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© Boardworks Ltd 2003 Key Stage 4 Diffusion. © Boardworks Ltd 2003 A slide contains teacher’s notes wherever this icon is displayed - To access these.

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Presentation on theme: "© Boardworks Ltd 2003 Key Stage 4 Diffusion. © Boardworks Ltd 2003 A slide contains teacher’s notes wherever this icon is displayed - To access these."— Presentation transcript:

1 © Boardworks Ltd 2003 Key Stage 4 Diffusion

2 © Boardworks Ltd 2003 A slide contains teacher’s notes wherever this icon is displayed - To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000). Normal ViewNotes Page View Teacher’s Notes Flash Files A flash file has been embedded into the PowerPoint slide wherever this icon is displayed – These files are not editable.

3 © Boardworks Ltd 2003 Have you ever wondered why cooked food can be smelt some distance away? …or why a glass of water will turn a different colour when concentrated juice is added to it? How do dissolved substances get across boundaries within the body?

4 © Boardworks Ltd 2003 Well the answer to these questions can be found if we look inside the human body. There is a special scientific process that is controlling these situations and without it, you would not be able to live your own life! It has an essential role in the organ systems that control your breathing, circulation, digestion and many other processes. The scientific process is known as…. DIFFUSION ….or why you can smell peoples aftershave or perfume when they walk past you?

5 © Boardworks Ltd 2003 The definition of diffusion is…. "…the spreading of a gas or any substance in solution from a higher to a lower concentration." What has this got to do with food, perfume, juice and more importantly us! Well, let's start at the beginning…. All things on Earth are made of particles. These are atoms and molecules. The particles that make up a substance have varying abilities to naturally move.

6 © Boardworks Ltd 2003 ICE LIQUID WATERWATER VAPOUR Which state, solid liquid or gas allows the particles to move most freely?

7 © Boardworks Ltd 2003 Gas particles will move the most freely, followed by liquid particles and in last place are the tightly bound solid particles. SolidLiquidGas The fact that particles can move at all means that they will naturally spread out to fill a given space. For liquids, remember how they will fill a beaker, whatever the shape For gases, remember how smoke from a small fire will fill a room

8 © Boardworks Ltd 2003 We can represent this movement by using a simple diagram: Here is the position of particles before movement. Particle Area A Area B QUESTION Over time, what will happen to the particles?

9 © Boardworks Ltd 2003 Diffusion Animation

10 © Boardworks Ltd 2003 They will spread out to fill the space in which they are allowed to move. We can call this movement diffusion because the particles moved from Area A where they were highly concentrated to area B where they were in low concentration. ANSWER

11 © Boardworks Ltd 2003 If the particles were part of a solid, the bonds between each particle and the next would be too strong to allow movement. Therefore, diffusion is concerned with the movement of gas and dissolved particles. In other words, gases and dissolved substances will tend to move from areas where they are in high concentration, to areas where they are in low concentration. Remember, in this case, concentration means the number of particles of a substance in a particular area.

12 © Boardworks Ltd 2003 If we consider the situation where someone could smell cooked food; the chemicals being produced from the food were highly concentrated around the food. At the same time, the chemicals around the person, who is standing further away, are low in concentration. So the chemicals move from high concentration to low concentration. The person does not have to move. The smell will move toward them. Question Do these particles just keep moving?

13 © Boardworks Ltd 2003 The particles move at the same rate in opposite directions. This situation is known as EQUILIBRIUM They will move this way And when the concentration is equal Yes! They keep moving in a certain direction until the particle concentration of the place they are moving to is equal to that of the place they are moving from…. ANSWER

14 © Boardworks Ltd 2003 …because there is equal movement in both directions, it would appear that overall; nothing was moving (however, we know better!). O.K., so we understand diffusion, but what has this got to do with vital life processes? Well, remember that we rely on certain gases and dissolved substances getting to the cells of our body every minute of the day. It is the process of diffusion that is helping us to achieve this. The most important substances we need are….

15 © Boardworks Ltd 2003 FOOD OXYGEN Once these chemicals are inside our body, they are in the form of either gases or dissolved substances. Both the dissolved food particles and the oxygen particles must pass into our blood stream so that they can be transported around the body to the cells. Each particle diffuses (performs diffusion) at particular sites within the body.

16 © Boardworks Ltd 2003 Let's consider them both in detail Chemical :Oxygen (O 2 ) Site of Diffusion : Alveoli Cross-section of an alveolus with arrows showing the movement of oxygen particles from the bronchiole to the end of the air sac.

