Water potential is a measure of the concentration of water molecules that are able to diffuse.
Unit - kPa
- Water moves from a high water potential to a low water potential, down a water potential gradient
- Osmosis - movement of water molecules by diffusion, from a high water potential to a low water potential, across a partially permeable membrane.
- Movement of water molecules will occur until the water potential is the same on both sides of the membrane.
Cells in solution of high water potential
- partially permeable membrane
- the cell is in a solution with a water potential higher than the cell content
- water molecules will move down the water potential gradient, into the cell, by osmosis
- from a higher water potential to a lower water potential
- the cell will swell
- animal cell - burst (haemolyse)
- plant cell - the cell wall prevents the cell from getting any larger; osmosis stops at this point (turgid)
Cells in solution of low water potential
Placing the cell in a concentrated salt/sugar solution - i.e. water potential is lower than the cell content.
- water will diffuse out of the cell down the water potential gradient by osmosis, from a higher water potential to a lower potential
- the cell will shrink
- animal cell - cell contents will shrink and shrivel up
- plant cell - cytoplasm and vacuole will shrink as they lose water (crenated) and the cell surface membrane will pull away from the cell wall (plasmolysis)
Under ANAEROBIC conditions in yeast cells
- pyruvate loses a CO2 molecule - DECARBOXYLATED
- becomes ethanal
- enzyme - pyruvate decarboxylase
- ethanal accepts hydrogen atoms from reduced NAD
- reduced to ethanol
- enzyme - ethanol dehydrogenase
- NAD is reoxidised - can accept more hydrogen atoms from glucose during glycolysis
** rate of growth in yeast cells is faster in aerobic respiration
- yeast produces CO2
- CO2 reacts with water to form carbonic acid, lowering pH
- denatures the proteins within the yeast cell (enzymes, Na/K pumps)