(a) Explain the term osmosis. (b) Describe an experiment to show that the xylem tissue conducts water from the rot to the shoot. (c)(i) Explain what happens...

(a) Osmosis

Osmosis is the movement of water (solvent) molecules from a region of higher water potential (dilute / more water) to a region of lower water potential (concentrated / less water) across a semi-permeable (selectively permeable) membrane, until the concentrations on both sides are equal.

(b) Experiment to show that xylem conducts water from root to shoot

1. Take a young, leafy balsam or Impatiens plant (or a white flower such as a Coleus) with its roots intact, or a freshly cut leafy shoot.
2. Place it in a beaker containing water to which a red dye (eosin) has been added.
3. Leave it in a well-lit place for a few hours so that transpiration draws the coloured water up.
4. Observation: the leaf veins and the flower become tinged red, showing water has risen up the stem.
5. Cut a thin transverse section of the stem and examine it. Only the xylem tissue is stained red, while the phloem and other tissues remain unstained.
6. Conclusion: since only the xylem is coloured, it is the xylem that conducts water from the root up to the shoot.

(c)(i) Red blood cell in a hypotonic solution

A hypotonic solution has a higher water potential than the cell contents. Water therefore enters the red blood cell by osmosis. The cell swells, becomes spherical, and finally its membrane bursts. This bursting is called haemolysis.

(c)(ii) Three conditions that may cause haemolysis

1. Placing the cells in a hypotonic solution or in distilled water.
2. Action of certain toxins, venoms (e.g. snake venom) or bacterial poisons.
3. Incompatible blood transfusion (mismatched blood groups) or certain infections such as malaria.