How is the Concept of Simple Diffusion Possible

How is the Concept of Simple Diffusion Possible

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How can a substance pass through a lipid membrane in a cell through simple diffusion? In order for something to be able to go through the membrane, in simple diffusion, it must be hydrophobic, or non polar. The reason for this is that in order for it to easily interact and pass the fatty acid part of the lipid membrane, which is hydrophobic or non polar, it must also be hydrophobic and non polar. But in order for this substance to pass the phosphate part of the membrane, which is hydrophilic, or polar, doesn't it also need to be polar and hydrophilic?

Simple diffusion is the movement of solutes from an area of high solute concentation to an area of low solute concentration.

Here are a few ways molecules will make it through membrane into cell:

(A) Fuse with the membrane and enter due to like polarities i.e non-polar subtance fuses with non-polar membrane and passes through membrane

(B) Small polar molecules CAN move into cell but at a much slower rate

(C) Through transmembrane integral protein channels such as aquaporins, K+ leak channels, and other channels that DO NOT require energy, and are ALWAYS open

If you want to understand how it was determined which molecules were able to pass through the membrane look for some resources about Overton's Plasmolytic Method.

Alongside that the definition of the partition coefficient will give you some insight on how fast molecules will pass through membrane, if at all.

Good luck on the searching!

@ Ro Siv - Simple diffusion is not facilitated diffusion

How is the Concept of Simple Diffusion Possible - Biology

The concept of cellular transport (diffusion, osmosis, hypotonic, hypertonic, active transport, passive transport) is fundamental to a biology class. There are so many great ideas for labs that teach and explore these concepts. Just this week, our biology students completed an activity that is so very simple, but it really illustrates the concept of semipermeable membranes.

Students are given 2 pieces of dialysis tubing. One is filled with a starch solution and the other is filled with a glucose solution. Each is placed into a cup containing tap water. The cup that contains the starch tube also has iodine added to the water in the cup.

After setting up the experiment, students are asked to wait 20 minutes before recording their results. The students immediately recognize that the dialysis tube containing starch is beginning to turn purple. The iodine molecules in the cup of water are diffusing across the dialysis tube membrane and are causing the starch molecules to turn dark blue or black. In addition to this observation, students are given a glucose test strip and asked to test for the presence of glucose in water of the second cup.

Students allow their experiment to sit overnight. Observations are made and results are recorded again after 24 hours. All students quickly determine that: (1) Starch did not leave the bag. (2) Iodine moved into the bag. (3) Glucose moves out of the bag slowly. (This is evidenced by the fact that the initial test for glucose after 20 minutes is negative, but the test is positive after 24 hours.) (4) Much water moves into the bag.

From these observations, students are asked to make predictions about the size of the molecules, and must place the molecules in order of their size from smallest to largest.

The lab that I use with my students can be found here: Diffusion Through a Non-Living Membrane .

FREEBIE: You might also want to try this lab: The Effect of Concentration on the Rate of Diffusion . This is a free download. Enjoy!


Examples of convection include rising smoke or steam, wind or water currents created as a result of varying temperatures, and the spread of cream added to coffee. In an attempt to elucidate this concept, it might be beneficial to understand the following: convection is really a combination of two things – advection and diffusion. Advection is the bulk movement of molecules in a fluid (i.e., liquid or gas) not as a result of diffusion. Different fluids placed in the same container can move for various reasons other than diffusion. Gases or liquids that have low densities will rise when placed in fluids that are denser. The momentum of a drop of ink falling into a beaker of water can propel that drop through the water as a result of gravity, like cannonballing into a pool. Differences in temperature can also create these bulk movements. For example, as water is heated, warmer water rises to the top while colder water sinks because warm water is slightly less dense than cold water.

The second half of the explanation for the movement of particles in a fluid is diffusion, but calling it “half” is misleading. Diffusion accounts for an almost insignificant portion of the net movement of fluids. Since the 1930s, researchers have established that diffusion due solely to Brownian motion (i.e., the random scattering of atoms or molecules) is an extremely slow process, on the scale of centimeters per hour (Jacobs, 1932). For example, tiny dissolved oxygen molecules in water require 12 days to diffuse 10 cm (Vogel, 1994). Larger molecules diffuse even more slowly (Storey, 1992). True diffusion of ink molecules through water, assuming that both fluids are the exact same temperature and density, would take hours if not days to reach equilibrium.

In general, common uses of the term convection refer to movement due to advection. The term does not typically take into account the minute movement that may be a direct result of diffusion. For the sake of consistency throughout this document, the term convection will be used in its common context. Also, this term should not be confused with convective heat transfer.

