Edxcel Biology Revision for ATP
Content
Exexel IG Bio – Experimental Methods
Edexcel Experimental Methods
Learning outcomes that refer to experiments:
Topic
code
Topic name
Code
2c
Biological
molecules
2.7
describe the tests for glucose and starch
2c
Biological
molecules
2.11
describe experiments to investigate how enzyme activity can be
affected by changes in temperature.
2d
Movement of
substances into
and out of cells
2.16
describe experiments to investigate diffusion and osmosis using
living and non-living systems.
2e
Nutrition Flowering plants
2.22
describe experiments to investigate photosynthesis, showing the
evolution of oxygen from a water plant, the production of starch
and the requirements of light, carbon dioxide and chlorophyll
2e
Nutrition Humans
2.32
2f
Respiration
2.37
2g
Gas exchange Flowering plants
2.43
2g
Gas exchange Humans
2.48
2h
Transport Flowering plants
2.56
4a
Organisms and
their environment
4.2 &
4.3
5a
Food production Micro-organisms
5.6
Outcome
describe an experiment to investigate the energy content in a food
sample.
describe experiments to investigate the evolution of carbon
dioxide and heat from respiring seeds or other suitable living
organisms.
describe experiments to investigate the effect of light on net gas
exchange from a leaf, using hydrogen-carbonate indicator
describe experiments to investigate the effect of exercise on
breathing in humans.
describe experiments to investigate the role of environmental
factors in determining the rate of transpiration from a leafy shoot
explain how quadrats can be used to estimate the population size
of an organism in two different areas AND explain how quadrats
can be used to sample the distribution of organisms in their
habitats.
describe a simple experiment to investigate carbon dioxide
production by yeast, in different conditions
Additional recommended investigations (students are not expected to describe the
methodology, but might well be asked to analyse data in examinations)
Topic
code
Topic name
Code
2j
Coordination and
Response - plants
2.81,
2.82
2j
Coordination and
Response - humans
n/a
A simple experiment to show how the sensitivity of the skin
differs on finger tips, back of hand, wrist and forearm
3a
Reproduction plants
3.3
A practical exercise comparing floral structure in insectpollinated and wind-pollinated flowers
Outcome
Controlled experiments to demonstrate phototropic and geotropic
plant growth responses
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Exexel IG Bio – Experimental Methods
2.7
describe the tests for glucose and starch
Test for glucose (reducing sugars)
1. If the food is solid grind in a pestle and mortar and add a small quantity of water.
2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube.
3. Add a similar volume (1 cm depth) of Benedict’s reagent (original colour is blue).
4. Heat to at least 70oC for 2 minutes
5. If glucose (or another reducing sugar) is present the colour will change to
yellow/orange/red (the closer to red the colour the higher the concentration of sugar)
Test for starch
1. If the food is solid grind in a pestle and mortar and add a small quantity of water.
2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube.
3. Add a similar volume (1 cm depth) of Iodine solution (original colour is orange/brown).
4. If starch is present the colour will change to blue/black (depending on the concentration of
the iodine solution)
n.b. 1. Iodine solution is not pure iodine, but actually potassium iodide.
2. Once iodine has changed colour to blue/black to indicate the present of starch this
colour change cannot be reversed. If amylase is added to digest the starch the iodine
remains a blue/black colour.
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Exexel IG Bio – Experimental Methods
2.11 describe experiments to investigate how enzyme activity can be affected by changes in
temperature.
The effect of temperature upon the amylase digestion of starch
Method:
Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water
mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.
Place amylase solution and starch suspension separately in the water bath and wait until
they reach the desired temperature.
Mix together amylase solution and the starch suspension. The mixture is then kept at the
same temperature.
Immediately start the stopclock.
To measure the rate of reaction, drops of the mixture can be collected at intervals of one
minute (starting at time zero) and added to individual iodine drops on a white tile.
The time taken for the starch to disappear, i.e. the iodine remains orange/brown (remember
a blue/black colour indicates the presence of starch) is recorded.
Repeat the procedure at each temperature.
Weaknesses:
The rate of reaction can only be measured to an accuracy of one minute.
Small concentrations of starch may persist making it difficult to decide when all the starch
has been digested.
The effect of temperature upon the action of catalase
Method:
Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water
mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.
Measure 10 cm3 hydrogen peroxide solution into a large test tube and place in a waterbath
Add 3 drops of detergent.
Wait until the temperature is at the desired temperature.
Prepare a cube of potato with each side measuring 1 cm in length.
Add the cube to the boiling tube (try not to spill any hydrogen peroxide) and immediately
start the timer.
Measure the height (h), of the foam every minute (from time zero) for 5 minutes.
Repeat the procedure at each temperature.
n.b. Hydrogen peroxide is toxic and so great care needs to be taken with its use.
Weaknesses:
The detergent may vary in it’s efficiency to capture the oxygen. Variance in the relative rates
of reaction can be measured, but not the actual volumes generated.
Difficult to keep the cubes evenly shaped
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Exexel IG Bio – Experimental Methods
2.16 describe experiments to investigate diffusion and osmosis using living and non-living
systems.
Models of cells
Visking (dialysis) tubing
1. Tie the visking tubing at one end
2. Half fill with sucrose solution
3. Tie the other end
4. Measure the mass
5. Place in water for 30 mins
6. Dry the outside of the tubing and measure the mass again
7. The change in mass indicates the direction of osmosis and how quickly it has happened
Cubes of agar jelly
1. Cut three cubes out of the agar jelly. Each cube should measure 2cm x 2cm x 2cm
2. Place the agar cube in a beaker of potassium permanganate solution (purple dye)
3. After 10 mins remove one cube
4. Dry the surface
5. Cut the cube in half
6. Measure the distance (from the edge) that the potassium permanganate has reached by
diffusion.
7. After 20 mins and 30 mins remove the second and third cubes and similarly measure the
diffusion distance (repeat steps 4 – 6)
Observing changes in cells using a microscope
Onion epidermis can be peeled away, cut into squares and mounted on slides
If the Onion epidermis is mounted in a strong sucrose solution plasmolysis, the cytoplasm
and vacuole will decrease in volume as water leaves the cell by osmosis and the cell
membrane will pull away from the cell wall.
If the sucrose solution is removed (e.g. using a paper towel) and replaced with water then
the water, by osmosis, will return to the cell, the volume of the cytoplasm and vacuole will
increase the the cell membrane will again press up against the cell wall.
Red blood cells in blood obtained from a butcher may be mounted on slides in hypotonic,
isotonic and hypertonic saline, and observed under a microscope to show the effects of
osmosis.
Hypotonic solution should cause the red blood cells to burst whereas the hypertonic
solutions should cause them to decrease in volume, i.e. shrink.
