Chapter 2 – Cells & Their Specialised Tissues

Introduction

• All living organisms are composed of cells.
  • Cells = basic structural & functional units of life.
• Universal features of ALL cells
  • Plasma membrane – selective phospholipid barrier.
  • Cytosol – semifluid, jelly-like matrix.
  • Chromosomes – DNA that carries genetic information.
  • Ribosomes – "protein factories".
• Cell sizes
  • Prokaryotes: 0.1\text{–}0.5\,\mu\text{m}.
  • Eukaryotes: 10\text{–}100\,\mu\text{m}.

2.1 Cell Theory

• Two universally accepted postulates
  1. Cells are the fundamental living units of organisation & function in all organisms.
  2. All cells arise only from pre-existing cells (biogenesis).
• Modern implications
  • Explains continuity of life, heredity and evolution.
  • Basis for microscopy, pathology and biotechnology.

2.2 Prokaryotes vs Eukaryotes

Terminology

• Pro-karyote = “before nucleus”. Domains: Bacteria & Archaea.
• Eu-karyote = “true nucleus”. Kingdoms: Plants, Animals, Fungi, Protists.

Prokaryotic Characteristics

• No true nucleus; DNA in nucleoid (single circular chromosome, +/- plasmids).
• No membrane-bound organelles; internal infoldings (mesosomes) or photosynthetic lamellae in cyanobacteria.
• 70 S ribosomes free in cytoplasm.
• Cell wall of peptidoglycan (murein); may have outer gelatinous capsule.
• Shapes of bacteria
  • Cocci (diplococci, streptococci, staphylococci, tetrads, sarcina).
  • Bacilli (single, chains, flagellate rods, spore-formers).
  • Spirals (vibrios, spirilla, spirochaetes).

Eukaryotic Characteristics

• DNA enclosed in double-membranous nucleus.
• Extensive endomembrane-bound organelles (ER, Golgi, mitochondria, chloroplasts, vacuoles, etc.).
• 80 S ribosomes: free + bound to ER/nuclear envelope.
• Cytoskeleton present; larger cell size.

Comparative Summary

• Cell division: prokaryotes → binary fission; eukaryotes → mitosis &/or meiosis.
• Energy conversion: prokaryotes use plasma membrane/mesosomes; eukaryotes use mitochondria (and chloroplasts in photo-autotrophs).
• Photosynthesis: cyanobacteria use lamellae; plants/algae use chloroplasts.
• Cell wall composition: bacteria → peptidoglycan; plants/algae → cellulose; fungi → chitin; animals → none.

Animal vs Plant Cells

2.3 Structure & Function of Organelles

Classification by Function

• Manufacturing/breakdown (endomembrane system).
• Energy conversion (mitochondria, chloroplasts).
• Oxidative detox (peroxisomes).
• Structural support & motility (cytoskeleton, ECM, cell wall).

Nucleus

• Usually spherical, centrally located; stores majority of DNA.
• Nuclear envelope: double membrane, continuous with rough ER; perforated by nuclear pores (pore complex regulates macromolecule traffic).
• Nuclear lamina & matrix: protein scaffolds maintaining shape & organising chromatin.
• Chromatin = DNA + histones; condenses into chromosomes prior to division.
• Nucleolus: non-membranous site of rRNA synthesis & ribosome subunit assembly.

Ribosomes

• rRNA + proteins; 2 subunits.
• Free ribosomes → cytosolic proteins.
• Bound ribosomes (to RER/nuclear envelope) → membrane proteins, secretory proteins, lysosomal enzymes.

Endomembrane System

Components: Nuclear envelope → ER → Golgi → lysosomes, vacuoles, plasma membrane (connected physically or via vesicles).

Endoplasmic Reticulum (ER)

• Continuous with nuclear envelope; network of cisternae.
• Smooth ER (SER)
  • No ribosomes; tubular network.
  • Functions: lipid & steroid synthesis, lipid metabolism, detoxification (liver/kidney), glycogenolysis, absorption & transport of fats, formation of lipoproteins.
• Rough ER (RER)
  • Ribosome-studded cisternae.
  • Newly synthesized polypeptides enter lumen → glycosylated → folded.
  • Packs secretory proteins into transport vesicles; membrane factory (adds phospholipids & membrane proteins).

