0610 · 3.1Diffusion

Defining diffusion and passive transport

0:00Spray some perfume in one corner of a room, and a few seconds later someone on the other side of the room can smell it. Nobody carried the perfume across, and there was no fan blowing it. The particles of perfume spread out on their own. That spreading has a name, diffusion, and your body depends on it every second. It is how oxygen gets from your lungs into your blood, and how it gets from your blood into your cells.

0:22By the end of this video you will be able to describe diffusion using the exact words the syllabus expects, say where the energy for it comes from, explain why it matters to living organisms, and describe how to investigate the four factors that change how fast it happens. You need one idea before we start. All particles, whether they are atoms, molecules or ions, are constantly moving. In a gas or a liquid they move about randomly, in straight lines, until they bump into each other or into the walls of their container, and then they bounce off in a new direction.

0:53Nobody controls where each particle goes. Its movement is random. Now imagine a box with a barrier down the middle. On the left side there are lots of particles, crowded together. On the right side there are none. We say the left side has a high concentration of particles, and the right side has a low concentration. Concentration just means how many particles there are in a certain space. Now take the barrier away.

1:15Every particle keeps moving randomly. Some of the particles on the left happen to move to the right. Because there are so many particles on the left and so few on the right, far more particles move from left to right than from right to left. Over time, the particles spread out until they are evenly mixed across the whole box. That is diffusion. The syllabus describes diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration, in other words down a concentration gradient, as a result of their random movement.

1:46Two parts of that sentence need a closer look. The first is the word net. Particles move in both directions all the time. Some move from the crowded side to the empty side, and some move back. Net movement means the overall movement, once you take the movements in both directions into account. Because more particles move from high to low concentration than the other way, the net movement is from high to low.

2:08The second is the phrase down a concentration gradient. A concentration gradient is a difference in concentration between two regions. Picture it as a slope, with the high concentration at the top and the low concentration at the bottom. In diffusion, particles move down the slope, from high to low, just as a ball rolls downhill. So what happens when the particles are evenly spread? They do not stop moving.

2:30They keep moving randomly, but now just as many move from left to right as from right to left. There is no longer a concentration gradient, so the net movement is zero. A common mistake is to write that the particles stop moving. They never stop. It is only the net movement that stops. Where does the energy for diffusion come from? The syllabus wants you to state that it comes from the kinetic energy of the random movement of molecules and ions.

2:56Kinetic energy is the energy of movement. The particles are already moving, so no extra energy has to be supplied. In particular, a cell does not have to use energy from respiration for diffusion to happen. That is why diffusion is called a passive process.

Diffusion in cells and organisms

3:12Diffusion also happens across cell membranes. The syllabus wants you to state that some substances move into and out of cells by diffusion through the cell membrane. Small molecules like oxygen and carbon dioxide can pass straight through the membrane. Think about a cell that is respiring. It is using up oxygen all the time, so the concentration of oxygen inside the cell stays low. Outside, in the blood or the tissue fluid, the oxygen concentration is higher.

3:39So oxygen diffuses into the cell, down its concentration gradient. At the same time, the cell is making carbon dioxide, so carbon dioxide is more concentrated inside the cell, and it diffuses out. Diffusion matters to living organisms because it is how many substances get to where they are needed. Here are the key examples. Start with gases. In your lungs, the air in the tiny air sacs has a high concentration of oxygen, and the blood arriving at the lungs has a low concentration.

4:05So oxygen diffuses from the air into the blood. Carbon dioxide does the opposite. It is more concentrated in the blood, so it diffuses out into the air sacs and you breathe it out. Plants rely on diffusion of gases too. In a leaf in the light, photosynthesis uses up carbon dioxide, so the concentration inside the leaf is low. Carbon dioxide diffuses in from the air through tiny holes in the leaf surface, called stomata.

4:31The oxygen made by photosynthesis diffuses out the same way. Diffusion of solutes matters too. A solute is a substance dissolved in a liquid, like glucose dissolved in your blood. After a meal, digested food molecules such as glucose are at a high concentration in your small intestine, so some of them diffuse into your blood. In the same way, waste substances made in cells, such as urea, diffuse out of the cells into the blood, which carries them away to be removed.

4:59And for a single-celled organism living in a pond, diffusion is how almost everything gets in and out. Oxygen diffuses in from the water, and carbon dioxide diffuses out, straight across its cell membrane.

