IB Biology - Curriculum Notes 


​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Option A - Neurobiology and Beahviour


A.1 Neural Development                                                
Nature of science: 
Use models as representations of the real world—developmental neuroscience uses a variety of animal models. (1.10)
Ted talk on Neurobiology and consciousness


∑ - Understandings:

∑ - The neural tube of embryonic chordates is formed by the infolding of ectoderm followed by elongation of the tube.

  • All chordates develop a dorsal nerve cord through a process called neurulation in the early stages of development
  • The cells located in a portion of the middle of the ectoderm (dorsally located) differentiate to form the neural plate
  • The ectoderm is separated from the neural plate by the neural plate border
  • The cells of the neural plate change shape, causing the plate to bend inwards and form a groove. The border is now called the neural crest.
  • The infolded neural crest closes and separates from ectoderm forming the neural tube
  • The neural tube will elongate as the embryo develops
  • The cells of the neural crest will differentiate to form the majority of the peripheral nervous system
  • The neural tube forms the narrow canal in the centre of the spinal cord

∑ - Neurons are initially produced by differentiation in the neural tube.

  • The ectoderm contains a portion called the neural plate that contains neuroectodermal cells that eventually develop into the nervous system
  • This neural plate forms the neural tube, which eventually differentiates into functioning neurons
  • The specific cells that develop into neurons are called neuroblasts
  • Cell proliferation through mitosis continues as the CNS develops into the brain and spinal cord

β - Application: Incomplete closure of the embryonic neural tube can cause spina bifida.

  • Spina bifida is a neural tube birth defect that occurs at an early stage of embryonic development
  • Each vertebrate has a strong centrum for support and a thinner vertebrate arch

  • During development, the centrum develops on the ventral side of the neural tube
  • These cells form tissue that migrates around the neural tube, eventually meeting up to form the vertebrate arch
  • If these tissues never meet up and fuse together, a gap will form between them
  • This disease or condition is called spina bifida
  • The severity can range from mild to severely debilitating



∑ - Immature neurons migrate to a final location.

  • Two types of cells develop from the neural tube called neurons and glial cells
  • Neurons carry messages and glial cells provide physical and nutritional support for the neurons
  • Glial cells provide a scaffolding network along which neurons can migrate to their final location mature and ready to send out their axons and dendrites
  • This is important in brain development as some neurons produced in one part of the developing brain, migrate and find their final position in another part
  • These neurons move somewhat like an Amoeba

Videos on migration:


β -Skill: Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.

Draw and label a diagram of Xenopus (right)



Ectoderm, mesoderm, and endoderm

Neural Tube development

Archenteron - Developing gut and gut cavity


∑ - An axon grows from each immature neuron in response to chemical stimuli.

  • Immature neurons consist of a cell body containing a nucleus and cytoplasm
  • Axons grow out from the cell body
  • At the tip of the axon is a growth cone
  • Chemical stimuli determine how the axon differentiates and grows and in what direction
  • As they grow, the axons will synapse with their target cells

Axons can be highly branched and carry signals to other neurons

∑ - Some axons extend beyond the neural tube to reach other parts of the body.

  • Neurons such as mammalian motor neurons have to send their axons far past the neural tube towards their target cells
  • These axons have to extend out of the CNS to form circuits with muscles in order to control voluntary muscle movement
  • These are some of the longest neurons in the body
  • The muscles attract the neurons by producing chemicals called CAMs
  • This activates enzymes in the axon that cause the growth cone to grow towards the target muscles

∑ - A developing neuron forms multiple synapses.

  • As neurons develop, they form multiple synapses with other neighbouring nerve cells and any other target cells.
  • Neurons keep trying new connections until compatible connections are made. Other connections are eliminated.
  • These axons develop synapses between the neuron and the other cell
  • Synapse development involves the creation of special structures being assembled in the membranes on either side of the synapse with a space between called the synaptic cleft
  • The smallest number of synapses can be two, but most neurons develop multiple synapses, even hundreds, to allow complex communication


∑ - Synapses that are not used do not persist.

  • Many different synapses are formed during early development; however, if they are not used they can be terminated or disappear
  • As synapses are used more frequently, chemical markers cause the connection to be strengthened
  • If they are not used, the connections become weakened as no chemical markers are present and therefore disappear.

∑ - Neural pruning involves the loss of unused neurons.

  • In some areas of the brains of newborns, there are more neurons than adulthood
  • This indicates that the removal of some neurons in childhood occurs
  • This is called neural pruning
  • Sometimes dendrites and axon branches are also removed over time

Video: https://www.youtube.com/watch?v=1uG4rItxyT8


∑ - The plasticity of the nervous system allows it to change with experience.

  • Plasticity is the ability of the nervous system to change and rewire its connections. It is known as neuroplasticity

This involves:

  • The growing of axons and dendrites
  • The establishment of new synapses and the elimination of others
  • The pruning of dendrites, branches of axons and even whole neurons

The changes come from experiences and how an individual uses their nervous system
This allows us to form new memories, is the basis for some types of reasoning and repairs damage to the brain and spinal cord

Video: https://www.youtube.com/watch?v=wI388XoCp48

Application: Events such as strokes may promote the reorganization of brain function.

  • Most strokes are caused by blocking or disruption of the supply of blood to a part of the brain
  • This is usually through a blood clot blocking a blood vessel in the brain or sometimes through bleeding from a vessel
  • This causes a part of the brain to be deprived of oxygen and glucose and therefore no cell respiration can take place in the neurons
  • These neurons will become damaged beyond repair and will die
  • Strokes can vary in their degree of severity from very mild to very severe and debilitating
  • Recovery from strokes involves different parts of the brain taking on new functions
  • Most recovery happens over the first 6 months to a year and can involve relearning writing and speaking spatial awareness and carrying out skilled physical activities

Short video: https://www.youtube.com/watch?v=pcmrgwNCPwM

Long video:

Minor stroke on video


A.2 The human brain

Nature of science:

  • Use models as representations of the real world—the sensory homunculus and motor homunculus are models of the relative space human body parts occupy on the somatosensory cortex and the motor cortex. (1.10)


∑ - The anterior part of the neural tube expands to form the brain.

