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.
1. In what type of solution does plasmolysis occur? 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? 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? 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 physiology, osmosis (Greek for push) is the net movement of water across a semipermeable membrane.[1][2] Across this membrane, water will tend to move from an area of high concentration to an area of low concentration. It is important to emphasize that ideal osmosis requires only the movement of pure water across the membrane without any movement of solute particles across the semipermeable membrane. Osmosis can still occur with some permeability of solute particles, but the osmotic effect becomes reduced with greater solute permeability across the semipermeable membrane. It is also true that, at a specific moment in time, water molecules can move towards either the higher or lower concentration solutions, but the net movement of water will be towards the higher solute concentration. The compartment with the highest solute and lowest water concentration has the greatest osmotic pressure. Osmotic pressure can be calculated with the van 't Hoff equation, which states that osmotic pressure depends on the number of solute particles, temperature, and how well a solute particle can move across a membrane. Its measured osmolality can describe the osmotic pressure of a solution. The osmolality of a solution describes how many particles are dissolved in the solution. The reflection coefficient of a semipermeable membrane describes how well solutes permeate the membrane. This coefficient ranges from 0 to 1. A reflection coefficient of 1 means a solute is impermeable. A reflection coefficient of 0 means a solute can freely permeable, and the solute can no generate osmotic pressure across the membrane.[2] The compartment with the greatest osmotic pressure will pull water in and tend to equalize the solute concentration difference between the compartments. The physical driving force of osmosis is the increase in entropy generated by the movement of free water molecules. There is also thought that the interaction of solute particles with membrane pores is involved in generating a negative pressure, which is the osmotic pressure driving the flow of water.[3] Reverse osmosis occurs when water is forced to flow in the opposite direction. In reverse osmosis, water flows into the compartment with lower osmotic pressure and higher water concentration. This flow is only possible with the application of an external force to the system. Reverse osmosis is commonly used to purify drinking water and requires the input of energy. [4] The concept of osmosis should not be confused with diffusion. Diffusion is the net movement of particles from an area of high to low concentration. One can think of osmosis as a specific type of diffusion. Both osmosis and diffusion are passive processes and involve the movement of particles from an area of high to low concentration.[2][5] The rate of osmosis always depends on the concentration of solute. The process is illustrated by comparing an environmental or external solution to the internal concentration found in the body. A hypertonic solution is any external solution that has a high solute concentration and low water concentration compared to body fluids. In a hypertonic solution, the net movement of water will be out of the body and into the solution. A cell placed into a hypertonic solution will shrivel and die by a process known as plasmolysis. An isotonic solution is any external solution that has the same solute concentration and water concentration compared to body fluids. In an isotonic solution, no net movement of water will take place. A hypotonic tonic solution is any external solution that has a low solute concentration and high water concentration compared to body fluids. In hypotonic solutions, there is a net movement of water from the solution into the body. A cell placed into a hypotonic solution will swell and expand until it eventually burst through a process known as cytolysis. These three examples of different solute concentrations provide an illustration of the spectrum of water movement based on solute concentration through the process of osmosis. The body, therefore, must regulate solute concentrations to prevent cell damage and control the movement of water where needed. Summary of Red Blood Cell Placed into Hypertonic, Isotonic, and Hypotonic Solutions Hypertonic A hypertonic solution has a higher solute concentration compared to the intracellular solute concentration. When placing a red blood cell in any hypertonic solution, there will be a movement of free water out of