Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Ribosomes: rRNA and protein, no membrane, found in all cells. They make proteins by reading mRNA. Free ribosomes make proteins for the cytosol; ribosomes on rough ER make proteins for export, membranes and lysosomes.
- Endomembrane system: nuclear envelope, ER, Golgi, lysosomes, vacuoles, transport vesicles and plasma membrane. Together they modify, package and transport proteins, lipids and polysaccharides.
- ER: gives mechanical support and a route for transport inside the cell. Rough ER = bound ribosomes, protein synthesis, compartmentalization. Smooth ER = lipid synthesis and detoxification.
- Golgi: stack of flattened sacs; checks folding, chemically modifies (e.g. adds sugar chains: glycosylation) and packages products for their destinations.
- Lysosomes: sacs of hydrolytic enzymes; digest and recycle material; help carry out apoptosis (programmed cell death).
- Vacuoles: plants have one large central vacuole that stores water and nutrients and keeps turgor pressure; animal cells have smaller, more numerous storage vacuoles.
- Mitochondria: double membrane; smooth outer, folded inner (cristae); two compartments for aerobic respiration.
- Chloroplasts: double membrane; in plants and photosynthetic algae; site of photosynthesis.
Key relationships
| Structure | Feature | Why it helps the cell |
|---|---|---|
| Rough ER | ribosomes on membrane | new export proteins go straight into a separate compartment |
| Golgi | flattened stack with entry and exit faces | products are finished and sorted in order |
| Lysosome | membrane around hydrolytic enzymes | digestion happens without harming the rest of the cell |
| Mitochondrion | folded inner membrane | more area for ATP-making proteins |
| Double membranes | two compartments | different reactions kept apart |
| Plant central vacuole | large water store | turgor pressure supports the plant |
Assumptions behind claims from data
- A large volume of an organelle suggests a heavy use of its function; it does not prove it.
- In a pulse-chase experiment, only molecules made during the pulse carry the label, so the label tracks one batch along the route.
Mistakes to avoid
- Calling ribosomes membrane-bound or “eukaryote only”.
- Saying the Golgi makes proteins.
- Saying proteins cross membranes between ER and Golgi; vesicles carry them.
- Saying cristae hold more fluid; they add membrane area.
- Forgetting that plant cells have mitochondria as well as chloroplasts.
- Listing an organelle without linking its structure to its function.
Quick self-check
- A cell’s mitochondria have an inner membrane area of 30 µm² and an outer area of 10 µm². What does the ratio of 3 tell you? (The inner membrane is folded, giving three times the outer area for ATP-making proteins.)
- A secreted protein is radioactively labelled. Where is the label first, and where last? (First in the rough ER; last outside the cell, after Golgi and secretory vesicles.)
- Why does a wilting plant cell have a smaller central vacuole? (It has lost water, so turgor pressure has fallen.)
Next: practice questions.