“Water water everywhere, nor any drop to drink.” Episode 30 reviews the passive transport of water and the calculation of water potential
“Water water everywhere, nor any drop to drink.” Episode 30 reviews the passive transport of water and the calculation of water potential - with many water sound effects! Hypertonic solutions have a greater concentration of solutes than a hypotonic solution (1:30). It does not end well for a paramecium placed in freshwater (2:15). Water potential allows us to predict the movement of water (4:00) and is measured in bars. Don’t memorize any equations! (5:20)
The Question of the Day asks (6:56) “What is the process called when a cell membrane pulls away from a plant cell wall after being placed in a hypertonic solution?”
Thank you for listening to The APsolute RecAP: Biology Edition!
(AP is a registered trademark of the College Board and is not affiliated with The APsolute RecAP. Copyright 2020 - The APsolute RecAP, LLC. All rights reserved.)
Website:
EMAIL:
Follow Us:
Hi and welcome to the APsolute Recap: Biology Edition. Before we start today’s episode, I want to let you know that we are planning a few Instagram Live review sessions leading up to the exam on May 18th. Please make sure to follow us on Instagram to stay up to date with the announcements.
Today’s episode will recap Osmosis and Water Potential
Lets Zoom out:
Water, water everywhere, nor any drop to drink. Samuel Coleridge may have known his biology when writing “The Rime of the Ancient Mariner.” He warned his readers not to drink the salty ocean water! But why not? This episode will review the movement of water into and out of cells and how to calculate water potential.
Lets Zoom in:
Osmosis - the passive diffusion of water. Remember that passive transport involved the movement of molecules from a high to low concentration without the input of cellular energy, or ATP. Water, H2O, is small - and although it has polarity, it is able to freely cross the phospholipid membrane. When water moves in larger quantities, it passes through membrane proteins known as a aquaporins. Areas that have a greater concentration of solutes (such as ions, sugars etc.) are hypertonic compared to areas with lesser solutes. The prefixes hyper and hypo refer to the relative amount of dissolved solutes in a solution. Since hypertonic solutions have more solutes, they have a lesser water concentration. Assuming that two solutions are separated by a semipermeable membrane, water will flow from a hypotonic area to a hypertonic area until the water concentrations are equal. or isotonic. Water is still moving between the isotonic areas, but at an equal rate and in both directions. Be careful - equal concentrations does not mean equal amounts. This means that water will enter or exit an area, or cell whether there is room for the water volume or not. This may cause some problems.
Consider a paramecium - a unicellular protist that is adapted for life in salt water. Meaning that in a salty environment, a paramecium is at equilibrium with its cytoplasm isotonic to the extracellular fluid. However, if you were to surround the paramecium with pure water, an environment hypotonic to the cell - water would rush in. Remember, water moves from high water concentration to low water concentration. While a paramecium does have a contractile vacuole to pump out excess water, the water would likely enter the cell too quickly, causing lysis or bursting of the organism. The opposite is also true - if a red blood cell were placed into a hypertonic solution, the cell would shrivel or crenate as water exited. If you've ever had to receive IV fluids from a doctor, you might have noticed that it is not pure water, but a saline or salt solution that is isotonic with your blood plasma.
Does the same hold true for plant cells? Not quite. Plant roots store several solutes, making them hypertonic to the surrounding environment. When you water your houseplants, the water moves from an area of higher concentration, outside the roots, to an area of lesser water concentration, inside the roots. The cells are reinforced by their cell wall. As cell vacuoles fill with water, turgor pressure increases. The cell walls are rigid and the plant will not wilt. Water also has to make a remarkable journey throughout the plant - defying the law of gravity. For that explanation, we need to dig deeper into water potential.
Water potential allows us to predict the movement of water, due to osmosis, pressure, surface tension, or gravity. It is indicated by the greak letter psi and looks like a trident, just the Poseidon’s symbol, the Greek god of the sea. Water potential equals the pressure potential plus the solute potential. Solute potential decreases as more solutes are added to a solution. Pressure potential is a physical force, such as the turgor pressure experienced in a plant cell due to a full central vacuole. The unit of measurement for pressure is bars. If you are given a sample problem that is in an open container, such as a beaker, the pressure potential is zero!
The greatest value that water potential can be is zero. Therefore, any solution that has solutes in it, will have a negative value. The concepts we reviewed in osmosis haven’t changed, we are just adding more vocabulary. Water flows from an area of high water potential (less negative value, hypotonic solution) to an area of low water potential (more negative value, hypertonic solution).
You may have to first solve for solute potential before solving for water potential in a given problem. The solute potential of a solution is equal to -iCRT. Do not memorize this, it will all be in the math sheet. i is the ionization constant, is equal to the number of ions the solute will make in water. For NaCl this would be 2 (sodium and chlorine ions) whereas for glucose this number is 1, since it doesn’t ionize. . An increase in ionization constant will decrease the solute potential. C is the molar concentration, measured as moles per liter. R is the pressure constant and is always equal to 0.0831 liter bar/mole K. NO MEMORIZING THIS! And lastly, T is temperature in degrees Kelvin, which is 273 + °C. Plug and play in your equation, and nearly all of your units will cancel out - leaving you with bars.
Back to that houseplant you’ve watered. The least amount of water potential is at the leaves, as water evaporates through stomata by transpiration. The greatest water potential is at the roots, with some dissolved solutes. However all of these areas are less than zero, the water potential of pure water. So what you are left with is a gradient of water potential from roots to shoots, high to low, causing osmosis throughout the plant, against gravity.
To recap….
Water potential is represented by the greek letter psi and calculated in bars. Osmosis is the movement of water from an area of high water potential to low water potential. “The Rime of the Ancient Mariner” knew it all along - the ocean is a hypertonic solution with a low water potential. Drinking it would only cause water to exit your cells!
Today’s Question of the day is about plant cells
Question “What is the process called when a cell membrane pulls away from a plant cell wall after being placed in a hypertonic solution?”
Coming up next on the Apsolute RecAP Biology Edition: Mutations