Macromolecules serve many roles in living systems

Macromolecules serve many roles in living systems. They can be very,
very small like the nucleic acids that form your DNA or they can be very large like the protein that makes a muscle cell. Many macromolecules are devoted to how cells interact with other cells and their environment.
A very important reason that cells need to communicate is that one role of macromolecules is to protect the cell. The intracellular compartment of a cell is a very controlled environment. Molecules crucial to cell survival reside in the intracellular space, while other molecules (including water, ions, steroids, and other small molecules) are permitted limited access to the intracellular compartment through specialized protein channels embedded in the cell’s protective, semipermeable lipid bilayer.

The interesting consequences of having an intracellular compartment that has different contents than the extracellular compartment is that a difference in concentration (or gradient) is set up against that membrane. For example, an excess of sodium chloride in the extracellular space wants to balance with the sodium or other ions in the intracellular space. The cell’s protective membrane actively prevents a re-balancing by having specific receptor channels that are closed or open depending on the needs of the cells.
As you might expect, water is very important to cells. The human body is 50-75% water and most of that water (67%)1 is in the intracellular compartment of the cells. Unlike the restrictions placed on other macromolecules, water is given a free pass to enter and exit the intracellular space as often as necessary. Water balance for cells and tissues is achieved through a process called osmosis.

Osmosis
Osmosis can be defined as the movement of water which balances the ‘pressure’ due to concentration differences between two neighboring aqueous compartments (such as the intracellular and extracellular spaces).
If you intake salty foods, the extracellular space becomes highly concentrated with salt. What do you think happens next?
Remember that (1) cell membranes freely permit the flow of water through specialized channels in the membrane, and (2) water can move between compartments when there is a concentration imbalance between compartments.
The answer to what happens next: Water will move through water channels in the lipid bilayer from the less salty intracellular compartment to the saltier extracellular compartment which leads to a balance of the concentration of salt (solute) between the intracellular and extracellular aqueous environments..
What do you think happens if too much water leaves the intracellular compartment?
There is a delicate, interdependent relationship between the intracellular compartment and the extracellular compartment. Answer: Crenation ‐ or cell shriveling ‐ occurs if cells lose too much water, which can be the death of a cell unless the organism is re-hydrated in a timely matter. Cytolysis, or cell bursting, occurs if a cell takes in too much water through osmosis.

Tonicity and Water Imbalance
In living systems, biological processes are often compared in reference to each other. As mentioned above, concentration balance across cell membranes is required for homeostasis (normal biological function).
A state of concentration balance is referred to as isotonicity and two compartments of equal concentration separated by a semi-permeable membrane are isotonic to each other.
A compartment (such as an intracellular compartment) is regarded as hypotonic (under-tone) compared to the extracellular space if the extracellular spaces has, for example, many more salt particles in it. When regarding the extracellular compartment in this same scenario, the extracellular space would be referred to as hypertonic (over-toned). These scenarios are shown in the diagram above (red blood cells are shown in hypertonic, isotonic, and hypotonic environments along with the flow of water into and out of the cells). The same scenarios for a plant cell are shown below.

Osmotic Pressure
Differences in the concentrations of various ions and molecules between the intracellular and extracellular compartments of cells causes a pressure across the semipermeable, lipid bilayer membrane. This pressure is referred to as osmotic pressure.
Because volume and the number of particles (solutes) in a solution are also governed by the laws of physics (just like all other matter on Earth), the differences in concentrations interact with gravity, air pressure, shape of the containers (such as the shape of a cell) and other physical features of the natural world to give rise to osmotic pressure. The osmotic pressure eventually results in a change in the volume of the compartments. In cells, for example, osmotic pressure can manifest itself as swelling or as shrinkage (crenation).
In a rigid container such as a laboratory beaker, the osmotic pressure between two compartments separated by a semipermeable membrane will be evident as changes in the height (volume) of the solutions.

The diagram shown above illustrates this process. The beaker on the left shows initial equal volumes of solutions separated by a semipermeable membrane. The difference in solute concentration (represented by red spheres) causes osmotic pressure that drives water through the semipermeable membrane and causes changes in height and volume on both sides of the membrane. The final equilibrium heights and volumes are shown in the beaker on the right.
Final heights (and volumes) occur when pressure is balanced across the semipermeable membrane. At osmotic pressure equilibrium, water continues to flow in both directions ‐ but at the same rate in both directions. That is, there is no net change in the volume of the solutions after equilibrium is reached.

Orientation to the Osmosis Model System
In this set of laboratory procedures, you will work with water as a solution and two test solutions of different concentrations. Water is a universal solvent but, for purposes of this laboratory, the water solution can be regarded as solute-free. Osmosis, strictly speaking, refers specifically to the movement of water. Water will move through protein channels across a semipermeable membrane to achieve an equilibrium in osmotic pressure. In this laboratory, you will observe changes in volume in the water solution and in the test solutions which mimic changes in volume that might occur in the different fluid compartments of a living organism. The final changes in the volume reflect the amount of water that moved to the test solution compartment to achieve equilibrium.
Procedure I Overview
Test Solution 1 – Water: In this procedure you will explore osmosis when two solutions separated by a semipermeable membrane are identical (water).
Procedure II Overview
Test Solution 2 – Guanine solution (known concentration): In this procedure you will explore osmosis when different solutions are separated by a semipermeable membrane.
Procedure III Overview
Test Solution 3 – Cytochrome C solution (unknown concentration): In this procedure you will explore osmosis when different solutions are separated by a semipermeable membrane. Your data will allow you to determine the concentration of this test solution.

Summary of Formulas and Concepts Needed for Calculations
Pressure across the semipermeable membrane
Initial differences in heights leads to gravitational pressure. Differences in solution concentrations leads to osmotic pressure.
Differences in concentrations → cause differences in osmotic pressures → cause differences in final volumes.
Final heights (and volumes) occur when pressure is balanced across the semipermeable membrane. Note that at equilibrium water continues to flow in both directions ‐ but at the same rate in both directions.
Different concentrations and volume differences are related by the ratio
concentration of solution B
concentration of solution A

=difference in final volumes for solution B
difference in final volumes for solution A

concentration of solution Bconcentration of solution A=difference in final volumes for solution Bdifference in final volumes for solution A
Sample Calculation: Determine the concentration of solution 3 given the data below:

-concentration of solution 3 = unknown
-concentration of solution 2 = 0.225 mmol/L
-difference in final volumes for solution 3 = 2.80 L
-difference in final volumes for solution 2 = 3.50 LSetup the ratio as follows
concentration of solution 3
concentration of solution 2

=difference in final volumes for solution 3
difference in final volumes for solution 2

concentration of solution 3concentration of solution 2=difference in final volumes for solution 3difference in final volumes for solution 2
Insert the known values
concentration of solution 3
0.225mmol/L

=2.80L
3.50L

concentration of solution 30.225mmol/L=2.80L3.50L
Solve for the unknown concentration
concentration of solution 3=0.225mmol/L×2.80L
3.50L

concentration of solution 3=0.225mmol/L×2.80L3.50L
=0.225mmol/L×0.800
=0.225mmol/L×0.800
=0.180mmol/L
=0.180mmol/L
1. http://howmed.net/physiology/body-fluid-compartments-water-balance/

As discussed in the Background material, water is an important biological molecule. Do you expect water to continue to flow across the semipermeable; lipid bilayer after osmotic equilibrium is reached? Why or why not?

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