Practical work and student investigations
1 Tests for glucose and starch, lipid and protein
2 Controlled experiments to illustrate how enzyme activity can be affected by changes in temperature
and pH
3 Simple experiments on diffusion and osmosis using living and non-living systems
4 A simple experiment to determine the energy content of a food sample
5The use of quadrats to estimate the population size of an organism in two different areas
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1 Tests for glucose, starch, lipid and protein
The tests that are expected:
• Benedict’s test for glucose
Testing for sugars
Benedict’s solution is blue.
Add Benedict’s solution and heat in a water bath. Solution should turn from blue to green to yellow and
finally red if glucose is present.
If solution remains blue, then glucose is not present.
• iodine test for starch (test not required by specification)
Testing for starch
Iodine is orange/brown
Add iodine solution, if it turns blue black it means starch is present. If it stays yellow/orange means there
is no starch present
• the emulsion test for lipid
Testing for lipid (fat)
Dissolve the lipid in ethanol then add water and shake vigorously, if it goes cloudy white lipid is present.
• biuret reagent for protein(test not required by specification).
Testing for protein
Biuret reagent is pale blue.
Add Biuret reagent to a solution of food, if it turns purple protein is present. If it remains pale blue no
protein is present
These tests could be carried out:
• on prepared samples of pure substances
• on foods that contain the substances
• in a context.
Example
To demonstrate health applications, a fake ‘urine sample’ can be made by colouring water with iodine
solution. A similar solution, with glucose added, could mimic the urine of an untreated diabetic. Students
can use Benedict’s test to identify the diabetic – if glucose is present in urine the patient is diabetic.
The iodine test could be introduced when teaching that plants manufacture starch by photosynthesis, or
when investigating the effect of amylase on starch digestion.
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Controlled experiments to illustrate how enzyme activity can be affected by changes in
temperature
1. The effect of amylase on starch digestion
At each temperature selected, from 0°C to 100°C, samples of amylase solution and of starch solution are
brought to temperature before being added together. The mixture is then kept at the same temperature.
To measure the rate of reaction, drops of the mixture can be collected at intervals of one minute and
added to individual iodine drops on a white tile. Iodine tests for the presence of starch – iodine goes black
when starch is present. The time taken for the starch to disappear is recorded for each temperature –
indicated when the iodine no longer changes colour (remains orange/brown). The temperature can be
controlled using water baths.
The digestive enzyme amylase breaks down starch into the sugar maltose. If the speed at which the
starch disappears is recorded, this is a measure of the activity of the amylase.
2. The effect of catalase on the breakdown of hydrogen peroxide.
Catalase is found on the surface of potato and liver. Catalase converts hydrogen peroxide into water and
oxygen. The rate of oxygen production can be measured as an indication of enzyme activity. An upturned
boiling tube previously filled with water can be used to collect and measure the volume of oxygen
evolved. Or the time take for ‘bubbling’ to stop can be measured. The liver/potato can be exposed to
different temperature to demonstrate the effect of temperature.
Enzymes are proteins that speed up chemical reactions in our cells.
Enzymes work best at their optimum temperature. This is why homeostasis is important - to keep our
body temperature at a constant 37°C.
As the temperature increases, so does the rate of chemical reaction. This is because heat energy causes
more collisions, with more energy, between the enzyme molecules active sites and other molecules.
However, if the temperature gets too high, the enzyme is denatured and stops working.
A common error in exams is to write that enzymes are killed at high temperatures. Since enzymes are not
living things, they cannot be killed.
Graph showing the effect of temperature on enzyme reactions
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3. Effect of changing pH on enzyme’s rate of reaction.
1 cm3 sodium
carbonate solution
0.5 cm3 sodium
carbonate solution
2 cm3 acetic acid
4 cm3 acetic acid
Place 5 cm3 starch solution in each tube. Note the time and add 1 cm3 amylase solution to each
Then test for starch using iodine – take a sample every 30 seconds and time how long it takes for the
iodine to no longer turn blue/black.
The most rapid breakdown of starch will be in the region of pH 6-7.
When the pH in the mouth was tested, the result was probably about pH 7.
Logically you would expect the pH in the mouth to be the one which favoured a rapid breakdown of
starch. The other pH values seem to slow/inhibit the action of the enzyme – pH can denature the enzyme
so that the enzyme’s active site changes shape and the substrate can no longer bind to it..
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Simple experiments on diffusion and osmosis using living and non-living systems
a Cubes of agar jelly placed into solutions of methylene blue or potassium permanganate will absorb the
pigment by diffusion. The cubes are left in the pigmented solution for different measured periods of time
and are then sliced open. The distance between the edge of each cube and the edge of the coloured agar
may be used as a measure of the distance the pigment molecules have moved by diffusion.
Demonstration of diffusion in a jelly
Agar jelly has a consistency similar to the cytoplasm of a cell. Like cytoplasm, it has a high water content.
Agar can be used to show how substances diffuse through a cell.
This demonstration uses the reaction between hydrochloric acid and potassium permanganate solution.
