osmosis

Mike Dalrymple dalrymple at pplros.demon.co.uk
Fri Feb 10 12:33:41 EST 1995


In article <3gu9pa$cuj at rebecca.albany.edu> , tivol at tethys.ph.albany.edu
writes:
>In article <3gpala$8ia at Owl.nstn.ca>,
>cquinn at fox.nstn.ca (Cam Quinn) writes:
>>Hi!  My name is Erin Quinn and I am doing a Grade 8 science project on 
>>osmosis.  I put celery in glasses of baking soda, cornstarch, salt and 
>>sugar.
>>I observed that the salt dried it up, baking soda one was dying slowly, 
>>cornstarch just sank to the bottom and didn't do anything and sugar was 
>>really healthy BUT I DON'T KNOW WHY!
>>I've tried to get information from the library but don't know where to 
>>look and can't get any information.
>>
>>Can anyone help me by explaining what happened or tell me where to look 
>>for an explanation?

Dear Erin,

Osmosis is the movement of water molecules across a semi-permeable
membrane from a region of low salt concentration to a
region of high salt concentration.  The interior of plant
cells is low salt, so when you put the celery in high salt,
the water leaves the plant cells across the cell membrane by
osmosis, moving to the salt solution and leaving the selery
dried up.  Bill is right though, salt could also be intrinsically
bad for the plant anyway.  The baking soda one is more tricky.
 Slowly dying could mean a few different things, and could
 be a natural result of having cut the celery in the first
 place.  You need to put one stick in water and one in
 baking soda and observe the difference.  The stick in
 water is called the control stick.  The use of a control is
 important in any experiment.  The cornstarch is insoluble,
 and so can't have any effect on the celery because it
 can't be taken up by the plant.  The sugar is taken up
 and used as a food by the plant, which is why it does so
 well.  The plant takes up dissolved stuff like sugar
 by transpiration, which goes like this:  The plant has
 tubes (a bit like veins) running up it, and ending in
 the leaf.  The leaf is thin and in the sun, and has pores
 in it, so water vapour is lost from the leaf surface. 
 This effectively sucks more water up from the ground,
 just like if you put a paper wick into a bowl of water.
 Any dissolved compounds in the water which is sucked up
 (like your sugar) are processed by the plant cells to
 make what it needs to grow.
 
 I hope that helps.  Being British I don't know how old
 8th grade is, but a guess it's rather young.  Good for
 you, Erin, for being brave enough to get on the Net and
 ask your question!
 
 Anthony, Edinburgh, Scotland.



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