17 © Boardworks Ltd 2003 When we breathe in (inhale), oxygen particles enter our breathing system and eventually find their way to the alveoli. Here, the oxygen particle concentration builds up. Surrounding the outer lining of the alveoli's are many tiny blood vessels called capillaries. The blood within these vessels is low in oxygen (deoxygenated).

18 © Boardworks Ltd 2003 The brief difference in concentration between the inside and the outside of the alveolus is enough for the oxygen to start diffusing. It passes across the lining of the alveolus and into the blood.

19 © Boardworks Ltd 2003 Question Think back to the process of respiration. Which gas is produced during the reaction and carried away from the cells in the blood. Answer Carbon Dioxide (CO 2 ) Therefore, the blood that reaches the alveolus may be deoxygenated, but it does have high levels of CO 2 The air we inhale does not contain a large concentration of CO 2 GasAir breathed inAir breathed out O2O2 21%17% CO 2 Almost zero 4% N2N2 79%

20 © Boardworks Ltd 2003 Therefore, we have another situation where a substance is in high concentration in one place and low concentration in another place. The result of this is that whilst O 2 is diffusing into the blood, CO 2 is diffusing in the opposite direction. The blood is exchanging one gas for another. We refer to the lining of the alveolus as a… GAS EXCHANGE SITE

21 © Boardworks Ltd 2003 Chemical : Dissolved food particles e.g. Glucose Fatty Acids Glycerol Phosphate Amino Acids Site of Diffusion :Villi within the Small Intestine Once the process of digestion has neared completion, the enzymes have chemically broken down Fats, Proteins and Carbohydrates into useful nutrients. These nutrients are small enough to be able to pass across the lining of the small intestine. As you will remember from the section on the digestive system, the lining of the small intestine is heavily folded to create finger like projections called villi.

22 © Boardworks Ltd 2003 Small intestine Diagram of magnified villus Each villus separates the digested food from a network of capillaries.

23 © Boardworks Ltd 2003 These capillaries approach the villi, carrying blood that has a low concentration of digested nutrients. Again, we have a situation where certain particles are highly concentrated in one area and in low concentration in another. Here again, diffusion occurs. The small nutrient particles diffuse across the lining of the small intestine and enter the blood.

24 © Boardworks Ltd 2003 The process of diffusion has resulted in useful substances entering the blood from the alveoli and the villi. This blood now carries these useful substances to the cells of the body where they will be required. When the blood reaches the cells, it is traveling within capillaries. You may have noticed that diffusion is only occurring into and out of one type of blood vessel, namely capillaries…. capillaries intestine food particle

25 © Boardworks Ltd 2003 Take a look at the cross section of arteries, veins and capillaries to remind yourself why! ArteryVeinCapillary The arrow shows the route substances would have to take to diffuse and leave the vessel. It is clear that the capillary offers the least obstruction to diffusion. Reaching a capillary bed, we yet again find a site of material exchange, powered by the process of diffusion.

26 © Boardworks Ltd 2003 The capillary bed sees the exchange of useful and waste substances between the cells and the blood. Waste Useful Cell BLOODBLOOD

27 © Boardworks Ltd 2003 From the blood, the dissolved nutrients and the oxygen diffuse into the cell. The cell is probably running low and needs a new supply. They will be used to generate energy during respiration As the cell is a unit of life, it will have been performing respiration prior to the blood arriving with the new nutrients and oxygen gas. Therefore, it will have been storing up waste materials. These will now be excreted from the cell and given to the blood for eventual removal. We have now seen how the process of diffusion is behind the movement of substances within the body.

28 © Boardworks Ltd 2003 Does everything diffuse at the same speed? In every situation we have considered, substances are moving from an area of high concentration to an area of low concentration, ACROSS something. This is usually a membrane of a cell. This is only possible if the cell membrane has minute gaps through which the substances can pass. Large particles (too big) Membrane Small Particles (these can fit through)

29 © Boardworks Ltd 2003 Firstly, small particles will diffuse faster than large particles. But what about two different particles which are the same size. Which one will diffuse fastest? Well, imagine two football pitches sitting side by side, separated by a fence with gates in it. On pitch A, 3 people are playing with footballs. They have plenty of room to themselves. On pitch B, no one is playing and the gates in the fence are shut.

30 © Boardworks Ltd 2003 If the gates are opened, there is no real rush for the people to move from pitch A to pitch B. They already have the room they need to move around. Pitch APitch B It is likely that maybe later, one of the people makes their way over to pitch B.

31 © Boardworks Ltd 2003 Now, if the situation was slightly different and pitch A was crowded with people, we would produce a different outcome. When the gates are opened, there would be a rush of people from pitch A to pitch B. This would be due to the players’ desire for more individual space. The speed of movement from A to B would be far greater that the speed of movement in the original situation.

