Cell biology

• The structure and functioning of cells and how they divide by mitosis and meiosis from sections Cell biology and Meiosis.
• Eukaryotic cells


  Cells with genetic material (DNA) enclosed in a nucleus. Plant and animal cells.
• Prokaryotic cells


  Cells with genetic material (DNA) not enclosed in a nucleus. Bacterial cells. Much smaller in comparison.
• Genetic material in a prokaryotic cell


  A single loop of DNA. May also have small rings of DNA called plasmids.
• Calculating difference in orders of magnitude

  • Amoeba diameter: 10 μm = 1 x 10^-5 m, Egg cell diameter: 0.1 mm = 1 x 10^-4 m, Difference in order of magnitude: 5 - 4 = 1
• Calculating how many times larger and orders of magnitude larger a liver cell is compared to a bacterial cell

  • Liver cell diameter: 2.5 x 10^-5 m, Bacterial cell diameter: 2.0 x 10^-7 m, Liver cell is 125 times larger, 2 orders of magnitude larger
• Sub-cellular structures in animal and plant cells

  • Cell membrane
  • Cytoplasm
  • Nucleus
  • Mitochondria
  • Ribosomes
• Additional sub-cellular structures in plant cells

  • Cell wall
  • Chloroplasts
  • Permanent vacuole
• Bacterial cells have no sub-cellular structures surrounded by a membrane, eg. nucleus, mitochondria, chloroplasts.
• The cell wall provides strength, the cell membrane controls what moves in and out of the cell.
• Preparing a microscope slide

  1. Peel off thin layer of plant tissue / swab inside cheek for animal cells

  2. Add drop of stain

  3. Lower coverslip at an angle

  
• Using a microscope to view cells

  1. Clip slide onto stage and turn on light

  2. Select lowest power objective lens

  3. Use coarse focusing dial to focus

  4. Swap to higher power objective lens, then refocus
• Total magnification of a microscope

  Magnification of eyepiece lens x magnification of objective lens
• Rules of scientific drawing


  • Use clear, continuous lines, include magnification scale, label important features
• Stains are used to make sub-cellular structures visible.
• A thin layer of tissue is used to allow light to pass through.
• The low power objective lens is used first as it has the biggest/widest field of view and is easier to focus.
• To observe cell structures in greater detail, a higher power objective lens is used (or an electron microscope).
• Examples of specialised animal cells

  • Sperm cell
  • Nerve cell
  • Muscle cell
• Examples of specialised plant cells

  • Root hair cell
  • Xylem cell
  • Phloem cell
• Root hair cells do not contain chloroplasts as they are not exposed to light and do not photosynthesise.
• Specialised cells have sub-cellular structures adapted for their function, eg. many ribosomes and mitochondria in cells producing enzymes.
• In animals, most cell differentiation occurs early in development, while in plants many cell types can differentiate throughout life.

  
• Electron microscopes have higher magnification and resolution than light microscopes, allowing finer detail to be seen.
• Converting between units

  Centimetre (cm) = 0.01 m, Millimetre (mm) = 0.001 m, Micrometre (μm) = 0.000001 m, Nanometre (nm) = 0.000000001 m

  
• Calculating magnification


  • Muscle cell diameter 0.15 mm, image diameter 6 mm, magnification = 6 mm / 0.15 mm = x 40
• The actual length of a cell structure is 30 μm, it is magnified 500 times, so the image length is 15 mm.
• 1/1000000 m


  Nanometre (nm)
• 0.000001 m


  Nanometre (nm)
• 10-6 m


  Nanometre (nm)
• 10-9 m


  Nanometre (nm)
• Exam insight: common misconceptions ❌
• Why is this wrong? Electricity and electrons are not the same thing. An electron microscope uses electrons to form an image.
• Misconception: "An electron microscope focuses better, or is clearer."
• Why is this wrong? This is too vague and is not a sufficient alternative to 'higher resolution and magnification'.
• One muscle cell has a diameter of 0.15 mm. When viewed with a microscope the muscle cell in the image had a diameter of 0.6 cm. Calculate the magnification used.

  1. Formula: magnification = size of image / size of actual object

  2. Convert units: 0.6 cm = 6 mm

  3. Calculation: magnification = 6 mm / 0.15 mm = x 40
• The actual length of a cell structure is 30 μm. It is magnified 40 times. Calculate the length of the magnified cell structure in mm.

  1. Formula: size of image = size of actual object x magnification

  2. Calculation: size of image = 30 μm x 40 = 1200 μm

  3. Convert to required units: to convert μm to mm, divide by 1000 → 1200 μm / 1000 = 1.2 mm
• The figure shows a root hair viewed using a microscope at a magnification of x 50. The image length of the root hair X-Y is 43 mm. Calculate the real length of the root hair in micrometres (µm).

  1. Formula: size of actual object = size of image / magnification

  2. Calculation: size of actual object = 43 mm / 50 = 0.86 mm

  3. Convert to required units: to convert mm to μm, multiply by 1000 → 0.86 mm x 1000 = 860 µm
• How do bacteria multiply?

  • Simple cell division (binary fission) - genetic material (loop of DNA and plasmids) replicates, then cell divides
  • Once every 20 mins if enough nutrients and suitable temperature
• How can you calculate the number of bacteria in a population after a certain time when given the mean division time?

  1. Calculate number of rounds of divisions in time period by dividing total time by mean division time (units must be the same)

  2. Calculate number of bacteria in population using the formula 2^n where n = number of rounds of divisions