Year 12 Biology Notes – Communication

1. Humans, and other animals, are able to detect a range of stimuli from the external

environment, some of which are useful for communication

Communication is the transfer of information or messages from one organism to another.

Identify the role of receptors in detecting stimuli

A stimulus is a change in the environment. Examples of stimuli include light, sound,

temperature, pressure, pain and certain chemicals.

The cells of the nervous system are not all the same, some have the role of detecting

changes in the environment and converting information into electrochemical signals for

translation in the brain, these are called receptors.

Stimulus Type of receptor Organ

Light Photoreceptors Eye

Touch Mechanoreceptors Skin

Temperature change Thermoreceptors Skin, hypothalamus

Explain that the response to a stimulus involves:

- Stimulus

Stimulus that reflects changes in the environment

- Receptor

Receptor that detects the stimulus. Each type of sensor is responsible for detecting a

certain type of stimulus.

- Messenger

Messenger that involves receptors that change the energy of the stimulus into an

electrochemical signal that is used to start a nerve impulse. The nerve impulse is the

messenger that is sent via the sensory neuron to the central nervous system (CNS) via

the spinal cord.

- Effector

Simultaneously, while the message is transmitted to the brain the CNS sends a message

via the motor neuron to the effector organ.

Effector that is the organ that receives the message and carries out the response.

- Response

Response is carried out in reaction to the stimulus.

Identify data sources, gather, and process information from secondary sources to identify the

range of senses involved in communication

Sense Explanation

Sight (visual) - Detected by photoreceptors

- Sight is frequently the means by which animals obtain information

about their environment

- Used to measure distance, determine colour and recognise

potential threat

- Facial expression and posture in humans communicate aggression

or affection

Sound (auditory) - Mechanoreceptors respond to mechanical energy and can detect

pressure waves

- Communication by sound

- Many species cannot produce sound or detect a wide range of

sounds frequencies

- Used to attract mates and distress calls to alert others of dangers

Smell (olfactory) - Chemoreceptors detect chemicals

- Communication occurs via chemical signals

- Many animals rely on smell to find food, find a mate or as a means

of identification

Touch (tactile) - Mechanoreceptors for touch are abundant under the skin

- Sensory nerve endings in the skin respond to touch

- Used in avoiding obstacles, fighting, defence mechanisms,

friendship behaviour and courtship

Taste (gestation) - Chemoreceptors detect chemicals

- Many animals use taste if they have a poor sense of smell (these

two are closely related senses)

2. Visual communication involves the eye registering changes in immediate environment

Describe the anatomy and function of the human eye, including the; conjunctiva, cornea, sclera,

choroid, retina, iris, lens, aqueous and vitreous humour, ciliary body, optic nerve

The eye functions as a sense organ by detecting light stimuli from the environment and transforming

this information received into

nerve impulses that are carried to

the brain. Humans have two eyes

for binocular vision. Each eye

sees a different image of an

object in the light path. The two

images are fused into one image

in the brain, allowing the

perception of depth.

Associated with the eyeballs are

numerous parts that help

maintain adequate functioning of

the eye. The eyeball is essentially

surrounded by a coat, made up of

three layers of tissue: an inner,

middle and outer layer (see

diagram).

Posterior refers to the back part

of the eye

Anterior refers to the front part of the eye

The outer coat:

The conjunctiva is a thin, transparent membrane that protects the front

of the eye. The membrane helps keep the outer surface of the eyeball

moist.

The sclera is the outmost layer of the eye. It is composed of tough, non-

elastic tissue that protects the inner layers of the eye, and maintains the

shape of the eyeball. It is also the site of attachment for external muscles

of the eye, which enables the eyeball to move in the socket. Towards the back of the eye, the sclera

is opaque (forming the white part of the eye); towards the front, it becomes a transparent structure

called the cornea.

The cornea contains no blood vessels and is complete transparent, allowing light to pass through. Its

curvature helps bend/refract incoming light rays so they converge at the back of the eyeball.

