.

• Electron microscope = Knoll & Ruska.

Quick Checkpoints

• What defines a cell's basic role?

• Who discovered the nucleus?

• Who invented the electron microscope?

Cell Theory

Historical Development

• 1838: Matthias Schleiden observed all plants are made of cells forming tissues.

• 1839: Theodore Schwann found animal cells have a thin outer layer called plasma membrane.

• Schwann noted cell wall is unique to plant cells.

• Schleiden and Schwann formulated the cell theory.

• 1855: Rudolf Virchow stated new cells arise from pre-existing cells (Omnis cellula-e cellula), finalizing the theory.

Features of Cell Theory

• All living organisms consist of cells and their products.

• All cells arise from pre-existing cells.

• Viruses are exceptions as they lack cellular organization.

Memory Aids

• Schleiden = Plants, Schwann = Animals.

• Virchow = \\\"Omnis cellula-e cellula\\\" = Cells from cells.

• Cell wall = Plant cells only.

Quick Checkpoints

• Who proposed that all plants are made of cells?

• What is unique about plant cells compared to animal cells?

• What does \\\"Omnis cellula-e cellula\\\" mean?

• Are viruses considered in cell theory?

An Overview of Cell

Cell Structure and Types

• Plant cells have a cell wall outside the cell membrane; animal cells have only a cell membrane.

• The nucleus is a membrane-bound structure containing chromosomes with DNA.

• Cells with membrane-bound nuclei are eukaryotic; those without are prokaryotic.

• Cytoplasm is a semi-fluid matrix where cellular activities occur in both cell types.

• Eukaryotic cells contain membrane-bound organelles: endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.

• Prokaryotic cells lack membrane-bound organelles.

• Ribosomes are non-membrane-bound organelles present in all cells; found in cytoplasm, chloroplasts, mitochondria, and on rough ER.

• Animal cells have centrioles, non-membrane-bound organelles aiding cell division.

• Cell size varies: smallest are Mycoplasmas (~0.3 µm), largest single cell is ostrich egg; human red blood cells ~7 µm diameter.

• Cell shapes vary: disc-like, polygonal, columnar, cuboid, thread-like, or irregular, depending on function.

Memory Aids

• Eukaryotic = \\\"Eu\\\" means true nucleus present.

• Prokaryotic = \\\"Pro\\\" means before nucleus (no membrane-bound nucleus).

• Ribosomes = \\\"Organelle within organelle\\\" (found inside mitochondria and chloroplasts).

• Cell shape matches function: e.g., nerve cells are long for signal transmission.

Quick Checkpoints

• What distinguishes eukaryotic from prokaryotic cells?

• Which organelles are membrane-bound?

• Where are ribosomes located inside the cell?

• How does cell shape relate to its function?

• What is the role of centrioles in animal cells?

Prokaryotic Cells

Characteristics and Types

• Represented by bacteria, blue-green algae, mycoplasma, and PPLO.

• Smaller and multiply faster than eukaryotic cells.

• Shapes: Bacillus (rod), Coccus (spherical), Vibrio (comma), Spirillum (spiral).

• No true nucleus; genetic material is naked circular DNA.

• May have plasmids—small circular DNA conferring traits like antibiotic resistance.

• No membrane-bound organelles except 70S ribosomes.

• Unique membrane infoldings called mesosomes.

Memory Aids

• Prokaryotes = No nucleus, Plasmids present.

• Bacterial shapes: \\\"Bacillus, Coccus, Vibrio, Spirillum\\\" (BCVS).

• Mesosome = Membrane infolding aiding cell functions.

Quick Checkpoints

• What shapes classify bacteria?

• Role of plasmids in bacteria?

• Function of mesosomes?

Cell Envelope

Structure and Types

• Composed of glycocalyx, cell wall, and plasma membrane.

• Glycocalyx: slime layer (loose) or capsule (thick).

• Cell wall provides shape and prevents bursting.

• Plasma membrane is selectively permeable, similar to eukaryotes.

• Bacteria classified as Gram-positive (retain stain) or Gram-negative (do not retain).

• Mesosomes: plasma membrane extensions (vesicles, tubules, lamellae) aiding cell wall formation, DNA replication, respiration, secretion.

• Cyanobacteria have chromatophores—pigment-containing membrane extensions.