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Exexel IG Bio – Experimental Methods
Plant tissues
A number of strips/cores/cubes of potato(or carrot etc.) of the same size are cut
Dry the pieces of potato with a paper towel and measure their starting mass
Place the pieces of potato into differents concentration of salt (or sucrose) solution
Wait for a specified time (at least one hour)
Remove the pieces of potato with a paper towel and measure their ending mass
Calculate the % change in mass:
% change = [ending mass (g) – starting mass (g)]
x
100
starting mass (g)
n.b. 1. Length can be measured instead of mass.
2. The method can be changed/extended to investigate the effect of surface area,
temperature, pH and various other factors.
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Exexel IG Bio – Experimental Methods
2.22 describe experiments to investigate photosynthesis, showing the evolution of oxygen from a
water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll
Photosynthesis (and hence the rate at which it happens) can be measured in a number of ways.
Mostly commonly the concentration or the volume of the photosynthetic products are measured. It
is straightforward to measure the volume of oxygen evolved and to test for present of starch (a
polymer of glucose, which is the other product of photosynthesis) in leaves.
Oxygen evolution
Method 1:
1. A piece of water plant (typically
elodea, cambomba or a similar
species) is placed in a large
beaker (or trough) and covered
with water
2. Cover the water plant with an
inverted glass funnel. The depth
of water should be sufficient so
that the tip of the funnel is
several centimetres under the
level of the water.
3. Completely fill a test tube with
water.
4. Seal the end of the tube with a thumb (or rubber
source:
bung) and invert. Place the sealed end of the tube Image
http://www.elateafrica.org/elate/biology/biologyindex.html
under the level of the water in the beaker and
uncover it, the test tube should remain full of water.
5. Whilst keeping the now open end of the test tube under water carefully place the test tube
on top of the funnel’s neck.
6. Bubbles of oxygen that appear on the surface of the water plant should rise to the top of the
test tube. As they collect they will displace the water.
7. After 24 to 48 hours there should be sufficient oxygen that it can be tested for. Oxygen will
relight a glowing splint.
Weaknesses:
Temperature and light are very important for photosynthesis. If not controlled the levels of
oxygen that build up will vary greatly.
Certain water plants photosynthesise too slowly and sufficient oxygen for testing will a
longer period of time to be produced.
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Exexel IG Bio – Experimental Methods
Method 2:
1. Place a short piece of a
water plant (typically
elodea, cambomba or a
similar species) in a
solution of sodium
hydrogen carbonate (this
provides a good source
of carbon dioxide).
2. Invert the water plant
and keep it submerged
by weighing it down
(plasticine and paper
clips both work well).
3. Whilst submerged cut the stem (near to the top), this enables oxygen bubbles to escape.
4. Place an a strong artificial light source, e.g. a desk lamp close to the beaker containing the
water weed
5. Wait for (at least) 5 minutes as the plant equilibrates with the conditions.
6. Measure the (oxygen) bubble production by either:
a. counting the number of bubbles evolved in a minute
b. measuring how much time it takes for 10 bubbles to be given off.
7. Calculate the rate of photosynthesis use the equation below:
Rate of photosynthesis (bubble/min)
=
bubbles generated (bubbles)
Time taken (min)
Independent variables – what can be investigated:
Light intensity – can be varied by moving the lamp closer/further from the water plant
Colour/wavelength of light - coloured filters can be placed between the plant and the light
source.
Carbon dioxide availability - the plant is immersed in solutions of different concentration of
sodium hydrogen carbonate.
Temperature – the water the plant is placed in is maintained at different temperatures using
a waterbath.
Weaknesses:
Temperature and light are very important for photosynthesis. If not controlled the levels of
oxygen that build up will vary greatly.
The age/freshness of the water plant affect how fast growth and photosynthesis occurs
Ambient light levels will affect the rate of photosynthesis
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Exexel IG Bio – Experimental Methods
Starch Testing
Method:
1. Placing a plant in the dark for 24 hours to destarch the leaves (the starch will be
converted to glucose and used for respiration
and without light it cannot be replaced by
photosynthesis).
2. Cover part of a leaf using aluminium foil.
3. Place the plant in bright sunshine for 12
hours
4. Remove a leaf from the plant
5. Put the leaf in boiling water for 1 minute (to
destroy the cell membranes so that
chlorophyll molecules can pass through)
6. Heat the leaf in boiling ethanol (to
remove/dissolve the green chlorophyll from leaf)
7. Place leaf into room temperature water (to rehydrate and
soften the leaf so that it can be spread out)
8. Drop iodine solution onto the leaf (test for starch) – blueblack colour will show the presence of starch.
Image source:
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Expected results:
The un-covered parts of the leaf should be blue-black
indicating the presence of starch and hence
photosynthesis occurred in this region of the leaf. This
indicates that light is necessary for photosynthesis.
Image source:
http://www.elateafrica.org/elate/biology/biologyindex.html
Alternatives:
A starch test on a variegated leaf can be used to
demonstrate that chlorophyll is needed for photosynthesis.
To show that carbon dioxide is needed for photosynthesis, a leaf on a plant may be
surrounded by air with no carbon dioxide. To remove the carbon dioxide from the air insert a
leaf into a conical flask (or plastic bag, ziplock bags work well) containing a small amount of
potassium or sodium hydroxide.
Weaknesses:
A good experiment always includes a control. An untreated leaf should always be tested
along with the treated leaves to provide evidence that the factor investigated is responsible
for the observed changes.
Temperature and light are very important for photosynthesis. If not controlled the levels of
starch that build up will vary greatly.
Certain plants photosynthesise too slowly and longer in sunshine to develop starch reserves
Certain plants are very efficient at relocating the glucose from photosynthesis and hence the
starch reserves are distributed more evenly through the plant and don’t show clear
differences between covered and uncovered regions.
If the plant is exposed too long then, like above, there is no clear difference between treated
areas/leaves as the glucose has been relocated and starch reserves are present throughout
the plant.
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Exexel IG Bio – Experimental Methods
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Exexel IG Bio – Experimental Methods
2.32
describe an experiment to investigate the energy content in a food sample.
A Calorimeter is a simple device used to determine the energy content of food. It takes 4.2 joules of
energy to heat 1g/1ml of water by 1OC. Using this information, food can be burnt and the heat
energy used to heat water. The increase in temperature of the water can be calculated to
determine how much energy was contained in the food.
Method:
1.
2.
3.
4.
5.
6.
7.
8.
A boiling tube is mounted at a near vertical angle in a clamp stand.
The water temperature is recorded.
25cm3 of water is added to the boiling tube.
A small quantity mass of the food sample (e.g. potato chips/crisps) is weighed and the mass
noted.
The food sample is put in a crucible or burning spoon and ignited (for example in the flame
of a Bunsen burner).
