Fundamental Principles of Cell Biology
All living organisms are composed of one or more cells, establishing the cell as the basic unit of life. This principle, central to the cell theory, also states that new cells arise only by the division of pre‑existing cells. Consequently, there is no spontaneous generation of life; every cell traces its lineage to a parent cell through processes such as mitosis in eukaryotes or binary fission in prokaryotes.
Cell Size and the Surface‑to‑Volume Ratio
Cell size is constrained by the surface‑to‑volume ratio. As a cell grows, its volume increases faster than its surface area, limiting the efficiency of diffusion and transport across the plasma membrane. This geometric constraint explains why most cells remain small and why specialized transport mechanisms evolve in larger cells.
Cellular Organelles and Their Functions
Key organelles define the functional capabilities of eukaryotic cells:
- Mitochondria: The central energy producer, generating ATP through oxidative phosphorylation.
- Nucleolus: Synthesizes ribosomal RNA and assembles ribosomal subunits, acting as the cell’s ribosome factory.
- Plasma Membrane: A selectively permeable barrier that regulates the passage of substances, maintaining cellular homeostasis.
Animal cells contain mitochondria but lack a rigid cell wall, distinguishing them from plant and bacterial cells.
Genetic Material and Chromosome Count
Human somatic cells possess 46 chromosomes organized into 23 pairs. Within the nucleus, the DNA stretches to approximately 2 meters in length, yet it is compacted into a nucleus only 5–10 µm in diameter, illustrating the remarkable degree of DNA packaging.
Chromatin Organization
Chromatin exists in two major forms. Heterochromatin is densely packed and generally transcriptionally silent, whereas euchromatin is less condensed and actively transcribed.
Cell Wall Composition
Cell walls differ among kingdoms:
- Bacteria: Composed primarily of peptidoglycan.
- Plants: Built mainly from cellulose.
Animal cells lack a cell wall altogether.
Ribosomes: Structure and Sedimentation
Ribosomes differ between prokaryotes and eukaryotes. Prokaryotic ribosomes sediment at 70 S, while eukaryotic ribosomes sediment at 80 S, reflecting structural complexity. Both types are essential for protein synthesis, but eukaryotic ribosomes are larger and contain additional proteins and rRNA components.
Membrane Composition and Physical Properties
Cholesterol integrates into the phospholipid bilayer, increasing membrane rigidity. This effect modulates fluidity, influencing membrane protein function and cellular signaling.
Endocytosis: Phagocytosis vs. Pinocytosis
Cells internalize external material through distinct mechanisms:
- Phagocytosis: Engulfs large solid particles, such as bacteria or debris.
- Pinocytosis: Ingests fluids and small dissolved substances.
Eukaryotes also employ diffusion and active transport, expanding their nutrient uptake repertoire beyond that of prokaryotes.
Prokaryotic Versus Eukaryotic Nutrient Uptake
Prokaryotes rely on diffusion and active transport to acquire nutrients. Eukaryotes supplement these methods with endocytic processes, including phagocytosis and pinocytosis, allowing the internalization of larger particles and fluids.
Plasmids and Genetic Elements
Plasmids are extrachromosomal DNA molecules found in prokaryotes. Eukaryotic cells do not contain plasmids within their nuclei, underscoring a key genetic distinction between the two domains of life.
Endosymbiotic Theory
The origin of eukaryotic cells is explained by endosymbiosis: an archaeal host engulfed a bacterium that provided a reliable energy source. Over evolutionary time, this partnership gave rise to modern eukaryotes, with mitochondria retaining many bacterial characteristics.
Cell Growth: Hypertrophy vs. Hyperplasia
Cellular enlargement can occur through two distinct mechanisms:
- Hypertrophy: Increases the size of existing cells.
- Hyperplasia: Increases the number of cells via proliferation.
Both processes contribute to tissue growth but differ in their cellular strategies.
Key Measurements in Microbiology
The bacterium Mycoplasma typically measures about 200 nm, illustrating the small scale of many prokaryotic organisms.
Selective Permeability of the Plasma Membrane
The plasma membrane does not allow any substance to pass freely; instead, it exhibits selective permeability, permitting only certain molecules to cross while restricting others. This selectivity is vital for maintaining internal conditions distinct from the external environment.
Summary Table
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Ribosome sedimentation | 70 S | 80 S |
| Cell wall composition | Peptidoglycan | Cellulose (plants) / none (animals) |
| Genetic elements | Plasmids present | Plasmids absent |
| Membrane rigidity | Variable, cholesterol less common | Increased by cholesterol |
| Nutrient uptake | Diffusion, active transport | Diffusion, active transport, endocytosis |
This lesson consolidates essential concepts of cell biology, providing a foundation for further study of cellular structure, function, and evolution.
