Sense organs: structure and function of the eye — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 43Sense organs and receptors

Receptors

• Receptors groups of specialised cells that detect a change in the environment and stimulate electrical impulses.

Sense organs

• Sense organs organs that contain groups of receptors responding to specific stimuli.

Section 2 of 43Sense organs

Sense organ	Sensitive to	Sense
Skin	Pressure, heat, cold and pain	Touch and temperature
Tongue	Chemicals in food and drink	Taste
Nose	Chemicals in the air	Smell
Ear	Sound and movement	Hearing and balance
Eye	Light	Sight

Section 3 of 43From stimulus to response

1

The receptor cell in the sense organ is stimulated and generates an electrical impulse.

2

A sensory neurone carries the impulse to the central nervous system.

3

A response is decided on, and the impulse passes via a relay neurone to a motor neurone.

4

The motor neurone carries the impulse to the effector.

5

The effector, a muscle or a gland, carries out the response.

Section 4 of 43Structure of the eye

Structure	Function
Cornea	Transparent covering at the front of the eye that refracts light
Iris	Muscle that controls how much light enters the pupil
Lens	Transparent disc that changes shape to focus light onto the retina
Retina	Layer containing the light receptor cells
Optic nerve	Sensory neurone that carries impulses from the eye to the brain
Fovea	Region of the retina with the most cones, which forms sharp coloured images

Section 5 of 43Pupil reflex

Role

• Role the pupil reflex controls the light entering the eye by changing the pupil diameter.

Dim light

• Dim light the pupil dilates to let in as much light as possible.

Bright light

• Bright light the pupil constricts to prevent too much light entering and damaging the retina.

Section 6 of 43Iris muscles in the pupil reflex

Condition	Radial muscles	Circular muscles	Pupil	Light entering
Dim light	Contract	Relax	Dilated	More
Bright light	Relax	Contract	Constricted	Less

• Antagonistic
  • the circular and radial muscles work against each other, so when one contracts the other relaxes.

Section 7 of 43Accommodation

Accommodation

Accommodation the way the eye focuses on near or distant objects by changing the shape of the lens.

Control

• Control the change is brought about by the ciliary muscles and the tension in the suspensory ligaments.

Section 8 of 43Near and distant objects

Part of the eye	Near objects	Distant objects
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Fatter and more rounded	Thinner
Light	Refracted more	Refracted less

Section 9 of 43Rods and cones

Rods

• Rods detect light at low levels, so they are important for night vision.

Cones

• Cones three types detect different wavelengths, which gives colour vision.

Distribution

• Distribution rods are found all over the retina except at the blind spot, and cones are concentrated in the fovea.

Section 10 of 43Pupil reflex practical

1

Sit a partner in dim light for a minute and estimate the pupil diameter.

2

Shine a torch briefly into one eye from the side.

3

Record the new pupil diameter.

4

Switch off the torch and watch the pupil dilate again.

Section 11 of 43More from the specification

More from the specification

Endocrine homeostasis and blood glucose control

• Blood glucose concentration is maintained via negative feedback;
  • elevated glucose triggers insulin secretion from the pancreas, stimulating hepatic glycogen synthesis.

Hormones and systemic coordination

• Endocrine glands secrete chemical hormones into the blood, coordinating metabolic functions throughout the body slower and longer-term than nervous impulses.

Plant growth regulation by auxin

• In plants, the hormone auxin diffuses towards shaded tissue in shoots, promoting differential cell elongation so stems grow towards unilateral light.
  • (positive phototropism)

Extended integration of nervous and endocrine systems

• Both the nervous system and circulating hormones coordinate organism responses to environmental changes, preserving homeostatic stability through negative feedback loops.

Synapse

• A junction between two neurones.

Negative feedback

• A control that reverses a change to restore the set point.

Glucose

• Sugar that is the fuel for respiration.

Growth

• Permanent increase in size.

Insulin

• The hormone that lowers blood glucose.

Hormone

• A chemical messenger carried in the blood.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Gravity

• Force of attraction between objects with mass.

Section 12 of 43Retina and optic nerve

Retina

• Retina the layer of light receptor cells that detect light intensity and colour.

Optic nerve

• Optic nerve a sensory neurone that carries impulses from the eye to the brain.

Fovea

• Fovea the region of the retina with the most cone cells, forming sharp, coloured images.

