Grade 7-9 Only
Mitochondria Structure
Mitochondria have a double membrane. The inner membrane is highly folded into projections called cristae, which massively increase the surface area for the enzymes and proteins involved in aerobic respiration. The central fluid-filled space is called the matrix, where many of the reactions of aerobic respiration take place.
Chloroplast Structure
Chloroplasts also have a double outer membrane. Inside, they contain flattened membrane sacs called thylakoids which are stacked into columns called grana. Chlorophyll is found in the thylakoid membranes. The fluid-filled space between the grana is called the stroma, where the reactions of photosynthesis that make glucose take place.
How Electron Microscopy Revealed This Detail
None of this internal detail was visible using a light microscope. Light microscopes use visible light and have a resolution of only about 200 nanometres (nm) — the smallest distance at which two points can still be told apart. Electron microscopes use electrons instead, which have a much shorter wavelength, giving a resolution of around 0.2 nm — roughly 1000 times better. This huge improvement in resolution is what allowed scientists to see the cristae, the double membrane, and the grana for the first time. Seeing this structure is also what allowed scientists to explain function: the large surface area created by cristae and grana explains why mitochondria and chloroplasts can carry out their reactions at such a high rate. The observation of structure, made possible by electron microscopy, came first — and the explanation of function followed from it.
Quick Check: Insulin is a protein hormone made by cells in the pancreas. Which three organelles would be most active in a pancreatic cell making large amounts of insulin? Explain why for each.
1. Ribosomes — protein synthesis: the mRNA code for insulin is translated into a polypeptide chain here, which is then folded into its correct 3D shape. 2. Golgi apparatus — modifies and packages the folded insulin into secretory vesicles, which fuse with the cell membrane and release insulin into the bloodstream (exocytosis). 3. Mitochondria — provide the ATP energy needed to power protein synthesis, folding, and transport throughout this whole process.
This higher tier covers Advanced: Organelle Detail for Grade 7-9 within Cell Organelles for GCSE Biology. Revise Cell Organelles in Cell Biology for GCSE Biology with 12 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 11 of 14 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Practice questions for Cell Organelles
Scientists studying mitochondria use electron microscopes rather than light microscopes. Which statement correctly explains why electron microscopes are more useful for studying cell ultrastructure?
The diameter of a ribosome is approximately 0.00002 mm. (a) Write this diameter in standard form, in millimetres. (b) Convert this diameter into nanometres (nm).