Chapter 5: The Unit of Life

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

Memory Aids

• Cell theory unites all 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 History

• Cells are the smallest units capable of independent life functions.
• Cells are the basic structural and functional units of life.
• Robert Hooke first observed cell walls in cork (dead cells).
• Anton von Leeuwenhoek first observed living cells (bacteria, protozoa, sperm, RBCs).
• Robert Brown discovered the nucleus in orchid root cells.
• Microscope improvements revealed cell structure; Leeuwenhoek is father of microscopy.
• Electron microscope invented by Max Knoll and Ernst Ruska in 1931.

Memory Aids

• Hooke = cork cell walls; Leeuwenhoek = living cells; Brown = nucleus.
• Electron microscope = Knoll & Ruska, 1931.

Quick Checkpoints

• What defines a cell as a life unit?
• 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 plasma membrane.
• Schwann noted cell wall is unique to plant cells.
• Schleiden and Schwann proposed that animals and plants are composed of cells and their products.
• 1855: Rudolf Virchow stated new cells arise from pre-existing cells (Omnis cellula-e cellula), finalizing cell 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 = Cells from cells (Omnis cellula-e cellula).
• Cell wall = Plant cells only.

Quick Checkpoints

• Who proposed that 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 living cells?

An Overview of Cell

Cell Structure and Types

• The plant cell has a cell wall outside and a cell membrane inside; human cells have only a cell membrane as the outer boundary.
• Each cell contains a nucleus with chromosomes carrying DNA, the genetic material.
• Cells with membrane-bound nuclei are eukaryotic; those without are prokaryotic.
• Both cell types have cytoplasm, a semi-fluid matrix where cellular activities occur.
• Eukaryotic cells have membrane-bound organelles: endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles.
• Prokaryotic cells lack membrane-bound organelles.
• Ribosomes are non-membrane organelles found in all cells, present in cytoplasm and some organelles like chloroplasts and mitochondria.
• Animal cells have centrioles, non-membrane organelles aiding cell division.
• Cell size varies: Mycoplasmas are smallest (~0.3 µm), bacteria 3-5 µm, human red blood cells ~7 µm, nerve cells are very long.
• 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).
• Ribosome = "Organelle within organelle" (found inside mitochondria and chloroplasts).
• Cell shape matches function: e.g., nerve cells are long for signal transmission.

Quick Checkpoints

• What defines a eukaryotic cell?
• Which organelles are membrane-bound?
• Where are ribosomes found inside the cell?
• How does cell shape relate to its function?
• What is the role of centrioles?

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).
• Cell wall present except in mycoplasma.
• Cytoplasm lacks membrane-bound nucleus; DNA is naked and circular.
• Plasmids: small circular DNA outside genomic DNA, confer traits like antibiotic resistance.
• No organelles except 70S ribosomes.
• Mesosome: infoldings of plasma membrane, unique to prokaryotes.

Memory Aids

• Prokaryotes = No nucleus, No organelles except ribosomes.
• Bacterial shapes: B-C-V-S (Bacillus, Coccus, Vibrio, Spirillum).
• Plasmids = Extra DNA for special traits.

Quick Checkpoints

• What shapes do bacteria exhibit?
• Which prokaryotes lack a cell wall?
• What is the role of plasmids?
• What is a mesosome?

Cell Envelope and Modifications

Structure and Types

• Cell envelope: glycocalyx, cell wall, plasma membrane.
• Glycocalyx: slime layer (loose) or capsule (thick).
• Cell wall: maintains shape, prevents bursting.
• Plasma membrane: selectively permeable, similar to eukaryotes.
• Gram staining divides bacteria into Gram-positive (retain stain) and Gram-negative (do not retain).
• Mesosome: plasma membrane infoldings as vesicles, tubules, lamellae.
• Mesosome functions: cell wall formation, DNA replication, distribution, respiration, secretion, surface area increase.
• Cyanobacteria have chromatophores with pigments.

Memory Aids

• Glycocalyx: Slime = loose, Capsule = tough.
• Gram-positive = purple stain, Gram-negative = pink/red.
• Mesosome = membrane infoldings aiding multiple cell functions.

Quick Checkpoints

• What layers form the cell envelope?
• Difference between slime layer and capsule?
• Functions of mesosome?
• How are bacteria classified by Gram stain?

Bacterial Surface Structures

Motility and Attachment

• Flagella: thin filaments for motility; parts: filament (longest), hook, basal body.
• Number and arrangement of flagella vary among bacteria.
• Pili: tubular protein structures, not for motility.
• Fimbriae: small bristle-like fibers, help attachment to surfaces and host tissues.

