B1: Cell Structure
Plant and animal cells, prokaryotic and eukaryotic cells, microscopy
B1 Cell Structure
📋 Key Concepts
All living organisms are made of cells. Cells are the basic building blocks of life. Some organisms are unicellular (made of one cell, like bacteria) and some are multicellular (made of many cells, like humans).
Two Types of Cell
Eukaryotic cells have a nucleus and membrane-bound organelles. Animals, plants, fungi and protists are eukaryotic.
Prokaryotic cells do not have a nucleus. Bacteria are prokaryotic.
Prokaryotic cells do not have a nucleus. Bacteria are prokaryotic.
| Feature | Eukaryotic Cell | Prokaryotic Cell |
|---|---|---|
| Nucleus | Yes | No (nucleoid region) |
| Cell membrane | Yes | Yes |
| Cytoplasm | Yes | Yes |
| Ribosomes | Yes (larger, 80S) | Yes (smaller, 70S) |
| Cell wall | Plants and fungi only | Always present (peptidoglycan) |
| Mitochondria | Yes | No |
| Chloroplasts | Plant cells only | No |
| DNA | Linear chromosomes in nucleus | Circular chromosome + plasmids |
| Size | Larger (5-100 μm) | Smaller (0.2-2.0 μm) |
📝 Animal Cells
Animal cells are eukaryotic cells that contain: a nucleus, cell membrane, cytoplasm, mitochondria, and ribosomes. They do NOT have a cell wall, chloroplasts, or a permanent vacuole.
Functions of Animal Cell Components
- Nucleus - contains DNA (genetic material) and controls the activities of the cell
- Cell membrane - controls what enters and leaves the cell (partially permeable)
- Cytoplasm - jelly-like substance where chemical reactions take place
- Mitochondria - where aerobic respiration happens, releasing energy for the cell
- Ribosomes - where protein synthesis happens (making proteins)
Example 1
Question: Name the part of an animal cell that controls what enters and leaves.
Answer: The cell membrane. It is partially permeable, meaning it allows some substances through but not others.
Example 2
Question: Where does aerobic respiration take place in an animal cell?
Answer: Mitochondria. This is where glucose and oxygen react to release energy for the cell.
📝 Plant Cells
Plant cells have everything an animal cell has, PLUS three extra structures: a cell wall, chloroplasts, and a permanent vacuole.
Extra Components in Plant Cells
- Cell wall - made of cellulose, provides strength and support (fully permeable, NOT partially permeable)
- Chloroplasts - contain chlorophyll (green pigment) and are where photosynthesis happens
- Permanent vacuole - contains cell sap (a solution of sugars and salts), helps keep the cell turgid (firm)
| Structure | Animal Cell | Plant Cell |
|---|---|---|
| Nucleus | ✓ | ✓ |
| Cell membrane | ✓ | ✓ |
| Cytoplasm | ✓ | ✓ |
| Mitochondria | ✓ | ✓ |
| Ribosomes | ✓ | ✓ |
| Cell wall | ✗ | ✓ (cellulose) |
| Chloroplasts | ✗ | ✓ |
| Permanent vacuole | ✗ | ✓ (cell sap) |
Example 3
Question: Give three differences between a plant cell and an animal cell.
Answer:
- Plant cells have a cellulose cell wall; animal cells do not
- Plant cells have chloroplasts for photosynthesis; animal cells do not
- Plant cells have a permanent vacuole containing cell sap; animal cells do not
Example 4
Question: Why do plant cells need a cell wall but animal cells do not?
Answer: Plant cells need a cell wall to provide strength and support. Without it, the cell would burst when it absorbs water by osmosis. Animal cells do not have a cell wall because they have other support structures (like a skeleton) and are not under the same pressure.
📝 Bacterial Cells (Prokaryotic)
Bacterial cells are prokaryotic. They are much smaller than eukaryotic cells and have a different structure. They have no nucleus - their DNA is a single circular chromosome found free in the cytoplasm.
