Learning Objectives
By the end of this chapter, you should be able to:
- State the three principles of cell theory and explain their significance
- Compare and contrast prokaryotic and eukaryotic cell structure and organization
- Describe the structure and function of major eukaryotic organelles
- Explain how cellular compartmentalization enables specialized functions
- Analyze the endosymbiotic theory and supporting evidence
- Describe methods used to study cellular structure and function
- Apply concepts of surface area to volume ratio to explain cellular adaptations
Introduction
The cell represents the fundamental unit of biological organization. All living organisms are composed of cells, and all cells arise from pre-existing cells. This chapter explores the historical development of cell theory, examines cellular diversity and unity, and investigates how cellular organization enables the complex functions of life. Understanding cellular structure provides the foundation for comprehending how cells transform energy, process information, and maintain the processes of life.
Historical Development of Cell Theory
Early Observations
Robert Hooke (1665): First used the term “cell” to describe compartments in cork observed through a microscope.
- Contribution: Coined the term “cell”
- Limitation: Observed only dead plant cell walls
Anton van Leeuwenhoek (1674-1683): First to observe living cells (bacteria, protozoa, sperm cells)
- Contribution: Described “animalcules” in various substances
- Advancement: Used superior lens grinding techniques
Modern Cell Theory
- All living organisms are composed of one or more cells
- Unicellular organisms: Single cell performs all functions
- Multicellular organisms: Cells specialize and cooperate
- The cell is the basic unit of structure and function in organisms
- Smallest unit that can perform all life processes
- Cells vary in structure based on function
- All cells arise from pre-existing cells through cell division
- No spontaneous generation of cells
- Genetic continuity maintained through replication
Exceptions and Modern Additions
Viruses: Challenge cell theory as they are not cellular but require cells to reproduce
Syncytial organisms: Some fungi and muscle tissues have multinucleated cells
Coenocytic organisms: Algae with multiple nuclei in continuous cytoplasm
Modern additions:
- Cells contain hereditary information (DNA) passed to daughter cells
- Energy flow (metabolism) occurs within cells
- All cells have similar chemical composition - Cells in multicellular organisms are specialized and interdependent
Basic Cell Structure
Common Features of All Cells
Plasma Membrane: Selectively permeable barrier separating cell from environment
- Composition: Phospholipid bilayer with embedded proteins
- Function: Regulates transport, receives signals, maintains homeostasis
Cytoplasm: Semi-fluid substance inside cell containing organelles
- Cytosol: Aqueous component with dissolved substances
- Cytoskeleton: Protein filaments providing structural support
Genetic Material: DNA containing hereditary information
- Prokaryotes: Circular DNA in nucleoid region
- Eukaryotes: Linear DNA in membrane-bound nucleus
Ribosomes: Sites of protein synthesis
- Composition: RNA and protein
- Location: Free in cytoplasm or attached to membranes
Comparing Cell Sizes
Typical sizes:
- Bacteria: 0.5-5.0 μm diameter
- Animal cells: 10-30 μm diameter
- Plant cells: 10-100 μm diameter
Surface Area to Volume Ratio:
- As cell size increases, volume increases faster than surface area
- Limits maximum cell size due to transport constraints
- Adaptations: Folding membranes, elongated shapes
Calculating SA:V ratio: For a cube with side length L:
- Surface Area = 6L²
- Volume = L³
- SA:V ratio = 6/L
Prokaryotic Cells
General Characteristics
Size: Typically 0.5-5.0 μm
Organization: Lack membrane-bound organelles
Genetic material: Single circular DNA molecule in nucleoid
Reproduction: Binary fission
Metabolism: Diverse metabolic capabilities
Structural Components
Cell Wall: Rigid structure outside plasma membrane
- Bacteria: Peptidoglycan (polymer of sugars and amino acids)
- Archaea: Pseudopeptidoglycan or other polymers
- Function: Protection, shape maintenance, prevents bursting
Plasma Membrane: Phospholipid bilayer with embedded proteins
- Function: Selective permeability, energy production (in bacteria)
