Effects of Osmosis in Living Organisms

Names: David Hobson Jacob Laroque

Sarosh Shaheen

Date Due: October 1, 2010

Teacher: Mrs. Jennings

Purpose: To find the isotonic salt concentration of celery tissue

Introduction (David): Whether a plant cell, or an animal cell, in order to function properly, conditions within the cell

must remain stable and constant. This steady state within the cell is referred to as homeostasis. Cells will always strive to obtain this steady state, and the structure within the cell responsible

for maintaining this is the cell membrane. The cell membrane is composed of a bilayer of phospholipid molecules which creates a semi-permeable membrane controlling what substances can enter and exit the cell. Substances can enter the cell by a process called diffusion which is the movement of substances from an area of high concentration to an area of lower concentration.

An important substance in maintaining the state of homeostasis within the cell is water. Water, because it is a polar molecule and has a distribution of charge within the molecule, is an important solvent and is used to dissolve materials both on the inside and the outside of the cell.

Water is also a very small molecule and can diffuse through the cell membrane through a process called osmosis. Osmosis is a specific type of diffusion, however the difference being that diffusion can occur for any chemical or substance that moves from high to low concentration, whereas osmosis is the diffusion of water through a semi-permeable or permeable membrane (Diffusion and Osmosis) (Difference between Diffusion and Osmosis).

The cell membrane is completely permeable to water and osmosis is a type of passive diffusion (Brown). Passive diffusion is a type of diffusion where no energy is required to transport substances from one region to the other because it is fuelled by a concentration gradient, or, in other words, a difference in concentrations between two regions.

Water molecules and other particles in general, will always try and move along a concentration gradient naturally. Molecules in liquid or gas state are always in constant motion and travel in straight lines until they collide with one another and cause each other to change directions, or until they meet another particle with the same charge as themselves and will therefore repel each other and cause each other to change directions that way (Emerson). This movement of particles is random and is what fuels diffusion, because particles will move away from areas where there are large concentrations of particles and into areas where there aren’t as many particles (Emerson). This is why passive diffusion requires no energy, and why water molecules will diffuse into areas of lower concentration naturally.

Cells will always attempt to achieve equal levels of water on the outside of the cell and on the inside and thus maintain a state of homeostasis. There are three conditions involved in osmosis to try and obtain this state. The first is called hypertonic condition. This is when the concentration of water inside the cell is greater than the concentration of water outside the cell (Brown). In order to achieve a balanced state, water from within the cell will diffuse through the cell membrane towards the outside in order to balance the concentrations. There is a risk involved in this however, because too much water can escape the cell and cause the cell to shrivel and shrink. This is called plasmolysis. Another condition is the hypotonic condition. This is when the concentration of water on the outside of the cell is higher than on the inside of the cell (Brown). When this occurs, water will diffuse through the cell membrane into the cell. However there is a risk involved in this as well, because too much water can diffuse into the cell and the cell can burst. This is called lysis. And the third condition of osmosis is when the concentration of water on the outside of the cell is equal to that on the inside of the cell (Brown). When this occurs, the NET or overall movement of water inside and outside of the cell is zero, and the cell has achieved a balanced state of homeostasis.

Materials:

- safety glasses - six petri dishes- one scalpel - celery stalk- one 900mL beaker - five 1000mL glass jars - six 25mL beakers- pure water - salt water solutions (5 different concentrations) - tape- ruler

Procedure: See Pre-Lab

Observations:

Table 1. Changes in Stem Length

Test Solution (g/100mL)

Initial Length (mm)

Final Length (mm)

Changes in length (+/- mm)

Class average change in length (+/- mm)

0 30 34 +4 +2.60.4 30 30 0 +1.80.8 30 30 0 +0.91.2 30 32 +2 +0.31.6 30 33 +3 -0.62.0 30 30 0 -1.1

0 0.5 1 1.5 2 2.5 3

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Graph 1. Class Average in Stem Length

