Sense OrgansCambridge IGCSE Biology: Revision notes
Section 1
What Are Sense Organs?
Sense organs are groups of receptor cells that respond to specific stimuli.
Different sense organs detect different stimuli:
| Stimulus | Example sense organ |
|---|---|
| Light | Eyes |
| Sound | Ears |
| Touch | Skin |
| Temperature | Skin |
| Chemicals | Nose, tongue |
Section 2
Structures of the Eye
The eye contains several key structures, each with a specific function:
| Structure | Function |
|---|---|
| Cornea | Refracts (bends) light as it enters the eye |
| Iris | Controls how much light enters through the pupil |
| Pupil | The opening through which light enters |
| Lens | Focuses light onto the retina |
| Retina | Contains light receptors, some sensitive to different colours of light |
| Optic nerve | Carries impulses from the retina to the brain |
| Blind spot | The point where the optic nerve leaves the eye; contains no light receptors |
Learn structure-function pairs together: cornea refracts, iris controls light entry, lens focuses, retina detects, optic nerve carries impulses to the brain.
Section 3
The Pupil Reflex
The pupil reflex automatically adjusts the amount of light entering the eye, depending on light intensity.
- In bright light, the pupil constricts (gets smaller) to reduce the amount of light entering, protecting the retina
- In dim light, the pupil dilates (gets bigger) to let more light in, improving vision
This is controlled by antagonistic muscles in the iris:
- Circular muscles contract to make the pupil smaller (bright light)
- Radial muscles contract to make the pupil larger (dim light)
Antagonistic muscles work in opposite directions - as one contracts, the other relaxes.
Do not mix up the muscle types: circular muscles constrict the pupil in bright light; radial muscles dilate the pupil in dim light.
Section 4
Accommodation: Focusing on Near and Far Objects
Accommodation is the process by which the eye changes the shape of the lens to focus light from objects at different distances.
Focusing on a near object:
- Ciliary muscles contract
- Suspensory ligaments become slack
- The lens becomes fatter (more rounded), refracting light more strongly
Focusing on a distant object:
- Ciliary muscles relax
- Suspensory ligaments become tight (taut)
- The lens becomes thinner, refracting light less strongly
Section 5
Rods, Cones and the Fovea
The retina contains two types of light receptor cell:
- Rods - sensitive to dim light, used for night vision; cannot distinguish colour
- Cones - responsible for colour vision; there are three types, each absorbing a different colour of light (red, green, blue)
The fovea is a small area of the retina with a very high concentration of cone cells. It is the point of sharpest, most detailed colour vision.
Must Know
- Sense organs are groups of receptor cells responding to specific stimuli: light, sound, touch, temperature, chemicals
- Eye structures: cornea (refracts), iris (controls light entry), lens (focuses), retina (detects light), optic nerve (carries impulses)
- Pupil reflex: circular muscles constrict pupil in bright light; radial muscles dilate pupil in dim light
- Accommodation: ciliary muscles contract and lens fattens for near objects; ciliary muscles relax and lens thins for distant objects
- Rods detect dim light (night vision); cones detect colour; the fovea has a high concentration of cones for sharp vision
That's the notes covered.
Carry on to the next subtopic.