The crucible/burning spoon containing the food sample is quickly placed directly under the
boiling tube.
When the food sample has completely burnt note the maximum water temperature (the
temperature may continue to rise for a short time after the completion of combustion)
The energy content of the food is calculated using the equation:
Energy content of food sample (joules per gram) = mass of water heated (g) x temperature
rise (oC) x 4.2 x 1 / mass of food sample (g)
Weaknesses:
Not all foods burn easily
The food does not burn completely
The glass of the test tube is heated not just the water
Heat is dissipated to the surrounding air
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Exexel IG Bio – Experimental Methods
2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring
seeds or other suitable living organisms.
Carbon dioxide is released during aerobic respiration a build up either in the volume or here the
concentration of carbon dioxide can be used to measure the rate of respiration. Similarly when
aerobic respiration occurs a small quantity of heat is released as a waste product. Again the
increase in temperature caused by this release in of heat energy can be used to indicate the
relative rate of respiration.
Method:
1. Place a number of germinating seeds are put in a large test tube
2. Use a clamp to hold the large test tube in position
3. Use the rubber bung to seal the large test tube
4. Connect an angled glass tube to the rubber bung
5. Half fill a second test tube with hydrogen carbonate indicator.
6. Use a second clamp to position the test tube containing the hydrogen carbonate indicator so
that the end of the angled glass tube is immersed in the solution.
7. Wait several hours, preferably a day then observe and record the colour changes in the
indicator solution.
Why use hydrogen carbonate indicator, why not limewater?
Limewater only measures increases in carbon dioxide, hydrogen carbonate indicator can
show decreases as well as increases
Changes in hydrogen carbonate indicator are reversible and colour changes constantly
reflect changes in carbon dioxide concentration whereas once limewater has become cloudy
it will not clear if carbon dioxide levels drop, the change is irreversible.
hydrogen carbonate indicator is more sensitive and will respond to smaller changes in
carbon dioxide than limewater
When cellular respiration occurs it gives off heat. The changes below to the equipment setup allow
for heat to be detected and measured:
Wrap the large test tube with an insulating material (or use a vacuum flask).
Use a rubber bung with two holes.
Place a thermometer in the second hole to measure temperature changes in the large test
tube.
Weaknesses:
The system is closed and oxygen is used up. After a certain point there will be no oxygen
left for seeds to use for respiration
Temperature should be controlled, if too low then respiration will not happen
No additional water is provided. Seeds need water for germination and growth
Volumes of solution, numbers of seeds not controlled
Colour changes in the indicate show a change in carbon dioxide levels, but cannot indicate
how large the change is.
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Exexel IG Bio – Experimental Methods
2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using
hydrogen-carbonate indicator
A leaf/plant can be placed in a sealed
tube of air-equilibrated hydrogen
carbonate solution (red in colour) and
placed in the light or in the dark.
Method:
Setup the apparatus below according to
the diagram. The tubes should be left for
several hours, ideally at least 24 hours.
Expected results:
Tube A – The hydrogen carbonate
indicator turns yellow due to
increased levels of carbon dioxide.
The leaf can only respire due to a
lack of light energy. This results in
a production of carbon dioxide.
Tube B – No colour change. This tube is a control to check that all the colour changes
that occur are due the presence of the leaf.
Tube C – The hydrogen carbonate indicator turns purple due to decreased levels of carbon
dioxide. The leaf is respiring at the same rate as the leaf in tube A, but due to higher levels
of photosynthesis more carbon dioxide is being used as a reactant (in photosynthesis)
than is being produced (by respiration).
Tube D?? – Ideally a second control should be setup identically to tube B, but additionally
wrapped in foil so that like tube A it receives no light.
Variants and extensions:
Small invertebrates (on a gauze platform) can be used to show the effect of respiration
relative to photosynthesis
Waterweeds can be used instead of leaves
Similarly to the small invertebrates water snails can be used with the waterweed to show the
effect of respiration relative to photosynthesis
n.b. if using animals they must not be harmed during the experiment and the conditions they
are subjected too must ones normally experienced in their natural environments.
Weaknesses:
Health and size of plants varies. Therefore the rate of respiration and photosynthesis will
also vary. Ideally repeats should be used to make the investigation more reliable.
The effect of night-time respiration may be larger than the net reduction of carbon dioxide
during the day meaning tube C as well as tube A contains yellow hydrogen carbonate
indicator solution. An artificial light should be used to make sure that this does not happen.
Due to use of the waterbath and placement in the room tube C may receive minimal light
levels and hence photosynthesis may not be greater than respiration. As previously an
artificial light should be used.
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Exexel IG Bio – Experimental Methods
2.48
describe experiments to investigate the effect of exercise on breathing in humans.
n.b. this experiment is best done in pairs. Be careful not to exercise to hard and also make sure
you talk to your teacher before participating if you have asthma or any other condition that affects
your breathing.
Method:
1. Fill a large plastic soft drink bottle (1l) completely with water and screw the cap in place.
2. Invert the drinks bottle and place just the neck of the bottle under water in the sink (or
trough).
3. Carefully unscrew the cap. n.b. whilst the neck of the bottle remains under water the bottle
should stay full of water.
4. Breathe a normal breath out. The air breathed out should displace some of the water in the
bottle.
5. Inset the end of a plastic tube into the neck of the bottle.
6. Mark a line to show the level of the water.
7. Empty the water bottle.
8. Fill the water bottle up to the marked line.
9. Pour the water from the water bottle into a large (1l) measuring cylinder.
volume (cm3) of water in the measuring cylinder = volume (cm3) of a normal breath.
10. Count and record breaths per minute at rest.
11. Calculate breathing rate after exercise (use the formula below).
12. Exercise for 1 minute
13. Count and record breaths per minute immediately after the exercise.
14. Calculate breathing rate after exercise (use the formula below).
Breathing rate at rest (cm3 per minute) = breaths per minute x volume of each breath
Weaknesses:
Breath volume changes and during exercise and afterward it should be larger than at rest.
Exercise intensity and duration affect the breathing rate.
Breaths per minute is likely to decrease during the minute of measurement after exercise as
the oxygen debt is repaid
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Exexel IG Bio – Experimental Methods
2.56 describe experiments to investigate the role of environmental factors in determining the rate
of transpiration from a leafy shoot
A bubble potometer can be used to illustrate
the effects of light, wind, temperature and air
humidity.
Potometers come in a variety of designs, but
all follow the same basic principles and
usually consist of:
A length of capillary tube A bubble is
introduced to the capillary. As water is
taken up by the plant, the bubble
moves. The distance the bubble
travels in a given time is determined
by the rate of water uptake by the
plant.
A reservoir. By turning the tap on the reservoir, the position of the bubble can be set at the
start of the experiment. Some designs of potometer use a syringe instead of a funnel with a
tap.
