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Lee Yi Zhan

on 27 March 2014

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Transcript of SCIENCE FORM 2

1.1 Sensory Organs
1.2 Sense of Touch
Structure of human skin
1.4 Sense of Taste
The position of sensory cells in the nose
1.3 Sense of Smell
Our five sense are very important to us. They help us to be sensitive to the changes around us. These changes in the surroundings are known as stimuli.

Example: Sense of touch
Sense of smell
Sense of taste
Sense of hearing
Sense of sight
The sensory organ for taste is the tongue. Our tongue can detect four basic tastes: salty ,sweet ,sour and bitter.
The skin is the sensory organ for touch. It is the largest organ in the body. The skin can detect changes in temperature ,pain ,touch and pressure. The skin has special receptors to detect each of these stimuli. The skin consists of two main layers, the outer epidermis and the inner dermis. The epidermis is waterproof and tough ,acting as a protective layer. The dermis contains many blood vessels ,sweat glands and receptors.
-Touch receptors are sensitive to slight pressure. For example ,they can tell you whether an object is rough or smooth.
-Pain receptors detect pain. They lie very lose to the skin surface so as to detect the slightest pain.
-Heat receptor are sensitive to heat.
-Cold receptors are sensitive to cold.
-Pressure receptors lie deep within the skin and are sensitive only to heavy pressure. For example ,you feel the weight when you carry a heavy object.
Our nose not only help us with our breathing but it is also a sensory organ of smell. Smells are actually chemicals present in the air. Food ,perfume and flowers release chemicals into the air. When we breath in ,the air passes through the nasal cavity. The chemical present in the air dissolve in the mucous lining and stimulate the sensory cells. These sensory cells send out nerve impulses to the brain which interprets them as a smell. We can get used to a smell. After smelling something for a long time ,the sensory cells stop sending messages to the brain. Then we can no longer smell it. The sensitivity to smell varies with individuals.
1.1 Sensory Organs
1.2 Sense of Touch
1.3 Sense of Smell
1.4 Sense of Taste
1.5 Sense of Hearing
1.6 Sense of Sight
1.7 Light and Sight
1.8 Sound and
1.9 Stimuli and Responses in Plant
1.5 Sense of Hearing
The ear is the sensory organ of hearing. It has three main parts - the outer ear ,the middle ear and the inner ear. Most parts of the ear are situated in the skull.
The hearing mechanism
-The pinna collects sound
waves and directs them along
the ear canal to eardrum.
-When the sound waves hit
the eardrum ,it vibrates.
-The ossicles amplify the vibration about 20 times before
transferring them to the oval window.
-Vibration of the oval window set up waves which travel
through the fluid in the cochlea.
-Receptors in the cochlea are stimulated to produce nerve
-The auditory nerve carries the impulses to the brain.
-The brain interprets the impulses as sounds.
1.6 Sens of Sight
Our eyes are responsible for our sense of sight. Each eye is a slightly flattened ball. The eyelid ,a thin layer of skin ,protects the front of the eye. The eyeball is held in the eye sockets of skull by muscles.
1.7 Light and Sight
Reflection of light occurs when light bounces off the surfaces of an object. Light can travel through transparent media such as air ,water ,glass and plastic. When light travels from one medium to another ,its speed change. This cause the light to bend or change direction. The bending of a light ray when it enters a medium of different density is know as the refraction of light.
1.8 Sound and Hearing
Sound are produced by vibration. The vocal cords in our voice box ,the ruler an the tuning fork ,all vibrate to produce sound. Sound needs a medium to travel. Sound can travel through solids ,liquids and gases. This is why when air is pumped out of the bell jar ,no sound is heard. This shows that sound cannot travel through a vacuum. When sound hits a surface ,it can be reflected or absorbed. Hard and smooth surface like walls are good reflectors of sound. Soft and rough surface like curtains are good absorbers of sound. Reflected sound is called an echo. Where can you hear an echo? In a recording studio ,echoes would spoil the quality of recording. So ,the whole studio is usually covered with soft materials such as carpets ,soft boards and curtains to reduce echo.
1.9 Stimuli and Responses in Plants
Plant do not have eyes ,nose and ears like we do. But they can detect stimuli too. Plant are sensitive to three main kinds of stimuli: water ,gravity and light. Other stimuli include touch ,temperature and chemicals. Let us look at some common tropisms.

Example of Tropism: Hydrotropism -Response to water
Geotropism-Response to gravity
Phototropism -Response to light
Thigmotropism -Response to touch
Nastic movements
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