Both plants and animals have cells, and one of the main differences between them is that plant cells have a cell wall. This helps the cells retain their shape even if their environment changes considerably. Animal cells are more flexible, and without the cell wall, they can react more adversely to changes in their environment, such as the concentration of a solution around them.
All cells are surrounded by a plasma membrane, which is a double layer of phospholipids that regulate the transport of materials into and out of the cell. In a plant, the cell membrane is next to the cell wall, which helps give it structure. The plasma membrane of a cell is said to be differentially permeable because it allows certain substances, such as water, to flow freely through it, but not others, such as ions and large molecules.
Diffusion and Osmosis
A solution contains the solvent, the liquid, such as water and the solute, the solids dissolved into the solvent. In a solution, particles in an area of high concentration will naturally move to an area of low concentration. This is known as diffusion. When diffusion happens across a differentially permeable membrane, it is known as osmosis. This is important to cells because if the solute cannot move across the differentially permeable membrane, water will freely move into or out of a cell to balance out the concentration.
If a cell is in a solution that has the same concentration outside the cell as it is inside the cell, it is said to be isotonic. This is the ideal situation for a cell, and the flow of water into the cell equals the flow of water out of the cell. However, cells often encounter environments that have different concentrations of solution, such as organisms found in fresh water (low concentration of solute) or ocean water (high concentration of solute).
If the solution surrounding a cell is less concentrated than that inside the cell, it is said to be hypotonic. In a hypotonic solution, water will move from outside to the inside of the cell across its membrane. This process will continue until the solutions reach equal concentration, the cell ruptures from internal pressure or organelles inside the cell exert energy to pump water out of the cell.
If a cell is in a solution that has a higher concentration than what is inside the cell, it is said to be hypertonic. When this happens, water will flow out of the cell and into the solution. This will continue until the concentrations are balanced, the cell dehydrates and collapses or the organelles in the cell exert energy to pump water back into the cell.
Animal Cells vs. Plant Cells
When animal cells are placed in a hypotonic solution, they will take in water due to osmosis. If the solution in which they are placed is a low enough concentration, such as distilled water, the intake of water will make the cells swell up and eventually burst. This can, of course, be a very dangerous result for cells placed in a solution of low concentration. Plant cells do not rupture because the rigid cell walls hold them together.