Osmosis

Aim: The aim of this experiment is to investigate the movement of water into and out of plant cells by osmosis. The cells chosen for study will be taken from potato tubes as they provide a ready supply of homogeneous material.

I did the investigation in two parts, the first part of my investigation was my preliminary investigation and then I did my official investigation. In both of my investigations there are several similarities, such as fair testing, variables, key variables, reliability of evidence and safety precautions. In both of my investigation there were obviously some differences which were in the method, apparatus, results and means of measuring etc.

Essential Background Reading

Osmosis is the diffusion of water molecules across a partially permeable membrane from a high concentration of water to a low concentration of water.

Movement of substances takes place through the cell membrane, which separates the different substances in the cell from its surroundings. This type of membrane allows small molecules such as water to pass through it, but it denies entry to large particles to pass. This membrane is called a partially permeable membrane. The amount of water inside a cell is called water potential.

Turgor is the name given when osmosis takes place between the cytoplasm and the solution outside the cell. This happens when a plant cell is placed in a high concentrated solution of water, water then passes through the cell wall, the cell membrane, and the cytoplasm and into the vacuole. The increased pressure of water inside the vacuole is called Turgor pressure. Then the cell becomes turgid.

Plasmolysis is the opposite of turgor. This happens when plant cells may be placed in a less concentrated solution of water, although this is very unlikely to happen in nature. Water passes out of the vacuole, the cytoplasm, the cell membrane and the cell wall and into the solution outside the cell. The pressure of the vacuole on the cytoplasm decreases until the cytoplasm pulls away from the cell wall.

Diagrams

I have illustrated the diagrams of turgor and Plasmolysis on a separate page.

Fair Testing

This is when the result is only related to the variable being tested.

For e.g. if there are six variables, five variables must be kept identically the same and only one is changed/tested to see how this affects the outcome. This is ensuring that this investigation shall only be testing one variable (Key Variable). I have listed the variables below and the key variable.

Variables

A variable is something, which can be measured or controlled. A dependent

variable is the variable you must keep identically the same. I must only test one variable at a time. The independent variable is the one I alter/test. These are my variables:

Concentration, this will change from beaker to beaker and will be the main factor that will determine the change in results

Size of potato i.e. cross -sectional area of the potato cylinders. This can change the rate of osmosis over a certain period of time, because if there are two identical potato cylinders but one has a larger surface area over the other then, the rate of osmosis happening over a certain period of time will be much quicker for the plant tissue which has a larger surface area.

PH of solution and potato.

Same type of potatoes i.e. from the same fields. This could affect the test in many ways, e.g. the ph of the potatoes can vary depending where and how they are grown.

Temperature of surroundings. If the beaker is left in a warm place then the water may evaporate and this will change the concentration.

Solvent, my solvent is water.

Solute, my solute is sugar

Time. The potato cylinders must be kept in their beaker for the same period of time. If not then the concentration will diffuse more or less depending on the time.

My potatoes must be as fresh as possible. If they are left for storage over a long period of time then this may result in the potatoes rotting and then the plant tissue will become faulty. This is because bacteria will start to live on of the potatoes.

Key variable

This is the variable, which is the most important to the activities outcome. This will always be the one you change. My key variable is concentration of sugar.

Reliability Of Evidence

This is proved/validated by repetition of the observation or measurement. For each of the changes of the tested variables (there should be a minimum of 5) each one of the five must be repeated at least once and then the two values averaged. This will also eliminate any human error. If the two results are very different then I must do it a third time to confirm which is a human error. I will ignore the faulty value and average the two, which are similar in results. I have chosen five concentrations. This will ensure that my results will be accurate as possible. This will also prove that concentration is a factor affecting osmosis in plant tissue, because if the potatoes are in a highly concentrated solution of sugar, then the potatoes will shrink. And if it is placed in highly concentrated solution of water, then the potatoes will increase in size.

Safety Precautions

When I carry out my investigation I will have to take under consideration some safety precautions. These safety precautions are very vital, because if I don't take these simple measures in mind then there would be serious danger, not only to my self but also to my other classmates.

I must wear safety goggles to protect my eyes from harmful chemicals.

After the scalpel has been used I must insert the scalpel into an unused potato, so that there is no danger of me cutting my self or someone else. The scalpel is a razor sharp knife, this is why I must be very careful whist handling it.

I must