Unit 1: An overview of cells

I. Orientation — the cell as the basic unit of life

Cell biology is founded on cell theory, developed through microscopy and refined during the nineteenth century. It states that living organisms are composed of cells, that the cell is the basic unit of life, and that new cells arise from pre-existing cells. Modern biology extends this view by recognizing genetic continuity, energy transformation, and evolutionary relationships among cells.

• Structural basis: Every organism consists of one or more cells; a bacterium has one cell, whereas a human body contains approximately (3.7 \times 10^{13}) cells.
• Functional basis: Cells perform metabolism, growth, response, reproduction, and regulation.
• Genetic continuity: Cellular information is stored mainly in DNA and passed to daughter cells during division.
• Bounded organization: A plasma membrane separates the cell from its environment and controls exchange.
• Common chemistry: Cells use water, proteins, lipids, carbohydrates, nucleic acids, ATP, and similar biochemical reactions.
• Evolutionary principle: Differences between cells reflect adaptation, ancestry, gene expression, and environmental conditions.

II. Discovery of Cell — from microscopy to cell theory

The discovery of cells depended on the invention and improvement of microscopes. Early observations revealed compartments, while later work established that cells are universal units of living matter.

A. Discovery of Cell

The term “cell” originated from Robert Hooke’s observations of cork in 1665, while later microscopists observed living cells and microorganisms.

• Robert Hooke: In Micrographia (1665), Hooke described box-like spaces in cork and called them “cells” because they resembled small rooms; the structures were mostly dead cell walls.
• Antonie van Leeuwenhoek: From the 1670s, improved single-lens microscopes allowed him to observe living “animalcules,” including bacteria, protozoa, sperm cells, and red blood cells.
• Plant observations: Matthias Schleiden proposed in 1838 that plants are composed of cells.
• Animal observations: Theodor Schwann concluded in 1839 that animals are also cellular.
• Cell theory: Rudolf Virchow stated in 1855 that new cells arise from pre-existing cells, rejecting spontaneous generation at the cellular level.
• Modern significance: Electron microscopy, fluorescence microscopy, and molecular imaging later revealed organelles and macromolecular structures invisible to light microscopy.

B. Significance and limitations

The historical development of cell theory explains both the unity of life and exceptions requiring careful interpretation.

• Scientific significance: Cell theory links anatomy, physiology, genetics, and evolution through a common structural unit.
• Non-cellular genetic material: Viruses contain genetic material but lack independent cellular metabolism, so they are not generally classified as cells.
• Coenocytic organisms: Some fungi and algae contain many nuclei within a continuous cytoplasm rather than separate cellular compartments.
• Modern refinement: Mitochondria and chloroplasts possess their own DNA, supporting their evolutionary origin from bacterial ancestors.

III. Basic Properties of Cell — organized systems of life

A cell is the smallest unit capable of carrying out the essential activities associated with life. Its properties arise from coordinated molecular systems rather than from any single component.

A. Basic Properties of Cell

Cells maintain internal order while exchanging matter and energy with their surroundings.

• Plasma membrane: A selectively permeable lipid bilayer controls ions, nutrients, wastes, and signals; phospholipids create a hydrophobic barrier.
• Metabolism: Enzymes catalyze reactions such as glycolysis, which converts glucose into pyruvate and produces ATP.
• Homeostasis: Feedback systems stabilize conditions such as pH, ion concentration, temperature, and water balance.
• Growth: Cells increase in mass by synthesizing proteins, membranes, nucleic acids, and other components.
• Response: Receptors detect external signals; for example, insulin receptors influence glucose uptake.
• Reproduction: Cells divide by binary fission in bacteria or mitosis in eukaryotic somatic cells.
• Information storage: DNA sequences encode proteins and functional RNAs; gene expression converts information into cellular activity.
• Compartmentalization: Organelles and membrane domains separate incompatible reactions and increase efficiency.
• Self-repair: Damaged molecules are degraded or repaired, such as DNA repair enzymes correcting some replication errors.

IV. Unity and diversity of Cells — common principles and specialized forms

All cells share fundamental molecular features, but their structures differ according to ancestry, environment, and function. This combination of similarity and variation is a central theme of cell biology.

A. Unity and diversity of Cells

Cellular unity reflects common ancestry, whereas diversity reflects evolutionary specialization.

