Biology Form 4 Notes : RECEPTION, RESPONSE AND CO-ORDINATION IN PLANTS AND ANIMALS

Reception, Response and Co-ordination in Plants and Animals

Introduction

• The structures involved in detecting changes may be located far from those that respond.
• There is a need for a communication system within the body.
• The nervous system and the endocrine system perform this function, linking the parts of the body that detect changes to those that respond.

Irritability

• Living organisms can detect changes in their internal and external environments and respond appropriately.
• This characteristic is called irritability and is vital for survival.

Stimuli

• A stimulus is a change in the internal or external environment to which an organism responds.
• Examples include light, heat, sound, chemicals, pH, water, food, oxygen, and other organisms.

Response

• A response is any change shown by an organism in reaction to a stimulus.
• The response involves movements of the whole or part of the body either towards or away from the stimulus.
• It also results in secretion of substances such as hormones or enzymes by glands.

Co-ordination

• Co-ordination is the working together of all parts of the body to bring about appropriate responses to environmental changes.

Reception

• Reception is the detection of changes in the environment through receptors.

Irritability in Plants

• Response in plants is less pronounced than in animals but equally important for survival.
• Plants respond to stimuli such as light, moisture, gravity, chemicals, and touch.

Tropisms

• Plants often respond by growing in a particular direction; these growth movements are called tropisms.
• They result from unequal growth caused by uneven distribution of growth hormones (auxins).
• One side grows more than the other, causing bending towards (positive tropism) or away from (negative tropism) the stimulus.

Phototropism

• Seedlings exposed to light from one direction have shoots that grow towards the light (positive phototropism).
• The shoot tip receives the unilateral light stimulus, but the response occurs below the tip.
• The hormone auxin, produced at the tip, diffuses down and causes cell elongation.
• Light causes auxin to migrate to the darker side, where it is more concentrated, causing faster growth on that side and bending the shoot towards the light.

Survival Value

• Positive phototropism ensures sufficient light absorption by leaves for photosynthesis.

Geotropism

• Geotropism is growth response to gravity.
• Roots are positively geotropic (grow downwards), while shoots are negatively geotropic (grow upwards).
• When seedlings are placed horizontally, auxin accumulates on the lower side of roots and shoots due to gravity.
• Shoots respond to higher auxin concentration by growing faster on the lower side, bending upwards.
• Roots are inhibited by high auxin concentration, so the lower side grows slower, bending roots downwards.

Survival Value

• Roots grow downwards to absorb water and anchor the plant, aiding nutrient uptake.

Hydrotropism

• Hydrotropism is the growth of roots towards water (moisture).

Survival Value

• Ensures roots grow towards moisture to obtain water needed for photosynthesis and mineral transport.

Chemotropism

• Chemotropism is the response of plant parts towards chemical substances.
• Example: growth of the pollen tube towards the ovule in flowering plants.

Survival Value

• Ensures fertilisation and continuation of the species.

Thigmotropism

• Thigmotropism is a growth response to touch.
• Example: tendrils of climbing plants bend around objects they contact.

Survival Value

• Provides support and positions leaves for optimal light absorption and gas exchange.

Tactic Movements in Plants and Other Organisms

• Tactic movement is movement by a whole organism or motile part in response to a stimulus.
• Named according to the stimulus nature:
• Phototaxis: movement in response to light direction and intensity. Example: Chlamydomonas swim towards optimal light.
• Osmotaxis: movement in response to osmotic conditions, e.g., freshwater amoeba.
• Chemotaxis: movement in response to chemical concentration.

Survival Value

• Ensures favourable conditions for existence and maximises benefits to the organism.
• Example: In bryophytes, antherozoids move towards archegonia for fertilisation.

Nastic Movements

• Nastic movements are responses by plant parts to stimuli not coming from a particular direction.
• Named according to the stimulus:
• Seismonasty/haptonasty: response to shock. Example: Mimosa pudica folds its leaves when touched.

