Basic Cell Structure – Lecture Review

Learning Outcomes

• Recall and explain:

  • The historical development and core statements of the Cell Theory.

  • The universal structural features shared by every cell.

  • The identity, structure, and function of each eukaryotic organelle.

  • Similarities and differences between prokaryotic and eukaryotic cells.

  • Real-world implications (e.g., antibiotic resistance, bio-energetics, tissue organisation).

Defining the Cell

• Fundamental unit of life; smallest entity capable of independent metabolism, growth, and reproduction.

• Serves simultaneously as the basic structural and functional unit in all organisms.

Historical Milestones Leading to Cell Theory

• 1665 – Robert Hooke coined the term “cells” while observing cork.

• 1673 – Antonie van Leeuwenhoek visualised living microorganisms in pond water & dental scrapings.

• 1838 – Matthias Schleiden proclaimed that all plants are cellular.

• 1839 – Theodor Schwann generalised the concept to animals; co-founder (with Schleiden) of Cell Theory.

• 1855 – Rudolf Virchow observed cell division, asserting “Omnis cellula e cellula” (all cells arise from pre-existing cells).

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Canonical Statements of Modern Cell Theory

• All living beings are composed of one or more cells.

• The cell is the basic unit of structure and function (i.e., of life).

• New cells originate only from the division of existing cells.

Diversity of Cell Morphology (Illustrative Examples)

• Amoeba proteus (unicellular protozoan; flexible shape).

• Plant stem cells (elongated, cell-wall reinforced).

• Bacteria (e.g., E. coli — prokaryotic rods).

• Neurons (highly polarised with dendrites/axons).

• Human erythrocytes (biconcave discs lacking nuclei).

Size Spectrum of Biological & Non-Biological Entities

Scale (approx.):
 0.1 nm – atom
   1 nm – amino acid
  10 nm – protein, virus capsid
 100 nm – typical virus; light-microscope limit begins
   1 µm – most bacteria; mitochondrion
  10 µm – average animal cell
 100 µm – human egg; thickness of a sheet of paper
   1 mm – frog egg (visible)
>
 Electron microscopes resolve ≲0.1\,\text{nm}, light microscopes down to \sim 200\,\text{nm}, and the unaided eye begins at \sim 100\,\mu\text{m}.

Two Broad Cellular Taxa

• Prokaryotes – bacteria & archaea

  • No membrane-bound nucleus; genome resides in nucleoid.

  • Generally 0.1–5\,\mu\text{m} diameter; structurally simpler, yet metabolically diverse.

• Eukaryotes – protists, fungi, plants, animals

  • Possess nucleus and extensive internal membrane system.

  • Typically 10–100\,\mu\text{m}; compartmentalisation enables specialised functions.

Architectural Blueprint of a Generic Eukaryotic Cell

• Plasma membrane

• Nucleus

• Cytoplasm

  • Cytosol (aqueous matrix)

  • Organelles (endomembrane, energy-related, and cytoskeletal structures)

Plasma (Cell) Membrane

• Phospholipid bilayer interspersed with proteins, cholesterol, and carbohydrates.

• Functions

  • Maintains shape & integrity.

  • Selective permeability – regulates trafficking of ions, nutrients, waste.

  • Facilitates cell signalling & adherence.

  • Preserves homeostasis via feedback mechanisms.

Nucleus

• Diameter \approx 10–25\,\mu\text{m}; double membrane (nuclear envelope) perforated by nuclear pores (controlled exchange with cytoplasm).

• Contains

  • Chromatin (DNA + histones) – diffused in interphase, condenses into chromosomes during division.

  • Nucleoplasm (semi-fluid interior).

  • Nucleolus – dense rRNA hub where ribosomal subunits are assembled; disassembles during mitosis.

Cytoplasm

• Cytosol: \sim 80\% water plus salts, metabolites, enzymes; site of glycolysis and myriad reactions.

• Organelle population drives compartmentalised metabolism.

