Tag Archives: diffusion

What is osmosis — the most important principle in biology

Osmosis is a biophysical phenomenon in which water (or another solvent) moves from a less concentrated solution to a more concentrated solution through a partially permeable membrane (in other words, it lets some particles pass, while blocking others).

The solvent will maintain this migration until equilibrium in concentration is reached.

So whenever there’s a net migration of the water molecules from a solution that has a low solute concentration towards one that has a higher solute concentration, we call this phenomenon osmosis. This movement is also sometimes referred to as “down the concentration gradient”.

Osmotic pressure is the force required to prevent water movement across the semipermeable membrane.

The term osmosis, which is Greek for ‘thrust’ or ‘impulse’, was first coined by J.A. Nollet, who in 1747 described an experiment in which he used an animal bladder to separate two chambers containing water and wine. He noticed that the volume in the chamber containing wine increased and, if the chamber was closed, pressure rose.

How osmosis works

A classic experiment for osmosis involves splitting a beaker of water into two halves, with a semipermeable membrane in between and salt added to one of the sides. You’ll soon notice water migrating from the side of the beaker with no salt at all to the side with the saline solution. This movement of water will continue until the concentration of salt is the same on both sides.

It’s the same reason why you should never put a snail near salt, which would cause the poor creature to die as its water is extracted.

Key to osmosis is the presence of a semipermeable membrane that makes it more likely for water molecules in a low concentration solution to collide with the membrane and pass through, whereas water molecules in a concentrated solution will have far fewer molecules of water colliding with the membrane and passing through. This mismatch means that there’s a greater statistical probability of more water molecules passing through the membrane from a less concentrated solution. Once the statistical probability of water molecules passing through the membrane is equal, osmosis stops.

Osmosis in nature

Osmosis is one of the essential processes of life. Each cell of our body, plants, and animals around us owe their survival to osmosis.

Take plants, for instance. When we water them, we pour it on the stem end and soil. If the plant’s cells are surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cells, water will enter the leaves, fruits, and flowers by osmosis. During this process, the plant cell will become firm.

However, if a plant is surrounded by a solution that contains a lower concentration of water, then the water molecules of the solution inside the plant’s cells will be expelled by osmosis, turning the plant flaccid.

When we water plants, we usually water the stem end and soil in which they are growing. Hence, the roots of the plants absorb water and from the roots, water travels to different parts of the plants; be it leaves, fruits or flowers. Every root acts as a semipermeable barrier, which allows water molecules to transfer from high concentration (soil) to low concentration (roots). Roots have hair, which increases surface area and hence the water intake by the plants.

Perhaps a more relatable example is within our own bodies. When we drink water, cells absorb it by osmosis just like plant roots. The cell wall acts as a semipermeable membrane, creating osmotic pressure between the inside and outside of the cell. Blood is a more dilute solution than the cell’s cytoplasm, so water will cross through the cell wall. The same applies for nutrients and minerals, which are also transferred by osmosis.

Humans have recognized the potential of osmosis since antiquity, employing it to preserve foods. The ancients observed that adding salt or sugar removes water from tissues. At the time, the process was called imbibition due to the fact that solutes like salt and sugar attracted the water from the material they touched.

What’s the difference between osmosis and diffusion

Diffusion and osmosis are both passive transport processes, meaning they require no energy input to move substances. Both processes are essential to the proper functioning of biological processes such as the transport of water or nutrients between cells.

The main difference between the two is that diffusion can occur in any mixture, even when two solutions aren’t separated by a semipermeable membrane, whereas osmosis exclusively occurs across a semipermeable membrane.

Diffusion makes air composition uniform by redistributing chemical species, such as oxygen in the air, until equilibrium is reached: in other words, until the concentration gradient — the difference in concentration between two areas — has been eliminated. If the concentration of a species is not initially uniform, over time, diffusion will cause a mass transfer in favor of a more uniform concentration.

Bottom line: osmosis — the natural movement of water into a solution through a semipermeable membrane — is central to all of biology. It is a passive transport process like diffusion, but the two are distinct.

osmosis-and-diffusion-venn-diagram

What’s the difference between diffusion and osmosis

osmosis-and-diffusion-venn-diagram

Ven diagram of osmosis vs diffusion.

