Science: Osmosis

Introduction

To find out how a limp lettuce can be made crisp, I have investigated into osmosis as this is the process of water moving through cell walls. I was not clear on the exact process, so I used books and the internet to broaden my understanding of osmosis for this GCSE coursework.

I have researched into osmosis and found that all living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases, usually CO₂ and O₂, taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell.

Passive and active transport

There are two ways that the molecules move through the membrane: passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously.

Diffusion

Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout an object. An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place. Dynamic equilibrium occurs when two opposing actions occur at the same rate. Diffusion occurs when a system is not at equilibrium.

As an example, one drop of ink is dropped into a beaker of water. At first, all of the ink molecules are in a small space and they are moving around in a random way. They move in straight lines and change direction only when they collide with each other or the surrounding water molecules. Some of the ink molecules near the edge of the drop move away from the centre of the drop. As a matter of fact, most of the molecules move away from the centre of the drop. Most of the molecules continue to move away from the original centre of the drop. They move in all different directions, and some may even move back toward the centre. Still, more are moving away from the drop than toward it until they find the wall of the beaker. Then they start moving back toward the centre again. More and more molecules bounce off of the glass until they start moving toward the centre, then they pass the centre and move toward the other side. Eventually the number of molecules moving away from the centre equals the number moving toward the centre, and equilibrium is established. At this point the molecules are evenly spread throughout the water, and diffusion stops.

Investigation into effects of osmosis

I will investigate the effects of osmosis with various concentrations of salt solution, using two different samples, cucumber and potato. This will ensure that there are double the variables and hopefully will enforce conclusions later. The samples will be weighed to see how much water has diffused into or out of the sample. This should highlight the concentration of the sample.

Variables show the points which will be changed in the process of experimenting. Non-variables show points which will be changed in the process of experimenting. For instance: all samples will be tested at roughly the same time of a certain day and they will be taken out of the solution, to be weighed, for no longer than one minute.

Method

The potato was cut with a corer of 7.5mm in diameter. The length depended on the weight which was consistently 5.8g. The weight of the cucumber was 10g and the dimensions are shown below.

A molar of salt is calculated by RAM (Relative Atomic Mass) of the two elements. Salt contains two elements: sodium and chlorine, sodium has a RAM of 23 and chlorine a RAM of 35.5. The formula for salt is NaCl, it contains one atom of each in one salt compound. To find the total RAM of NaCl the two elements’ RAM have to be added. Na + Cl = NaCl, 23 + 35.5 = 58.5. Molar is measured in grams per litre, so the RAM of NaCl is 58.5g/l. Our experiment was measured in units of molar, so if 1M = 58.5g, ½M = 58.5/2 = 29.25g and ¼M = 58.5/4 = 14.625g.

The sample will need to be submerged in the brine so that all the surface area of the sample are submerged, making it a fair test. 100ml of water is enough to totally submerge the five samples of potato and cucumber in the separate beakers, so exposure to oxygen will not affect the outcome.

I will use 500g of water rather than 1 litre, this means less salt is wasted when the brine is made up for each sample. To find the correct ratio per 500ml instead of 1l, I just have to divide the molar by 2.

Results

Here is a table of the results spread over a week. The results were recorded on three days, the weight and appearance of the samples were recorded.

The weights of the two types of samples up to day 5 were recorded, on day 7 we were unable to record the results as the samples often became broken and were saturated with water. The thick mould on top of the solution on day 7 made it very unfavourable for weighing to take place.

Your comments

The experient took place in 1997 with diagrams redone in 2020 as vector graphics for the new version of Nahoo. Otherwise, this is pretty old and the project doesn’t have a conclusion. I still appreciate comments if you want to send me your views.