MadSci Network: Botany

Re: When is a cell flaccid compared to turgid compared to plasmolysed?

Date: Tue Feb 10 15:02:42 2004
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1076280722.Bt

Some of the confusion might be the similar terms hypotonic and hypertonic. It 
can get confusing because when a cell is hypertonic relative to the 
surrounding solution, the surrounding solution is hypotonic relative to the 
cell. Let's apply the -tonic terms just to the solution for clarity.

Hypertonic solution - cell less salty than surrounding solution, net water 
movement out of cell

Isotonic solution - cell and surrounding solution equally salty, no net water 

Hypotonic solution - cell saltier than surrounding solution, net water 
movement into cell

It depends on whether it is a plant or animal cell. Here's what to expect: 

Plant cell
Hypertonic solution - Plasmolysed cell
Isotonic solution - Nonturgid or wilted cell
Hypotonic solution - Turgid cell (Usual environment)

Animal cell
Hypertonic solution - Cell shrinks
Isotonic solution - Normal (Usual environment) 
Hypotonic solution - Cell swells and may burst

The differences are due to the fact that plant cells have a cell wall and 
animal cells do not. Remember that the usual condition is different for plant 
and animal cells. When considering effects of changing the external solution, 
the normal assumption is you are transferring a cell from its usual 
environment to a different environment. 

A plant cell is normally "saltier" than its external environment so the cell 
swells slightly due to osmosis as water enters the cell. The cell wall limits 
swelling and allows a positive turgor pressure or tugor potential to develop. 
Osmotic potential plus turgor potential equals the water potential. If the 
osmotic potential was -8 and the turgor potential was +2 then the water 
potential would be -6.

A plant cell transferred from a hypotonic solution to an isotonic solution 
will lose a little bit of water, and the turgor potential will go to zero. 
With zero turgor, the cell is wilted. The water potential will be the same 
inside the cell and in the external solution.

A plant cell transferred from a hypotonic solution to a very hypertonic 
solution will immediately lose a large volume of water by osmosis. The cell 
volume is greatly reduced, and the cell membrane shrinks away from the cell 
wall. This is often demonstrated under a microscope with an elodea leaf or 
onion bulb epidermis.

An animal cell is normally as salty as its external solution. Animal cells do 
not have cell walls so have no turgor pressure. Plants use turgor pressure for 

An animal cell transferred from an isotonic to a hypertonic solution will 
shrink as water moves out of the cell via osmosis.

An animal cell transferred from isotonic to hypotonic will gain water via 
osmosis and swell. If the solution is dilute enough, the cell may burst 
because it has no cell wall to prevent the cell membrane from stretching as 
the cell expands.

Turgid and flaccid are terms that really only apply to cells with cell walls, 
such as plant cells. Animal cells may shrink or swell when placed in 
hypertonic and hypotonic solutions, respectively, but they never have turgor 
because the cell membrane is too weak. A cell wall is required for a cell to 
have a turgor potential. 

Plasmolysis is a term that applies only to cells with cell walls. Plasmolysis 
of plant cells is a very extreme and rather artifical situation found mainly 
in biology classes. In a typical plasmolysis demonstration, an elodea leaf on 
a microscope slide is flooded with a concentrated (10% by weight) salt 
solution. Water rapidly exits the cell, and the cell membrane shrinks away 
from the cell wall. However, the cell membrane is still surrounded by 
solution. Under natural conditions, plants are rarely exposed to concentrated 
salt solutions. When plant cells are subjected to extreme drought, their cells 
may shrink but the outside of the cell membrane would be surrounded by air, 
not solution. 

Plant cells in isotonic and hypertonic solutions both lack turgor. 


Animal versus Plant Cells in Isotonic, Hypertonic and Hypotonic Solutions


Plasmolysis definition

Photos of Red Blood Cells in Hypotonic Solution

Photos of Red Blood Cells in Isotonic Solution

Photos of Red Blood Cells in Hypertonic Solution

Photos of Plasmolysis in Elodea Leaf Cells and Onion Bulb Epidermal Cells


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