Movement of substances

Factors affecting the rate of diffusion:

• Surface area: increasing the surface area increases the rate of diffusion, 
• Concentration gradient: Increasing the concentration gradient increases the rate of diffusion, 
• Temperature: Increasing the temperature increases the rate of diffusion. 
• Diffusion distance: Increasing the diffusion distance decreases the rate of diffusion. 

Rate of diffusion = Concentration gradient x surface area / diffusion distance 

• Osmosis: Water molecules are small enough to pass between phospholipid molecules, down a water potential gradient. The molecules move from an area of high water potential to an area of low water potential across the membrane. 
• Active Transport: Water-soluble substances that must be moved against a concentration gradient require carrier protein and ATP. 
• Facilitated diffusion: Polar, water soluble substances that cannot pass through the membrane can be transported bus specific channel proteins down a concentration gradient.
• Diffusion: Small lipid soluble substances can pass throughout the lipid bilayer - between the phospholipid molecules - down a concentration gradient. 
• Facilitated diffusion: Large water soluble substances cannot pass between the lipid molecules so are carried through the membranes by carrier proteins down a concentration gradient.

Water Potential

Osmosis is the movement of water molecules down a water potential gradient across a partially permeable membrane. Water molecules will move from an area of high water potential (e.g. pure water), to an area of low water potential (e.g. saline or sugar solutions) Water molecules move from a position of less negative to more negative water potential.