Diffusion / osmosis in general

Diffusion / osmosis in general

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The osmotic pressure

The osmotic pressure of a solution is always related to the pure solvent. It depends only on the number (concentration) of the dissolved particles, but is independent of the type of particles. A one molar aqueous glucose solution therefore has the same osmotic pressure as a one molar solution of urea in water.

The osmotic pressure (pOsm) is temperature dependent. It can be calculated from the concentration using the following equation:


A one molar glucose solution, i.e. a solution of 1 molGlucose in 1 L.Water provides an osmotic pressure of 24.5 bar, a solution of 1 molGlucose in 24.5 L. a pressure of 1 bar (at 25 ° C).

Similarities and differences between osmosis and diffusion

Osmosis and diffusion play an essential but different role in the human body. Diffusion is the process of moving molecules in an area of ​​high concentration to areas of lower concentration, while osmosis refers to the process by which water or other solvents migrate through a semi-permeable membrane and leave other pieces of matter behind. For example, oxygen diffuses into red blood cells, and salt that is outside a cell uses osmosis to draw moisture from the cell's water and dehydrate it. Although they appear to be similar, they have different mechanisms of action and goals in the many species on earth.

Diffusion follows a gradient in concentration

Gases and substances dissolved in a liquid diffuse from an area of ​​high concentration to one of low concentration. For example, when you spray perfume in the air, the volatile perfume molecules spread out in the air from the concentrated point of origin. Diffusion also occurs with or without a permeable membrane in a liquid such as water. The diffusion of small molecules across plant or animal cell membranes follows a concentration gradient. For example, if the oxygen outside a cell is higher, it will diffuse into the cell until the oxygen concentrations outside and inside the cell are equal.

Osmosis follows an increasing concentration gradient

During osmosis, water flows from a low concentration of solutes across a semi-permeable membrane to a high concentration of solutes. For example, if you add water to a blood sample that consists of plasma and red blood cells, water gets into the red blood cells and causes them to swell because the blood plasma is less concentrated than the inside of the red blood cells. However, if you add sugar or salt to the blood sample, water will leave the red blood cells, causing them to shrink and warp.

Both processes do not require any energy

Diffusion and osmosis are passive processes, which means that they do not require any energy to occur. Both are spontaneous processes. Diffusion depends on the random movement of particles or molecules. It increases with increasing temperature, as heat increases the random movement of molecules. In osmosis, water moves freely across a membrane from an area with a low concentration of solute or hypotonic solution to an area with a high concentration of solute or hypertonic solution. When the concentration of solute is the same on both sides of the membrane, the solution is said to be "isotonic". Osmosis does not achieve isotonicity in plant cells, as they are surrounded by a rigid shell, which means that a pressure builds up in the cells, an electric charge of molecules. Smaller molecules diffuse faster than larger molecules. Charged molecules do not diffuse across animal or plant cell membranes. They have to enter or leave cells through other mechanisms, as cell membranes are made up of hydrophobic lipids and repel charged molecules, much like oil repels vinegar. Osmosis is the flow of water molecules and depends on the concentration of particles - not the type of molecule on either side of the membrane.

Osmosis requires a semi-permeable membrane

Diffusion occurs with or without a membrane between two areas of different concentrations of molecules. However, osmosis only occurs through a semi-permeable membrane, a membrane that prevents many particles or molecules from moving freely between the two sides while water is being passed through. In nature, osmosis is essential for many biological processes that depend on the movement of water, such as: B. cell shape or pressure, essential

Diffusion and osmosis

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To fully understand why we can smell the cookies baking in the kitchen from the bedroom we also have to consider another process at work here - advection. Advection involves the transfer of a material or heat due to the movement of a fluid. So, because people walk through the rooms of your house and because heat rises from your radiators, the air is constantly moving, and that movement carries and mixes the scent molecules in your house. In many situations (such as your house), the effects of advection exceed those of diffusion, but these processes work in tandem to bring you the cookie smell.

From the traveling smells of cookies to the dissolving of salt into water, diffusion is a process happening around (and within!) Us every second of every day. It is a process that is critical to moving oxygen across the membranes of our lungs, moving nutrients through soil to be taken up by plants, dispersing pollutants that are released into the atmosphere, and a whole host of other events that are necessary for life to exist.


The process of diffusion is critical to life, as it is necessary when our lungs exchange gas during breathing and when our cells take in nutrients. This module explains diffusion and describes factors that influence the process. The module looks at historical developments in our understanding of diffusion, from observations of “dancing” particles in the first century BCE to the discovery of Brownian motion to more recent experiments. Topics include concentration gradients, the diffusion coefficient, and advection.

Key Concepts

Diffusion is the process by which molecules move through a substance, seemingly down a concentration gradient, because of the random molecular motion and collision between particles.

Many factors influence the rate at which diffusion takes place, including the medium through with a substance is diffusing, the size of molecules diffusing, the temperature of the materials, and the distance molecules travel between collisions.

The diffusion coefficient, or diffusivity, provides a relative measure at specific conditions of the speed at which two substances will diffuse into one another.