Golgi Apparatus

• Stacks of flattened cisternae.
  • Cis face (receiving), medial cisternae, trans face (shipping).
• Modifies ER products (glycosylation, phospho-tagging), manufactures polysaccharides, sorts & packages into vesicles.
• Cisternal maturation model: cisternae migrate cis→trans while carrying cargo; retrograde vesicles recycle ER & Golgi enzymes.

Lysosomes

• Acidic (pH≈5) single-membrane sacs containing hydrolytic enzymes.
• Functions
  • Phagocytosis: fuse with food vacuoles, digest macromolecules.
  • Autophagy: recycle damaged organelles; crucial for cellular renewal.
  • Autolysis if membrane ruptures (injury, vitamin A excess, hypoxia).

Vacuoles

• Large vesicles derived from ER & Golgi.

1. Food vacuoles (phagocytosis).
2. Contractile vacuoles – pump excess water in freshwater protists (osmoregulation).
3. Central vacuole (plants) – stores water, ions, pigments, toxins; enclosed by tonoplast; generates turgor pressure.

Summary: Secretory Pathway

1. Translation on RER-bound ribosome.
2. Protein enters ER lumen → folding + glycosylation.
3. Budding of transport vesicle at transitional ER.
4. Vesicle → cis-Golgi → modifications.
5. Trans-Golgi sorts into vesicles → (i) exocytosis \rightarrow plasma membrane; (ii) lysosome formation; (iii) vacuole; (iv) retrieval to ER/Golgi.

Energy-Converting Organelles

Mitochondria

• Double membrane; inner folds into cristae (↑ surface for ATP synthase).
• Compartments: inter-membrane space, matrix (contains DNA, ribosomes, enzymes).
• Site of aerobic respiration; abundant in motile/contractile cells.

Chloroplasts (Plastids)

• Plants & algae only; double membrane + internal thylakoid system.
  • Thylakoids ⇒ stacked grana; lumen inside.
  • Stroma ⇒ enzyme-rich fluid with DNA & ribosomes.
• Photosynthesis converts CO2 + H2O → sugars using light & chlorophyll.
• Larger & disc-shaped compared with mitochondria.

Peroxisomes

• Single-membranous oxidative compartments; numerous in liver/kidney.
• Contain oxidase → O2 accepts H producing H2O2; catalase converts 2H2O2 \rightarrow 2H2O + O_2.
• Beta-oxidation of fatty acids; detoxification of alcohol.

Cytoskeleton

Components

1. Microtubules (tubulin dimers) – 25\,\text{nm} diameter, hollow.
2. Microfilaments (actin) – 7\,\text{nm} diameter.
3. Intermediate filaments (keratins etc.) – 8\text{–}12\,\text{nm}.

General Functions

• Mechanical support, shape maintenance, organelle anchorage.
• Tracks for motor proteins (dynein, kinesin, myosin) → vesicle transport using ATP.
• Involved in cell motility (cilia, flagella, amoeboid movement) & cell division (mitotic spindle).

Microtubule Specialisations

• Centrosome (MTOC) with pair of centrioles (9×3) duplicates before mitosis.
• Basal bodies of cilia/flagella also 9×3.
• Cilia/Flagella: axoneme 9×2 + 2 central microtubules; dynein arms produce bending.

Microfilament Specialisations

• Muscle contraction: actin + myosin sliding filaments.
• Cytoplasmic streaming (plants), cleavage furrow (animal cytokinesis), pseudopodia.

Intermediate Filaments

• More permanent; bear tension; form nuclear lamina; anchor organelles.

Extracellular Structures & Junctions

Plant Cell Wall

• Layers: primary wall → middle lamella (pectin) → secondary wall (lignified).
• Functions: protection, shape, prevents excessive water uptake.
• Plasmodesmata: cytoplasmic channels connecting adjacent cells; symplastic transport of water, ions, RNA, proteins.

Animal Extracellular Matrix (ECM)

• Components: collagen fibres embedded in proteoglycan network; fibronectin links ECM to integrins (trans-membrane proteins) connected to cytoskeleton → signal transduction & adhesion.

Animal Cell Junctions

• Tight junctions – seal membranes eliminating intercellular leakage.
• Desmosomes (anchoring) – rivet cells via keratin filaments; mechanical strength while permitting solute passage.
• Gap junctions – connexon channels for ions & small molecules; electrical/chemical communication.

2.4 Specialised Tissues in Animals

• Tissue = group of similar cells performing a common function.
• Multicellular animals show cellular differentiation forming 4 fundamental tissue types.

