BIOL 1017: Cell Biology - Lecture 1: The Cell Theory

Characteristics of Living Organisms

• Living organisms must be capable of carrying out seven essential functions of life simultaneously and independently:

  • Feeding

  • Energy transduction

  • Growth

  • Reproduction

  • Excretion

  • Irritability

  • Locomotion

• Core characteristics present across all living organisms include:

  • Homeostasis

  • Organization

  • Metabolism

  • Growth

  • Adaptation

  • Response to stimuli

  • Reproduction

Life, Energy Balance, and the Environment

• Entropy represents the natural tendency toward disorder present within the physical world.

• Enclosing biochemical reactions inside a cell was a pivotal event in the origin and ongoing maintenance of life:

  • Concentrates biochemical reactions within a defined spatial and temporal volume.

  • Separates internal reactions from the chaotic external environment.

  • Establishes an ordered internal environment within a broader physical system governed by entropy.

  • Requires continuous, controlled exchange of information and materials with the surrounding environment.

• Systems Framework of Living Systems:

  • Life exists in constant disequilibrium with its physical environment.

  • A cellular system is defined by a distinct Boundary separating the internal System from its external Surroundings.

  • System Operational Components:

  • Environment: All external elements outside the system holding the potential to influence all or part of the system.

  • Input: Resources received or actively taken in from the external environment.

  • Throughput: The internal processes of conversion or transformation of acquired resources within the system.

  • Output: The work or processed materials of the system exported back into the environment.

  • Feedback: Continuous flow of information regarding system performance relative to the external environment, utilized to execute vital adaptations for growth and survival.

• Thermodynamic Equilibrium vs. Cellular Integrity:

  • Time elapses under spontaneous reactions leading toward maximum entropy and disorder in the environment.

  • Preserving cell integrity and order requires organized effort powered by continuous energy input.

The Cell Theory

• The Cell Theory was formulated in the 19th century and serves as a fundamental cornerstone of biological science.

• Three Critical Tenets of Cell Theory:

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

  • The cell is the basic structural and functional unit of all living organisms.

  • All cells arise exclusively from the division of pre-existing cells, establishing that spontaneous generation of life does not occur.

The Cell as the Basic Unit of Life

• Primary Biological Role:

  • The cell is the fundamental vehicle carrying hereditary information that defines a species.

  • Specified by genetic instructions, the cell contains complete molecular machinery to process information and self-perpetuate; no entity smaller than a cell possesses this independent capacity.

  • Metabolism and tissue development in complex organisms are direct results of organized cellular activity.

  • Cells represent the closest entity to an autonomous biological unit.

• Universal Properties Shared by All Cells:

  • All cells store hereditary information in the same linear chemical code: Deoxyribonucleic Acid (DNA).

  • All cells replicate hereditary information using templated polymerization.

  • All cells transcribe portions of hereditary information into an intermediary molecule: Ribonucleic Acid (RNA).

  • All cells translate RNA into functional proteins using the exact same mechanisms.

  • A defined fragment of genetic information corresponding to a single protein is designated as a gene.

  • All cells utilize proteins as enzymatic catalysts.

  • All cells function as complex biochemical factories dealing with identical basic molecular building blocks, requiring free energy input.

  • All cells are encapsulated by a plasma membrane controlling the passage of nutrients and waste products.

Cell Function and Energy Requirements

• Operational Optimization:

  • To operate as a self-perpetuating system, cells manage two essential flows:

  • Flow of energy

  • Flow of information

• Energy Dynamics and ATP:

  • Cell integrity depends on an uninterrupted supply of free energy.

  • Environmental energy is accessed directly or indirectly via autotrophic processes.

  • Energy is stored and utilized within the cell in the form of Adenosine Triphosphate (ATP).

  • Systemic energy requirements:

  • Mechanisms for energy absorption/input from surroundings.

  • Intracellular energy transducers capable of converting energy to perform biological work (e.g., mitochondria and chloroplasts).

  • Mechanisms to actively resist entropic breakdown.

  • Adaptive strategies to compete with other living organisms.

• Five Major Types of Cellular Work:

  • Chemical Work: Synthesis of cellular macromolecules and structural components.

  • Osmotic Work: Maintenance of steep chemical and concentration gradients across internal and external membranes.

  • Electrical Work: Generation and maintenance of membrane potentials and ion distribution.

  • Mechanical Work: Physical movement and locomotion (e.g., movement of cilia, flagella, and internal transport).

  • Regulatory Work: Precise regulation of macromolecular synthesis, degradation, and molecular interactions.

• Energy Cycling Flow in Biological Systems:

  • Autotrophic inputs: Plant autotrophs capture light, $H_2O$, $CO_2$, and $NH_3$.

  • Heterotrophic inputs: Animal heterotrophs consume organic food; microbial heterotrophs utilize sugars, fatty acids, and amino acids.

  • Macromolecules (polysaccharides, fats, proteins) are broken down or synthesized via enzyme activity.

