Cells: Diffusion and Osmosis

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Cells Diffusion and Osmosis
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Cells have to control what leaves and what enters
the cytoplasm.
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Water and solutes move according to their
concentration gradient

• EVERYTHING naturally moves away from where it is
  concentrated and towards where it is less
  concentrated
• This natural movement is called diffusion.

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Concentration gradients

• Concentration gradients form when there is a
  difference in concentration between two places
• Diffusion takes place until the concentration
  reaches equilibrium

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concentration
Source
distance
The farther you move from a source, the lower the
concentration
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Diffusion naturally tries to equalize
concentration of water and solutes
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• Carbon dioxide naturally diffuses out of animal
  cells because animal cells produce CO2 as they
  make energy from food.
• The concentration inside is higher, so the net
  movement of CO2 is outward.

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CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
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CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
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CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
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CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
CO2
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• Oxygen naturally diffuses into animal cells
  because they use up oxygen as they turn food into
  energy
• The concentration outside is higher, so the net
  movement of O2 is inward.

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O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
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O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
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O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
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The cell membrane does not stop the diffusion of
CO2, O2, or water molecules
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• CO2, O2, and H2O are small molecules
• they each move down their individual
  concentration gradients until equilibrium is
  reached.

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Water molecules are attracted to dissolved
solutes like salts and sugars
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Salts and sugars cannot get through the bilayer,
but water can.

• This makes the cell membrane semipermeable.
• The movement of water through a semipermeable
  membrane that blocks solutes from moving is
  called osmosis.

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solute(like salt)
membrane
water
high concentration of solute
lower concentration of solute
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solute(like salt)
water
high concentration of solute
lower concentration of solute
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solute(like salt)
water
high concentration of solute
lower concentration of solute
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solute(like salt)
water
high concentration of solute
lower concentration of solute
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The net flow of water is towards the higher
solute concentration
high concentration of solute
lower concentration of solute
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Osmosis is the diffusion of water through a
semipermeable membrane
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Osmosis is used by plants to lift water upward.
All the plants cells must do is maintain a higher
solute concentration inside their cells.
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Osmosis can pull water into of a cell if the cell
is placed in an environment with a lower solute
concentration
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Hypotonic means lower solute concentration

• animal cells can rupture in hypotonic
  environments
• plant cells are protected from rupture by their
  strong cell walls

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normal blood cell
blood cell in hypotonic environment
blood cell ruptures due to water flooding inward
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Osmosis can also pull water out of a cell if the
cell is placed in an environment with a higher
solute concentration
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Hypertonic means higher concentration of solutes

• A cell placed in a hypertonic environment will
  lose water and shrink.
• Plant cells plasmolyze when placed in very salty
  water.

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Egg placed in hypertonic syrup
Egg placed in syrup after 24 hours
Osmosis dehydrated the egg.
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chloroplasts
Normal plant cell
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Plant cell placed in hypertonic environment
causes water to move outward
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cell wall
Plasmolyzed plant cell drained of water by
osmosis
cell membrane
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Isotonic means balanced

• Cells in an isotonic environment experience no
  net flow of water due to osmosis.
• The water flowing in is balanced by the water
  flowing out

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Human brains work to keep the blood isotonic

• If the blood gets hypertonic (too much salt),
  then osmosis will draw water out of body cells
  and dehydrate them.
• The brain signals thirst to dilute the blood
  with water.

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Human brains work to keep the blood isotonic
If the blood gets hypotonic (too much water),
then osmosis will over-fill the body cells and
cause bloating. The brain speeds up urine output
in the kidneys.
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Summary

• Hypertonic higher dehydrating environment,
  cells shrink
• Hypotonic lower bloating environment, cells
  swell
• Isotonic equal homeostatic environment, cells
  happy