15
Detecting the
environment
15
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Detectinginthe
humans
environment
Think about…
15.1 Irritability
15.2 Human eye
15.3 Human ear
15.4 Phototropism of plants
Recall Think about…
Concept map
2
15
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Detectinginthe
humans
environment
Contact lenses for controlling short sight
A special contact lens has
been developed to control
short sight.
3
15
6 Nutrition
Detectinginthe
humans
environment
Contact lenses for controlling short sight
lenses put in
before sleeping
cornea (角膜)
reshaped
during sleep
4
15
6 Nutrition
Detectinginthe
humans
environment
Contact lenses for controlling short sight
People with short sight can
see clearly the next day
even after the lenses are
removed.
5
15
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Detectinginthe
humans
environment
What is the cause of short sight?
6
15
6 Nutrition
Detectinginthe
humans
environment
What is the role of the cornea in
our eyes?
7
15
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Detectinginthe
humans
environment
15.1 Irritability
15.1 Irritability
• organisms can
stimuli (刺激)
detect changes
in the environment
and respond to them
8
15
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Detectinginthe
humans
environment
15.1 Irritability
15.1 Irritability
• irritability (感應性):
ability of detecting stimuli and
giving responses
9
15
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Detectinginthe
humans
environment
15.1 Irritability
A Importance of irritability
• helps organisms obtain food, e.g.
butterfly detects
the smell of nectar
(stimulus)
flies to the flower
(response)
10
15
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Detectinginthe
humans
environment
15.1 Irritability
A Importance of irritability
• helps organisms obtain food, e.g.
shoot of plant
detects a light
source (stimulus)
grows towards the
light source
(response)
11
15
6 Nutrition
Detectinginthe
humans
environment
15.1 Irritability
A Importance of irritability
• helps organisms find mates, e.g.
peahen sees the
feather display of
peacock (stimulus)
may approach the
peacock
(response)
peacock
peahen
12
15
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Detectinginthe
humans
environment
15.1 Irritability
A Importance of irritability
• helps organisms escape from danger, e.g.
rabbit hears the
sound produced
by its predators
(stimulus)
runs away quickly
(response)
13
15
6 Nutrition
Detectinginthe
humans
environment
15.1 Irritability
B Detection of stimuli
consist of sensory cells (感覺細胞)
• by receptors (感受器)
some are concentrated to form
part of a sense organ (感覺器官)
14
15
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Detectinginthe
humans
environment
15.1 Irritability
Receptors in humans
Sense organ
Type of receptor
Stimulus
detected
Light
Eye
Photoreceptor
(光感受器)
Ear
Mechanoreceptor Sound
(機械感受器)
15
15
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Detectinginthe
humans
environment
15.1 Irritability
Receptors in humans
Sense organ
Type of receptor
Stimulus
detected
Nose
Chemoreceptor
(化學感受器)
Chemicals
in the air
Tongue
Chemoreceptor
Chemicals
in food
16
15
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Detectinginthe
humans
environment
15.1 Irritability
Receptors in humans
Sense organ
Type of receptor
Stimulus
detected
Mechanoreceptor Pressure
Skin
Thermoreceptor
(温度感受器)
Temperature
change
17
15
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Detectinginthe
humans
environment
15.1 Irritability
C From stimulus to response
How do we detect stimuli and then
give responses?
18
15
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Detectinginthe
humans
environment
15.1 Irritability
Example
 Light (stimulus)
from the bus is
detected by the
photoreceptors in
the boy’s eyes.
19
15
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Detectinginthe
humans
environment
15.1 Irritability
Example
 The photoreceptors
generate and send
nerve impulses
(神經脈衝) along the
nerves (神經) to the
brain (coordinator).
20
15
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Detectinginthe
humans
environment
15.1 Irritability
Example
 The brain
(coordinator)
interprets the
nerve impulses
and produces the
sense of sight
(sensation 感覺).
21
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Detectinginthe
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environment
Example
15.1 Irritability
 The brain
(coordinator) sends
nerve impulses to
the leg muscles
(effector 效應器).
 The leg muscles
contract and the boy
runs towards the bus
(response).
22
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Detectinginthe
humans
environment
15.1 Irritability
Example
 The brain
links receptors and (coordinator) sends
effectors so that they nerve impulses to
are well coordinated the leg muscles
(effector 效應器).
 The leg muscles
contract and the boy
runs towards the bus
(response).
23
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Detectinginthe
humans
environment
15.1 Irritability
Major events from detecting a stimulus
to producing a response in humans
stimulus
detected by
receptor
sends nerve impulses to
coordinator
• nervous system (神經系統)
• endocrine system (內分泌系統)
sends nerve impulses to
(if nervous system
effector
is involved)
produces
response
24
15
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Detectinginthe
humans
environment
15.1 Irritability
plants also detect and respond to stimuli,
but their responses are much slower
25
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Detectinginthe
humans
environment
1
15.1 Irritability
Irritability is the ability of
detecting stimuli and giving
responses in organisms. It is
important for survival.
