Water Potential of a Potato Chip

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ater Potential of a Potato Chip

Aim: To find out the water potential of potato cells using different concentrations of sucrose solutions.

Scientific knowledge

Water Potential can be described in terms of osmosis. Osmosis is best regarded as a special type of diffusion involving water molecules only.

Diffusion is the movement of particles from a region of their high concentration to a region of their low concentration

In 1827 a Scottish botanist Robert brown reported that when pollen grains are mounted in water and viewed at high magnification through a microscope they are seen to make continuous movement this was caused by the constant motion of minute particles within pollen grains. This movement of suspended particles is now called the Brownian motion.

In the Brownian motion the small particles which we can see are bombarded on all sides by randomly moving molecules which are far too small for us to see.

Einstein in 1879 suggested that Brownian motion provided evidence for the kinetic (energy of movement) theory of mater. According to the theory, all matter is composed of tiny invisible particles; the continuous movement of particles in liquids and gases causes the transport and mixing that we call diffusion.

During diffusion molecules move down a concentration gradient. (From high concentration to low concentration).it happens because kinetic energy passed by molecules or ions, which makes them move about at random. As a result of this diffusion molecules tend to reach an equilibrium situation where they are evenly spread within a given volume or space. This means where there is a difference of

concentration in a gas or of solute in a liquid, net movement will occur to areas of lower concentration resulting in molecules becoming evenly spread. However once particles are evenly spread there is no longer net movement of particles in any particular direction. Facilitated diffusion occurs when substance is help across a membrane by action of special molecule in the membrane. This diffusion may be selective, allowing certain molecules, but not others.



Text Box: Partition removed


Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in diagram Y, the gases will mix because of the random movement of their molecules. In time a uniform mixture of A and B molecules will be produced in the container.


The diagram above can be used to describe Osmosis. If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to the smaller solvent molecules (water molecules) but not to the larger solute molecules (sucrose molecules), then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis.

[IMAGE]Osmosis is the diffusion of water from a region of high water concentration to a region of low water concentration through partially permeable membrane.

Water Potential is used to measure the tendency of water molecules to move from one place to another. Water always moves from a region of higher water potential to a region of lower water potential. It therefore moves down a water potential gradient. Equilibrium is reached when water potential in one region is the same as the other. There will then be no net movement of water molecules. Pure Water is given a water potential of zero. Adding solute molecules to the water lowers the water potential, making it negative. This is because the presence of the solute molecules lowers the concentration of the water molecules thus reducing the number of the water molecules that can diffuse out of it (Water potential is the capacity of a system to lose water). If...
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