  • Cellular respiratory metabolism extracts energy to generate ATP from $\text{ADP} + \text{phosphate}$, releasing $CO_2 + H_2O$.

  • ATP hydrolysis drives chemical, regulatory, osmotic, electrical, and mechanical cellular work.

Information Relations and the Central Dogma

• The Central Dogma of Molecular Biology defines the directional flow of genetic information:

  • Hereditary information is encoded within linear sequences of nucleotides in DNA.

  • Replication: DNA directs its own synthesis via templated polymerization.

  • Transcription: Information in DNA is transcribed into intermediary RNA molecules (mRNA, tRNA, rRNA).

  • Translation: Ribosomes decode RNA nucleotide sequences into specific amino acid sequences, synthesizing proteins.

  • A discrete DNA fragment coding for a functional protein is defined as a gene.

• Intracellular Networks and Feedback Controls:

  • DNA transcription generates Messenger RNA (mRNA), Transfer RNA (tRNA), and Ribosomal RNA (rRNA).

  • Ribosomal RNA and ribosomal proteins undergo assembly into functional ribosomes.

  • Polypeptide chains synthesized during translation fold into native protein conformations.

  • Synthesized proteins fulfill diverse intracellular roles:

  • Assembly of multi-chain complexes and higher-order multimolecular structures.

  • Enzymatic catalysis: Enzymes catalyze metabolic steps (e.g., converting Compound A to Compound B via Enzyme 1, and Compound B to Compound C via Enzyme 2).

  • Metabolic feedback inhibition: Downstream products regulate upstream enzymatic activity.

  • Transcriptional regulation: End products execute feedback repression directly on DNA transcription.

Factors Governing Cell Size and Surface Area to Volume Ratio

• Scale of Cellular Dimensions:

  • Most cells range in diameter from $1\,\mu m$ to $100\,\mu m$.

  • Lower Size Limit: Imposed by the minimal structural volume required to house essential molecular machinery (DNA, ribosomes, enzymes) necessary to maintain life.

  • Upper Size Limit: Constrained by four physiological factors:

  • Nucleus-to-cytoplasm ratio.

  • Metabolic activity level of the cell.

  • Surface Area-to-Volume ($SA:V$) ratio.

  • Overall cell shape.

• Geometry of Surface Area-to-Volume Ratio ($SA:V$):

  • As a cell increases in volume, its surface area increases at a proportionally slower rate, causing the $SA:V$ ratio to decline.

  • Mathematical Comparison using Cubes:

  • $1\,mm$ Cube:

    • Surface Area: $6 \times (1\,mm)^2 = 6\,mm^2$

    • Volume: $(1\,mm)^3 = 1\,mm^3$

    • $SA:V$ Ratio: $6:1$

  • $2\,mm$ Cube:

    • Surface Area: $6 \times (2\,mm)^2 = 24\,mm^2$

    • Volume: $(2\,mm)^3 = 8\,mm^3$

    • $SA:V$ Ratio: $3:1$

  • $4\,mm$ Cube:

    • Surface Area: $6 \times (4\,mm)^2 = 96\,mm^2$

    • Volume: $(4\,mm)^3 = 64\,mm^3$

    • $SA:V$ Ratio: $1.5:1$

• Biological Significance of $SA:V$ Constraints:

  • Internal volume dictates the rate of cellular metabolism and heat/waste generation.

  • Surface area determines exchange rates across the plasma membrane:

  • Raw material uptake (via passive diffusion or active transport).

  • Elimination of waste products (via excretion) and release of functional compounds (via secretion).

Non-Cellular Entities and Diversity of Life

• Viruses: Non-Cellular Entities

  • Viruses are non-cellular infective particles and are not classified as cells.

  • Structure:

  • Composed of nucleic acid (DNA or RNA) enclosed in a protein capsid.

  • Consist of distinct structural components including a head (capsule), sheath/tail, end plate, and tail fibers.

  • Completely lack plasma membranes, cytoplasm, ribosomes, and protoplasm.

  • Function:

  • Inert outside host cells; exhibit biological activity and replication exclusively after infecting a living host cell.

• Organization across Life Forms:

  • Unicellular Organisms: Single-celled entities capable of independent existence (e.g., Bacteria, Paramecium).

  • Multicellular Organisms: Complex organisms consisting of specialized cellular tissues (e.g., Flowering Plants, Mammals such as Dolphins).

• Spectrum of Cell Types in Nature:

  • Filamentous fungal cells

  • Treponema bacteria

  • Human blood components: Red blood cells, platelets, white blood cells

  • Radiolarians

  • Protozoans (e.g., Stentor)

  • Human reproductive gametes: Egg and sperm cells

  • Intestinal epithelial cells

  • Plant vascular cells (xylem)

  • Retinal neurons

Microscopy Principles and Modalities

• Role of Microscopy in Cell Biology:

  • Microscopes are optical instruments designed to view objects below the resolution threshold of the human eye.

  • Essential Performance Parameters:

  • Magnification: The degree to which the visual size of an object is enlarged relative to its actual size.

  • Resolution (Resolving Power): The minimum distance by which two point sources can be separated and still be distinguished as distinct entities.

