To understand Osmosis and Diffusion processes of Physics having a role in life sciences, before that let us recall something, we observe daily happening around us, day and night. We may call it a sort of “experiment we go through but rarely register it”. Try to visualize a room on whose window has a fine net installed to ensure flies, mosquitoes, etc., are unable to enter the room. So, during the daytime, when there is sun outside, a person walking outside across that room cannot see through the fine net of things happening inside the room, as it is relatively darker in the room compared to outside. But during nighttime, when it is dark outside and there is light in the same room, then the same person now walking on the road can see what is happening inside the room! WHY?
Well, in the first case the concentration or amount of Sunlight was more outside, hence a person inside the room was able to see outside through the fine net on the window as the light was entering the room through the process of Diffusion, but not vice versa. While at night, the light inside the room was more than outside, so the outsider can see inside through the net, as the Diffusion process has now reversed. Hence, one can say the net installed on the window is a sort of “semi-permeable membrane” allowing certain things to move across the net, but not everything, and we will find a similar thing existing in living organisms throughout the earth.
For a detailed derivation of why and how diffusion of light shown above happens, it requires a mathematical derivation of wave motion. But in a nutshell diffusion of light above is nothing but its “bending” at edges and corners resulting in the way above images appear. Now, having conceptually understood the process of Diffusion across a “membrane or partition or obstruction”, let us move to organic and biological entities.
The image below is of a fried egg. Interestingly before it is fried, we hardly notice the existence of a membrane inside an egg. Then, how can one experience the fact there are biological membranes within a cell. This gets confirmed when the yellow yolk of a raw egg is pricked say by the sharp point of a knife. The fluid inside the yellow yolk meaning the “nucleus” flows out and spreads. Hence it confirms the existence of a “biological membrane in a cell”!
Every living single-celled organism carries out the primary essential processes of life to exist viz., it must take in nutrients (energy capture), excrete wastes, detect and respond to its environment, move, breathe, grow, and reproduce. An egg is the closest example of a biological cell without going into the intricacies to look at a cell through a microscope.
Now, a human body is made up of trillions of cells of different types but having the common feature being made up of three parts: the cell biological membrane, the nucleus, and, between the two, the cytoplasm. Within the cytoplasm lie intricate arrangements of fine fibers and hundreds or even thousands of minuscule but distinct structures called organelles as shown in the general cell structure image below.
Source: 2012 Book Archive
But let us remain focused on the human body as it will be simpler to understand how nutrients flow in it. The movements of nutrients across the cellular chain in any living organism are governed by the process of Osmosis. Osmotic pressure is defined as the pressure that must be applied to the solution side to stop fluid movement when a semipermeable membrane separates a solution from pure water. So, the key element which plays a primary role is “the semi-permeable membrane” which we must explore in the context of living biological cells.
The human body as an organic entity requires nutrients to perform, survive and grow. This holds good for the growth of all organic materials we find in everyday life around us, may it be plants, animals, ants, insects, etc. In the case of human beings, the nutrients flow in the blood across the arteries to reach all cells, but let us confine two facts here. First, the human body’s blood has more water than molecules of glucose, minerals, amino acids, fatty acids, vitamins, etc. While glucose is responsible for providing “energy” in a cell to carry out its required functions. In comparison, one finds the molar mass of glucose is (180.156 gm/mol) and it is almost 10 times higher than that of water (18.02 g/mol).
The image above then describes the process of osmosis across membranes in living organisms viz., biological cells. One explanation for this is through Gibb’s free energy calculations of thermodynamics we use in Quantitative Chemistry. But, for a common man without going into the mathematical calculations and intricate Chemistry, using the first principles of Physics, can it be explained broadly. YES!
I leave it to readers to now think why the water in the above image moves from left to right but the same water molecules existing in glucose solution do not move from right to left.
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June 21, 2021