Considering the aforementioned, a second look at classroom perfume experiments reveals that the slow rate of diffusion cannot possibly account for the detectable smell of perfume on the far side of a room after only a minute or two. This phenomenon is most likely due to air currents created by the gases being forced out of the perfume bottle, the movement and breathing of the students, the heating/cooling ventilation system, and – perhaps the most influential – the formation of currents due to differences in density of air masses within the room that spread the perfume molecules around. Nevertheless, high school textbooks and activities continue to tout this and similar demonstrations as strict examples of diffusion. As mentioned earlier, educators often use dye dropped in a beaker of water as the “go-to” demonstration. This, too, is an example of convection currents created by gravity acting on two liquids of slightly varying densities in addition to the dispersal pattern caused by the momentum of the drop entering the water (Vogel, 1994).


  • A sugar cube is left in a beaker of water for a while.
  • The smell of ammonia spreads from the front of the classroom to the back of the room.
  • Fumes of perfume rise from the bottle when the top is removed.
  • Food coloring dropped on the beaker spreads out.
  • the smell of food spread in the whole house

Molecules tend to move from places of high concentration to places of low concentration, just by moving randomly. For example, there is more oxygen in a lung than there is oxygen in the blood so oxygen molecules will tend to move into the blood. Similarly, there is more carbon dioxide molecules in the blood than in the lung so carbon dioxide molecules will tend to move into the lung. It happens in cell biology, where small molecules simply diffuse through the cell membrane, but larger molecules only get through by using energy: see active transport.

The random movement of fluid molecules makes them spread out until a boundary stops them.

Diffusion is a passive process, therefore does not require energy as it occurs down a concentration gradient.

Osmosis and heat transfer are types of diffusion.

  • the concentration gradient - diffusion will be greater where gradient is larger
  • the temperatures - diffusion will happen faster when temperatures are higher as there is more kinetic energy
  • the surface area - diffusion will be greater where it's greater
  • the diffusion distance - diffusion will be greater where there is a short diffusion distance

In small unicellular organisms, simple diffusion can exchange molecules quickly enough to keep them alive. A high surface area to volume ratio helps.

However, for multicellular organism simple diffusion is not enough. They need to move more material over longer distances to remain alive. They have evolved to have internal structures and systems for rapid distribution movement. For example, humans have lungs to make diffusion happen rapidly. The same happens in plants with the leaf.

New Models of the Cell Nucleus: Crowding, Entropic Forces, Phase Separation, and Fractals


Diffusion is the basic mode of transport for molecules in living cells. Diffusion leads to dispersion of individual molecules, but it is also the driving force behind biochemical reactions and pattern formation as diffusional motion mediates reactant encounters. Owing to macromolecular crowding in all cellular fluids and biomembranes, diffusion of molecules in cells is quite different from the motion observed in dilute solutions in a test tube. Hindered and anomalous diffusion are seen in cells, and biochemical reactions are affected by these. This review is intended to give an introduction and a brief overview about causes and consequences of crowding-induced diffusion anomalies and their impact on biochemical reactions.

10 Examples Of Diffusion In Everyday Life

Have you ever wondered why the fragrance of your perfume or the incense sticks lightened by your mom during prayers spread all over to your home? It all happens due to diffusion. Diffusion is a fundamental factor in both natural and human-made processes. Being a universal physical phenomenon, we deal with it throughout our daily life.

“Diffusion is the movement of particles from the area of higher concentration to lower concentration area, continuing until equilibrium is reached”.

Let’s check some notable examples of diffusion that occurs in our daily life.

1. Perfumes/Incense Sticks

When perfume (scents, incense sticks, room sprays, fragrance sprays) are sprayed at one part of the room, it spreads throughout the whole room due to diffusion. Perfume particles travel from higher concentrated area to the entire room where the concentration is less.

2. Helium Balloons

Helium balloons deflate slowly and lose their lift. This happens because helium diffuses from the helium-rich area (balloon) to low helium area (outside environment).

3. Tea Bags

When we put tea bags into a cup of water, it automatically mixes in the whole cup of tea, and it happens due to diffusion. Tea bag contents diffuse from its higher concentration to lower concentration (water in the mug).

4. Soda/Cold Drinks

After a few seconds of opening a soda bottle, soda goes flat. This is because the CO2 (Carbon dioxide) concentration is higher in the bottle than the outside environment and hence, CO2 diffuses from its higher concentration to its lower concentration.

5. Breathing

When we breathe the air, the inhale of oxygen and exhale of carbon dioxide is possible only because of the process of diffusion. Therefore, diffusion is a vital process in breathing.

6. Air Pollution

Apart from having many household uses of diffusion, it also brings some cons with it air pollution being the most prominent cons caused by diffusion. When harmful gases, fumes, and toxic particles are released from various human-made sources including factories (like cement factory, chemical factories, brick kilns, etc), vehicles, and waste burning then .they pollute the normal air with the process of diffusion.

