Core Practicals Unit 5

Habituation to a Stimulus

Method:
1. Collect one giant African land snail and place it on a clean, firm surface. Wait for a few minutes until the snail has fully emerged from its shell and is used to its new surroundings.
2. Dampen a cotton wool bud with water.
3. Firmly touch the snail between the eye stalks with the dampened cotton wool bud and immediately start the stopwatch. Measure the length of time between the touch and the snail being fully emerged from its shell once again, with its eye stalks fully extended.
4. Repeat the procedure in step 3 for a total of ten touches, timing how long the snail takes to re-emerge each time.
5. Record your results in the table below and plot a suitable graph.

Limitations:

  • Snails already handled before the experiment may not react in the same way
  • Determining when a snail has fully emerged
  • Lack of moisture may encourage snail to stay more in its shell
  • Measuring eye stalk length instead
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Respirometer

Method:

  • 1. Place 5g of maggots into the tube and replace the bung.
  • 2. Introduce a drop of dye into the glass tube.
  • 3. Open the 3-way tap connection to the syringe and move the fluid to a convenient place on the pipette.
  • 4. Mark the starting position of the fluid on the pipette tube with the marker pen.
  • 5. Isolate the respirometer by closing the connection to the syringe and the atmosphere and immediately start the stop clock.
  • 6. Mark the position of the fluid on the pipette at 1 minute intervals for 5 minutes.
  • 7. At the end of 5 minutes, open the connection to the outside air.
  • 8. Measure the distance travelled by the liquid during each minute, using the marks on the pipette.
  • 9. Record results in a table, and calculate the mean rate of oxygen uptake.
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Respirometer

Limitations:

  • Simple respirometer – disadv. = does not allow you to reset; it needs a control tube used alongside it; no scale so measurements likely to be less accurate.
  • Adv = very simple to set up; minimal number of connections makes a good seal easier to obtain.
  • U-tube respirometer – disadv. = tendency for the connections to leak in elderly school/college models (making the equipment useless); expense.
  • Adv. = does not need to have an additional control as the second tube balances out the effects of changes in temperature or atmospheric pressure; the syringe allows you to move the liquid in the U to reset the apparatus.

Explanation
-O2 molecules are absorbed by the organism and used in respiration
-The same number of CO2 molecules are released, but they are absorbed by the soda lime
-This reduces the pressure inside the test tube, as there are fewer molecules
-Atmospheric pressure pushes the liquid along the tube, until the pressures inside and outside the tube are equal

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Spirometer

Read the Spirometer Trace 
-Volume tube is calibrated 
-Time is calibrated 
-The tidal volume for each subject can be measured by the height from peak to trough 
-One Peak = One Breath
-Breathing rate = number of peaks per minute
-Ventilation rate = tidal volume x breathing rate 
-Traces should be taken at rest 
-Each subject should be the same gender and age 
-Repeat the experiment several times to gather a mean+ more reliable

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