  • A neural tube forms along the whole dorsal side, above the gut, near the surface
  • Most of the neural tube becomes the spinal cord; however, the anterior end expands and develops into the brain during cephalization (development of the human head)
  • The brain is a central control centre for the body.

∑ - Different parts of the brain have specific roles.


  • Medulla Oblongata – controls the autonomic and homeostatic functions of the body including swallowing, digestion, vomiting, breathing and heart rate
  • Cerebellum – coordinates subconscious functions such as balance and movement
  • Hypothalamus – Interface between the brain and the pituitary gland. It synthesizes the hormones secreted by the posterior pituitary gland and releases factors that regulate the secretion of hormones by the anterior pituitary gland
  • Pituitary gland – the posterior lobe stores and releases hormones produced by the hypothalamus. The anterior lobe produces and secretes hormones that regulate many homeostatic body functions
  • Cerebral Hemispheres – integrating centre for high complex functions such as learning, emotions and memory

β - Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts.

  • Lesions are due to tumours, strokes or other damage to the brain that can be studied post-mortem through autopsies. Lesion studies have occurred since the 19th century
  • By studying where these lesions are located, observed changes in behaviour and mental capacities, one can make conclusions about the damaged areas of the brain and what effects the damaged portions had when the individual was living.
  • Some neuroscientists study animals by removing the skull cap and stimulating the brain directly to study the effects of stimulating certain portions of the brain on animal behaviour, temperament and long term capacities
  • Experiments such as JP Flourens’ of the 19th century, removed different pieces of certain animals brains such as rabbits to observe impaired function from the removal
  • Experiments with pigeons were also performed. He removed their cerebellum and discovered it was the movement centre of the brain
  • This information gathered through animal testing is useful for understanding and treating diseases such as epilepsy, Parkinson’s and multiple sclerosis
  • This has definite ethical implications on using animals as the animal test subjects generally die
  • MRI’s are used today to investigate the internal structure of the body, looking for abnormalities such as tumours
  • fMRI (functional magnetic resonance imaging), allows parts of the brain that are activated by specific thought processes to be identified
  • It uses radio waves and a strong magnetic field, not x-rays
  • The radioactive dye makes the parts of the brain visible that receive an increase in blood flow when stimulated
  • A famous study was on a region of the brain known as Broca's area. In the 1860's, a patient named Leborgne could only speak one word, whereas another, Lelong, could only speak a few. When they died, Broca examined their brains and found lesions in the same location.
  • He deduced that this region was responsible for language. Since then many studies (including fMRI) have confirmed this.

fMRI video on how it works - https://www.youtube.com/watch?v=lLORKtkf2n8

Monkey controls robotic arm with brain

Warning Graphic content ** Video shows brain experiments on monkeys at Weizmann Institute***

β - Application: Visual cortex, Broca’s area, nucleus accumbens as areas of the brain with specific functions.

Visual Cortex – the area of the brain that receives neural impulses from light-sensitive cells called rods (peripheral vision and dim light) and cones (visual acuity). This also includes pattern recognition and speed and direction of moving objects

Broca’s Area – This area control’s production of speech. It is located in the left cerebral hemisphere. If this area is damaged, the person can recognize speech and know what they want to say; however, they cannot put together meaningful words and sentences

Nucleus accumbens – This is the pleasure reward centre that is located in both cerebral hemispheres. Pleasurable stimuli such as food or sex cause the release of dopamine, which produces feelings of pleasure and satisfaction. Drugs such as cocaine and heroin also cause a high level of release of dopamine by the nucleus accumbens   

∑ - The autonomic nervous system controls involuntary processes in the body using centres located mainly in the brain stem.

  • The central nervous system is composed of the brain and spinal cord
  • The peripheral nervous system is composed of all nerves outside the central nervous system
  • It is divided into the voluntary and autonomic nervous system (involuntary)
  • The brain stem controls the involuntary functions of the body and consists of the pons, the midbrain and the medulla oblongata
  • The medulla oblongata is the area of the brain that controls many of these involuntary actions including swallowing, digestion, vomiting, breathing and heart rate
  • The autonomic nervous system is comprised of two divisions called the parasympathetic and the sympathetic nervous systems
  • Parasympathetic nerves (rest and digest) increase blood flow to the digestive system in periods of rest and relaxation
  • Sympathetic nerves (fight or flight) decrease blood flow to the digestive system and increase blood flow to the heart, brain and muscles necessary for fight or flight



Video:Watch 50 sec until 3 min 30 second

β - Application: Swallowing, breathing and heart rate as examples of activities coordinated by the medulla.

  • After food is swallowed, it is passed involuntarily from the pharynx to the stomach via the esophagus through muscular actions (peristalsis)  controlled by the medulla oblongata
  • The medulla oblongata also controls breathing. One area of the medulla controls inspiration and one controls expiration
  • Chemoreceptors in the medulla sense changes in the blood pH. Increases in carbon dioxide, causes the blood pH to drop and breathing becomes deeper and more frequent
  • Heart rate is also regulated by the medulla. Blood pressure and pH changes are sensed by receptors in the medulla and blood vessels. In response, the cardiovascular centre of the medulla increases or decreases the heart rate by sending signals to the pacemaker of the heart.
  • Signals from the sympathetic nervous system speed up the heart rate and signals from the parasympathetic nervous system (Vagus nerve) slow down the heartrate. 

Medulla Oblongata - Waterboy - https://www.youtube.com/watch?v=cu7A8LIzL1o

Brainstem function – Little long -

β - Application: Use of the pupil reflex to evaluate brain damage.