Light path

• Light path the cornea and lens refract light and focus it onto the retina, which converts it into electrical impulses.

Section 13 of 43Rods and cones

Feature	Rods	Cones
Detect	Light at low intensity	Bright light and colour
Colour	No, black and white	Yes, three types sensitive to different wavelengths
Best for	Night vision	Detailed daytime vision
Position	All over the retina except the blind spot	Concentrated in the fovea

Section 14 of 43Pupil reflex

Pupil reflex

Pupil reflex a reflex action that controls the light entering the eye by altering the pupil diameter.

Iris muscles

• Iris muscles circular muscles form rings around the pupil and radial muscles radiate outwards;
  • they are antagonistic.

Light level	Radial muscles	Circular muscles	Pupil	Light entering the eye
Dim	Contract	Relax	Dilates (wider)	More
Bright	Relax	Contract	Constricts (narrower)	Less, so the retina is protected

Section 15 of 43Accommodation

Accommodation

Accommodation the way the eye focuses on near or distant objects by changing the shape of the lens.

Mechanism

• Mechanism the ciliary muscles contract or relax, which changes the tension in the suspensory ligaments.

Part of the eye	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Fatter, more rounded	Thinner, flatter
Light refraction	More	Less

Section 16 of 43Everyday signs of accommodation

Near object

• Near object staring at your hand tires the eyes because the ciliary muscles are contracted.

Distant object

• Distant object looking far away feels comfortable because the ciliary muscles are relaxed.

Section 17 of 43Hormones and the eye

Adrenaline

• Adrenaline it dilates the pupils when the body is in danger.

Hormone

Hormone a chemical messenger released by a gland into the blood, and only cells with complementary target receptors respond.

Section 18 of 43More from the specification

More from the specification

Histology and optical refractive components of the eye

• Light rays refract through the transparent cornea, aqueous humour, crystalline lens,
• and vitreous humour to focus an inverted image on the retina.

Accommodation reflex for distant and near vision

• Ciliary muscles contract to slacken suspensory ligaments,
• allowing the elastic lens to become more convex and converge divergent rays from nearby objects.

Photoreceptor distribution on the retina

• High-acuity color-sensitive cone cells concentrate in the central fovea,
• while high-sensitivity rod cells occupy the peripheral retina for monochromatic vision in low light.

Negative feedback homeostasis

• Negative feedback homeostasis autonomic nervous system regulates body temperature and circulating hormone levels around stable physiological setpoints.

Insulin blood glucose control

• Insulin blood glucose control beta endocrine cells release insulin to promote glucose uptake by hepatic cells when blood levels rise.

Auxin asymmetric distribution

• Auxin asymmetric distribution lateral light causes auxin to accumulate on shaded shoot sides, promoting unequal cell elongation.

Synapse

• A junction between two neurones.

Negative feedback

• A control that reverses a change to restore the set point.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Glucose

• Sugar that is the fuel for respiration.

Temperature

• Average energy of the particles in an object.

Growth

• Permanent increase in size.

Insulin

• The hormone that lowers blood glucose.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Gravity

• Force of attraction between objects with mass.

Section 19 of 43Light entering the eye

1

The cornea refracts the light.

2

The pupil is the opening through which light enters, and the iris changes its size.

3

The lens focuses the light onto the retina.

4

Receptor cells in the retina change light into electrical impulses.

5

The impulses pass along the optic nerve to the brain, which interprets the image.

Section 20 of 43Rods and cones

Feature	Rods	Cones
Detect	Light at low intensity	Bright light and colour
Colour	No, black and white	Yes, three types sensitive to different wavelengths
Best for	Night vision	Detailed daytime vision
Position	All over the retina except the blind spot	Concentrated in the fovea

Section 21 of 43Fovea and blind spot

Fovea

• Fovea the retina region with the highest density of cones, giving sharp coloured images.

Blind spot

• Blind spot the point where the optic nerve leaves the eye has no receptors.

Section 22 of 43Colour vision

Colour vision

Three cone types

• Three cone types each is sensitive to a different wavelength.
  • (red, green or blue)

Brain

• Brain it combines their signals to see all other colours.

Colour blindness

• Colour blindness one cone type is missing or does not work normally.

Section 23 of 43Pupil reflex

Pupil reflex

• Pupil reflex a reflex that controls the light entering the eye by altering the pupil diameter.

