VCE Biology (Year 11) · Unit 1 — Cells and Systems · 12m read

Cell Structure — Prokaryotes vs Eukaryotes

Ever wondered why bacteria can survive almost anywhere while your cells need way more help to stay alive? The answer lies in two fundamentally different cell designs. Prokaryotic cells (like bacteria) are simpler and lack a nucleus, while eukaryotic cells (yours!) are packed with specialized compartments called organelles that divide up the work. Understanding this difference is crucial for VCE biology and explains how life evolved from simple organisms to complex multicellular creatures like us.

Learning objective: Compare prokaryotic and eukaryotic cells and their organelles.

By the end of this lesson you can:

I distinguish prokaryotes and eukaryotes.
I describe key organelles and their functions.
I link structure to function.

Prokaryotes are studio apartments; eukaryotes are mansions. Prokaryotes do everything in one room. Eukaryotes have separate rooms (organelles) for different jobs.

Two types of cells

PROKARYOTES (e.g., bacteria, archaea):
• SMALL (~1-5μm).
• NO membrane-bound nucleus — DNA in cytoplasm.
• NO membrane-bound organelles.
• Simple cell wall.
• Ribosomes (smaller — 70S).
• Single circular chromosome + plasmids.
• Divide by BINARY FISSION.

EUKARYOTES (plants, animals, fungi, protists):
• LARGER (~10-100μm).
• MEMBRANE-BOUND nucleus.
• Many organelles.
• Linear chromosomes (multiple).
• Divide by MITOSIS / MEIOSIS.

KEY EUKARYOTIC ORGANELLES:

NUCLEUS:
• Contains DNA.
• Nuclear membrane with pores.
• Nucleolus — ribosome assembly.

RIBOSOMES:
• Protein synthesis.
• Free in cytoplasm OR attached to rough ER.

ENDOPLASMIC RETICULUM (ER):
• ROUGH ER — ribosomes attached; processes proteins.
• SMOOTH ER — lipid synthesis, detoxification.

GOLGI APPARATUS:
• Modifies, packages, ships proteins/lipids.

MITOCHONDRIA:
• "Powerhouse" — cellular respiration, produces ATP.
• Double membrane; own DNA.

LYSOSOMES (animal):
• Contain digestive enzymes; recycle / dispose of cellular waste.

CHLOROPLASTS (plant):
• Photosynthesis → glucose + O₂.
• Chlorophyll captures light.

CELL MEMBRANE:
• Phospholipid bilayer.
• Selectively permeable.

CELL WALL:
• Plants: cellulose.
• Bacteria: peptidoglycan.
• Fungi: chitin.

VACUOLES:
• Plants: large central vacuole (turgor).
• Animals: smaller, temporary.

CYTOSKELETON:
• Microtubules, microfilaments.
• Cell shape, internal transport, division.

SURFACE AREA : VOLUME RATIO:
• Smaller cells have higher SA:V.
• Supports fast exchange across membrane.
• Explains cell size limits.

Quick compare

Prokaryote: Small, simple, no nucleus.
Eukaryote: Big, complex, nucleus + organelles.
Structure→function: Each organelle has a job.

Quick reference

| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | No | Yes |
| Membrane-bound organelles | No | Yes |
| Size | Small | Larger |
| DNA | Circular, plasmids | Linear chromosomes |
| Examples | Bacteria | Plants, animals, fungi |

Worked example (Medium)

A cell has a nucleus, mitochondria, chloroplasts and cell wall. What type is it?

Nucleus. Present → eukaryote.
Chloroplasts. Present → photosynthetic.
Cell wall. Present → plant (cellulose).
Conclusion. Eukaryotic plant cell.

Answer: Eukaryotic plant cell (nucleus + chloroplasts + cell wall).

Worked example (Hard)

Mitochondria and chloroplasts both have their own DNA and double membranes. Using the endosymbiotic theory, explain how this supports the idea that they were once free-living prokaryotes.

Shared features. Both organelles have circular DNA (like prokaryotes), 70S ribosomes, and replicate independently via binary fission.
Double membrane. The inner membrane may be the original prokaryote membrane; the outer membrane may have formed when the host cell engulfed it via endocytosis.
Evolutionary implication. These features strongly suggest ancestral prokaryotes were engulfed by early eukaryotic cells and became permanent endosymbionts — a mutualistic relationship where the host provided protection and the endosymbiont provided energy (mitochondria) or sugars (chloroplasts).

Answer: The independent DNA, 70S ribosomes, double membrane and binary fission of mitochondria and chloroplasts all point to a prokaryotic origin, supporting the endosymbiotic theory.

Common mistakes

Saying prokaryotes have "no organelles" Prokaryotes DO have ribosomes (70S) — they lack MEMBRANE-BOUND organelles like nuclei and mitochondria, not all organelles. Fix: Say "no membrane-bound organelles" instead of "no organelles".
Confusing cell wall materials Plant cell walls are cellulose, bacterial walls are peptidoglycan, and fungal walls are chitin. Students mix these up. Fix: Memorise: Plants = Cellulose, Bacteria = Peptidoglycan, Fungi = Chitin (C-P-C alphabetical trick).
Thinking all eukaryotic cells have the same organelles Animal cells lack chloroplasts and cell walls; plant cells lack centrioles and lysosomes (generally). Not all eukaryotic cells are identical. Fix: Always specify "animal eukaryotic" or "plant eukaryotic" in exam responses.

The endosymbiotic theory — how cells became complex

The endosymbiotic theory, proposed by Lynn Margulis in 1967, argues that mitochondria and chloroplasts were once independent prokaryotes that were engulfed by an ancestral eukaryotic cell. Rather than being digested, these prokaryotes survived inside the host, eventually becoming permanent residents — endosymbionts.

The evidence is compelling:

Both organelles have their own circular DNA, separate from the nuclear genome.
They replicate via binary fission, just like bacteria.
Their ribosomes are 70S (prokaryotic size), not 80S (eukaryotic).
They are bounded by a double membrane — the inner membrane likely from the original prokaryote, the outer from the host's engulfing vesicle.

Mitochondria are thought to descend from aerobic proteobacteria, while chloroplasts trace back to photosynthetic cyanobacteria. This theory explains why virtually all eukaryotes have mitochondria (aerobic respiration was a huge advantage) and why only plants and algae have chloroplasts (photosynthesis was incorporated later and only in some lineages).

The endosymbiotic theory is one of biology's most elegant examples of how cooperation, not just competition, drives evolution.

Practice questions

Which organelle is the "powerhouse" of the cell?
1. Nucleus
2. Mitochondria
3. Golgi apparatus
4. Lysosome
(1 marks)
Why does a larger cell have a harder time exchanging materials with its environment?
(3 marks)

Summary

Distinguish prokaryotes and eukaryotes.
Describe key organelles and their functions.
Link structure to function.