Structure of a Bacterial Cell
- Chromosomal DNA - one single loop of DNA free in the cytoplasm (not in a nucleus)
- Plasmids - small extra loops of DNA that can carry antibiotic resistance genes
- Cell membrane - controls what enters and leaves
- Cytoplasm - where chemical reactions happen
- Cell wall - made of peptidoglycan (not cellulose like plant cells)
- Flagella - (some bacteria) tail-like structures for movement
- Slime capsule - (some bacteria) protective layer outside the cell wall
Key difference: Bacteria have NO nucleus, NO mitochondria, NO chloroplasts.
Their DNA is free in the cytoplasm as a single circular chromosome.
Their DNA is free in the cytoplasm as a single circular chromosome.
Example 5
Question: Describe two differences between a bacterial cell and a plant cell.
Answer:
- A bacterial cell has no nucleus (its DNA is free in the cytoplasm), whereas a plant cell has a nucleus containing its DNA
- A bacterial cell has no chloroplasts, whereas a plant cell has chloroplasts for photosynthesis
📝 Microscopy
Microscopes let us see cells and their structures. Light microscopes use visible light and glass lenses. Electron microscopes use electrons and can magnify much more.
| Feature | Light Microscope | Electron Microscope |
|---|---|---|
| Magnification | Up to ~2000x | Up to ~2,000,000x |
| Resolution | Lower | Much higher |
| Specimen | Living or dead | Dead only (vacuum required) |
| Cost | Cheap | Very expensive |
| Size | Small, portable | Large |
| Can see | Cells and some organelles | Internal structures of organelles |
Calculating Magnification
Magnification = size of image ÷ size of real object
Or rearranged: size of real object = size of image ÷ magnification
Or: size of image = magnification × size of real object
Or rearranged: size of real object = size of image ÷ magnification
Or: size of image = magnification × size of real object
Example 6
Question: A cell is 0.05 mm in real life. Under a microscope it appears to be 50 mm across. What is the magnification?
Solution:
Magnification = image size ÷ real size = 50 ÷ 0.05 = 1000
Magnification = ×1000
Example 7
Question: A mitochondrion is 2 μm long in real life. It is viewed under a microscope at ×5000 magnification. How long will it appear in the image?
Solution:
Image size = magnification × real size = 5000 × 2 μm = 10,000 μm
Convert: 10,000 μm = 10 mm
Image size = 10 mm
Example 8
Question: A red blood cell appears to be 8 mm in diameter in a photograph taken at ×4000 magnification. What is its real diameter?
Solution:
Real size = image size ÷ magnification = 8 mm ÷ 4000 = 0.002 mm
Convert: 0.002 mm = 2 μm
Real diameter = 2 μm
📝 Unit Conversions for Microscopy
1 mm = 1000 μm (micrometres)
1 μm = 1000 nm (nanometres)
1 mm = 1,000,000 nm
1 μm = 1000 nm (nanometres)
1 mm = 1,000,000 nm
Tip: Always check the units in the question and answer. Convert everything to the same unit before calculating. Exam questions often mix mm and μm.
Example 9
Question: Convert 0.025 mm to μm.
Solution: 0.025 × 1000 = 25 μm
❓ Practice Questions
Q1: Name three structures found in both animal and plant cells.
Q2: Name three structures found in plant cells but NOT in animal cells.
Q3: Describe two differences between a prokaryotic cell and a eukaryotic cell.
Q4: A cell is measured as 0.02 mm in real life. It appears as 40 mm in a micrograph. Calculate the magnification.
Q5: Explain why electron microscopes can show more detail than light microscopes.
Q6: What is the function of ribosomes in a cell?
Q7: A bacterial cell has a plasmid. What is a plasmid and why might it be important in medicine?
✅ Answers
- Nucleus, cell membrane, cytoplasm, mitochondria, ribosomes (any three)
- Cell wall (cellulose), chloroplasts, permanent vacuole (cell sap)
- Prokaryotic cells have no nucleus (DNA is free in cytoplasm); prokaryotic cells are smaller (0.2-2.0 μm vs 5-100 μm); prokaryotic cells have circular DNA and may have plasmids (any two)
- Magnification = 40 ÷ 0.02 = ×2000
- Electron microscopes have higher resolution (ability to distinguish between two points) because electrons have a shorter wavelength than visible light. This means they can show smaller structures like internal organelle detail.