The middle coat:

The choroid layer is located in the middle coat of the eyeball. Most of the blood vessels in the eye

are located in this layer. Posteriorly (towards back of the eye), the choroid layer is black and reduces

scattering and reflection of light within the eye. Anteriorly (towards front of the eye), the choroid

forms the ciliary body and lens. In front of this is the iris.

The ciliary body forms a ‘ring’ around the front of the eye, and contains the circularly arrange ciliary

muscles. The ciliary muscles attach to the lens by suspensory ligaments. The muscles and ligaments

are important in adjusting the curvature of the lens for near and far vision. The ciliary body also

secretes aqueous humour.

Aqueous humour is a transparent, watery liquid found in the anterior part of the eye between the

cornea and the lens. It provides nutrients for the lens and the cornea (both of which do not have

their own blood supply. It also helps refract light.

Vitreous humour is a clear, jelly-like material filling the remainder of the eyeball. It contains

dissolved nutrients, refracts light, and helps maintain shape of the eyeball.

The lens is a transparent structure made of cells enclosed in a membrane called the lens capsule.

The lens refracts light rays and directs them onto the retina to form a focused image. The lens is

highly elastic – allowing it to change shape (either rounder or flatter). This allows the eye to

accommodate for near and far vision.

The iris is the coloured part of the eye, situated behind the cornea and in front of the lens. It is

surrounded by aqueous humour. The iris is made up of connective tissue and smooth muscles, which

allow it to perform its main function – that is, controlling the size of the pupil. The pupil is an

opening in the iris through which light passes in order to reach the retina at the back of the eye.

The inner coat

The retina contains photoreceptor cells, nerves and blood vessels; the photoreceptor cells (cones-

which respond to colour, and rods – which do not respond to colour) respond to light before

transmitting the information towards the central nervous system.

The fovea is a particularly sensitive area (near the centre) of the retina that focuses images most

sharply. It contains densely packed cone cells, but no rod cells at all. The fovea is the part of the

retina where the greatest detail can be detected

The blind spot is an area of the retina corresponding to the exit point for the optic nerve. Because

there are no rod/cone cells, light cannot be detected in this area.

The optic nerve transmits visual signals from the retina to the brain.

Identify the limited range of wavelengths of the electromagnetic spectrum detected by humans

and compare this range with those of other vertebrates and invertebrates

Use available evidence to suggest reasons for the differences in range of electromagnetic radiation

detected by humans and other animals

The electromagnetic spectrum is a major stimulus that impacts on our sense. It is a range of energy

forms that all travel at the speed of light, and in waves. However, electromagnetic waves differ in

wavelength (distance between successive crests of a wave) and frequency (amount of waves passing

through a given point in one second).

Visible light (for humans) lies towards the middle of the electromagnetic spectrum with wavelengths

of 400-700nm. The human eye is limited to the detection of only the wavelengths that lie in this

range; all other forms of electromagnetic radiation cannot be detected by the naked eye.

Most living organisms have a visual range close to that of humans; however, some are very different.

For example:

Honeybees are able to detect wavelengths in the ultraviolet range (300-650 nm). Some

flowers have ultraviolet markings on them which the bees use to find pollen; to guide them

to the nectar of a plant.

Rattle snakes can detect infrared light (400-850 nm), in order to detect prey (heat is emitted

in the form of infrared waves)

Deep sea fish can only detect blue light (450-500 nm); little light penetrates to the depth at

which they live so they use bioluminescence (470 nm) to communicate

The sense receptors possessed by a particular population or species is typically suited to the

environment in which they live. This is because over time, the effectiveness of the organism’s

receptors inevitably impacts their ability to survive and reproduce. In this manner, selecting pressure

causes those with receptors most suited to their environment to survive. The way different

organisms’ senses adapt to their needs is clearly highlighted in the above examples.

