2.7 Tonicity and Osmoregulation
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Osmosis | 渗透作用 | The diffusion of free water across a selectively permeable membrane. |
| Tonicity | 张力/渗透压 | The ability of a surrounding solution to cause a cell to gain or lose water. |
| Hypertonic | 高渗的 | A solution with a higher concentration of solutes compared to the inside of the cell. |
| Hypotonic | 低渗的 | A solution with a lower concentration of solutes compared to the inside of the cell. |
| Isotonic | 等渗的 | A solution with an equal concentration of solutes compared to the inside of the cell. |
| Water Potential (\(\Psi\)) | 水势 | The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure. |
| Osmoregulation | 渗透调节 | The regulation of solute concentrations and water balance by a cell or organism. |
1. Tonicity and the Direction of Osmosis
Growth and homeostasis are maintained by the constant movement of molecules across membranes. While solutes often need transport proteins to move, water can move across the membrane via osmosis.
Understanding which way water will flow depends entirely on the tonicity of the cell's environment. The external environment can be hypotonic, hypertonic, or isotonic to the internal environment of the cell.
The Golden Rule of Osmosis: Water Chases Solutes!
To easily predict the movement of water, remember this simple rule: Water moves from regions of low solute concentration to regions of high solute concentration. Water wants to go wherever the party is (where there are more solute particles) to dilute them!
Therefore, the movement of water can be accurately described as moving from hypotonic to hypertonic regions:
- Hypotonic Environment: Water rushes INTO the cell. (Animal cells will swell and burst/lyse; Plant cells become turgid/firm, which is healthy for them).
- Hypertonic Environment: Water rushes OUT of the cell. (Both animal and plant cells will shrivel/plasmolyze).
- Isotonic Environment: Water moves in and out at equal rates. There is no net movement of water. (Animal cells are stable; Plant cells become flaccid/limp).
2. Osmoregulation
Organisms must constantly fight against their environments to survive. Osmoregulation maintains water balance and allows organisms to control their internal solute composition and water potential.
For example, a paramecium living in a hypotonic pond is constantly taking on water. To avoid bursting, it uses a specialized organelle called a contractile vacuole to actively pump water back out.
3. The Math of Water Potential (\(\Psi\))
In AP Biology, the movement of water is quantified using the concept of Water Potential, represented by the Greek letter Psi (\(\Psi\)).
The Rule: Water always moves by osmosis from regions of high water potential to regions of low water potential.
A. The Water Potential Equation
Water potential is determined by combining the effects of pressure and solutes:
- \(\Psi_P\) (Pressure Potential): The physical pressure applied to a solution. (In an open beaker, \(\Psi_P = 0\). In a turgid plant cell, the cell wall pushes back, creating a positive \(\Psi_P\)).
- \(\Psi_S\) (Solute Potential): The effect of dissolved solutes. Adding solutes always lowers the water potential (making \(\Psi_S\) a negative number). Pure water has a solute potential of \(0\).
B. The Solute Potential Equation
To calculate the solute potential (\(\Psi_S\)) of a solution, use the following formula provided on your AP formula sheet:
- \(i\) = Ionization constant: How many pieces the solute breaks into in water. (e.g., Sucrose does not ionize, so \(i = 1\). \(\ce{NaCl}\) breaks into \(\ce{Na+}\) and \(\ce{Cl-}\), so \(i = 2\)).
- \(C\) = Molar concentration: The molarity of the solution (moles/Liter).
- \(R\) = Pressure constant: Always \(0.0831 \frac{L \cdot bar}{mol \cdot K}\).
- \(T\) = Temperature in Kelvin: Celsius + \(273\). (e.g., \(20^\circ\text{C} = 293\text{K}\)).
Because of the negative sign in the equation, an increase in solute concentration (\(C\)) will lead to a more negative solute potential (\(\Psi_S\)), thereby lowering the total water potential (\(\Psi\)).
Quiz
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