Water as a solvent in organisms

Water dissolves many substances. When a substance dissolves, its particles spread through the water to form a solution. The substance has not disappeared: its particles can move with the water and interact with other substances.

Absorption, transport and excretion

During digestion, food is broken down into products that can dissolve in digestive fluids. Soluble products can be absorbed through the wall of the gut into the blood, which carries them to other parts of the body.

Transport fluids carry dissolved substances between parts of an organism. For example, blood carries dissolved substances around the body, while plant sap carries dissolved substances through a plant.

Some waste substances also dissolve in water. For example, urea dissolves in the blood and is carried to the kidneys, where it can be removed from the body in urine. In these roles, the substances must be present in solution rather than remaining undissolved.

Osmosis

Water can move across a membrane that allows water molecules through but prevents some dissolved substances from crossing. Such a membrane is called partially permeable. The diffusion of water through it is called osmosis.

When the solutions on either side have different concentrations, there is a net movement of water from the more dilute solution to the more concentrated solution. Water molecules move in both directions, but more move towards the concentrated solution than in the opposite direction.

A cell membrane is partially permeable, so water can move into or out of a cell by osmosis. If the solution outside is more dilute than the solution inside, there is a net movement of water into the cell. If the solution outside is more concentrated, there is a net movement of water out of the cell.

Investigating osmosis with dialysis tubing

Dialysis tubing acts as a partially permeable membrane: it allows water molecules to pass through, but prevents some dissolved particles from crossing. A solution sealed inside the tubing can therefore be surrounded by a solution of a different concentration to investigate osmosis.

Measuring the mass change

1. Place a solution inside a piece of dialysis tubing and seal it. Measure the mass of the filled tubing.
2. Immerse the tubing in a solution of a different concentration.
3. After a period of time, remove the tubing, gently blot water from its outside and measure its mass again.

Blotting removes water on the surface that would otherwise affect the final measurement. A gain in mass indicates that water has moved into the tubing overall. A loss in mass indicates that water has moved out overall.

Explaining the direction of osmosis

Water moves across the membrane in both directions, but there is a net movement from the more dilute solution towards the more concentrated solution. This assumes that the dissolved particles responsible for the concentration difference cannot cross the tubing.

• If the solution inside is more concentrated than the surrounding solution, water moves into the tubing overall, so its mass increases.
• If the solution inside is more dilute than the surrounding solution, water moves out overall, so its mass decreases.
• If the solutions have the same concentration, there is no net movement of water and no overall mass change due to osmosis.

Osmosis and support in plant tissues

Plant tissue becomes firm when its cells contain enough water to press against their cell walls. The concentration of the solution surrounding the tissue affects whether water enters or leaves the cells by osmosis.

Osmosis is the net movement of water through a partially permeable membrane from a more dilute solution towards a more concentrated solution. The cell membrane is partially permeable, so water can move across it into or out of the cell.

Water movement and firmness

When the surrounding solution is more dilute than the cell contents, water enters the cells by osmosis. The cell contents press outwards against the cell walls. The walls resist this pressure, so the cells become turgid. Many turgid cells together help support the tissue and keep it firm.

When the surrounding solution is more concentrated than the cell contents, water leaves the cells by osmosis. The outward pressure against the cell walls falls, so the cells are less turgid and the tissue becomes less firm. Water loss may also cause the tissue to shrink.

Investigating plant tissue in different concentrations

Comparable pieces of the same plant tissue can be placed in solutions of different concentrations for the same length of time. Record their initial size and firmness, then compare these with their size and firmness after immersion. Keeping the pieces similar in size makes their changes easier to compare.

A piece that becomes larger or firmer has taken in water overall. A piece that becomes smaller or less firm has lost water overall. Comparing the pieces shows how the concentration of the surrounding solution affects water movement and support in the tissue.

Water potential and osmotic water balance

Water potential describes the tendency of water to move. A dilute solution has a higher water potential than a more concentrated solution, provided other conditions are the same.

When a partially permeable membrane separates solutions with different water potentials, water molecules move across it in both directions. However, more water moves from the region of higher water potential to the region of lower water potential. This overall movement is the net movement of water and is called osmosis.

Water balance in an organism

The water potential inside an organism compared with that of its surroundings determines whether it gains or loses water overall:

• If the surroundings have a higher water potential, water moves into the organism overall. Its cells retain more water.
• If the surroundings have a lower water potential, water moves out of the organism overall. Its cells retain less water.
• If the water potentials are equal, water still moves in both directions, but there is no net movement of water.

Plasmolysis in plant cells

A plant cell placed in a sufficiently concentrated solution loses water by osmosis. Water moves out across the partially permeable cell membrane because the surrounding solution is more concentrated than the cell contents.

As the cell loses water, its contents press less strongly against the cell wall. This outward pressure is called turgor pressure. When it falls, the cell becomes flaccid: it is no longer firm because its contents no longer exert significant outward pressure on the wall.

From a flaccid cell to plasmolysis

If the cell loses more water, its contents shrink further. The cell membrane and the contents it encloses pull away from the cell wall. This separation is called plasmolysis. The cell wall remains in place as the contents shrink away from it.

A flaccid cell is not necessarily plasmolysed. A cell can lose its outward pressure before its membrane has separated from the cell wall.