B1: Cell Structure
Eukaryotic and prokaryotic cells, microscopy, and magnification calculations
Key Definitions
Eukaryotic cell — A cell that contains a nucleus and membrane-bound organelles (e.g. animal, plant, fungal cells).
Prokaryotic cell — A cell that does not have a nucleus or membrane-bound organelles (e.g. bacterial cells).
Organelle — A specialised structure within a cell that performs a specific function.
Magnification — How many times larger an image appears compared to the real object.
Resolution — The ability to distinguish between two separate points; the detail a microscope can show.
Prokaryotic cell — A cell that does not have a nucleus or membrane-bound organelles (e.g. bacterial cells).
Organelle — A specialised structure within a cell that performs a specific function.
Magnification — How many times larger an image appears compared to the real object.
Resolution — The ability to distinguish between two separate points; the detail a microscope can show.
Eukaryotic vs Prokaryotic Cells
| Feature | Eukaryotic Cell | Prokaryotic Cell |
|---|---|---|
| Nucleus | Present — contains DNA in linear chromosomes | Absent — circular DNA free in cytoplasm |
| Size | Typically 10–100 μm | Typically 1–5 μm |
| Membrane-bound organelles | Present (mitochondria, chloroplasts, etc.) | Absent |
| DNA | Linear chromosomes inside nucleus | Single circular chromosome + plasmids |
| Cell wall | Plants: cellulose; Fungi: chitin | Peptidoglycan |
| Ribosomes | Larger (80S) | Smaller (70S) |
| Cell membrane | Present | Present |
| Examples | Animal, plant, fungal, protist cells | Bacteria (e.g. E. coli) |
Animal Cell Structures
Nucleus — Contains genetic material (DNA); controls cell activities and gene expression.
Cell membrane — Controls which substances enter and leave the cell; partially permeable.
Cytoplasm — Gel-like substance where chemical reactions (metabolism) occur; contains enzymes.
Mitochondria — Site of aerobic respiration; transfers energy from glucose to ATP.
Ribosomes — Site of protein synthesis; translate mRNA into polypeptide chains.
Cell membrane — Controls which substances enter and leave the cell; partially permeable.
Cytoplasm — Gel-like substance where chemical reactions (metabolism) occur; contains enzymes.
Mitochondria — Site of aerobic respiration; transfers energy from glucose to ATP.
Ribosomes — Site of protein synthesis; translate mRNA into polypeptide chains.
Exam tip: Animal cells do NOT have a cell wall, chloroplasts, or a permanent vacuole. Only plant cells have these three structures.
Plant Cell Extra Structures
Cell wall — Made of cellulose; provides structural support and is fully permeable (not selectively permeable).
Chloroplasts — Contain chlorophyll (absorbs light); site of photosynthesis; found in green parts of plants.
Permanent vacuole — Contains cell sap (a solution of sugars and salts); maintains turgor pressure to keep cell rigid.
Chloroplasts — Contain chlorophyll (absorbs light); site of photosynthesis; found in green parts of plants.
Permanent vacuole — Contains cell sap (a solution of sugars and salts); maintains turgor pressure to keep cell rigid.
Example 1: Labelling an Animal Cell
A typical animal cell contains: nucleus (controls the cell), cell membrane (selectively permeable barrier), cytoplasm (site of metabolic reactions), mitochondria (aerobic respiration), and ribosomes (protein synthesis). It does NOT contain a cell wall, chloroplasts, or a permanent vacuole.
Example 2: Labelling a Plant Cell
A typical plant cell contains ALL the structures of an animal cell PLUS: a cellulose cell wall (outside the cell membrane, provides rigidity and support), chloroplasts (contain chlorophyll for photosynthesis), and a permanent vacuole filled with cell sap (maintains turgor). Note: root hair cells lack chloroplasts because they are underground and do not photosynthesise.
Bacterial Cell Structure (Prokaryotic)
Circular DNA — One long circular chromosome; floats free in the cytoplasm (not in a nucleus).
Plasmids — Small extra rings of DNA; can carry antibiotic resistance genes; can be transferred between bacteria.
Flagella — Long tail-like projections for movement (not all bacteria have these).
No nucleus, no mitochondria, no chloroplasts — Bacteria are prokaryotes and lack membrane-bound organelles.
Cell wall — Made of peptidoglycan (not cellulose).
Slime capsule — Protective layer outside the cell wall; helps bacteria stick to surfaces and evade immune systems.
Plasmids — Small extra rings of DNA; can carry antibiotic resistance genes; can be transferred between bacteria.