26
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Detectinginthe
humans
environment
2
15.1 Irritability
Types of receptors in humans:
Photoreceptors :
• (in eye) detect light
27
15
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Detectinginthe
humans
environment
2
15.1 Irritability
Types of receptors in humans:
Chemoreceptors :
• (in nose) detect chemicals
in the air
• (in tongue) detect chemicals
in food
28
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Detectinginthe
humans
environment
2
15.1 Irritability
Types of receptors in humans:
Mechanoreceptors :
• (in ear) detect sound
• (in skin) detect pressure
29
15
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Detectinginthe
humans
environment
2
15.1 Irritability
Types of receptors in humans:
Thermoreceptors :
• (in skin) detect temperature
change
30
15
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Detectinginthe
humans
environment
3
15.1 Irritability
stimulus
detected by
receptor
sends nerve impulses to
coordinator
(e.g. brain) (interprets
nerve impulses)
sends nerve impulses to
effector
(e.g. muscles)
produces
response
31
15
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Detectinginthe
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environment
15.2 Human eye
15.2 Human eye
detected
by eye
32
15
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
eyebrow (眼眉)
• prevents sweat from
running into the eye
eyelash (眼睫毛)
• traps dust and prevents
it from entering the eye
33
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Detectinginthe
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environment
15.2 Human eye
A Structures around the eye
eyelid (眼瞼)
• can be closed to
protect the eye from
dirt and strong light
• spreads tears over
the eye surface
when we blink
34
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
tear gland (淚腺)
• produces tears which
- contain sodium,
chloride and
lysozyme (溶菌酶)
that can kill bacteria
35
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
tear gland (淚腺)
• produces tears which
- keep the eye moist
and clean
36
15
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
tear duct (淚管)
• drains tears into the
nasal cavity
37
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
skull (顱骨)
orbit (眼窩)
38
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
skull orbit
eyeball
39
15
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Detectinginthe
humans
environment
15.2 Human eye
A Structures around the eye
conjunctiva (結膜)
eye muscles
• enable the
eyeball to
rotate
(not cover the cornea)
• keeps the front part of the
eye moist and lubricated
40
15
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
3D model 15.1 Animation 15.1
41
15
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Sclera (鞏膜)
• outermost layer
• tough, white
42
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Sclera (鞏膜)
• protects inner
structures
• maintains the
shape of eyeball
• provides a surface
for attachment of
eye muscles
43
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Cornea (角膜)
• transparent
 allows light to
pass through
• curved
 refracts and
focuses light
• no capillaries
44
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Choroid (脈絡膜)
• middle layer
• with a black pigment
which absorbs light
 reduces reflection
of light within the
eye
45
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Choroid (脈絡膜)
• rich in capillaries
 supply nutrients
and oxygen, and
remove wastes
46
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Iris (虹膜)
• continuous with
the choroid
• made up of muscles
47
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Iris (虹膜)
• with a pigment
less
pigment
e.g.
blue
more
pigment
grey
iris
green
brown
48
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Iris (虹膜)
• controls the size
of pupil
 regulates the
amount of light
entering the eye
pupil
49
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Pupil (瞳孔)
• an opening
that allows
light to enter
the eye
50
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Retina (視網膜)
• innermost layer
• contains photoreceptors
(light-sensitive cells)
- rod cells (視桿細胞)
- cone cells (視錐細胞)
51
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Optic nerve (視神經)
• transmits nerve
impulses generated
from photoreceptors
to the cerebrum (大腦)
52
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Yellow spot (黃點)
• central region of retina
• high density of
cone cells
• no rod cells
53
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Blind spot (盲點)
• where the optic nerve
leaves the eyeball
• no photoreceptors
54
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Detectinginthe
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15.2 Human eye
B Structures of the eye
Lens (晶體)
• transparent,
elastic and
biconvex (雙凸)
• refracts and
focuses light
onto the retina
55
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Lens (晶體)
• made up of
living cells with
no nuclei
• no capillaries
56
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Suspensory ligaments
(懸韌帶)
• hold the lens in
position
• connected to the
ciliary body
ciliary body
57
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Detectinginthe
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environment
15.2 Human eye
B Structures of the eye
Ciliary body (睫狀體)
• consists of a ring of
ciliary muscles (睫狀肌)
• controls the tension
of suspensory
ligaments
 changes the
thickness of the lens
allows light from
objects at different
distances to be focused
onto the retina
58
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
cornea
iris
ciliary body
suspensory
ligaments
lens
59
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Aqueous humour
(水狀液)
• a watery fluid
• supplies nutrients
and oxygen to the
cornea and the lens
60
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Aqueous humour
(水狀液)
• maintains the
shape of the eyeball
• refracts light onto
the retina
61
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Detectinginthe
humans
environment
15.2 Human eye
B Structures of the eye
Vitreous humour
(玻璃狀液)
• a jelly-like fluid
• maintains the
shape of the eyeball
• refracts light onto
the retina
62
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Detectinginthe
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15.2 Human eye
15.1
Practical 15.1
Dissection of ox eye
Procedure
1 Remove the eye muscles and fatty tissue.
fatty
tissue
63
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Detectinginthe
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environment
15.2 Human eye
15.1
2 Identify the sclera and optic nerve.
optic nerve
sclera
64
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15.2 Human eye
15.1
2 Cut the eye in half.
65
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15.2 Human eye
15.1
3 Examine the front half of the eye.
Identify the cornea, iris and pupil.
Front view Back view
pupil
iris
cornea
cornea
66
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15.2 Human eye
15.1
4 Examine the lens and vitreous humour.
Note their texture.
lens
vitreous
humour
67
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environment
15.2 Human eye
15.1
5 Examine the back half of the eye.
Locate the blind spot.
blind
spot
optic
nerve
68
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
cornea
• allows light to
enter the eye
• refracts and
focuses light onto
the retina
69
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
iris
• controls the size
of the pupil so
as to regulate the
amount of light
entering the eye
70
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
pupil
• allows light to
enter the eye
71
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
lens
• refracts and
focuses light
onto the retina
72
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
aqueous humour
• supplies nutrients
and oxygen to
the cornea and
the lens
73
15
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
aqueous humour
• maintains the
shape of the
eyeball
• refracts light onto
the retina
74
15
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
suspensory
ligament
• connects the
lens to the
ciliary body
75
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
ciliary body
• consists of ciliary
muscles
• changes the
thickness of the
lens
76
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
sclera
• protects inner
structures
77
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
sclera
• maintains the
shape of the
eyeball
78
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
sclera
• provides a
surface for
attachment of
eye muscles
79
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
choroid
• contains a black
pigment which
absorbs light to
reduce reflection
of light
80
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Detectinginthe
humans
environment
15.2 Human eye
Structure of the human eye:
choroid
• contains
capillaries which
supply nutrients
and oxygen to the
retina and sclera
81
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
retina
• contains
photoreceptors
to detect light
82
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Detectinginthe
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15.2 Human eye
Structure of the human eye:
yellow spot
• has a high
density of cone
cells
83
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
blind spot
• has no
photoreceptors
84
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
optic nerve
• transmits
nerve impulses
from the retina to
the cerebrum
85
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Detectinginthe
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environment
15.2 Human eye
Structure of the human eye:
vitreous humour
• maintains the
shape of the
eyeball
• refracts light onto
the retina
86
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15.2 Human eye
C Process of how we see
87
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15.2 Human eye
C Process of how we see
aqueous
humour
cornea
Animation 15.2
retina
lens
vitreous
humour
refract light
88
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15.2 Human eye
C Process of how we see

 Light rays enter the eye and are
refracted and focused onto the retina.
