Biology: Cells, Microscopy and Genetics
Cells, microscopy and genetics, at 13+ Biology, covers how cells become specialised for a particular job, how to use a light microscope and calculate magnification, and the basics of how characteristics are inherited.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn that a cell becomes specialised (differentiated) for a particular job, and be able to link the structure of a named specialised cell to its function, for example a sperm cell has a tail (flagellum) to swim and many mitochondria to release energy for swimming.
- Learn the parts of a light microscope (eyepiece lens, objective lens, stage, focusing wheel, light source) and the correct method for preparing a slide: place a thin sample on a slide, add a drop of a stain such as iodine to make structures visible, and lower a coverslip at an angle using a mounted needle to avoid trapping air bubbles.
- Always view a specimen on the lowest-power objective lens first to find it in a wide field of view, then switch to a higher-power objective lens for a more detailed view; starting on high power makes the specimen hard to locate.
- Learn and be able to apply the magnification formula: magnification equals size of image divided by size of real object, making sure both measurements are converted to the same unit before dividing.
- Learn that the total magnification of a microscope is the eyepiece lens magnification multiplied by the objective lens magnification in use.
- Learn that chromosomes, found in the nucleus, are made of DNA and carry genes; a gene is a section of DNA that codes for a particular characteristic, and different versions of the same gene are called alleles.
- Learn how to complete a simple Punnett square to predict the possible genotypes of offspring from a monohybrid cross, using a capital letter for a dominant allele and the matching lowercase letter for the recessive allele.
- Learn the difference between variation caused only by genes (inherited, such as blood group), variation caused only by the environment (such as a scar or a suntan), and variation caused by a combination of both genes and environment (such as height).
Worked example
A photograph of an onion cell viewed under a light microscope shows the cell is 6 mm wide in the image. The real width of the onion cell is 0.12 mm. Calculate the magnification of the image.
- Write down the formula: magnification equals size of image divided by size of real object.
- Check both measurements are in the same unit: the image is 6 mm and the real object is 0.12 mm, so both are already in mm.
- Substitute the values into the formula: magnification = 6 / 0.12.
- Calculate: 6 / 0.12 = 50.
- Final answer: the magnification of the image is x50 (the image is 50 times bigger than the real onion cell). Magnification is a ratio of two lengths, so it has no unit.
Practice questions
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Q1Name the specialised cell in humans that is adapted with a long tail and many mitochondria to swim toward an egg cell.Show answer
Answer: Sperm cell
Q2State one way a red blood cell is adapted to carry oxygen efficiently.Show answer
Answer: Any one of: it has no nucleus, giving more space to carry oxygen, it is a biconcave disc shape giving a larger surface area, or it contains haemoglobin, which binds to oxygen
Q3On a microscope, the eyepiece lens has a magnification of x10 and the objective lens in use has a magnification of x40. Calculate the total magnification.Show answer
Answer: x400 (10 x 40 = 400)
Q4A cell has a real width of 0.02 mm. In a drawing, the same cell is shown 4 mm wide. Calculate the magnification of the drawing.Show answer
Answer: x200 (4 / 0.02 = 200)
Q5A pupil views onion cells under a microscope after adding a drop of iodine solution to the slide. State why the iodine solution was added.Show answer
Answer: To stain the cell structures (such as the nucleus and cell wall) so they are easier to see under the microscope
Q6In pea plants, the allele for tall (T) is dominant over the allele for short (t). State the possible genotype(s) of a tall pea plant.Show answer
Answer: TT or Tt
Q7State whether human eye colour is an example of inherited variation, environmental variation, or both.Show answer
Answer: Inherited variation (it is controlled by genes and is not affected by the environment)
Exam-style questions
Written in the style of a 13+ Common Entrance exam paper, with a full mark scheme.
A student compares an unfertilised human egg cell with a sperm cell. (a) State one structural feature that helps a sperm cell to swim to an egg cell. (b) State one structural feature that gives the egg cell a store of nutrients for the early embryo. (c) A normal human body cell contains 46 chromosomes. State the number of chromosomes found in a human gamete (sperm or egg cell), and explain why this number must be half the usual amount.
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A student examines a plant cell under a microscope. The real length of the cell is 0.05 mm. When drawn in her lab book, the cell is 15 mm long. (a) Calculate the magnification of her drawing, showing your working. (b) State what happens to the field of view (the area you can see) when the microscope's magnification is increased. (c) Suggest one reason why a specimen should always be found using the lowest-power objective lens first.
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In hamsters, the allele for brown fur (B) is dominant over the allele for white fur (b). A brown hamster with genotype Bb is crossed with a white hamster with genotype bb. (a) State the possible genotypes of the offspring from this cross. (b) State the ratio of brown offspring to white offspring you would expect. (c) State the term used to describe an organism, such as one with genotype Bb, that has two different alleles for a gene.
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Free printable worksheet
Want more practice on paper? Download the biology: cells, microscopy and genetics worksheet pack - 16 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 4 of 13+ Science Workbook, the whole course as one free printable PDF.
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