Investigating osmosis on swede cells.

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Julie Salsbury 12s

Investigating osmosis on swede cells.

Aim: To investigate the effect of sucrose solution on swede cells in order to determine the water potential of them.

Background information:

‘Osmosis is the diffusion of water only. It is the net movement of solvent (water) molecules from a region of high concentration to a region of their lower concentration, through a partially permeable membrane.’ 1

Robert. M, Reiss. M, Monger. G, Biology Principles, Nelson, 1993

A diagram summarising the conditions on the two sides of a partially permeable membrane, and the terms used to describe them. The large black blobs represent sucrose molecules, the smaller white circles represent water molecules. The terms ‘high’ and ‘low’ relate.

In the diagram above the solute molecules are too large to diffuse through the partially permeable membrane but as water molecules are a lot smaller they can diffuse down the concentration gradient to where there is a lower concentration of water molecules. Although there are water molecules on the right side the solute molecules, because they carry a charge become surrounded by a shell of water molecules, and so become hydrated and the result is that the water molecule is no longer free to move around. Therefore osmosis can also be described as ‘the movement of water molecules down a concentration gradient of water, from a region of high kinetic energy to a region of lower kinetic energy.’ 1

Prediction: 

Pure water has the highest water potential, which is zero. If swede chips were placed into pure water, the water potential inside the cells would be exceeded by the water potential of the external solution, resulting in a net flow of water molecules into the cells by the process of osmosis. This will be visible as an increase in length and mass of the swede chips.
It is predicted that as the solute potential of the external solution is decreased (i.e. the solution becomes more concentrated) less and less water will move by osmosis into the cells and the net flow will begin to move out of the cell and as a result the increases in length and mass of the swede chip will be smaller. This will continue until the isotonic point is reached, which is where the internal and external water potentials are equal, and will be visible as no change in mass or length of the swede chips. After this point the solute potential in the external solution will be less than that of inside the cell and therefore there will be a net movement of water molecules out of the cell, resulting visibly in a decrease in length and mass of the swede cells from their original size. My theory behind this can be explained as follows.
Water molecules possess kinetic energy, which means that in gaseous or liquid form they move about more rapidly and randomly from one location to another. The greater the concentration of water molecules in a system, the greater the total kinetic energy of water molecules and the higher its water potential will be. In thermodynamic terms, ‘water molecules move according to the laws of entropy from a region of higher to lower ‘free energy’.’
ii Water potential (ψ, the Greek letter psi) is most commonly measured in pressure units, i.e. Pascal’s. Pure water has the maximum water potential, which is defined as zero, therefore all other solutions have lower water potentials than pure water and therefore have negative values of ψ at atmospheric pressure an at a defined temperature. Water will always move from a region of higher water potential to a lower one.

   The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence lower the water potential. All solutions therefore have lower water potentials than pure water, lowering the water potential is known as solute potential ψs, and this value is always negative. The more solute molecules present, the more negative is the ψs. For a solution at atmospheric pressure, ψ = ψs.

  If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. Also when water enters plant cells by osmosis, pressure may build up inside the cell making the cell turgid and increasing the pressure potential. Pressure potential is usually positive, but in certain circumstances, as in xylem when water is under tension it may be negative.

    Water potential is affected by both solute potential and pressure potential, and the following equation summarises the relationship between the two.

ψ    =             ψs          +        ψp

                                       Water potential = solute potential +  pressure potential

Osmosis in plant cells

The main part of the body of many kinds of plants is a tissue called parenchyma. Parenchyma cells have a wall made of a jelly-like matrix in which fibres of cellulose are enmeshed. The cell wall is porous and permeable to aqueous solutions. In contrast the outer boundary of the protoplast, the live matter of the cell, is a cell membrane which is more permeable to water than to many kinds of solutes. The protoplast contains sap, an aqueous solution of inorganic and organic substances. They cause the sap to have a negative water potential.

Simpkins. J, William. J.I, Advanced Biology, Collins Educational, 1990

A diagram to show a typical parenchyma cell.

 When water enters the cell, the volume of the cell increases and the protoplast starts to push against the cell wall and pressure starts to build up rapidly. This is the pressure potential and it increases the water potential of the cell until the water potential inside the cell equals the water potential outside the cell, and equilibrium is reached. The cell wall will prevent the cell from bursting. When a plant cell is fully inflated with water it is described as turgid. When a plant cell is placed in a solution of lower water potential, e.g. concentrated sucrose solution, when water will leave the cell by osmosis. As it does, the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. At this point the pressure potential is zero, so the water potential of the cell is equal to its solute potential. As the protoplast continues to shrink it begins to pull away from the cell wall. This process is called plasmolysis and a cell is then said to be plasmolysed. Both the solute molecules and the water molecules of the external solution can pass through the freely permeable cell wall, and so the external solution remains in contact with the shrinking protoplast. Eventually an equilibrium is reached when the water potential has decreased to that of the external solution. The point at which pressure potential has just reached zero and plasmolysis is about to occur is referred to as incipient plasmolysis. A solution that contains more solute particles than another, and hence has a low water potential, is referred to as being hypertonic, whilst the less concentrated solution is hypotonic.

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Simpkins. J, William, J.I, Advanced Biology, Collins Educational, 1990

A diagram to show a plant cell in full turgidity and total plasmolysis.

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