32 © Boardworks Ltd 2003 A B AB In this situation, the difference in the number of players on each side of the fence was small In this situation, the difference in the number of players on each side of the fence was large In living organisms, the same relationship occurs if we consider the movement of substances across boundaries.

33 © Boardworks Ltd 2003 To see this in action, let’s replace the word ‘players’ with particles and ‘number of players’ with concentration of particles. Therefore, the speed of diffusion will depend on the difference in concentration of particles on each side of the membrane (boundary) Slow movement of playersFast Movement of players The greater the concentration difference between two areas, the greater the rate of diffusion will occur across the membrane. Rule Remember that this will occur until a state of equilibrium is reached.

34 © Boardworks Ltd 2003 We call this difference in concentrations, a concentration gradient. Inside Outside Out In Concentration Gradient Out Inside Outside Concentration Gradient In The steeper the gradient, the greater the difference in concentration. We can represent this using a diagram.

35 © Boardworks Ltd 2003 You can probably imagine that it is in the interest of the body to try to maintain a high concentration difference between two sides of a membrane. Especially, if the body is trying to absorb a useful chemical or remove a toxic one. By having a large concentration gradient, diffusion would occur more quickly and the body would have its demands satisfied.

36 © Boardworks Ltd 2003 Here is one site employing a maintained concentration gradient…. Capillaries within Villi The lumen of the small intestine will be highly concentrated in dissolved food. It is essential that a continuous supply of this material diffuse across the lining of the villus and into the blood capillary. This is because… If the blood were stationary within the capillaries, this continuous process would stop.

37 © Boardworks Ltd 2003 Small Intestine BLOODBLOOD Food particles …Initially the concentration gradient would be high with food particles diffusing in to the blood. But, if the blood just received these particles and remained stationary, a state of equilibrium would then occur. Small Intestine BLOODBLOOD Food particles

38 © Boardworks Ltd 2003 The blood and the small intestine would have equal concentrations of dissolved food. Once this situation was reached, no extra dissolved food particles would move into the blood! In fact, some diffusion would occur in the opposite direction. Useful materials would be LEAVING the blood and heading back into the small intestine. (Remember the body needs a constant supply of dissolved food so that cellular respiration can occur)

39 © Boardworks Ltd 2003 Therefore to overcome this, the blood is constantly moving through the capillaries, bringing new blood to the villi and taking blood Blood carrying dissolved food away from the small intestine Dissolved food (now containing dissolved food) away. Small Intestine Concentration Gradient Blood

40 © Boardworks Ltd 2003 The concentration gradient remains large and ensures that the blood constantly receives dissolved food. Although this fast moving blood makes sure that dissolved substances are diffusing into the blood in high enough quantities to satisfy the bodies needs, there is another process at work, also doing its bit to get these nutrients into the blood. ACTIVE TRANSPORT We must now consider…… The cell membrane of the cells that line the villi of the small intestine can use ENERGY to actively move dissolved substances across from one side to the other.

41 © Boardworks Ltd 2003 With this extra process, the rate of diffusion will increase. Therefore active transport can speed up the diffusion of substances across membranes. Small Intestine Diffusion Active Transport Blood

42 © Boardworks Ltd 2003 This means that Active transport can be used to move substances in both directions: with and against the concentration gradient. High Concentration Low Concentration High Concentration Low Concentration Diffusion down concentration gradient However, because Active transport is powered by energy, it does not have to move substances in the same direction as diffusion. Active transport

43 © Boardworks Ltd 2003 Where does the energy for active transport come from? As you can see, the production and use of this energy is linked in a cycle. This is one way the body helps itself perform important functions. The energy for active transport comes from respiration Aerobic Respiration requires a supply of glucose and oxygen These two substances are carried in the blood to cells. They have to move from the blood to the cells of the body. The quicker they move, the quicker the cell can perform respiration The faster the cell performs respiration, the faster it generates energy

44 © Boardworks Ltd 2003 Are there common structural features shared by all exchange sites ? Let’s finally consider… what makes a good exchange site? Is it possible to look at the shape of a living structure and establish whether it would make an effective site for exchanging materials? Well, let’s look at the two exchange sites that we have been learning about. AlveoliVilli

45 © Boardworks Ltd 2003 AlveoliVilli Small Intestine Lining Both sites share a common function. They are there to enable substances to pass into and out of the blood as quickly as possible. In response to this, they both have enormous surface areas. The small intestine has an enormously folded lining. These folds have further microscopic folds, increasing the number of possible exchange sites. These microvilli are lined with cells. The process of diffusion occurs across these cell’s membranes. Diagram of microvilli

46 © Boardworks Ltd 2003 The lungs consist of millions of individual alveoli. Each alveolus is a small air sac, covered with blood vessels. Diagram of the cross section of many alveoli Therefore, in humans, the overall surface area of the lungs is enormously increased by the presence of alveoli and that of the small intestine by the folded villi.