the cell and into the solution. This movement occurs through osmosis because the cell has more free water than the solution. After the solutions are allowed to equilibrate, the result will be a cell with a lower overall volume. The remaining volume inside the cell will have a higher solute concentration, and the cell will appear shriveled under the microscope. The solution will be more dilute than originally. The overall process is known as plasmolysis. Isotonic An isotonic solution has the same solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in an isotonic solution, there will be no net movement of water. Both the concentration of solute and water are equal both intracellularly and extracellularly; therefore, there will be no net movement of water towards the solution or the cell. The cell and the environment around it are in equilibrium, and the cell should remain unchanged under the microscope. Hypotonic A hypotonic solution has a lower solute concentration compared to the intracellular solute concentration. When a red blood cell is placed in a hypotonic solution, there will be a net movement of free water into the cell. This situation will result in an increased intracellular volume with a lower intracellular solute concentration. The solution will end up with a higher overall solute concentration. Under the microscope, the cell may appear engorged, and the cell membrane may eventually rupture. This overall process is known as cytolysis. Note that osmosis is a dynamic equilibrium, so at any given moment, water molecular can momentarily flow toward any direction across the semipermeable membrane, but the overall net movement of all water molecules will be from an area of high free water concentration to an area of low free water concentration.[5][6] Water is known as the "universal solvent," and almost all known life depends on it for survival. Therefore, the principle of osmosis, though seemingly simple, plays a large role in almost all physiological processes. Osmosis is specifically important in maintaining homeostasis, which is the tendency of systems toward a relatively stable dynamic equilibrium. Biological membranes act as semipermeable barriers and allow for the process of osmosis to occur. Osmosis underlies almost all major processes in the body, including digestion, kidney function, nerve conduction, etc. It allows for water and nutrient concentrations to be at equilibrium in all of the cells of the body. It is the underlying physical process that regulates solute concentration in and out of cells, and aids in excreting excess water out of the body.[2][7][8][9][10][11] Review QuestionsThe image shows the process of osmosis. Contributed from Cornell, B. 2016. Referencing. [ONLINE] Available at: http://ib.bioninja.com.au/standard-level/topic-1-cell-biology/14-membrane-transport/osmosis.html 1. Chen JS, Sabir S, Al Khalili Y. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 9, 2021. Physiology, Osmoregulation and Excretion. [PubMed: 31082152] 2.Marbach S, Bocquet L. Osmosis, from molecular insights to large-scale applications. Chem Soc Rev. 2019 Jun 04;48(11):3102-3144. [PubMed: 31114820] 3.Kiil F. Molecular mechanisms of osmosis. Am J Physiol. 1989 Apr;256(4 Pt 2):R801-8. [PubMed: 2705569] Greenlee LF, Lawler DF, Freeman BD, Marrot B, Moulin P. Reverse osmosis desalination: water sources, technology, and today's challenges. Water Res. 2009 May;43(9):2317-48. [PubMed: 19371922] 5.Goodhead LK, MacMillan FM. Measuring osmosis and hemolysis of red blood cells. Adv Physiol Educ. 2017 Jun 01;41(2):298-305. [PubMed: 28526694] 6.Maldonado KA, Mohiuddin SS. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Aug 17, 2021. Biochemistry, Hypertonicity. [PubMed: 31082139] 7.Kiil F. Mechanism of osmosis. Kidney Int. 1982 Feb;21(2):303-8. [PubMed: 7069994] 8.Meir E, Perry J, Stal D, Maruca S, Klopfer E. How effective are simulated molecular-level experiments for teaching diffusion and osmosis? Cell Biol Educ. 2005 Fall;4(3):235-48. [PMC free article: PMC1200778] [PubMed: 16220144] 9.Schultz SG. Epithelial water absorption: osmosis or cotransport? Proc Natl Acad Sci U S A. 2001 Mar 27;98(7):3628-30. [PMC free article: PMC33327] [PubMed: 11274376] 10.Ogobuiro I, Tuma F. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jul 26, 2021. Physiology, Renal. [PubMed: 30855923] 11.Trigo D, Smith KJ. Axonal morphological changes following impulse activity in mouse peripheral nerve in vivo: the return pathway for sodium ions. J Physiol. 2015 Feb 15;593(4):987-1002. [PMC free article: PMC4398533] [PubMed: 25524071] |
What occurs when a cell is placed in a solution that has a lower concentration of solute than its cytoplasm?
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