Plan, choose equipment or resources, and perform a first-hand investigation of a mammalian eye

to gather first hand data to relate structures to functions

Aim: To dissect a bull’s eye and relate structures to functions

Materials:

Bull’s eye

Dissecting equipment (sharp scalpel, dissecting scissors, forceps, probe)

Disposable rubber gloves

Dissecting tray

Newspaper

Safety:

Risk Controlling risk

Sharp scalpel/scissors could cut skin Great care must be taken when using scalpel/scissors. Use forceps to hold the eye whilst dissecting to minimise risk of cutting your own fingers.

Entry of infective microbes via any cuts in skin Wear rubber gloves when performing dissection

Method

1. Put on a pair of disposable gloves, collect and eye specimen and place on newspaper on top

of dissecting tray

2. Remove fatty tissue from around the eyeball with the scissors and scalpel

3. Examine the external features of the eye (optic nerve, sclera, cornea, conjunctiva, iris, pupil

etc.)

4. Cut a long around the eyeball parallel to the lens. The clear liquid that escapes is the

aqueous humour. Observe the pupil and iris at the front.

5. Remove the lens. The vitreous humour, which is denser and more jelly like than the aqueous

humour, can also be removed.

6. Clean the lens. Observe words on newspaper with the lens; try squeezing the lens and see

what happens.

7. Rinse the eyeball and examine the retina

8. Wrap the eye and eye parts in newspaper and dispose of them.

9. Place dissecting equipment in disinfectant, and dispose of dissecting gloves

3. The clarity of the signal transferred can affect interpretation of the intended visual

communication

Identify the conditions under which refraction of light occurs

The bending of light is called refraction. Refraction occurs when light travels from one medium to

another of a different density at an angle other than 90 degrees (perpendicular). This is because the

light travelling at differing speeds in the different mediums. If light travels to a more dense medium,

it bends towards the normal; if it enters a less dense medium, it bends away from the normal.

Identify the cornea, aqueous humour, lens and vitreous humour as refractive media

Light refraction occurs at each boundary

between the cornea, aqueous humour, lens and

vitreous humour due to their varying densities.

The refraction is essential to form a clear image

on the retina.

Refraction occurs when light passes from the air

into the denser material of the cornea. When

the light then passes into less dense aqueous

humour, it is refracted again. The same thing

occurs when the light passes through the denser

lens, and then finally through the vitreous

humour.

Identify accommodation as the focusing on objects at different distances, describe its achievement

through the change in curvature of the lens and explain its importance

Accommodation refers to the focusing of objects at different distances.

A convex lens is one that is thicker in the middle,

thinner on the outside. It causes light to converge. The

lenses in our eyes are convex.

A concave lens is one that is thinner in the middle,

thicker on the outside. It causes light to diverge.

Most of the refraction occurs when light passes

through the cornea; however fine focusing is achieved

through the lens. The lens is attached via suspensory

ligaments in the middle of a ring of muscle called

suspensory ligaments. The contraction or relaxation of the ciliary muscles in the ciliary body causes

the shape of the lens to change, and hence alters the focal length/distance.

If we wish to see a close object, the ciliary body contracts, the suspensory ligaments

become loose, and the lens becomes more rounded in shape.

If we wish to see a distant object, the ciliary body relaxes; the suspensory ligaments

become tighter and pull on the lens. The lens gets flatter in shape, giving a clear image of

the object.

This ability to make the lens just the right thickness in order to see objects at different distances is

called the power of accommodation.

Focusing is the result of accommodation. It is essential for an image to be focused to achieve clear

vision. In this way, accommodation allows organisms to see both near and far objects clearly. This is

important for many organisms to be able to detect predators, food sources etc.

Compare the change in the refractive power of the lens from rest to maximum accommodation

Maximum accommodation (in terms of the ciliary muscles)

occurs when focusing on very close objects and the lens is

rounded in shape.

Relaxed/rest state occurs when looking at distant objects, and

the lens is flatter in shape.

Refractive power is basically the degree to which the lens

bends/refracts light. It is inversely proportional to the focal

length of the lens, and is measured by the unit dioptre.

