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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

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As a result of their random movements the molecules become evenly distributed

(a) (b)

Representation of molecules in a gas3

Next slide

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As a result of this random movement, themolecules 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

5Diffusion

One of the ways substances enter and leave cellsis by diffusion

If a substance is more concentrated outside a cell than inside, the molecules will tend to diffuseinto the cell

If a substance is more concentrated inside thecell than outside, the molecules will tend todiffuse out of the cell

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A diffusion gradient

The molecules are more densely packed on the left and so they tend to diffuse into thespace on the right. This is a diffusion gradient

Diffusion gradient7

The scale of the following drawings is greatly distorted.

Even if the cells were as large as they appearon the screen, the molecules would still beinvisible particles

8Scale

The concentration of oxygen molecules isgreater outside the cellthan inside

So the oxygen moleculesdiffuse into the cell

Diffusion of oxygen into a cell 9

Because the cell is using up oxygen, the concentration of oxygen inside the cell isalways lower then the concentration outside.

So oxygen continues to diffuse in

The diffusion gradient is maintained

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If all kinds of substance could diffuse into a cellthere would be a danger that poisonous substances could diffuse in and kill the cell

If all the substances in a cell could diffuse outthe cell would lose essential substances (e.g. glucose) needed to keep the cell alive

In fact, although the cell membrane does allowsome substances (e.g. oxygen and carbon dioxide) to diffuse freely, it controls the exit and entry of nearly all other substances

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For living processes it is effective only overshort 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 cellsupplied with oxygen and food

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Diffusion is slow

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

Single-celled organisms are rarely more than1mm 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

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Inside the bodies of large animals, diffusion of oxygen into their cells is rapid enough

For transport across the whole body, diffusionwould be much too slow

Large organisms have evolved transportsystems (e.g. blood circulatory systems)that carry oxygen from outside the body to the cells inside

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CO2 diffuses

outO2 diffuses in

Section throughworm’s skin

the blood vesselsabsorb the O2 andcarry it to the body

0.04mm

Earthwormdiffusion takes place through the thin skin of the worm

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Did you notice anything in particular about the earthworm’s ‘skin’ as seen in the section ?

Have another look.

How might this affect diffusion ?

17Question

Humans obtain their oxygen by diffusion

But not through the skin

Although the skin is well supplied with bloodvessels, 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

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position of lungsin thorax

windpipe

lung

diaphragm

heart

human lungs

Human lungs 19

the air passages in the lungbranch into finer and finer tubes

each tube ends up ina cluster of tiny airsacs.

Lung Structure 20

blood supply to air sac

air breathedin and out

diffusion ofoxygen

diffusion ofcarbon dioxide O2

CO2

A single air sac 21

0.03 mm

question 4

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 bydiffusion through the gills

The oxygen dissolved in the water diffuses into the blood vessels in the gills.

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Fish

gill cover

gill cover cut away

gills

gill filaments

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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 isgreatly increased

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

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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

In darkness, O2 will diffuse in and CO2 will diffuse out as a result of respiration

25Plants

Leaf

O2 and CO2 diffuseinto the spaces between cells

the ‘veins’bring water

O2 and CO2

diffuse throughpores in theepidermis

In a thin leaf, thediffusion distanceis short

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Question 1

Diffusion can normally take place in

(a) a liquid

(b) a solution

(c) a solid

(d) a gas

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Question 2

A fish breathes

(a) water

(b) oxygen dissolved in water

(c) oxygen in the air

(d) carbon dioxide dissolved in water

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Question 3

Diffusion takes place as a result of

(a) convection currents

(b) air movements

(c) natural movement of molecules

(d) changes in temperature

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Question 4

What is the approximate diameter of an air sac in the human lung ?

(a) 0.006 mm

(b) 0.06 mm

(c) 0.6 mm

(d) 6.0 mm

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(Slide 21)

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

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Question 6

Which of these would you expect to diffusefreely through a cell membrane ?

(a) carbon dioxide

(b) water

(c) proteins

(d) oxygen

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Question 7Through which of these layers would you expect diffusion to be most rapid ?

(a)

(b)

(c)

(d)

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Question 8The 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 thecells, (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

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Answer

Correct

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Answer

Incorrect

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single cell

The earthworm’s ‘skin’ is only one cell thick.To reach a blood vessel, the oxygen has to diffuseover a very short distance and so is rapid enough tomeet the earthworm’s needs.

The single cell layer is an epidermis rather than a ‘skin’.

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