9/27/2015

Diffusion

•• Movement of a substance from an area of high concentration to an area of low concentration Biology 102 (along a diffusion gradient) •• Net movement stops when substance is evenly distributed Cells and Osmosis

Diffusion Diffusion

•• Diffusion can occur across a barrier, IF the •• Different substances can diffuse independently barrier is permeable to the substance •• May be in the same direction, may be in different directions

Diffusion Activity

•• This is useful for cells •• Predict the direction of diffusion relative to the •• Some substances can diffuse across the cell separation (to the left or to the right) membrane •• Requires no energy 2M 4M

22mM 5mM

5% 1.3%

2.4M/kg 1.5M/kg

350mM 0.35M

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Diffusion Osmosis

•• Scenario: •• Diffusion of water molecules •• Concentration gradient exists across a barrier •• Occurs when… •• Barrier is not permeable to the substance •• Concentration gradient exists •• Lowest energy state of the system is to •• Membrane is not permeable to substance equalize the gradient •• Membrane is permeable to water •• What to do? •• If you can’t move the substance, move the water!

Osmosis Osmosis

•• Osmosis: Movement of water from an area of low •• Use arrows to indicate the direction that water solute concentration to an area of high solute will move across the membrane concentration •• Remember: the goal is to reach the same •• Again, won’t happen if the barrier is permeable concentration on both sides to the dissolved substance 2M 4M 22mM 5mM

5% 1.3%

2.4M/kg 1.5M/kg

350mM 0.35M

Osmosis Tonicity

•• This frequently happens in cells •• A way to describe solutions surrounding a living •• Water diffuses in or out through a membrane cell, and the osmotic effect on the cell protein called aquaporin •• Three possibilities: •• Hypertonic •• Solution has higher osmotic concentration than cell •• Hypotonic •• Solution has lower osmotic concentration than cell •• Isotonic •• Solution has equal osmotic concentration to cell

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Tonicity Tonicity •• Consider a cell suspended in a solution of sucrose •• Consider a cell suspended in a solution of sucrose •• Sucrose can’t get in or out through the •• Sucrose can’t get in or out through the membrane membrane

•• Cell: 300mM •• Cell: 300mM •• Solution: 450mM •• Solution: 265mM •• Hypertonic •• Hypotonic 300mM 300mM •• Which way will the •• Which way will the water move? water move? •• What will happen 450mM •• What will happen 265mM to cell shape? to cell shape

Tonicity Tonicity •• Consider a cell suspended in a solution of sucrose •• Consider a cell suspended in a solution of sucrose •• Sucrose can’t get in or out through the •• Sucrose can’t get in or out through the membrane membrane

•• Cell: 300mM •• Cell: 0.85% •• Solution: 300mM •• Solution: 1.2% •• Isotonic •• Tonicity? 300mM 0.85% •• Which way will the •• Which way will water move? water move? •• What will happen 300mM •• What will happen 1.2% to cell shape? to cell shape?

Tonicity Tonicity

•• Consider a cell suspended in a solution of sucrose •• Isotonic •• Sucrose can’t get in or out through the •• Solution does not cause osmotic flow of water membrane •• No change in cell shape

•• Cell: 300mOs •• Solution: 375mOs •• Tonicity? 300mOs •• Which way will water move? •• What will happen 375mOs to cell shape?

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Tonicity Tonicity

•• Hypotonic •• Hypertonic •• Solution with lower osmotic concentration •• Solution with higher osmotic concentration than the cell than the cell •• Cell will swell •• Cell will shrink

Thought Question

•• How would adding salt to the soil affect a plant? (The Romans did this after defeating Carthage in 149 BC)

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