• Shared features: Almost all cells contain a plasma membrane, cytoplasm, ribosomes, DNA, and a mechanism for ATP production.
• Molecular unity: The genetic code is nearly universal; codons specify the same amino acids in bacteria, plants, and animals.
• Structural diversity: Cell diameters range from roughly (0.1\,\mu\text{m}) in some bacteria to many centimeters in certain eggs.
• Functional specialization: Neurons extend long processes for signaling, erythrocytes carry oxygen, and root-hair cells increase absorptive surface area.
• Environmental adaptation: Halophilic archaea tolerate high salt, while thermophiles possess proteins stable at high temperatures.
• Organismal levels: Unicellular organisms perform all life functions in one cell; multicellular organisms divide labor among specialized cells.
• Evolutionary evidence: Similar ribosomes, ATP-producing pathways, and membrane chemistry indicate deep relationships among cellular lineages.

V. Cell under the microscope — methods for visualizing cells

Microscopy enlarges and resolves structures that cannot be seen with the unaided eye. The method selected determines whether the observer emphasizes living behavior, surface form, internal ultrastructure, or specific molecules.

A. Cell under the microscope

Microscope performance depends chiefly on magnification, resolution, contrast, and specimen preparation.

• Magnification: Enlargement increases apparent size; a (10\times) eyepiece and (40\times) objective produce (400\times) total magnification.
• Resolution: Resolution is the ability to distinguish two close points; visible-light microscopes resolve approximately (200\text{ nm}), whereas transmission electron microscopes can approach (0.1\text{ nm}) under ideal conditions.
• Bright-field microscopy: Stained or naturally pigmented specimens appear against a bright background; fixed cells are commonly used.
• Phase-contrast microscopy: Differences in refractive index become brightness differences, allowing observation of living, unstained cells.
• Fluorescence microscopy: Fluorophores emit light after excitation; antibodies tagged with fluorescein can locate a chosen protein.
• Confocal microscopy: A focused laser and pinhole reject out-of-focus light, enabling optical sections and three-dimensional reconstructions.
• Electron microscopy: TEM reveals internal ultrastructure in thin sections; SEM produces detailed surface images.
• Preparation limitation: Fixation, dehydration, staining, and vacuum conditions may distort living structures or eliminate dynamic behavior.

VI. Prokaryotic and eukaryotic cells — two major cellular organizations

Prokaryotic and eukaryotic cells share core biological processes but differ in internal organization. The distinction is based primarily on the presence or absence of a membrane-bound nucleus and other organelles.

A. Prokaryotic and eukaryotic cells

Prokaryotes include Bacteria and Archaea; eukaryotes include protists, fungi, plants, and animals.

• Nucleus:
  1. Prokaryotic cells: DNA lies in a nucleoid region without a surrounding nuclear membrane; a typical bacterial chromosome is circular.
  2. Eukaryotic cells: DNA is enclosed in a nucleus and organized into multiple linear chromosomes associated with histone proteins.
• Organelles:
  1. Prokaryotic cells: Lack membrane-bound organelles but contain ribosomes, usually (70S).
  2. Eukaryotic cells: Contain organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus; cytosolic ribosomes are usually (80S).
• Size: Bacteria commonly measure (0.5–5\,\mu\text{m}); eukaryotic cells commonly measure (10–100\,\mu\text{m}).
• Cell division: Prokaryotes usually divide by binary fission; eukaryotes use mitosis or meiosis involving spindle fibers.
• Cell walls: Bacterial walls commonly contain peptidoglycan; archaeal walls lack bacterial peptidoglycan; plant walls contain cellulose.
• Surface structures: Bacteria may possess capsules, pili, or flagella; eukaryotic cilia and flagella contain a microtubule-based (9+2) arrangement.
• Endosymbiotic evidence: Mitochondria and chloroplasts have circular DNA, bacterial-type ribosomes, and double membranes.

VII. Cell Architecture: Animal cell and plant cell — compartmental organization

Cell architecture describes how membranes, organelles, cytoskeletons, and extracellular structures are arranged to support cellular functions. Plant and animal cells share eukaryotic organization but have distinct structural adaptations.

