Easy Water Relations
Problems: Solutions
UNI Plant Physiology
Spring 2008
Three Rules
• WP = PP + SP
• Membrane?
– Yes?…Osmosis happens: WP determines flow
– No?... Bulk flow happens: PP determines flow
• Water flows from high to low
Ask yourself
• Is there a membrane?
– Choose WP or PP
• What are the WPs or
PPs?
• Which one is lower?
Write it down, Say it out loud
•
•
•
•
•
Write down the arithmetic
Draw the arrow showing flow
Explain it in words
Helps you understand and remember
Prepares you for tests
Now do the problem set
Don’t go any further in this presentation
until you have done everything
you can on your own.
Reading about solving problems is like reading
about swimming. You will drown if you read,
then jump in the deep end of the pool.
Done the problem set yet?
OK, time to check the thinking and the answers.
Read and answer the questions before
clicking to continue.
Watch carefully: something happens with each click.
Page Up to go back & see it again.
Two limp, wilting leaf cells
• Is there a membrane here?
– Yes (one for each cell)
– Water potential is what counts
-4
-2
• Calculate water potentials
– SP + PP = WP
– Write them in as you go
– -4 + 0 = ?
• -4
– -2 + 0 = ?
• -2
• Which water potential is lower?
Draw arrow to
show water flow.
– -4 is lower than -2 (like temperature)
Two ordinary (stiff) leaf cells
• Is there a membrane here?
– Yes (one for each cell)
– Water potential is what counts
-2
-1
Draw arrow to
show water flow.
• Calculate water potentials
– SP + PP = WP
– Write them in as you go
– -6 + 4 = ?
• -2
– -4 + 3 = ?
• -1
• Which water potential is lower?
– -2 is lower than -1 (like temperature)
Two xylem “cells” (pipes)
• Is there a membrane here?
– No (membranes gone—only walls left)
– Bulk flow (everything together)
– Pressure potential is what counts
• Don’t have to calculate water
potentials
– Just look at the pressure potential
– -1 and -2
• Which pressure potential is lower?
Draw arrow to
show water flow.
– -2 is lower than -1 (like temperature)
Ordinary cell next to xylem “cell”
• Is there a membrane here?
– Yes (for the live cell)
– Water potential is what counts
-3
-2
Draw arrow to
show water flow.
• Calculate water potentials
– SP + PP = WP
– Write them in as you go
– -4 + 1 = ?
• -3
– -1 + (-1) = ?
• 0
• Which water potential is lower?
– -3 is lower than 0 (like temperature)
That’s all
• Just follow all the steps
– One at a time
– In order
• Write, draw, explain
• Let me know if this was helpful
• Tell me if you spot any errors