7. Transport Of Minerals and Biomolecules in Plants and Animals

Our survival is very much dependent upon the process of diffusion as in absence of diffusion our body cannot function properly because it’s diffusion which facilitates the smooth transport of minerals and biomolecules in our body.

8. Removal of Toxins and Waste Substances from Our Body

Our body continuously needs to remove the toxic and waste products produced during metabolism and the most important organ to accomplish this activity are kidneys however, Kidneys alone can’t do this task, here also the concept of diffusion come into play. Kidneys are made up of nephrons, which are the structural and functional units of kidneys. They are microscopic tubules which filter toxic substance from the blood. Nephron first separate waste chemicals and toxic from the blood and then reabsorb water and nutrients in the blood through diffusion. Hence, diffusion plays a significant role also in the filtration of the blood.

9. Food Coloring

The benefits of diffusion have been widely harnessed by the food industry, where coloring of edible foods is common in use.

10. Heat Conduction

Heat conduction or transfer also occurs through diffusion. Heat is transferred from higher temperature to lower temperature.

Why Does Diffusion Occur?

A characteristic feature of diffusion in chemistry is that it requires no net input of energy to occur. Molecules will spontaneously spread out to fill a space without being pushed around by any external force or input. Of course, the question arises, if diffusion requires no energy to occur, then what drives the process?

Consider the point this way, for any sample of gas in a space, there is a finite amount of ways that the particles in that gas could occupy that space. The overwhelming majority of these possible arrangements are ones where the gas is more evenly distributed in the space than not. Therefore, any random motion of molecules is more likely to put the system in a state that is closer to diffusion equilibrium simply because the vast majority of possible states the system could be in are ones where the particles in the gas are spread out evenly.

In simpler terms, diffusion is a statistical result of the random motion of particles. Nothing “causes” diffusion to occur in the sense of an external agent influencing a system. Given enough time, two substances will diffuse into each other simply because it is more likely to occur than not. Diffusion is a statistical truth about the possible physical arrangements of a system.

Top 5 Experiments on Diffusion (With Diagram)

The following points highlight the top five experiments on diffusion. The experiments are: 1. Diffusion of Solid in Liquid 2. Diffusion of Liquid in Liquid 3. Diffusion of Gas in Gas 4. Comparative Rates of Diffusion of Different Solutes 5. Comparative rates of diffu­sion through different media.

Experiment # 1

Diffusion of Solid in Liquid:

A beaker is almost filled with water. Some crystals of CuSO4 or KMnO4 are dropped carefully without disturbing water and is left as such for some time.

The water is uniformly coloured, blue in case of CuSO4 and pink in case of KMnO4.

The molecules of the chemicals diffuse gradually from higher concentration to lower concentration and are uniformly distributed after some time. Here, CuSO4 or KMnO4 diffuses independently of water and at the same time water diffuses independently of the chemicals.

Experiment # 2

Diffusion of Liquid in Liquid:

Two test tubes are taken. To one 30 rim depth of chloroform and to the other 4 mm depth of water are added. Now to the first test tube 4 mm depth of water and to the other 30 mm depth of ether are added (both chloroform and ether form the upper layer).

Ether must be added carefully to avoid disturbance of water. The tubes are stoppered tightly with corks. The position of liquid layers in each test tube is marked and their thickness measured.

The tubes are set aside for some time and the thickness of the liquids in each test tube is recorded at different intervals.

The rate of diffusion of ether is faster than that of chloroform into water as indicated by their respective volumes.

The rate of diffusion is inversely proportional (approxi­mately) to the square root of density of the substance. Substances having higher molecular weights show slower diffusion rates than those having lower molecular weights.

In the present experiment ether (C2H5-O-G2H5, J mol. wt. 74) diffuses faster into water than chloroform (CHCI3, mol. wt. 119.5). This ratio (74: 119-5) is known as diffusively or coefficient of diffusion.

Experiment # 3

Diffusion of Gas in Gas:

One gas jar is filled with CO2 (either by laboratory method: CaCO3 + HCL, or by allowing living plant tissue to respire in a closed jar). Another jar is similarly filled with O2 (either by laboratory method: MnO2 + KClO2, or by allowing green plant tissue to photosynthesize in a dosed jar). The gases may be tested with glowing match stick.

The oxygen jar is then inverted over the mouth of the carbon dioxide jar and made air-tight with grease. It is then allowed to remain for some time. The jars are carefully removed and tested with glowing match stick.

The glowing match sticks flared up in both the jars.

The diffusion of CO2 and O2 takes place in both the jars until finally the concentrations are same in both of them making a mixture of CO2 and O2. Hence the glowing match sticks flared up in both the jars.

