1. To observe and investigate the effects of osmosis across the cell membranes of a plant.
2. To investigate the ability of different molecules to diffuse across a selectively permeable membrane.
All living cells are enclosed by an outer covering known as plasma or cell membranes. In addition, these plasma membranes serve as barriers between different compartments in the body to confine processes and materials to certain locations. Examples of these processes are diffusion and osmosis which are the main physiological processes that play important roles in the body. Diffusion can be defined as the process by which molecules move down against the concentration gradient that is, from a region of high concentration to a region of low concentration while osmosis is a special type of diffusion that occurs when water molecules move from a region of high concentration to a region of low concentration across a semi-permeable membrane (Linda, 2009). In addition, plant cells have an additional cell wall which is permeable and very rigid.
Part 1: Osmosis in Plant Cells
_5 pieces of potato
_10 ml Graduated cylinder
_5 paper cups
_20% NaCl solution
5 pieces of potato, each approximately 8mm× 8 mm× 50mm long, were obtained using the displacement method. This was done by placing the pieces of potato each at a time in a graduated cylinder containing a known volume of distilled water then observing and recording the rise in volume of the distilled water in ml that occurred which was the volume of each piece of potato. After measuring the volumes of the pieces of potatoes, they were each placed in the 5 cups containing distilled water, 20% NaCl, 10% NaCl, 5 % NaCl and 1 %NaCl and time was recorded. 10% NaCl was prepared by adding the remaining 40 ml of 20% NaCl with 40ml of distilled water while 5% NaCl was prepared by measuring 40 ml of the already prepared 10% NaCl and adding to 40ml distilled water, and finally 1%NaCl was prepared by measuring 8ml of the prepared 5% NaCl and adding to 32 ml of distilled water. After placing the pieces of potatoes in the solution for one hour, their volumes were recorded again after which the % changes in the volume of potatoes were calculated.
The following table shows the results that were obtained.
SOLUTION WHERE WE PUT THE POTATP
The change in volume of each piece of the potato as a percentage of the original was calculated using the following formula:
Final Volume – Initial Volume
Therefore, the changes of the potatoes were as follows:
3.1 – 2.0
Potato 1: × 100% = 55%
2.5 – 2.05
× 100% = 21.95%
2.5 – 2.2
× 100%= 13.64%
2.5 – 2.9
1.7 – 1.9
Plant and animal cells are enclosed by outer coverings called cell or plasma membranes which are semi-permeable. In addition, semi-permeability can be referred to as the ability to allow only smaller molecules to pass through (Joyce et al., 2002). Osmosis can be defined as the movement of water molecules from a region of high concentration to a region of lower concentration across a semi-permeable membrane. Potatoes 1, 2 and 3 were hard or turbid when placed between fingers while potatoes 4 and 5 were soft in texture. In addition, the changes in volume of the pieces of the potatoes as a percentage of their original calculated were found to be 55%, 21.95%, 13.64%, -13.79 and -10.53 for potatoes 1, 2, 3, 4 and 5 respectively. This was because; distilled water is a highly hypotonic solution containing higher number of water solvent molecules than solute molecules which created a higher concentration gradient when the potato was dipped in (Philip, 2007). This resulted to water molecules moving into the cells of the potato across the cell membrane by osmosis due to the lower number of water molecules in the cytoplasm of the potato cells which lead to the high change in the percentage weight of the potato 1. This was the same reason why solutions containing 1% and 5% NaCl solutions produced also a percentage increase in the weights of the potato although the percentage changes in their weights was decreasing because 5% NaCl solution is less hypotonic than distilled water but, more hypotonic than 1%NaCl solution. On the other hand, 10% and 20% NaCl solutions were hypertonic to the cell cytoplasm of the potato cells that is, contained a higher number of solute molecules than water molecules. This lead to an increase in the osmotic pressure hence water molecules moved out of the potato cells into these solutions by osmosis across the cell membrane. In addition, 20% NaCl solution is more hypertonic than 10%NaCl solution hence it had a greater change in the percentage weight.
Plant cells have a cell wall in addition to the cell membrane which comprises of cellulose. The cell wall is tough, rigid, and permeable to all molecules and does not rupture when exposed to wall pressure by osmosis due to water moving into the cell (Janat, 1994). This was the reason why potatoes 1, 2 and 3 were turbid with a coarse texture while potatoes 4 and 5 were soft after gaining or loosing water by osmosis. On the contrary, animal cells do not have the cell wall; therefore they rupture and burst when they gain excess water.
In conclusion, plant and animal cells are enclosed by an outer semi-permeable covering referred to as a cell or plasma membrane. Plant cells have a cell wall in addition to the cell membrane which is permeable to all molecules. Also, osmosis and diffusion are significant physiological processes in living things. A solution with a greater total solute concentration is referred to as hypertonic, while a solution with a lower solute concentration is referred to as hypotonic. Additionally, these two factors influence the osmotic pressure of a solution (Santos, 2003).
Part 2: Diffusion and Osmosis across a semi-permeable membrane
_2ml iodine solution
_50 ml distilled water
_2 bags of dialysis tubing
_A stop watch
Two bags of dialysis tubing were filled with solutions of starch and glucose; one was placed into a beaker containing iodine solution while the other into a beaker containing distilled water. Their initial masses at the beginning of the experiment were recorded and their subsequent masses after 15, 30, and 45 minutes and also 1 hour were recorded. Finally, an investigation was carried out to determine which molecules crossed the membrane.
The following table shows the results that were recorded:
Starch in water
In 1st second the initial mass = 9.05g
After 15 min the mass = 9.97g
Glucose in the solution was = 100 and the color did not change
30 min the mass = 10.36g
After 45 min the mass = 10.576g and the Glucose in the solution was 100
After one hr the mass = 10.817g
Starch in 50 ml Di water + .2ml iodine
In 1st second the initial mass = 8.59 g
After 15 min the mass = 9.39g
Glucose in the solution was = 300 and the color was purple
30 min the mass = 9.84g
After 45 min the mass = 10.140g
After one hr the mass = 10.158g
Dialysis tubing is an artificially manufactured semi-permeable membrane. In addition, movement of particles across a semi-permeable membrane is affected by several factors including the size of particles. In the above experiment, there was an increase in the mass of dialysis tubing placed in distilled water at subsequent times although; there was no increase in glucose or any color change exhibited. This was because, water molecules moved by osmosis from the beaker where they were highly concentrated to the dialysis tubing where they were less concentrated. In addition, there was no color change due to absence of the iodine molecules to react with the glucose molecules. In the second dialysis tubing that was placed in a solution containing 2ml iodine and 50 ml distilled water, there was an a subsequent increase in mass after 15, 30, and 45 minutes and also after one hour. In addition, there was also a color change to purple which was as a result of iodine molecules diffusing from the solution in the beaker, a region of high concentration, into the dialysis tubing, a region of low concentration, and reacting with glucose molecules to produce a purple color. It is good to note that, iodine molecules are small and can pass through a semi-permeable membrane while starch molecules are large and cannot pass through a semi-permeable membrane (Joyce, 2002).
Diffusion and movement of particles a cross a semi-permeable membrane are affected by several factors including the size of the particles, concentration gradient and cross-sectional area is good to note that, smaller particles like iodine can diffuse through a semi-permeable membrane while large particles like starch molecules cannot pass through. Additionally, diffusion and osmosis can be termed as significant in various physiological processes in the body like a absorption of food in the small intestine, a absorption of water and mineral salts in the soil by plant roots and reabsorption of mineral salts and nutrients in the kidneys (Research and Education Association (2006).
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