• Resolving Power Thresholds:

  • Unaided Human Eye: $200\,\mu m$ ($0.2\,mm$).

  • Light Microscope: $0.2\,\mu m$ ($200\,nm$); allows observation of overall cell morphology, shape, and large organelles.

  • Electron Microscope: $2\,nm$ ($0.002\,\mu m$); allows resolution of fine ultrastructural details (ribosomes, membranes, macromolecular assemblies).

• Light Microscopy Modalities:

  • Bright-field Microscopy: Unaltered light passes directly through the specimen. Provides poor contrast in unstained biological samples unless natural pigments exist.

  • Stained Bright-field Microscopy: Uses selective dyes to enhance contrast and highlight specific cellular components.

  • Dark-field Microscopy: An opaque disc blocks central light in the condenser, directing light onto the specimen at oblique angles. Scattered light enters the objective lens, producing a bright image against a dark background. Ideal for viewing minute aquatic organisms (e.g., diatoms), protozoa, bone/hair fibers, small insects, and micropropagation cultures.

  • Fluorescence Microscopy: Specimen fluorophores absorb short-wavelength light from lamps (mercury/xenon) or lasers and re-emit longer-wavelength light across UV, visible, or IR spectra. Dyes can be selectively targeted to specific cellular proteins or nucleic acids.

  • Phase-contrast Microscopy: Amplifies subtle differences in refractive index (light-bending capacity) across cellular components, translating phase shifts into intensity variations to visualize living, unstained cells.

• Common Microscopic Stains and Specific Uses:

  • Iodine: Stains starch granules blue.

  • Haemotoxylin: Stains cell nuclei violet-brown.

  • Eosin: Stains cytoplasm and cellular membranes red-pink.

  • Crystal Violet: Stains peptidoglycan cell walls of Gram-positive bacteria dark purple.

  • Phloroglucinol-HCl: Stains lignified sclerenchyma plant cell walls red.

• Comparative Analysis: Light Microscopy vs. Electron Microscopy:

  • Source of Illumination: Light rays with longer wavelengths ($\lambda$) vs. Electron beams with significantly shorter wavelengths ($\lambda$).

  • Image Appearance: Colored images vs. Monochromatic (black and white) images.

  • Specimen Preparation: Rapid preparation (minutes to hours) vs. Extensive preparation (typically several days).

  • Specimen Viability: Live or fixed/dead specimens vs. Exclusively dead specimens (due to high vacuum).

  • Lens Construction: Glass optical lenses vs. Electromagnetic lenses.

  • Resolving Power: Lower resolving power vs. Approximately $250\times$ higher resolving power.

  • Magnification Range: $500\times - 1500\times$ vs. $100,000\times - 300,000\times$.

  • Electrical Requirements: Standard electrical supply vs. High-voltage power supply required.

  • Observation Method: Direct viewing through ocular eyepieces vs. Indirect viewing on a fluorescent screen or digital monitor.

  • Safety Hazards: Negligible radiation risk vs. Risk of high-voltage radiation leakage.

Characteristics of Living Organisms

• 7 Essential Functions: Feeding, energy transduction, growth, reproduction, excretion, irritability, locomotion.

• Core Characteristics: Homeostasis, organization, metabolism, growth, adaptation, response to stimuli, reproduction.

Life, Energy Balance, and Environment

• Entropy: Physical tendency toward disorder.

• Cellular Compartmentalization: Concentrates biochemical reactions, isolates internal order from environmental entropy, and regulates material/information exchange.

• Systems Framework:

  • Boundary: Separates internal system from surroundings; maintains life in disequilibrium.

  • Components: Environment (external influences), Input (resources), Throughput (transformation), Output (exported work), Feedback (adaptation info).

The Cell as the Basic Unit of Life

• Biological Role: Fundamental vehicle of genetic instructions; smallest autonomous unit capable of self-perpetuation.

• Universal Features:

  • Linear DNA code replicated via templated polymerization.

  • Transcribes DNA to RNA; translates RNA into proteins.

  • Genes code for single proteins; proteins act as enzymatic catalysts.

  • Enclosed by a selective plasma membrane and requires free energy.

Cell Function and Energy Requirements

• Flow Management: Controls flows of energy and information.

• ATP Dynamics: Continuous free energy in the form of ATP is required to resist entropic decay.

• 5 Types of Cellular Work:

  • Chemical: Macromolecule synthesis.

  • Osmotic: Concentration gradient maintenance.

  • Electrical: Membrane potential generation.

  • Mechanical: Movement of cilia, flagella, and internal transport.

  • Regulatory: Macromolecular interaction and expression control.

• Energy Cycling: Autotrophic/heterotrophic inputs yield macromolecules broken down via respiration to generate ATP from $\text{ADP} + \text{phosphate}$.

Non-Cellular Entities

• Viruses: Non-cellular particles containing nucleic acid (DNA or RNA) inside a protein capsid. Lacks membranes, cytoplasm, and ribosomes; active only inside a host cell.