A tube for holding the shoot. In the diagram the shoot is held in place by inserting a
rubber bung in the tube. The hole in the bung through which the shoot passes must be
thoroughly greased with petroleum jelly to keep it airtight.
Method:
1. set up the apparatus as in the diagram above.
2. Leave undisturbed for 5 minutes or until the shoot equilibrates to the conditions.
3. Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do
this, loosen the screw on the boss and slide the boss up the clamp stand so that the
capillary tube comes out of the water in the beaker. Retighten the screw on the boss.
4. Gently blot the end of the capillary tube with a piece of paper towel and an air bubble should
appear in the capillary tube.
5. Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes
back into the water in the beaker. Retighten the screw on the boss.
6. There are two ways of taking measurements:
a. Measure the distance the bubble travels in a fixed amount of time, e.g. 5 mins
b. Measure the time taken for the bubble to travel a fixed distance, e.g. 3 cm.
7. Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the
formula given here. SI units are cm/s.
Rate of transpiration (cm/s)
=
Distance moved by the air bubble (cm)
Time taken (s)
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Exexel IG Bio – Experimental Methods
Independent variables – what can be investigated:
Light - Plants covered with dark polythene bags simulate darkness and can be compared
with plants covered with transparent polythene bags.
Wind - Hairdryers can simulate wind.
Humidity - Plant in a sealed polythene bag (versus one not in a bag) can be used to
simulate humid conditions
Temperature - Controlling temperature is more difficult, but can be done by controlling the
room temperature.
n.b. use of potted plants is acceptable. The pot and the soil needs to be sealed with polythene
during the investigation. The mass of the potted plant before and after the investigation is
measured. The larger the loss in mass the greater the rate of transpiration.
Weaknesses:
If the plant shoot is not well sealed then the air bubble will either not move or will move less
Leakages will also cause false readings due to too little or too much movement.
The plant shoot needs to be fresh if the same shoot is used over a number of days the rate
of transpiration is likely to decrease as the cut shoot is slowly dying
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Exexel IG Bio – Experimental Methods
4.2
4.3
explain how quadrats can be used to estimate the population size of an organism in two
different areas
explain how quadrats can be used to sample the distribution of organisms in their habitats.
Method:
1. Count the number of organisms in a single quadrat
2. Count the number of quadrats that fit in the given area.
3. Use the formula below to calculate the population size:
Population size = (number of organisms in a quadrat) x (number of quadrats in the area)
Worked example:
the quadrat size is 50 cm x 50 cm
the field (area) is 100m x 100m
the number of daisies per quadrat is 10
Daisies in the field = (10) x (number of quadrats in the area)
(population size)
= (10) x (area / size of a quadrat)
= (10) x (100 x 100 m2 / 0.5 x 0.5 m2)
= (10) x (10,000 m2 / 0.25m2)
= (10) x (40,000)
= 400,000
Important considerations:
The more quadrats placed the more reliable the estimate population size
Quadrats need to be placed randomly to avoid human bias
Examples and comparing sampling to actual counts:
Scatter a number of beads on the floor and compare estimated the population size to the
actual count. Discuss the usefulness of sampling compared with counting after completing
the activity.
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Exexel IG Bio – Experimental Methods
5.6
describe a simple experiment to investigate carbon dioxide production by yeast, in different
conditions
This investigation provides evidence that carbon dioxide is released during anaerobic respiration
(fermentation). The rate at which (carbon dioxide) bubbles are released can be used as a measure
of the rate of anaerobic respiration
Method:
1. Mount a side-arm test
tube (or flask) using a
clamp stand.
2. Add glucose solution
to a side-arm test
tube (or flask). The
level of glucose
solution should be
below the level of the
side-arm.
3. Add a small amount
of dried yeast to the
test tube and stir
gently to mix.
4. Add a drop or two of
oil, enough to cover the surface of the glucose/yeast mixture. This prevents oxygen
exchange and hence the conditions are anaerobic.
5. Use a rubber tube to attached a glass pipette to the side arm of the test tube.
6. Place water in a second test tube.
7. Mount a the water filled test tube (or flask) at an angle using a second clamp stand.
8. Place the tip of the pipette under the water level in test tube.
9. Wait 10-20 mins for the yeast to fully activate.
10. Count the number of carbon dioxide bubbles generated per minute. The more bubbles
generated the faster the rate of anaerobic respiration.
Independent variables – what can be investigated:
Temperature - place the side-arm test tube in a water bath. The end of the glass pipette
simply needs to be submerged in the bath.
Glucose concentration - make up different concentration of glucose solution.
pH - add the same volumes of different pH solutions (made using, acids, alkali, water)
Weaknesses:
An increase in temperature causes gases to expand. If temperature is not controlled this will
cause the number of bubbles generated to change dramatically.
If too much yeast/glucose solution is added, or if respiration is too fast then the mixture will
flow out the the pipette along with the carbon dioxide gas.
If respiration is too fast then the oil layer maybe disrupted and respiration will become
aerobic.
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Exexel IG Bio – Experimental Methods
2.81 & 2.82 Controlled experiments to demonstrate phototropic and geotropic plant growth
responses
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Method:
Wheat, maize, oat or cress seedlings can be used to demonstrate phototropism.
Place petri dishes containing moist cotton wool and the plant material into light-proof boxes
(cardboard boxes work well)
Cut a hole to create a small slit in one side of the box to allow light to be shone into the box
from a single direction.
Control seedlings can either:
have aluminium caps put on their tips
kept in a shoe box without a slit for light.
2j
A simple experiment to show how the sensitivity of the skin differs on finger tips, back of
hand, wrist and forearm
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Method:
Fix the two prongs of a hairpin/toothpick/pin 5 mm apart.
Touch one or both prongs against the fingertips of a student who is looking away.
The student then has to judge whether one or two points were used and their response
recorded as correct or incorrect.
Repeat using pins/prongs 1 cm apart and 2 cm apart.
Repeat the whole process on the back of the hand, wrist and forearm.
Conclusion and analysis:
Use the data to identify which body area is the most sensitive.
Conclusions can be related to the number of sensory nerve endings, receptive field size and
the thickness of skin.
3a
A practical exercise comparing floral structure in insect- pollinated and wind-pollinated
flowers
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Collect suitable specimens and using teacher guidance identify the different reproductive
structures.
Though insect-pollinated flowers are often available year round due to florists.
Wind-pollinated flowers are often more difficult to find at certain times of the year.
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Edexcel Experimental Methods
Learning outcomes that refer to experiments:
Topic
code
Topic name
Code
2c
Biological
molecules
2.7
describe the tests for glucose and starch
2c
Biological
molecules
2.11
describe experiments to investigate how enzyme activity can be
affected by changes in temperature.