Iris

• Iris circular muscles form rings and radial muscles radiate outwards, and they work antagonistically.

Light level	Radial muscles	Circular muscles	Pupil	Light entering the eye
Dim	Contract	Relax	Dilates (wider)	More
Bright	Relax	Contract	Constricts (narrower)	Less, so the retina is protected

Section 24 of 43Adapting to dim light

Pupil

• Pupil it widens to let in more light.

Rods

• Rods they gradually become more sensitive as pigment re-forms, so vision improves over several minutes.

Section 25 of 43Bright light compared with dim light

Condition	Pupil	Receptors working	Vision
Dim light	Wide	Rods	Shades of grey, less sharp
Bright light	Narrow	Cones	Sharp and in colour, best at the fovea

Section 26 of 43More from the specification

More from the specification

Nervous system coordination and neurones

• Sensory receptors detect stimuli and transmit electrical impulses along sensory neurones to the central nervous system,
• where relay and motor neurones coordinate the effector response.

Blood glucose regulation by insulin

• In endocrine homeostasis, high blood glucose levels stimulate pancreatic beta cells to secrete insulin, promoting cellular uptake and glycogen storage to restore set-point levels.

Temperature regulation and homeostatic control

• Thermoreceptors monitor core body and skin temperature, initiating physiological autonomic responses via the nervous system to maintain optimal thermal stability.

Auxin-mediated plant growth vs animal coordination

• While animals rely on fast electrical impulses through neurones, plants regulate directional growth responses using chemical messengers such as auxin.

Nervous system coordination and neurones

• Sensory receptors detect stimuli and transmit electrical impulses along sensory neurones to the central nervous system,
• where relay and motor neurones coordinate the effector response.

Blood glucose regulation by insulin

• In endocrine homeostasis, high blood glucose levels stimulate pancreatic beta cells to secrete insulin, promoting cellular uptake and glycogen storage to restore set-point levels.

Temperature regulation and homeostatic control

• Thermoreceptors monitor core body and skin temperature, initiating physiological autonomic responses via the nervous system to maintain optimal thermal stability.

Auxin-mediated plant growth vs animal coordination

• While animals rely on fast electrical impulses through neurones, plants regulate directional growth responses using chemical messengers such as auxin.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Gravity

• Force of attraction between objects with mass.

Synapse

• A junction between two neurones.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Glucose

• Sugar that is the fuel for respiration.

Temperature

• Average energy of the particles in an object.

Neurone

• A nerve cell that carries electrical signals.

Growth

• Permanent increase in size.

Insulin

• The hormone that lowers blood glucose.

Hormone

• A chemical messenger carried in the blood.

Section 27 of 43Accommodation

Accommodation

Accommodation focusing on near or distant objects by changing the lens shape, which changes how much light is refracted.

Mechanism

• Mechanism contraction or relaxation of the ciliary muscles changes the tension in the suspensory ligaments.

Part of the eye	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Fatter, more rounded	Thinner, flatter
Light refraction	More	Less

Section 28 of 43Looking at near and distant objects

Near object

• Near object the ligaments loosen and the lens becomes fatter.
• The ciliary muscles contract.
• Light is refracted more.

Distant object

• Distant object the ligaments tighten and pull the lens thinner.
• The ciliary muscles relax.
• Light is refracted less.

Section 29 of 43Where refraction takes place

Cornea

• Cornea it provides most of the refraction of light entering the eye.

Lens

• Lens it is the only part that changes shape, so it fine-tunes the focus.

Section 30 of 43Sight defects and correction

Defect	Problem	Cause	Correction
Short sight (myopia)	Distant objects blurred	Eyeball too long or lens too curved, so the image forms in front of the retina	Concave (diverging) lens
Long sight (hyperopia)	Near objects blurred	Eyeball too short or lens too weak, so the image would form behind the retina	Convex (converging) lens

Section 31 of 43Ageing and the lens

Stiffer lens

• Stiffer lens with age the lens cannot become thick enough, so close focusing is harder.

Reading glasses

• Reading glasses they supply the extra bending that older people need.

Section 32 of 43Other methods of correction

Contact lenses

• Contact lenses they work like glasses but sit on the cornea.

Laser surgery

• Laser surgery it reshapes the cornea so the focus falls on the retina.