Diffusion and osmosis are both passive transport processes, meaning they require no energy input to move substances. Both processes are essential to the proper functioning of biological processes such as the transport of water or nutrients between cells.

The main difference between the two is that diffusion can occur in any mixture, even when two solutions aren’t separated by a semipermeable membrane, whereas osmosis exclusively occurs across a semipermeable membrane.

There are actually three types of passive transport processes. Besides diffusion and osmosis, there’s also facilitated diffusion. While diffusion and osmosis do not involve proteins when transporting substances, facilitated diffusion needs the assistance of proteins.

What’s diffusion?

Animation of a volume of solution which is initially nonuniform. Red is a high concentration of solvent while blue is the pure solute. Over time, diffusion causes the solution to even up in concentration. Credit: Comsol.

Animation of a volume of solution which is initially nonuniform. Red is a high concentration of solvent while blue is the pure solute. Over time, diffusion causes the solution to even up in concentration. Credit: Comsol.

Diffusion is the passive movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. Inside cells, diffusion is the transport of small molecules across the cell membrane.

Molecules are always in motion. Temperature, a physical quality people commonly reference in their daily lives, is directly related to molecular motion. It is a measure of the average kinetic energy of the molecules in a material. The energy of the molecules causes random motion which in turn triggers diffusion. Collisions between molecules are common: even in the air at atmospheric pressure, a molecule collides with a neighbor every few nanoseconds.

Across the planet, the air inside the atmosphere has the same composition and is comprised of nitrogen (78%), oxygen (about 21%), argon (almost 1%), and other gases like CO2 that are present in minute quantities (but still enough to warm the planet at an accelerating rate).

Diffusion makes air composition uniform by redistributing chemical species, such as oxygen in the air, until equilibrium is reached: in other words, until the concentration gradient — the difference in concentration between two areas — has been eliminated. If the concentration of a species is not initially uniform, over time diffusion will cause a mass transfer in favor of a more uniform concentration.

Yellow food coloring diffusing through water. The glass on the left contains hot water while the glass on the right is filled with colder water. Despite coloring was added to the hot water slightly after the cold water, it diffused more thoroughly in this glass. This effect is due to the higher kinetic energy of the hot water. (animation is 2x real-time).

Yellow food coloring diffusing through water. The glass on the left contains hot water while the glass on the right is filled with colder water. Even though the food coloring was added to the hot water slightly after the cold water, it still diffused more thoroughly in this glass. This effect is due to the higher kinetic energy of the hot water. (Animation is 2x real-time). Credit: Austin Community College.

Once in equilibrium, the movement of molecules does not stop because their kinetic energy is the same. There is now an equal movement of chemical species in both directions.

The factors affecting diffusion are:

  • concentration gradient;
  • temperature;
  • distance particles must travel.

Let’s look at some examples of diffusion in action. Spraying perfume in a room will make it smell nice for a little while, but over time diffusion will distribute the odor molecules until their concentration is imperceptible to the human nose. Dropping food coloring in a cup of water, which will change the color of the whole solvent (water), is another great example of diffusion

Diffusion is a widespread and important process for both nonliving and living systems. To enter and exit a cell, substances like water or nutrients have to pass through the semipermeable membrane. Diffusion is one of the processes that enable this. A semipermeable or selectively permeable membrane is a membrane that allows some substances to pass through easily while other substances travel through very slowly or not at all.

Since diffusion occurs under a variety of conditions, scientists classify several types of diffusion.

  • Simple diffusion is the most common kind of diffusion, where substances are transported without the help of proteins.
  • Facilitated diffusion requires transport proteins to diffuse substances across a cell’s membrane.
  • Dialysis is the diffusion of solutes across a selectively permeable membrane.
  • Osmosis is usually defined as the diffusion of water, the solvent of choice in all living systems, across a selectively permeable membrane.

What is Osmosis

Osmosis, a type of diffusion, represents the movement of water across a partially-permeable membrane, from an area of high water concentration to an area of low water concentration.