Factors affecting diffusion rate

5:11Diffusion always happens down a concentration gradient, but it can be fast or slow. The syllabus asks you to investigate four factors that affect how fast it happens. These are surface area, temperature, concentration gradient and distance. For each one, let us see why it matters, and then how you could test it. The first factor is surface area. The surface is where particles cross from one region to another.

5:35A bigger surface means more particles can cross at the same time, so diffusion is faster. This is why your lungs contain millions of tiny air sacs. Together they give a surface about the size of a tennis court. The second factor is temperature. At a higher temperature, particles have more kinetic energy, so they move faster. Faster-moving particles spread out more quickly, so diffusion is faster. Notice that temperature does not cause diffusion.

6:01It changes how fast diffusion happens. The third factor is the concentration gradient. The bigger the difference in concentration between two regions, the steeper the slope, and the faster the net movement. This is why your blood keeps flowing past your air sacs. Fresh blood with very little oxygen keeps arriving, so the difference in concentration stays large, and oxygen keeps diffusing in quickly. The fourth factor is distance. The shorter the distance the particles have to travel, the faster they get there.

6:29That is why the walls of your air sacs and blood capillaries are extremely thin, only one cell thick. Oxygen has only a tiny distance to diffuse across. Now, how do you investigate these factors? A classic experiment uses small cubes of agar jelly. The agar is made pink by adding an alkali and an indicator called phenolphthalein, which is pink in alkali and colourless in acid. When you drop a cube into hydrochloric acid, the acid diffuses into the cube.

6:55As it does, the pink colour disappears, from the outside inwards. You time how long it takes for the cube to turn completely colourless. To investigate surface area and distance, you cut cubes of different sizes, for example one centimetre, two centimetres and three centimetres along each side, and put each one into the same volume of acid. Smaller cubes turn colourless first. A small cube has more surface for its size, and the acid has a shorter distance to travel to reach its centre.

7:24To investigate temperature, you use cubes of the same size, in acid of the same concentration, but in water baths at different temperatures, such as 20, 30 and 40 degrees Celsius. The warmer the acid, the faster the cube loses its colour. And to investigate the concentration gradient, you use cubes of the same size at the same temperature, but put them into acid of different concentrations. The more concentrated the acid, the bigger the concentration gradient, and the faster the colour disappears.

7:52In every version, change only one factor and keep all the others the same. If you are testing temperature, the cubes must be the same size and the acid must be the same concentration and volume. Otherwise you could not tell which factor caused the difference.

Agar cube exam question

8:06Let us try an exam-style question. A student timed how long agar cubes of different sizes took to turn colourless in acid. Here are some example results. The one centimetre cube took 4 minutes, the two centimetre cube took 14 minutes, and the three centimetre cube took 30 minutes. The question asks you to describe and explain the results. First, describe the pattern, using the data. As the size of the cube increases, the time taken to turn colourless increases, from 4 minutes for the one centimetre cube to 30 minutes for the three centimetre cube.

8:38Quoting numbers from the table makes your description precise. Then explain it, using the idea of diffusion. The acid enters the cube by diffusion. In a larger cube, the acid has a greater distance to diffuse to reach the centre, and the cube has less surface area compared with its volume. So it takes longer for the acid to reach the middle of the cube. Notice that a good answer uses the key words, diffusion, distance and surface area.

9:02Let us collect the common mistakes. The first is leaving out the word net. Particles move in both directions, so always say net movement. The second is saying that particles stop moving when they are evenly spread. They keep moving. Only the net movement stops. The third is saying that particles move along a concentration gradient, or up it. Diffusion is always down a concentration gradient, from higher to lower concentration.

9:29And the fourth is saying that diffusion needs energy from respiration. No, it does not. The energy comes from the kinetic energy of the particles themselves.

Diffusion recap

9:38Let us recap. Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement. The energy comes from the kinetic energy of the random movement of molecules and ions, so no energy from respiration is needed. Some substances, such as oxygen and carbon dioxide, move into and out of cells by diffusion through the cell membrane.

10:03Diffusion of gases is vital in the lungs and in leaves, and diffusion of solutes moves substances such as glucose and urea around the body. Diffusion is faster with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter distance. And when you investigate any one of these, change only that factor and keep the others the same. In the next video, we will look at a special kind of diffusion that involves water, called osmosis.