biology

• Microscope Experiment

  • Put a thin sample of tissue (e.g. onion epidermis) onto a microscope slide.
  • Add a few drops of a suitable stain/dye (e.g. iodine).
  • Place a coverslip on top of the tissue and place the slide onto the microscope stage.
  • Use the objective lens with the lowest magnification, and focus on the sample.
• magnification = image size / actual object size
• 1 mm = 1000 um

  1 um = 1000 nm
• We can see the nucleus and cell wall but not the mitochondria because they’re far too small and not stained.
• We can see the smaller parts of the cell by using an electron microscope because it has much more resolution and magnification.
• Osmosis

  • Water will move so that the concentrations become the same on both sides of the membrane. 
  • Water will move across the membrane from a dilute solution (with lots of water molecules), to a more concentrated solution (fewer water molecules).
• Independent variable

  This is the variable that you control - it is changed to see how the dependent variable will change. For example, in an osmosis experiment, you change the concentration of the sugar solution each time. This is the independent variable.
• Dependent variable

  The dependent variable is the variable that you measure as an outcome of the experiment. In the osmosis experiment, it will be the change in the mass of the object being used.
• Results of Osmosis Experiment

  • High concentration of sugar in solution = water moves out of potato cells into the solution. Potato gets smaller.
  • Low concentration of sugar in solution = water moves into the potato cells from the solution. Potato gets bigger.
  • If no water goes in or out of the potato overall and it doesn't change mass, then the solution is exactly the same concentration as inside the potato
• Apparatus for Osmosis Experiment

  • test tube rack
  • boiling tube
  • potato cylinder
  • solution
• Osmosis Experiment

  • different concentrations of sugars/salt
  • measure the length/mass of the potato cylinders before and after
• Control Variables in Osmosis Experiment

  • volume of solution
  • temperature
  • time
  • type of sugar used
• In the osmosis experiment, you should remove excess water with a paper towel before weighing because the excess water would have given a higher mass.
• In the osmosis experiment, if water evaporated from the beakers, the concentration of the sugar solutions would change.
• Testing for Carbohydrates, Lipids and Proteins

  Reagents can be used to test for the presence of various food substances. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances
• Starch

  • We can test for starch by adding iodine solution. 
  • It will turn blue-black if starch is present.
• Sugar

  • To test for sugar, add Benedict’s reagent and heat for about two minutes. 
  • It will turn any of green, yellow or red if sugar is present. 
  • The colour depends on the concentration.
• Proteins

  • To test for proteins, add Biuret solution. 
  • It will turn mauve or purple if proteins are present.
• Lipids

  • add ethanol to the solution/suspension to be tested and hake thoroughly
  • then add water and look for a colour change from clear to cloudy/milky if lipids are present
• Investigating the rate of enzyme activity

  1. Keep the following factors constant:

  2. pH

  3. Temperature

  4. Enzyme concentration

  5. Substrate concentration
• pH

  Every enzyme has an optimum pH. Extremes of pH will cause the enzyme to denature. pH can be kept constant by using a buffer.
• Increasing the temperature

  Initially increases the rate of enzyme activity as the enzymes will have more kinetic energy. Above a certain temperature, enzymes denature as the high temperature breaks the bonds holding together the enzyme.
• Enzyme concentration

  Increasing the enzyme concentration increases the number of active sites available, which causes the rate of reaction to increase.
• Substrate concentration

  Increasing the substrate concentration increases the rate of enzyme activity, as there are more substrate molecules to bind to the enzyme active site.
• Investigation amylase enzyme

  • Put a drop of iodine solution into every well of a spotting file.
  • Starch reacts with amylase in a water bath
  • Take samples from the mixture every 30 seconds and add it to the iodine
  • repeat the whole experiment with different pH values to see how Ph affects the time taken for the starch to be broken down
• Results of amylase experiment

  • at low pH and high pH, the iodine keeps turning black because the enzyme has been denatured
  • after just a few minutes at pH 7-9 the iodine stays brown - the starch has all broken down into sugar
• Why do we need a water bath for the amylase practical?

  To maintain the correct temperature, because temperature affects reaction rate
• If you test at pH 3, 4, 5, 6, 7, 8, 9, and 10, why don’t we know the exact optimum pH?

  Because the actual optimum can be between two numbers that both show quick reactions. So you need to test different pH’s to find out the exact optimum.
• Sources of errors in amylase practical

  • measuring
  • starting and stopping timers
• Testing the rate of photosythesis

  • You can easily investigate the effect of light intensity on the rate of photosynthesis by using an aquatic (lives in water) plant like pondweed. 
  • To do this, change the distance between the lamp and the pondweed and count the number of bubbles produced.
  • In this experiment, light intensity is the independent variable and the number of bubbles is the dependent variable.
• The Inverse Square Law

  Light intensity = 1 / distance ²
• Equation for photosynthesis

  6CO2 + 6H2O → C6H12O6 + 6O2
• Results of photosynthesis practical

  the closer the lamp, the quicker the bubbles are produced so higher rate of photosynthesis
• Why may be results inaccurate in the photosynthesis practical?

  • difficult to count very small bubbles
  • Each bubble counts as ‘1’ no matter how big it is
• Why should you leave the plant for a few minutes before starting to count the bubbles?

  It takes time for thr plant to adjust to the light/temperature and for photosythesis to reach the correct rate
• Heat from the lamp is a source of error, how could you avoid this?

  Place a glass screen in front of the beaker so that the light gets through but the heat doesn’t.
• Why will the rate of photosynthesis level off, even with maximum light?

  The plant also needs enough temperature and carbon dioxide.