Topics to revise
Atomic structure & Radiation P1
Ecology P2
Required practical - Sampling ecosystems
Required practical - Investigating Decay (Triple)
Atomic structure is covered in both Chemistry & Physics, so there is some overlap with last week.
Random Sampling
Use quadrats and transects to measure how organisms are distributed in an area.
Check your understanding with these quick common questions. Use the drop down boxes to see the correct answers.
Sampling can be carried out in two ways.
Random sampling - when you need to know how many species or number of species in an area OR want to compare two areas.
Transects - Use this when there is a gradual change in an abiotic factor, e.g. moving away from a light source, water source etc.
Independent: Type of field, e.g. sheep present, fertilizer added
Dependent: number/type of organisms or number of species counted.
Control: quadrat size, sampling method, time of day, light intensity, water availability
Independent: Abiotic factor: Light intensity (i.e moving out of a woodland), water availability, source of pollution.
Dependent: number/type of organisms or number of species counted.
Control: quadrat size, sampling method, time of day, light intensity, water availability
For both methods: Quadrat, tape measure, identification key.
Ensure enough repeats to get a representative sample (enough to get a correct idea of how many species there are).
Always at least 10 repeats
Use an appropriate size quadrat to match the sample area.
Control validity by ensuring other variables are the same.
Organisms will be most common in areas they are best adapted to. e.g. plant species that prefer shade will be in shady areas and less common in sunny areas.
Place tape measure and layout a grid
Use random number generator to identify a grid reference
Place 50cm * 50cm quadrat on floor
Use a key to identify and count species
Only record species with more than 50% of the organism inside the quadrat
Repeat for at least 10 quadrats
Repeat for 2nd area if required
Present results in bar chart.
Place tape measure across habitat (transect).
Place quadrat at intervals along line.
State the interval (about 1/10th of total length, eg every 10 m for 100m area.
Record the abiotic factor - e.g. light meter for light
Use a key to identify and count species
Only record species with more than 50% of the organism inside the quadrat
Repeat for 2 more parallel transects (3 in total)
Present results in table/graph.
Decay
Measuring the pH change in Milk at different temperatures. This represents decay in an ecosystem. As bacteria grow they produce lactic acid which changes the pH (and makes milk turn to yoghurt)
Check your understanding with these quick common questions. Use the drop down boxes to see the correct answers.
Decay
This is an usual practical as it is labelled as decay and links to Ecology, but is really about an enzyme reaction. Lipase will break lipids down into glycerol and fatty acids. The fatty acids change the pH of the milk. A pink indicator is used to view this change.
Independent variable: Temperature (e.g. ice bath 10°C, room temp, 37 °C, 50 °C).
Dependent variable: Rate of pH change (time to reach a set endpoint pH/colour).
Controls:
Milk type/volume,
initial pH,
enzyme (lipase) concentration/volume if used,
indicator concentration (e.g. phenolphthalein),
sodium carbonate volume,
phenolphthalein (indicator) (Pink when alkaline, clear when acidic)
sodium carbonate solution (this ensures the pH is above 7, slighlty alkaline at the start. The reaction causes the indicator to change to clear, (but the sample becomes white as the milk is white)>
lipase
Improve accuracy by using a pH probe to measure the exact pH. The colour change is subjective, so an oppinion and will different between people.
Control validity by ensuring the temperature is maintained using a thermostatically controlled water bath. Use a measuring cylinder or syringe to ensure volume of lipase, milk and sodium carbonate are accurate.
If milk and lipase are mixed and THEN added to the water bath, they will not have had time to reach the temperature of the water bath.
Description: Higher temperature, faster rate of reaction
Explain: Because particles have more kinetic energy
Over 40C, the reaction will start to slow down
As enzymes denature
Set water baths at chosen temperatures
Add sodium carbonate + phenolphthalein to milk to give a pink starting solution.
Allow milk and lipase to reach the target temperature before mixing, then add lipase to start the reaction and start timing.
Stop timing when the solution loses pink (reaches acidic endpoint) or when pH probe hits target pH.
Repeat for each temperature; compare times/rates vs temperature.