When hydrochloric acid comes into contact with potassium permanganate, the purple colour of the
permanganate disappears.
A Petri dish is prepared which contains a 2 cm deep layer of agar jelly, dyed purple with potassium
permanganate. Three cubes of different sizes are cut out of the jelly, with side lengths 2 cm, 1 cm and 0.5
cm.
The cubes are carefully dropped, at the same time, into a beaker of dilute hydrochloric acid (Figure 1.10).
The time is taken for each cube to turn colourless.
The smaller the cube the larger its surface area to volume ratio, so the quicker the rate of diffusion (the
shorter the time for the cube to go colourless).
b A crystal of potassium permanganate can be dropped into a beaker of water and the appearance of the
water noted over time.
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c To demonstrate osmosis, Visking tubing (dialysis tubing) can be tied at one end and filled with 20 per
cent sucrose solution. The other end is attached to a capillary tube. The level of the sucrose can be noted
before and after the tubing has been placed in a beaker of water for about 30 minutes. Visking tubing has
microscopic holes in it, which let small molecules like water pass through (it is permeable to them) but is
not permeable to some larger molecules, such as the sugar sucrose. This is why it is called ‘partially’
permeable.
The sucrose molecules are too big to pass through the holes in the partially permeable membrane. The
water molecules can pass through the membrane in either direction, but those on the right are attracted
to the sugar molecules. This slows them down and means that they are less free to move – they have less
kinetic energy. As a result of this, more water molecules diffuse from left to right than from right to left.
In other words, there is a greater diffusion of water molecules from the more dilute solution (in this case
pure water) to the more concentrated solution.
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d Onion epidermis can be peeled away, cut into squares and mounted on slides in different
concentrations of sucrose solution. Observation under a microscope will show the effects of osmosis.
Plasmolysis - The state of a plant cell in a hypertonic solution, the cell shrinks.
Turgid - Plant cells in a hypotonic solution, the cell swells against the cell wall and the cell is rigid and firm
Flaccid - Plant cells in a hypertonic solution, the cell shrinks away from the cell wall and the cell is limp.
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e 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.
Blood plasma has a concentration equivalent to a 0.85% salt solution. If fresh blood is placed into
solutions with different concentrations, the blood cells will gain or lose water by osmosis. This can be
demonstrated using sterile animal blood
(available from suppliers of biological materials).
Three test tubes are set up, containing these solutions:
A 10 cmᶟ of distilled water (hypotonic)
B 10 cmᶟ of 0.85% salt solution (isotonic)
C 10 cmᶟ of 3% salt solution (hypertonic)
1 cmᶟ of blood is added to each tube, and the tubes are shaken. A sample from each tube is examined
under the microscope. The sample from tube A is found to contain no intact cells
The blood cells on the right (tube C) were placed in a 3% salt solution and the normal blood cells on the
left(tube B) were in a 0.85% salt solution.
The cells from tube B look normal, but those from tube C are shrunken, with crinkly edges
It is important that animal cells are surrounded by a solution containing the correct concentration of
dissolved solutes. If the surrounding solution does not have the right concentration, cells can be damaged
by the effects of osmosis. The red blood cells placed in water absorb the water by osmosis, swell up and
burst, leaving a red solution of haemoglobin in the test tube. When placed in 3% salt solution, the red
blood cells lose water by osmosis and shrink.
The three tubes are now placed in a centrifuge and spun around at high speed to separate any solid
particles from solution.
Tube A contains a clear red solution and no solid material at the bottom of the tube = the cells have burst
due to water entering via osmosis and haemoglobin is released into the water
Tubes B and C both contain a colourless liquid and a red precipitate at the bottom = the cells have
remained intact so haemoglobin remains in the cells.
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f Osmosis can be demonstrated by using strips of potato, and this basic experimental method provides a
good opportunity for students to carry out individual whole investigations.
Because of the difficulty of the osmosis concept, it is better to keep this investigation until the latter part
of the course so that students will have had previous experience of carrying out investigations on simpler
topics. Students enjoy the reference to ‘chips’, but should quickly realise that it can be difficult to keep
the size constant – to achieve consistency lengths of potato tissue can be drilled from a potato using a
cork borer. The ‘chips’ are measured by mass or by length and are placed into sucrose solutions of
different concentrations for at least one hour. The percentage change in mass or length is a measure of
the degree of osmosis that has occurred.
A potato tuber is a plant storage organ. It is a convenient tissue to
use to investigate the effects of osmosis on the mass of the
tissue.
A boiling tube is half-filled with tap water and a second with
concentrated (Molar) sucrose solution. A third tube is left empty.
A potato is ‘chipped’ – cut into chips 5cm x 1cm x 1cm, making
these measurements as accurate as possible to ensure the chips
have the same surface area. No skin is left on the potato (it would
provide a waterproof layer).
Each chip is gently blotted to remove excess moisture and
weighed to find the starting mass of each. One chip is placed into each of the three tubes, and after a
fixed time the chips are removed, gently blotted to remove excess moisture and reweighed. The chips
can be felt to see and change in texture and re-measured to see if there is any changed in dimensions
(length/width/breadth).