When the eye is looking at close objects, the light rays tend to

diverge as they reach the eye. For proper focusing, the refractive power of the lens must be

increased, by making the lens more convex (rounded).

When the eye is looking at distant objects, light reaches the eyes in almost parallel rays. This light is

focused on the retina when the lens has little refractive power (i.e. when it is quite flat). A minimal

amount of refraction or bending of light occurs when it passes through the lens as it is not required.

Lens Maximum accommodation Rest

Shape Bulges/round Thin/flat

Distance from object Near Distant

Refractive power High (approx. 67 dioptres) Lower (20-34 dioptres)

Distinguish between myopia and hyperopia and outline how

technologies can be used to correct these conditions

Myopia, or short sightedness, is when it is possible to see near

objects clearly, but more distant objects are blurred and

indistinct. It occurs when the distance between the lens and the

retina is too great (i.e. the eyeball is too long) or when the lens

cannot get thin enough to focus distant objects correctly (instead,

the image is focused in front of the retina).

Myopia can be corrected by wearing glasses with concave lenses (thinner middle, thicker ends –

diverges light). They spread the light rays out before entering the eye; allowing the lens to focus

them correctly.

Hyperopia, or long sightedness, is when it is possible to see distant

objects clearly, but near objects are blurred and indistinct. It occurs

when the distance between the lens and the retina is too short (i.e.

eyeball is too short) or when the lens cannot get fat enough to focus

near objects correctly (instead, the image is focused on an imagery

spot behind the retina).

Hyperopia can be corrected by wearing glasses with convex lenses.

These bend the light rays in a bit extra to allow them to focus on the

retina.

Other corrective technologies

Laser surgery can also be used treat myopia and hyperopia. The

treatment involves reshaping the curvature of the cornea. A thin flap or

corneal tissue is cut, folded back, and a laser beam is applied to the

exposed corneal tissue (reshaping the layers underneath to treat

refractive error). When the laser is finished, the flap is returned.

Contact lenses are similar to spectacle lenses (either concave or convex). However, they are

designed to model the curvature of the eyeball, and sit directly on the surface on the eye – just

covering the cornea.

Explain how the production of two different images of a view can result in a depth perception

Depth perception depends on binocular vision, where the field of view of each eye overlaps –

allowing each eye to observe the same object simultaneously. The images formed by each eye are

superimposed by the brain, and because each view is slightly different, it allows us to see it in three-

dimensions. When objects are close enough, we are also able to judge its distance from us.

Stereoscopic vision refers to vision where the same object is viewed from slightly different angles –

creating an impression of depth. Predators also have stereoscopic vision (eyes at front of the head)

to allow them to estimate distances from prey. Tree-dwelling primates have it to allow them to

estimate depth when moving from branch to branch. Animals that are likely to be preyed upon,

however, usually have eyes placed on each side of the head – allowing for a wider total visual field at

the expense of losing the ability to judge distances.

Plan, choose equipment or resources and perform a first-hand investigation to model the process

of accommodation by passing rays of light through convex lenses of different focal lengths

Aim: To model the process of accommodation by passing rays of light through convex lenses of

differing lengths

Equipment:

Two convex lenses of different shape (one thin, one thick)

Lens holder

Sheet of white paper clipped on solid support (acts as a screen where the object is focused)

Candle as a source of light

Variables:

Independent variable: shape of the lens

Dependent: Distance between the screen and the lens

Control: Light source, screen and distance between the light and screen

Method:

1. Darken the room and set up thin lens holder as above

2. Move the lens forwards and backwards to find a position that produces a clear, focused

image of the light source on screen.

3. Measure and record the distance of the screen from the lens

4. Keep the lens holder the same distance, and change the lens to the thick lens. Observe the

screen and not the appearance of the image

5. Repeat steps 2-3 for the thick lens

Results: The thicker lens (one with more curvature) had a smaller focal length (was closer to the

screen). The thinner lens (one will less curvature) had a longer focal length (held further from the

screen)

Analyse information from secondary sources to describe changes in the shape of the eye’s lens

when focusing on near and far objects

When a person is looking at something close, the ciliary body contracts, the ligaments

loosen, and the lens becomes rounded. Accommodation and refractive power of the lens are

at a maximum.