Memory Aids

• Flagella = movement; Pili and fimbriae = attachment.
• Flagellum parts: F-H-B (Filament, Hook, Basal body).

Quick Checkpoints

• What are the parts of bacterial flagellum?
• Which structures aid bacterial attachment?
• Do pili help in motility?

Ribosomes and Inclusion Bodies

Structure and Function

• Prokaryotic ribosomes are 70S (50S + 30S), size 15-20 nm.
• Site of protein synthesis.
• Polyribosomes (polysomes): multiple ribosomes on one mRNA for simultaneous translation.
• Inclusion bodies: reserve materials like phosphate, cyanophycean, glycogen granules; not membrane-bound.
• Gas vacuoles in photosynthetic bacteria for buoyancy.

Memory Aids

• 70S ribosomes = Prokaryotes.
• Polysomes = multiple ribosomes on mRNA.
• Inclusion bodies = storage granules.

Quick Checkpoints

• What is the size and type of prokaryotic ribosomes?
• What are polysomes?
• What types of inclusion bodies exist?
• Function of gas vacuoles?

Eukaryotic Cells

Structure and Components

• Eukaryotes include protists, plants, animals, and fungi.
• Cells have a well-organized nucleus with nuclear envelope.
• Contain several membrane-bound organelles.
• Genetic material is organized into chromosomes and chromatin.

• Cell wall: outer layer providing shape and protection.
• Plasma membrane: controls entry and exit of substances.
• Nucleus: contains genetic material, controls activities.
• Nuclear envelope: protects nucleus.
• Nucleolus: site of ribosome synthesis.
• Cytoplasm: jelly-like substance housing organelles.
• Chloroplasts: site of photosynthesis.
• Mitochondrion: produces cellular energy.
• Vacuole: stores water and nutrients.
• Golgi apparatus: packages and transports proteins.
• Ribosomes: protein synthesis.
• Endoplasmic reticulum (rough and smooth): protein and lipid synthesis and transport.
• Lysosomes: waste breakdown.
• Peroxisomes: detoxify harmful substances.
• Microtubules: maintain cell shape.
• Plasmodesmata: channels for cell communication.
• Middle lamella: glues plant cells together.

Memory Aids

• “Nucleus controls cell activities.”
• “Chloroplasts make food via photosynthesis.”
• “Mitochondria are the cell’s powerhouses.”
• “Golgi apparatus packages proteins.”

Quick Checkpoints

• What organelle controls cell activities?
• Where does photosynthesis occur?
• Which organelle produces energy?
• What is the function of Golgi apparatus?

Cell Membrane

Structure and Composition

• Composed mainly of phospholipids and proteins; also contains cholesterol.
• Lipids arranged with polar heads outward and hydrophobic tails inward.
• Contains lipids (20-79%), proteins (20-70%), carbohydrates (1-5%), and water (20%).
• Proteins classified as integral (embedded) or 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 without energy.
• Active transport: energy-dependent movement against concentration gradient (e.g., Na⁺/K⁺ pump).

Memory Aids

• “Fluid mosaic: proteins float in lipid sea.”
• “Integral proteins embedded; peripheral on surface.”
• “Passive transport needs no energy; active transport does.”

Quick Checkpoints

• What are the main components of the cell membrane?
• Define fluid mosaic model.
• Difference between passive and active transport?
• Role of cholesterol in membrane?

Cell Wall

Structure and Composition

• Non-living, rigid, protective outer covering in plants, fungi, some protists.
• Algal walls: cellulose, galactans, mannans, calcium carbonate.
• Plant walls: cellulose, hemicellulose, pectins, proteins.
• Three parts: primary wall (young cells), middle lamella (calcium pectate, glues cells), secondary wall (mature cells).
• Plasmodesmata connect cytoplasm of adjacent cells.

Functions

• Provides shape and rigidity.
• Protects protoplasm from injury and macromolecules.
• Prevents bursting by counteracting osmotic pressure.

Memory Aids

• “Microfibrils are structural elements.”
• “Middle lamella dissolves during fruit ripening.”
• “Plasmodesmata discovered by Strasburger.”

Quick Checkpoints

• What are the three parts of the cell wall?
• Function of middle lamella?
• Role of plasmodesmata?

Endomembrane System

Components and Functions

• Includes endoplasmic reticulum, Golgi complex, lysosomes, vacuoles.
• Nuclear envelope lumen continuous with ER lumen.
• Functions in processing, sorting, and recycling proteins.