Memory Aids

• Glycocalyx = Slime or Capsule.

• Gram stain differentiates bacteria by cell envelope.

• Mesosome = Membrane extension for vital processes.

Quick Checkpoints

• Components of bacterial cell envelope?

• Difference between Gram-positive and Gram-negative?

• Functions of mesosomes?

Bacterial Surface Structures

Motility and Attachment

• Flagella: filamentous extensions for motility; composed of filament, hook, basal body.

• Number and arrangement of flagella vary among bacteria.

• Pili: tubular protein structures; not for motility.

• Fimbriae: small bristle-like fibers aiding attachment to surfaces and host tissues.

Memory Aids

• Flagella = Movement; Pili and Fimbriae = Attachment.

• Flagellum parts: Filament (longest), Hook, Basal body.

Quick Checkpoints

• Structure and function of flagella?

• Difference between pili and fimbriae?

• Role of fimbriae in bacteria?

Ribosomes and Inclusion Bodies

Protein Synthesis and Storage

• Prokaryotic ribosomes are 70S (50S + 30S), size 15-20 nm.

• Ribosomes synthesize proteins; multiple ribosomes on mRNA form polyribosomes (polysomes).

• Inclusion bodies store reserve materials like phosphate, cyanophycean, glycogen granules.

• Gas vacuoles present in photosynthetic bacteria for buoyancy.

Memory Aids

• 70S ribosomes = Prokaryotes.

• Polysomes = Multiple ribosomes on one mRNA.

• Inclusion bodies = Storage granules.

Quick Checkpoints

• Size and composition of prokaryotic ribosomes?

• Function of polyribosomes?

• Types of inclusion bodies in bacteria?

Eukaryotic Cells

Cell Components and Functions

• Eukaryotes include protists, plants, animals, and fungi with well-organized nucleus and membrane-bound organelles.

• DNA is organized into chromosomes and chromatin.

• Plant cell structure includes cell wall, plasma membrane, nucleus, nuclear envelope, nucleolus, cytoplasm, chloroplasts, mitochondrion, vacuole, Golgi apparatus, ribosomes, endoplasmic reticulum, lysosomes, peroxisomes, microtubules, plasmodesmata, and middle lamella.

• Cell wall provides shape and protection; plasma membrane controls entry and exit; nucleus controls cell activities; nucleolus makes ribosomes; cytoplasm holds organelles; chloroplasts perform photosynthesis; mitochondrion produces energy; vacuole stores water and nutrients; Golgi apparatus packages proteins; ribosomes synthesize proteins; ER synthesizes and transports proteins and lipids; lysosomes digest waste; peroxisomes detoxify; microtubules maintain shape; plasmodesmata allow cell communication; middle lamella glues cells together.

Memory Aids

• Remember key organelles: Nucleus (control), Mitochondria (energy), Chloroplast (photosynthesis), Golgi (packaging).

• Plasmodesmata connect plant cells for communication.

Quick Checkpoints

• What organelles are unique to plant cells?

• Which organelle is the site of protein synthesis?

• Function of plasmodesmata?

Cell Membrane

Structure and Composition

• Composed mainly of phospholipids, proteins, cholesterol, carbohydrates, and water.

• Phospholipids arranged with polar heads outward and hydrophobic tails inward.

• Proteins classified as integral (embedded) and peripheral (surface).

• Fluid mosaic model: proteins float in a lipid bilayer, allowing lateral movement.

Functions

• Selective permeability controls molecule transport.

• Passive transport: diffusion and osmosis along concentration gradient without energy.

• Active transport: movement against gradient using ATP (e.g., Na⁺/K⁺ pump).

Memory Aids

• Fluid mosaic = proteins float in lipid sea.

• Passive transport = no energy; active transport = energy required.

Quick Checkpoints

• What molecules make up the plasma membrane?

• Difference between integral and peripheral proteins?

• Examples of passive and active transport?

Cell Wall

Structure and Composition

• Non-living, rigid outer covering in plant cells, fungi, and some protists.

• Made of cellulose, hemicellulose, pectins, proteins in plants; algae have cellulose and minerals.

• Three parts: primary wall (young cells, flexible), secondary wall (mature cells, inner side), middle lamella (calcium pectate, glues cells).

• Plasmodesmata traverse cell wall connecting cytoplasm of adjacent cells.

