This document describes an experiment investigating how the surface area to volume ratio of a cell influences the movement of substances into and out of the cell. Students use differently sized cubes of agar jelly, which are placed in sulfuric acid. As the acid diffuses into the jelly, it changes color. By measuring how far the acid penetrates, students can determine how the surface area to volume ratio affects diffusion. The results show that diffusion is more efficient for smaller cubes with higher surface area to volume ratios, demonstrating the importance of this ratio for cellular transport.

1. Name
Su rface Area to Volume Ratio
Practical
lntroduction e
cell for use in respiration'
Cells require substances to enrer and leave them; for example, oxygen enters the
A, ..r,rlt of respiration cells produce wastes such as carbon dioxide that need to be removed. However,
" Cells need to
c
the rate at which th.se sr-rbsta.rces are able to enter and leave the cells is also very important'
be able to gain and remove substances at an efficient rate so that normal cell
functioning is maintained' c
If there is too little oxygen available, then the amount of aerobic respiration will be affected; if there
is
too high a concentrarion of ."rbon dioxide within the cell, then the cell may die. Is
there a connection c
b.t,"..., the rate of movement of molecules in and out of the cell and the actual size of a cell? e
a
Pu rpose
of substances in
To inriestigate how the surface area ro volume ratio of a cell influences the movements e
and out ofthat cell'
a
Materials O
Each group will require:
E + ,r"y, of agarlsodium hydroxide/phenolphthalein jelly (pink-purple d
coloured) that have been poured 10, 20, 30 and 40 mm thick An alternative to using agar blocks
and sulfuric acid is cubes of aPPle e
D O.t M sulfuric acid (100 mL per group)
in RibenarM. This works just as well,
E ruler but you need to remember that }/ou CI
fl single-edged razor blade will be looking atthe coloured part
E zoo-zlo mL beaker of the apple, rather than atthe clear a
part as referred to in this activity.
Q clock or watch
d
E ,poon or large spatulas to stir and remove the agar cubes in the acid
D p"p.t towelling e
&t
r* E calculator.
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NelsonBiolo$lvcEl)nits7&2studentAC(NWManuat@2006NelsonAustraliaPtyLtd'
Not to be reproduced without the permission ofthe publisher'

2. )
,
) "'.€) The concentrations of sodium hydroxide and sulfuric acid used in this experiment are
regarded as'dilute'. However, avoid getting them in your eye, into cuts in your skin, etc.
) Wash your hands after you have finished handling the blocks of jelly. Take care when using
) the razor blade to cut the jelly blocks.
)
) Procedu re
Note: Phenolphthalein is an indicator like litmus or universal, and changes colour depending on how
D acidic its surroundings are. The jelly that you are using in this activity contains sodium hydroxide and
t phenolphthalein. In this experiment, acid is able to move gradually into this jelly preparation. In doing so
it neutralises the sodium hydroxide and the phenolphthalein changes from pink-purple to colourless.
D E tf thir has not already been done, the teacher should gently pry a-way the agar jelly from the tray sides
and turn the trays upside down, revealing the four slabs of purple jelly, 10, 20, 30 and 40 mm thick.
D E Por'r. about 100 mL of the 0.1 M sulfuric acid into the 250 mL beaker.
I E Uri"g razor blades, cut out blocks of no less than 10, 20, 30 and 40 mm in size to take away to your
area to rrim more accurately. (Try not to destroy the remainder of the jelly so that others may also
) use it!)
E ttl- the blocks of jelly into 10, 20,30 and 40 mm cubes. Do this as accurately as possible.
D E Gently add the cubes to the acid in the beaker without making splashes and stir gently every few
minures.
D
E Nor. the time when the cubes were added and also the time when they should come out in 10 minutes.
Pretest the jelly cubes to ensure that 10 minutes is adequate testing time for the jelly preparation to be
)
used by students. If not, the time can always be extended or shortened to highlight the ideas behind
D the activity.
E C"lculate the surface area, volume and SA:V for each of your cubes and complete Ta6le3,4 (under
I 'Observations' below).
D
D Wh.tt the 10 minutes are up, remove the cubes with a spoon, and pat them dry on paper towelling.
E Immediately cut each cube in half and measure the depth, to the nearest millimetre, of the clear parts.
D Decide on a typical figure and record it in Table 3.4.
[l Follo* your teacher's instructions when disposing of the materials.
D E Compl.teTable3.4.
I
Ohservations
I Table 3,4 Results and calculations.
D
I
D
D
D
I i
j30 mm
I
D I
t_
I
i
o i40 mm
i
i
40
I
I
I
o
6 Nelson BiologlvcE Unlts 1&2 Student Activiy Manual@ 2006 Nelson Australia Pty Ltd.
Not to be reproduced without the permission ofthe publisher. 35

3. Discussion questions
ll Explain why parts of the cubes become clear in the activity.
I Draw a graph based on your results, showing the relationship between SA:V ratio and the increase of
size of the cubes. On the horizontal axis, label the size of the cubes you have tested. On the vertical
axis, label the SA:V ratio.
f| From your graph, what happens to the SA:V ratio as the cubes get bigger?
I Draw a graph of your results. On the horizontal axis, label the size of the cubes you have tested. On
the vertical axis, label the percentage of volume uncoloured.
tl
Nelson Bblogy VCi UniE 7&2 Student Activy Manual @ 2OOG Nelson Australia Pty Ltd.
Notto be reproduced withoutthe permission ofthe publisher.

4. )
)
E From your graph, what happens to the diffusion of chemicals into the cubes as they increased in size?
) Explain.
)
)
I How do your answers to Questions 3 and 5 relate to nutrients diffusing into a cell through their
)
surface?
)
)
)
)
) What did you norice about the depth of the clear part generally for all the cubes?
[
)
)
)
ft Explain why organisms, when they are bigger than 1-2 mm, need to have other ways, apart from only
) diffusion, of exchanging materials with their surroundings'
)
)
)
)
D !t Could a cell grow to the size of a football? Explain your reasoning fully.
)
t
D
D
D
D
Conclusion
Refer back to the purpose of this activity. Have you achieved this purpose? frite a conclusion based on
D what you have learnt by undertaking this activity.
D
D
I
)
)
)
0 Nelson BiologlvcE Unrts 1&2 Student Activry Manual @ 2006 Nelson Australia Pty Ltd.
Not to be reproduced withoutthe permission ofthe publisher.
37