Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.

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IGCSE Biology 0610 - 3.2 Osmosis

Objective: Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.

What is Osmosis?

Osmosis is the net movement of water across a semi-permeable membrane from a region of high water potential to a region of low water potential.

A semi-permeable membrane allows some substances (like water) to pass through but restricts the passage of others (like larger molecules or ions).

Water Potential

Water potential (Ψ) is a measure of the tendency of water to move from one area to another. It's related to the pressure potential and solute potential.

Solute potential is the decrease in water potential caused by the presence of solutes. The more solutes, the lower the water potential.

Water potential is always expressed in Pascals (Pa) or megapascals (MPa). 1 MPa = 1000 kPa.

Pure water has a water potential of 0 MPa.

Table summarizing water potential:

Solution Water Potential (MPa)
Pure Water 0
Sugar Solution < 0
Salt Solution < 0

Osmotic Concentration

Osmotic concentration refers to the concentration of solutes in a solution. A higher osmotic concentration means more solutes are present.

The osmotic concentration is directly related to the solute potential, which in turn affects the water potential.

Types of Solutions and Water Movement

The movement of water across a semi-permeable membrane depends on the relative water potential of the solutions on either side. We can classify solutions as:

  • Hypotonic Solution: A solution with a lower concentration of solutes than the inside of a cell. Water will move into the cell.
  • Isotonic Solution: A solution with the same concentration of solutes as the inside of a cell. There is no net movement of water.
  • Hypertonic Solution: A solution with a higher concentration of solutes than the inside of a cell. Water will move out of the cell.

Osmosis in Plants

Osmosis plays a crucial role in the uptake and loss of water in plants.

Water Uptake

Water is absorbed by the roots of plants through osmosis. The soil water typically has a higher water potential than the root cells (due to lower solute concentration within the root cells).

This causes water to move into the root cells, increasing their turgor pressure.

Turgor pressure is the pressure exerted by the water inside the plant cells against the cell wall. It's essential for maintaining the rigidity of non-woody plants.

Suggested diagram: A diagram showing water moving from the soil into a plant root cell via osmosis, illustrating the difference in water potential.

Water Loss

Water is lost from plants primarily through transpiration, which is the evaporation of water from the leaves.

The water potential inside the leaves is typically higher than the water potential in the air surrounding the leaves. This difference in water potential drives water movement out of the leaves by osmosis.

The loss of water through transpiration helps to keep the plant cool and maintains turgor pressure.

Importance of Osmosis

Osmosis is vital for:

  • Uptake of water and nutrients by plant roots.
  • Maintenance of turgor pressure in plant cells, which is essential for plant rigidity.
  • Regulation of water balance in organisms.
  • Absorption of water in animal cells (e.g., in the small intestine).
  • Excretion of water in animal cells (e.g., in the kidneys).