Capsule/Slime Layer: Outer coating of polysaccharides
- Function: Protection, adherence, resistance to desiccation
Flagella: Protein filaments for movement
- Structure: Rotary motor embedded in membrane
- Function: Propulsion toward nutrients or away from toxins
Pili/Fimbriae: Hair-like protein appendages
Function: Attachment to surfaces, conjugation (transfer of DNA)
- Nucleoid: Region containing chromosomal DNA
- Organization: Supercoiled circular DNA with associated proteins
Ribosomes: 70S type (smaller than eukaryotic ribosomes)
Function: Protein synthesis - Antibiotic target: Many antibiotics inhibit bacterial ribosomes
Eukaryotic Cells
General Characteristics
Size: Typically 10-100 μm
Organization: Membrane-bound organelles compartmentalize functions
Genetic material: Multiple linear chromosomes in nucleus
Reproduction: Mitosis and meiosis Cytoskeleton: Complex network of protein filaments
Organelle Structure and Function
Nucleus
Structure: Double membrane (nuclear envelope) with pores
Function: Houses genetic material, controls gene expression
Components:
- Chromatin: DNA complexed with histone proteins
- Nucleolus: Site of ribosomal RNA synthesis
- Nuclear pores: Regulate transport between nucleus and cytoplasm
Endomembrane System
Endoplasmic Reticulum (ER):
- Rough ER: Ribosome-studded surface, protein synthesis and modification
- Smooth ER: Lipid synthesis, detoxification, calcium storage
Golgi Apparatus: Stack of flattened membrane sacs (cisternae)
- Function: Modifies, sorts, packages proteins and lipids
- Directionality: Cis face (receiving) to trans face (shipping)
Lysosomes: Membrane-bound vesicles containing digestive enzymes
- Function: Intracellular digestion, autophagy, programmed cell death
- pH: ~4.5 (maintained by proton pumps)
Vacuoles: Large membrane-bound compartments
- Plant central vacuole: Storage, waste disposal, turgor pressure
- Contractile vacuoles: Water expulsion in freshwater protists
Energy-Converting Organelles
Mitochondria: Sites of cellular respiration
- Structure: Double membrane, inner membrane folded into cristae
- Function: ATP production through oxidative phosphorylation
- Own DNA: Circular genome encoding some mitochondrial proteins
Chloroplasts: Sites of photosynthesis (plant cells and algae)
- Structure: Double membrane, internal thylakoid membranes
- Function: Convert light energy to chemical energy (sugars)
- Own DNA: Circular genome encoding some chloroplast proteins
Other Organelles
Peroxisomes: Single membrane vesicles containing oxidative enzymes
- Function: Breakdown of fatty acids, detoxification of hydrogen peroxide
Cytoskeleton: Network of protein filaments
- Microfilaments (actin): Cell movement, division, shape
- Intermediate filaments: Structural support, organelle anchorage
- Microtubules: Cell shape, intracellular transport, cell division
Centrosome/Centrioles: Microtubule organizing centers
- Function: Organize spindle fibers during cell division
Plant vs. Animal Cells
Plant cell unique features: - Cell wall (cellulose)
- Chloroplasts - Central vacuole
- Plasmodesmata (channels between cells)
- No centrioles (in most plants)
Animal cell unique features:
- Centrioles - Lysosomes
- Extracellular matrix
- Tight junctions, desmosomes, gap junctions
Endosymbiotic Theory
Theory Overview
Proposed by: Lynn Margulis (1967)
Basic premise: Certain organelles originated as free-living bacteria engulfed by ancestral eukaryotic cells
Evidence for Endosymbiosis
Mitochondria and chloroplasts:
- Size: Similar to bacteria
- Double membranes: Suggestive of engulfment
- Own DNA: Circular, similar to bacterial DNA
- Ribosomes: 70S type (bacterial) vs. 80S (eukaryotic cytoplasmic)
- Reproduction: Divide independently by binary fission
- Genetic code: Slight variations from nuclear code
- Antibiotic sensitivity: Similar to bacteria
Primary vs. Secondary Endosymbiosis
Primary endosymbiosis: Engulfment of prokaryote by ancestral eukaryote
- Resulted in mitochondria and chloroplasts
Secondary endosymbiosis: Engulfment of eukaryotic cell by another eukaryote
- Resulted in complex plastids with multiple membranes
- Example: Euglenoids with chloroplasts surrounded by three membranes
Modern Support
Genomic evidence: Mitochondrial genes show closest relationship to α-proteobacteria
Phylogenetic analysis: Supports single origin of mitochondria
Biochemical similarities: Similar electron transport chains in mitochondria and certain bacteria
