CIE iGCSE Biology-14.2 Sense organs- Study Notes- New Syllabus
CIE iGCSE Biology-14.2 Sense organs- Study Notes – New syllabus
CIE iGCSE Biology-14.2 Sense organs- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals
- Identify in diagrams and images the structures of the eye, limited to: cornea, iris, pupil, lens, retina, optic nerve and blind spot
- Describe the function of each part of the eye, limited to:
(a) cornea – refracts light
(b) iris – controls how much light enters the pupil
(c) lens – focuses light on to the retina
(d) retina – contains light receptors, some sensitive to light of different colours
(e) optic nerve – carries impulses to the brain - Explain the pupil reflex, limited to changes in light intensity and pupil diameter
Supplement
- Explain the pupil reflex in terms of the antagonistic action of circular and radial muscles in the iris
- Explain accommodation to view near and distant objects in terms of the contraction and relaxation of the ciliary muscles, tension in the suspensory ligaments, shape of the lens and refraction of light
- Describe the distribution of rods and cones in the retina of a human
- Outline the function of rods and cones, limited to:
(a) greater sensitivity of rods for night vision
(b) three different kinds of cones, absorbing light of different colours, for colour vision - Identify in diagrams and images the position of the fovea and state its function
Sense Organs and Receptor Cells
Key Statement: Sense organs are groups of receptor cells that respond to specific stimuli.
📌 What Are Receptor Cells?
- Receptor cells are specialized cells that detect changes (stimuli) in the environment.
- Each receptor responds to a specific type of stimulus.
🧠 Major Sense Organs & Their Stimuli:
| Sense Organ | Type of Stimulus Detected | Receptor Type |
|---|---|---|
| Eye | Light | Light receptors (retina) |
| Ear | Sound and balance | Sound receptors |
| Skin | Touch, pressure, temperature | Touch and temperature receptors |
| Nose | Chemicals in the air (smell) | Olfactory receptors |
| Tongue | Chemicals in food (taste) | Taste receptors |
🧾 Summary:
Each sense organ contains receptors that detect specific types of stimuli like light, sound, touch, temperature, or chemicals, helping the body respond to the environment.
Each sense organ contains receptors that detect specific types of stimuli like light, sound, touch, temperature, or chemicals, helping the body respond to the environment.
Identifying Structures of the Human Eye (in Diagrams)
1. Cornea
- Location: Transparent front part of the eye.
- Function: Refracts (bends) light as it enters the eye.
2. Iris
- Location: Coloured ring around the pupil.
- Function: Controls the amount of light entering the eye by adjusting pupil size.
3. Pupil
- Location: Central black hole within the iris.
- Function: The opening through which light passes into the eye.
4. Lens
- Location: Just behind the iris and pupil.
- Function: Focuses light onto the retina by changing shape.
5. Retina
- Location: Lining at the back of the eye.
- Function: Contains light-sensitive cells (rods and cones) that detect light and send signals to the brain.
6. Optic Nerve
- Location: Extends from the back of the eye to the brain.
- Function: Carries electrical impulses from the retina to the brain.
7. Blind Spot
- Location: Where the optic nerve leaves the retina.
- Function: No photoreceptor cells here, so no image is detected in this small area.
Functions of the Eye – Key Structures
(a) Cornea:
- Transparent front part of the eye.
- Refracts (bends) incoming light to help focus it.
(b) Iris:
- Colored part of the eye.
- Controls how much light enters by adjusting the size of the pupil.
- In bright light: pupil gets smaller (constricts).
- In dim light: pupil gets wider (dilates).
(c) Lens:
- Transparent, flexible structure behind the iris.
- Focuses light precisely onto the retina.
- Changes shape (thicker or thinner) to focus on near or distant objects (called accommodation).
(d) Retina:
- Inner light-sensitive layer at the back of the eye.
- Contains light receptor cells:
- Rods – detect light intensity (black & white)
- Cones – detect colour
- Converts light into electrical impulses.
(e) Optic Nerve:
- Connects the retina to the brain.
- Carries electrical impulses from the eye to the visual centre of the brain for interpretation.
🧾 Summary:
The eye detects and focuses light using the cornea, lens, and retina, while the iris controls light entry and the optic nerve sends signals to the brain.
The eye detects and focuses light using the cornea, lens, and retina, while the iris controls light entry and the optic nerve sends signals to the brain.
Pupil Reflex – Response to Light Intensity
Key Concept:
The pupil reflex is an automatic response that controls the amount of light entering the eye by changing the diameter of the pupil.
The pupil reflex is an automatic response that controls the amount of light entering the eye by changing the diameter of the pupil.
🌞 In Bright Light:
Too much light can damage the retina, so the eye must protect itself.
