Diffusion © Centura Foods Ltd “Ah! Bisto” The ‘Bisto Kids’ featured on advertising for ‘Bisto’ gravy powder from the 1920s - 90s. After you have studied this presentation you may be able to decide whether this is a good example of diffusion. © Centura Foods Ltd

In solids the molecules are more or less stationary 2 All substances are made up of sub-microscopic particles called molecules In gases (like air) the molecules can move freely In liquids (like water) the molecules can also move In solutions (e.g sugar dissolved in water) the molecules of the dissolved substance can move. In solids the molecules are more or less stationary

Representation of molecules in a gas 3 Representation of molecules in a gas (a) (b) As a result of their random movements the molecules become evenly distributed

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Diffusion As a result of this random movement, the 5 As a result of this random movement, the molecules of a gas become evenly dispersed This movement is called DIFFUSION The scent of a hyacinth diffuses throughout a room In an area as large as a room, there will be air currents and convection currents which will play a greater part than diffusion The scent molecules diffuse from a region where they are concentrated (the flower) to regions where they are absent or in low concentration

One of the ways substances enter and leave cells is by diffusion 6 One of the ways substances enter and leave cells is by diffusion If a substance is more concentrated outside a cell than inside, the molecules will tend to diffuse into the cell If a substance is more concentrated inside the cell than outside, the molecules will tend to diffuse out of the cell

Diffusion gradient The molecules are more densely packed on 7 The molecules are more densely packed on the left and so they tend to diffuse into the space on the right. This is a diffusion gradient The bigger the difference in concentration, the greater will be the diffusion gradient. A diffusion gradient

Scale The scale of the following drawings is greatly distorted. 8 The scale of the following drawings is greatly distorted. Even if the cells were as large as they appear on the screen, the molecules would still be invisible particles

Diffusion of oxygen into a cell 9 The concentration of oxygen molecules is greater outside the cell than inside So the oxygen molecules diffuse into the cell The concentration of carbon dioxide is greater inside the cell than outside. Carbon dioxide molecules will diffuse out of the cell.

Because the cell is using up oxygen, the 10 Because the cell is using up oxygen, the concentration of oxygen inside the cell is always lower then the concentration outside. The diffusion gradient is maintained The oxygen is used in respiration. Carbon dioxide is produced by respiration. The concentration of carbon dioxide inside the cell increases, so carbon dioxide diffuses out of the cell. So oxygen continues to diffuse in

If all kinds of substance could diffuse into a cell 11 If all kinds of substance could diffuse into a cell there would be a danger that poisonous substances could diffuse in and kill the cell If all the substances in a cell could diffuse out the cell would lose essential substances (e.g. glucose) needed to keep the cell alive In fact, although the cell membrane does allow some substances (e.g. oxygen and carbon dioxide) to diffuse freely, it controls the exit and entry of nearly all other substances

Diffusion is slow For living processes it is effective only over 12 Diffusion is slow For living processes it is effective only over short distances The distance from a cell membrane to the centre of the cell may be 0.1mm or less Diffusion is rapid enough to keep a cell supplied with oxygen and food

Single-celled organisms 13 In a single-celled organism (such as Amoeba) the distance is so small that diffusion is rapid enough for the cell’s needs maximum distance is 0.1 mm oxygen Compared with its bulk, a cell like this has a huge surface area through which diffusion can occur carbon dioxide

Single-celled organisms are rarely more than 1mm in diameter 14 Single-celled organisms are rarely more than 1mm in diameter Bacteria range from 1 - 10 microns (1-10 µ ) ( 1 µ is one thousandth of a millimetre) Diffusion is rapid enough for such small organisms

Inside the bodies of large animals, diffusion of 15 Inside the bodies of large animals, diffusion of oxygen into their cells is rapid enough For transport across the whole body, diffusion would be much too slow Large organisms have evolved transport systems (e.g. blood circulatory systems) that carry oxygen from outside the body to the cells inside

Earthworm diffusion takes place through the thin skin of the worm 16 diffusion takes place through the thin skin of the worm CO2 diffuses out O2 diffuses in Section through worm’s skin 0.04mm the blood vessels absorb the O2 and carry it to the body

Question Did you notice anything in particular about the 17 Did you notice anything in particular about the earthworm’s ‘skin’ as seen in the section ? Have another look. How might this affect diffusion ?