Experiment # 4

Comparative Rates of Diffusion of Different Solutes:

3.2gm of agar-agar is completely dissolved in 200 ml of boiling water and when partially cooled, 30 drops of methyl red solution and a little of 0.1 N NaOH are added to give an alkaline yellow colour. 3 test tubes are filled three-fourth full with agar mixture and allowed to set.

The agar is covered with 4 ml portion of the following solutions, stoppered tightly and kept in a cool place:

(a) 4 ml of 0-4% methylene blue,

(b) 4 ml of 0.05 N HCl, and (4.2 ml of 0.1ml HCL plus 2 ml of 0-4% methylene blue.

The diffusion of various solutes is recorded in millimeters after 4 hours. The top of the gel should be marked before the above solutions are added.

The rate of diffusion of HCL alone (tube b) is faster compared to the combination of methylene blue and HCl (tube c) and minimum in case of methylene blue alone (tube a).

Different substances like gases, liquids and solutes can diffuse simultaneously and independently at different rates in the same place without interfering each other.

HCL being gaseous in nature and of lower molecular weight can diffuse much faster than methylene blue which is a dye of higher molecular weight having an adsorptive property. Hence in combination, these two substances diffuse more readily than methylene blue alone.

Experiment # 5

Comparative rates of diffu­sion through different media:

Two apparatus for measuring comparative rates of diffusion through gas and liquid are set up (Figure 2). Tube 1 is filled with a 2% agar-sol containing 1 ml of methyl red indicator and 1 drop of 0.1 N NaOH (in alkaline medium methyl red is yellow and in acidic medium red).

When the agar sets in the tube, it is held over a small bottle containing conc. HCL. The distance to the diffu­sion front as indicated by the red colour line is measured at suitable time intervals and the rate of diffusion of HCL gas into agar-gel in millimeter per hour is recorded.

A strip of filter paper approximately of equal length and internal diameter of tube 2 is cut, soaked in methyl red indicator containing a little 0.1 N NaOH (the strip is coloured yellow) and suspended in tube 2.

The tube is held over a small bottle containing conc. HCL as in tube 1. The rate of diffusion of HCl through the gaseous medium surrounding the strip is recorded by noting the colour change of the strip.

The rate of diffusion of HCl gas is faster in case of tube 2 containing filter paper strip and slower in case of tube 1 containing agar-gel.

The rate of diffusion of gases through a medium is inversely related to the density of the medium. Hence HCL diffuses faster in gaseous medium (tube 2) than in semisolid medium (tube 1).

What Is Cell Diffusion?

Cellular diffusion is the process that causes molecules to move in and out of a cell. Molecules move from an area of high concentration to an area of low concentration. When there is a higher concentration of molecules outside of a cell, then more molecules enter the cell than leave. When there is a higher concentration of molecules inside of a cell, then more molecules leave the cell than enter.

When molecules are evenly distributed, then an equal number of molecules are entering and leaving the cell. This is called equilibrium.

It is possible to observe osmosis, or the diffusion of water across a cell membrane, by performing a simple experiment using two slices of potatoes, two glasses of water and table salt. Fill one glass with water, and place a slice of potato in the water. Fill another glass with water, and add a slice of potato and 2 tablespoons of table salt. Let the potato slices soak. Then observe the slices.

The potato slice in the salt water look smaller and feel mushier. This is because more water molecules left the potato's cells than entered them. The water molecules moved to where there was less concentration of water molecules (the salt water solution surrounding the potato).

The potato slice in water only looks bigger and feels firmer. This is because there is more salt and other dissolved chemicals (solutes) within the potato. The water molecules moved from the outside area where there is a higher concentration of water molecules to the potato's cells, where there is more solute and less concentration of water molecules.

Small Intestine

The small intestine is part of the digestive tract and is responsible for the digestion of food and absorption of nutrients. The lining of the small intestine is covered by epithelial cells with tiny hair-like follicles known as micro-villi. Lipids can diffuse directly into the epithelial cells lining the small intestine where they are then processed by organelles. Other molecules such as amino acids are transferred into the epithelial cells with a process known as facilitated diffusion. In this process special transfer proteins within the membranes of epithelial cells help to remove the molecules from the small intestine.

The cornea in the eye does not have any blood vessels supplying oxygen to its cells. This makes the eye unusual in that it instead obtains the required oxygen by diffusion from the atmosphere. Oxygen first dissolves within the tears of the eye and then diffuses into the cornea. Similarly, carbon dioxide waste diffuses out of the cornea and into the atmosphere.

Watch the video: What is Diffusion. Simple Diffusion. Facilitated Diffusion. Biology (September 2022).


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