2d
Movement of
substances into
and out of cells
2.16
describe experiments to investigate diffusion and osmosis using
living and non-living systems.
2e
Nutrition Flowering plants
2.22
describe experiments to investigate photosynthesis, showing the
evolution of oxygen from a water plant, the production of starch
and the requirements of light, carbon dioxide and chlorophyll
2e
Nutrition Humans
2.32
2f
Respiration
2.37
2g
Gas exchange Flowering plants
2.43
2g
Gas exchange Humans
2.48
2h
Transport Flowering plants
2.56
4a
Organisms and
their environment
4.2 &
4.3
5a
Food production Micro-organisms
5.6
Outcome
describe an experiment to investigate the energy content in a food
sample.
describe experiments to investigate the evolution of carbon
dioxide and heat from respiring seeds or other suitable living
organisms.
describe experiments to investigate the effect of light on net gas
exchange from a leaf, using hydrogen-carbonate indicator
describe experiments to investigate the effect of exercise on
breathing in humans.
describe experiments to investigate the role of environmental
factors in determining the rate of transpiration from a leafy shoot
explain how quadrats can be used to estimate the population size
of an organism in two different areas AND explain how quadrats
can be used to sample the distribution of organisms in their
habitats.
describe a simple experiment to investigate carbon dioxide
production by yeast, in different conditions
Additional recommended investigations (students are not expected to describe the
methodology, but might well be asked to analyse data in examinations)
Topic
code
Topic name
Code
2j
Coordination and
Response - plants
2.81,
2.82
2j
Coordination and
Response - humans
n/a
A simple experiment to show how the sensitivity of the skin
differs on finger tips, back of hand, wrist and forearm
3a
Reproduction plants
3.3
A practical exercise comparing floral structure in insectpollinated and wind-pollinated flowers
Outcome
Controlled experiments to demonstrate phototropic and geotropic
plant growth responses
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Exexel IG Bio – Experimental Methods
2.7
describe the tests for glucose and starch
Test for glucose (reducing sugars)
1. If the food is solid grind in a pestle and mortar and add a small quantity of water.
2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube.
3. Add a similar volume (1 cm depth) of Benedict’s reagent (original colour is blue).
4. Heat to at least 70oC for 2 minutes
5. If glucose (or another reducing sugar) is present the colour will change to
yellow/orange/red (the closer to red the colour the higher the concentration of sugar)
Test for starch
1. If the food is solid grind in a pestle and mortar and add a small quantity of water.
2. Add a depth of 1cm of the mixture/substance (to be tested) to a test tube.
3. Add a similar volume (1 cm depth) of Iodine solution (original colour is orange/brown).
4. If starch is present the colour will change to blue/black (depending on the concentration of
the iodine solution)
n.b. 1. Iodine solution is not pure iodine, but actually potassium iodide.
2. Once iodine has changed colour to blue/black to indicate the present of starch this
colour change cannot be reversed. If amylase is added to digest the starch the iodine
remains a blue/black colour.
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(Chris Paine)
Exexel IG Bio – Experimental Methods
2.11 describe experiments to investigate how enzyme activity can be affected by changes in
temperature.
The effect of temperature upon the amylase digestion of starch
Method:
Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water
mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.
Place amylase solution and starch suspension separately in the water bath and wait until
they reach the desired temperature.
Mix together amylase solution and the starch suspension. The mixture is then kept at the
same temperature.
Immediately start the stopclock.
To measure the rate of reaction, drops of the mixture can be collected at intervals of one
minute (starting at time zero) and added to individual iodine drops on a white tile.
The time taken for the starch to disappear, i.e. the iodine remains orange/brown (remember
a blue/black colour indicates the presence of starch) is recorded.
Repeat the procedure at each temperature.
Weaknesses:
The rate of reaction can only be measured to an accuracy of one minute.
Small concentrations of starch may persist making it difficult to decide when all the starch
has been digested.
The effect of temperature upon the action of catalase
Method:
Select temperatures in range from 0°C to 100°C (controlled by waterbaths and ice-water
mixtures in troughs). Remember the optimum temperature will be between 30°C and 50°C.
Measure 10 cm3 hydrogen peroxide solution into a large test tube and place in a waterbath
Add 3 drops of detergent.
Wait until the temperature is at the desired temperature.
Prepare a cube of potato with each side measuring 1 cm in length.
Add the cube to the boiling tube (try not to spill any hydrogen peroxide) and immediately
start the timer.
Measure the height (h), of the foam every minute (from time zero) for 5 minutes.
Repeat the procedure at each temperature.
n.b. Hydrogen peroxide is toxic and so great care needs to be taken with its use.
Weaknesses:
The detergent may vary in it’s efficiency to capture the oxygen. Variance in the relative rates
of reaction can be measured, but not the actual volumes generated.
Difficult to keep the cubes evenly shaped
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Exexel IG Bio – Experimental Methods
2.16 describe experiments to investigate diffusion and osmosis using living and non-living
systems.
Models of cells
Visking (dialysis) tubing
1. Tie the visking tubing at one end
2. Half fill with sucrose solution
3. Tie the other end
4. Measure the mass
5. Place in water for 30 mins
6. Dry the outside of the tubing and measure the mass again
7. The change in mass indicates the direction of osmosis and how quickly it has happened
Cubes of agar jelly
1. Cut three cubes out of the agar jelly. Each cube should measure 2cm x 2cm x 2cm
2. Place the agar cube in a beaker of potassium permanganate solution (purple dye)
3. After 10 mins remove one cube
4. Dry the surface
5. Cut the cube in half
6. Measure the distance (from the edge) that the potassium permanganate has reached by
diffusion.
7. After 20 mins and 30 mins remove the second and third cubes and similarly measure the
diffusion distance (repeat steps 4 – 6)
Observing changes in cells using a microscope
Onion epidermis can be peeled away, cut into squares and mounted on slides
If the Onion epidermis is mounted in a strong sucrose solution plasmolysis, the cytoplasm
and vacuole will decrease in volume as water leaves the cell by osmosis and the cell
membrane will pull away from the cell wall.
If the sucrose solution is removed (e.g. using a paper towel) and replaced with water then
the water, by osmosis, will return to the cell, the volume of the cytoplasm and vacuole will
increase the the cell membrane will again press up against the cell wall.
Red blood cells in blood obtained from a butcher may be mounted on slides in hypotonic,
isotonic and hypertonic saline, and observed under a microscope to show the effects of
osmosis.
Hypotonic solution should cause the red blood cells to burst whereas the hypertonic
solutions should cause them to decrease in volume, i.e. shrink.
