• All cells are surrounded by a cell membrane which is?partially permeable
• Water?can move in and out of cells by osmosis
• Osmosis is the net movement of water molecules from a region of?higher water potential?(dilute solution) to a region of?lower water potential?(concentrated solution), through a?partially permeable membrane
• In doing this, water is moving down its?concentration gradient
• The cell membrane is partially permeable which means it allows?small molecules?(like water) through but not larger molecules (like solute molecules)

Osmosis and the partially permeable membrane

• It can get a little confusing to talk about the 'concentration of water' when we also talk about solutions being ‘concentrated’ (having a lot of solute in them) so instead, we can say that a?dilute solution?has a?high water potential?(the right-hand side of the diagram below) and a?concentrated solution?has a?low water potential?(the left-hand side of the diagram below):

How osmosis works

• Animal cells?lose and gain water as a result of osmosis
• As animal cells?do not have a supporting cell wall, the results of osmosis can be severe
• If an animal cell is placed into a?strong sugar solution?(with a lower water potential than the cell), it will lose water by osmosis and become?crenated?(shrivelled up)
• If an animal cell is placed into?distilled water?(with a higher water potential than the cell), it will gain water by osmosis as it has?no cell wall to create turgor pressure
• It will continue to gain water until the cell membrane is stretched too far and?it bursts

Effect of osmosis on animal cells

Osmosis and the human body

• It is important that osmosis is?carefully controlled?in organisms to avoid damage to cells through?lysis
• The human body is adapted to maintain the optimum?osmotic balance?using processes such as sweating or increasing and decreasing urine concentration
  • This is all part of?osmoregulation

   

• Plant cells?lose or gain water as a result of osmosis
• As plant cells?have a supporting cell wall, they are protected from cell lysis
• If a plant cell is placed into a?strong sugar solution?(with a lower water potential than the cell), it will lose water by osmosis
  • The vacuole gets?smaller?and the cell membrane?shrivels away from the cell wall
  • It becomes?flaccid or plasmolysed?(shrivelled up)

   

• If a plant cell is placed into?distilled water?(with a higher water potential than the cell), it will gain water by osmosis
  • The vacuole gets?bigger,?pushing the cell membrane against the cell wall
  • The plant cell is described as being?turgid?or as containing a?high turgor pressure?(the pressure of the cytoplasm pushing against the cell wall)

   

The effect of osmosis on plant cells

Osmosis and plant cells

• Water entering the cell by osmosis makes the cell?rigid and firm
• This is important for plants as the effect of all the cells in a plant being firm is to?provide support and strength for the plant?- making the plant stand upright with its leaves held out to catch sunlight
• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant?wilts

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