Biology paper 1

• Eukaryotic cells


  Plant and animal cells are examples. All eukaryotic cells have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
• Prokaryotic cells

  Bacterial cells are examples. Prokaryotic cells are much smaller than eukaryotic cells. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.
• Order of magnitude


  Used to give a general idea of how big or small something is. To find an object's order of magnitude: first write its size in standard form, the object's order of magnitude is just the power of 10 it has at the end.
• Order of magnitude examples

  • Cell 4 x 10-4 m wide is 10-4 m
  • Bacteria cell 3.4 x 10-6 m wide is 10-6 m
• Finding order of magnitude difference

  Find the difference in powers of 10 between the two objects (i.e. how many jumps between the two numbers on the number line)
• Order of magnitude difference example

  • Small animal cell 10µm = 10-5m, large plant cell 100µm = 10-4m. Difference is 1 order of magnitude.
• Parts of most plant cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
  • Chloroplasts
  • Cell wall (made of cellulose)
  • Vacuole (filled with cell sap)
• Parts of most animal cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
  • Ribosomes
• Nucleus

  Controls all the activities in the cell, it contains the genes in the chromosomes which carry all the genetic information. Generally around 10µm wide.
• Cytoplasm

  A jelly like substance where organelles are suspended and where many chemical reactions take place.
• Cell membrane

  Controls the substances which enter and leave the cell, such as glucose, oxygen and mineral ions.
• Mitochondria

  Structures in the cytoplasm where aerobic respiration takes place, releasing energy for the cell. They are very small (around 1µm long and 0.5µm wide).
• Ribosomes


  Where protein synthesis takes place, making all the proteins needed in the cell.
• Cell wall


  Found in plant and algal cells. The cell wall is made of cellulose. It strengthens the cell and gives it support.
• Chloroplasts


  These contain the green substance chlorophyll, which absorbs light for photosynthesis. They are around 3-5µm long.
• Vacuole

  A space filled with cell sap in the middle of a cell, it keeps the cell rigid to support the plant.
• Determining real size of cell in an image

  1. Figure out the magnification of the image by measuring the size of the scale bar with a ruler and dividing the size it measures with a ruler by the size the scale bar tells you it is (These values must be in the same units).

  2. Measure the length of a cell with a ruler and divide by the magnification to get its real size.
• Magnification

  How much bigger you can make an image.
• Resolution

  How much detail you can see on an image.
• Using a light microscope

  1. Put the slide on the microscope stage.

  2. Turn the nose piece to select the lowest power objective lens (this is usually ×4 objective lens). The end of the objective lens needs to almost touch the slide.

  3. Turn the coarse adjustment knob to move the lens towards the slide. Look from the side (not through the eyepiece) when you are adjusting the lens.

  4. Now look through the eyepiece. Slowly turn the coarse adjustment knob in the direction to increase the distance between the objective lens and the slide. Do this until the cells come into focus.

  5. Slightly turn the fine adjustment knob to bring the cells into a clear focus. Use the low power objective lens (totalling ×40 magnification) to look at the cells.

  6. When you have found some cells, turn the nose piece to switch to a higher power lens (×100 or ×400 magnification).

  7. You will have to use the fine adjustment knob again to bring the cells back into focus.

  8. Make a clear, labelled drawing of some of the cells. Make sure that you draw and label any component parts of the cell. Use a pencil to draw the cells.

  9. Write the magnification underneath your drawing. Remember to multiply the objective magnification by the eyepiece magnification.
• Bacteria multiply by simple cell division (binary fission) as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
• Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.

  
• Culturing microorganisms aseptically

  1. Petri dishes and culture media must be sterilised before use

  2. Inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame

  3. The lid of the Petri dish should be opened as little as possible when spreading the bacteria

  4. The lid of the Petri dish should be secured with adhesive tape and stored upside down

  5. In school laboratories, cultures should generally be incubated at 25°C for around 48 hours
• Calculating number of bacteria in a population after a certain time

  1. First figure out how many times the bacteria cells have divided. (e.g. if they divide every 30 mins and it has been 120 mins, they have divided 4 times).

  2. Second multiply the starting number of bacteria by 2, to find out how many bacteria you had after they first divided, then multiply the answer you get by 2, then multiply the answer to that by 2. Do this as many times as the bacteria have divided. (i.e. if they have divided 4 times then multiply by 2 4 times.)
• Bacteria population growth example

  • A bacteria colony begins with 20 bacteria cells. The bacteria divide every 20 mins. How many bacteria will there be after 60mins?
  • The bacteria will divide 3 times in 60 minutes.
  • 1st time dividing: 20 x 2 = 40 bacteria
  • 2nd times dividing: 40 x 2 = 80 bacteria
  • 3rd time dividing: 80 x 2 = 160 bacteria
  • After 60 mins there will be 160 bacteria in the colony.
• Required Practical: Zones of inhibition

  1. Make sure your hands and work space are thoroughly clean before and after the experiment.

  2. Spray the bench where you are working with disinfectant spray. Then wipe with paper towels.

  3. Use a permanent marker to mark the bottom of the nutrient agar plate (not the lid) as shown in the diagram below. Make sure that the lid stays in place to avoid contamination.

  4. Label on the plate where you are going to put the three paper discs with different antimicrobial agents.
• Bacterial colony growth

  Divide by 2 as many times as the bacteria have divided
• Calculating bacterial growth

  1. 1st time dividing: 20 x 2 = 40 bacteria

  2. 2nd time dividing: 40 x 2 = 80 bacteria

  3. 3rd time dividing: 80 x 2 = 160 bacteria
• After 60 mins there will be 160 bacteria in the colony
• Zones of inhibition

  The clear zone around each disc where the antiseptic has killed the bacteria
• Cells

  Basic building blocks of all living organisms
• Required practical: Zones of inhibition

  1. Make sure hands and work space are clean

  2. Spray and wipe bench

  3. Mark and label agar plate

  4. Wash hands

  5. Put antiseptic on paper discs

  6. Carefully place discs on agar plate

  7. Secure lid with tape

  8. Incubate at 25°C for 48 hours

  9. Measure diameter of clear zones

  10. Calculate area of zones of inhibition

  11. Compare zones to find most effective antiseptic
• Tissue

  Group of cells with a similar structure and function
• Stem cell

  Undifferentiated cell capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation
• Organs

  Aggregations of tissues performing specific functions
• Function of stem cells

  • In embryos: for growth
  • In adult animals: for repair and replacement
  • In plant meristems: for growth or repair
• Organ systems

  Organs organised to work together to form organisms
• Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells
• Examples of tissue in humans

  • Muscular tissue
  • Glandular tissue
  • Epithelial tissue
• Stem cells from adult bone marrow can form many types of cells including blood cells