89
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15.2 Human eye
C Process of how we see

 A real and inverted image is formed on
the retina.
90
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15.2 Human eye
C Process of how we see
nerve
impulse

 The photoreceptors on the retina are
stimulated by the light. They generate
nerve impulses.
91
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15.2 Human eye
C Process of how we see
brain
optic nerve
 The photoreceptors
on the
retina
nerve impulses travel
along
theare
stimulated
bytothe
They
generate
optic nerve
thelight.
visual
centre
(視覺中心)
in the cerebrum of the brain.
nerve
impulses.
92
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15.2 Human eye
C Process of how we see
brain

optic nerve
 The visual centre interprets the nerve
impulses as an upright image of the
object.
93
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15.2 Human eye
D Detection of light by retina
light
94
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15.2 Human eye
1 Types of photoreceptors choroid
optic nerve
blind spot
nerve fibres to
optic nerve
rod cell
layers of nerve cells
(neurones)
cone cell
95
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Detectinginthe
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environment
15.2 Human eye
1 Types of photoreceptors
optic nerve
Rod cells and
cone cells are
stimulated
generate
nerve
impulses
brain
light
rod cell
cone cell
96
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15.2 Human eye
1 Types of photoreceptors
cone cells rod cells
rod cell
cone
(1500)
cell
97
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15.2 Human eye
Rod cells
segment
containing
pigment
mitochondria
nucleus
Cone cells
segment
containing
pigment
mitochondria
nucleus
(1500)
98
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15.2 Human eye
Rod cells
segment
containing
pigment
Cone cells
segment
containing
pigment
• more numerous
• less numerous
• pigment sensitive to
light of low intensity
 important for vision
in dim light
• pigment sensitive to
light of high intensity
 work best in
bright light
99
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15.2 Human eye
Rod cells
segment
containing
pigment
• cannot detect colour
 responsible for
black and white
vision
Cone cells
segment
containing
pigment
• can detect colour
 responsible for
colour vision
100
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15.2 Human eye
Cone cells
• 3 types: red, green and blue
101
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15.2 Human eye
sensitivity
(% of light absorbed)
Cone cells
blue cone cell most sensitive
to blue light
80
100
60
40
20
400
500
600
wavelength
(nm)
700
102
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15.2 Human eye
sensitivity
(% of light absorbed)
Cone cells
100
80
green cone cell most sensitive
to green light
60
40
20
400
500
600
wavelength
(nm)
700
103
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15.2 Human eye
sensitivity
(% of light absorbed)
Cone cells
100
80
red cone cell most sensitive
to red light
60
40
20
400
500
600
wavelength
(nm)
700
104
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15.2 Human eye
Cone cells
• combined stimulation of different types
of cone cells
 different colours perceived
105
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15.2 Human eye
Cone cells
Cone cells
Red
Green
Blue
+
+
+
+
+
Colour
perceived
red
green
blue
yellow
106
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15.2 Human eye
Cone cells
Cone cells
Red
Green
Blue
+
+
+
+
+
+
+
-
Colour
perceived
magenta
cyan
white
black
107
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environment
15.2 Human eye
number of
photoreceptors per mm2
2 Distribution of photoreceptors on
• mainly located on the periphery
retina
• none at the yellow spot and the
blind spot
150 000
rod cell
100 000
50 000
away from centre of away from
retina
108
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Detectinginthe
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15.2 Human eye
number of
photoreceptors per mm2
2 Distribution of photoreceptors on
• concentrated at the yellow spot
retina
• only a few on the periphery
150 000
rod cell
100 000
50 000
cone cell
away from centre of away from
retina
109
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Detectinginthe
humans
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15.2 Human eye
number of
photoreceptors per mm2
2 Distribution of photoreceptors on
Yellow spot
retina
• concentrated with cone cells
• no rod cells
150 000
rod cell
100 000
50 000
cone cell
away from centre of away from
retina
110
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Detectinginthe
humans
environment
15.2 Human eye
number of
photoreceptors per mm2
2 Distribution of photoreceptors on
Blind spot
retina
• no rod cells
and cone cells
150 000
rod cell
100 000
50 000
cone cell
away from centre of away from
retina
111
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15.2 Human eye
Blind spot
blind spot
• where the
optic nerve
leaves
the eyeball
112
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15.2 Human eye
Blind spot
retina
blind spot
no photoreceptors
no nerve impulses
sent to the brain
image can be formed
on the blind spot but
cannot be seen
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15.2 Human eye
Blind spot
Let’s experience the presence of the blind
spot through an activity.
114
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15.2 Human eye
Blind spot
1 Hold your textbook at arm’s length and
at horizontal level with your eyes.
115
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15.2 Human eye
Blind spot
2 Close your left eye and keep staring at
the
with your right eye.
3 At the same time, slowly move the
book towards yourself.
What happens?
The disappears at
a certain distance.
116
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15.2 Human eye
Blind spot
At a certain distance…
image formed on
blind spot
 cannot see
blind spot
image formed on
another part of the
retina
 can see
retina
117
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1
15.2 Human eye
Light rays that enter the eye are
refracted and focused on the
retina.
118
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1
15.2 Human eye
The image formed on the retina
is detected by rod cells and
cone cells .
119
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1
15.2 Human eye
Rod cells and cone cells send
nerve impulses along the
optic nerve to the brain.
120
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1
15.2 Human eye
The brain interprets the nerve
impulses and gives the sensation
of sight .
121
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2
Shape
15.2 Human eye
Rod cells
Rod -shaped
Cone cells
Cone -shaped
Relative
More / Less
abundance numerous
More / Less
numerous
Number of
type
Three type(s)
One type(s)
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15.2 Human eye
2
Rod cells
Cone cells
Sensitivity Sensitive to light Sensitive to light
of low / high
of low / high
intensity
intensity
Vision
Black and
white
vision
Colour vision
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15.2 Human eye
Rod cells
Distribution Mainly on the
in retina
periphery
of retina;
none at
yellow spot
and
blind spot
Cone cells
Concentrated at
yellow spot ;
none at
blind spot
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15.2 Human eye
E Seeing in bright light and in
dim light
Iris
• consists of
circular muscles (環肌) and
radial muscles (放射肌)
contract or relax
control the size of the pupil
regulate the amount of light entering the eye
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15.2 Human eye
In bright light
Animation 15.3
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15.2 Human eye
In bright light
1a circular muscles contract
1b radial muscles relax
2 pupil constricts (縮小)
 less light enters the eye
This prevents the photoreceptors in the retina
from being damaged by bright light.
