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3. a) i) Define irritability, stimulus and response irritability

-also called sensitivity

– Responsiveness to change in environment


A change in the environment of organism which causes change in organism’s activity


  • change in activity of an organism caused by a stimulus

ii) State importance of irritability to living organisms

  • Adjusting to environmental conditions. Sensitive/defect/responding
  • List the examples of external stimuli to organisms
  • air/oxygen (aero)
  • light(photo)
  • osmotic pressure (osmo)
  • current (Rheo)
  • chemical concentration (chemo)
  • \water/moisture (hydro)
  • Touch/contact (hapto/thigmo)
  • Gravity/soil (geo)
  • Temperature (thermo)

b) i) What are tactic responses?

  • response in which whole organism or its motile parts move e.g. gamete

ii) What causes tactic responses?

  • caused by unidirectional stimulus
  • usually doesn’t involve growth
  • response is either positive or negative
  • named according to source of stimulus
  • e.g phototaxis, aerotaxis, chemotaxis

iii) State the importance of tactic response to:

Members of kingdom protista

  • move towards favorable environment/move away  from unfavorable environment
  • move towards their prey/food

Microscopic plants

– escape injurious stimuli/seek favorable habitats

  1. Name the type of response exhibited by:

Euglena when they swim towards the source of light

  • phototaxis
  • sperms when they swim towards the ovum
  • chemotaxis
  • State the advantages of tactic responses to organisms
  • to avoid unfavorable environment/injurious stimuli
  • escape from predators
  • to seek favorable environment
  • to seek for food/prey

c) i) Define the term tropism

  • growth movement of plants in response to external unilateral/unidirectional stimuli

ii) Explain the various types of tropism in plants


  • growth movements of plant shoots in response to unilateral sources of light
  • the tip of the shoots produce auxins down the shoot
  • light causes auxins to migrate to outer side/darker side causing growth  on the side away from light hence growth curvature towards source of light roots are negatively phototrophic


  • response of roots/parts of a plant to the direction of force of gravity
  • auxins grow towards the direction of force of gravity causing positive geotropism in roots while shoot grows away from force of gravity (negatively geotrophic)


  • growth response of plant when in contact with an object
  • contact with support causes migration of auxins to outer side causing faster growth on the side away from contact surface
  • this causes tendrils/stem to twin around a support


  • growth movement of roots in response to unilateral source of water/moisture
  • the root grows towards the source of water/ positively hydrotropic while leaves are negatively hydrotropic


  • growth movement of parts of plant to unilateral source of chemicals
  • the chemicals form a gradient between two regions e.g. pollen tube growing towards the ovary through the style

iii) State the ways in which tropisms are important to plants

  • expose leaves/shoots in positions for maximum absorption of sunlight for photosynthesis
  • enables roots of plants to seek/look/search for water
  • enables plant stems/tendrils to obtain mechanical support especially those that lack woody stems
  • enables roots to grow deep into the soil for anchorage
  • enables pollen tube grow to embryo sac to facilitate fertilization

iv) Explain the differences between tropic and tactic responses

growth curvature in responseslowinfluenced by hormoneslocomotory responsefastexternal influence

d) The diagram below represents growing seedlings which were subjected to unilateral light at the beginning of an experiment


  1. State the results of P, Q and R after 5 days
  2. P will bend/grow towards light
  3. Q will remain straight/have little or no growth
  4. R will remain/grow straight/grow upwards
  5. Account for your results in (i) above

P- Growth substance/growth hormone/IAA/auxin are produced by the stem tip

  • they move (downwards and get distributed) to the side away from light where they cause rapid/more growth/cell division/elongation that results in bending

Q- Source of auxin has been removed

R- The auxins cannot be affected by light because the tip has been covered

  1. If the tin foil were removed from the tip of seedling R, what results would be observed after two days
  2. it will bend/grow towards light
  3. State the expected results after 3 day is if the box were removed
  4. all seedlings will grow straight/upwards

e) In an experiment to investigate a certain aspect of plant response, a seedling was placed horizontally as shown in diagram I below. After seven days the appearance of the seedling was as shown in diagram 2

Account for the curvature of the shoot and root after the seven days

  1. Shoot
  2. auxins accumulate on the lower side of the seedling due to gravity
  3. high concentration of auxins in shoot stimulates faster growth causing more elongation on the lower side than the upper side hence curvature occurs upwards
  4. Root
  5. the high concentration of auxins inhibits growth hence the upper side with less auxins grows faster than the lower side therefore the curvature occurs downwards

f) What is etiolation?