Flagella — Long tail-like projections for movement (not all bacteria have these).
No nucleus, no mitochondria, no chloroplasts — Bacteria are prokaryotes and lack membrane-bound organelles.
Cell wall — Made of peptidoglycan (not cellulose).
Slime capsule — Protective layer outside the cell wall; helps bacteria stick to surfaces and evade immune systems.
Example 3: Comparing a Bacterial Cell to a Plant Cell
A bacterial cell has circular DNA and plasmids but lacks a nucleus, mitochondria, and chloroplasts. A plant cell has linear DNA enclosed in a nucleus, mitochondria for respiration, chloroplasts for photosynthesis, a cellulose cell wall, and a permanent vacuole. Both have a cell membrane, cytoplasm, and ribosomes (though bacterial ribosomes are smaller — 70S vs 80S). The bacterial cell wall is made of peptidoglycan, while the plant cell wall is made of cellulose.
Example 4: Identifying Cell Type from a Description
A cell has a cell wall, a nucleus, mitochondria, but no chloroplasts. What type of cell is it?It is a plant cell from a non-photosynthetic tissue (e.g. root cell). The presence of a cell wall and nucleus rules out animal and bacterial cells. The absence of chloroplasts means it is not from a green part of the plant.
Microscopy: Light vs Electron Microscopes
| Feature | Light Microscope | Electron Microscope |
|---|---|---|
| Maximum magnification | About ×2,000 | About ×2,000,000 |
| Resolution | About 200 nm | About 0.2 nm |
| Specimen | Living or dead | Dead only (vacuum required) |
| Cost | Relatively cheap | Very expensive |
| Size and portability | Small, portable | Large, requires special room |
| Preparation | Simple staining | Complex preparation required |
| Image | Colour image (direct view) | Black and white image (on screen) |
| Can see | Cells, large organelles | Internal structure of organelles, ribosomes, plasmids |
Exam tip: Electron microscopes have higher magnification AND higher resolution than light microscopes. Resolution is the ability to see fine detail — it is NOT the same as magnification! Higher resolution means you can distinguish between two points that are closer together.
Magnification Calculations
Magnification = Size of image ÷ Size of real object
Image size = Magnification × Real size
Real size = Image size ÷ Magnification
Image size = Magnification × Real size
Real size = Image size ÷ Magnification
Unit Conversions
1 mm = 1,000 μm (millimetres to micrometres: multiply by 1,000)
1 μm = 1,000 nm (micrometres to nanometres: multiply by 1,000)
1 mm = 1,000,000 nm (millimetres to nanometres: multiply by 1,000,000)
1 μm = 1,000 nm (micrometres to nanometres: multiply by 1,000)
1 mm = 1,000,000 nm (millimetres to nanometres: multiply by 1,000,000)
Exam tip: Always convert both measurements to the same unit before calculating magnification. The most common error in exams is forgetting to convert units. Write out the conversion step to get method marks even if you make an arithmetic error.
Example 5: Calculating Magnification
A cell is 50 μm wide in real life and appears 25 mm wide in an image.Convert image size: 25 mm = 25 × 1,000 = 25,000 μm
Magnification = 25,000 ÷ 50 = ×500
Example 6: Calculating Real Size
A mitochondrion in a photograph measures 20 mm. The magnification is ×10,000.Real size = 20 mm ÷ 10,000 = 0.002 mm = 0.002 × 1,000 = 2 μm
Example 7: Unit Conversion and Magnification
A bacterium appears 40 mm long in an electron micrograph. Its actual length is 2 μm. Calculate magnification.Convert: 40 mm = 40,000 μm
Magnification = 40,000 ÷ 2 = ×20,000
Example 8: Finding Image Size
A red blood cell has a diameter of 8 μm. Under a microscope with magnification ×2,500, how large will it appear?Image size = 8 × 2,500 = 20,000 μm = 20,000 ÷ 1,000 = 20 mm
Example 9: Multi-step Unit Conversion
A virus particle has a diameter of 200 nm. It appears 60 mm in an image. Calculate magnification.Convert: 60 mm = 60 × 1,000,000 = 60,000,000 nm
Magnification = 60,000,000 ÷ 200 = ×300,000
Practice Questions
1. Foundation Name three structures found in both animal and plant cells.
Nucleus, cell membrane, cytoplasm, mitochondria, ribosomes (any three).
2. Foundation State two structures found in plant cells but not in animal cells.
Chloroplasts, permanent vacuole, cellulose cell wall (any two).