Diffusion / osmosis in general - chemistry and physics

The one-sided diffusion of a substance (usually water) through a semipermeable membrane is called osmosis (Greek osmos = penetration). A semipermeable (semi-permeable) membrane is characterized by the fact that it is only permeable to certain substances. Water can always pass through the membrane, but the substances dissolved in it, such as sugar or salts, cannot.
Osmosis is based on the effort of the particles to create a concentration balance between the interior and exterior of the membrane. Therefore, the water always flows from the location of the higher water potential (fewer dissolved particles), in the direction of the lower water potential (more dissolved particles). Osmotic pressure exists until the concentrations on both sides of the membrane are equalized. From this point on, the same amount of water flows in both directions (isotonic state).

Cherries are ideal for illustrating the process of osmosis, because their outer skin functions as a semipermeable membrane. In late summer, when the cherries ripen, the sugar content inside the cells is highest. A rain shower can then make the cherries burst. This is how it works: water droplets on the cherry flow into the interior of the cherry because the concentration of dissolved sugar particles is much higher on the inside than on the outside. A balance of the concentration can only be achieved by inflowing water (to a certain extent by diluting the cell sap with water), because the sugar molecules are too large to be able to leave the semipermeable membrane to the outside nnen. As a result, the cherry sucks itself full of water (osmotic pressure) until the concentration is equalized (the same number of dissolved particles inside and outside the membrane). However, since the cells of the cherry cannot expand as much as desired, the cherry will burst at some point.

Incidentally, osmosis is also responsible for wrinkled skin when bathing. The concentration of dissolved salt particles is much higher in the skin cells than in bath water. Then water flows into the cells and causes the skin to swell.

Why do English vegetables have to taste bland?

To get straight to the point: English cuisine is much better than its reputation. And I love English food, and England anyway. Breakfast is a real feast for which you should take plenty of time. And I confess: I think the mushy white English & # 8222Chips & # 8220, something like our fries, are great and enjoy them with vinegar and with a little bit of salt. Also baked beans, sausages, poached egg, lamb chops with mint sauce and the traditional Christmas pudding, a cake made from raisins and plums, are & # 8218heaven on earth & # 8216 for me. In my numerous stays in England I have never really eaten badly. And since Jamie Oliver at least, English cuisine has gained a lot in diversity.

But what about the English vegetables? This is really mostly weak. Soft, pale and tasteless.
What's wrong?
It's physics again that works against the English chefs. More precisely, there are two physical processes: osmosis and diffusion.

Osmosis is the term used to describe the movement of molecules through a semi-permeable membrane. A cell is enclosed by such a membrane. This membrane lets water through in both directions. However, other molecules are not allowed through.

Let us now consider, for example, the cells of a pea. The cells contain water and sugar and other substances. On the other side of the membrane, i.e. outside, there is only water.
Now there is a principle in this constellation: a solvent, in this case water, always strives for the side of a semipermeable membrane on which there is a higher concentration of dissolved molecules. In the pea cell, there are more molecules dissolved in the water than outside. So the water strives into the pea and dilutes the concentration there. This causes the pea to lose its taste.

In connection with the heat when cooking, however, the cell walls are also burst open because the pressure in the cell increases. This will put the pea's nutrients back into the water in the saucepan. And this is where diffusion comes in. The water molecules move very quickly when the water is boiling. As a result, the flavor and nutrients of the pea are distributed throughout the pot and after a while only the coarse cell structures remain in the pea. The water is then drained off and what remains is a relatively tasteless former pea.

But how can this be prevented? We don't want to eat the peas raw.
Here we can start at various points. First, we need to convince the water to stay outside the pea. So we have to dissolve more larger molecules in water and thus increase its density. That works with sugar and salt.

We can also ensure that the pea's cell walls remain largely intact.
This can be achieved with a lower temperature. In addition, we can work with very little water and then use it to prepare our food, e.g. for the sauce.
We can also reduce the cooking time. In this way, the cell structure of the pea is largely preserved and it tastes strong and crisp.

I don't put fresh vegetables, e.g. carrots, peppers, broccoli or cauliflower in & # 8217's water, they are only cooked with steam. An insert for the saucepan is sufficient, if necessary a sieve that fits exactly on the saucepan.
There is then only a little water with a little sugar and salt in the pot. The plate is only set to the highest level until the water boils very slightly, then switch back to the lowest level, put the lid on and the vegetables are cooked after a maximum of ten minutes. The natural color is also beautifully preserved.

So, dear Englishmen, do your continental guests a favor and be nice to your vegetables. If that's too difficult, take canned vegetables. It is cooked very gently and the flavors and vitamins are still there.

Do you still have vegetables in the fridge? Then write to me and I'll think about what you can do with it.


Osmosis is one unidirectional diffusion, mostly from water. The diffusion usually takes place through a semi-permeable membrane, i.e. a semi-permeable membrane. This only allows certain substances through and mostly this is just water.

flickr | Radarsmum67

The regulates osmotic pressure the water balance between the cells. That is why your Skin shrinks when bathing. The salt content of the bath water is much lower than that of your skin. To balance the concentration, water flows into the cells and ultimately creates a salt solution that is very similar in concentration to bath water. However, due to the additional water in the cells, they are now swollen.

As can be guessed from this example, is osmosis of central importance in biology and is often used to regulate the water balance of living things and their cells.

Video: Osmosis and Water Potential Updated (May 2022).