Functions

• Provides shape and rigidity.

• Protects protoplasm from injury and unwanted molecules.

• Prevents bursting by counteracting osmotic pressure.

Memory Aids

• Middle lamella = glue between cells.

• Primary wall = flexible; secondary wall = rigid.

Quick Checkpoints

• What is the main component of plant cell wall?

• Role of plasmodesmata?

• Difference between primary and secondary wall?

Endomembrane System

Components and Functions

• Includes endoplasmic reticulum (ER), Golgi complex, lysosomes, vacuoles, and nuclear envelope.

• Coordinates processing, sorting, and recycling of proteins.

• Nuclear envelope lumen continuous with ER lumen.

Memory Aids

• Endomembrane system = cell's protein processing and transport network.

Quick Checkpoints

• Which organelles are part of the endomembrane system?

• Function of nuclear envelope in this system?

Endoplasmic Reticulum

Structure and Types

• Network of tubules dividing cytoplasm into luminal (inside ER) and extra luminal (cytoplasm) compartments.

• Three structures: cisternae (flat sacs), tubules (lipid synthesis), vesicles (membrane-bound vacuoles).

• Two types: Rough ER (RER) with ribosomes, Smooth ER (SER) without ribosomes.

Functions

• RER: protein synthesis and secretion; glycosylation; lysosome enzyme precursors.

• SER: lipid and steroid hormone synthesis; detoxification of drugs and xenobiotics.

Memory Aids

• RER = rough with ribosomes, protein factory.

• SER = smooth, lipid factory and detox center.

Quick Checkpoints

• Difference between RER and SER?

• Role of ER in protein and lipid synthesis?

Golgi Apparatus

Structure

• Stack of flat, disc-shaped cisternae with vesicles, tubules, and vacuoles.

• Has distinct cis (forming) and trans (maturing) faces.

• Present in all eukaryotic cells except some specialized cells.

Functions

• Packages and stores materials from ER.

• Modifies proteins (glycosylation) before release.

• Forms acrosomes during spermatogenesis.

• Produces glycoproteins and glycolipids for cell recognition and adhesion.

Memory Aids

• Golgi = cell's post office: modifies, sorts, ships proteins.

Quick Checkpoints

• What are cis and trans faces?

• Role of Golgi in protein modification?

Lysosomes

Structure and Composition

• Single membrane-bound vesicles containing hydrolytic enzymes (acid hydrolases).

• Formed by Golgi apparatus packaging.

• Present in all animal cells except RBCs; vacuoles perform similar function in plants and fungi.

• Known as suicidal bags due to digestive enzymes.

Types

• Primary lysosomes, secondary lysosomes, residual bodies, autophagic vacuoles.

Memory Aids

• Lysosomes = cell's waste disposal and recycling units.

Quick Checkpoints

• What enzymes do lysosomes contain?

• Why are lysosomes called suicidal bags?

Vacuoles

Structure and Types

• Membrane-bound spaces containing water, sap, excretory products; bounded by tonoplast.

• In plants, can occupy up to 90% of cell volume.

• Types: sap vacuole, food vacuole, contractile vacuole, air vacuole.

• Contractile vacuole in Amoeba regulates osmoregulation and excretion.

Functions

• Maintains osmotic pressure and turgidity.

• Stores nutrients and waste.

Memory Aids

• Tonoplast = vacuole membrane controlling ion passage.

Quick Checkpoints

• What is the function of contractile vacuole?

• Role of tonoplast?

Mitochondria

Structure

• Cylindrical, double-membraned organelles (1-4 μm length, 0.2-1 μm diameter).

• Outer membrane forms boundary; inner membrane forms cristae (folds) increasing surface area.

• Matrix contains circular DNA, RNA, 70S ribosomes, enzymes for protein synthesis.

• Inner membrane has elementary particles (F₀-F₁ particles) with electron transport enzymes.

• Divide by fission.

Functions

• Site of cellular respiration and ATP production.

• Powerhouse of the cell.

Memory Aids

• Mitochondria have their own DNA and ribosomes (semi-autonomous).

• Cristae increase surface area for energy production.

Quick Checkpoints

• What is the function of cristae?

• Why are mitochondria called semi-autonomous?