1 Epithelial Tissue

• Continuous sheets covering body surfaces & lining cavities; basement membrane attachment.
• Cell shapes: squamous (flat), cuboidal (dice), columnar (tall).
• Arrangements: simple, stratified, pseudostratified.
• Functional examples
  • Simple squamous – diffusion surfaces (alveoli, capillaries).
  • Simple columnar – absorption/secretion (intestinal mucosa).
  • Simple cuboidal – secretion (kidney tubules, glands).
  • Stratified squamous – protection against abrasion (skin, mouth).
  • Pseudostratified ciliated columnar – respiratory tract mucociliary escalator.
• Specialised glands (modified epithelia)
  • Goblet cells (unicellular, mucus).
  • Exocrine glands (ducted: sweat, salivary).
  • Endocrine glands (ductless: pituitary) → hormones to blood.

2 Connective Tissue

• Components: cells + extracellular matrix (ground substance + fibres).
• Types & Features
  1. Loose (areolar) – packing material, cushions organs.
  2. Fibrous (dense regular/irregular) – collagen for tensile strength (tendons, ligaments).
  3. Adipose – fat storage, insulation, energy reserve.
  4. Cartilage – chondrocytes in chondroitin matrix; flexible support; embryonic skeleton.
  5. Bone – osteoblasts deposit collagen + mineral salts (Ca, Mg, PO_4^{3-}).
  6. Blood – plasma matrix with erythrocytes, leukocytes, platelets; functions in transport & immunity.

3 Muscle Tissue

• Actin & myosin allow contraction.
  1. Skeletal – striated, voluntary, multi-nucleated; attached to bones by tendons.
  2. Cardiac – striated, branched, intercalated discs (gap junctions) synchronize heartbeat.
  3. Smooth – non-striated, involuntary; walls of GI tract, bladder, vessels; peristalsis, vasoconstriction.

4 Nervous Tissue

• Excitable neurons transmit electrochemical impulses; glial cells support, nourish & insulate.

2.5 Specialised Tissues in Plants

• Three basic organs: roots, stems, leaves.
• Each organ contains continuous tissue systems: dermal, vascular, ground.

Dermal Tissue System

• Epidermis (non-woody) or periderm (woody) forms protective outer layer.
• Features
  • Cuticle (waxy) limits water loss.
  • Root hairs increase absorptive surface.
  • Guard cells regulate stomatal gas exchange.
  • Trichomes reduce transpiration & reflect light.

Vascular Tissue System

• Embedded within ground tissue; conducts materials & provides support.

Xylem

• Conducts water & dissolved minerals root→shoot.
• Cells: tracheids (all vascular plants) & vessel elements (angiosperms; more efficient); lignified, dead at maturity.

Phloem

• Translocates photosynthates (sugars) source→sink.
• Cells: sieve-tube elements (living, anucleate; sieve plates) + companion cells (nucleated; metabolic support & loading/unloading of sugars).

Ground Tissue System

• Bulk of plant body; functions in metabolism, storage & support.


• Three cell types distinguished by walls
  

• Cell theory underpins medical diagnostics, tissue engineering, antibiotics targeting prokaryotic ribosomes, herbicides acting on plant cell walls, cancer therapies exploiting cell division machinery.
• Understanding organelles informs genetic engineering (protein secretion pathways), metabolic disease (lysosomal storage disorders), crop improvement (xylem efficiency, companion cell transport).
• Tissue knowledge critical to pathology (epithelial cancers), regenerative medicine (stem-cell derived tissues), organ transplants.

Key Numerical / Statistical Data

• Prokaryote size: 0.1\text{–}0.5\,\mu\text{m}.
• Eukaryote size: 10\text{–}100\,\mu\text{m}.
• Flagella/cilia axoneme: 9 + 2 arrangement; basal body: 9 × 3.
• Lysosomal pH: \approx 5 vs cytosolic \approx 7.2.
• Muscle fibre diameter ≈ 10\,\mu\text{m}; length can reach \text{cm} order.

Concept Maps / Connections

• Endomembrane continuity links nucleus → ER → Golgi → plasma membrane.
• Cytoskeleton integrates with ECM via integrins for mechanotransduction.
• Plant plasmodesmata analogous to animal gap junctions for intercellular communication.

Summary

Grasping cell fundamentals—from molecular organelles to specialised tissues—provides the foundation for understanding physiology, pathology, biotechnology and ecology across all domains of life.