- Ribosomes are where protein synthesis takes place - they make proteins from amino acids.
- A plasmid is a small, circular loop of DNA separate from the main chromosome. It is important in medicine because plasmids can carry genes for antibiotic resistance, which can be passed between bacteria.
🎯 Exam Tips
- Learn the comparison tables - questions often ask you to compare cell types
- "Plant vs animal cell" and "eukaryotic vs prokaryotic" are the two most common comparison questions
- For microscopy calculations, always convert to the same unit first
- Don't confuse cell membrane (partially permeable) with cell wall (fully permeable)
- Remember: bacteria have a cell wall but it is made of peptidoglycan, NOT cellulose
- When asked to "describe" differences, give both sides (e.g. "plant cells have X, but animal cells do not")
- Plasmids are only found in prokaryotic cells - they carry extra genes like antibiotic resistance
🔬 Required Practical
Using a Light Microscope
- Prepare a slide: place a thin sample on a microscope slide, add a drop of stain (e.g. iodine for plant cells) and lower a coverslip
- Turn the objective lens to the lowest magnification
- Place the slide on the stage and secure with clips
- Look through the eyepiece and use the coarse focusing knob to bring the image into view
- Use the fine focusing knob to sharpen the image
- Switch to a higher magnification objective lens and refocus
- Draw what you see, labelling the magnification and any cell structures you can identify
Common errors: Coverslip not lowered at an angle (traps air bubbles); using too much sample; not starting on lowest magnification.
🔢 Maths Skills
Mathematical Skills
Magnification = image size ÷ actual size
Actual size = image size ÷ magnification
Image size = magnification × actual size
Actual size = image size ÷ magnification
Image size = magnification × actual size
Unit conversions:
1 mm = 1000 μm | 1 μm = 1000 nm | 1 mm = 1,000,000 nm
1 mm = 1000 μm | 1 μm = 1000 nm | 1 mm = 1,000,000 nm
Maths Example
A cell appears 40 mm wide in a micrograph at ×2000 magnification. What is its actual size?
Actual = 40 ÷ 2000 = 0.02 mm = 20 μm
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Prokaryotes are simpler so they must have evolved first Correct: Simplicity does not determine evolutionary order — prokaryotes and eukaryotes evolved along different lines
2. Wrong: Mitochondria make energy Correct: Mitochondria transfer energy from glucose to ATP — energy cannot be created or destroyed
✍️ 6-Mark Question
Extended Answer
6 marks: Compare prokaryotic and eukaryotic cells.
Eukaryotic cells have a nucleus containing linear DNA in chromosomes, whereas prokaryotic cells have a single circular chromosome free in the cytoplasm and may carry plasmids. Eukaryotic cells contain membrane-bound organelles such as mitochondria and (in plants) chloroplasts, which prokaryotic cells lack. Eukaryotic cells are typically larger (5–100 μm) while prokaryotic cells are smaller (0.2–2.0 μm). Both types have ribosomes, a cell membrane and cytoplasm, but eukaryotic ribosomes (80S) are larger than prokaryotic ones (70S). Plant eukaryotic cells have a cellulose cell wall; prokaryotic cell walls are made of peptidoglycan.
Mark scheme: 1 mark for nucleus/DNA difference; 1 mark for organelles; 1 mark for size; 1 mark for ribosomes; 1 mark for cell wall material; 1 mark for comparative structure throughout
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student measures a red blood cell in a micrograph as 6 mm diameter. The magnification is ×3000.
(a) Calculate the actual diameter of the cell in μm.
(b) The student then measures a white blood cell in the same micrograph as 9 mm. The student concludes the white blood cell is 50% larger. Evaluate this conclusion.
Answers: (a) Actual = 6 ÷ 3000 = 0.002 mm = 2 μm. (b) The white blood cell image is 50% larger in the image (9 mm vs 6 mm), so its actual size is also 50% larger (3 μm vs 2 μm) since both are at the same magnification. The conclusion is correct.
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