47 © Boardworks Ltd 2003 Diffusion Let’s get moving! This activity will look at the movement of substances into and out of the cells. It is essential that you pay close attention to the diagram of the cell on the next page. The questions are based on this diagram. Good luck!

48 © Boardworks Ltd 2003 Look closely at this cell. It is surrounded by tissue fluid. We can see various chemicals inside and outside the cell. Key Water Oxygen Glucose Carbon Dioxide

49 © Boardworks Ltd 2003 1. Draw and label the picture of the cell. 2. What process does the cell use to release energy from food? 3. Which 2 substances are needed for this process to happen? 4. What are the waste products of this process? Water Oxygen Glucose Carbon Dioxide It is very important that you count the number of each chemical very carefully.

50 © Boardworks Ltd 2003 5. If there was a large concentration difference between the inside and outside of the cell, do you think that diffusion would occur faster or slower? 6. Count the number of particles of each substance inside and outside the cell. Put your results in the following table. SubstanceConcentration Concentration inside Outside Carbon Dioxide Oxygen Glucose Water

51 © Boardworks Ltd 2003 7. For each of the four substances, draw an arrow across the table to indicate which way it will move. e.g. SubstanceConcentration Concentration inside outside or Make the thickness of the arrow represent how fast the substance would diffuse I.e. a fat arrow represents a greater rate of diffusion. 8. Define the term diffusion.

52 © Boardworks Ltd 2003 Diffusion Multiple choice questions

53 © Boardworks Ltd 2003 1. Which of the following is the proper definition of the term diffusion? A The spreading of any substance from where it is in lower concentration to where it is in higher concentration. B The spreading of a gas or any substance in solution from where it is in lower concentration to where it is in higher concentration. C The spreading of any substance from where it is in higher concentration to where it is in lower concentration. D The spreading of a gas or any substance in solution from where it is in higher concentration to where it is in lower concentration.

54 © Boardworks Ltd 2003 2. Choose the correct statement from the following (only one is correct). A Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at a higher concentration in the blood. B Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at a higher concentration in the air. C Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at a lower concentration in the blood. D Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at a higher concentration in the blood.

55 © Boardworks Ltd 2003 3. Choose the correct statement from the following (only one is correct). A Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at higher concentration in the blood. B Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at higher concentration in the air. C Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at lower concentration in the blood. D Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at higher concentration in the blood.

56 © Boardworks Ltd 2003 4. A chip is weighed and then placed in a very concentrated salt solution for 24 hours. After this period of time the chip is re-weighed and it is discovered that the chip weighs less. The reason for this is: A Water has entered the chip, moving from an area of higher concentration to an area of lower concentration. B Water has moved out of the chip, moving from an area of higher concentration to an area of lower concentration. C Water has moved out of the chip, moving from an area of lower concentration to an area of higher concentration. D Water has entered the chip, moving from an area of lower concentration to an area of higher concentration.

57 © Boardworks Ltd 2003 5. Look at the diagram below. Choose the correct statement to explain what is happening. AB membrane A The rate of diffusion of particles with be greater in A because the difference in concentration is smaller. B The rate of diffusion of particles with be greater in B because the difference in concentration is smaller. C The rate of diffusion of particles with be greater in A because the difference in concentration is greater. D The rate of diffusion of particles with be greater in B because the difference in concentration is greater.

58 © Boardworks Ltd 2003 6. The surface area of the lungs as an exchange surface is increased by the possession of… A millions of glandular cells. B millions of villi. C millions of alveoli. D millions of blood cells.

59 © Boardworks Ltd 2003 7. Gas and solute exchange surfaces (e.g. fish gills, human lung wall and small intestine wall, placenta of a pregnant woman) maximise their effectiveness by… A having a minimal surface area, being permeable to the particular gases/solutes, being thin and maximising the concentration difference. B having a massive surface area, being permeable to the particular gases/solutes, being thin and minimising the concentration difference. C having a massive surface area, being permeable to the particular gases/solutes, being thick and maximising the concentration difference. D having a massive surface area, being permeable to the particular gases/solutes, being thin and maximising the concentration difference.

60 © Boardworks Ltd 2003 8. Active transport is the… A movement of a substance across a cell membrane against the direction in which it would tend to diffuse and the process releases energy. B movement of a substance across a cell membrane against the direction in which it would tend to diffuse and the process uses energy. C movement of a substance across a cell membrane in the same direction in which it would tend to diffuse and the process releases energy.


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