When a person is looking at something distant, the ciliary muscles relax, the ligaments

tighten, and the lens becomes flatter and thinner. The muscles are in a relaxed state, and

refractive power is minimal.

Process and analyse information from secondary sources to describe cataracts and the technology

that can be used to prevent blindness from cataracts and discuss the implications of this

technology for society

Cataracts are a condition where the lens grows cloudy and eventually becomes opaque. When part

or the whole of the lens becomes opaque, the transmission of light through the eye is obstructed,

causing both near and far objects to become blurred. Cataracts mostly develop slowly as a result of

aging. The development of cataracts is often linked to eye injury, extended exposure of the eyes to

the sun’s UV light, excessive smoking, radiation and particular diseases (e.g. diabetes).

When the presence of cataracts begins to interfere with daily activities and the quality of one’s life,

treatment by means of surgery is the only option

Technologies used

Today, cataract surgery involves IOL implantation – that is, replacing the cloudy lens with a

plastic/silicone intraocular lens (lens within the eye), similar in shape to a natural human lens.

The most common technique (phacoemulsification) involves making a 3mm incision where the

cornea meets the sclera, and small, vibrating probe is inserted into the eye. This probe breaks up the

lens into small particles, which are then suctioned out using an aspirator. The artificial lens is then

inserted into the space left in the existing lens capsule. The incision in the eye may be so small that

no stiches are needed.

Implications

The cataract surgery takes very little time, is performed under local anaesthetic, and can be done

anywhere. It has revolutionised the treatment of cataracts so that people who are cataract blind,

can now see. The implications of cataract surgery are huge; regaining sight often increases an

individual’s life span, and allows older people to live more independent and active lives, thus

reducing the financial burden required to look after the elderly.

This safe, precise and successful technique has been made available to thousands of people in

developing countries through groups like the Fred Hollows Foundation, which send teams to those

isolated and poor communities to perform cataract surgery. As such, the surgery was made available

to those who could not previously afford them.

4. The light signal reaching the retina is transformed into an electrical impulse

Identify photoreceptors cells as those containing light sensitive pigments and explain that these

cells convert light images into electrochemical signals that the brain can interpret

The retina is a thin sheet of cells that contain photoreceptor cells.

Photoreceptor cells are those containing light sensitive pigments and these cells convert

light into electrochemical signals that the brain can interpret.

There are two types of photoreceptor cells in the retina:

Rods and cones contain photosensitive chemical substances that undergo chemical reactions

when they absorb light energy.

When the rods and cones are stimulated by light, electrochemical signals are transmitted

through successive neurones on the retina and finally into the optic nerve and then to the

cerebral cortex in the brain.

Describe the differences distribution, structure and function of the photoreceptor cells in the eye

Rod Cone

Feature Rods Cones

Distribution - 125 million in the human retina - Spread across the retina but

more dense around the edges of the retina (periphery)

- none in the fovea

- 6-7 million in the human retina - Located mostly on the fovea (centre)

of the retina and a small depression in the centre of the macula lutea at the back of the eyeball.

- In the fovea each cone cell is connected to one nerve cell to give the greatest acuity

Structure - Elongated - Narrower, longer and straighter - Both contain visual pigments

(rhodopsins) in stacks of disc-shaped membrane at one end of the cell. The other end connects to a nerve cell

- Elongated - Conical - Shorter - Both contain visual pigments

(photopsins) in stacks of disc-shaped membrane at one end of the cell. The other end connects to a nerve cell

Function - In low light conditions the pupil dilates, allowing light to fall on the rods

- So in low light levels, our vision is more ‘grainy’ and we cannot discriminate colour.

