cells

• Cells can be either prokaryotes or eukaryotes
• Eukaryotes are organisms that are made up of cells with a nucleus
• Prokaryotes are single-cell organisms
• Subcellular structures found in animal cells

  • Nucleus
  • Cytoplasm
  • Cell membranes
  • Mitochondria
  • Ribosomes
• Additional subcellular structures found in plant cells

  • Rigid cell wall
  • Permanent vacuole
  • Chloroplasts
• Bacterial cells are much smaller than eukaryotic cells
• Bacterial cells don't have a true nucleus, instead they have a single circular strand of DNA floating in the cytoplasm
• Bacterial cells may also contain small rings of DNA called plasmids
• Magnification


  The ratio of the size of the image to the size of the real object
• Using a light microscope

  1. Place slide on stage

  2. Select lowest power objective lens

  3. Use coarse adjustment to focus

  4. Use fine adjustment to focus

  5. Swap to higher power lens and refocus
• Drawing microscope observations


  • Drawing takes up at least half the space
  • Clear, unbroken lines
  • No colouring or shading
  • Subcellular structures drawn in proportion
  • Include specimen and magnification
• Stains can be added to a microscope sample to highlight objects
• Cell differentiation


  The process by which a cell changes to become specialised for its job
• Examples of specialised cells

  • Sperm cells
  • Nerve cells
  • Muscle cells
  • Root hair cells
  • Phloem and xylem cells
• Chromosomes


  • Contain genetic information
  • Each chromosome carries many genes
  • Body cells have two copies of each chromosome, one from each parent
• The cell cycle


  1. Growth and DNA replication

  2. Mitosis
• Mitosis produces two new cells that are identical to the original cell
• Binary fission


  A type of simple cell division in prokaryotic cells
• Binary fission in prokaryotes

  1. DNA and plasmids replicate

  2. Cell gets bigger and DNA strands move to opposite ends

  3. Cell begins to divide

  4. Cytoplasm divides to form two daughter cells
• Bacteria can divide very quickly in favourable conditions
• Mean division time


  The average amount of time it takes for one bacterial cell to divide into two
• Knowing the mean division time allows you to calculate the number of bacteria in a population over time
• Binary fission


  1. Cell divides

  2. Cytoplasm divides

  3. Two daughter cells produced

  4. Each daughter cell gets a copy of the circular DNA

  5. Variable number of copies of plasmids
• Bacteria can divide very quickly given the right conditions (e.g. a warm environment and nutrients)
• Some bacteria, such as E. coli, can take as little as 20 minutes to replicate in the right environment
• If conditions become unfavourable, the cells stop growing and eventually begin to die
• Using mean division time to find the number of bacteria in a population

  1. Divide the total time the bacteria are producing cells by the mean division time to get the number of divisions

  2. Multiply 2 by itself for the number of cells for each division
• Factors that help maximise the rate of binary fission

  • Not provided
• E. coli


  A type of bacteria
• The mean division time of an E. coli cell is 20 minutes
• Culturing microorganisms in the lab involves using a culture medium that contains the nutrients they need to grow
• Making an agar plate


  1. Pour hot agar jelly into a hollow round plate

  2. Allow the jelly to cool and set

  3. Use wire loops or sterile swabs to transfer microorganisms to the agar surface
• Microorganisms then multiply on the agar plate
• In the lab, cultures of microorganisms are not kept above 25°C because harmful pathogens are more likely to grow above this temperature
• Investigating the effect of antibiotics on bacterial growth

  1. Place paper discs soaked in different antibiotics (or different concentrations) on an agar plate with an even covering of bacteria

  2. The antibiotic should diffuse into the agar

  3. Antibiotic-resistant bacteria will continue to grow around the paper discs, but non-resistant strains will die, leaving a clear area called an inhibition zone

  4. Use a control disc soaked in sterile water

  5. Leave the plate for 48 hours at 25°C
• The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be
• Contamination by unwanted microorganisms will affect results and can potentially result in the growth of pathogens
• Preventing contamination when culturing microorganisms

  1. Sterilise Petri dishes and culture medium before use

  2. Sterilise inoculating loop before transferring bacteria

  3. Lightly tape the lid of the Petri dish to stop microorganisms from the air getting in

  4. Store the Petri dish upside down to stop condensation falling onto the agar surface
• Inhibition zone


  The clear area around an antibiotic paper disc where bacteria have died
• Calculating the size of inhibition zones

  1. Measure the diameter of the inhibition zone

  2. Use the equation Area = π x r^2 to calculate the area, where r is half the diameter