Cellular Compartmentalization
Advantages of Compartmentalization
- Specialized environments: Different pH, ion concentrations, enzyme sets
- Increased efficiency: Localized reactants and enzymes
- Protection: Isolation of harmful processes (e.g., digestive enzymes in lysosomes)
- Regulation: Controlled transport between compartments
- Simultaneous processes: Incompatible reactions can occur simultaneously
Protein Targeting and Transport
Signal sequences: Amino acid sequences directing proteins to specific locations
- Nuclear localization signal: Targets proteins to nucleus
- ER signal sequence: Directs proteins to endoplasmic reticulum
- Mitochondrial targeting sequence: Guides proteins to mitochondria
Transport mechanisms:
- Gated transport: Through nuclear pores
- Transmembrane transport: Across organelle membranes
- Vesicular transport: Via membrane vesicles
Methods in Cell Biology
Microscopy Techniques
Light Microscopy: Uses visible light
- Brightfield: Stained specimens against bright background
- Phase contrast: Enhances contrast of transparent specimens
- Fluorescence: Uses fluorescent dyes or proteins
- Confocal: Eliminates out-of-focus light, creates optical sections
Electron Microscopy: Uses electron beams
- Transmission EM (TEM): Electrons pass through thin sections
- Scanning EM (SEM): Electrons scan surface, 3D appearance
Super-resolution Microscopy: Breaks diffraction limit
- STED: Stimulated emission depletion
- PALM/STORM: Photoactivated localization microscopy
Cell Fractionation
Homogenization: Breaking cells open
Differential centrifugation: Separating components by size/density
Density gradient centrifugation: Separating by buoyant density
Molecular Techniques
Immunofluorescence: Using antibodies to localize specific proteins
Green Fluorescent Protein (GFP): Tagging proteins to visualize in living cells
Live-cell imaging: Time-lapse microscopy of living cells
Electron tomography: 3D reconstruction from multiple EM images
Chapter Summary
Key Concepts
- Cell theory establishes cells as fundamental units of life
- Prokaryotic cells lack membrane-bound organelles but perform all life functions
- Eukaryotic cells compartmentalize functions in membrane-bound organelles
- Endosymbiotic theory explains origins of mitochondria and chloroplasts
- Compartmentalization enables specialized functions and increases efficiency
- Surface area to volume ratio limits cell size and influences cell shape
- Advanced microscopy and biochemical techniques reveal cellular structure and function
Comparison of Cell Types
| Size |
0.5-5.0 μm |
10-100 μm |
| Nucleus |
No (nucleoid region) |
Yes (membrane-bound) |
| Organelles |
None membrane-bound |
Multiple membrane-bound |
| DNA form |
Circular, single chromosome |
Linear, multiple chromosomes |
| Ribosomes |
70S |
80S (cytoplasmic) |
| Cell division |
Binary fission |
Mitosis/meiosis |
| Cytoskeleton |
Simple filaments |
Complex network |
Organelle Functions Summary
| Nucleus |
Genetic control center |
Nuclear envelope, chromatin, nucleolus |
| Mitochondria |
ATP production |
Double membrane, cristae, own DNA |
| Chloroplasts |
Photosynthesis |
Thylakoids, stroma, own DNA |
| ER |
Protein/lipid synthesis |
Rough (ribosomes), smooth (no ribosomes) |
| Golgi |
Modification/sorting |
Cisternae stacks, cis-trans orientation |
| Lysosomes |
Digestion |
Acidic pH, hydrolytic enzymes |
| Peroxisomes |
Oxidation reactions |
Catalase, β-oxidation of fatty acids |
Review Questions
Level 1: Recall and Understanding
- State the three principles of modern cell theory.
- List three differences between prokaryotic and eukaryotic cells.
- Name the function of each: nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus.
- What evidence supports the endosymbiotic theory?
- Why does surface area to volume ratio limit cell size?
Level 2: Application and Analysis
- A cell has a large number of mitochondria. What can you infer about its energy requirements?
- Compare the functions of rough and smooth endoplasmic reticulum.
- Explain how compartmentalization increases cellular efficiency.
- A scientist discovers a new single-celled organism. What observations would help determine if it is prokaryotic or eukaryotic?
- Calculate the surface area to volume ratio for a spherical cell with radius 10 μm and another with radius 20 μm. Which has a more favorable ratio for transport?