  • Muscles contained within the iris control the size of the pupil in response to light
  • This is called the pupillary reflex
  • Neurons of the sympathetic system cause the radial muscles to contract, resulting in the dilation of the pupil
  • Neurons of the parasympathetic nervous system, cause the circular muscles of the iris to contract, resulting in the constriction of the pupils
  • Receptors: Photoreceptors in the retina
  • Stimulus: Bright light hitting the retina
  • Sensory neuron: optic nerve
  • Relay neurons: medulla oblongata in the brain
  • Motor neurons: Back to iris
  • Effect: Circular muscles contract, the pupil constricts, light entering eye is reduced.
  • Doctors use the pupil reflex to test if a patient has brain function or not
  • If the light is shone into each eye and the pupils don’t constrict, the medulla region of the brain is probably damaged
  • If other tests fail, the patient has suffered brain death

Pupil reflex https://www.youtube.com/watch?v=E2XzBaOOX8g



∑ - The cerebral cortex forms a larger proportion of the brain and is more highly developed in humans than other animals.

  • The cerebral cortex is the highly complex area of the brain that is located in the outer layer of the cerebral hemispheres
  • It is approximately two to four mm thick and contains six different layers of neurons
  • Only mammals have a cerebral cortex and in humans, the cerebral cortex comprises a larger proportion of the brain than any other mammal



∑ - The human cerebral cortex has become enlarged principally by an increase in total area with extensive folding to accommodate it within the cranium.

  • Through evolution, the cerebral cortex in humans has become greatly enlarged and now contains more neurons thank any other animal
  • The increase is due mainly to an increase in the total surface area caused by extensive folding during development
  • Most of the surface area is within the folds, rather than the outer surface
  • The area is approximately .18 m2 and is so large, it can only fit inside an enlarged cranium forming the distinctive shape of our skull
  • Lower mammals such as rats and mice have a smooth cortex and some other mammals have some folds
  • Primates that are closely related to humans have a greater degree of folding and hence more surface area

β - Skill: Analysis of correlations between body size and brain size in different animals.

There is a positive correlation between body and brain size; however, it is not directly proportional

***Do data based questions on page 523***

∑ - The cerebral hemispheres are responsible for higher order functions.

  • The cerebral hemispheres are responsible for the brain’s most complex tasks
  • These higher-order functions include memory, speech and emotions
  • These functions involve stimuli from the eyes and ears and also retrieval of memories
  • Thought processes such as reasoning and planning occur in the pre-frontal and frontal lobe of the cerebral cortex
  • When we use this part of our brain, we can organize our thoughts into a logical sequence, predict their outcomes, be aware of our existence and develop a sense of right or wrong

∑ - The left cerebral hemisphere receives sensory input from sensory receptors in the right side of the body and the right side of the visual field in both eyes and vice versa for the right hemisphere.

  • Sensory inputs are received from different sense organs in our body
  • Inputs from skin, muscle and other internal organs pass to the somatosensory area of the parietal lobe via the spinal cord
  • Impulses from either side of our body cross at the base of the brain so that the left hemisphere receives impulses from the right side of the body
  • Sensory input from our eyes pass to the occipital lobe via the optic nerve
  • Impulses from the right side of our field of vision in each eye go to the visual cortex in the left hemisphere
  • Impulses from the left side of our field of vision pass to the right hemisphere
  • These images are integrated allowing us to gauge distance and perspective

∑ - The left cerebral hemisphere controls muscle contraction in the right side of the body and vice versa for the right hemisphere.

  • The posterior portion of the frontal lobe called the primary motor cortex controls voluntary muscle movement
  • There are overlapping areas in this region that control muscle movement throughout our bodies
  • The primary motor cortex in the left hemisphere of our brain controls the right side of our body and the right hemisphere controls our left side.
  • A stroke that occurs in the right hemisphere would, therefore, affect the left side of our body

∑ - Brain metabolism requires large energy inputs.

  • Since the brain has a large number of neurons, a large amount of oxygen and glucose is needed for aerobic cellular respiration to create ATP
  • Energy is needed to maintain and re-establish resting potential in neurons
  • It is also needed to synthesize a large number of signal molecules like neurotransmitters
  • In adult humans, 20% of our energy is used by our brain
  • Infants and small children use even more than 20%
  • In comparison, other vertebrates use only about 10%

Applications and skills:

β - Skill: Identification of parts of the brain in a photograph, diagram or scan of the brain.


• Image of the brain should include the medulla oblongata, cerebellum, hypothalamus, pituitary gland and cerebral hemispheres.

• Although specific functions can be attributed to certain areas, brain imagery shows that some activities are spread in many areas and that the brain can even reorganize itself following a disturbance such as a stroke


• The definition of living varies depending on local and national laws and culture.

Theory of knowledge:

• In medicine, the concept of death is defined in terms of brain function, but sometimes conflicts can occur when the medical criteria for death differ from the family’s criteria for death. To what extent should the views of the family members be given priority when making decisions in medical ethics? What criteria should be used to make ethical decisions?


• Angelman syndrome is a genetically inherited condition that is diagnosed from characteristically abnormal patterns on an electroencephalogram.


A.3 Perception of stimuli

Nature of science:

Understanding of the underlying science is the basis for technological developments—the discovery that electrical stimulation in the auditory system can create a perception of sound resulted in the development of electrical hearing aids and ultimately cochlear implants. (1.2)

∑ - Understandings:

∑ - Receptors detect changes in the environment.

Respond to changes in mechanical forces and movements, such as pressure, texture and vibration

- Touch and pain receptors in your skin

- Inner ear receptors for hearing and balance

Baroreceptors in your arteries that sense a change in blood pressure
Changes in temperature, sense heat

Thermoreceptive nerve endings in your skin

Respond to a chemical substance like chemical vapours and solutes

- Taste buds in the tongue

- The CO2 concentration in the blood (pH)

- smell detected by your nose
Electromagnetic radiation (respond to visible light in humans)

- Rods and cones of the retina

β - Application: Detection of chemicals in the air by the many different olfactory receptors.

