2.4.1Inquiry Process
Cell Organelles — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 55 of 160
Section 1 of 5Membrane-Bound Compartments
The cytoplasm of a eukaryotic cell is divided into compartments, each specialised for particular jobs.
Nucleus
- Surrounded by a double membrane with pores;
- it contains the DNA and is the site of transcription and ribosome subunit assembly.
Mitochondria
- Bounded by two membranes; the site of aerobic respiration, where most ATP is made in animal and plant cells.
Chloroplasts
- Bounded by two membranes and found in plants and algae;
- they carry out photosynthesis using pigments held in thylakoid membranes.
Single-membrane organelles
- Rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles and vesicles each have one membrane.
| Organelle | Membranes | Main function |
|---|---|---|
| Nucleus | 2 | Stores DNA, makes mRNA |
| Mitochondrion | 2 | Aerobic respiration |
| Chloroplast | 2 | Photosynthesis |
| Rough ER | 1 | Makes proteins for export or membranes |
| Golgi apparatus | 1 | Modifies and packages proteins |
| Lysosome | 1 | Digests waste and worn-out structures |
Section 2 of 5Endomembrane System
Several organelles are linked by the movement of vesicles and work as one production line.
Endomembrane System
Rough ER
- Has ribosomes on its surface and makes proteins that enter its lumen for folding and processing.
Smooth ER
- Has no ribosomes and makes lipids, including phospholipids and steroids, and takes part in detoxification.
Golgi apparatus
- A stack of flattened sacs that modifies proteins and packages them in vesicles.
Lysosomes
- Vesicles from the Golgi containing hydrolytic enzymes that digest worn-out organelles and engulfed material.
Section 3 of 5Organelles Without Membranes
Ribosomes
- Made of RNA and protein, they build polypeptides;
- those of eukaryotes are 80S and those in mitochondria are 70S.
Cytoskeleton
- Protein fibres such as microtubules give shape, move organelles and form the spindle.
Centrosome
- Organises microtubules in animal cells during cell division.
Cell wall
- In plants a cellulose wall outside the membrane gives support, but it is not an organelle.
Section 4 of 5A Journey of a Secreted Protein
1
The gene for the protein is transcribed in the nucleus and mRNA leaves through a nuclear pore.
2
A ribosome on the rough ER translates the mRNA, and the polypeptide enters the ER lumen.
3
A transport vesicle carries the protein from the ER to the Golgi apparatus.
4
The Golgi apparatus modifies the protein and packages it into a secretory vesicle.
5
The vesicle fuses with the plasma membrane and releases the protein by exocytosis.
Section 5 of 5Exam Checklist & Tips
2.4.1 – Key Points for the Exam
- Organelle A specialised, usually membrane-bound structure inside a cell with a specific function.
- Endomembrane system The network of membranes and vesicles connecting the ER, Golgi, lysosomes and plasma membrane.
- Lysosome A vesicle containing digestive enzymes that breaks down waste and old organelles.
- Ribosome A structure of RNA and protein that makes polypeptides.
- Cytoskeleton A network of protein fibres that supports the cell and moves structures within it.
Common Mistakes
- Saying the nucleus has one membrane; it has two.
- Giving ribosomes a membrane; they have none.
- Stating that smooth ER makes proteins.
- Assuming all eukaryotes have chloroplasts; animal cells do not.
Last updated:
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.