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15.2 Human eye
In dim light
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15.2 Human eye
In dim light
1a circular muscles relax
1b radial muscles contract
2 pupil dilates (擴張)
 more light enters the eye
This allows the photoreceptors to be
stimulated so that a clear image can be seen.
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15.2 Human eye
In bright light In dim light
Circular
muscles of iris
Radial muscles
of iris
Pupil
Contract
Relax
Relax
Contract
Constricts
Dilates
Amount of light Decreases
entering the eye
Increases
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15.2 Human eye
In bright light In dim light
Importance
Prevents
damage to
photoreceptors
Allows
photoreceptors
to be
stimulated so
that a clear
image can be
seen
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15.2 Human eye
F Seeing objects at different
distances
• the lens is elastic
action of ciliary muscles
change the thickness (or curvature) of lens
focus on objects at different distances
eye accommodation (視覺調節)
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15.2 Human eye
Focusing on near objects
Animation 15.4
side view
front view
133
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15.2 Human eye
Focusing on near objects
 Ciliary muscles
contract.
diverging
light rays from a near object
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15.2 Human eye
Focusing on near objects
 Tension in suspensory ligaments 
(i.e. they become slackened).
135
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15.2 Human eye
Focusing on near objects
 Lens becomes thicker
(more convex).
136
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15.2 Human eye
Focusing on near objects
 Lens becomes thicker
(more convex).
thicker lens
refracts light more
light rays are
focused onto the
retina
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15.2 Human eye
Focusing on near objects
• If we look at a near object for a long time
ciliary muscles contract for a long time
without rest
eye strains (眼睛疲勞)
• Signs of eye strains:
headache
tired eyes
blurred vision
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15.2 Human eye
Focusing on distant objects
 Ciliary muscles
relax.
parallel
light rays from a distant object
139
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15.2 Human eye
Focusing on distant objects
 Tension in suspensory ligaments 
(i.e. they become tightened).
140
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15.2 Human eye
Focusing on distant objects
 Lens becomes thinner
(less convex).
141
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15.2 Human eye
Focusing on distant objects
 Lens becomes thinner
(less convex).
thinner lens
refracts light less
light rays are
focused onto the
retina
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Interpreting a graph of changes in
curvature of the lens over a period of
time
The graph on the next slide shows the changes
in curvature of the lens of a person who is
staring at a moving object in 25 seconds.
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lens remains thick (most curved)
 the person is looking at a
stationary near object
most curved
curvature of lens
(arbitrary unit)
least curved
time (s)
0
5 10 15 20 25
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lens becomes less convex
( in curvature)
 the object is moving away
from the person
most curved
curvature of lens
(arbitrary unit)
least curved
time (s)
0
5 10 15 20 25
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lens becomes more
convex ( in curvature)
 the object is moving
towards the person
most curved
curvature of lens
(arbitrary unit)
least curved
time (s)
0
5 10 15 20 25
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lens remains thin
 the person is
looking at a
stationary
distant object
most curved
curvature of lens
(arbitrary unit)
least curved
time (s)
0
5 10 15 20 25
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curvature of lens
(arbitrary unit)
A girl is watching a butterfly. The graph below
shows the changes in curvature of the lens of her eye.
time (s)
0
2
4
6
8
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6 Nutrition in humans
a During which period(s) is the butterfly flying
towards the girl?
(2 marks)
curvature of lens
(arbitrary unit)
0 – 2 s and 4 – 6 s (2)
time (s)
0
2
4
6
8
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b At what time is the tension in the suspensory
ligaments the greatest?
(1 mark)
curvature of lens
(arbitrary unit)
8 s (1)
time (s)
0
2
4
6
8
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1
15.2 Human eye
Eye accommodation is the ability
of the eye to focus on objects at
different distances.
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2
15.2 Human eye
Focusing on
near objects
Contract
Ciliary
muscles
Suspensory Tension
decreases ;
ligaments
become
slackened
Focusing on
distant objects
Relax
Tension
increases ;
become
tightened
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2
Lens
Refraction
of light
15.2 Human eye
Focusing on
near objects
Thicker
(more convex)
Increases
Focusing on
distant objects
Thinner
(less convex)
Decreases
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15.2 Human eye
G Eye defects
Animation 15.5
Some types of eye defects (眼睛毛病):
• short sight (近視)
• long sight (遠視)
• colour blindness (色盲)
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15.2 Human eye
1 Short sight
Vision problem
near objects
are clear
distant objects
are blurred
155
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15.2 Human eye
1 Short sight
Cause
Lens too thick
light rays
from a
distant object
Eyeball too long
image formed
in front of the retina
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15.2 Human eye
1 Short sight
Correction
Lens too thick
Eyeball too long
light rays
diverged
concave
lens
image formed
on the retina
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15.2 Human eye
2 Long sight
Vision problem
near objects
are blurred
distant objects
are clear
158
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15.2 Human eye
2 Long sight
Cause
Lens too thin
light rays
from a
near object
Eyeball too short
image formed
behind the retina
159
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15.2 Human eye
2 Long sight
Correction
Lens too thin
Eyeball too short
light rays
converged
convex
lens
image formed
on the retina
160
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15.2 Human eye
3 Colour blindness
Vision problem
• cannot distinguish some or all colours
161
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15.2 Human eye
3 Colour blindness
Vision problem
Normal vision
Red-green colour
blindness (紅綠色盲)
162
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15.2 Human eye
3 Colour blindness
Vision problem
• most common
• cannot distinguish
between red and
green
Red-green colour
blindness (紅綠色盲)
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15.2 Human eye
3 Colour blindness
Vision problem
Normal vision
Total colour
blindness (全色盲)
164
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15.2 Human eye
3 Colour blindness
Vision problem
• rare
Total colour
blindness (全色盲)
165
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15.2 Human eye
3 Colour blindness
Cause
• caused by the deficiency or defect of one or
more of the three cone cell types
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15.2 Human eye
3 Colour blindness
Cause
• caused by
deficiency or defect
of the red or green
cone cells, or both
Red-green colour
blindness (紅綠色盲)
167
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15.2 Human eye