  • phenomenon exhibited by plants when grown in darkness
  • such plants are pale yellow due to absence of chlorophyll, have small leaves, long stems/hypocotyle and slender stems
  • plants exhibit etiolation to reach light/obtain light
  • this is a survival response
  • a) i) What is coordination in animals
  • – The linking together of all physiological activities that occur in the body so that they take place at the right time and in the correct place

ii) Name the main systems for coordination in animals

– Nervous system/sensory system

– Endocrine (hormonal system)

iii) List the components of the mammalian sensory system

– Central nervous system (CNS), brain & spinal cord

– Peripheral nervous system (PNS) cranial and spinal nerves

– Sense organs

– Autonomic nervous system (ANS) nerve fibers and ganglia

iv).Explain the terms receptors, conductors and effectors

– Receptors are structures that detect stimuli i.e. sense organs

– Conductors transmit impulses from receptors to effectors e.g. neurons

– Effectors are the responding parts e.g. muscles, glands

v) What are the functions of the central nervous system?

– provides a fast means of communication between receptors and effectors

– coordinates the activities of the body

vi) State the differences between somatic and autonomic systems of peripheral nervous system

– Somatic is concerned with controlling the conscious or voluntary actions of the body i.e. skin, bones, joints and skeletal muscles

– the autonomic (automatic) nervous system controls involuntary actions of internal organs, digestive system, blood vessels, cardiac muscles and glandular products.

b) i) What is a neurone?

  • the basic unit of the nervous system
  • also called  nerve cell
  • conducts impulses
  • include monitor sensory and relay neurons

ii) Name the parts of a typical neurone and state the functions of each part

  • cell body/centron contains nucleus and cytoplasm
  • axon transmits impulses away from cell body
  • dendrites relays impulses across adjacent neurons
  • myelin sheath insulates axon and speeds up transmission of impulses
  • schwan cells forms myelin sheath and aid in nutrition and regeneration of axon
  • node of ranvier occur between schwan cells, where axon is not covered, speeds up impulse transmission
  • nissils granules contain mitochondria that provide cell body with energy for metabolic process
  • Describe the structure and function of a motor neurone
  • motor neurone relays impulses from CNS (brain/spinal cord) to effectors ( muscles/glands)
  1. Describe the structure and function of sensory neurone
  2. sensory neurone relays impulses from receptors (sense organs) to CNS
  1. a. State structural differences between motor and sensory neurons
  2. Cell body in motor neurone is terminal (at the end) and inside central nervous system.
  3. Cell body in sensory neurone is terminal but has axon at both ends (bipolar)

B. Describe the structure and function of a relay neurone

  • also called intermediate/internuncial/associate/connector/interneurone
  • locate inside central nervous system and spinal cord
  • usually lack myelin sheath

c) State the functions of the major parts of the human brain

i) Cerebrum

  • called forebrain
  • occupies most of the brain
  • consists of four lobes each with specific function
  • temporal lobe controls taste smell hearing learning and memory
  • partial lobe controls sensory output and touch
  • occipital lobe controls vision, motor output and speech
  • frontal lobe controls personality, learning thought and speech
  • also has parts called thalamus and hypothalamus
  • thalamus helps to sort sensory information
  • hypothalamus controls hunger, heartbeat body temperature and aggression

ii) Mid brain

  • quite small in humans
  • relay centre for audio and visual information
  • also involves in some sight, hearing and orientation responses
  • Hind brain
  • consists of cellebral and medulla oblongata
  • cerebellum is responsible for coordinating impulses, posture and balance, motor coordination and muscle tone
  • medulla oblongata controls heartbeat, blood pressure breathing rate, coughing and sneezing
  • i) What is reflex action?
  • an automatic response to an external stimulus e.g. sneezing or withdrawing hand from a hot object

ii) Describe a reflex action that will lead to the withdrawal of a hand from a hot object

  • Receptors in the skin respond to stimuli. Are stimulated
  • an impulse is transmitted through the sensory neurone, across a synapse to the central nervous system (white matter), through the relay neurone into grey matter, then to the motor neurone and finally to the effect muscle which contracts
  • the hand is then withdrawn

iii) Explain how an impulse is transmitted across the synapse (gap)

  • impulse initiates release of transmitter substance acetylcholine at the end of the sensory neurone
  • acetylcholine diffuses across the synapse and generates an impulse in the next neurone
  1. Briefly describe the transmission of a nervous impulse across a neuro-muscular junction
  2. impulse arrives at synoptic knob and causes vesicle to move to the pre-synaptic membrane
  3. vesicle discharges transmitter substance into synaptic cleft
  4. transmitter substance/acetylcholine diffuses across the cleft and attaches to post-synaptic membrane
  5. the membrane is depolarized, generating the action potential
  6. What are the functions of a synapse?
  7. allows transmission of nerve impulses from neurone to neurone
  8. ensures nerve impulses travel in only one direction
  9. in the brain they store information/memory
  10. i) What is a conditioned reflex?
  11. A response caused by a unilateral stimulus (associated stimulus) which substitutes the normal stimulus

ii) Explain a conditioned reflex

  • it is automatic
  • it involves the spinal cord
  • it is usually learned e.g. writing, cycling, dancing
  • it involves the interaction of highly specialized centers of the brain with a large number of neurone  necessary to bring about conditioning
  • example is experiments carried out by Pavlov using dogs

iii) Compare a simple reflex action with a conditioned reflex

Simple reflexConditioned reflex
independent of experienceone stimulus to evoke responsesome sensory and motor neurons usedreflex is simpledependent on experienceboth substitute and original reflex evoke responsesensory component replaced but motor remains unchangedreflex is modified
  • i) What are endocrine glands?
  • ductless glands that produce hormones in animals
  • hormones are chemical substances which help to coordinate the functions of the body

ii) State the functions of hormones in animals

  • regulate growth and development
  • control behavior during breeding
  • proper functioning of cells
  • regulate metabolic activities

iii) Name the main endocrine glands, their secretions and functions in the human body

