Osmosis and Plant Transport

• Osmosis is the diffusion of water particles from where they are in higher concentration to where they are in lower concentration, across a selectively permeable membrane. That is, from a dilute solution to a more concentrated solution until equilibrium is reached.
• For osmosis to occur, you need two solutions with different concentrations, and a selectively permeable membrane
• When a plant cell is placed in water or a dilute solution, water moves into the vacuole in a plant cell. The vacuole increases in size, pushing the cell membrane against the cell wall.
• During osmosis, The cell wall prevents too much water from entering, and the cell does not burst open, like an animal cell would
• • The force of the membrane pushing against the cell wall makes the cell firm or turgid
  • This turgor pressure gives the cell support and is essential in keeping the plant upright(If the stem has no wood in it)
  • When plant cells do not have enough water, wilting occurs and they are no longer turgid. They are said to be flaccid
• • If a plant cell is placed in a concentrated solution, it loses too much water and plasmolysis will occur
  • So much water leaves the vacuole of the cell that the cell contents shrink, pulling away from the cell wall
  • Plasmolysed cells are unlikely to survive.
• There are 3 main reasons why water moves up a plant:

  1. The supply of water to the leaves as raw material for photosynthesis
  2. The transport of minerals in the water through the root and up to other parts of the plant
  3. As water passes through the plant it enters cells by osmosis to provide support (turgor)
• Transport of water from soil to leaf:

  • Plants obtain water from the soil through their roots. To increase their surface area through which water is absorbed, roots are covered with tiny root hairs. Water enters the root hairs by osmosis.
  • The Water flows up the plant in tiny, narrow pipes, rather like capillary tubes(xylem). The walls of these are made of lignin (hard and waterproof)
  • Water rises up the stem mainly being "pulled" from above. The "pull" is created by evaporation of water from the leaves, through the stomata

  This is called transpiration.
• Transpiration can be defined as: The evaporation of water from mesophyll cells followed by diffusion through air spaces and stomata
• Factors that affect the rate of transpiration:

  • Air movement - Transpiration is faster on windy days. Wind blows away water vapour from around the leaf, allowing better diffusion through stomata
  • Humidity - Transpiration is faster when the air is dry, because diffusion of water vapour is quicker
  • Temperature - Hot weather increases transpiration
  • Soil water - When soil water is scarce, transpiration is obviously reduced
  • Light - Generally, transpiration is reduced a lot at night time because the stomata closes in the dark.
• Factors that affect the rate of transpiration:

  • Air movement - Transpiration is faster on windy days. Wind blows away water vapour from around the leaf, allowing better diffusion through stomata
  • Humidity - Transpiration is faster when the air is dry, because diffusion of water vapour is quicker
  • Temperature - Hot weather increases transpiration
  • Soil water - When soil water is scarce, transpiration is obviously reduced
  • Light - Generally, transpiration is reduced a lot at night time because the stomata closes in the dark.