Production of Auxins and Their Effects on Plant Growth

• Auxins are produced by plant apices (root and shoot apex).
• They cause cell elongation and growth.
• Auxins are diffusible substances effective in small amounts.
• Roots require lower auxin concentrations than shoots.
• Auxins also influence other aspects of growth and development.
• Other chemical substances also influence plant growth and development.

Effects of Auxin on Plant Growth

Apical Dominance

• Auxins inhibit growth of side branches (apical dominance).
• If the terminal bud is removed, side branches develop from lateral buds.
• This principle is applied in pruning to control branch development.

Growth of Adventitious Roots

• Adventitious roots develop from the stem.
• Auxins stimulate growth of adventitious roots.

Parthenocarpy

• Formation of fruits without fertilisation.
• Can be induced by treating unpollinated flowers with auxin.
• Used in developing seedless fruit varieties.
• Auxins, along with other hormones, are involved in secondary growth, leaf fall, and fruit ripening.

Reception, Responses and Coordination in Animals

• The nervous and endocrine systems (neuro-endocrine system) coordinate body functions.
• They link receptors to effectors and regulate their activities.

Receptors

• Receptors are cells that detect or receive stimuli.
• They may be scattered over the body surface (e.g., pain, touch, temperature receptors) or located in special sense organs (e.g., light, sound, taste, smell).
• Motor nerves link the Central Nervous System (CNS) to effectors.
• The motor nerve cell body is located at one end of the axon.
• They transmit impulses from the CNS to effectors.

Effectors

• Effectors are cells, organs, or organelles enabling the organism to respond.
• They include muscles, glands, cilia, and flagella.

The Nervous System

Components of the Nervous System in Humans

• Every organ in the human body is connected to nerves.
• The nervous system consists of nerve cells (neurons) transmitting impulses between body parts.
• It includes:
• The Central Nervous System (CNS): brain and spinal cord.
• The Peripheral Nervous System: nerves linking CNS to receptors and effectors.
• Sensory nerves transmit impulses from receptors to CNS.

Structure and Functions of Neurons

• A nerve cell consists of a cell body (centron) containing the nucleus, with dendrites arising from it.
• One projection is the axon, the longest process.
• The axon contains axoplasm continuous with the cell body cytoplasm.
• The axon is enclosed in a fatty myelin sheath secreted by Schwann cells.
• The myelin sheath is interrupted by nodes of Ranvier at approximately 1 mm intervals.
• The myelin sheath is enclosed by the neurilemma, part of the Schwann cell in contact with the axon.
• The myelin sheath and nodes of Ranvier enhance impulse transmission.

Types of Neurons

Sensory Neuron

• Also called afferent neuron.
• Transmits impulses from sensory cells to CNS.
• Cell body is located along the axon outside the CNS.

Motor Neuron

• Also called efferent or effector neuron.
• Transmits impulses from CNS to effectors (muscles and glands).
• Cell body is located inside the CNS.

Intermediate or Connector Neuron

• Also called relay neuron.
• Found inside the CNS.
• Connect sensory and motor neurons and other nerve cells in the CNS.

Functions of the Neuron

• The nerve impulse is electrical.
• Transmission depends on electrical potential differences between inside and outside of the axon.
• Outside is positive; inside is negative.
• Stimulus changes membrane permeability, altering ion composition.
• Sodium ions rush in, reversing polarity (inside positive, outside negative), creating a nerve impulse.
• Impulse transmits along sensory neuron to CNS at high speed (up to 100 m/s in mammals).
• Dendrites do not connect directly but communicate across synapses (small gaps).
• Impulse transmission across synapses uses chemical transmitter substances stored in synaptic vesicles.
• Common transmitters: acetylcholine or noradrenaline.
• Synaptic vesicles release transmitters when impulse arrives at synaptic knob.
• Motor neuron impulses transmit to effectors across the neuromuscular junction.

Functions of Major Parts of the Human Brain

• The CNS consists of the brain and spinal cord.
• The CNS coordinates body activities by receiving impulses from sensory cells and sending impulses to effectors.
• The brain is enclosed in the cranium and protected by meninges.
• Infection of meninges causes meningitis.