ThyroidThyroxineIncreases rate of metabolism
ParathyroidParathyroid hormoneRegulates calcium and phosphate levels
PituitaryHormone growthRegulate growth of body
 Gonadotrophic hormoneStimulates the development of male and female sex organs
Lactogenic hormone (prolactin)Stimulates secretion of milk after child birth
Thyrotropic hormone(TSH)– proper functioning of thyroid gland/thyroxine production
Adrenocorthicotropic hormone (ACTH)– stimulates release of adrenal cortex hormone
Oxytocinregulates blood pressurestimulates smooth musclesstimulates contraction of uterus during childbirthaids in flow of milk from mammary glands
Follicle stimulating hormone(FSH)causes maturation of egg in femalesstimulates sperm production in males
Vasopressin (ADH) Antiduretic hormone– regulates water balance by kidneys
AdrenalAdrenaline (epinephrine)for emergencyprepares body to cope up with stress
 Aldosterone– maintains balance of salt and water in blood
 Cortisonebreaks down stored proteins to amino acidsaids in breakdown of adipose tissueregulates sugar level in bloodprevents inflammation      
 Sex hormonessupplements sex hormones produced by gonadspromotes development of sexual characteristics
PancreaseInsulinregulates level of sugar in bloodenables liver to store sugar
Glucagonsregulates level of sugar in blood
OvariesOestrogencauses sexual secondary characteristics in females prepares uterus for pregnancy
Progesteronegrowth of mucus lining of uterusmaintains uterus during pregnancy
TestesAndrogens(testosterone)causes secondary sexual characteristics in males
Stomach cellsGastrinstimulates release of gastric juice
Intestinal cellsSecretinstimulate release of pancreatic juice

iv) Give the differences between nervous and endocrine (hormonal) communication

NervousHormonal (endocrine)
Response confined to effector organs (localized targets)Response more widespread (various targets)
Speed of response is rapidResponse less rapid
Nervous impulse thro\\ugh nerves/nerve cell/neuronsHormones transferred through blood
Duration of response is shortPersist for long
Speed of transmission is rapidSpeed of transmission is slower
Transmission is electricalTransmission is chemical

v) State the effects of over secretion and under secretion of adrenaline and thyroxine in humans

HormoneOver secretionUnder secretion
Adrenalinethin toneless muscles high blood pressureweak bonesobesityearly onset of sexual developmentlow blood pressureinability to withstand stressfatiguemuscular weaknessmuscle wastingincreased dark pigmentation of skin
Thyroxineincreased metabolismincreased heartbeatphysical restlessnessmental restlessnessprotruding eyeballsenlarged thyroid glandcretinism(retarded growth and low mental developmentlowered metabolismlow ventilation rate of lungslow body temperaturelowered mental activitycoarse hairpuffy eyesenlarged thyroid gland

g) i) Define the following terms


  • a substance that causes a change in body function

Drug abuse

  • indiscriminate use of drugs without minding their side effects
  • misuse or wrong use of drugs

ii) State the types of drugs, examples and side effects


  • also called depressant
  • a drug that decreases the action of the central nervous system
  • reduce anxiety, and tension, induce sleep and act as muscle relaxants
  • when abused they cause withdrawal effects such as anxiety, delirium and death
  • includes barbiturates, other sedatives, tranquilizers and alcohol


  • suppress centers of pain in the brain


  • given to people with hallucination or mentally ill patients to calm then down
  • when abused they lead to a feeling of confusion, agitation, depression and violent behavior that can lead to murder or suicide
  • examples include valium, LSD, bhang, narcotics and cannabis


  • drugs that temporarily increase the action of the central nervous system
  • they create a feeling of alertness, wakefulness, a sense of self confidence and well being
  • used to decrease fatigue and mild depression
  • when abused they cause feelings of persecution, hallucination and addiction
  • include amphetamines, cocaine, caffeine, miraa and nicotine

iii) State the general effects of drug abuse on human health

  • damage to body organs e.g. liver cirrhosis
  • drug addiction
  • impaired judgment resulting in clumsiness
  • socio-economic problems e.g. crime, loss of jobs, divorce, prostitution, HIV/AIDS
  • may cause poor health

h) i) List the special sense organs in mammals and the major function of each

– Eye for sight

– Ear for hearing and balance

– Nose for smell

– Skin for touch, temperature detection, pain detection

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