Cells as the basis of life
What this note covers
- Prokaryotic and eukaryotic cells and their organelles
- Stem cells, differentiation and levels of organisation
- Body systems and specialised cells suited to function
- The cell membrane, transport and SA:V ratio
4 sections · 8 key terms & formulas · 5 common mistakes
Every living thing is built from one or more cells, and every cell comes from an existing cell. Biologists sort cells into two broad groups. Prokaryotic cells (bacteria and archaea) are small, usually 1–10 µm across, and have no membrane-bound nucleus. Their DNA is a single circular chromosome sitting in a region called the nucleoid, often with extra small DNA loops called plasmids. They contain small 70S ribosomes, a cell membrane, cytoplasm and usually a cell wall made of peptidoglycan.
Some have a capsule, pili or a flagellum.
Eukaryotic cells (animals, plants, fungi and protists) are typically 10–100 µm across. Their linear chromosomes are enclosed in a nucleus with a double membrane (the nuclear envelope). They contain larger 80S ribosomes and many membrane-bound organelles, which divide the cell into compartments so that incompatible reactions can run at the same time.
Key eukaryotic organelles and their functions:
- Nucleus – stores DNA and controls cell activity; the nucleolus assembles ribosome subunits.
- Ribosomes – sites of protein synthesis; free in the cytosol or attached to rough ER.
- Rough endoplasmic reticulum – membrane network studded with ribosomes; folds and transports proteins destined for export or membranes.
- Smooth endoplasmic reticulum – no ribosomes; synthesises lipids and steroids and detoxifies drugs.
- Golgi apparatus – stacks of flattened sacs that modify, sort and package proteins into vesicles.
- Mitochondria – double-membraned; site of aerobic cellular respiration, producing ATP.
- Lysosomes – vesicles of digestive enzymes that break down worn-out organelles and engulfed material.
- Vacuoles – storage sacs; plant cells have one large central vacuole that holds water and keeps the cell turgid.
- Chloroplasts – double-membraned, contain chlorophyll; site of photosynthesis in plants and algae.
Mitochondria and chloroplasts have their own circular DNA and 70S ribosomes, which is strong evidence that they descended from free-living prokaryotes (endosymbiotic theory).
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent (nucleoid) | Present, double membrane |
| DNA | Circular, plus plasmids | Linear, with histone proteins |
| Ribosomes | 70S | 80S (70S in organelles) |
| Membrane-bound organelles | None | Many |
| Typical size | 1–10 µm | 10–100 µm |
Step 1: Check for a nucleus – absent, so DNA is free in a nucleoid.
Step 2: Check for membrane-bound organelles – none (no mitochondria).
Step 3: Check size – 3 µm fits the prokaryote range.
Answer: The cell is prokaryotic (likely a bacterium) because it lacks a membrane-bound nucleus and organelles and is small. A cell wall alone does not decide it, because plant and fungal cells also have walls.
Common questions
What does the QCE Year 11 Biology note "Cells as the basis of life" cover?
This note has 4 sections on Cells as the basis of life. It includes 8 key terms and formulas, plus 5 common mistakes. It covers 1, Unit 1 Topic 1: Cells as the basis of life. The full section list is shown under “What this note covers”.
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