Watch TED talk on smell and olfactory receptors:

  • Olfactory receptors are located in the epithelium inside the upper part of the nose (back of the nose)
  • Adults can distinguish around 10,000 different smells
  • Cilia from these cells protrude into the air inside the nostril and contain chemical receptor proteins which detect chemicals in the air
  • Odour molecules dissolve in the mucous covering the olfactory epithelium.
  • They then bind to the olfactory receptors cells
  • Each olfactory receptor cells binds to only one type of odourant; however, there are many types of receptor cells encoded by a different gene
  • A human nose has around 400 types of scent receptors, but can sense a much greater number of odours; however, humans can sense much less than many other mammals.

∑ - Rods and cones are photoreceptors located in the retina.

  • Light enters the eye via the lens and is reflected by the choroid cells at the back of the eye.
  • This reflected light is absorbed by two different sets of photoreceptors cells called rods and cones
  • Rod and cone cells convert light stimuli into an electrical signal called an action potential
  • Bipolar neurons are specialized sensory neurons that take the information from the photoreceptor cells and transmit this information through the retinal ganglion cells which form the optic nerve
  • The optic nerve takes this information to the occipital lobe

∑ - Rods and cones differ in their sensitivities to light intensities and wavelengths.

  • Rod cells function better in low-light conditions and are used for peripheral vision.
  • Rod cells become bleached in bright light (sunny and bright days)
  • Rod cells contain the same pigment which absorbs a wide range of wavelengths of light, and therefore do not distinguish between a variety of different colours (monochrome vision)
  • Rod cells produce undefined images as many rod cells synapse with one bipolar neuron
  • Cone cells are used for colour vision, high-light conditions and produce a sharp, clear image.
  • There are three types of cone cells, all with different pigments and can, therefore, distinguish between different colours as they absorb different wavelengths of light (blue, green and red)
  • All cone cells synapse with one bipolar neuron
  • Cone cells require a greater number of photons of light to activate them to send a signal to the brain than do rod cells

Brain Games - Colour Vision: https://vimeo.com/84207602

Vision -

Compare rods and cones


β - Skill: Labelling a diagram of the structure of the human eye (answers at the bottom)

Label the following structures on the above diagram of the eye.

• Diagram of human eye should include the sclera, cornea, conjunctiva, eyelid, choroid, aqueous humour, pupil, lens, iris, vitreous humour, retina, fovea, optic nerve and blind spot.

β - Application: Red-green colour-blindness as a variant of normal trichromatic vision.

  • Photoreceptor pigments in red, green and blue cone cells are part of a group of proteins called opsins
  • Three different genes code for the three different types of opsins
  • The genes for the red and green wavelength of light are located on the X chromosome
  • Red/green colour blindness is caused by the lack of a functioning pigment for that particular wavelength of light
  • If either of these genes is not working properly, red and green wavelengths of light cannot be distinguished
  • Since these are on the X chromosome, they are an inherited sex-linked condition
  • The alleles that code for the creation of the proper pigment are dominant over the alleles that cause red/green colourblindness.
  • This is why the condition is more often seen in boys, as they have only one X chromosome (inherited from their mother)
  • Girls are therefore generally carriers that give the condition to their sons

∑ - Bipolar cells send the impulses from rods and cones to ganglion cells.

  • Many rod cells can synapse with one bipolar neuron and as few as one cone cell can synapse with a bipolar neuron
  • Bipolar neurons receive nerve impulses from rods and cones
  • These impulses are processed in the bipolar neuron and ganglion cells and are then sent to the visual cortex in the brain via the optic nerve
  • Bipolar neurons have their cell body and nucleus in the middle of the axon

β - Skill: Annotation of a diagram of the retina to show the cell types and the direction in which light moves.

 ∑ - Ganglion cells send messages to the brain via the optic nerve.

  • Ganglion cells in the retina have cell bodies that synapse with bipolar neurons through their surrounding dendrites.
  • The ganglion cells also have long axons that send nerve impulses back to the brain via the optic nerve to the optic chiasma and finally to the visual cortex
  • The axons pass through the retina in a bundle at the back of the eye, which is known as the “blind spot”, as photoreceptor cells are not located here

∑ - The information from the right field of vision from both eyes is sent to the left part of the visual cortex and vice versa.

  • Stimuli from the right field of vision in both eyes pass to the left side of the visual cortex
  • Stimuli from the left field of vision in both eyes pass to the right side of the visual cortex
  • This is achieved when some of the axons from the nerve fibres cross over at the optic chiasma in the brain
  • As a result of some of these axons crossing over at the optic chiasma, the right visual cortex processes all the information from the left field of view and vice versa
  • This allows the distance and size of objects to be judged more precisely

See the diagram below

β - Skill: Labelling a diagram of the structure of the human ear.

∑ - Structures in the middle ear transmit and amplify sound.

  • How the ear works videohttps://www.youtube.com/watch?v=qgdqp-oPb1Q
  • The middle ear is separated from the outer ear by the eardrum
  • It is an air filled chamber has a series of small bones called ossicles, which consist of the malleus (hammer), incus (anvil), and the stapes (stirrup)
  • The malleus is attached to the eardrum and makes contact with the incus, which in turn makes contact with the stapes
  • The stapes is attached to the oval window
  • The ossicles transmit sound waves from the eardrum to the oval window
  • These bones reduce the amplitude of the sound waves by increasing their force, which amplifies the sounds by about 20 times
  • The oval window is smaller than the eardrum which also helps to amplify the sound
  • Contractions by the muscles attached to the ossicles help protect the ear when loud sounds occur. These contractions dampen the vibrations from the loud noises      

Hearing test - http://tonometric.com/adaptivepitch/


 ∑ - Sensory hairs of the cochlea detect sounds of specific wavelengths.