Osmosis takes place in all cells. For instance, when placed in water, red blood cells will let the water creep through their membrane. When placed in a concentrated solution of sugar, the red blood cell actually shrinks because the water moves out by osmosis towards the area of lower water concentration. This is why the cells appear wrinkled when viewed through a microscope. Luckily, this never happens inside the body because the kidneys make sure the concentration of the blood stays about the same as the concentration of the solution inside the red blood cell.

Unlike red blood cells, plant cells have a far stronger and more rigid cell wall on the outside of the cell membrane. This enables the plant cells to absorb more water by osmosis without bursting. Without osmosis, plants wouldn’t be able to absorb water from the soil.  As more water is absorbed, the cell themselves become rigid due to the pressure — this is very useful since plants don’t have skeletons. If plant cells lose too much water by osmosis, they become less rigid, and eventually, the cell membrane shrinks away from the cell wall.

osmotic_pressure

Credit: Wikimedia Commons.

When osmosis is used to equalize concentrations on both sides of the membrane, it exerts a force called osmotic pressure. For instance, picture two compartments in a tank separated by a semipermeable membrane that only allows water molecules to pass through. One compartment is filled with a salt solution, while the other adjacent compartment is a pure water solution. The only way equilibrium can be reached is by transporting water from the pure water compartment to the saltwater compartment. In doing so, osmosis raises the level of liquid in the saltwater compartment until enough pressure caused by the difference in levels between the two compartments stops the processes. The pressure it takes to reach this equilibrium is called the osmotic pressure.

There’s also such a thing as reverse osmosis, which is literally the reverse process of osmosis, where the solvent filters out of the high concentrate into the lower concentrate solution. In other words, instead of seeking an equal balance of solvent and solute in both solutions, reverse osmosis separates the solute from the solvent.

Reverse osmosis is very handy for applications like water desalination (removing salt from seawater). Worldwide, there are now over 13,000 desalination plants in the world. In reverse osmosis, we are (literally) just reversing the process by making our solvent filter out of our high concentrate and into the lower concentrate solution, so instead of creating an equal balance of solvent and solute in both solutions, it is separating out solute from the solvent.

Greek and Armenian orphan refugees experience the sea for the first time, Marathon, Greece.

The ‘Last Pictures’ project: a time capsule set to orbit Earth, built to outlive the human race by billions of years

Glimpses of America, American National Exhibition, Moscow World's Fair, 1959.

Glimpses of America, American National Exhibition, Moscow World’s Fair, 1959.

Do you remember Carl Sagan’s Voyager Golden Records? When the now iconic first  Voyager spacecraft launched in 1977, a series of phonograph records containing 116 sounds and images selected to portray the diversity of life and culture on Earth were also attached onboard. These were inserted with the idea in mind that some alien race or possibly even an other human civilization that lost its roots might find it and explore the planet Earth of that time. Currently, Voyager-1 is slated to become the first man-made object to leave our solar system.

Most likely drawing inspiration from Sagan’s work, a recent project called Last Pictures seeks to put a time capsule in Earth’s orbit, harboring exactly 100 images that represent the human race. The idea is that billion of years from now the human race would cease to exist, at least in the current form, and thus such a time capsule would offer the possibility to an alien explorer to browse through the lives of what was once the dominant species on planet Earth.

Greek and Armenian orphan refugees experience the sea for the first time, Marathon, Greece.

Greek and Armenian orphan refugees experience the sea for the first time, Marathon, Greece.

Two huge problems surfaced for Trevor Paglen and colleagues. First of all, the logistics are difficult to come by, especially for an unfunded project such as this. The photos would have to last for millions of years, after of course they wind up in space…somehow. Secondly, how to choose the perfect 100 images that portray our race?

“Any group of people would come up with 100 totally different images, but that is part of the fun. It’s an impossible project. Part of it was to engage peoples’ imaginations,” says artist Trevor Paglen,

Eventually, the team whittled down the image selection from 100,000, then to 10,000 and lastly to 100. These were then arranged in a tiny 10-by-10 nanogrid, etched on a single silicon disk. This highly important part of the project was undertook by MIT scientists, Brian Wardle, associate professor at MIT’s Department of Aeronautics and Astronautics and director of the Nano-Engineered Composite aerospace Structures (NECST) Consortium, and his colleague  Professor Karl Berggren, a quantum nano-structures expert. Their greatest challenge was to make the nano-etched disk stand the test of time.

