SA:V prac

Name

Su rface Area to Volume Ratio

Practical

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lntroductionCells require substances to enrer and leave them; for example, oxygen enters the cell for use in respiration'

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..r,rlt of respiration cells produce wastes such as carbon dioxide that need to be removed. However,

the rate at which th.se sr-rbsta.rces are able to enter and leave the cells is also very important' Cells need to

be able to gain and remove substances at an efficient rate so that normal cell functioning is maintained'

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

b.t,"..., the rate of movement of molecules in and out of the cell and the actual size of a cell?

Pu rposeTo inriestigate how the surface area ro volume ratio of a cell influences the movements of substances in

and out ofthat cell'

MaterialsEach group will require:

E + ,r"y, of agarlsodium hydroxide/phenolphthalein jelly (pink-purple

coloured) that have been poured 10, 20, 30 and 40 mm thick

D O.t M sulfuric acid (100 mL per group)

E ruler

fl single-edged razor blade

E zoo-zlo mL beaker

Q clock or watch

E ,poon or large spatulas to stir and remove the agar cubes in the acid

D p"p.t towelling

E calculator.

NelsonBiolo$lvcEl)nits7&2studentAC(NWManuat@2006NelsonAustraliaPtyLtd'Not to be reproduced without the permission ofthe publisher'

An alternative to using agar blocks

and sulfuric acid is cubes of aPPle

in RibenarM. This works just as well,

but you need to remember that }/ou

will be looking atthe coloured part

of the apple, rather than atthe clear

part as referred to in this activity.

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"'.€) 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.

mm

Procedu reNote: Phenolphthalein is an indicator like litmus or universal, and changes colour depending on how

acidic its surroundings are. The jelly that you are using in this activity contains sodium hydroxide and

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.

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.

E Por'r. about 100 mL of the 0.1 M sulfuric acid into the 250 mL beaker.

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.

E Gently add the cubes to the acid in the beaker without making splashes and stir gently every few

minures.

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

the activity.

E C"lculate the surface area, volume and SA:V for each of your cubes and complete Ta6le3,4 (under

'Observations' below).

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.

Decide on a typical figure and record it in Table 3.4.

[l Follo* your teacher's instructions when disposing of the materials.

E Compl.teTable3.4.

Ohservations

Table 3,4 Results and calculations.

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40 mm

Nelson BiologlvcE Unlts 1&2 Student Activiy Manual@ 2006 Nelson Australia Pty Ltd.

Not to be reproduced without the permission ofthe publisher. 35

Discussion questionsll 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 ofsize 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. Onthe vertical axis, label the percentage of volume uncoloured.

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Nelson Bblogy VCi UniE 7&2 Student Activy Manual @ 2OOG Nelson Australia Pty Ltd.

Notto be reproduced withoutthe permission ofthe publisher.

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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'

!t Could a cell grow to the size of a football? Explain your reasoning fully.

ConclusionRefer back to the purpose of this activity. Have you achieved this purpose? \frite a conclusion based on

what you have learnt by undertaking this activity.

Nelson BiologlvcE Unrts 1&2 Student Activry Manual @ 2006 Nelson Australia Pty Ltd.

Not to be reproduced withoutthe permission ofthe publisher. 37