  • The cochlea is a fluid-filled spiral tube of the inner ear where vibrations are converted to neural impulses by tiny receptor hair cells
  • The sensory cells have bundles of hair attached to them which stretch from one of the membranes to another
  • When vibrations from sound in the middle ear strike the oval window, these vibrations are transmitted from the oval window into the cochlea stimulating the hair cells
  • Gradual variations in the width and thickness of the membranes allows for the activation of different hair cells enabling humans to distinguish different frequencies of sound
  • When these hair cells vibrate, they send impulses to the brain via the auditory nerve

∑ - Impulses caused by sound perception are transmitted to the brain via the auditory nerve.

  • Depolarizations caused by vibrations in the hair cells of the cochlea, release neurotransmitters across a synapse stimulating the adjacent sensory neuron
  • An action potential is triggered which propagates along the auditory nerve to the brain
  • The auditory neuron is one of the cranial nerves


∑ - Hair cells in the semicircular canals detect movement of the head.

  • There are 3 fluid-filled semicircular canals in the inner ear
  • Each canal has a swelling called an ampulla at one end which contains sensory hair cells embedded in a gel
  • When a person moves their head in the plane of one of these fluid-filled canals, the rigid wall moves along with the head; however, the fluid lags behind due to inertia
  • This fluid that passes over the hair cells, sends a signal to the brain
  • Each canal is at a ninety-degree angle and therefore the movement of the head in all three planes can be detected
  • The brain can deduce the position and the movement of the head, by the relative amount of stimulation of the hair cells in the three semicircular canals

β – Application: Use of cochlear implants in deaf patients.

  • Hearing aids that amplify sound can’t help people hear if the hair cells of the cochlea are defective or not working properly
  • Cochlear implants can help people with non-functioning cochlear hair cells  hear
  • Cochlear implants consist of external parts, which are a microphone to detect sounds, a speech processor to filter out frequencies not used in speech and a transmitter
  • The internal parts are implanted into the mastoid bone behind the ear, and consist of the receiver that picks up the sounds from the transmitter, a stimulator to convert the signals into electrical impulses and electrodes used to carry the impulses to the cochlea
  • The electrodes stimulate the auditory nerve directly instead of the sensory hair cells


Applications and skills:


• Humans’ sensory receptors should include mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors.

• Diagram of the human eye should include the sclera, cornea, conjunctiva, eyelid, choroid, aqueous humour, pupil, lens, iris, vitreous humour, retina, fovea, optic nerve and blind spot.

• Diagram of the retina should include rod and cone cells, bipolar neurons and ganglion cells.

• Diagram of the ear should include pinna, eardrum, bones of the middle ear, oval window, round window, semicircular canals, auditory nerve and cochlea.

Diagram Answers


A.4 Innate and learned behaviour

Nature of science:

Looking for patterns, trends and discrepancies—laboratory experiments and field investigations helped in the understanding of different types of behaviour and learning. (3.1)


∑ - Innate behaviour is inherited from parents and so develops independently of the environment.

  • Instinctive genetically based behaviour
  • Innate behaviour is inherited from an individual’s parents and is not modified by the individual and is generally not affected by the organism’s environment, including an organism’s experiences
  • Innate behaviour is generally uniform and has low variation in the population
  • Beneficial innate behaviour evolves through natural selection, survival and reproduction
  • If an allele that generates a certain behaviour, gives an organism a survival advantage, it will reproduce and pass this allele on to its offspring and the allele will, therefore, increase in frequency within the population

Examples are suckling instinct in newborns and migration of Canadian Geese

β - Skill: Analysis of data from invertebrate behaviour experiments in terms of the effect on chances of survival and reproduction.

  • Taxis – is a direct movement towards or away from a stimulus eg. Pillbugs movement away from light (negative phototaxis)
  • Earthworms will move towards a moist environment (positive hydrotaxis)
  • Kinesis – is a non-directional movement when an invertebrate encounters a stimulus in terms of speed of the movement and the number of times an organism turns
  • Orthokinesis: The speed of movement of the organism is dependent upon the intensity of the stimulus. For example, the movement of woodlice in relation to humidity. With increased humidity, there is an increase in the percentage time that the woodlice will remain stationary.
  • Klinokinesis: frequency or rate of turning is proportional to stimulus intensity. An example involves the behaviour of the flatworm which turns more frequently in response to increasing light thus ensuring that it spends more time in dark areas.

****Design and carry out an experiment that involves a taxis or a kinesis with an invertebrate such as Planarian, pill bug, or beetles. ****

Explanation -

A couple of experiments



∑ - Learning is the acquisition of skill or knowledge.

∑ - Learned behaviour develops as a result of experience.

  • Learning is the acquisition of a skill or knowledge
  • Learned behaviour is based upon the experience that an organism undergoes
  • Learned behaviour is modified by the individual by trial and error
  • There is a high variation of learned behaviour within the population
  • Learned behaviour is highly influenced by the environment
  • The ability or capacity to learn may be a product of animals genes, rather than the specific behaviour; however, without experiences, the organism does not develop the behaviour

Examples: Acquisition of language & social skills and domesticated behaviour in pets

∑ - Autonomic and involuntary responses are referred to as reflexes.

  • An internal or external change in the environment that is detected by a receptor and elicits a response is known as a stimulus
  • Responses are changes in an organism that are carried out by an effector such as a gland or a muscle
  • If these changes are carried out without conscious thought, they are involuntary responses and are carried out by the autonomic nervous system
  • These involuntary responses are known as reflexes
  • A reflex is a rapid unconscious response to a stimulus
  • Examples are the pupil reflex and the patellar reflex

∑ - Reflex arcs comprise the neurons that mediate reflexes.

β - Application: Withdrawal reflex of the hand from a painful stimulus.

