© Centura Foods Ltd. All substances are made up of sub-microscopic particles called molecules In...
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Transcript of © Centura Foods Ltd. All substances are made up of sub-microscopic particles called molecules In...
© CenturaFoods Ltd
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
As a result of their random movements the molecules become evenly distributed
(a) (b)
Representation of molecules in a gas3
Next slide
4
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
10
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
12
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
14
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
15
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
16
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
18
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.
22
Fish
gill cover
gill cover cut away
gills
gill filaments
23
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
24
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
26
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
28
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
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
32
Question 7Through which of these layers would you expect diffusion to be most rapid ?
(a)
(b)
(c)
(d)
33
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
35
Answer
Incorrect
36
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’.
37