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Exexel IG Bio – Experimental Methods
Plant tissues
A number of strips/cores/cubes of potato(or carrot etc.) of the same size are cut
Dry the pieces of potato with a paper towel and measure their starting mass
Place the pieces of potato into differents concentration of salt (or sucrose) solution
Wait for a specified time (at least one hour)
Remove the pieces of potato with a paper towel and measure their ending mass
Calculate the % change in mass:
% change = [ending mass (g) – starting mass (g)]
x
100
starting mass (g)
n.b. 1. Length can be measured instead of mass.
2. The method can be changed/extended to investigate the effect of surface area,
temperature, pH and various other factors.
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Exexel IG Bio – Experimental Methods
2.22 describe experiments to investigate photosynthesis, showing the evolution of oxygen from a
water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll
Photosynthesis (and hence the rate at which it happens) can be measured in a number of ways.
Mostly commonly the concentration or the volume of the photosynthetic products are measured. It
is straightforward to measure the volume of oxygen evolved and to test for present of starch (a
polymer of glucose, which is the other product of photosynthesis) in leaves.
Oxygen evolution
Method 1:
1. A piece of water plant (typically
elodea, cambomba or a similar
species) is placed in a large
beaker (or trough) and covered
with water
2. Cover the water plant with an
inverted glass funnel. The depth
of water should be sufficient so
that the tip of the funnel is
several centimetres under the
level of the water.
3. Completely fill a test tube with
water.
4. Seal the end of the tube with a thumb (or rubber
source:
bung) and invert. Place the sealed end of the tube Image
http://www.elateafrica.org/elate/biology/biologyindex.html
under the level of the water in the beaker and
uncover it, the test tube should remain full of water.
5. Whilst keeping the now open end of the test tube under water carefully place the test tube
on top of the funnel’s neck.
6. Bubbles of oxygen that appear on the surface of the water plant should rise to the top of the
test tube. As they collect they will displace the water.
7. After 24 to 48 hours there should be sufficient oxygen that it can be tested for. Oxygen will
relight a glowing splint.
Weaknesses:
Temperature and light are very important for photosynthesis. If not controlled the levels of
oxygen that build up will vary greatly.
Certain water plants photosynthesise too slowly and sufficient oxygen for testing will a
longer period of time to be produced.
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Exexel IG Bio – Experimental Methods
Method 2:
1. Place a short piece of a
water plant (typically
elodea, cambomba or a
similar species) in a
solution of sodium
hydrogen carbonate (this
provides a good source
of carbon dioxide).
2. Invert the water plant
and keep it submerged
by weighing it down
(plasticine and paper
clips both work well).
3. Whilst submerged cut the stem (near to the top), this enables oxygen bubbles to escape.
4. Place an a strong artificial light source, e.g. a desk lamp close to the beaker containing the
water weed
5. Wait for (at least) 5 minutes as the plant equilibrates with the conditions.
6. Measure the (oxygen) bubble production by either:
a. counting the number of bubbles evolved in a minute
b. measuring how much time it takes for 10 bubbles to be given off.
7. Calculate the rate of photosynthesis use the equation below:
Rate of photosynthesis (bubble/min)
=
bubbles generated (bubbles)
Time taken (min)
Independent variables – what can be investigated:
Light intensity – can be varied by moving the lamp closer/further from the water plant
Colour/wavelength of light - coloured filters can be placed between the plant and the light
source.
Carbon dioxide availability - the plant is immersed in solutions of different concentration of
sodium hydrogen carbonate.
Temperature – the water the plant is placed in is maintained at different temperatures using
a waterbath.
Weaknesses:
Temperature and light are very important for photosynthesis. If not controlled the levels of
oxygen that build up will vary greatly.
The age/freshness of the water plant affect how fast growth and photosynthesis occurs
Ambient light levels will affect the rate of photosynthesis
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Exexel IG Bio – Experimental Methods
Starch Testing
Method:
1. Placing a plant in the dark for 24 hours to destarch the leaves (the starch will be
converted to glucose and used for respiration
and without light it cannot be replaced by
photosynthesis).
2. Cover part of a leaf using aluminium foil.
3. Place the plant in bright sunshine for 12
hours
4. Remove a leaf from the plant
5. Put the leaf in boiling water for 1 minute (to
destroy the cell membranes so that
chlorophyll molecules can pass through)
6. Heat the leaf in boiling ethanol (to
remove/dissolve the green chlorophyll from leaf)
7. Place leaf into room temperature water (to rehydrate and
soften the leaf so that it can be spread out)
8. Drop iodine solution onto the leaf (test for starch) – blueblack colour will show the presence of starch.
Image source:
http://www.elateafrica.org/elate/biology/biologyindex.html
Expected results:
The un-covered parts of the leaf should be blue-black
indicating the presence of starch and hence
photosynthesis occurred in this region of the leaf. This
indicates that light is necessary for photosynthesis.
Image source:
http://www.elateafrica.org/elate/biology/biologyindex.html
Alternatives:
A starch test on a variegated leaf can be used to
demonstrate that chlorophyll is needed for photosynthesis.
To show that carbon dioxide is needed for photosynthesis, a leaf on a plant may be
surrounded by air with no carbon dioxide. To remove the carbon dioxide from the air insert a
leaf into a conical flask (or plastic bag, ziplock bags work well) containing a small amount of
potassium or sodium hydroxide.
Weaknesses:
A good experiment always includes a control. An untreated leaf should always be tested
along with the treated leaves to provide evidence that the factor investigated is responsible
for the observed changes.
Temperature and light are very important for photosynthesis. If not controlled the levels of
starch that build up will vary greatly.
Certain plants photosynthesise too slowly and longer in sunshine to develop starch reserves
Certain plants are very efficient at relocating the glucose from photosynthesis and hence the
starch reserves are distributed more evenly through the plant and don’t show clear
differences between covered and uncovered regions.
If the plant is exposed too long then, like above, there is no clear difference between treated
areas/leaves as the glucose has been relocated and starch reserves are present throughout
the plant.
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Exexel IG Bio – Experimental Methods
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Exexel IG Bio – Experimental Methods
2.32
describe an experiment to investigate the energy content in a food sample.
A Calorimeter is a simple device used to determine the energy content of food. It takes 4.2 joules of
energy to heat 1g/1ml of water by 1OC. Using this information, food can be burnt and the heat
energy used to heat water. The increase in temperature of the water can be calculated to
determine how much energy was contained in the food.
Method:
1.
2.
3.
4.
5.
6.
7.
8.
A boiling tube is mounted at a near vertical angle in a clamp stand.
The water temperature is recorded.
25cm3 of water is added to the boiling tube.
A small quantity mass of the food sample (e.g. potato chips/crisps) is weighed and the mass
noted.
The food sample is put in a crucible or burning spoon and ignited (for example in the flame
of a Bunsen burner).
