3.1.7Topic 3 · Voice of the Genome
Recognising Organelles — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 9 parts Lesson 9 of 26 1 quiz
Lesson summary
3.1.7 Recognising Organelles — Edexcel A Level Biology A (SNAB) 9BI0 notes: Identify cell organelles from micrographs and diagrams and use their appearance to deduce the function and activity of the cell.An electron micrograph is a black and white image in which dense structures …
- Exam-style question 1
- Exam-style question 2
- Exam-style question 3
- Heterochromatin
- Phagocytosis
- Abundant smooth ER suggests lipid or steroid production, as in the cells of the adrenal cortex and gonads
Part 1 of 7Approach to a micrograph
An electron micrograph is a black and white image in which dense structures look dark.
Approach to a micrograph
When asked to identify an organelle, use three clues: its size and shape, whether it has one or two membranes, and its internal features.
Remember also that you are viewing a two-dimensional slice, so the same organelle can appear round, oval or long depending on how it was cut.
Part 2 of 7Key identification features
| Organelle | Appearance in an electron micrograph |
|---|---|
| Nucleus | Large (about 5 to 10 µm), round, double envelope with pores, dark nucleolus, patches of dense heterochromatin |
| Mitochondrion | Oval or rod, about 1 to 5 µm, double membrane, inner membrane folded into cristae, grey matrix |
| Chloroplast | Lens shape, double membrane, stacks of thylakoids (grana) and starch grains |
| Rough ER | Parallel flattened sacs with small dark dots (ribosomes) on the outer surface |
| Smooth ER | Network of tubes or sacs without dots |
| Golgi apparatus | Curved stack of flat sacs with vesicles budding from the edges |
| Lysosome | Small round, single membrane, dark uniform contents |
| Vacuole | Large, round, single membrane (tonoplast), pale contents |
| Ribosomes | Tiny dark dots free or on ER |
Part 3 of 7Deducing the cell's function
The abundance of an organelle shows what the cell does.
1
Many mitochondria with dense cristae suggest a high demand for ATP, as in muscle cells, sperm midpieces and cells of the proximal convoluted tubule.
2
Extensive rough ER and several Golgi stacks point to a cell that secretes protein,
- such as an enzyme-secreting pancreatic cell or antibody-secreting plasma cell.
3
Abundant smooth ER suggests lipid or steroid production, as in the cells of the adrenal cortex and gonads.
4
Many lysosomes suggest phagocytosis, as in neutrophils and macrophages.
5
Many chloroplasts show a photosynthetic cell.
6
A lot of vesicles near the cell surface indicates active exocytosis or endocytosis.
Part 4 of 7Eukaryotic and prokaryotic clues
If a micrograph shows no nucleus and no membrane-bound organelles, but there is a cell wall, a darker nucleoid and perhaps a flagellum, the cell is a bacterium.
Eukaryotic and prokaryotic clues
Very small cells (around 1 to 2 µm) are a strong clue.
If there is a nucleus and organelles, it is eukaryotic.
A large vacuole and a wall mean plant; centrioles and no wall suggest an animal cell.
Part 5 of 7Calculating real size from an image
Use actual size = image size / magnification.
Calculating real size from an image
Measure the image in millimetres using a ruler, convert to micrometres (multiply by 1000), then divide by the magnification.
If the magnification is not given but there is a scale bar, measure the bar and compare it with the length being measured: size = (measured length / bar length) x value of the bar.
Check units carefully.
Part 6 of 7Worked approach and common traps
When you are given a micrograph in an examination, first note whether a scale bar or magnification is shown.
Worked approach and common traps
Then identify the largest and most obvious structure, usually the nucleus, and use it as a reference for relative size.
Next look for clues such as a double membrane, folds or ribosome dots.
State the features you can see in your answer,
- because marks are usually given for naming the structure and for a visible feature that supports your identification.
Worked approach and common traps
Questions may link organelle abundance to a disease.
- For example, a person with a mitochondrial disorder may have muscles that tire quickly, because less ATP is made.
- Cells lacking functioning lysosomal enzymes accumulate undigested material.
- Such links show that structure and function are connected at the level of the cell.
Summary of method: identify, justify with a visible feature, deduce the function of the cell from the most abundant organelles,
- and where required calculate the real size.
- State the units you use, and write the working so that credit can be given even if the final answer is incorrect.
Practice by labelling a diagram of an animal cell and a plant cell from memory, then compare your drawing with a micrograph.
Worked approach and common traps
Notice that organelle proportions change between tissues: a hepatocyte, a muscle fibre and a root cell look very different even though they share the same basic components.
Questions may also ask you to explain why a particular structure is absent, for example why a mature red blood cell has no mitochondria and therefore relies on anaerobic respiration.
Part 7 of 7Exam-style questions
An electron micrograph shows a cell containing large amounts of rough ER, many Golgi stacks and numerous vesicles near the surface membrane. Suggest the function of this cell. Explain your answer. [3]3 marks
Exam-style questions1. The cell secretes protein (for example an enzyme or hormone). [1]2. Rough ER with ribosomes makes proteins and the Golgi modifies and packages them. [1]3. Vesicles carry the secretory product to the membrane for exocytosis. [1]A mitochondrion in a micrograph is 45 mm long on the image. The magnification is x15 000. Calculate its actual length in µm. [2]2 marks
45 mm = 45 000 µm. [1]
45 000 / 15 000 = 3 µm. [1]
State two features that would let you identify a chloroplast in a micrograph. [2]2 marks
Double membrane (envelope). [1]
Stacks of thylakoid membranes (grana) in a stroma (starch grains also acceptable). [1]
Key terms
| Term | Meaning |
|---|---|
| Micrograph | A photograph or image taken through a microscope. |
| Scale bar | A line on an image representing a given real length. |
| Heterochromatin | Densely packed, mostly inactive chromatin that stains dark. |
| Thylakoid | A flattened membrane sac in a chloroplast containing chlorophyll. |
| Secretion | Release of a useful substance from a cell, usually by exocytosis. |
| Phagocytosis | Engulfing of solid particles by a cell. |
Avoid these mistakes
1
Mixing up rough ER and Golgi
- rough ER has ribosomes (dots) and parallel sacs;
- the Golgi is a curved stack with vesicles and no dots.
2
Forgetting to convert units before dividing by magnification.
3
Treating one section as the whole organelle. A single round section can be part of a long tube.
4
Saying a cell with many mitochondria 'makes energy'. Energy cannot be made; mitochondria make ATP by transferring energy.
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