The Brain Consists of the Following Parts:

Cerebrum

• The largest brain part, consisting of two cerebral hemispheres.
• Highly folded to increase surface area.
• Controls learning, intelligence, thought, imagination, and reasoning.

Medulla Oblongata (Brain Stem)

• Controls breathing rate, heartbeat rate, swallowing, salivation, blood pressure, temperature regulation, hearing, taste, and touch.

Cerebellum

• Located in front of the medulla; a folded dorsal expansion of the hindbrain.
• Controls posture, movement, and balance.

Hypothalamus

• Controls body temperature and osmoregulation.

Pituitary Gland

• An endocrine organ secreting hormones controlling osmoregulation, growth, metabolism, and sexual development.

Optic lobes control sight.

Olfactory lobes control smell.

Spinal Cord

• Located within the vertebral column.
• Consists of grey matter (central part with nerve cell bodies and intermediate nerve fibres) and white matter (carries sensory nerve fibres).
• Ventral root carries motor nerve fibres.

Simple and Conditioned Reflex Actions

Simple Reflex Action

• An automatic response to a stimulus.
• The impulse route is called a reflex arc.

Reflex Action Sequence

• Receptor is stimulated; impulse travels along sensory nerve fibre to spinal cord.
• Impulse is picked by intermediate neuron in CNS.
• Intermediate neuron transmits impulse to motor nerve fibre connected to effector.
• Effector responds.

Examples of Reflex Actions

• Pulling hand away from hot object.
• Knee jerk.
• Sneezing.

Conditioned Reflexes

• Learned responses.
• When two or more stimuli are repeatedly presented together, the animal eventually responds to either.
• Example: a hungry animal salivates at food; if a bell rings with food, it learns to salivate at bell alone.
• This is called a conditioned reflex and is a learning mechanism.

The Role of the Endocrine System in Humans

• Consists of glands that secrete hormones.
• Endocrine glands have no ducts; exocrine glands have ducts.
• The pancreas has both exocrine and endocrine parts.
• Hormones are protein chemical substances secreted at one part of the body affecting other parts.
• Secreted directly into blood and transported by blood.
• Each hormone has either a general coordinating effect or a specific response in a target organ.

Hormones Produced in Humans and Their Effects

Adrenaline

• Enhances activity of the sympathetic nervous system.

Oversecretion

• Increased heartbeat.
• High blood pressure.
• Thin, toneless muscles.

Undersecretion

• Low blood pressure.
• Inability to withstand stress.
• Muscular weakness.

Thyroxine

Oversecretion (Hyperthyroidism)

• Increased basal metabolic rate (BMR) and temperature.
• Person becomes angry, nervous, and hands may shake.
• Increased heartbeat leading to cardiac failure.

Undersecretion (Hypothyroidism)

• Poor growth and mental retardation (cretinism).
• Reduced metabolic rate and temperature.
• Person becomes inactive and slothful.
• Eyes and face become puffy due to fluid retention.
• Swollen tongue and rough skin in extreme cases.
• Enlarged thyroid gland.

Comparison Between Endocrine and Nervous Systems

Similarities

• Both endocrine and nervous systems coordinate body functions.
• Both have target organs.
• Both are controlled via negative feedback mechanisms.

Effects of Drug Abuse on Human Health

• Drug abuse is misuse of drugs.
• Drugs are chemical compounds affecting body functions or killing microorganisms.

Prescription Drugs

• Prescribed by doctors.
• Overdose can cause death.

Over-the-Counter Drugs (OCD)

• Self-prescribed drugs.
• Can have harmful effects and lead to tolerance requiring higher doses.

Effects of Hard Drugs on Human Health

• Lung cancer caused by nicotine.
• Emphysema.
• Liver cirrhosis caused by alcohol.
• Interference with vision due to alcohol.
• Sterility caused by khat (miraa).
• Sleeplessness (insomnia) caused by khat.
• Hallucinations caused by Cannabis sativa (Bangi).
• Digestive upset, nausea.
• Diarrhoea and vomiting.
• Headache and double vision.
• Skin tone changes (e.g., very dark).
• Extreme appetite changes (very poor or very great).
• Weight loss.
• Personality changes (irritable, confused).
• Convulsions, lethargy, and depression due to inhalation of solvents (e.g., glue).