- Circular muscles in the iris contract
- Radial muscles relax
- The pupil constricts (becomes smaller)
🌑 In Dim Light:
The eye needs to let in more light to see clearly.
- Radial muscles contract
- Circular muscles relax
- The pupil dilates (becomes wider)
🧠 Type of Reflex:
- This is an involuntary response (a reflex action).
- Controlled by the brain via the optic nerve.
🧾 Summary:
The pupil reflex helps protect the eye by adjusting pupil size in response to light intensity smaller in bright light, larger in dim light.
The pupil reflex helps protect the eye by adjusting pupil size in response to light intensity smaller in bright light, larger in dim light.
Pupil Reflex: Antagonistic Muscle Action in the Iris
What is the Pupil Reflex?
The pupil reflex is an automatic response that controls the amount of light entering the eye by adjusting the pupil’s diameter.
The pupil reflex is an automatic response that controls the amount of light entering the eye by adjusting the pupil’s diameter.
🔄 Antagonistic Muscle Pairs in the Iris
The iris contains two muscle types:
- Circular muscles
- Radial muscles
These muscles work as antagonistic pairs: When one contracts, the other relaxes.
🌞 In Bright Light
Goal: Reduce light entering the eye to protect the retina.
- Circular muscles contract
- Radial muscles relax
- Pupil constricts (gets smaller)
🌑 In Dim Light
Goal: Increase light entering the eye to improve vision.
- Radial muscles contract
- Circular muscles relax
- Pupil dilates (gets wider)
🧾Summary
The pupil reflex involves the antagonistic action of circular and radial muscles in the iris to control pupil size:
Bright light → Circular muscles contract, radial relax → Pupil constricts
Dim light → Radial muscles contract, circular relax → Pupil dilates
Accommodation – Focusing on Near and Distant Objects
Definition:
Accommodation is the process by which the lens changes shape to focus light from objects at different distances onto the retina.
Accommodation is the process by which the lens changes shape to focus light from objects at different distances onto the retina.
🔄 Key Parts Involved:
- Ciliary muscles
- Suspensory ligaments
- Lens
- Light refraction
📍 To View a Near Object:
| Component | Action |
|---|---|
| Ciliary muscles | Contract |
| Suspensory ligaments | Loosen (less tension) |
| Lens shape | Becomes thicker (more curved) |
| Refraction of light | More refraction needed to sharply focus nearby objects |
🌐 To View a Distant Object:
| Component | Action |
|---|---|
| Ciliary muscles | Relax |
| Suspensory ligaments | Tighten (more tension) |
| Lens shape | Becomes thinner (less curved) |
| Refraction of light | Less refraction needed for distant light rays |
🧾 Key Summary:
- To focus on near objects, the lens becomes thicker by contraction of ciliary muscles and loosening of suspensory ligaments, increasing refraction.
- To focus on distant objects, the lens becomes thinner due to relaxed ciliary muscles and tight suspensory ligaments, reducing refraction.
Distribution of Rods and Cones in the Retina
Two Types of Light Receptors:
- Rods – Detect light intensity (black, white, shades of grey)
- Cones – Detect colour (red, green, blue types)
📍 Distribution in the Retina:
| Region | Rods | Cones |
|---|---|---|
| Fovea (central retina) | Very few rods | High density of cones – for sharp, detailed colour vision |
| Peripheral retina | High density of rods – especially towards the edges | Fewer cones |
🔎 Why This Distribution Matters:
- Fovea: Gives sharp, colour vision in bright light (e.g., reading or seeing details).
- Peripheral retina: Allows better night vision and detection of movement in dim light, but without colour.
🧾 Key Summary:
The fovea contains mostly cones for detailed colour vision, while the peripheral retina contains mostly rods, enabling vision in dim light and detecting movement.
Function of Rods and Cones in the Retina
(a) Rods – Night Vision
- Highly sensitive to light
- Can detect very low levels of light
- Allow black-and-white vision in dim light or at night
- Do not detect colour
📝 Key Point:
Rods are more sensitive to light than cones, making them essential for night vision.
(b) Cones – Colour Vision
- Work best in bright light
- There are three types of cones:
- Red-sensitive cones
- Green-sensitive cones
- Blue-sensitive cones
- Together, they allow us to see a full range of colours
📝 Key Point:
Cones provide colour vision by detecting different wavelengths of light (red, green, blue).
🧾 Summary:
Rods help with black-and-white night vision due to their high light sensitivity, while cones enable colour vision using three types that respond to red, green, and blue light.
Rods help with black-and-white night vision due to their high light sensitivity, while cones enable colour vision using three types that respond to red, green, and blue light.