1 - STRUCTURES AND FUNCTIONS IN LIVING ORGANISMS

Osmosis 

• The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
• Partially permeable membrane - small holes in it (only small molecules - eg: water can get through), eg: cell membrane 
• Water molecules pass both ways during osmosis (move randomly) 
• Steady net flow of water into region with fewer water molecules 
• Other solution gets more dilute 

Osmosis in cells - 

• Water moves in and out of cells by osmosis 
• Tissue fluid (water, with oxygen, glucose etc. dissolved in it) surrounds cells in body. Squeezed out of blood capillaries to supply cells 
• Tissue fluid has different concentration to fluid inside cell - water will move into/out of cell by osmosis (to/from tissue fluid) 

Turgid cells - 

• Turgid - when the cells in a plant are all hydrated (plump and swollen) 
• Hydrated plant - all cells draw in water by osmosis = plump and swollen (turgid)  
• Contents of cell push against cell wall - turgor pressure - helps support plant tissues 
• Dehydrated plant - cells lose water, lose turgor pressure - cells become flaccid (plant wilts) 
• Plant doesn't completely lose shape - inelastic cell well keeps it supported

Osmosis experiments - 

1. Living system - potato cylinders 

• Cut potato into equal cylinders, measure and record lengths
• Put cylinders in beakers with different sugar solutions (eg: pure water, different levels of concentrated sugar solution, very concentrated sugar solution) 
• Leave in beaker for at least half an hour 
• Take out cylinders - measure and record 
• Drawn in water (osmosis) = will be longer 
• Water drawn out = slightly smaller 

2. Non-living system - visking tubing 

• Tie wire around 1 end of visking tubing, put glass tube in other end, tie visking tubing around it 
• Pour sugar solution down glass tube into visking tubing 
• Measure where sugar solution comes up to in glass tube 
• Put in beaker of pure water 
• Leave overnight 
• Measure liquid in glass tube again 
• Water drawn in by osmosis - level in glass tube higher