The Artifact is a ten-by-ten grid of 100 nano-etched images.

The Artifact is a ten-by-ten grid of 100 nano-etched images.

Diffusion causes molecules to move away over the course of time, which is why very old photographs look pale, lose their sharpness and so on. The scientists surpassed this problem, and made the time capsule diffusion free for millions of years ahead.

“By using a single material, Silicon, and etching physical features in that material, the Artifact will maximally resist diffusion. Usually the ‘sands of time’ erase writings through erosion, but in this case we used sand/Silicon against time to resist its effect,” says Wardle.

The last and final challenge of the project was getting the time capsule in orbit. After many failed attempts, they eventually managed to convince EchoStar Corporation, a Colorado-based telecommunications company responsible for maintaining Dish Network’s satellite fleet, to let them hitch a ride. So, on November 20th the Last Pictures time capsule will be launched into orbit, attached to the  6,600 kilograms EchoStar XVI. The host satellite will only broadcast signal and orbit Earth for 15 years. In 2027, at the end of its mission, the satellite is scheduled to retire in a safe orbit, just beyond the Clarke Belt. Hopefully space junk or meteorites won’t impact the satellite, despite over the course of millions of years this becomes a rather sound possibility.

Like the project founders themselves admit, this is a highly subjective list of images. Anyone else in the world would each pick a set of different images, still I get the felling this is more of an artsy project than an actually practical time capsule. See more images on Wired, and comment below this post with your opinion.

 

This figure shows that the object in the center of the cloak stays cold, while the heat diffuses elsewhere. The source of the heat, which is at a constant temperature of 100 °C, is on the left-hand side, while the material inside the invisibility region remains cold. (Image: Sebastien Guenneau, Institut Fresnel, CNRS/AMU)

Thermal cloaking renders heat invisible

Cloaking has turned into a subject of great interest for scientists in the past decade, most likely because of its military potential. We’ve seen some exciting prototypes developed, from optical invisibility cloaks to temporal cloaks, and now French scientists at the University of Aix-Marseille have added a new member to the cloaking family, one that renders heat invisible.

This figure shows that the object in the center of the cloak  stays cold, while the heat diffuses elsewhere. The source of the heat, which is at a constant temperature of 100 °C, is on the left-hand side, while the material inside the invisibility region remains cold. (Image: Sebastien Guenneau, Institut Fresnel, CNRS/AMU)

This figure shows that the object in the center of the cloak stays cold, while the heat diffuses elsewhere. The source of the heat, which is at a constant temperature of 100 °C, is on the left-hand side, while the material inside the invisibility region remains cold. (Image: Sebastien Guenneau, Institut Fresnel, CNRS/AMU)

Whereas cloaking research so far was concerned with manipulating wave trajectories, like electromagnetic, sound, elastodynamic or  hydrodynamic  waves, the French researchers, lead by Sebastien Guenneau at Centre National de la Recherche Scientifique (CNRS), focused on the  physical phenomenon of diffusion, rather than wave propagation.

“Our key goal with this research was to control the way heat diffuses in a manner similar to those that have already been achieved for waves, such as lightwaves or sound waves, by using the tools of transformation optics,” Guenneau said.

The researchers devised a mathematical model which proves a set of concentric rings,  each made out of different materials of varying diffusion coefficients, can cloak heat. Arranging the materials, the heat was found to flow centrally, so that heat diffuses around an invisibility region, which is protected from heat. Thus, an infrared camera wouldn’t be able to pick up the object since there is no temperature difference between that region and the ambient.

“Heat isn’t a wave — it simply diffuses from hot to cold regions,” he said. “The mathematics and physics at play are much different. For instance, a wave can travel long distances with little attenuation, whereas temperature usually diffuses over smaller distances.”

Thermal cloaking of man-sized objects wasn’t attempted yet, though. Instead, the researchers focused on micro-sized objects for which their mathematical model was conceived. However, this is in the size range of most electronic components, where thermal cloaking might have a tremendous impact by improving cooling. Also, the ability to concentrate heat could prove useful to the solar industry. Of course, scaling the model to larger sized objects will be attempted in the future.

The findings were reported in the journal Optics Express.

[source Photonics]