obsidianvault/biology/Cell Structure and Function/Tonicity and Osmoregulation.md

Cell Structure and Function

Tonicity compares solute concentrations in and out of a cell, telling you which direction the water will move. Cells and organisms use osmoregulation to keep water and solutes balanced.

Tonicity is essentially comparing solute concentration inside and outside a cell.

Osmoregulation is how cells handle water balance and solute concentration.

Summary:

• Tonicity compares solute concentration inside and outside a cell.
• Osmosis moves water from high water potential to low water potential
• Water potential: \psi=\psi_s+\psi_p
• Adding solute lowers water potential, pressure raises it.
• Solute potential: \psi=-iCRT
• As solute concentration rises, \psi becomes more negative.

Tonicity

Tonicity compares solute concentration inside and outside a cell.

• Hypertonic is when a cells solute concentration is higher than its environment, causing the water inside to leave by osmosis and making animal cells crenate and plant cells undergo plasmolysis.
• Hypotonic is when a cells solute concentration is lower than the environment so the surrounding water flows in, making animal cells lyse and plant cells turgid.
• Isotonic is when solute concentrations match, so the cell keeps its shape.

Osmosis

Passive movement of water across a selectively permeable membrane. Water moves from hypotonic (low solute concentrations) to hypertonic (higher) (equivalent to moving from high water potential to low). Water is polar, so it moves through aquaporins, which provide an hydrophilic path. Aquaporins are important in plant cells, red blood cells, etc.

Water Potential

\psi=\psi_s+\psi_p

\psi is total water potential, \psi_s is solute potential, \psi_p is pressure potential. Adding solute lowers solute potential, pressure potential is physics pressure on solution.

In an open container, pressure potential is zero, in a plant cell turgor pressure increases pressure potential.

Solute Potential

\psi=-iCRT

i is ionization constant (number of molecules a particle makes in a solution) C is molar concentration R is pressure constant (R=0.0831 L*bars/mol*K) T is temperature in Kelvin

Solute potential is always zero or negative. Solute concentration has a negative relation with solute potential, drawing more water to the region.

Osmoregulation and Homeostasis

Organisms must regulate water potential and solute composition to maintain homeostasis.

• Contractile vacuoles in freshwater protists: freshwater environments are hypotonic to protist's cytoplasm, so they constantly pump this water out.
• Central vacuole in plant cell: Large central vacuole stores water and dissolved substances, making the plasma membrane push against cell wall and making the cell rigid and strong.

How Water and Solutes Cross Membranes

Channel Proteins: Provides a hydrophilic passage through the membrane for certain ions and molecules. Aquaporins are a channel protein for water. Movement of these ions can charge the membrane.

Carrier Proteins: Change shape to shuttle molecules across the membrane. Transport rate is slower than channel proteins. They let large polar molecules move down concentration gradient.

Active Transport (Related Context): Transport above is passive because it moves along the gradient. Active transport requires ATP and moves against gradient.