Endomembrane System – Sequential Flow of Membranes & Cargo

• Nuclear envelope – continuous with rough ER.

• Endoplasmic Reticulum (ER)

  • Rough ER: ribosome-studded; synthesises secretory & membrane proteins → lumenal folding/modification.

  • Smooth ER: lipid biosynthesis, detoxification (cytochrome P450), carbohydrate metabolism; forms transport vesicles.

• Ribosomes

  • rRNA + proteins; two subunits (large + small); free (cytosolic proteins) vs bound (RER-targeted proteins); polyribosomes enhance throughput.

• Golgi Apparatus

  • Stacks of cisternae (\sim4–8) with cis→medial→trans polarity.

  • Glycosylates, trims, phosphorylates, and sorts proteins/lipids into vesicles destined for lysosomes, plasma membrane, or secretion.

• Vesicles

  • Small lipid bilayer sacs; shuttle and store cargo; include transport, secretory, and storage vesicles.

• Lysosomes

  • Golgi-derived; single membrane; pH \approx 5; acid hydrolases digest macromolecules, old organelles (autophagy), pathogens (phagocytosis).

• Peroxisomes

  • Contain oxidases + catalase; execute β-oxidation of very-long-chain fatty acids and detoxify \mathrm{H2O2\;\to\;H2O + O2}.

Energy-Related Organelles

• Mitochondria

  • Double membrane; inner membrane folds (cristae) envelop matrix.

  • Possess circular DNA & 70S-type ribosomes (endosymbiotic origin).

  • Site of aerobic respiration & ATP synthesis via oxidative phosphorylation.

• Chloroplasts (plants & algae)

  • Double membrane + internal thylakoid membranes stacked as grana; chlorophyll absorbs photons → drives photosynthesis (light-dependent ATP & NADPH production; Calvin cycle in stroma).

Cytoskeleton

• Dynamic lattice of protein filaments providing mechanics & motility.

  • Microtubules (tubulin) – tracks for vesicle transport; organise mitotic spindle.

  • Microfilaments (actin) – muscle contraction, cytokinesis, cell crawling.

  • Intermediate filaments (keratin, vimentin, etc.) – tensile strength.

Microtubule-Based Arrays

• Centrosome = microtubule organising centre (MTOC); houses a pair of perpendicular centrioles (9×3 microtubule triplets).

• Cilia & Flagella

  • Axoneme structure 9\,+\,2 microtubule arrangement.

  • Cilia – numerous, short, synchronous “oar-like” motion (e.g., respiratory epithelium clears mucus/dust).

  • Flagella – few, long, whip/propeller motion (e.g., sperm).

Plant-Specific Modifications

• Cell Wall

  • Rigid exoskeleton outside plasma membrane; cellulose + hemicellulose + pectin.

  • Functions: shape, protection, turgor management, pathogen barrier.

• Central Vacuole

  • Large, membrane-bound (tonoplast); stores water, ions, pigments, waste; generates turgor pressure.

  • In animals, vacuoles are small, numerous, or transient; lysosomes often substitute their degradative role.

• Plastids

  • Family including chloroplasts, chromoplasts (pigments), amyloplasts (starch storage).

Prokaryotic Cell Architecture (Using a Bacterial Model)

• External Layers

  • Capsule (glycocalyx) – polysaccharide sheath; anti-phagocytic, environmental adhesion.

  • Cell wall – peptidoglycan lattice; defines Gram + (thick) vs Gram – (thin + outer membrane) phenotype.

  • Plasma membrane – phospholipid bilayer regulating transport & respiration (no mitochondria present).

• Cytoplasmic Region

  • Nucleoid – circular DNA chromosome.

  • Plasmids – accessory genetic elements (≈5$–100 genes) conferring e.g., antibiotic resistance (R-plasmids), virulence, conjugation functions.

  • Ribosomes – 70\,S (30S + 50S); antibiotic target (e.g., tetracycline).

  • Granules – nutrient stores (e.g., poly-β-hydroxybutyrate, glycogen, volutin).