Cataracts

• Cataracts the lens becomes cloudy, and eye tests allow early detection.

Section 33 of 43More from the specification

1Photoreceptors and the Pupillary Light Reflex

• The retina contains light-sensitive rod and cone cells that detect changes in light intensity, triggering a reflex impulse along sensory neurones to adjust the pupil diameter and regulate light entering the eye.

2Neuronal Transmission in Involuntary Responses

• In involuntary reflex pathways, electrical impulses travel across synapses between sensory, relay,
• and motor neurones, generating a rapid muscular response that protects tissues from damage.

3Hormonal Homeostasis vs. Nervous Responses

• While nervous reflexes coordinate fast, short-lived electrical responses like pupil constriction, homeostasis of blood glucose levels and core body temperature relies on prolonged hormonal and autonomic mechanisms.

4Chemical Coordination

• Human Hormones vs. Plant Auxin — Unlike animal coordination where hormones travel in the blood to regulate glucose levels, plants lack nervous systems and rely on plant growth substances like auxin to coordinate growth responses to directional stimuli.

5Motor neurone

• A neurone carrying impulses from the CNS to an effector.

6Impulse

• Electrical signal along a neurone

7Concentration

• Amount of solute dissolved in a volume of solution.

8Homeostasis

• Maintaining a constant internal environment.

9Glycogen

• The storage carbohydrate in animals.

10Gravity

• Force of attraction between objects with mass.

11Synapse

• A junction between two neurones.

12Pupil reflex

• The automatic change in pupil size in response to light.

13Negative feedback

• A control that reverses a change to restore the set point.

14Glucose

• Sugar that is the fuel for respiration.

15Temperature

• Average energy of the particles in an object.

16Neurone

• A nerve cell that carries electrical signals.

17Growth

• Permanent increase in size.

18Insulin

• The hormone that lowers blood glucose.

Section 34 of 43Structure of the eye

Structure	Function
Cornea	Transparent front covering that refracts light the most
Iris	Muscle that controls how much light enters the pupil
Pupil	Opening through which light enters
Lens	Transparent disc that changes shape to focus light onto the retina
Retina	Layer of light receptor cells (rods and cones)
Optic nerve	Sensory neurone carrying impulses from the eye to the brain
Fovea	Retina region with the most cones; forms sharp coloured images
Ciliary muscles and suspensory ligaments	Change the shape of the lens
Sclera	Tough white outer coat
Choroid	Pigmented layer that stops reflection

Section 35 of 43Rods and cones

Feature	Rods	Cones
Detect	Light at low intensity	Bright light and colour
Colour	No, black and white	Yes, three types sensitive to different wavelengths
Best for	Night vision	Detailed daytime vision
Position	All over the retina except the blind spot	Concentrated in the fovea

Section 36 of 43Focusing by accommodation

Part of the eye	Near object	Distant object
Ciliary muscles	Contract	Relax
Suspensory ligaments	Loosen	Tighten
Lens	Fatter, more rounded	Thinner, flatter
Light refraction	More	Less

• Light
  • the cornea and lens refract it to form a sharp image on the retina.

Section 37 of 43Pupil reflex

Role

• Role the iris controls how much light enters, which protects the retina.

Automatic

• Automatic it is a reflex and needs no conscious control.

Light level	Radial muscles	Circular muscles	Pupil	Light entering the eye
Dim	Contract	Relax	Dilates (wider)	More
Bright	Relax	Contract	Constricts (narrower)	Less, so the retina is protected

Section 38 of 43Short sight and long sight

Defect	Image forms	Cause	Correction
Short sight	In front of the retina; distant objects blurred	Eyeball too long or lens too strong	Concave lens
Long sight	Behind the retina; near objects blurred	Eyeball too short or lens too weak	Convex lens

Section 39 of 43Image interpretation and blind spot

Inverted image

• Inverted image the cornea and lens form an upside-down image that the brain interprets as upright.

Impulses

• Impulses light striking rods and cones triggers impulses along the optic nerve.

Blind spot

• Blind spot where the optic nerve leaves there are no receptors, so no image is detected.

Section 40 of 43Ageing and other conditions

Ageing

• Ageing the lens loses elasticity and cannot become round enough, so near focusing is difficult.

Cataract

• Cataract the lens becomes cloudy, and it can be treated by replacing the lens with an artificial one.

