Question

1. water potential (2.8)what is water potential? give the equation and explain its components. how do you know what value to use for i in the equation? provide examples of organisms modifying their water potential to maintain homeostasis.

Explanation:

Water potential ($\Psi$) is the potential energy of water per unit volume relative to pure - water in reference conditions. The equation is $\Psi=\Psi_s+\Psi_p+\Psi_g+\Psi_m$, where $\Psi_s$ is solute potential (negative value as solutes lower water potential), $\Psi_p$ is pressure potential (positive in turgid cells, negative in xylem under tension), $\Psi_g$ is gravitational potential (usually negligible in short plants), and $\Psi_m$ is matric potential (due to adhesion of water to surfaces). The van't Hoff factor ($i$) in the solute - potential equation $\Psi_s=-iCRT$ represents the number of ions or particles a solute dissociates into. For example, for glucose ($i = 1$ since it doesn't dissociate), for $NaCl$ ($i=2$ as it dissociates into $Na^+$ and $Cl^-$). Organisms like plants use osmoregulation. In a hypertonic environment, plant cells may lose water and become flaccid, but they can also accumulate solutes to lower their water potential and prevent excessive water loss. In animals, kidney cells can adjust the reabsorption of water and solutes to maintain homeostasis. In a hypotonic environment, cells may gain water; some single - celled organisms have contractile vacuoles to expel excess water. In an isotonic environment, there is no net movement of water across the cell membrane as the water potential inside and outside the cell is equal. To diagram water movement, in a hypotonic solution, water moves into the cell (from high water potential outside to low inside). In a hypertonic solution, water moves out of the cell (from high water potential inside to low outside). In an isotonic solution, water moves in and out at an equal rate with no net change.

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