Memory Aids

• “Endomembrane system processes and sorts proteins.”

Quick Checkpoints

• Which organelles form the endomembrane system?
• How is nuclear envelope related to ER?

Endoplasmic Reticulum

Structure and Types

• Network of tubular structures dividing cytoplasm into luminal (inside ER) and extra luminal (cytoplasm) compartments.
• Composed of cisternae (protein synthesis), tubules (lipid synthesis), and vesicles.
• Two types: Rough ER (RER) with ribosomes, Smooth ER (SER) without ribosomes.

Functions

• RER: protein synthesis and secretion.
• RER: glycosylation and lysosome enzyme precursor formation.
• SER: lipid and steroid hormone synthesis.
• SER: detoxification of drugs and xenobiotics.

Memory Aids

• “RER has ribosomes; SER does not.”
• “SER detoxifies and synthesizes lipids.”

Quick Checkpoints

• Difference between RER and SER?
• Role of ER in protein synthesis?
• Function of SER in animal cells?

Golgi Apparatus

Structure and Occurrence

• Discovered by Camillo Golgi in 1898.
• Present in all eukaryotic cells except male gametes of bryophytes, mammalian RBCs, sieve tubes, fungi cells.
• Animal cells: single or connected complex; plant cells: unconnected dictyosomes.
• Composed of cisternae, vesicles, tubules, and vacuoles.
• Cisternae are flat sacs stacked parallel with distinct cis (forming) and trans (maturing) faces.

Functions

• Packages and stores materials from ER.
• Modifies proteins before release.
• Forms acrosomes during spermatogenesis.
• Glycosylates proteins and lipids for cell recognition and adhesion.

Memory Aids

• “Golgi acts like a post office of the cell.”
• “Cis face receives; trans face ships.”

Quick Checkpoints

• What are the parts of Golgi apparatus?
• Function of cis and trans faces?
• Role in protein modification?

Lysosomes

Structure and Composition

• Discovered by Christian de Duve in 1955.
• Single membrane-bound vesicles containing hydrolytic enzymes (lipases, proteases, carbohydrases).
• Formed by Golgi apparatus packaging.
• Known as suicidal bags due to digestive enzymes.
• Present in all animal cells except RBCs; vacuoles perform similar function in plants and fungi.

Types

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

Memory Aids

• “Lysosomes digest waste and old organelles.”
• “Called suicidal bags due to enzymes.”

Quick Checkpoints

• What enzymes do lysosomes contain?
• Why are lysosomes called suicidal bags?
• Types of lysosomes?

Vacuoles

Structure and Types

• Membrane-bound spaces in cytoplasm containing water, sap, excretory products.
• Bounded by tonoplast membrane.
• Plant vacuoles 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.
• Food vacuoles formed by engulfing food in protists.
• Tonoplast controls ion passage, maintains osmotic pressure and turgidity.

Memory Aids

• “Tonoplast surrounds vacuole.”
• “Contractile vacuole controls water balance in protists.”

Quick Checkpoints

• What is tonoplast?
• Functions of vacuoles?
• Types of vacuoles?

Mitochondria

Discovery and Occurrence

• Observed by Kolliker (1857); named by Benda (1898).
• Present in aerobic eukaryotes; absent in prokaryotes and mature RBCs.
• Known as powerhouses of the cell for energy production.

Structure

• Cylindrical, double-membraned organelles (1-4.1 μm length, 0.2-1 μm diameter).
• Outer membrane: smooth, continuous boundary.
• Inner membrane: folded into cristae to increase surface area.
• Matrix: dense fluid inside inner membrane containing circular DNA, RNA, 70S ribosomes, enzymes.
• Inner membrane and cristae have elementary particles (F₀-F₁ particles) with electron transport enzymes.
• Divide by fission.

Memory Aids

• “Cristae increase surface area for respiration.”
• “Mitochondria have their own DNA and ribosomes.”
• “Divide by fission like bacteria.”

Quick Checkpoints

• Describe mitochondrial structure.
• Function of cristae?
• Why are mitochondria semi-autonomous?

Plastids

Types and Functions

• Found in plant cells and euglenoids; contain pigments.
• Three types based on pigments: leucoplasts (colorless, store food), chloroplasts (green, photosynthesis), chromoplasts (yellow/red, pigment storage).
• Leucoplasts: amyloplasts (starch), elaioplasts (fat/oil), aleuroplasts (protein).
• Chloroplasts: lens-shaped, double membrane, contain stroma and thylakoids arranged in grana.
• Chromoplasts: carotenoid pigments, color flowers/fruits, aid pollination and seed dispersal.
• Chloroplasts have DNA, 70S ribosomes, and enzymes; semi-autonomous.