Colour blindness

• Colour blindness it arises from faulty cones.

Section 41 of 43More from the specification

More from the specification

Coordination Systems Comparison

• The mammalian nervous system uses electrical impulses for rapid, localized responses,
• while the endocrine system secretes hormones like insulin into the blood for slower, widespread regulation.

Blood Glucose Regulation

• High blood glucose levels stimulate pancreatic beta cells to secrete insulin, promoting cellular uptake and conversion of glucose into stored glycogen in the liver.

Thermoregulation Homeostasis

• The nervous system detects core body temperature variations via the hypothalamus, triggering physiological responses like sweating or vasoconstriction.

Plant Growth Control by Auxin

• Auxin is a plant growth hormone synthesized in shoot tips that stimulates cell elongation, coordinating directional growth responses such as phototropism.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Gravity

• Force of attraction between objects with mass.

Synapse

• A junction between two neurones.

Negative feedback

• A control that reverses a change to restore the set point.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Glucose

• Sugar that is the fuel for respiration.

Temperature

• Average energy of the particles in an object.

Growth

• Permanent increase in size.

Insulin

• The hormone that lowers blood glucose.

Hormone

• A chemical messenger carried in the blood.

Section 42 of 43Key terms

Term	Meaning
Cornea	The transparent front of the eye that refracts light.
Iris	The muscle that controls the size of the pupil.
Retina	The layer at the back of the eye with light receptors.
Fovea	The part of the retina with the most cone cells.
Accommodation	The change in lens shape to focus on near or distant objects.
Pupil reflex	The automatic change in pupil size in response to light.
Rod cell	A receptor cell that works in dim light.
Cone cell	A receptor cell that detects colour.

Term	Meaning
Retina	Light-sensitive layer of the eye
Optic nerve	Carries impulses from the retina to the brain
Fovea	Region of the retina giving sharpest vision
Suspensory ligaments	Fibres joining the lens to the ciliary muscles
Iris	Muscle controlling pupil size
Accommodation	Focusing by changing lens shape

Term	Meaning
Rod cell	Receptor sensitive to dim light
Cone cell	Receptor for colour and bright light
Fovea	Retina region with the most cones
Blind spot	Region with no receptors where the optic nerve leaves
Iris	Muscular ring that controls pupil size
Optic nerve	Carries impulses to the brain

Term	Meaning
Accommodation	Changing lens shape to focus
Ciliary muscle	Muscle that changes the lens shape
Suspensory ligament	Fibre connecting the lens to the ciliary muscle
Short sight	Condition in which distant objects are blurred
Long sight	Condition in which near objects are blurred
Refraction	Bending of light

Term	Meaning
Retina	Light-sensitive layer at the back of the eye
Accommodation	Changing the lens shape to focus on near or distant objects
Iris	Muscle controlling pupil size
Cornea	Clear front layer of the eye
Rod	Receptor cell for dim light
Cone	Receptor cell for colour in bright light

Section 43 of 43Exam technique

1

Saying cones work in dim light — rods do.

2

Saying the lens becomes thicker for distant objects — it becomes thinner.

3

Confusing the optic nerve with the retina — the retina has the receptors and the nerve carries the impulses.

4

Saying the iris focuses light — the lens does this and the iris controls the amount.

1

Saying the lens flattens for near vision — it becomes fatter.

2

Believing the pupil reflex needs a conscious decision — it is automatic.

3

Mixing up the ciliary muscles with the iris muscles.

4

Saying the fovea has many rods — it has the most cones.

1

Saying cones are for night vision — rods are.

2

Saying the optic nerve detects light — the retina does.

3

Saying the lens does all of the refraction — the cornea does most.

4

Saying the pupil has muscles — the iris has them, the pupil is the hole.

1

Saying the lens thickens for distant objects — it gets thinner.

2

Saying the ciliary muscles relax for near objects — they contract.

3

Mixing up convex and concave lenses — short sight needs concave, long sight needs convex.

4

Saying the cornea changes shape to focus — the lens does.

1

Saying the lens does most of the bending of light — the cornea does.

2

Saying near objects need a thin lens — they need a thicker lens.

3

Mixing up ciliary muscle contraction with ligament tension — contraction loosens the ligaments.

4

Saying rods detect colour — cones do.

5

Describing the pupil as a structure — it is a hole in the iris.