Graph 1. Class Average in Stem LengthLinear (Graph 1. Class Average in Stem Length)

Test Solution (g/100mL)

Chan

ges i

n St

em Le

ngth

(+/-

mm

)

Conclusion:

(Jacob) When a celery piece is placed in pure distilled water the concentration of water on the outside of the celery is greater than the amount inside the celery. Therefore the celery piece is in a hypotonic condition of osmosis (Osmosis: Background). Water molecules, due to the concentration gradient, will diffuse through the cell membrane of the celery to balance the concentration gradient (Osmosis: Background). This results in the vacuole or vacuoles of the individual cells of the celery pieces to expand and fill up with water to the point where the vacuole or vacuoles will put pressure on the cell wall of the celery cells making all the cells turgid. The cells will become enlarged and swollen causing the celery to grow in length, width and height. However, when the concentration of salt within the water is increased, the salt ions, being a charged particle, will attract the water molecules on the outside and the inside of the cells towards themselves due to the electrostatic attractions between the molecules (Clark). Water will diffuse out of the cell and as water exits the cell, the vacuoles in the cells (membranes containing water) will lose water (Diffusion and Osmosis). As the vacuoles lose water the pressure that is exerted by the vacuoles on the cell wall, called turgor pressure (which provides stability and support to the cell) will decrease causing the cell to shrink and shrivel (Diffusion and Osmosis)(Osmosis (Cellular) – Osmosis in Plant Cells).

(Sarosh) Lysis did not occur in the celery pieces for two primary reasons. One of them being that plant cells can store a great deal more water than an animal cell due to the fact that plant cells have larger vacuoles, as well as having the additional feature of a cell wall (Plant Cells vs. Animal Cells).When pure distilled water enters the cell, the plant cell expands, causing noticeable growth, however the cell wall will prevent the cell from bursting (Osmosis (Cellular) – Osmosis in Plant Cells). Unlike in an animal cell that lacks a cell wall, all the water entering the cell would have caused the cell to burst and lysis would have occurred. However in a plant cell, the plant cell wall, made of cellulose, limits the amount of water the cell can intake, because the cell wall exerts pressure as water enters the cell, preventing too

much water from entering the cell and causing the cell to burst (Osmosis (Cellular) – Osmosis in Plant Cells). This pressure is called turgor pressure (Osmosis (Cellular) – Osmosis in Plant Cells). Another major reason why lysis didn’t occur in the celery pieces is due to having a very large vacuole, which can store large quantities of water. Up to 90% of a plant cells volume can be occupied by a vacuole or several vacuoles (Plant Cells vs. Animal Cells). A contractile vacuole takes up water from the cytosol and, unlike an animal vacuole, periodically discharges its contents through fusion with the plasma membrane (Different Cells have Different Have Various Mechanisms for Controlling Cell Volume). A cellulose composed cell wall as well as having a water storing and ejecting vacuole allows the cell to remain turgid at all times when filled with water and making lysis impossible.

(Jacob) According to our graph, the isotonic salt concentration is 1.35 g/100mL. This is the concentration where the celery molecules achieve dynamic equilibrium, where “water diffuses in and out of the cell at the same rate” (Brown).

(Jacob) According to graph 1, the length of the celery if it started at 30mm and was put in a 2.4g/ 100mL of salt solution for the same amount of time as the rest of the pieces; it would have had a decreased length of 2mm. The resulting length would have been 28mm.

(Sarosh) When dry sugar is sprinkled on a bowl of strawberries, an artificial concentration gradient of glucose is being created where there are more glucose molecules on the outside of the cell than on the inside. Due to the polar charge of a glucose molecule, water molecules will be attracted to the glucose molecules on the outside the strawberry cells and will diffuse to the outside of the cell to try and dissolve the sugar (Intermolecular Forces). “Depending on the amount of sugar added and the water content of the strawberries, the result is a very sweet juice surrounding the pieces of strawberry” (Hoff).

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