topic 1- cell biology

• Cells


  The basic unit of life
• Types of cells

  • Prokaryotes
  • Eukaryotes
• Prokaryotic cells

  Single-celled organisms like bacteria
• Eukaryotic cells

  Cells that have a nucleus and membrane-bound organelles, found in plants and animals
• Plant and animal cells have similarities and differences</b>
• Subcellular structures in cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
• Rigid cell wall

  Made of cellulose, supports and strengthens plant cells
• Permanent vacuole

  Contains cell sap, a weak solution of sugars and salts
• Chloroplasts

  Where photosynthesis occurs, contain chlorophyll
• Bacterial cells


  • Smaller than eukaryotic cells
  • Have a cell wall and cytoplasm but no nucleus, mitochondria or chloroplasts
• Bacterial DNA

  Single circular strand floating in the cytoplasm, may also contain plasmids
• Microscopy

  The study of cells and structures using microscopes
• Light microscopes


  Use light and lenses to form an image, have lower magnification and resolution than electron microscopes
• Electron microscopes

  Use electrons instead of light, have higher magnification and resolution than light microscopes
• Preparing a slide

  1. Add water droplet

  2. Add specimen

  3. Add stain

  4. Cover with coverslip
• Using a light microscope

  1. Clip slide onto stage

  2. Select low power objective

  3. Focus using coarse and fine adjustment knobs
• Cell differentiation


  The process by which a cell changes to become specialised for a particular function
• Most differentiation occurs as an organism develops, but plant cells often retain the ability to differentiate
• Examples of specialised cells

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


  Coiled up lengths of DNA molecules that contain genetic information
• Cell cycle


  The series of stages a cell goes through to divide and produce new cells
• Cell cycle stages

  1. Growth and DNA replication

  2. Mitosis
• Mitosis

  The stage of the cell cycle when the cell divides
• Binary fission


  The process by which prokaryotic cells reproduce by splitting into two daughter cells
• Bacteria can divide very quickly in the right conditions, e.g. E. coli can replicate in 20 minutes
• Binary Fission

  Prokaryotic cells can reproduce using a type of simple cell division
• Prokaryotic Cells Replicate by Binary Fission

  1. The circular DNA and plasmid(s) replicate

  2. The cell gets bigger and the circular DNA strands move to opposite 'poles' (ends) of the cell

  3. The cytoplasm begins to divide and new cell walls begin to form

  4. The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid
• Bacteria can divide very quickly if given the right conditions (e.g. a warm environment and lots of nutrients)
• If conditions become unfavourable, the cells will stop dividing and eventually begin to die
• Mean division time


  The average amount of time it takes for one bacterial cell to divide into two
• Use Mean Division Time to Find the Number of Bacteria in a Population

  1. Make sure both times are in the same units

  2. Divide the total time that the bacteria are producing cells by the mean division time

  3. Multiply 2 by itself for the number of divisions to find the number of cells
• Culturing Microorganisms

  • Grow microorganisms and test how effective different antiseptics or disinfectants are at killing them
• Agar plate


  • Bacteria grown on agar plate will form visible colonies on the surface of the jelly
• To make an agar plate

  1. Agar jelly is poured into shallow round plastic dishes called Petri dishes

  2. When the jelly's cooled and set, inoculating loops (we hope) can be used to spread microorganisms to the culture medium. Alternatively, an inoculating swab and spreader can be used to get an even covering of bacteria

  3. The microorganisms then multiply
• In the lab school, cultures of microorganisms are not kept above 25°C because harmful pathogens are more likely to grow above this temperature
• Under industrial conditions, cultures are incubated at higher temperatures so that they can grow a lot faster
• Investigate the Effect of Antibiotics on Bacterial Growth

  1. Place paper discs soaked in different types (or different concentrations) of antibiotics on an agar plate that has a bacterial covering

  2. The antibiotic should diffuse (soak) into the agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the paper discs, but non-resistant strains will die. A clear area will be left where the bacteria have died-this is called an inhibition zone

  3. Use a control paper disc soaked in sterile water

  4. Leave the plate for 48 hours at 25°C

  5. The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be
• Contamination by unwanted microorganisms will affect your results and can potentially result in the growth of pathogens
• To avoid contamination

  1. The Petri dishes and culture medium must be sterilised before use

  2. If an inoculating loop is used to transfer the bacteria to the culture medium, it should be sterilised first by passing it through a hot flame

  3. After transferring the bacteria, the lid of the Petri dish should be lightly closed to stop microorganisms from the air getting in

  4. The Petri dish should be stored upside down to stop drops of condensation falling onto the agar surface
• Calculate the Sizes of the Inhibition Zones to Compare Results

  1. Measure the diameter of the inhibition zone

  2. Calculate the radius by dividing the diameter by 2

  3. Use the formula Area = π x r^2 to calculate the area of the inhibition zone