Level 3: Synthesis and Evaluation
- Defend or challenge: “Viruses should be considered living organisms based on modern cell theory.”
- How might cellular structure differ between cells specialized for secretion vs. cells specialized for contraction?
- Design an experiment to test whether a particular organelle is essential for cell survival.
- Evaluate the statement: “Endosymbiosis represents a major evolutionary transition in the history of life.”
Key Terms
- Cell theory: All organisms composed of cells; cell is basic unit; cells arise from pre-existing cells
- Prokaryote: Organism whose cells lack a nucleus and membrane-bound organelles
- Eukaryote: Organism whose cells contain a nucleus and membrane-bound organelles
- Organelle: Specialized structure within a cell that performs specific functions
- Plasma membrane: Selectively permeable barrier surrounding the cell
- Cytoplasm: Material inside cell between plasma membrane and nucleus
- Nucleus: Membrane-bound organelle containing genetic material
- Mitochondria: Organelle site of cellular respiration and ATP production
- Chloroplast: Organelle site of photosynthesis in plants and algae
- Endoplasmic reticulum: Network of membranes involved in protein and lipid synthesis
- Golgi apparatus: Organelle that modifies, sorts, and packages proteins
- Lysosome: Vesicle containing digestive enzymes
- Endosymbiotic theory: Proposal that certain organelles originated as symbiotic bacteria
- Compartmentalization: Separation of cellular functions into distinct membrane-bound areas
- Surface area to volume ratio: Relationship affecting transport efficiency in cells
Further Reading
Books
- Alberts, B. et al. (2022). Molecular Biology of the Cell (7th ed.). W. W. Norton & Company.
- Cooper, G. M., & Hausman, R. E. (2019). The Cell: A Molecular Approach (8th ed.). Oxford University Press.
- Margulis, L. (1981). Symbiosis in Cell Evolution. W. H. Freeman.
Scientific Articles
- Lane, N., & Martin, W. (2010). The energetics of genome complexity. Nature, 467(7318), 929-934.
- Sagan, L. (1967). On the origin of mitosing cells. Journal of Theoretical Biology, 14(3), 225-274.
- Wickner, W., & Schekman, R. (2005). Protein translocation across biological membranes. Science, 310(5753), 1452-1456.
Online Resources
- Cell Image Library: http://www.cellimagelibrary.org
- Allen Institute for Cell Science: https://www.allencell.org
- HHMI BioInteractive: https://www.biointeractive.org
Quantitative Problems
- Surface Area to Volume Calculations:
- Calculate the surface area and volume of a spherical cell with diameter 20 μm.
- What is the SA:V ratio?
- If the cell divides into two equal daughter cells, what is the new total surface area and SA:V ratio?
- Explain the physiological significance of your calculations.
- Organelle Number Estimation: A liver cell contains approximately 1,700 mitochondria occupying about 20% of the cell volume.
- If the cell diameter is 25 μm, what is the average mitochondrial diameter (assuming spherical mitochondria)?
- If each mitochondrion produces 10^7 ATP molecules per second, estimate total ATP production per cell per day.
- Transport Rate Analysis: A substance diffuses across the plasma membrane at a rate of 0.01 μmol/s per cm² of membrane area.
- How long would it take for 1 μmol to enter a spherical cell with radius 10 μm?
- How would this time change if the cell had microvilli increasing surface area 10-fold?
- What cellular processes might be limited by this diffusion rate?
Case Study: Mitochondrial Diseases
Background: Mitochondrial disorders result from dysfunction of mitochondrial oxidative phosphorylation.
Questions:
- Why are tissues with high energy demands (muscle, brain) most affected by mitochondrial diseases?
- How does maternal inheritance pattern of some mitochondrial diseases support endosymbiotic theory?
- What cellular functions besides ATP production might be impaired in mitochondrial disorders?
- Propose a therapeutic strategy for treating mitochondrial diseases based on your understanding of mitochondrial function.
Data for analysis:
- Normal mitochondrial ATP production: ~30 kg ATP/day in humans
- Mitochondrial DNA mutation rate: 10× higher than nuclear DNA
- Heteroplasmy: Cells contain mixed populations of normal and mutant mitochondria
- Threshold effect: Symptoms appear when mutant mitochondria exceed 60-90% of total
Next Chapter: Metabolism and Energy Transformation