Osmosis

define osmosis

the random net movement of water molecules from an area of high water potential to an area of low water potential (down a water potential gradient across a partially permeable membrane)

what is net movement?

the overall movement of the molecules.

why is there always movement both ways across the membrane?

Molecules move randomly all the time

what time of membrane can osmosis only occur over?

• Osmosis will only occur across a partially permeable. In nature, this is usually the cell surface membrane.  

• Whenever water enters or leaves a cell, it is moving across the cell surface membrane. 

Is osmosis passive?

Osmosis is passive and so therefore does not require energy.  

what water has the highest water potential?

pure water

what is the highest water potential?

0kPa 

what type of numbers is water potential always in?

negative numbers

which way will water always move in?

Water will move from an area of negative water potential to an area with more negative water potential.

Water will therefore always move from an area of pure water to an area where there are solutes (e.g. glucose, sucrose, ions) in the water. 

the higher the conc of solutes the more ________ the water potential

negative 

what are the 4 factors affecting osmosis?

• temperature

• water potential 

• distance 

• surface area 

how does temp affect osmosis?

the higher the temperature the higher the kinetic energy so the faster rate of osmosis. 

how does water potential affect osmosis?

the steeper the water potential gradient, the faster the rate of osmosis. 

how does distance affect osmosis?

the greater the distance the water molecules have to travel, the slower the rate of osmosis. 

how does surface area affect osmosis?

• the larger the surface area, the greater the rate of osmosis.  

what happens to a plant cell in a dilute solution (has a higher water potential than the cell)?

• water moves in via osmosis 

• creates turgor pressure which is very important for plants 

• cell is turgid. 

what happens to a plant cell in an isotonic solution?

• no osmosis

• water potential in solution is same as inside cell.

what happens to a plant cell in a concentrated solution (solution has a lower water potential then the cell)?

• water leaves the cell via osmosis 

• plant starts to wilt as cells are flaccid. 

what happens to a plant cell in a really concentrated solution (solution has a far lower water potential then the cell)?

• water leaves cell via osmosis 

• plant wilts as cells become flaccid

• Plasmolyed cell - cytoplasm is pulled away from the wall

what is a hypotonic solution?

a solution which has a higher water potential then inside the cell.

what is a isotonic solution?

a solution which has the same water potential as inside the cell.

what is a hypertonic solution?

a solution which has a lower water potential then inside the cell.

what type of terms are hypotonic, hypertonic and isotonic?

comparative

what happens to an animal cell in a hypotonic solution?

• water moves into the cell

• they become turgid and then burst

• they will undergo cell lysis

• they become ‘lysed’

what is lysis?

bursting or breaking of the cell

what happens to animal cells in an isotonic solution?

• no osmosis

• water potential outside = water potential inside

what happens to animal cells in a hypertonic solution?

• water leaves the cell via osmosis

• the cell crenates (shrink, shrivel and deform)

• no enzyme reactions can take place, cell will die

why is it so important that storage molecules (such as starch in plants and glycogen in animals and fungi) to be insoluble?

So they don’t effect the water potential of the cell (they are osmotically inactive).