Memory Aids

• “Chloroplasts for photosynthesis.”
• “Leucoplasts store food.”
• “Chromoplasts give color to plants.”

Quick Checkpoints

• Types of plastids and their functions?
• Structure of chloroplast?
• Why are chloroplasts semi-autonomous?

Ribosomes

Structure and Function

• Discovered by George Palade (1953).
• Composed of RNA and proteins; no membrane.
• Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic are 70S (50S + 30S).
• Sites of protein synthesis; act as protein factories.

Memory Aids

• “80S in eukaryotes, 70S in prokaryotes.”
• “Two subunits form functional ribosome.”

Quick Checkpoints

• What are ribosomes made of?
• Difference between eukaryotic and prokaryotic ribosomes?
• Function of ribosomes?

Cytoskeleton

Structure and Types

• Network of protein filaments in cytoplasm.
• Maintains cell shape, organelle distribution, intracellular transport, and movement.
• Types: microfilaments, microtubules, intermediate filaments.

Memory Aids

• “Cytoskeleton supports and moves the cell.”
• “Three filament types for structure and transport.”

Quick Checkpoints

• What are the three types of cytoskeletal elements?
• Functions of cytoskeleton?

Cilia and Flagella

Structure and Function

• Hair-like outgrowths of plasma membrane.
• Cilia: short, oar-like movement of cell or fluid.
• Flagella: longer, responsible for cell movement.
• Prokaryotic flagella structurally different from eukaryotic.
• Core (axoneme) has 9+2 microtubule arrangement: nine pairs around two central microtubules.
• Connected by radial spokes and interdoublet linkers; arise from basal bodies.

Memory Aids

• “9+2 arrangement in axoneme.”
• “Cilia short, flagella long.”
• “Basal bodies anchor cilia and flagella.”

Quick Checkpoints

• Describe axoneme structure.
• Difference between cilia and flagella?
• Origin of basal bodies?

Centrosome and Centrioles

Structure and Occurrence

• Centrosome contains two centrioles arranged perpendicularly.
• Centrioles present in animal cells and motile plant cells (e.g., zoospores, sperm cells).
• Centrioles have nine triplet microtubules arranged in a cartwheel pattern.
• Central hub connected to triplets by radial spokes.
• Centrioles form basal bodies of cilia/flagella and spindle fibers during cell division.

Memory Aids

• “Centrioles have 9 triplet microtubules.”
• “Form basal bodies and spindle fibers.”

Quick Checkpoints

• Structure of centrioles?
• Function of centrosome?
• Where are centrioles found?

Nucleus

Structure and Components

• Discovered by Robert Brown (1831).
• Largest organelle with double membrane.
• Contains chromatin, nucleoplasm, nucleoli.
• Nuclear envelope has pores for RNA and protein transport.
• Outer membrane continuous with ER; bears ribosomes.
• Usually one nucleus per cell; some binucleate or multinucleate cells exist.
• Chromatin: DNA + histones + non-histone proteins + RNA.
• Chromosomes have centromere and kinetochores.

Chromosome Types

• Telocentric: centromere at terminal end.
• Acrocentric: centromere near end.
• Sub-metacentric: centromere slightly off center.
• Metacentric: centromere in middle.
• Some chromosomes have satellites (secondary constrictions).

Memory Aids

• “Chromatin: euchromatin (active), heterochromatin (inactive).”
• “Nucleolus synthesizes rRNA.”
• “Nuclear pores allow molecule exchange.”

Quick Checkpoints

• What is chromatin made of?
• Types of chromosomes based on centromere?
• Function of nucleolus?

Microbodies

Structure and Function

• Membrane-bound vesicles containing enzymes.
• Present in both plant and animal cells.

Memory Aids

• “Microbodies contain various enzymes.”

Quick Checkpoints

• What are microbodies?
• Where are microbodies found?

Plant Cell v/s Animal Cell

Differences

• Plant cells have cell wall, plastids, large central vacuole; absent in animal cells.
• Animal cells have centrioles; absent in most plant cells.

Memory Aids

• “Plant cells: wall, plastids, big vacuole.”
• “Animal cells: centrioles present.”

Quick Checkpoints

• Key structural differences between plant and animal cells?
• Functions of plastids and vacuoles in plants?
• Role of centrioles in animal cells?