Edexcel IGCSE Biology 复习笔记 2.4.3 Osmosis

• 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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