3. Higher A cell has an actual diameter of 0.05 mm. In a micrograph it measures 30 mm. Calculate the magnification.
Magnification = image ÷ real = 30 ÷ 0.05 = ×600
4. Higher A ribosome is 25 nm in diameter. Under a magnification of ×40,000, what will its image size be in mm?
Image size = 25 nm × 40,000 = 1,000,000 nm = 1,000 μm = 1 mm
5. Higher Explain two differences between a bacterial cell and a plant cell.
1) A bacterial cell has circular DNA free in the cytoplasm and no nucleus, whereas a plant cell has linear DNA enclosed in a nucleus. 2) A bacterial cell lacks chloroplasts and mitochondria, whereas a plant cell has both. Also acceptable: bacterial cell wall is made of peptidoglycan, plant cell wall is cellulose; bacterial cells have plasmids, plant cells do not.
6. Foundation Explain why electron microscopes can show more detail than light microscopes.
Electron microscopes use electron beams with a much shorter wavelength than light, giving a much higher resolution. This means they can distinguish between points that are much closer together, revealing fine detail such as the internal structure of organelles (e.g. cristae inside mitochondria) that light microscopes cannot resolve.
🔬 Required Practical
Required Practical: Using a Light Microscope
Aim: To prepare and view a slide using a light microscope, and calculate magnification.
Method: 1) Prepare a slide — place a thin specimen on a slide, add a drop of stain, and lower a coverslip at an angle to avoid air bubbles. 2) Focus — start with the lowest power objective lens, use the coarse focus knob to bring the image into view, then fine-tune with the fine focus knob. 3) Increase magnification by rotating to a higher power objective lens and refocus. 4) Calculate magnification using the formula: magnification = image size / actual size.
Variables: IV: magnification (objective lens), DV: level of detail visible, Control: specimen type, slide preparation method, light intensity
🔢 Maths Skills
Mathematical Skills for this Topic
Magnification equation: magnification = image size / actual size. Rearrange to find any unknown.
Unit conversions: 1 mm = 1,000 μm; 1 μm = 1,000 nm; 1 mm = 1,000,000 nm. Always convert to the same unit before calculating.
Standard form for cell sizes: A typical bacterial cell is 1–5 μm = 1 × 10⁻⁶ m. A red blood cell is ~8 μm = 8 × 10⁻⁶ m. A virus is ~100 nm = 1 × 10⁻⁷ m. Practice converting between mm, μm, nm and standard form.
⚠️ Common Misconceptions
Watch Out!
Students often think prokaryotic cells are simpler so must have evolved first. Wrong: simpler structure means they evolved first Correct: both cell types evolved; prokaryotes appeared first based on fossil evidence, but complexity alone does not determine evolutionary order
Students often think mitochondria make energy. Wrong: mitochondria create energy Correct: energy cannot be created or destroyed; mitochondria transfer energy from glucose to ATP via respiration
✍️ 6-Mark Question
Extended Answer Question
6 marks: Compare and contrast prokaryotic and eukaryotic cells. Include at least four differences in your answer.
Eukaryotic cells contain a nucleus enclosing linear DNA, whereas prokaryotic cells have circular DNA free in the cytoplasm with no nucleus. Eukaryotic cells contain membrane-bound organelles such as mitochondria and chloroplasts, which prokaryotic cells lack. Eukaryotic cells are typically larger (10–100 μm) compared to prokaryotic cells (1–5 μm). Eukaryotic ribosomes are larger (80S) while prokaryotic ribosomes are smaller (70S). Prokaryotic cells may carry plasmids (small extra DNA rings) and have a cell wall made of peptidoglycan, whereas eukaryotic plant cell walls are made of cellulose. Both cell types have a cell membrane, cytoplasm, and ribosomes.
Mark scheme: 1 mark per valid comparison with both sides stated (max 6). Must include at least four distinct differences. 1 mark for overall quality of comparison language.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
You are given microscope images of two cells with scale bars. Cell A measures 40 mm in the image and the scale bar shows 10 μm = 20 mm. Cell B measures 15 mm in the image with the same scale bar. Calculate the actual size of each cell, determine whether each is eukaryotic or prokaryotic, and justify your answer using the calculated sizes and visible structural features.
Approach: First calculate magnification from the scale bar (magnification = scale bar image size / scale bar actual size = 20 mm / 10 μm = 20,000 μm / 10 μm = ×2,000). Then find actual sizes: Cell A = 40,000 μm / 2,000 = 20 μm (eukaryotic — too large for prokaryote, likely has nucleus visible). Cell B = 15,000 μm / 2,000 = 7.5 μm (could be either — check for nucleus and organelles in the image to decide).
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.