  • A stimulus such as pain is sensed by the receptors in the skin
  • A signal is sent by the sensory neuron back to the spinal cord, which enters the spinal cord through the dorsal root, where it synapses with relay/interneuron in the grey matter
  • The relay neuron synapses with the motor neuron and a signal are sent back through the ventral root by the motor neuron to the effector (muscle)
  • The muscle contracts removing your hand/body away from the pain stimulus
  • This is also known as the withdrawal reflex (the brain is not involved in a reflex arc)

  • Video on reflex arc - https://www.youtube.com/watch?v=wLrhYzdbbpE

β - Skill: Drawing and labelling a diagram of a reflex arc for a pain withdrawal reflex.

Include the receptor cell, sensory neuron, relay neuron, motor neuron and effector.

∑ - Reflex conditioning involves forming new associations.

β - Application: Pavlov’s experiments into reflex conditioning in dogs.

  • A classic example of reflex conditioning involves the salivation reflex in dogs done by Ivan Pavlov
  • Pavlov observed when a dog ate or saw its food, it salivated
  • The site of food is known as the unconditioned stimulus and the salvation by the dogs is the unconditioned response
  • Pavlov then used a neutral stimulus, such as ringing a bell, before he gave the unconditioned stimulus (food)
  • He repeated this process many times and found that the dogs began to salivate when he rang the bell without the presence of food
  • The sound of the bell is known as the conditioned stimulus and the salivation because of the ringing of the bell is now the conditioned response
  • The dogs learning to associate the two external stimuli (the bell and the arrival of food) is called reflex conditioning

Video -

Office Classical Conditioning

∑ - Operant conditioning is a form of learning that consists of trial and error experiences.

Big Bang Theory – Operant Conditioning -

  • Involves rewarding desired behaviour called Positive Reinforcement and sometimes punishing incorrect behaviour called Negative Reinforcement 
  • BF Skinner a psychologist developed something called the Skinner Box, where rats were rewarded for pushing a lever that gave them a food pellet to eat
  • The pushing of the lever was accidental and occurred through trial and error
  • The rat began to associate the pushing of the lever with the reward of food
  • The food is the reinforcement and the pushing of the lever is called the operant response
  • This is called operant conditioning

β - The role of inheritance and learning in the development of birdsong.

  • Birdsongs have been studied in many species of birds and it has been found that the development of the birdsong is partly innate and learned
  • All members of the same species share innate aspects of their song allowing members of the same species to recognize each other
  • In many species, young birds learn specific mating calls from their fathers that introduce slight differences in their songs
  • This is important in mate selection in many species as males are chosen by the quality of their song

Good article - http://phys.org/news/2013-08-human-language-birdsong-stepwise-imitation.html




****Data Analysis page 537***

∑ - Imprinting is learning occurring at a particular life stage and is independent of the consequences of behaviour.

  • Imprinting occurs in a sensitive period in an organism's development
  • It is a form of associative learning
  • Imprinting occurs at a stage that is vital to an individual’s survival and reproductive success
  • In birdsong, young chicks listen to and practice adult birdsong of the same species in the same area and modify their song to match its song to fit in
  • Captive birds that do not hear birdsong during this imprinting period are not able to reproduce the correct song when they are mature
  • Another example is geese that imprint on whatever they see as their mother

Lorenz experiment



∑ - Memory is the process of encoding, storing and accessing information.

  • Memory is the process of encoding, storing and accessing information
  • It is a higher-order function of the brain
  • Encoding is converting information into a form which can be stored
  • Short term memory lasts for a short period of time and can be converted or not-converted into long term memory (indefinite time period)
  • Accessing is the recall of a memory so it can be actively used in thought processes
  • The part of the brain called the hippocampus plays an important part in memory development and recall


• Drawing of reflex arc should include the receptor cell, sensory neuron, relay neuron, motor neuron and effector.

Theory of knowledge:

• It is easy for us to guess how the behaviour of an animal might influence its chance of survival and reproduction. Is intuition a valid starting point for scientists?


• Aim 7: Data logging using an electrocardiogram (ECG) sensor to analyze neuromuscular reflexes.

• Aim 8: Experiments with animals—implications of today’s animal policies for experimental science in Pavlov’s experiments.

Practice Questions

Banded wrens (Thryothorus pleurostictus) are known to sing actively in defence of their territories during the breeding season. Males possess over twenty different song-types. When two males approach each other near a boundary they engage in counter-singing and some song-types will be shared. The following diagram shows the pattern of song-types used during an interaction between two males at their territorial boundary in the Guanacaste Conservation Area, Costa Rica. The arrows indicate when both males sang identical song-types in succession. The interaction ended without a fight when the males retreated from the boundary.

Identify which song-types are shared between both males.

b.        Describe the changes in the song-type pattern during the entire interaction.
c.         Deduce how a male banded wren can communicate aggressive behaviour.

(Total 6 marks)

Two groups of 15 rats were trained to escape from an electric shock that was applied to one compartment of their cages. For one group (labelled EscD) the shock coincided with switching off the light, resulting in darkness in that compartment. The training was repeated for five sessions. The graphs below show the mean results for the two groups.

[Source: K. Zielinski and Savonenko, (2000), Acta Neurobiol. Exp, 60, pages 457-465]
(a)       (i)        Calculate the difference in escape times in session 1 between the two groups.

(ii)       Suggest a reason for the difference.

(b)       (i)        Compare the changes in escape times over the five sessions between the two groups.

(ii)       Deduce, giving a reason, which group shows more evidence of learned behaviour.

(c)       If the researchers were to continue their experiments with the group Esc and apply the same experimental conditions as for the group EscD, predict what would happen to the escape times for the group Esc.

(Total 6 marks)


A.5 Neuropharmacology  

Essential idea: Communication between neurons can be altered through the manipulation of the release and reception of chemical messengers.

Nature of science:

Assessing risks associated with scientific research—patient advocates will often press for the speeding up of drug approval processes, encouraging more tolerance of risk. (4.5)


∑ - Some neurotransmitters excite nerve impulses in postsynaptic neurons and others inhibit them.

