Theme A: Unity & Diversity
A2.2 - Cell Structure
Comprehensive Guide for IB Biology SL Students
Cell structure is fundamental to understanding how living organisms function at the microscopic level. This comprehensive guide covers prokaryotic and eukaryotic cell structures, the functions of life, microscopy techniques, and atypical cell examples that challenge traditional cell theory.
Prokaryotic Cell Structure
Prokaryotic cells are simpler cellular structures that lack membrane-bound organelles and a true nucleus. These cells are characteristic of bacteria and archaea, representing some of the earliest forms of life on Earth.
Key Structural Components
Nucleoid Region
The region in the cytoplasm where genetic material (circular DNA) is located, not enclosed by a nuclear membrane.
Cell Wall
Rigid outer layer composed of peptidoglycan that provides shape and protection to the cell.
Cell Membrane
Phospholipid bilayer that regulates the entry and exit of substances, maintaining cellular homeostasis.
Ribosomes (70S)
Smaller ribosomes responsible for protein synthesis, composed of RNA and protein.
Plasmids
Small, circular DNA molecules separate from chromosomal DNA, often containing genes for antibiotic resistance.
Flagella
Whip-like structures used for locomotion, allowing the cell to move toward nutrients or away from toxins.
Pili
Hair-like appendages that facilitate attachment to surfaces and genetic exchange during conjugation.
Capsule
Protective outer layer that helps with moisture retention and protects against phagocytosis.
Key Characteristic: Prokaryotic cells lack compartmentalization, meaning their cytoplasm is not divided into membrane-bound organelles. This simplicity allows for rapid reproduction through binary fission.
Eukaryotic Cell Structure
Eukaryotic cells are structurally complex with membrane-bound organelles and a defined nucleus enclosed by a nuclear envelope. These cells are found in animals, plants, fungi, and protists.
Major Organelles and Functions
Nucleus
Controls cellular activities and contains linear DNA organized into chromosomes. The nucleolus within produces ribosomes.
Mitochondria
The powerhouse of the cell where aerobic respiration occurs, producing ATP (adenosine triphosphate) for cellular energy.
Endoplasmic Reticulum (ER)
Rough ER: Studded with ribosomes; synthesizes and transports proteins.
Smooth ER: Lacks ribosomes; synthesizes lipids and carbohydrates.
Golgi Apparatus
Modifies, packages, and distributes proteins and lipids, forming vesicles for transport.
Lysosomes
Contain hydrolytic enzymes for digesting worn-out organelles, pathogens, and cellular debris.
Ribosomes (80S)
Larger than prokaryotic ribosomes; site of protein synthesis using mRNA templates.
Chloroplasts (Plants Only)
Site of photosynthesis where light energy is converted to chemical energy, containing chlorophyll pigments.
Cell Wall (Plants Only)
Rigid structure made of cellulose providing structural support and protection.
Vacuole (Plants)
Large central vacuole stores cell sap, maintains turgor pressure, and helps with cellular homeostasis.
Advantage of Compartmentalization: Membrane-bound organelles allow for specialized environments, concentrated enzymes, separation of incompatible reactions, and increased efficiency in cellular processes.
Light & Electron Microscopy
Microscopy techniques are essential for studying cell structure, with different types offering varying levels of magnification and resolution.
| Feature | Light Microscope | Electron Microscope |
|---|---|---|
| Illumination Source | Visible light | Beam of electrons |
| Maximum Magnification | ×500 to ×1,500 | ×100,000 to ×300,000 |
| Resolution | 0.2 μm (200 nm) | 0.001 μm (1 nm) - 250× better |
| Specimen Type | Living or dead specimens | Only dead/dried specimens |
| Image Color | Colored (with stains) | Black and white (grayscale) |
| Specimen Preparation | Minutes to hours | Several days |
| Vacuum Required | No | Yes - essential for operation |
| Voltage Required | Standard electricity | 50,000+ Volts |
| Cost | Relatively inexpensive | Very expensive |
| Best Use | Observing living cells, general structure | Ultra-structure, surface details |
Calculating Magnification
The magnification formula is essential for microscopy calculations:
Magnification = Image Size ÷ Actual Size
Units must be consistent when using this formula. Common conversions: 1 mm = 1,000 μm = 1,000,000 nm.
Atypical Cell Structures
Certain eukaryotic cells have atypical structures that challenge traditional cell theory, demonstrating specialized adaptations for specific functions.
Striated Muscle Fibres
Atypical Features: Multinucleated with multiple nuclei per cell, extremely long (up to 300 mm), formed by fusion of multiple cells.
Functional Significance: Multiple nuclei allow simultaneous protein synthesis for muscle contraction; challenges the concept that cells function independently.
Aseptate Fungal Hyphae
Atypical Features: Multinucleated with continuous cytoplasm, lack septa (cross-walls), appear as one long cell.
Functional Significance: Rapid cytoplasmic streaming allows quick nutrient distribution; continuous structure supports increased metabolic activity.
Red Blood Cells (Erythrocytes)
Atypical Features: Anucleate (no nucleus in mature cells), lack mitochondria and other organelles, biconcave disc shape.
Functional Significance: Absence of nucleus maximizes space for hemoglobin, increasing oxygen-carrying capacity; biconcave shape increases surface area for gas exchange.
Phloem Sieve Tube Elements
Atypical Features: No nucleus, lack ribosomes and mitochondria, have no end cell walls forming continuous tubes.
Functional Significance: Reduced organelles allow efficient transport of sugars; depend on companion cells for metabolic support and survival.
Cell Theory Challenge: These atypical examples question fundamental principles of cell theory, such as "all cells have one nucleus" and "cells function as independent units".
Test Your Knowledge
1. Which organelle is responsible for ATP production in eukaryotic cells?
2. What type of ribosome is found in prokaryotic cells?
3. Which atypical cell lacks a nucleus in its mature form?