The change in mass (+/-) is calculated for each chip and the % change is found. (% change in mass takes
into account the difference in starting masses of the chip so they can be compared).
% change = change in mass x 100
Starting mass
Boiling tube with water - water moves into the cells
The water in the tube is more concentrated than the water in the cells, so water molecules will move
from an area of higher concentration of water (in the tube) to an area of lower concentration of water (in
the potato cells).
The cell membrane is semi-permeable so the water molecules can get through and each cell will swell as
the water flows in.
As all the cells swell, the potato chip will increase in mass, and length/width/breadth and become more
rigid.
Boiling tube with concentrated sugar solution - water moves out of the cells
The water in the tube is a lower concentration than the water in the cells, so water molecules will move
from an area of higher concentration of water (in the potato cells) to an area of lower concentration of
water (in the tube).
The cell membrane is semi-permeable so the water molecules can get through and each cell will shrink as
the water flows out.
As all the cells shrink, the potato chip will decrease in mass, and length/width/breadth and become more
flexible/floppy.
Boiling tube with nothing - some water moves out of the cells
Water molecules will move from an area of higher concentration of water (in the potato cells) to an area
of lower concentration of water (in the tube).
The cell membrane is semi-permeable so the water molecules can get through and each cell will shrink as
the water flows out.
As all the cells shrink, the potato chip will decrease in mass, and length/width/breadth and become more
flexible/floppy. The effect will be less than in the tube with concentrated sugar solution.
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g A variation on this theme is to cut potato cubes of different sizes, which have different surface area to
volume ratios. After measuring and recording the masses of the cubes, they are immersed in water. After
one hour, the cubes are blotted dry and their masses measured and recorded again. The percentage
increase in mass for cubes of different surface area to mass ratio can be compared, to explore the
concept of how surface area to volume ratio influences water uptake.
The surface area over which the osmosis occurs can change the speed at which it happens dramatically.
The larger the surface area between the two solutions the faster osmosis will happen. This is due to more
of each solution will be in contact with the membrane at one moment. This
means that a larger amount of the free water molecules in the solution can move across the membrane
at the same time therefore making the process happen much faster.
Eight 1cm³ cubes
One 8cm³ cube
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A simple experiment to determine the energy content of a food sample
Fat-containing foods such as dried crisps work very well.
• A known mass of the food sample is weighed and the
mass noted.
• A boiling tube is prepared, containing a known volume of
water. The water temperature is recorded.
• The food sample is put in a crucible or burning spoon and
ignited (for example in the flame of a Bunsen burner).
• The food sample is quickly placed under the boiling tube.
As soon as the food sample has completely burnt the water
temperature in the boiling tube is re-measured.
• The equation used to calculate the energy content of the
food is:
Energy content of food sample (joules per gram) =
mass of water heated (g) x temperature rise (oC) x 4.2
mass of food sample (g)
To obtain an accurate result, all the energy in the food sample needs to be transferred to the water, but
unless you use a bomb calorimeter this won’t happen.
Bomb calorimeter
NB: The energy gets lost to the surroundings as not all energy is direct to heat the water, also there is a
need to heat the glass apparatus first.
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The use of quadrats to estimate the population size of an organism in two different
areas
Quadrats can be used to sample part of each area. Calculation will be needed to work out the estimated
population size. For example, if 10 quadrats have been used and the total area amounts to 100 quadrats,
the estimated population size will be the number of organisms counted in the 10 sample quadrats
multiplied by 10.
It is important to place the sample quadrats randomly – this avoids bias and improves the reliability of the
data.
An interesting way to practise the technique is to throw plastic beads on the floor of the classroom and
ask students to guess how many beads there are. The quadrat sampling procedure can be used in front of
students to get an estimate. The beads can then be collected and counted.
The actual number can be compared to the estimated number and used to see how accurate the
estimation was.
A quadrat is a square (of either metal,
wood, or plastic) used in ecology and
geography to isolate a sample, usually
about 1m2 or 0.25m2. The quadrat is
suitable for sampling plants, slowmoving animals (such as millipedes
and insects), and some aquatic
organisms.
When an ecologist wants to know
how many organisms there are in a
particular habitat, it would not be
feasible to count them all. Instead, he
or she would be forced to count a
smaller representative part of the
population, called a sample. Sampling
of plants or animals that do not move
much (such as snails), can be done
using a sampling square called a
quadrat. A suitable size of a quadrat
depends on the size of the organisms
being sampled. For example, to count
plants growing on a school field, one
could use a quadrat with sides 0.5 or
1 metre in length.
It is important that sampling in an area is carried out at random, to avoid bias. For example, if one were
sampling from a school field, but for convenience only placed quadrats next to a path, this might not give
a sample that was representative of the whole field. It would be an unrepresentative, or biased, sample.
One way one can sample randomly is to place the quadrats at coordinates on a numbered grid.
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