  • Neurotransmitters are released through exocytosis by the pre-synaptic neuron, diffuse across the synapse and bind to receptors on the post-synaptic membrane of the next neuron or effector
  • Depending on the synapse, neurotransmitters can have an excitatory effect on the post-synaptic neuron (causing a depolarization of that neuron) or an inhibitory effect on the post-synaptic neuron (hyperpolarization of that neuron)
  • If the depolarization (makes the membrane potential more positive) caused by the excitatory neurotransmitters reaches the threshold level, an action potential is created in the post-synaptic neuron
  • If hyperpolarization (makes the membrane potential more negative) caused by the inhibitory neurotransmitters remains below the threshold level, an action potential is prevented or inhibited
  • image

 Glutamate is a main excitatory neurotransmitter and GABA is the main inhibitory neurotransmitter

Video: https://www.youtube.com/watch?v=aDKz3GUVAzg

∑ - Nerve impulses are initiated or inhibited in post-synaptic neurons as a result of summation of all excitatory and inhibitory neurotransmitters received from presynaptic neurons.

  • Usually, more than one pre-synaptic neuron can synapse with a post-synaptic neuron
  • There can be hundreds or thousands of pre-synaptic neurons that synapse with one post-synaptic neuron
  • Some of these pre-synaptic neurons are excitatory and some are inhibitory
  • The total additive effect of the excitatory neurotransmitters and the decreasing effect of the inhibitory neurons will determine if the threshold potential is reached and an action potential is created
  • The combined effects of the inhibitory neurotransmitters and the excitatory neurotransmitters is called summation
  • If the total sum of the incoming signals is excitatory and it reaches the threshold level, the neuron fires and the signal is carried forward
  • If the total sum of the incoming signals is inhibitory, the neuron does not fire and the signal is not carried forward

Video: https://www.youtube.com/watch?v=LT3VKAr4roo

∑ - Many different slow-acting neurotransmitters modulate fast synaptic transmission in the brain.

  • There are two types of neurotransmitters found in the brain: fast-acting and slow-acting neurotransmitters
  • Fast-acting neurotransmitters have an effect on the target cell within 1 millisecond
  • Slow-acting neurotransmitters have an effect on the target cell from hundreds of milliseconds up to a minute
  • Slow-acting NT’s act on a second messenger molecule which then affects the target cell

  • Examples of slow-acting neurotransmitters are dopamine and serotonin
  • Slow acting neurotransmitters are called neuromodulators
  • They modulate fast-acting neurotransmitters in the brain by either regulating the release of NT’s from the pre-synaptic neuron or regulating the efficiency at the postsynaptic membrane
  • Neuromodulators bind to the receptor causing the release of an internal secondary messenger inside the post-synaptic neuron
  • This secondary messenger binds to a second receptor causing the channel protein to open and the diffusion of ions like Na+

∑ - Memory and learning involve changes in neurons caused by slow-acting neurotransmitters.

Slow-acting neurotransmitters have a role in memory and learning
As above, they can cause the release of secondary messengers.
These messengers can increase synaptic transmission by increasing the number of receptors in the membrane or modify these receptors to increase the rate of ion movement into the neuron when the neurotransmitter binds
Secondary messengers can persist for days, causing long term potentiation (LTP)
This may be essential for synaptic plasticity necessary for memory and learning
The learning of new skills has been linked to the formation of new synapses in the hippocampus
Memory is long term and requires the synthesis of proteins
The proteins change the form and function of the synapse, resulting in a memory


∑ - Psychoactive drugs affect the brain by either increasing or decreasing postsynaptic transmission.

β - Application: Effects on the nervous system of two stimulants and two sedatives.

  • Psychoactive drugs affect the brain and personality by altering the functioning of some of these synapses
  • Excitatory drugs increase post-synaptic transmission
  • For example, Nicotine, Cocaine and Amphetamines
  • Inhibitory drugs decrease post-synaptic transmission
  • For example, benzodiazepines, alcohol and Tetrahydrocannabinol (THC)


  • synapses using acetylcholine are called cholinergic synapses
  • Nicotine stimulates the transmission in cholergenic pathways by mimicking acetylcholine
  • However, nicotine is not broken down by the enzyme acetylcholinesterase
  • Since acetylcholine is part of the parasympathetic pathway, the use of nicotine has a calming effect on one’s personality
  • This is also why people become agitated when they don’t have a cigarette
  • Nicotine also raises the dopamine levels in the brain

Cocaine and Amphetamines

  • Synapses using Noradrenaline are called adrenergic synapses
  • Cocaine and amphetamines stimulate adrenergic pathways
  • Noradrenaline is part of the sympathetic pathway “fight or flight”.
  • When it depolarizes the postsynaptic neuron it causes an excitatory effect, and since cocaine and amphetamines stimulate this synapse, they cause an increase in alertness, energy and euphoria
  • Cocaine blocks the dopamine reuptake transporters of the pre-synaptic neuron, therefore dopamine cannot be pumped back into the pre-synaptic neuron. Dopamine levels remain high in the synapse and there is a continual excitation of the post-synaptic neuron.


  • Bind to an allosteric site on the GABA receptors on the post synaptic membrane, increasing the rate of Cl- diffusion into the postsynaptic neuron, thus increasing the hyperpolarization of the membrane.
  • Therefore Benzodiazepines have an inhibitory effect on the neuron and is considered a sedative
  • These are widely used for generalized anxiety disorder and panic attacks

THC (Tetrahydrocannabinol)

Present in cannabis
Binds to cannabinoid pre-synaptic receptors in various parts of the brain ( example cerebellum and cerebrum), preventing the release of neurotransmitters that would cause the excitation of the post-synaptic neuron
THC also binds to GABA receptors in the reward pathway decreasing the release of GABA
Less GABA released means there is less inhibition of dopamine and therefore more dopamine is released (gives the user feeling of euphoria and pleasure)

Good interactive link on synapses and drugs:


Crash Course on Addiction


Mouse Party


Alcohol and the Brain


∑ - Anesthetics act by interfering with neural transmission between areas of sensory perception and the CNS.

