5.8
動物の浸透圧
溶液の浸透圧は細胞がその溶液中で水分を得るか失うか決定します。浸透圧は、異なる溶質に対する細胞膜の浸透性と、細胞内外の溶液中の非浸透溶質の濃度に依存します。半透膜は一部の溶質の通過を妨げますが、水がその濃度勾配に従うことを許せば、水はモル浸透圧濃度の低い側（すなわち溶質濃度の低い側）からモル浸透圧濃…
Tonicity is the effective osmotic pressure gradient created by non-penetrating solutes across a semipermeable membrane, which influences water movement across it. This results in three possible scenarios that maintain or change the cell's volume.
When the solute concentration inside and outside the cell is equal, the solution is isotonic. There is no net movement of water because water enters and leaves the cell at equal rates, so the cell maintains its normal shape.
But, if the fluid outside the cell has fewer solutes and more water than the inside, the condition is hypotonic. Water moves into the cell, causing it to swell and possibly burst, a process called cell lysis.
On the other hand, in hypertonic conditions, the extracellular fluid contains more solutes and less water than the inside of the cell. Water moves out of the cell, causing it to shrink.
Q1: What is tonicity and how does it affect water movement across cell membranes?
Tonicity is the effective osmotic pressure gradient created by non-penetrating solutes across a semipermeable membrane and osmosis. It determines whether a cell gains or loses water based on solute concentration differences. Water moves from areas of low solute concentration to high solute concentration, influencing cell volume and shape.
Q2: What happens to animal cells in isotonic solutions?
In isotonic solutions, solute concentration inside and outside the cell is equal, so there is no net water movement. Water enters and leaves the cell at equal rates, maintaining the cell's normal shape and volume. The cell remains in osmotic balance with its environment.
Q3: Why do animal cells burst in hypotonic solutions?
Hypotonic solutions contain fewer solutes and more water outside the cell than inside. Water moves into the cell, causing it to swell and eventually burst in a process called cell lysis. Freshwater is an example of a hypotonic environment where animal cells face this risk.
Q4: What causes animal cells to shrink in hypertonic solutions?
Hypertonic solutions contain more solutes and less water outside the cell than inside. Water moves out of the cell to balance the solute concentration gradient, causing the cell to shrink. Saltwater is a common example of a hypertonic environment for animal cells.
Q5: How do osmoregulators maintain water balance differently from osmoconformers?
Osmoregulators actively control their internal osmotic conditions independent of their environment, like humans and fish. Osmoconformers, such as lobsters and jellyfish, maintain internal solute concentration equal to their surroundings. This allows osmoconformers to thrive in stable environments without frequent osmotic fluctuations.
Q6: What strategies do freshwater fish use to survive in hypotonic environments?
Freshwater fish mitigate constant water influx through frequent urination, which releases excess water from their bodies. They also maintain higher internal osmolarity than their surroundings, creating an osmotic gradient. These osmoregulatory strategies prevent cell swelling and maintain proper body fluid balance.
Q7: How do saltwater fish compensate for water loss in hypertonic environments?
Saltwater fish regulate water loss by drinking large quantities of seawater and actively excreting excess salt through their gills and urine. This osmoregulatory strategy maintains internal water balance despite the hypertonic environment. These adaptations allow them to survive in high-salinity conditions.