The aim of this experiment is to investigate the movement of water in and out of plant cells. The cells chosen for study will be taken from potato tubers. Firstly I will explain what osmosis is. Osmosis is the passage of water from a region of high water concentration through a semi permeable membrane to a region of low water concentration. This definition contains three important statements:
a) It is the passage of water through a semi permeable membrane
b) It is the passage of water from a region of high water concentration
c) It is the passage of water to a region of low water concentration.
All the above statements are included in the definition, but define certain aspects of it.
Semi-permeable membranes are very thin layers of material which allow some things to pass through, but prevent others. A cell membrane is semi permeable. They allow small molecules like oxygen, water, amino acids etc. to pass through but will not allow larger molecules like sucrose, starch, protein etc. through. A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: a high concentration of water. A region of low water concentration is the opposite of the above, i.e. a very high concentration of sucrose solution: a low water concentration.
The water content of plants varies depending on environmental conditions. In Land plants this water plays a vital role in the support of tissues and the transport of materials around the organism. Lack of water leads to wilting and eventually death. Water is mainly absorbed through the roots, which are covered in specially adapted root hair cells, with large surface areas and thin cell walls to aid absorption. It is drawn up the plant through xylem vessels by a pull resulting from the evaporation of water through the stomata on the leaves. This evaporation is called transpiration and the xylem flow resulting is called the transpiration stream. Soluble food substances formed during photosynthesis are transported around the plant in the phloem tubes. This movement of water through the plant in the xylem vessels or phloem tubes is similar to the flow of blood in humans as it transports soluble mineral salts, nutrients and auxins, (plant hormones), from place to place. The evaporation of water from the leaves also removes heat energy from the plant and helps to prevent overheating.
Transpiration pulls water up the plant stem but osmosis is the process whereby water is drawn into or out of cells and tissues. Osmosis is the flow of water by diffusion through a differentially permeable membrane from areas of high water concentration to regions of low water concentration. The diagram below illustrates this:
Water can freely penetrate all membrane. The cellulose cell wall does not act as a semi permeable membrane and will allow most substances that are dissolved in water to freely pass through it.
Whether water enters the cell by osmosis or not will depend on the balance between external and internal solute concentrations and the state of the cell. If the solutions on each side of the differentially permeable membrane are equally concentrated then there will be no net movement of water across the membrane. This is called an equilibrium state and the solutions are referred to as being isotonic. A solution that contains more solute particles than another, and is hence more concentrated, is referred to as being hypertonic. The less concentrated solution is hypotonic. This concentration of solute particles is usually described as a molarity.
Even if the solute concentration external to the cell is hypotonic to the vacuole contents the cell will not continue to take in water by osmosis for ever. The cellulose cell wall provides a rigid barrier to uncontrolled expansion. A cell that is full of water is called turgid and cannot expand further as the...