A. Cell Architecture: Animal cell and plant cell

Animal and plant cells contain a nucleus and endomembrane system, but their external structures and energy-related organelles differ.

• Animal cell:
  • Nucleus: Stores chromosomes and contains the nucleolus, where ribosomal subunits begin assembly.
  • Mitochondria: Produce ATP through oxidative phosphorylation; inner-membrane folds are called cristae.
  • Endoplasmic reticulum: Rough ER synthesizes secreted and membrane proteins; smooth ER supports lipid synthesis and detoxification.
  • Golgi apparatus: Modifies, sorts, and packages proteins into vesicles.
  • Lysosomes: Acidic compartments contain hydrolytic enzymes for intracellular digestion.
  • Cytoskeleton: Actin, intermediate filaments, and microtubules support shape, transport, and movement.
• Plant cell:
  • Cell wall: A cellulose-rich wall resists osmotic swelling and provides mechanical support.
  • Chloroplasts: Carry out photosynthesis; thylakoid membranes contain chlorophyll and are stacked into grana.
  • Central vacuole: Stores ions, pigments, metabolites, and water; its pressure contributes to turgor.
  • Plasmodesmata: Channels through cell walls connect neighboring plant-cell cytoplasms.
  • Shared structures: Plant cells also contain nuclei, mitochondria, ER, Golgi apparatus, ribosomes, and cytoskeletons.

VIII. Cell Size and components — dimensions, scaling, and essential parts

Cell size reflects a balance between the need for sufficient internal machinery and the efficiency of exchange with the environment. Components are organized to maintain metabolism, information flow, and structural stability.

A. Cell Size and components

Cell dimensions are limited largely by surface-area-to-volume relationships and by the distance over which materials must move.

• Surface-area-to-volume ratio: As a cell grows, volume increases faster than surface area; a cube of side (1) has ratio (6:1), whereas a cube of side (2) has ratio (24:8=3:1).
• Exchange limitation: Reduced surface-area-to-volume ratio makes nutrient uptake and waste removal less efficient.
• Diffusion distance: Small cells exchange materials rapidly because molecules travel shorter distances; large cells require cytoplasmic streaming or internal transport systems.
• Membrane components: Phospholipids, cholesterol, and proteins form the fluid mosaic membrane.
• Cytoplasmic components: Cytosol contains ions, metabolites, enzymes, and macromolecules; organelles occupy the cytoplasmic space.
• Ribosomes: These molecular machines translate messenger RNA into polypeptides by joining amino acids.
• Cytoskeleton: Microtubules provide tracks for vesicle transport, while actin supports contraction and cell shape.
• Cell junctions: Tight junctions seal epithelial layers, desmosomes provide mechanical attachment, and gap junctions permit animal-cell communication.
• Specialized enlargement: Microvilli increase absorptive surface area; intestinal epithelial microvilli are a concrete example.

IX. Viruses and Viroids — acellular genetic agents

Viruses and viroids are infectious entities that depend on host cells. They illustrate the boundary between biological information and independent cellular life because neither possesses the complete machinery for autonomous metabolism.

A. Viruses and Viroids

Viruses are nucleic acid genomes enclosed in protein, sometimes with a lipid envelope; viroids are much smaller infectious RNA molecules found primarily in plants.

• Viral genome: A virus may contain DNA or RNA, but not both as its primary genome; genomes may be single- or double-stranded.
• Capsid: Protein subunits called capsomeres protect viral nucleic acid and help determine particle shape.
• Envelope: Some viruses acquire a lipid envelope from the host membrane and insert viral glycoproteins into it; influenza virus is an enveloped example.
• Replication dependence: Viruses use host ribosomes, ATP-producing systems, and many host enzymes because they lack complete cellular machinery.
• Infection cycle: Attachment is followed by entry, uncoating, genome replication, protein synthesis, assembly, and release.
• Host specificity: Viral surface proteins recognize particular receptors, restricting infection to susceptible cell types.
• Viroid structure: A viroid consists of a short, circular, single-stranded RNA molecule without a capsid or protein-coding region.
• Viroid disease: Viroid RNA can disrupt plant gene regulation and development; potato spindle tuber disease is a recognized example.
• Biological status: Viruses and viroids are not cells because they lack independent metabolism, ribosomes, and cellular organization, although they evolve and reproduce through host-dependent replication.