Humans obtain their oxygen by diffusion 18 Humans obtain their oxygen by diffusion But not through the skin Although the skin is well supplied with blood vessels, there are too many layers of cells for diffusion to be fast enough Humans have lungs and it is in these lungs that diffusion occurs

Human lungs windpipe lung diaphragm heart position of lungs in thorax 19 windpipe lung The lungs are not hollow, but spongy. They are made up of thousands of tiny air pockets diaphragm heart position of lungs in thorax human lungs

Lung Structure each tube ends up in a cluster of tiny air sacs. 20 The windpipe (trachea) divides into two smaller passes (bronchi) which continue to divide repeatedly into smaller and smaller branches until they end up in tiny, thin-walled air pockets (air sacs).. each tube ends up in a cluster of tiny air sacs. the air passages in the lung branch into finer and finer tubes

A single air sac blood supply to air sac diffusion of oxygen 21 blood supply to air sac diffusion of oxygen air breathed in and out diffusion of carbon dioxide O2 0.03 mm The air is carried to the air sacs by the breathing movements. Oxygen then diffuses across the air sac, though the single cell layer lining the air sac, and into the blood vessels. Carbon dioxide diffuses in the opposite direction. CO2 question 4

In mammals, birds, reptiles and amphibia, 22 In mammals, birds, reptiles and amphibia, oxygen and carbon dioxide are exchanged by diffusion in the lungs In fish, this exchange of gases takes place by diffusion through the gills The oxygen dissolved in the water diffuses into the blood vessels in the gills.

Fish gill cover gills gill filaments gill cover cut away 23 Water is taken in through the mouth, passed over the gills and expelled from the gill cover. gill filaments gill cover cut away

Diffusion takes place through the surface of the gills 24 Diffusion takes place through the surface of the gills Diffusion is a slow process The branching gill filaments offer a big surface area through which oxygen and carbon dioxide can diffuse In this way, the total diffusion through the gills is greatly increased It is said that if all the air sacs were opened out and spread flat they would occupy an area the size of a football pitch. The millions of air sacs in the lungs also vastly increase the area through which the gases can diffuse

Plants Plants have no special organs for breathing 25 Plants have no special organs for breathing They have to rely on diffusion for their supplies of oxygen and carbon dioxide There are pores in the leaves and stems through which the gases diffuse In daylight, CO2 (for photosynthesis) will be diffusing in and O2 will be diffusing out The plant is respiring all the time, using up oxygen and producing carbon dioxide. In daylight, photosynthesis will use up the carbon dioxide as fast as it is produced by respiration, so there will be no outward diffusion of carbon dioxide. Similarly, the oxygen produced by photosynthesis is used up by respiration. In darkness, O2 will diffuse in and CO2 will diffuse out as a result of respiration

Leaf the ‘veins’ bring water In a thin leaf, the diffusion distance 26 O2 and CO2 diffuse through pores in the epidermis the ‘veins’ bring water The leaf needs carbon dioxide for photosynthesis and oxygen for respiration. These gases diffuse through pores in the leaf’s epidermis and enter the air spaces between cells. They then diffuse through the cell wall into the cytoplasm. In a thin leaf, the diffusion distance is short O2 and CO2 diffuse into the spaces between cells

Question 1 Diffusion can normally take place in (a) a liquid 27 Diffusion can normally take place in (a) a liquid (b) a solution (c) a solid (d) a gas

Question 2 A fish breathes (a) water (b) oxygen dissolved in water 28 A fish breathes (a) water (b) oxygen dissolved in water (c) oxygen in the air (d) carbon dioxide dissolved in water

Question 3 Diffusion takes place as a result of 29 Question 3 Diffusion takes place as a result of (a) convection currents (b) air movements (c) natural movement of molecules (d) changes in temperature

Question 4 What is the approximate diameter of 30 Question 4 What is the approximate diameter of an air sac in the human lung ? (Slide 21) (a) 0.006 mm (b) 0.06 mm (c) 0.6 mm (d) 6.0 mm

31 Question 5 In a plant leaf, CO2 diffuses (a) into the air space between cells, (b) into the cytoplasm, (c) through the cell wall, (d) through a pore in the epidermis. The correct sequence is (a) a,b,c,d (b) c, b, d, a (c) d, a, c, b (d) d, c, a, b

Question 6 Which of these would you expect to diffuse 32 Question 6 Which of these would you expect to diffuse freely through a cell membrane ? (a) carbon dioxide (b) water (c) proteins (d) oxygen

33 Question 7 Through which of these layers would you expect diffusion to be most rapid ? (a) (b) (c) (d)

Question 8 The cells inside an earthworm receive oxygen 34 The cells inside an earthworm receive oxygen via (a) diffusion into the blood vessels, (b) transport by the blood, (c) diffusion out of the blood into the cells, (d) diffusion through the epidermis. Which of the following is the correct sequence? (a) b, c, d, a (b) a, b, c, d (c) d, a, b, c (d) d, b, a, c

35 Answer Correct

36 Answer Incorrect

single cell The earthworm’s ‘skin’ is only one cell thick. 37 The earthworm’s ‘skin’ is only one cell thick. To reach a blood vessel, the oxygen has to diffuse over a very short distance and so is rapid enough to meet the earthworm’s needs. The single cell layer is an epidermis rather than a ‘skin’. single cell