What happens when a cell is placed into water?

Plasmolysis is when plant cells lose water after being placed in a solution that has a higher concentration of solutes than the cell does. This is known as a hypertonic solution. Water flows out of the cells and into the surrounding fluid due to osmosis. This causes the protoplasm, all the material on the inside of the cell, to shrink away from the cell wall. Severe water loss that leads to the collapse of the cell wall can result in cell death. Since osmosis is a process that requires no energy on the part of the cell and cannot be controlled, cells cannot stop plasmolysis from taking place.

Osmosis is responsible for the occurrence of plasmolysis. Osmosis is a special type of diffusion that occurs when water flows into or out of a membrane such as a cell’s plasma membrane. It occurs based on the type of solution that a cell is in. A solution is a mixture that contains a fluid, or solvent (usually water), and a solute that is dissolved in the solvent. When a cell is placed into a hypertonic solution, there is a higher concentration of solutes outside the cell, so water flows out of the cell to balance the concentration on both sides of the membrane. Since plasmolysis is the loss of water from a cell, it occurs when a cell is in a hypertonic solution. Conversely, when a cell is placed into a hypotonic solution, there is a lower solute concentration outside the cell than inside, and water rushes into the cell. In an isotonic solution, solute concentrations are the same on both sides, so there is no net gain or loss of water.

Plant cells fare best in hypotonic solutions. This is because when plant cells are full of water, they push against each other to form the basic support structure for the plant and allow it to stand upright. Plant calls full of water are known as turgid cells; they exert turgor pressure on each other. The cells’ rigid cell wall keeps them from bursting. Unlike plant cells, animal cells do not have a cell wall in addition to their cell membrane. When animal cells are placed in a hypotonic solution and too much water rushes in, they will lyse, or burst. They fare best in isotonic solutions instead.

This figure shows a plant cell in different types of solutions:

Concave plasmolysis is a process that can usually be reversed. During concave plasmolysis, the protoplasm and the plasma membrane shrink away from the cell wall in places due to the loss of water; the protoplasm is then called protoplast once it has started to detach from the cell wall. Half-moon-shaped “pockets” form in the cell as the protoplast peels from the surface of the cell wall. This can be reversed if the cell is placed in a hypotonic solution, which will cause water to rush back into the cell.

Convex plasmolysis is more severe than concave plasmolysis. When a cell undergoes complex plasmolysis, the plasma membrane and protoplast lose so much water that they completely detach from the cell wall. The cell wall collapses in a process called ctyorrhysis. Convex plasmolysis cannot be reversed, and results in the destruction of the cell. Essentially, this is what happens when a plant wilts and dies from lack of water.

Plasmolysis happens in extreme cases of water loss, and does not happen very often in nature. Plants have a couple mechanisms to protect against water loss. Stomata, which are small holes on the underside of a plant’s leaves, close to help keep water in the plant. Plants also naturally produce wax that is another defense against water loss.

Although plasmolysis more commonly happens in a laboratory setting, it can happen in real-life settings as well. For example, during periods of extreme coastal flooding, ocean water deposits salt onto land. Too much salt causes the water to flow out of any plants on the affected land, killing them. Chemical weedicides are also used to kill unwanted plants through plasmolysis. This same process is also used when a lot of salt and/or sugar is added to preserve food and make jams, jellies, and pickles. The cells lose water and become less conducive to the growth of microorganisms such as bacteria, allowing these food items to be preserved.

• Osmosis – Process by which water diffuses across a membrane to balance out the solute concentration on either side of the membrane.
• Cell wall – Found in plant and fungi cells, a tough layer surrounding the outside of the cell that provides structural support.
• Ctyorrhysis – Permanent and irreversible collapse of the cell wall due to too much water being lost through plasmolysis.
• Protoplasm – The material comprising the inside of the cell; it is called protoplast when it separates from the cell wall through plasmolysis.

1. In what type of solution does plasmolysis occur?
A. Hypertonic
B. Isotonic
C. Hypotonic

A is correct. In a hypertonic solution, water flows out of plant cells through osmosis, and this causes the protoplast to detach partially or fully from the cell wall.

2. What mechanisms do plants use to defend themselves against plasmolysis?
A. The plants’ stomata close to help keep water inside.
B. The plants produce wax that keep water inside.
C. The plants pump water into their cells through reverse osmosis.
D. Both A and B

D is correct. Stomata and wax are two defenses that plants have for maintaining enough water in their cells. Osmosis happens without any energy or control on the part of the cell, so the cell cannot reverse the process.

3. What type of solution is best for plant cells?
A. Hypertonic
B. Isotonic
C. Hypotonic

C is correct. Plant cells do best in a hypotonic solution because water flows into the cells and allows them to have full turgor pressure. In an isotonic solution, the cells are not as turgid as they could be since there is no net gain or loss of water, and the plant begins to droop. In a hypertonic solution, water flows out of the cells, and plasmolysis occurs.

In biology, osmosis refers to the movement of water. It is a passive process, meaning it requires no energy and happens automatically. The water moves from an area of greater concentration to an area of lesser concentration until equilibrium is reached. For cells, this movement is from one compartment to another through a semipermeable membrane. The solution is the area outside the cellular compartment, also called the extracellular environment.

Solute is the number of dissolved solids in a solution regardless of what they are. It can consist of proteins, carbohydrates, ions, hormones, etc. When discussing osmosis, comparison is made between the intracellular and extracellular (solution) solute concentrations. The concentration is also called the osmolarity of the solution. The difference between the number of dissolved solids creates an osmotic pressure gradient that forces water to move to achieve equilibrium between the inside and outside environments.

In a hypotonic solution, the solute concentration is lower than inside the cell. The prefix hypo means under or below in Latin. Under these conditions, the osmotic pressure gradient forces water into the cell. Depending on the amount of water that enters, the cell may look enlarged or bloated. If the water continues to move into the cell, it can stretch the cell membrane to the point the cell bursts (lyses) and dies.

Cells don’t have the ability to regulate their water content (remember that osmosis is a passive process), so they rely on the body to provide an environment where intracellular and extracellular solute concentrations are equal or isotonic. The body regulates the composition of extracellular fluid using the kidneys, adrenal glands, and the hypothalamus in the brain which triggers thirst and drives organisms to drink water. Slight fluctuations in the solute concentration of the extracellular fluid throughout the day cause small amounts of water to be exchanged between the intracellular and extracellular compartments to maintain homeostasis.
In contrast to hypotonic and isotonic solutions, a hypertonic solution has a higher solute concentration than inside the cell. When this happens, the osmotic gradient causes water to rush out of the cell and it becomes wrinkled or shriveled. If this happens to red blood cells, it is called crenation

Plant cells respond the same way as animal cells in a hypotonic solution, but the affects may not be as severe. Plants have rigid cell walls made of cellulose covering the plasma membrane. This makes it difficult for the cell to lyse, but the increased pressure causes the sides of the cell to bulge out.

References

• OpenStax College. (2018). Anatomy & Physiology. Houston, TX. OpenStax CNX. Retrieved from http://cnx.org/contents/
• Tonicity. (n.d.). In Wikipedia. Retrieved April 25, 2018 from https://en.wikipedia.org/wiki/Tonicity