Structure and Function of Parts of the Human Eye

Structure

• The human eye is spherical and situated within the skull’s orbit.
• Attached by three pairs of muscles controlling movement.
• Composed of three main layers: sclerotic layer, choroid, and light-sensitive retina.

Sclerotic Layer

• Outermost white part at sides and back of eye.
• Made of collagen fibres.
• Protects the eye and maintains shape.

Cornea

• Transparent front part of sclera allowing light passage.
• Curved and bulging, helps focus light onto retina.

Choroid

• Middle layer with many blood vessels supplying nutrients and removing wastes.
• Contains dark pigments absorbing stray light to prevent internal reflection.

Ciliary Body

• Glandular, secretes aqueous humour.
• Has blood vessels for nutrient supply, excretion, and gas exchange.
• Contains ciliary muscles that change lens shape during accommodation.

Suspensory Ligaments

• Inelastic ligaments attaching lens to ciliary body, holding it in position.

Lens

• Biconvex, crystalline, and transparent to refract and focus light onto retina.

Aqueous Humour

• Fluid between lens and cornea.
• Transparent and watery, helps focus and maintain eye shape.
• Conveys nutrients and oxygen to cornea and removes waste.

Iris

• Coloured part with central pupil opening.
• Contains circular and radial muscles controlling pupil size and light entry.

Vitreous Humour

• Fluid between lens and retina.
• Viscous, transparent, gives eye shape and refracts light.

Retina

• Light-sensitive layer at back of eye.
• Contains rods (sensitive to low light, black and white) and cones (sensitive to bright light and colour).
• Nocturnal mammals have more rods; diurnal mammals have more cones.

Fovea Centralis

• Most sensitive retina part, mainly cones for sharp vision (visual acuity).

Optic Nerve

• Transmits impulses from retina to brain for interpretation.

Blind Spot

• Point where optic nerve leaves eye; no rods or cones, not sensitive to light.

Eyelid

• Loose skin covering eye, closes by reflex to protect from damage and excessive light.

Eyelashes

• Prevent dust and particles from entering eye.

Conjunctiva

• Transparent thin layer allowing light passage.
• Continuous with eyelid epithelium, protects cornea.

Accommodation

• Change in lens shape to focus images.
• For distant objects: ciliary muscles relax, suspensory ligaments stretch, lens thins to focus light on retina.
• For near objects: ciliary muscles contract, suspensory ligaments slacken, lens becomes more convex to focus light on retina.

Control of Light Intensity Entering the Eye

• In bright light, circular iris muscles contract, pupil diameter decreases, reducing light entry to protect retina.
• In dim light, circular muscles relax, radial muscles contract, pupil diameter increases, allowing more light in.

Image Formation and Interpretation

• Light rays enter cornea, pass through pupil, refracted by cornea and lens.
• Lens focuses rays to form an inverted image on retina.
• Rods and cones stimulated; impulses transmitted via optic nerve to brain.
• Brain interprets image as upright.

Common Eye Defects and Their Correction

Short-sightedness (Myopia)

• Cannot focus distant objects properly; light focuses in front of retina.
• Caused by eyeball being too long.
• Corrected with concave lenses that diverge light rays before entering eye.

Long-sightedness (Hypermetropia)

• Cannot focus near objects properly; light focuses behind retina.
• Caused by eyeball being too short.
• Corrected with convex lenses that converge light rays before entering eye.

Astigmatism

• Cornea or lens is uneven, causing improper focus on retina.
• Corrected with cylindrical lenses.

Presbyopia

• Light rays from near objects not focused on retina due to lens hardening with age.
• Corrected with convex lenses.

Structure and Functions of Parts of the Human Ear

The Mammalian Ear

• Performs hearing and detects body position changes for balance and posture.
• Divided into three sections: outer ear, middle ear, and inner ear.

