2.7 Tonicity and osmoregulation (2025-2026).pptx

Tonicity and osmoregulation

1. UNIT 2: CELLS 2. 7 Tonicity and osmoregulation

2. One special type of simple diffusion is osmosis , which is the diffusion of free water across a selectively permeable membrane. While water can cross the cell membrane due to the fact that is a small molecule (although polar), large quantities of water are moved via protein channels called aquaporins . As we studied, water is great solvent and many solutes can be dissolved in it forming a solution.

3. Some concepts we need to be familiar with when we refer to osmosis: Osmolarity : the total solute concentration in a solution Tonicity : the measurement of the relative concentration of solute between two solutions (in our case, the inside and the outside of the cell) In the measurement of tonicity, we need to consider the difference in solute concentration inside and outside of the cell. There are three situations, which are comparative: Hypertonic environment: more solute and less solvent Isotonic environment: equal amounts of solute and solvent Hypotonic environment: less solute and more solvent In osmosis, water always moves from a hypotonic environment to a hypertonic environment , in an attempt to reach equilibrium (isotonicity) Predict what would happen if you placed a red blood cell in these two scenarios: A beaker with distilled water A beaker with salt water HINT: the cell membrane is selectively permeable, so the solute usually does not cross the membrane.

4. How to solve this type of scenarios? Identify the tonicity of each side of the cell membrane (which side is hypertonic compared to the other side, which in turn will be hypotonic? Knowing that water moves passively from high concentration of water (hypotonic) to low concentration of water (hypertonic), draw an arrow indicating the main movement of water Indicate the final result on the cell based on your prediction 1. Why don’t we have just one arrow for each diagram indicating the movement of water? 2. What would happen if you placed a plant cell in each one of the three solutions above? Crenation/shriveling Hemolysis

5. The cell wall prevents the cell from bursting when placed in hypotonic conditions. The outside is hypotonic, so the water will flow into the central vacuoles via osmosis, causing them to expand and press against the cell wall. The cell wall expands until it begins to exert pressure back on the cell, which is called turgor pressure . Turgidity is the optimum state for plants. Keep in mind that osmoregulation (the ability to regulate water and solute / salt concentrations) is essential for living organisms in order to maintain an internal balance ( homeostasis )

6. PRACTICE - OSMOTIC PREDICTIONS Given the scenarios below, what are your predicted results? Make sure to indicate the movement of water, and the final result on the cell. Cell type Place in a … solution Predicted results Liver cell Hypotonic Onion cell Hypertonic Cheek cell Isotonic Red blood cell Hypertonic Potato cell Hypotonic

7. AP question

8. Water potential measures the tendency of water to move by osmosis, and it is calculated from pressure potential and solute potential, as shown below: Since water moves from hypotonic to hypertonic, we can infer that: A hypotonic environment has a high water potential (it has many water molecules) A hypertonic environment has a low water potential (it does not have many water molecules). Therefore, we can infer that the movement of water happens like this, and that the more negative the water potential, the more likely it is that water move into that area. If Ѱ inside the cell is -1 Bars and the Ѱ outside the cell is -3 Bars, where will the water move to?

9. The water potential of pure water is 0 in an open container, because there are no solutes (Ѱ S is 0) and there is no pressure exerted by the container since it is open (Ѱ P is 0). In an open system, since Ѱ P is 0 then Ѱ = Ѱ S meaning that the water potential is equal to the solute potential. How do we calculate the solute potential? In this formula, i is the ionization constant (e.g. for NaCl is 2, and for sucrose is 1) C is the molar concentration (M), which is the amount of moles of solute per volume of solution R is a pressure constant, which is 0.0831 L Bars / mol K T is temperature in Kelvin (which is the same as temperature in Celsius + 273) What about the negative sign? This means that the more solutes, we have a more negative solute potential.

10. PRACTICE

11. PRACTICE

12. SECTION SUMMARY - MAIN IDEAS What are the different ways to describe relative tonicity of environments? External environments can be hypotonic, hypertonic or isotonic to the internal environment of cells. How do differences in osmolarity impact the movement of water in a cell? Water moves by osmosis from areas of low osmolarity/solute concentration to area of high osmolarity/solute concentration. How does the constant movement of molecules across a membrane maintain growth and homeostasis? Growth and homeostasis are maintained by the constant movement of molecules across membranes. What is osmoregulation and how does it contribute to the health and survival of organisms? Osmoregulation maintains water balance and allows organisms to control their internal solute composition.

13. SECTION SUMMARY - MAIN IDEAS How does water potential impact the movement of water? Water moves by osmosis from areas of high water potential to areas of low water potential. How does solute potential impact the movement of water? Water moves by osmosis from areas of low solute potential to areas of high solute potential How do organisms maintain water balance? Osmoregulation maintains water balance and allows organisms to control their internal solute composition/water potential.

14. UNIT 2: CELLS GRAPHING SKILLS

15. Components of a graph 1) Title : it describes what we see in the graph, mentioning both the independent and dependent variables. 2) Labeled axes with units The independent variable is always located on the X-axis The dependent variable is always located on the Y-axis 3) Scaling should be appropriate: uniform intervals and scale numbers present on the grid lines, 4) The lines or the bars in the graph should be identifiable (a legend is recommended) 5) When appropriate, there should be a trend line to indicate the overall trend of the data Go back to your graphs in the lab notebook and check whether all the elements on the left are present

16. Which graph to choose? Line graphs: Both variables are numerical (continuous) and quantitative. Often used to show changes over time.

17. Which graph to choose? Bar graphs: One variable is numerical and the other one is categorical. Often used to compare the means of different groups. Usually have error bars to indicate the dispersion of the data for each group.

18. Which graph to choose? Scatter Plot graphs: Used to determine the relationship between two variables (may not be linear)

19. Which graph to choose? Box and Whisker plots: Used to show the variability in a sample, especially related to the median

20. IN-CLASS ACTIVITY - Representing and Describin g data ( HANDOUT ) Molarity sucrose in beaker (M) Percent change in mass (%) 2SEM 0.0 16.0 2.9 0.2 5.0 1.7 0.4 -9.0 2.3 0.6 -16.0 2.9 0.8 -23.5 3.5 1.0 -24.0 4.0 Use the handout provided in class