Unit / Cell Structure and Function – Cell Size
Enduring Understanding /
- As cells increase in size, surface area to volume ratios decrease, making cells unable to obtain nutrients or remove wastes. To reduce the effects of this, cells divide to stay small or change shape to increase surface area or reduce volume.
SOL Objectives / BIO.3The student will investigate and understand relationships between cell structure and function. Key concepts include
a)evidence supporting the cell theory;
b)characteristics of prokaryotic and eukaryotic cells;
c)similarities between the activities of the organelles in a single cell and a whole organism;
d)the cell membrane model; and
e)the impact of surface area to volume ratio on cell division, material transport, and other life processes.
BIO.1The student will demonstrate an understanding of scientific reasoning, logic, and the nature of science by planning and conducting investigations.
Title / Why are cells so small?
Lesson Objective / Students will explore how surface area to volume ratios effect the functioning of cells They will design an experiment that mimics this through studying how heat transfer varies in different sized potatoes. They will find that smaller potatoes (cells) are more efficient in carrying out cellular process, such as heat transfer, osmosis, and waste elimination.
Inquiry Level / Level 3 – Students will be given the question to answer and materials, but will need to come up with their own methods.
Materials Required / Sugar cubes, oven or toaster oven, potatoes of varying sizes, rulers, calculators, probeware thermometers (optional)
Name______Period:______
Why are cells so small?
Today'slabisaboutthesizeofcells.Tounderstandthis,wefirsthavetounderstandsurfaceareato
volumeratios.
Let'sstartwithsurfacearea.Getaboxofsugarcubes.Thesurfaceareaofthecubeiscalculatedby
findingthearea(lengthxwidth)ofoneside.Butwewanttoknowthewholesurface.Onacube,there
are6sides,sowemultiplythatareax6.So
Surfaceareaofacube=lengthxwidthx6
Volumeisanothersimplecalculation-wejustmultiplylengthxwidthxheight.Buildthefollowingfour
structuresfromyoursugarcubes,andfilloutthetablebelow.
ABC D
1UNIT2UNITS 3 4UNITS
ORCM UNITS
FigureTotalnumberSurfaceareaof Volumeof Surfaceareato Totalsurface
A
B
C
D
ofCubes
figure
(=6xlengthx
width)
figure
(=lengthx
widthxheight)
VolumeRatio
SA/V
ofindividual
cubes
SurfaceAreatoVolumeLabQuestions
1.Whathappenedtothesurfaceareaasthesizeincreased?
2.Whathappenedtothevolumeasthesizeincreased?
3.Whathappenedtothesurfaceareatovolumeratioasthesizeincreased?
4.Nowimagineyou'relookingataverylargecell.Nexttoitisaverysmallcell.Whichonehasalarger
surfaceareatovolumeratio?
SurfaceAreatoVolumeLabPartII:HotPotato-Inquiry
Nowyouaregoingtoapplywhatyoujustlearnedaboutsurfaceareatovolumeratios.Yourjob
istofigureouthowthiswholethingappliestobiology.
Thecellsyou'velookedatinbiologyhaveallbeenwiththeuseofamicroscope:they'veallbeenvery,verysmall.Butsomecellsarenot-theyaremuchbigger,andyoucanevenseethemwithoutanyhelpfrommagnifyingglassesoranything‐forexample,a
chickenegg.That'sallonecell!Whatisthelargestcell?It'sdisputed,butsomesayitis
theostrichegg.Othergigantoidcellsincludethenervecellofthegiantsquid(overameterlong!)and
somealgaewhichhavelargecells.Butmostcellsaretiny.
Why?
You'velearnedaboutosmosisanddiffusion.Thisishowcellsgetalmosteverythingtheyneed:
nutrients,ions,water,etc,andhowtheygetridofwastes.Willdiffusionandosmosisbeenoughto
supportjustanycell?
Questions
1.Doyouthinklargecellsandsmallcellscarryoutdiffusionandosmosisatthesamerate?Whyorwhy
not?
2.Ifacellhasahighconcentrationofsomething,say,waste,thatitwantstogetridof,whichdoyou
predictwillbeabletogetridofthewastesooner-asmallercelloralargeone?
Now,designanexperimenttotestyourhypothesisfromquestion#2.
Wehavelargeandsmallpotatoestomodelcells.Theyarehot.Justlikeincellularprocesses,heat
diffusesoutofareasofhighconcentrationtoareasoflowconcentration,whichmakesitagreat
demonstrationfortestingquestionslikethis,sincecellsareveryhardtoworkwith.
Wewillapplythehypothesisfromquestiontwotothepotatoes.Ifyouhavealargehotpotatoanda smallhotpotato,andtwothermometers(andusinganythingelsearoundtheroomyoucanthinkofif
youwant),howcanyoutestyourhypothesisthatonepotato(whicheveryoupredictedabove)willlose
heatfasterthantheother?Whatwillyoumeasureandhowwillyoumeasureit?Describeyour
experimentbrieflybelow,using1‐2completesentences.
Onceyouhavecomeupwithanexperiment,bringyourpapertoyour teachertoapproveyourexperimentandtogetyourtwohotpotatoesandbegin.
Data
Graphyourdatabelow.Makesuretolabelyourxandyaxis.Useadifferentcolorlineforyour
differentpotatoes.
Questions
3.Whichpotatocooledofffaster? Why do you think?
4.Ifthepotatowasacell,andtheheatwaswastethecellwastryingtogetridof,whichcelldoyou
thinkwouldhaveanadvantage?
5.Howmightsurfaceareatovolumeratioshelpdecidethesizeofcells?
6.Basedonwhatyouhavelearnedtoday,inwhatwaymaymulticellularorganismshaveanadvantage
oversingle‐celledones?
7.Consideramouseandanelephant.Ifbothwereleftinthecoldovernight,whichwouldbeinmore
dangeroffreezingtodeath?Why?
8. How could this experiment be improved or expanded?