The Outer Ear

• Pinna: outer flap made of cartilage, catches and directs sound.
• External auditory canal: tube through which sound travels; secretes wax trapping dust and microbes.
• Tympanum: membrane at inner end of canal that vibrates with sound waves.

The Middle Ear

• Chamber containing three small bones (ossicles): malleus, incus, and stapes.
• Ossicles amplify vibrations and transmit them from tympanum to oval window.
• Oval window: membrane at chamber end that vibrates with ossicle movement.
• Eustachian tube connects middle ear to pharynx, equalising pressure inside and outside tympanum.

The Inner Ear

• Series of fluid-filled chambers including cochlea and semicircular canals.
• Cochlea: coiled tube with many sensory cells connected to brain via auditory nerve; detects vibrations for hearing.

Hearing

• Sound waves vibrate tympanum, transformed into vibrations.
• Ossicles transmit vibrations to oval window.
• Oval window vibrations cause cochlear fluid to vibrate.
• Sensory cells in cochlea convert vibrations to nerve impulses.
• Impulses transmitted to brain via auditory nerve; hearing perceived in brain.

Balance and Posture

• Three semicircular canals in each ear, at right angles, sensitive to movement in different planes.
• Filled with fluid; each has an ampulla containing sensory cells.
• Movement of head moves fluid, stimulating sensory hairs.
• Impulses sent to brain via auditory nerve to register movement.

Defects of the Ear

Acute Labyrinthitis

• Inflammation of middle ear and cochlea.
• May cause deafness.
• Treated with drugs or surgery if necessary.

Tinnitus

• Sensation of noises in the ear.
• Caused by wax accumulation or certain drugs (e.g., quinine).
• Treated by wax removal and stopping causative drugs.

Vertigo (Giddiness)

• Disorientation of body in space, sometimes caused by dilation of endolymph.
• Treated with appropriate drugs.

Deafness

• Inability to hear, varying from partial to complete.
• Causes include chronic cochlea infection, lack of sensory cells, excess wax, or ossicle fusion.

Otitis Media

• Inflammation of middle ear due to fluid build-up.
• Marked by swelling around Eustachian tube from infection or congestion.
• Creates vacuum in middle ear.
• Treated with antibiotics or surgery.

Practical Activities

• Investigate tactic response using choice chambers with fly maggots.
• Observe responses to stimuli such as chemical substances (chemotaxis).
• Place dried beef/fish on one side and rotting meat/fish on the other.
• Place ten maggots in center, cover chamber, and count maggots at each end after 10 minutes.
• Most maggots move to chamber with rotting meat.

Tropisms

• Soak and germinate maize or bean seeds until radicle and plumule appear (about 5 days for beans, 7 for maize).
• Use seedlings with straight radicles and plumules.

Geotropism

• Place seedlings horizontally on soil, vermiculite, sawdust, or sand.
• Observe and record results after three days.

Phototropism

• Place potted plant or young seedling near a window as the only light source or in a dark box.
• Observe after 3-5 days; shoots bend towards the light.

Etiolation

• Place young seedlings in a dark box, keep moist but not exposed to light.
• After two weeks, observe yellow, small leaves; long internodes; elongated, thin stems.
• Compare with seedlings grown in light (control) which are green with larger leaves, shorter internodes, and thicker stems.

Experiment to Determine Distance of the Blind Spot

• Work in pairs; one measures, the other observes.
• Mark a cross and a dot on white paper 6-9 cm apart.
• Hold paper 50 cm from face.
• Close left eye; fix right eye on cross; slowly move paper towards face.
• At 50 cm, both cross and dot are visible.
• As paper moves closer, dot disappears at a certain distance.
• Measure this distance; it is the blind spot distance.
• Dot disappears because light rays focus on blind spot, which lacks photoreceptors.

The Knee Jerk Experiment

• Work in pairs; one sits with one leg crossed over the other.
• The other taps the crossed knee just below the kneecap with palm edge or wooden ruler.
• Observe the knee jerk reflex.
• This is a spinal reflex.

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