- Best in dim light, do not distinguish colour (discriminates between shades of light and dark), used for night vision

- More sensitive to light - Sensitive to movement - Formation of images

- In bright light the pupil is contracted and most light falls on the cones

- This means that in bright light we can see detailed, coloured images

- Require more light than rods to be stimulated

- Day vision, colour vision and visual tasks requiring visual acuity (e.g. reading)

- Formation of images

Outline the role of rhodopsin in rods

Rod cells contain a photosensitive pigment that absorbs light waves called rhodopsin which

is made of up of the protein opsin in loose chemical combination with a pigment called

retinal.

When light strikes a rod cell it stimulates a response, the retinal changes shape and loses its

attachment to the opsin molecule and splits rhodopsin molecules into its components.

Rhodopsin allows seeing the shades of grey, black and white.

Identify that there are three types of cones, each containing separate pigment to either blue, red

or green light

There are three types of cones in the human eye, each containing a pigment sensitive to

either blue, red or green light.

Because three photopigments are used to interpret colour by the various cone cells, humans

possess trichromatic vision.

There are three types of cones, each with its own kind of opsin associated with retinal.

These molecules are called photopsins and they are involved in colour vision, there are 3

types of photopsin present in these cones (each one having only one type of photopsin):

- Opsin blue short wavelengths (maximum at 420 nm)

- Opsin green medium wavelengths (maximum at 530 nm)

- Opsin red Long wavelengths (maximum at 560 nm)

Explain that colour blindness in humans results from the lack of one or more of the colour-

sensitive pigments in the cones

Colour blindness is the inability to see colour, there are a range of colour blindness

conditions from only a slight difficulty distinguishing different shades of the same colour to

the rare inability to distinguish any colours.

Colour blindness in humans results from the lack of one or more of the colour-sensitive

pigments in the cones, people who are colour blind still see some colour.

As rod cells detect light, they play no role in colour blindness so those who are colour blind

can usually see.

Most common type of colour blindness is red-green (people with the condition perceive red

and green as the same colour), where the cones most receptive to red light and green light

are missing.

Process and analyse information from secondary sources to compare and describe the nature and

functioning of photoreceptor cells in mammals, insects and in one other animal

Mammal: Human Insect: Dragonfly Flatworm: Planarian Worm

Diagram of photoreceptor

Type of photoreceptor

Single eye lens Compound eyes Eye cups

Type of image Image on retina is inverted and diminished

Mosaic image of large visual field, can see in colour

Image (if formed) is unclear and not inverted – gives information about light intensity and direction

Similarity in photoreceptors

Detects light - rhodopsin Detects light - rhodopsin Detects light - rhodopsin

Difference in photoreceptor

Has higher resolving power and greater visual acuity

Is made up of many units called ommatidia (each has its own cornea and transparent crystalline cone which acts as lens) pointing in different direction. Can be more efficient detecting colour than mammals and fast movement.

Has fewer photoreceptor cells than insect or mammals and has poor visual acuity and no colour vision. Two eye spots (Ocelli) are used to detect light and move away from it.

Structure that refracts light

Cornea, aqueous humour, lens (changes curvature), vitreous humour

Cornea, lens (fixed, cannot accommodate)

No refraction

Sensitivity to light

Rhodopsin is a photosensitive pigment

Greater – detects light, UV as well as visible light

Highly sensitive

Visual acuity (vision)

High visual acuity Lower visual acuity than mammals but more efficient in detecting movement

Poor visual acuity

Ocelli

Process and analyse information from secondary sources to describe and analyse the use of colour

for communication in animals and relate this to the occurrence of colour vision in animals

Animal Method Purpose

Blue-ringed octopus Glowing blue ring around their bodies

Signal an intention to attack

Rainbow lorikeets Warning mechanism by opening their wings

Bright underneath colours are used to threaten off rival male

Red Black Spider Warning mechanism highlighting their poisonous nature

Have a warning red strip on their abdomen to ward off predators or things looking to eat it

Primates Looking at colour of food Use colour to determine whether food is ripe and good to eat