• Surface Appendages

  • Flagella – helical propellers generating motility via proton-driven motor.

  • Fimbriae – short, numerous; adhesion; prominent in Gram – pathogens.

  • Sex (conjugation) pili – longer, mediate plasmid transfer during conjugation (horizontal gene flow).

Comparative Snapshot: Prokaryote vs Eukaryote

• Genome location – nucleoid vs membrane-bound nucleus.

• Chromosome structure – circular ± plasmids vs linear with histones.

• Organelles – absent vs present (endomembrane, mitochondria, etc.).

• Ribosome size – 70\,S vs 80\,S (cytosolic).

• Cell division – binary fission vs mitosis/meiosis.

• Typical size – <5\,\mu\text{m} vs 10–100\,\mu\text{m}.

Practical & Ethical Implications

• Antibiotic resistance emerges via plasmid-encoded genes transferred by sex pili—necessitates prudent antimicrobial stewardship.

• Organelle malfunctions underpin many diseases:

  • Lysosomal storage disorders (e.g., Tay-Sachs).

  • Mitochondrial myopathies.

• Understanding ciliary structure clarifies respiratory pathologies (primary ciliary dyskinesia) and infertility.

• Plant cell wall insights inform biofuel engineering and crop protection.

• Endosymbiotic theory frames evolutionary biology and guides research into artificial organelles.

Foundational Formulae / Numerical References (for memory cues)

• Diameter of human nucleus: \approx 10\text{–}25\,\mu\text{m}.

• pH of lysosome lumen: \sim5.

• Thylakoid charge separation → ATP synthase proton motive force \Delta pH\ \approx 2–3.

• Prokaryotic ribosome sedimentation coefficient: 70\,S = 30\,S + 50\,S (note: S is not additive, reflecting shape & mass).

High-Yield Connections

• Endomembrane continuity couples transcription (nucleus) ⇨ translation (RER) ⇨ modification/sorting (Golgi) ⇨ secretion (vesicles) — critical for antibody or hormone production.

• Cytoskeleton interacts with motor proteins (kinesin/dynein) to traffic vesicles generated by Golgi along microtubules.

• Oxidative reactions in peroxisomes complement mitochondrial β-oxidation; together maintain cellular redox balance.

• Chloroplast and mitochondrial DNA inheritance is typically maternal — leveraged in evolutionary and forensic studies.

The note outlines Cell Theory, defining cells as the fundamental units of life capable of independent metabolism, growth, and reproduction. Key historical figures like Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolf Virchow contributed to its development. The core tenets are: all living beings are composed of cells, the cell is the basic unit of life, and new cells arise only from existing cells.

Cells are broadly categorized into two types:

• Prokaryotes (bacteria, archaea): Generally smaller (0.1–5\,\mu\text{m}), they lack a membrane-bound nucleus (DNA in a nucleoid) and other membrane-bound organelles. They possess 70\,S ribosomes and a peptidoglycan cell wall, often with plasmids.

• Eukaryotes (protists, fungi, plants, animals): Significantly larger (10–100\,\mu\text{m}$$), they feature a true nucleus housing DNA and an extensive internal membrane system of organelles, enabling specialized functions.

Key eukaryotic organelles and their primary functions include:

• Plasma membrane: Regulates transport and signals.

• Nucleus: Contains genetic material (chromatin) and assembles ribosomal subunits.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Conduct intracellular digestion.

• Mitochondria: Generate ATP through aerobic respiration.

• Cytoskeleton: Provides structural support and aids cell movement and transport.

Plant cells have additional unique structures: a rigid Cell Wall (for support and protection), a large Central Vacuole (for storage and turgor pressure), and Chloroplasts (for photosynthesis).

Understanding cellular biology has significant practical implications, such as explaining antibiotic resistance (often linked to plasmid transfer in prokaryotes) and the pathogenesis of diseases resulting from organelle dysfunction (e.g., lysosomal or mitochondrial disorders).