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© Centura Foods Ltd. All substances are made up of sub-microscopic particles called molecules In gases (like air) the molecules can move freely In liquids.

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Presentation on theme: "© Centura Foods Ltd. All substances are made up of sub-microscopic particles called molecules In gases (like air) the molecules can move freely In liquids."— Presentation transcript:

1 © Centura Foods Ltd

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 solids the molecules are more or less stationary 2

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

4 Next slide 4

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 The scent molecules diffuse from a region where they are concentrated (the flower) to regions where they are absent or in low concentration 5 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 6

7 A diffusion gradient 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 Diffusion gradient 7

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 8 Scale

9 The concentration of oxygen molecules is greater outside the cell than inside So the oxygen molecules diffuse into the cell Diffusion of oxygen into a cell 9

10 Because the cell is using up oxygen, the concentration of oxygen inside the cell is always lower then the concentration outside. So oxygen continues to diffuse in The diffusion gradient is maintained 10

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 11

12 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 12 Diffusion is slow

13 oxygen carbon dioxide maximum distance is 0.1 mm In a single-celled organism (such as Amoeba) the distance is so small that diffusion is rapid enough for the cell’s needs Single-celled organisms 13

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

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 15

16 CO 2 diffuses out O 2 diffuses in Section through worm’s skin the blood vessels absorb the O 2 and carry it to the body 0.04mm Earthworm diffusion takes place through the thin skin of the worm 16

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 ? 17 Question

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 18

19 position of lungs in thorax windpipe lung diaphragm heart human lungs Human lungs 19

20 the air passages in the lung branch into finer and finer tubes each tube ends up in a cluster of tiny air sacs. Lung Structure 20

21 blood supply to air sac air breathed in and out diffusion of oxygen diffusion of carbon dioxide O2O2 CO 2 A single air sac mm question 4

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. 22

23 Fish gill cover gill cover cut away gills gill filaments 23

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 The millions of air sacs in the lungs also vastly increase the area through which the gases can diffuse 24

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, CO 2 (for photosynthesis) will be diffusing in and O 2 will be diffusing out In darkness, O 2 will diffuse in and CO 2 will diffuse out as a result of respiration 25 Plants

26 Leaf O 2 and CO 2 diffuse into the spaces between cells the ‘veins’ bring water O 2 and CO 2 diffuse through pores in the epidermis In a thin leaf, the diffusion distance is short 26

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

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

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 29

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

31 Question 5 In a plant leaf, CO 2 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 31

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 32

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

34 Question 8 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 34

35 Answer Correct 35

36 Answer Incorrect 36

37 single cell 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’. 37

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