Cell Structure and Microscopy
Cells are the smallest unit of living things, and eukaryotic cells (animal, plant and fungal) differ from prokaryotic cells (bacteria) in how they store their DNA. Eukaryotic cells keep their DNA in a nucleus and contain mitochondria and ribosomes, while prokaryotic cells are much smaller, have no nucleus, and carry a single loop of DNA plus small plasmid rings.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the sub-cellular structures with a function each: nucleus (contains DNA, controls the cell), cytoplasm (site of most chemical reactions), cell membrane (controls what enters and leaves), mitochondria (aerobic respiration, releasing energy), ribosomes (protein synthesis), and in plants the cell wall (support), chloroplasts (photosynthesis) and permanent vacuole (cell sap, keeps the cell turgid).
- Learn the prokaryote list separately: no nucleus, DNA as a single loop, plasmids, no mitochondria or chloroplasts, and much smaller, typically a few micrometres across against 10 to 100 micrometres for a eukaryotic cell.
- Learn the microscope comparison as a pair of reasons: an electron microscope has much higher magnification AND much higher resolution than a light microscope, so it reveals sub-cellular structures such as ribosomes and the internal structure of mitochondria; a light microscope is cheap, portable and can show living cells.
- Use the formula triangle: magnification = image size / real size, so real size = image size / magnification and image size = real size x magnification.
- Convert units before substituting, not after: 1 mm = 1000 micrometres, and 1 micrometre = 1000 nanometres. Write the number in standard form if it has more than four zeros.
- For the microscopy required practical, remember the order: prepare the slide with a thin sample and a stain such as iodine, lower the coverslip at an angle to avoid air bubbles, focus with the lowest power objective first using the coarse focus, then switch to higher power and use fine focus only.
Worked example
A cheek cell appears 45 mm wide in a photograph taken at a magnification of 1500. Calculate the real width of the cell in micrometres.
- Write the formula in the form needed: real size = image size / magnification.
- Convert the image size to micrometres first, because the answer is wanted in micrometres: 45 mm x 1000 = 45 000 micrometres.
- Substitute: real size = 45 000 / 1500.
- Calculate: 45 000 / 1500 = 30.
- Final answer: the cell is 30 micrometres wide, which is a sensible order of magnitude for an animal cell.
Practice questions
Try each question, then tap to reveal the answer.
Q1Name two structures found in a plant cell but not in an animal cell.Show answer
Answer: Any two of: cell wall (cellulose), chloroplasts, permanent vacuole.
Q2Where is the DNA found in a bacterial cell?Show answer
Answer: In a single loop of DNA free in the cytoplasm, plus small rings called plasmids. Bacteria have no nucleus.
Q3State the function of the mitochondria.Show answer
Answer: The site of aerobic respiration, which transfers energy for the cell to use.
Q4Give the equation linking magnification, image size and real size.Show answer
Answer: Magnification = image size / real size.
Q5Convert 0.05 mm into micrometres.Show answer
Answer: 0.05 x 1000 = 50 micrometres.
Q6Give one advantage of a light microscope over an electron microscope.Show answer
Answer: It is much cheaper and more portable, and it can be used to view living cells (specimens for an electron microscope must be dead and in a vacuum).
Q7Why is a stain such as iodine used when preparing a slide?Show answer
Answer: It makes structures such as the nucleus show up against the rest of the cell, which is otherwise transparent.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A sperm cell has many mitochondria and a tail. Explain how these two features are adaptations for its function.
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An image of a bacterial cell measures 60 mm in length. The real length of the cell is 2 micrometres. Calculate the magnification of the image.
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Compare a light microscope and an electron microscope, and explain why the electron microscope changed our understanding of cells.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the cell structure and microscopy worksheet pack - 11 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 9 of GCSE Biology Workbook, the whole course as one free printable PDF.
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