6.11
Tonicité chez les animaux
La tonicité décrit la quantité de soluté dans une solution. La mesure de la tonicité d'une solution, ou la quantité totale de solutés dissous dans une…
La tonicité est la capacité de la solution extracellulaire à modifier la forme de la cellule en faisant gagner ou perdre de l’eau à la cellule.
La tonicité dépend de la concentration de solutés imperméables à l’extérieur d’une cellule par rapport à l’intérieur, ce qui détermine la direction dans laquelle l’eau se déplace.
Il peut y avoir trois conditions possibles.
En isotonicité, les concentrations de soluté à l’extérieur et à l’intérieur de la cellule sont égales, sans mouvement net de l’eau. Ainsi, la forme et le volume des cellules restent inchangés.
En cas d’hypertonicité, la concentration de soluté à l’extérieur de la cellule est plus élevée qu’à l’intérieur, ce qui provoque l’évacuation de l’eau. En conséquence, la cellule se ratatine, provoquant la création cellulaire.
En hypotonicité, la concentration de soluté à l’extérieur d’une cellule est plus faible qu’à l’intérieur. Ainsi, l’eau pénètre, provoquant le gonflement et finalement la rupture de la cellule.
Pour traiter la perte de liquide chez les patients déshydratés, une solution isotonique de chlorure de sodium à 0,9 % est administrée par voie intraveineuse pour prévenir la création ou la lyse des globules rouges.
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Q1: What is tonicity and how does it affect cell shape?
Tonicity is the ability of an extracellular solution to change cell shape by causing the cell to gain or lose water. It depends on the concentration of impermeable solutes outside the cell relative to inside, which determines water movement direction. Changes in tonicity can cause cells to shrivel, swell, or maintain their original shape.
Q2: What happens to a cell in a hypertonic solution?
In a hypertonic solution, the solute concentration outside the cell is higher than inside, causing water to move out of the cell. This results in cell crenation, where the cell shrivels. The cell loses water because the extracellular fluid contains less water than the cell's interior.
Q3: How does a hypotonic solution affect animal cells?
In a hypotonic solution, the solute concentration outside the cell is lower than inside, so water moves into the cell. The cell swells as it absorbs water and may eventually rupture, a process called cell lysis. This occurs because water follows its concentration gradient into the cell.
Q4: Why is an isotonic solution used to treat dehydrated patients?
An isotonic solution, such as 0.9 percent sodium chloride, has equal solute concentrations inside and outside the cell, resulting in no net water movement. This prevents red blood cells from crenating or lysing, maintaining cell volume and shape during intravenous administration to dehydrated patients.
Q5: How do plant cells respond differently to hypotonic solutions than animal cells?
Plant cells have cell walls that prevent lysis, so they do not burst in hypotonic solutions. Instead, water enters the cell, creating turgor pressure that stiffens the cell wall and supports the plant. In non-woody plants, this turgor pressure is essential for structural support and plant rigidity.
Q6: What is osmolarity and how does it relate to tonicity?
Osmolarity is the measure of the total amount of solutes dissolved in a specific amount of solution. Tonicity describes how a solution's osmolarity compares to a cell's osmolarity. The terms hypotonic, isotonic, and hypertonic relate the osmolarity of the extracellular fluid to the osmolarity inside the cell, similar to how the glycocalyx and its functions influence cellular interactions.
Q7: What causes plants to wilt when water is unavailable?
When plants lack adequate water, cells become hypertonic and water leaves the cell. This causes the plant to lose turgor pressure, which normally stiffens the cell walls. Without sufficient turgor pressure, non-woody plants cannot maintain their rigid structure and wilt.