3 Colour blindness
Correction
• inherited
• cannot be cured or corrected by wearing
lenses
168
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15.2 Human eye
Short sight
Vision problem:
Can only see near objects clearly
as the images of distant objects are
formed in front of the retina
169
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15.2 Human eye
Short sight
Cause:
Lens too thick and/or
eyeball too long
170
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15.2 Human eye
Short sight
Correction:
Wear concave lenses
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15.2 Human eye
Long sight
Vision problem:
Can only see distant objects clearly
as the images of near objects are
formed behind the retina
172
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15.2 Human eye
Long sight
Cause:
Lens too thin and/or
eyeball too short
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15.2 Human eye
Long sight
Correction:
Wear convex lenses
174
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15.2 Human eye
Colour blindness
Vision problem:
Cannot distinguish some or all
colours
175
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15.2 Human eye
Colour blindness
Cause:
Deficiency or defect in one or more of
the three types of cone cells
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15.2 Human eye
Colour blindness
Correction:
Can / Cannot be cured or corrected
by wearing lenses
177
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15.3 Human ear
15.3 Human ear
sound
detected
by ear
178
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15.3 Human ear
A Structures of the ear
outer ear middle ear inner ear
3D model 15.2
3 regions
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15.3 Human ear
1 Outer ear
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15.3 Human ear
1 Outer ear
Pinna (耳廓)
• cartilage covered
by skin
• collects sound
waves
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15.3 Human ear
1 Outer ear
Auditory canal
(聽道)
• directs sound
waves to the
eardrum
182
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15.3 Human ear
1 Outer ear
Auditory canal
(聽道)
• produces wax which
lubricates the canal
and traps dirt and
bacteria
183
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15.3 Human ear
1 Outer ear
Eardrum (鼓膜)
• a thin and elastic
membrane
• converts
sound waves into
vibrations
184
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15.3 Human ear
2 Middle ear
185
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15.3 Human ear
2 Middle ear
Ear bones (聽小骨)
1 mm
smallest bones
in our body
186
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15.3 Human ear
2 Middle ear
Ear bones (聽小骨)
• amplify and transmit
vibrations from
the eardrum to
the oval window
187
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15.3 Human ear
2 Middle ear
Oval window (卵圓窗)
• transmits vibrations
from the ear bones to
the inner ear
188
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15.3 Human ear
2 Middle ear
Round window (圓窗)
• releases the fluid
pressure in the
cochlea into the air
in the middle ear
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15.3 Human ear
2 Middle ear
When we are on a plane
that is taking off…
air pressure
outside  quickly
when we go higher
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15.3 Human ear
2 Middle ear
lower pressure
higher pressure
outside
in middle ear
eardrum bulges outwards
 cannot vibrate freely
 cannot hear clearly
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15.3 Human ear
2 Middle ear
Eustachian tube
(耳咽管)
• equalizes the air
pressure on either
side of the eardrum
to pharynx
192
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15.3 Human ear
3 Inner ear
193
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15.3 Human ear
3 Inner ear
Auditory nerve
(聽神經)
• transmits nerve
impulses to the
auditory centre in
the brain
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15.3 Human ear
3 Inner ear
Cochlea (耳蝸)
• for hearing
• coiled tube with
3 parallel canals
separated by
membranes
195
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15.3 Human ear
3 Inner ear
196
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15.3 Human ear
3 Inner ear
central canal filled
with endolymph
(內淋巴)
auditory
nerve
upper and lower canals filled with
perilymph (外淋巴)
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15.3 Human ear
3 Inner ear
198
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15.3 Human ear
3 Inner ear
membrane
hair
sensory hair cell
(感覺毛細胞)
nerve fibres
of hair cells
form auditory nerve
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15.3 Human ear
3 Inner ear
When endolymph vibrates…
 Hairs are bent.
 Sensory hair cells
are stimulated
and generate
nerve impulses.
200
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15.3 Human ear
3 Inner ear auditory
auditory nerve
centre
 Nerve impulses travel to the
brain for interpretation.
201
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15.3 Human ear
3 Inner ear
hairs of healthy
sensory hair cells
hairs of damaged
sensory hair cells
repeated
exposure
(×2000)
to loud
sound
(×2000)
hearing loss
202
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15.3 Human ear
3 Inner ear
Semicircular
canals (半規管)
• not involved in
hearing
203
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15.3 Human ear
3 Inner ear
Semicircular
canals (半規管)
when
stimulated send nerve
• contain sensory
impulses to
hair cells to detect
the brain
the directions of
head movements
coordinates muscles
to maintain body
balance
204
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15.3 Human ear
Structure of the human ear:
pinna
• collects sound
waves in the air
205
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15.3 Human ear
Structure of the human ear:
auditory canal
• directs sound
waves to the
eardrum
206
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15.3 Human ear
Structure of the human ear:
eardrum
• converts sound
waves to sound
vibrations
207
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15.3 Human ear
Structure of the human ear:
ear bones
• amplify and transmit vibrations from
the eardrum to the oval window
208
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15.3 Human ear
Structure of the human ear:
oval window
• transmits vibrations from the
ear bones to the inner ear
209
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15.3 Human ear
Structure of the human ear:
Eustachian tube
• equalizes air
pressure on both
sides of the
eardrum
210
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15.3 Human ear
Structure of the human ear:
semicircular canals
• contain sensory
hair cells which
detect directions of
head movements
211
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15.3 Human ear
Structure of the human ear:
semicircular canals
• help maintain body
balance
212
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15.3 Human ear
Structure of the human ear:
cochlea
• contains sensory
hair cells which
detect vibrations
and send nerve
impulses to the
brain
213
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15.3 Human ear
Structure of the human ear:
auditory nerve
• transmits nerve
impulses from
the cochlea to
the brain for
interpretation
214
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15.3 Human ear
Structure of the human ear:
round window
• releases fluid pressure to
the air in the middle ear
215
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15.3 Human ear
B Process of how we hear
auditory canal
Animation 15.6

pinna
eardrum
 Sound waves collected by the pinna
are directed to the eardrum.
