Process of Life: DIffusion and Osmosis

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. 

Keywords: diffuses, concentration, gradient 

Concentration: amount of substance (mass g/kg) / volume of fluid (cm³/ ml) 

The change in concentration between two regions is known as the concentration gradient.

High concentration: amount of substance (higher), volume of fluid (low) 

Low concentration: amount of substance (low), volume of fluid (high) 

Factors of diffusion:

• Temperature
• Size of particles: The bigger the size of the particles, the slower the rate of diffusion.  
• Concentration gradient: The larger the concentration gradient, the faster the rate of diffusion. 

Example

Sugar molecules diffuses down the concentration gradient from Point A to Point B. 

Applications of Diffusion

1. Chemical substances must be able to move from one place to another in order to keep the living organisms alive and growing.
2. For example, food substances that were absorbed need to
   • move from one cell to another
   • move in & out of the cell
   • move from one part of the cell to another
   
   Other examples of diffusion in Biology include:
   - Movement of carbon dioxide during photosynthesis
   - Movement of oxygen and carbon dioxide in animals
   
   Conclusion

• Diffusion is an important process where substances are moved waithout use of energy (passive process).
• It is the net movement of particles (or moelcules; or ions) from a region of higher concentration to a region of a lower concentration. 
• Thus the movement is down a concentration gradient. 

Osmosis

Osmosis is the net movement of water molecules down the concentration gradient through a partially permeable membrane. 

• Water moves freely through pores in the partially permeable membrane
• Solute(green)too large to move across the membrane. 

Concentration = amount of solute / volume of solvent (water) 

Since water potential is a measure of the tendency of water molecules to move from one area to the another, 

We can also define osmosis as the net movement of water through a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Osmosis in animal cells 

Animal Cells 

• structure is simple
• cytoplasm is surrounded by a partially permeable membrane 

If places in a hypertonic solution (solution has a higher concentration of solutes than the cytoplasm) higher solute concentration (low water potential) -> water leaves the cell by osmosis -> cell loses colume and shrinks (crenates: means that the animal cell shrinks in volume. only used for animal cells!) 

• Water loss only ceases if the concentration of the cytoplasm rises to that of the surrounding solution. -> water leaves the cell by osmosis (no more water loss from the cell : no net movement of water seen) ONLY water loss is ceased, but osmosis still occurs. This is because the concentration of the cell and the surrounding is the same, therefore the cell would have to lose anymore water molecules. 

If places in a hypotonic solution (solution has a lower concentration of sloutes than the cytoplasm- high water potential) water enters the cell by osmosis -> since the cell membrane cannot resist expansion, the cell eventually bursts (cytolysis) 

The Plant Cell 

• each plant cell contains a large central vacuole which contains a solution of salt sugars and ions, is bound by a partially permeable membrane 

If placed in a hypotonic solution, (solution has a lower concentration of sloutes than the cytoplasm- high water potential) water enters the large vacuole by osmosis -> vacuole swells, pushing the cytoplasm against the cell wall -> pushing the inelastic cell wall resists expansion and the cell becomes rigid, or turgid. It can be described as in a state of turgor.  

Young plants, which have little woody tissue, rely on turgor for support against wind and gravity. 

If places in a hypertonic solution (solution has a higher concentration of solutes than the cytoplasm) water leaves the cell by osmosis -> the cytoplasm and vacuole shrinks, pulling the cell membrane away from the cell wall -> the cell is now plasmolysed or is in a state of plasmolysis, The tissue becomes flaccid.

This tissue is flaccid

cell is plasmolysed, tissue is flaccid

PLASMOLYSIS VS CRENATION 

Isotonic Solution

An isotonic solution has the same concentration of solutes as cytoplasm.

No net movement of water molecules into or out of the cell 

Therefore, cells neither shrinks nor expands when put into an isotonic solution. 

What type of solution are these cells in?

                      hypertonic          isotonic                hypotonic 

                 hypertonic       isotonic          hypotonic