β - Application: Endorphins can act as painkillers.

  • Pain receptors at the end of sensory neurons convey impulses to the cerebral cortex where they are recognized as pain
  • Endorphins which are natural painkillers produced by the pituitary gland, are released and bind to receptors at the synapses in the pain pathways effectively blocking the feeling of pain
  • Anesthetics, which are pain killers, act by interfering with the neural transmission from the pain receptors to the CNS
  • They cause a reversible loss of sensation in certain parts of the body (localized anesthetics) or all (general anesthetics)
  • General anesthetics is when they put somebody under or complete unconsciousness  

β - Application: The effect of anesthetics on awareness.

  • An individual under general anesthetics generally has no awareness during the surgery or procedure
  • Sometimes the person is partially conscious, like during some brain surgeries to remove tumours. The patient’s brain is monitored during the process
  • There is a risk during operations where the dose needs to be minimized
  • Sometimes, when not enough drug is administered, the patient might still maintain awareness and possibly feel pain

Video: https://www.youtube.com/watch?v=hX1eyGiBYck

∑ - Stimulant drugs mimic the stimulation provided by the sympathetic nervous system

  • The sympathetic nervous system is involved in the fight or flight response
  • Stimulants are drugs that promote the activity of the nervous system, similar to the sympathetic nervous system
  • They make a person more alert, energetic and increase heart rate, blood pressure and body temperature
  • Examples are caffeine, cocaine, amphetamines and nicotine

∑ - Addiction can be affected by genetic predisposition, social environment and dopamine secretion.

Genetic predisposition

  • many drugs are addictive; however, not everyone becomes an addict when they do these drugs
  • Addictions can run in families suggesting a genetic predisposition
  • Addiction must have some relationship to certain genes
  • DRD2 is a gene with multiple alleles that codes for the dopamine receptor protein
  • People with one or more copies of the A1 allele consumed less alcohol than those that were homozygous for the A2 allele

Social Environment

  • Peer pressure, cultural traditions, poverty and social deprivation, and mental health or traumatic experiences all contribute to possible addiction

Dopamine Secretion

  • Many addictive drugs affect the dopamine synapses and the secretion of dopamine
  • These synapses are involved in the reward pathway, so users of these drugs, find it difficult to stop as they become addicted to these feelings of euphoria


Brain and Addiction




β - Skill: Evaluation of data showing the impact of MDMA (ecstasy) on serotonin and dopamine metabolism in the brain.

*** Data analysis on page 544***




• Examples of stimulants are nicotine, cocaine or amphetamines.

• Examples of sedatives are benzodiazepines, alcohol or tetrahydrocannabinol



• Attitudes to drugs and the use of drugs differ globally. There are many cultures that use drugs to enhance rituals or religious experiences.




• Many psychoactive drugs have been used therapeutically to treat a range of mental illnesses and psychological disorders.

 A.6 Ethology 

Essential idea: Natural selection favours specific types of behaviour.

Nature of science:

Testing a hypothesis—experiments to test hypotheses on the migratory behaviour of blackcaps have been carried out. (1.9)


∑ - Ethology is the study of animal behaviour in natural conditions.

  • ​The objective and scientific study of animal behaviour observed under natural conditions
  • Ethology is usually interested in the behavioural process that leads to an adaptive trait due to evolution  

∑ - Natural selection can change the frequency of observed animal behaviour.

∑ - Behaviour that increases the chances of survival and reproduction will become more prevalent in a population.

  • ​Natural selection can cause a change in the frequency of behaviours and traits within a population
  • If the trait or behaviour is beneficial to an organism and helps survive and reproduce, the frequency of this observed animal behaviour will increase within a population
  • ​Good link: https://ncse.com/files/pub/evolution/Evolution--Futuyma--chap11--fb.pdf
  • Misconceptions about evolution: http://evolution.berkeley.edu/evolibrary/misconceptions_faq.php

∑ - Learned behaviour can spread through a population or be lost from it more rapidly than innate behaviour.

  • ​Innate behaviour can only be modified by a change in allele frequency through natural selection
  • It generally occurs quite slowly because there must be changes in these alleles that affect behaviour
  • Learned behaviour might take time to learn, but once learned, can spread quickly within a population as individuals learn from one another
  • ​Learned behaviour can be much more adaptable and produce a greater range of behaviour in comparison to innate behaviour

Applications and skills:

β - Application: Migratory behaviour in blackcaps as an example of the genetic basis of behaviour and its change by natural selection.

β - Application: Blood sharing in vampire bats as an example of the development of altruistic behaviour by natural selection.

Vampire Bats Sharing Blood: https://www.youtube.com/watch?v=8ZJOKJNjLuQ 

β - Application: Foraging behaviour in shore crabs as an example of increasing chances of survival by optimal prey choice.

β - Application: Breeding strategies in Coho salmon populations as an example of behaviour affecting chances of survival and reproduction.

β - Application: Courtship in birds of paradise as an example of mate selection.

BBC Birds of Paradise - https://www.youtube.com/watch?v=W7QZnwKqopo

β - Application: Synchronized oestrus in female lions in a pride as an example of innate behaviour that increases the chances of survival and reproduction of offspring.

β - Application: Feeding on cream from milk bottles in blue tits as an example of the development and loss of learned behaviour.


Topic 5.2 Natural selection Option D Medicinal chemistry

Topic D1 Pharmaceutical products and drug action

Topic D3 Opiates


Core: Biological level of analysis


• Aim 8: The social consequences of psychoactive drugs could be considered, for the user, his or her family and the wider society.


• The seven applications in this sub-topic are intended to reinforce understanding of the general principles. The applications include a range of types of behaviour and types of animals. Other examples, including local examples that can be observed, should also be studied if possible.



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