216
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15.3 Human ear
B Process of how we hear

eardrum
 Sound waves cause the eardrum to
vibrate.
217
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15.3 Human ear
B Process of how we hear
oval window

ear bones
 The ear bones amplify and transmit
the vibrations to the oval window.
218
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15.3 Human ear
B Process of how we hear
oval window

cochlea
perilymph
 The oval window vibrates, causing the
perilymph in the cochlea to vibrate.
219
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15.3 Human ear
B Process of how we hear

cochlea
perilymph endolymph
 Vibrations are transmitted to the
endolymph of the cochlea.
220
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15.3 Human ear
B Process of how we hear

sensory
hair cell
 The sensory hair cells in the central
canal are stimulated. They generate
nerve impulses.
221
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15.3 Human ear
B Process of how we hear
auditory nerve

nerve
impulse
to the
auditory
centre in
the brain
sensation
of hearing
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15.3 Human ear
B Process of how we hear

round window
 Vibrations are transmitted to the round
window to release the fluid pressure into
the air in the middle ear.
223
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15.3 Human ear
The pinna collects sound waves
and directs them along the auditory
canal to the eardrum.
224
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15.3 Human ear
Sound waves are converted to
vibrations by the eardrum.
225
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15.3 Human ear
The ear bones amplify and transmit
the vibrations to the oval window.
226
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15.3 Human ear
The oval window vibrates, causing
the fluids in the cochlea to vibrate .
227
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15.3 Human ear
Sensory hair cells in the cochlea
are stimulated and they send
nerve impulses along the auditory
nerve to the brain .
228
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15.3 Human ear
The brain interprets the nerve
impulses and gives the sensation of
hearing .
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15.4 Phototropism of plants
15.4 Phototropism of plants
What do
you notice
about the
shoots of
this plant?
They grow
towards
light.
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15.4 Phototropism of plants
15.4 Phototropism of plants
• like animals, plants can also detect
a number of stimuli (e.g. light) and
respond to them
• slower responses
• involve growth of certain parts of
the body
231
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15.4 Phototropism of plants
15.4 Phototropism of plants
• directional growth movement of
a part of a plant in response to a
unilateral stimulus (單側刺激)
tropism (向性)
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15.4 Phototropism of plants
A Responses of plants to
unilateral light
• directional growth movement of
a part of a plant in response to
unilateral light
phototropism (向光性)
233
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15.4 Phototropism of plants
A Responses of plants to
unilateral light
• shoots and roots respond differently to
unilateral light
unilateral light
234
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15.4 Phototropism of plants
A Responses of plants to
unilateral light
Shoot
• grows towards light
• positively phototropic
enables leaves to obtain
the maximum amount of
light for photosynthesis
235
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15.4 Phototropism of plants
A Responses of plants to
unilateral light
Root
• grows away from light
• negatively phototropic
enables roots to anchor
to the soil for support
236
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15.4 Phototropism of plants
15.2
Investigation of the phototropic responses
of shoots and roots
Procedure
1 Set up the apparatus as shown.
237
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15.4 Phototropism of plants
15.2
1
light-proof boxes
seedling
light
light
culture
solution
clinostat
(旋轉器)
stand
set-up A
set-up B
238
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15.4 Phototropism of plants
15.2
1
light
set-up A
light
set-up B
turn
on the
clinostat
239
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15.4 Phototropism of plants
15.2
2 Observe and record any differences in
the way the seedlings have grown in
both set-ups after 2 days.
240
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15.4 Phototropism of plants
15.2
Results and discussion
What do you observe in set-up A?
The shoots of the seedlings grow towards
the light (positively phototropic) while the
roots grow away from the light (negatively
phototropic).
241
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15.4 Phototropism of plants
15.2
Results and discussion
What do you observe in set-up B?
Set-up B is a control in which the clinostat
eliminates the effect of unilateral light.
In this set-up, the shoots grow vertically
upwards and the roots grow vertically
downwards.
242
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15.4 Phototropism of plants
B Early studies on phototropism
• coleoptiles (胚芽鞘)
are commonly used in
the investigations of
phototropism
243
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15.4 Phototropism of plants
B Early studies on phototropism
coleoptile
• sheath covering and
protecting the young leaf
in seedling of grass
coleoptile
first leaf
inside
• bursts open when the
first leaf develops
244
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15.4 Phototropism of plants
B Early studies on phototropism
coleoptiles are commonly
used because:
• their response to light is
easy to observe
coleoptile
first leaf
inside
• they grow rapidly
• they are small and easy
to handle
245
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Charles
Darwin
(1880)
15.4 Phototropism of plants
BoysenJensen
(1913)
Paal
(1919)
Went
(1926)
• scientists built their work upon earlier
findings of others
Simulation 15.1
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15.4 Phototropism of plants
Charles Darwin’s investigation (1880)
Aim:
To study the part of the coleoptile that
detects unilateral light
247
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15.4 Phototropism of plants
Charles Darwin’s investigation (1880)
intact
coleoptile
decapitated coleoptile
opaque cap
opaque collar
A
B
C
D
248
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15.4 Phototropism of plants
Charles Darwin’s investigation (1880)
unilateral light
A
B
C
D
A
B
C
D
249
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15.4 Phototropism of plants
Charles Darwin’s investigation (1880)
• The results show that
unilateral light
the tip is necessary
for growth.
A
B
C
D
A
B
C
D
250
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15.4 Phototropism of plants
Charles Darwin’s investigation (1880)
• The results show that
unilateral light
the tip is responsible
for detecting
unilateral light.
A
B
C
D
A
B
C
D
251
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
Aim:
To study the nature of signal transmission
involved in phototropism
252
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
intact
coleoptile
agar block (chemicals can
pass through)
tip placed on
agar block
mica plate
(chemicals cannot
pass through)
A
B
C
D
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
unilateral light
A
B
C
D
A
B
C
D
254
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
• A substance is
unilateral light
produced in the tip
and it is chemical in
nature.
agar
A B
block
mica
C D
plate
A
B
C
D
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
• A substance is
unilateral light
produced in the tip
and it is chemical in
nature.