The crucible/burning spoon containing the food sample is quickly placed directly under the
boiling tube.
When the food sample has completely burnt note the maximum water temperature (the
temperature may continue to rise for a short time after the completion of combustion)
The energy content of the food is calculated using the equation:
Energy content of food sample (joules per gram) = mass of water heated (g) x temperature
rise (oC) x 4.2 x 1 / mass of food sample (g)
Weaknesses:
Not all foods burn easily
The food does not burn completely
The glass of the test tube is heated not just the water
Heat is dissipated to the surrounding air
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Exexel IG Bio – Experimental Methods
2.37 describe experiments to investigate the evolution of carbon dioxide and heat from respiring
seeds or other suitable living organisms.
Carbon dioxide is released during aerobic respiration a build up either in the volume or here the
concentration of carbon dioxide can be used to measure the rate of respiration. Similarly when
aerobic respiration occurs a small quantity of heat is released as a waste product. Again the
increase in temperature caused by this release in of heat energy can be used to indicate the
relative rate of respiration.
Method:
1. Place a number of germinating seeds are put in a large test tube
2. Use a clamp to hold the large test tube in position
3. Use the rubber bung to seal the large test tube
4. Connect an angled glass tube to the rubber bung
5. Half fill a second test tube with hydrogen carbonate indicator.
6. Use a second clamp to position the test tube containing the hydrogen carbonate indicator so
that the end of the angled glass tube is immersed in the solution.
7. Wait several hours, preferably a day then observe and record the colour changes in the
indicator solution.
Why use hydrogen carbonate indicator, why not limewater?
Limewater only measures increases in carbon dioxide, hydrogen carbonate indicator can
show decreases as well as increases
Changes in hydrogen carbonate indicator are reversible and colour changes constantly
reflect changes in carbon dioxide concentration whereas once limewater has become cloudy
it will not clear if carbon dioxide levels drop, the change is irreversible.
hydrogen carbonate indicator is more sensitive and will respond to smaller changes in
carbon dioxide than limewater
When cellular respiration occurs it gives off heat. The changes below to the equipment setup allow
for heat to be detected and measured:
Wrap the large test tube with an insulating material (or use a vacuum flask).
Use a rubber bung with two holes.
Place a thermometer in the second hole to measure temperature changes in the large test
tube.
Weaknesses:
The system is closed and oxygen is used up. After a certain point there will be no oxygen
left for seeds to use for respiration
Temperature should be controlled, if too low then respiration will not happen
No additional water is provided. Seeds need water for germination and growth
Volumes of solution, numbers of seeds not controlled
Colour changes in the indicate show a change in carbon dioxide levels, but cannot indicate
how large the change is.
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Exexel IG Bio – Experimental Methods
2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using
hydrogen-carbonate indicator
A leaf/plant can be placed in a sealed
tube of air-equilibrated hydrogen
carbonate solution (red in colour) and
placed in the light or in the dark.
Method:
Setup the apparatus below according to
the diagram. The tubes should be left for
several hours, ideally at least 24 hours.
Expected results:
Tube A – The hydrogen carbonate
indicator turns yellow due to
increased levels of carbon dioxide.
The leaf can only respire due to a
lack of light energy. This results in
a production of carbon dioxide.
Tube B – No colour change. This tube is a control to check that all the colour changes
that occur are due the presence of the leaf.
Tube C – The hydrogen carbonate indicator turns purple due to decreased levels of carbon
dioxide. The leaf is respiring at the same rate as the leaf in tube A, but due to higher levels
of photosynthesis more carbon dioxide is being used as a reactant (in photosynthesis)
than is being produced (by respiration).
Tube D?? – Ideally a second control should be setup identically to tube B, but additionally
wrapped in foil so that like tube A it receives no light.
Variants and extensions:
Small invertebrates (on a gauze platform) can be used to show the effect of respiration
relative to photosynthesis
Waterweeds can be used instead of leaves
Similarly to the small invertebrates water snails can be used with the waterweed to show the
effect of respiration relative to photosynthesis
n.b. if using animals they must not be harmed during the experiment and the conditions they
are subjected too must ones normally experienced in their natural environments.
Weaknesses:
Health and size of plants varies. Therefore the rate of respiration and photosynthesis will
also vary. Ideally repeats should be used to make the investigation more reliable.
The effect of night-time respiration may be larger than the net reduction of carbon dioxide
during the day meaning tube C as well as tube A contains yellow hydrogen carbonate
indicator solution. An artificial light should be used to make sure that this does not happen.
Due to use of the waterbath and placement in the room tube C may receive minimal light
levels and hence photosynthesis may not be greater than respiration. As previously an
artificial light should be used.
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Exexel IG Bio – Experimental Methods
2.48
describe experiments to investigate the effect of exercise on breathing in humans.
n.b. this experiment is best done in pairs. Be careful not to exercise to hard and also make sure
you talk to your teacher before participating if you have asthma or any other condition that affects
your breathing.
Method:
1. Fill a large plastic soft drink bottle (1l) completely with water and screw the cap in place.
2. Invert the drinks bottle and place just the neck of the bottle under water in the sink (or
trough).
3. Carefully unscrew the cap. n.b. whilst the neck of the bottle remains under water the bottle
should stay full of water.
4. Breathe a normal breath out. The air breathed out should displace some of the water in the
bottle.
5. Inset the end of a plastic tube into the neck of the bottle.
6. Mark a line to show the level of the water.
7. Empty the water bottle.
8. Fill the water bottle up to the marked line.
9. Pour the water from the water bottle into a large (1l) measuring cylinder.
volume (cm3) of water in the measuring cylinder = volume (cm3) of a normal breath.
10. Count and record breaths per minute at rest.
11. Calculate breathing rate after exercise (use the formula below).
12. Exercise for 1 minute
13. Count and record breaths per minute immediately after the exercise.
14. Calculate breathing rate after exercise (use the formula below).
Breathing rate at rest (cm3 per minute) = breaths per minute x volume of each breath
Weaknesses:
Breath volume changes and during exercise and afterward it should be larger than at rest.
Exercise intensity and duration affect the breathing rate.
Breaths per minute is likely to decrease during the minute of measurement after exercise as
the oxygen debt is repaid
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Exexel IG Bio – Experimental Methods
2.56 describe experiments to investigate the role of environmental factors in determining the rate
of transpiration from a leafy shoot
A bubble potometer can be used to illustrate
the effects of light, wind, temperature and air
humidity.
Potometers come in a variety of designs, but
all follow the same basic principles and
usually consist of:
A length of capillary tube A bubble is
introduced to the capillary. As water is
taken up by the plant, the bubble
moves. The distance the bubble
travels in a given time is determined
by the rate of water uptake by the
plant.
