The Cell Theory
The Cell Theory is a cornerstone of biology, defining life at its most fundamental level. It’s built on three core principles derived from centuries of scientific observation and refinement.
Core Tenets
- Unity of Life: All living organisms are composed of one or more cells.
- Basic Unit: The cell is the basic structural and functional unit of life.
- Origin of Cells: All cells arise from pre-existing cells (*Omnis cellula e cellula*).
Modern Additions
- Energy Flow: Metabolism and biochemistry occur within cells.
- Hereditary Information: DNA is passed from cell to cell during division.
- Basic Composition: All cells have the same basic chemical makeup.
Prokaryotes vs. Eukaryotes
The most fundamental division in the living world is between simple prokaryotic cells and complex eukaryotic cells. The defining difference is the presence of a membrane-bound nucleus and other organelles in eukaryotes.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Average Size | Small (0.1β5 Β΅m) | Large (10β100 Β΅m) |
| Nucleus | Absent; DNA in nucleoid | Present; membrane-bound |
| DNA Structure | Single, circular chromosome | Multiple, linear chromosomes |
| Membrane-Bound Organelles | Absent | Present (e.g., mitochondria, ER) |
| Ribosomes | Smaller (70S) | Larger (80S) |
| Cell Division | Binary Fission | Mitosis & Meiosis |
Surface Area to Volume Ratio
A cell’s size is limited by the relationship between its surface area (the membrane) and its volume (the cytoplasm). As a cell grows, its volume (r3) increases faster than its surface area (r2), making efficient exchange of materials difficult.
Overcoming the Limit
- Cell Division: Remaining small maintains a high SA:V ratio.
- Altered Shape: Long, thin, or flat shapes increase surface area.
- Compartmentalisation: Organelles increase internal surface area for reactions.
This chart shows how a cell’s volume quickly outpaces its surface area as its radius increases, leading to a decreasing SA:V ratio.
The Eukaryotic Cell Explorer
Discover the “little organs” that make eukaryotic cells so complex. This compartmentalisation allows for efficient, specialised functions. Use the filters to see which organelles are unique to plant or animal cells.
The Plasma Membrane
The cell membrane is a dynamic, fluid structure described by the Fluid Mosaic Model. It’s a patchwork of phospholipids, proteins, cholesterol, and carbohydrates that controls what enters and leaves the cell.
Components of the Fluid Mosaic
Phospholipid Bilayer
Forms the basic fabric. Amphipathic molecules with hydrophilic heads and hydrophobic tails create a selectively permeable barrier.
Membrane Proteins
Carry out most functions. Integral proteins act as channels/pumps, while peripheral proteins act as enzymes or anchors.
Cholesterol (Animal Cells)
Acts as a ‘fluidity buffer’, maintaining membrane stability by preventing it from becoming too fluid at high temps or too rigid at low temps.
Glycocalyx
Carbohydrate chains on the outer surface (glycoproteins/glycolipids) crucial for cell-cell recognition, adhesion, and signaling.
Crossing the Border: Membrane Transport
A cell’s survival depends on regulating what enters and leaves. This section explores the different ways substances are transported across the selectively permeable plasma membrane.
Passive Transport
No energy needed. Moves down the concentration gradient.
- Simple Diffusion: Small, non-polar molecules (O2, CO2) pass directly through the membrane.
- Osmosis: Diffusion of water across a membrane.
- Facilitated Diffusion: Requires protein channels or carriers for specific molecules like glucose.
Active Transport
Requires energy (ATP). Moves against the concentration gradient.
- Primary Active Transport: Uses ATP directly. E.g., the Na+/K+ pump creates gradients.
- Secondary Active Transport: Uses the energy stored in a gradient (created by primary transport) to move another substance.
Vesicular (Bulk) Transport
For large molecules. Requires energy.
- Endocytosis: Cell membrane engulfs substances to bring them inside.
- Exocytosis: Vesicles fuse with the membrane to release substances outside.
The Cell Cycle
Explore the life of a cell, from its growth (Interphase) to its division (M Phase). Use the interactive viewer to see how a cell precisely divides its genetic material during mitosis.
Interactive Mitosis Viewer
Apoptosis & Cancer
The cell cycle is tightly controlled. Apoptosis (programmed cell death) removes damaged cells, while cancer is the result of this control system failing, leading to uncontrolled division.
Apoptosis: Programmed Death
Apoptosis is a clean, controlled process of cell suicide essential for development (e.g., forming fingers) and removing damaged cells. It prevents inflammation by packaging cell contents into apoptotic bodies for disposal by phagocytic cells.
Cancer: Uncontrolled Division
Cancer results from mutations in genes that control the cell cycle. Oncogenes (stuck accelerators) promote division, while faulty tumor suppressor genes (broken brakes) fail to halt it or trigger apoptosis. The accumulation of multiple mutations is required for cancer to develop.
Cellular Potential & Specialisation
All complex organisms arise from a single cell. This journey involves stem cells, which have the unique ability to become other cell types. Explore the hierarchy of stem cell potency, representing a path of increasing specialisation.
The Hierarchy of Stem Cell Potency
Totipotent
“Total Potential”
Pluripotent
“Many Potentials”
Multipotent
“Multiple Potentials”
Click on each potency level to reveal its description.