×
A
B
C
D
A
B
C
D
256
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
• The chemical is
unilateral light
transmitted to the
lower part of the
coleoptile where it
causes bending to
occur.
A
B
C
D
A
B
C
D
257
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
mica plates
A
B
258
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
unilateral light
A
B
A
B
259
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15.4 Phototropism of plants
Boysen-Jensen’s investigation (1913)
• The chemical
unilateral light
produced in the tip
can pass down the
shaded side of the
coleoptile, causing
bending towards the
illuminated side.
A
B
A
B
260
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15.4 Phototropism of plants
Paal’s investigation (1919)
Aim:
To study how the chemical produced in the
tip of a coleoptile works
261
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15.4 Phototropism of plants
Paal’s investigation (1919)
tip put on left
side of cut end
A
tip put on right
side of cut end
B
262
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15.4 Phototropism of plants
Paal’s investigation (1919)
in darkness
A
B
A
B
263
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15.4 Phototropism of plants
Paal’s investigation (1919)
• The side with the
displaced tip
receives a higher
concentration of the
chemical
grows more rapidly,
causing
A bending
B
A
B
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15.4 Phototropism of plants
Went’s investigation (1926)
Aim:
To study the effect of unilateral light on
the distribution of the chemical in the tip
of a coleoptile
265
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
tip removed and placed on
an agar block for some time
266
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
the agar block is
placed on the cut end
A
267
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
A
A
268
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
mica plate
X
Y
agar blocks
269
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
unilateral light
X
Y
270
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
Y
X
X
B
Y
B
271
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15.4 Phototropism of plants
Went’s investigation (1926)
in darkness
• While the chemical
from the tip diffuses
into the agar block,
light causes an
uneven distribution
unilateral light
of the chemical.
X
Y
272
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15.4 Phototropism of plants
Went’s investigation (1926)
• The shaded side
has a higher
concentration of the
chemical
 grows more
rapidly
 the shoot bends
towards light
Y
X
A
B
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15.4 Phototropism of plants
C Auxins
• Went identified the chemical
produced in the tip of the
coleoptile that causes phototropism.
He named it:
auxin (生長素)
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15.4 Phototropism of plants
1 Nature of auxins
• a group of plant growth substances
• the most common naturally occurring
auxin is indoleacetic acid (IAA)
275
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15.4 Phototropism of plants
1 Nature of auxins
produced in small
amounts in apical
meristem
travel down
region of
elongation
promote cell
elongation
coleoptile
elongating cell
276
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15.4 Phototropism of plants
2 Effect of light on the distribution
of auxins
Hypotheses:
i Light destroys auxins.
ii Light causes auxins to move away from
the illuminated side to the shaded side.
Which hypothesis is valid?
277
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15.4 Phototropism of plants
Experiment 1
decapitated
coleoptile
coleoptile tip
agar
block A
A
agar
block B
B
278
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15.4 Phototropism of plants
Experiment 1
uniform light
in darkness
A
B
279
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15.4 Phototropism of plants
Experiment 1
in darkness
in darkness
24°
A
24°
B
280
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15.4 Phototropism of plants
Experiment 1
• Both coleoptiles bend
to the same degree
 amount of auxins in
agar blocks are
the same
regardless of light
or dark condition
A
24°
B
281
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15.4 Phototropism of plants
Experiment 1
• Light does not
destroy auxins.
24°
A
B
282
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15.4 Phototropism of plants
Experiment 2
coleoptile tip
L
R
mica plate
L
C
D
R
mica plate
E
F
283
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15.4 Phototropism of plants
Experiment 2
unilateral light
L
unilateral light
R
L
C
D
R
E
F
284
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15.4 Phototropism of plants
Experiment 2
in darkness
in darkness
L
L
R
R
L
L
C
D
R
R
E
F
285
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15.4 Phototropism of plants
Experiment 2
in darkness
in darkness
L
R
L
R
12°
24°
31°
C
D
E
F
286
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15.4 Phototropism of plants
Experiment 2
• Mica plate stops
lateral transport of
auxins.
• Light does not
destroy auxins.
C
24° 24°
D
12° 31°
E
F
287
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15.4 Phototropism of plants
Experiment 2
• Coleoptiles C and D
bend to the same
degree
 agar blocks
L and R contain
the same amount
of auxins
24° 24°
C
D
12° 31°
E
F
288
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15.4 Phototropism of plants
Experiment 2
• Coleoptile F bends
the most
 auxins move
from the
illuminated side
to the shaded
side
24° 24°
C
D
12° 31°
E
F
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The diagram below shows a set-up prepared by a
student to study the effect of unilateral light on the
growth of oat coleoptiles.
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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6 Nutrition in humans
a In which region of coleoptile 1 (P, Q or R) did
bending occur?
(1 mark)
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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Suggested answer
a Region Q (1)
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b i Comparing results of which two of the
coleoptiles (1 to 5) would allow drawing a
conclusion that oat coleoptiles show positive
phototropism?
(1 mark)
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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Suggested answer
b i Coleoptiles 1 and 2 (1)
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b ii Which one of the coleoptiles in your answer
to i serves as a control? Explain your answer.
(2 marks)
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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Suggested answer
b ii Coleoptile 2 (1)
It is identical to coleoptile 1 except that the
factor under investigation (i.e. unilateral light)
is removed by the rotating clinostat. (1)
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Setting up controls in phototropism
experiments
In an experiment, an experimental set-up and
a control are often prepared.
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Setting up controls in phototropism
experiments
experimental
set-up
control
•
•
coleoptile
1
2
identical to the
experimental set-up
except that the factor
under investigation
is absent
ensures the result of
the experiment is due
to the factor under
investigation only
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Setting up controls in phototropism
experiments
For example:
experimental
set-up
unilateral
light
control
in a
rotating
clinostat
How should the
control be set up?
factor under
investigation
coleoptile
1
2
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Setting up controls in phototropism
experiments
For example:
experimental
set-up
unilateral
light
control
•
in a
rotating
clinostat
factor under
investigation
coleoptile
1
2
identical to the
experimental set-up
the effect
except that ________
_________________
of
unilateral light
is absent (being
removed by the rotating
clinostat)
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Setting up controls in phototropism
experiments
For example:
experimental
set-up
unilateral
light
control
•
in a
rotating
clinostat
factor under
investigation
coleoptile
1
ensures the bending in
coleoptile 1 is due to the
effect of
unilateral light
__________________
but not others
2
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A student designed an experiment to test the
hypothesis that auxins are the growth-promoting
substances in oat coleoptiles. The experimental
set-up is shown below.
agar block
with auxins
decapitated
coleoptile
experimental
set-up
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Draw a diagram to show how the control of this
experiment should be set up.