A reservoir. By turning the tap on the reservoir, the position of the bubble can be set at the
start of the experiment. Some designs of potometer use a syringe instead of a funnel with a
tap.
A tube for holding the shoot. In the diagram the shoot is held in place by inserting a
rubber bung in the tube. The hole in the bung through which the shoot passes must be
thoroughly greased with petroleum jelly to keep it airtight.
Method:
1. set up the apparatus as in the diagram above.
2. Leave undisturbed for 5 minutes or until the shoot equilibrates to the conditions.
3. Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do
this, loosen the screw on the boss and slide the boss up the clamp stand so that the
capillary tube comes out of the water in the beaker. Retighten the screw on the boss.
4. Gently blot the end of the capillary tube with a piece of paper towel and an air bubble should
appear in the capillary tube.
5. Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes
back into the water in the beaker. Retighten the screw on the boss.
6. There are two ways of taking measurements:
a. Measure the distance the bubble travels in a fixed amount of time, e.g. 5 mins
b. Measure the time taken for the bubble to travel a fixed distance, e.g. 3 cm.
7. Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the
formula given here. SI units are cm/s.
Rate of transpiration (cm/s)
=
Distance moved by the air bubble (cm)
Time taken (s)
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Exexel IG Bio – Experimental Methods
Independent variables – what can be investigated:
Light - Plants covered with dark polythene bags simulate darkness and can be compared
with plants covered with transparent polythene bags.
Wind - Hairdryers can simulate wind.
Humidity - Plant in a sealed polythene bag (versus one not in a bag) can be used to
simulate humid conditions
Temperature - Controlling temperature is more difficult, but can be done by controlling the
room temperature.
n.b. use of potted plants is acceptable. The pot and the soil needs to be sealed with polythene
during the investigation. The mass of the potted plant before and after the investigation is
measured. The larger the loss in mass the greater the rate of transpiration.
Weaknesses:
If the plant shoot is not well sealed then the air bubble will either not move or will move less
Leakages will also cause false readings due to too little or too much movement.
The plant shoot needs to be fresh if the same shoot is used over a number of days the rate
of transpiration is likely to decrease as the cut shoot is slowly dying
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Exexel IG Bio – Experimental Methods
4.2
4.3
explain how quadrats can be used to estimate the population size of an organism in two
different areas
explain how quadrats can be used to sample the distribution of organisms in their habitats.
Method:
1. Count the number of organisms in a single quadrat
2. Count the number of quadrats that fit in the given area.
3. Use the formula below to calculate the population size:
Population size = (number of organisms in a quadrat) x (number of quadrats in the area)
Worked example:
the quadrat size is 50 cm x 50 cm
the field (area) is 100m x 100m
the number of daisies per quadrat is 10
Daisies in the field = (10) x (number of quadrats in the area)
(population size)
= (10) x (area / size of a quadrat)
= (10) x (100 x 100 m2 / 0.5 x 0.5 m2)
= (10) x (10,000 m2 / 0.25m2)
= (10) x (40,000)
= 400,000
Important considerations:
The more quadrats placed the more reliable the estimate population size
Quadrats need to be placed randomly to avoid human bias
Examples and comparing sampling to actual counts:
Scatter a number of beads on the floor and compare estimated the population size to the
actual count. Discuss the usefulness of sampling compared with counting after completing
the activity.
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Exexel IG Bio – Experimental Methods
5.6
describe a simple experiment to investigate carbon dioxide production by yeast, in different
conditions
This investigation provides evidence that carbon dioxide is released during anaerobic respiration
(fermentation). The rate at which (carbon dioxide) bubbles are released can be used as a measure
of the rate of anaerobic respiration
Method:
1. Mount a side-arm test
tube (or flask) using a
clamp stand.
2. Add glucose solution
to a side-arm test
tube (or flask). The
level of glucose
solution should be
below the level of the
side-arm.
3. Add a small amount
of dried yeast to the
test tube and stir
gently to mix.
4. Add a drop or two of
oil, enough to cover the surface of the glucose/yeast mixture. This prevents oxygen
exchange and hence the conditions are anaerobic.
5. Use a rubber tube to attached a glass pipette to the side arm of the test tube.
6. Place water in a second test tube.
7. Mount a the water filled test tube (or flask) at an angle using a second clamp stand.
8. Place the tip of the pipette under the water level in test tube.
9. Wait 10-20 mins for the yeast to fully activate.
10. Count the number of carbon dioxide bubbles generated per minute. The more bubbles
generated the faster the rate of anaerobic respiration.
Independent variables – what can be investigated:
Temperature - place the side-arm test tube in a water bath. The end of the glass pipette
simply needs to be submerged in the bath.
Glucose concentration - make up different concentration of glucose solution.
pH - add the same volumes of different pH solutions (made using, acids, alkali, water)
Weaknesses:
An increase in temperature causes gases to expand. If temperature is not controlled this will
cause the number of bubbles generated to change dramatically.
If too much yeast/glucose solution is added, or if respiration is too fast then the mixture will
flow out the the pipette along with the carbon dioxide gas.
If respiration is too fast then the oil layer maybe disrupted and respiration will become
aerobic.
https://bioknowledgy.wikispaces.com/
(Chris Paine)
Exexel IG Bio – Experimental Methods
2.81 & 2.82 Controlled experiments to demonstrate phototropic and geotropic plant growth
responses
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Method:
Wheat, maize, oat or cress seedlings can be used to demonstrate phototropism.
Place petri dishes containing moist cotton wool and the plant material into light-proof boxes
(cardboard boxes work well)
Cut a hole to create a small slit in one side of the box to allow light to be shone into the box
from a single direction.
Control seedlings can either:
have aluminium caps put on their tips
kept in a shoe box without a slit for light.
2j
A simple experiment to show how the sensitivity of the skin differs on finger tips, back of
hand, wrist and forearm
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Method:
Fix the two prongs of a hairpin/toothpick/pin 5 mm apart.
Touch one or both prongs against the fingertips of a student who is looking away.
The student then has to judge whether one or two points were used and their response
recorded as correct or incorrect.
Repeat using pins/prongs 1 cm apart and 2 cm apart.
Repeat the whole process on the back of the hand, wrist and forearm.
Conclusion and analysis:
Use the data to identify which body area is the most sensitive.
Conclusions can be related to the number of sensory nerve endings, receptive field size and
the thickness of skin.
3a
A practical exercise comparing floral structure in insect- pollinated and wind-pollinated
flowers
n.b. students are not expected to describe the methodology, but might well be asked to analyse
data in examinations
Collect suitable specimens and using teacher guidance identify the different reproductive
structures.
Though insect-pollinated flowers are often available year round due to florists.
Wind-pollinated flowers are often more difficult to find at certain times of the year.
https://bioknowledgy.wikispaces.com/
(Chris Paine)
