Cell Theory Basics
Cell Theory
Cell theory states:
1. All living organisms are made of one or more cells or their products.
2. The cell is the smallest structural and functional unit of life.
3. New cells form only by division of pre-existing cells.
Quick check: Which principle explains why viruses are not considered living cells?
Anatomy of a Typical Cell
Four universal components present in every cell
All living cells share a core kit of parts. Master these and you can describe any cell you meet.
Key Points:
- Plasma membrane – selective border that regulates what enters and leaves.
- Cytoplasm – fluid matrix where metabolic reactions take place.
- DNA – genetic blueprint storing all instructions.
- Ribosomes – protein-building factories that read the DNA code.
Two Cell Kingdoms
Prokaryotic Cell
Eukaryotic Cell
Key Similarities
Inside a Bacterium
Label the key parts of a prokaryotic cell (p.41–45).
Key structures
A bacterium lacks a nucleus but holds all life-support parts in a compact space.
- Cell wall – rigid peptidoglycan shell that preserves shape.
- Plasma membrane – selective barrier just inside the wall.
- Nucleoid – bacterial chromosome in an open region, no envelope.
- Plasmid – small circular DNA carrying extra traits.
- 70S ribosome – smaller protein factory than eukaryotic 80S.
- Flagellum – rotating tail that propels the cell.
- Pili – hair-like projections for attachment or gene transfer.
Tip: Spotting the nucleoid (no membrane) instantly tells you the cell is prokaryotic.
Inside an Animal Cell
Identify the key membrane-bound organelles of eukaryotes.
What the image shows
An animal cell cut-away highlighting each major membrane-bound compartment.
- Nucleus: stores DNA, directs protein synthesis.
- Endoplasmic reticulum (ER): rough ER makes proteins; smooth ER builds lipids.
- Golgi apparatus: modifies and packages ER products for export.
- Mitochondria: generate ATP, the cell’s energy currency.
- Lysosome: enzyme sac that digests waste.
- Plasma membrane: phospholipid bilayer regulating entry and exit.
Tip: Memorise each organelle’s job—recognising them is key to understanding eukaryotic cell function.
Label the Eukaryotic Cell
Drag each label—nucleus, mitochondria, Golgi apparatus, rough ER and plasma membrane—to its correct spot on the cell outline.
Draggable Items
Drop Zones
Nucleus
Mitochondria
Golgi apparatus
Rough ER
Plasma membrane
Tip:
Match the organelle’s usual shape and position to the diagram.
Results
Multiple Choice Question
Question
Which ribosome size is characteristic of eukaryotic cytoplasm?
Hint:
Remember: prokaryotic ribosomes are 70S.
Correct!
80S ribosomes are found in eukaryotic cytoplasm; 70S occur in prokaryotes and organelles.
Incorrect
Review ribosome sizes: eukaryotic cytoplasm has 80S, whereas 70S belongs to prokaryotes and organelles.
Key Takeaways
Cell theory states every organism is cellular, and new cells arise only from existing cells.
Plasma membrane, cytoplasm, DNA and ribosomes form the universal core of all cells.
Prokaryotes lack a nucleus; eukaryotes keep DNA in a nucleus and contain membrane-bound organelles.
Nucleus, mitochondria, ER, Golgi and lysosomes coordinate information, energy, transport and recycling.
Next Steps
Review cell diagrams and practice naming organelles before exploring membrane transport.
Thank You!
We hope you found this lesson informative and engaging.