(3 marks)
agar block
with auxins
agar block
without auxins
decapitated
coleoptile
decapitated
coleoptile
experimental
set-up
Correct use of an
agar block
without auxins (1)
Correct labels (1)
Correct drawing (1)
control
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c What conclusion can be drawn from the results of
coleoptiles 1, 3 and 4?
(1 mark)
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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Suggested answer
c There may be a growth-promoting substance
passing from the tip to the growing region of the
coleoptile. (1)
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d What would be the growth response of
coleoptile 5? Explain your answer using your
knowledge about phototropism.
(4 marks)
P
unilateral
light
in a
rotating
clinostat
Q
agar
block
mica
plates
R
coleoptile
1
result
bent
after
towards
two days the left
2
3
4
5
growth
without
bending
bent
towards
the left
growth
without
bending
(not
recorded)
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Suggested answer
d Coleoptile 5 would grow vertically upwards. (1)
As the mica plate prevents the lateral transport of
auxins, (1)
the auxins in the region of elongation of the
coleoptile is distributed evenly. (1)
As a result, the illuminated side and the shaded
side of the coleoptile grew at the same rate and no
bending occurred. (1)
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15.4 Phototropism of plants
3 Effect of auxin concentration on
the growth of shoots and roots
Effects of auxins on growth vary with:
• concentration of auxins
• parts of the plant concerned
Animation 15.7
308
% stimulation
15.4 Phototropism of plants
200
150
100
50
% inhibition
growth response
15
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Detectinginthe
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0
50
100
shoots
roots
10-6 10-4 10-2
1
102 104
concentration of auxins (ppm)
309
% stimulation
15.4 Phototropism of plants
high200
auxin
concentrations
150
promote shoot
100 but inhibit
growth
root50
growth
% inhibition
growth response
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Detectinginthe
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0
50
100
shoots
roots
10-6 10-4 10-2
1
102 104
concentration of auxins (ppm)
310
% stimulation
200
150
100
15.4 Phototropism of plants
low auxin
concentrations
promote root
growth
50
% inhibition
growth response
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0
50
100
shoots
roots
10-6 10-4 10-2
1
102 104
concentration of auxins (ppm)
311
% stimulation
200
150
15.4 Phototropism of plants
but have little effect
on shoot growth
100
50
% inhibition
growth response
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0
50
100
shoots
roots
10-6 10-4 10-2
1
102 104
concentration of auxins (ppm)
312
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15.4 Phototropism of plants
D Mechanism of phototropism
313
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15.4 Phototropism of plants
light from all directions
1
shoot
auxins
root
314
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2
15.4 Phototropism of plants
light from all directions
light from all directions
auxins are
distributed
evenly
315
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Detectinginthe
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15.4 Phototropism of plants
light from all directions
3
shoot grows
straight upwards
root grows
straight
downwards
316
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Detectinginthe
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15.4 Phototropism of plants
unilateral light
1
shoot
auxins
root
317
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Detectinginthe
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15.4 Phototropism of plants
unilateral light
2
auxins move from
illuminated side
to shaded side
unilateral
light
318
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Detectinginthe
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15.4 Phototropism of plants
unilateral light
3 in shoot, high auxin concentration
on shaded side
promotes growth
(cell elongation)
shoot bends
towards light
cells on
shaded
side
elongate
more
319
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Detectinginthe
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environment
15.4 Phototropism of plants
unilateral light
3 in root, high auxin concentration
on shaded side
inhibits growth
(cell elongation)
root bends away
from light
cells on
shaded side
elongate less
320
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Detectinginthe
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1
15.4 Phototropism of plants
Phototropism is the directional
growth movement of a part of a
plant in response to unilateral
light .
321
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2
15.4 Phototropism of plants
Shoots: grow towards light
(i.e. positively phototropic)
Significance: enables leaves to
reach a position where they can
obtain the maximum amount of
light for photosynthesis
322
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2
15.4 Phototropism of plants
Roots: grow away from light
(i.e. negatively phototropic)
Significance: enables roots to
anchor to the soil for support
323
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3
15.4 Phototropism of plants
Auxins are produced at
shoot tips and root tips .
324
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Detectinginthe
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3
15.4 Phototropism of plants
Auxins move to the region of
elongation in shoots and roots
and affect growth there.
325
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Detectinginthe
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4
15.4 Phototropism of plants
Unilateral light causes auxins to
move from the illuminated side
to the shaded side of the shoot
and that of the root.
326
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4
15.4 Phototropism of plants
This results in a higher
concentration of auxins on the
shaded side of the shoot and
that of the root.
327
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environment
4
15.4 Phototropism of plants
The high auxin concentration
promotes shoot growth but
inhibits root growth.
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4
15.4 Phototropism of plants
As a result, the shoot grows
faster on the shaded side and it
bends towards the light. The
root grows slower on the
shaded side and it bends
away from the light.
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1 What is the cause of short sight?
Short sight is caused by the lens being
too thick or the eyeball being too long,
or both.
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2 What is the role of the cornea in our eyes?
The cornea helps refract and focus light
onto the retina.
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Stimuli
examples
light
in humans is
detected by
eye
sound
in plants is
detected by
shoot tip
& root tip
in humans is
detected by
ear
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eye
contains
rod cells & cone cells
send nerve impulses to
visual centre in brain
gives sensation of
sight
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eye
has the ability of
controlling the
amount of light
entering it
eye
accommodation
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shoot tip & root tip
produce
auxins
under unilateral light show
uneven distribution
results in
phototropism
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environment
ear
contains
sensory hair cells
send nerve impulses to
auditory centre in brain
gives sensation of
hearing
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