Plastids

Types and Functions

• Found in plant cells and euglenoids; contain pigments imparting color.

• Three types: leucoplasts (colorless, store starch, fat, protein), chloroplasts (green, photosynthesis), chromoplasts (yellow/red, carotenoids, color flowers/fruits).

• Chloroplasts have double membrane, stroma, thylakoids arranged in grana, stroma lamellae, DNA, and 70S ribosomes.

• Semi-autonomous like mitochondria.

Memory Aids

• Leucoplasts = storage; Chloroplasts = photosynthesis; Chromoplasts = color.

• Grana = stacks of thylakoids; stroma = fluid matrix.

Quick Checkpoints

• What pigments are found in chloroplasts?

• Functions of chromoplasts?

• Why are plastids semi-autonomous?

Ribosomes

Structure and Function

• Granular, non-membranous particles made of RNA and proteins.

• Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic are 70S (50S + 30S).

• Sites of protein synthesis; act as protein factories.

Memory Aids

• 80S ribosomes in eukaryotes; 70S in prokaryotes.

• Two subunits combine for protein synthesis.

Quick Checkpoints

• What is the sedimentation coefficient of eukaryotic ribosomes?

• Role of ribosomes in the cell?

Cytoskeleton

Structure and Types

• Network of protein filaments in cytoplasm providing structural support.

• Types: microfilaments, microtubules, intermediate filaments.

• Maintains cell shape, organelle positioning, intracellular transport, and cell movement.

Memory Aids

• Microtubules = tubulin; microfilaments = actin.

• Cytoskeleton = cell's skeleton and transport system.

Quick Checkpoints

• What are the three types of cytoskeletal elements?

• Functions of cytoskeleton?

Cilia and Flagella

Structure and Function

• Hair-like projections from plasma membrane aiding movement.

• Cilia are short and numerous; flagella are longer and fewer.

• Core (axoneme) has 9+2 arrangement: nine peripheral microtubule doublets and two central microtubules.

• Connected by radial spokes, interdoublet links, and dynein arms for movement.

• Originate from basal bodies (centriole-like structures).

Memory Aids

• 9+2 microtubule arrangement is characteristic.

• Cilia = oars; flagella = whip-like for movement.

Quick Checkpoints

• What is the axoneme structure?

• Difference between cilia and flagella?

Centrosome and Centrioles

Structure and Function

• Centrosome contains two centrioles arranged perpendicularly.

• Centrioles are cylindrical structures made of nine triplets of microtubules linked by proteins.

• Centrioles form basal bodies of cilia and flagella and spindle fibers during cell division.

Memory Aids

• Centrioles = microtubule organizing centers.

• Cartwheel structure with nine triplets.

Quick Checkpoints

• Role of centrioles in cell division?

• Structure of centrioles?

Nucleus

Structure and Components

• Largest organelle with double membrane (nuclear envelope) containing nuclear pores.

• Contains chromatin (DNA + proteins), nucleolus, and nucleoplasm.

• Chromatin exists as euchromatin (active, loosely packed) and heterochromatin (inactive, condensed).

• Chromosomes have centromere and kinetochores; classified as telocentric, acrocentric, sub-metacentric, and metacentric based on centromere position.

• Nucleolus synthesizes ribosomal RNA.

• Some cells are multinucleate or anucleate.

Memory Aids

• Chromatin = DNA + histones; euchromatin active, heterochromatin inactive.

• Centromere position defines chromosome type.

Quick Checkpoints

• Function of nuclear pores?

• Difference between euchromatin and heterochromatin?

• Types of chromosomes based on centromere?

Microbodies

Structure and Function

• Small membrane-bound vesicles containing enzymes.

• Present in both plant and animal cells.

Memory Aids

• Microbodies help in various metabolic processes.

Quick Checkpoints

• What are microbodies?

• Role of microbodies in cells?

Plant Cell v/s Animal Cell

Differences

• Plant cells have cell wall, plastids, and large central vacuole; animal cells lack these.

• Animal cells have centrioles; plant cells usually do not.

• Plant cells have rigid shape; animal cells are flexible.

Memory Aids

• Plant cells = wall + plastids + big vacuole.

• Animal cells = centrioles + lysosomes + small vacuoles.

Quick Checkpoints

• Which organelles are unique to plant cells?

• Presence of centrioles in animal cells?

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