Osmosis & Cell Structure

"Osmosis" is one of many scientific terms that has seeped into everyday language in a way that doesn't quite retain the original meaning.

For example, if you have a roommate who excels at a particular game that you don't play yourself, but discover that you have a flair for the game on your first try, you might joke that you picked up some skills "by osmosis" – that is, by watching your roommate play or merely by being in close physical proximity.

Osmosis in biology has a more formal and limited definition. It does not quite mean what its colloquial use in the above example implies, which would be a flow of something (skills and information) into some other region (your brain) as a result of mere physical proximity to the source. Instead, certain physical criteria have to be met.

Welcome to the world of water and solute transport in cells!

Osmosis is **the net movement of water (H₂O) from an area of high H₂O concentration to a region of low H₂O concentration through a selectively permeable membrane.** There are no wasted words here, so a deeper exploration of this definition is required in order to fully explain osmosis and how it differs from other forms of membrane transport.

First, fix in your mind the idea of a semi-permeable, or selectively permeable, membrane. It is a barrier, but one that allows some substances to pass while barring the passage of others. In some cases, water can flow freely back and forth across such a membrane, while solid particles of a certain size are excluded. This is precisely the principle of a common kitchen sieve strainer or colander.

Imagine a household aquarium divided into two equal halves by an impermeable membrane (basically, a wall). Each half is filled with pure water containing no other ingredients, or _solutes. Now imagine pouring x particles of fish food into one half of the tank and 2x particles of the same product into the other. A few minutes later, you press a switch and the membrane becomes permeable to water, but not to the fish food particles_.

What happens next?

Concentration, in the context of biological systems, is often called tonicity. This refers to the ratio of the amount of something that is dissolved in water (the solute) to the amount of free water, i.e., water alone.

The higher the tonicity, the "stronger" and more concentrated it is, because a larger amount of whatever is "tainting" the water is present. Thus sea water, which contains an abundance of salt, has a much higher tonicity than tap water, which contains only trace amounts of salt.

The solute plus the water in which it is dissolved together form a **solution**. It is often useful in biology to want to compare the tonicity of different solutions, in part to determine the direction of osmotic influence, if any. The terminology employed in this comparisons is as follows:

• Isotonic: The solutions compared have an equal concentration of solutes.
• Hypertonic: The solution with the higher concentration of solutes than the other.
• Hypotonic: The solution with the lower concentration of solutes than the other.

In the current context, your interest in osmosis lies in how this occurs within and between **cells**, and hence within living organisms. Cells are often described as "the building blocks of life," and indeed, they are the smallest distinct "things" that possess all of the properties of life as a whole. But what exactly are cells?

At a minimum, a cell has four elements: A plasma membrane (cell membrane) enclosing the cell; genetic (i.e., heritable) material in the form of deoxyribonucleic acid, or DNA; cytoplasm, which makes up the gelatinous majority of the cell's interior; and ribosomes, which manufacture proteins.

The simplest cells belong to prokaryotic organisms, such as bacteria; usually, the prokaryotic cell is the entire prokaryotic organism. In contrast, eukaryotic cells – found in eukaryotes such as fungi, plants and yourself – have a number of specialized inclusions called organelles. They also have their DNA enclosed in a nucleus.

The cell membrane, also called a plasma membrane, is functionally a semi-permeable membrane, allowing the passage of certain molecules ("solutes") but not all of them. Not all of them pass by the same mechanism, as you'll see. A perhaps more apt description of the cell membrane is "selectively permeable."

The cell membrane consists of two layers of **phospholipid molecules**. The tail ends of these molecules, the lipids, point toward each other to form the interior of the membrane; the phosphate heads of the phospholipids, on the other hand, face the exterior of the cell on one side and the cytoplasm on the other.

Importantly, other structures within the eukaryotic cell also have phospholipid bilayer, i.e., double plasma, membranes. These include the mitochondria, the chloroplasts found in plants and the nucleus.

Osmosis has been mentioned already, and is dealt with again soon enough. Another way things can move across a membrane – provided the membrane is at least semi-permeable – is through simple diffusion. In this case, molecules and water can both pass freely across the membrane. The solute molecules will tend to move from areas of higher concentration to areas of lower concentration, down what is called their diffusion gradient.

In **facilitated diffusion**, a protein "shuttle" is required in order to move the solute molecules across the membrane, owing to characteristics such as different electrostatic properties of the solute and the biological membrane. In active transport, a transmembrane protein embedded in the phospholipid bilayer uses energy to move the molecule across the cell membrane.

A detailed example of osmosis can be provided with the terms for solutions of differing tonicities having been offered.

Suppose you have a 1-liter solution of water containing 10 grams of dissolved sugar and a second 1-liter solution containing 20 grams of dissolved sugar. If these are separated by a membrane across which only water can pass, in what direction will the water move?

In this case, the 20g solution is hypertonic to the 10g solution, so water will tend to flow across the membrane toward the 20g solution. Water will accumulate on this side of the membrane until the concentration of sugar in the two compartments is balanced.

The process of osmosis functions to keep the cells in the body and the membrane-bound structures within them, healthy and operational. This requires keeping the tonicity of the interior of cells in a relatively narrow range.

Various experiments with **red blood cells** have demonstrated this nicely. The insides of these cells are isotonic to blood fluid, which is why they maintain a constant shape in these conditions. But if red blood cells are placed in plain water, they burst, because water rushes into the cell toward the extremely hypertonic interior.

If red blood cells are placed instead in extremely salty water, what do you suppose happens? If you guessed that water rushes out of the cells this time, you're right. The result is that the cells collapse inward and become "spiky" in appearance.