The Effect Of Sodium Chloride On Yeast Cells

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. 

• 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)

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)