!!!!!!!!!!NAME:!__________________________________________________!PER.!_____________!
Lab:%Determining)the)Water)Potential)of)Potato)Cells)!
BACKGROUND+
In#animal#cells,#the#movement#of#water#into#and#out#of#the#cell#is#influenced#by#the#relative#concentration#of#solute#on#
either#side#of#the#cell#membrane.#If#water#moves#out#of#the#cell,#the#cell#will#shrink.#If#water#moves#into#the#cell,#the#cell#
may#swell#or#even#bursts.#In#plant#cells,#the#presence#of#a#cell#wall#prevents#the#cells#from#bursting,#but#pressure#does#
eventually#build#up#inside#the#cell#and#affects#the#process#of#osmosis.#When#the#pressure#inside#the#cell#becomes#large#
enough,#no#additional#water#will#accumulate#in#the#cell#even#though#the#cell#still#has#a#higher#solute#concentration#than#
does#pure#water.#So#movement#of#water#through#the#plant#tissue#cannot#be#predicted#simply#through#knowing#the#
relative#solute#concentrations#on#either#side#of#the#plant#cell#wall.#Instead,#the#concept#of#water#potential#is#used#to#
predict#the#direction#in#which#water#will#diffuse#through#living#plant#tissues.##
+
In#a#general#sense,#the#water#potential#is#the#tendency#of#water#to#diffuse#from#one#area#to#another#under#a#given#set#of#
perimeters.#Water#potential#is#expressed#in#bars,#a#metric#unit#of#pressure#equal#to#about#1#atmosphere#and#measured#
with#a#barometer.#Water#potential#is#abbreviated#by#the#Greek#letter#psi#(ψ)#and#has#two#major#components:#solute#
potential#(ψS),#which#is#dependent#on#solute#concentration#and#pressure#potential,#(ψP)#which#results#from#the#exertion#
of#pressure#G#either#positive#or#negative#G#on#a#solution.#We#express#this#as:##
#
Figure+1.1#A#potato#cell#is#placed#in#pure#water.#Initially#the#water#potential#outside#the#cell#is#0#and#is#higher#than#the#
water#potential#inside#the#cell#(G3).#Under#these#conditions#there#will#be#a#net#movement#of#water#into#the#cell.#The#
pressure#potential#inside#the#cell#will#increase#until#the#cell#reaches#a#state#of#equilibrium.##
#
PRE6LAB+QUESTIONS+
1. What#would#happen#if#you#applied#saltwater#to#the#roots#of#a#plant?#Why?#
2. Will#water#move#into#or#out#of#a#plant#cell#if#the#cell#has#a#higher#water#potential#than#the#surrounding#environment?#
#
PROCEDURES:#(An#example#set#of#procedures#is#given#below;#yours#may#differ!)##
1. Make#5#sucrose#solutions#of#increasing#molarity:#0.2#M,#0.4#M,#0.6#M,#0.8#M,#1.0M.##
2. Pour#50#mL#of#each#unknown#solution#into#a#beaker.#Slice#a#potato#into#5#equal#cylinders.#
3. Determine#the#mass#of#the#5#potato#cylinders#and#record.##
4. Place#the#cylinders#into#the#beakers#with#solution#and#cover#with#plastic#wrap.#Leave#overnight.#
5. Record#the#room#temperature#in#Celsius.#
6. Remove#the#cylinders#from#the#beakers#and#carefully#blot#of#any#excess#solution.#
7. Determine#the#mass#of#the#potato#cylinders#and#record.##
