2.77 – Understand that organisms are able to respond to changes in their environment.
– Organisms are sensitive and are able to respond to changes (stimuli) in their
environment.
2.78 – Understand that homeostasis is the maintenance of a constant internal
environment and that body water content and body temperature are both examples of
homeostasis.
– Homeostasis: The maintenance of a constant internal environment.
• Regulation of body water content and body temperature are examples of
homeostasis.
2.79 – Understand that a coordinated response requires a stimulus, a receptor and an
effector.
– A coordinated response requires a stimulus, a receptor and an effector.
– Stimulus: A change in an organism’s surrounding environment.
– Receptor: An organ that detects the stimulus.
– Effector: An organ that carries out the response to the stimulus.
STIMULUS → RECEPTOR → COORDINATION → EFFECTOR → RESPONSE
FLOWERING PLANTS (2.80 – 2.82)
2.80 – Understand that plants respond to stimuli.
– Plants respond to stimuli.
• Plant stimuli are called tropisms.
• Major plant tropisms include geotropism (gravity), phototropism (light) and
hydrotropism (water).
• Tropisms are classified into positive (plants growing towards stimulus) and
negative (plants growing away from stimulus).
2.81 – Describe the geotropic responses of roots and stems.
– Geotropism: The response of plants to gravity.
• Roots carry out positive geotropism (growing downwards with gravity).
• Shoots carry out negative geotropism (growing upwards against gravity).
2.82 – Describe positive phototropism of stems.
– Phototropism: The response of plants to light.
– Auxins (a hormone) elongate plant cells (i.e. causes plant growth).
• In the shoots, the concentration of auxins promotes the growth of the plant.
• In the roots, a high concentration of auxins inhibit growth, and a low concentration of
auxins promotes growth.
Sunlight breaks down auxins.
HUMANS (2.83 – 2.90)
2.83 – Describe how responses can be controlled by nervous or by hormonal
communication and understand the differences between the two systems.
– Responses can be controlled by the nervous or endocrine (hormonal) system
Nervous Communication Hormonal Communication
Fast Slow
Relies on an electrical impulse to deliver response Relies on hormones (chemical messengers) to
deliver response
Travel along neurones Travels in blood
Response is short-lived Response is long lasting
Targets individual cells (localised) Targets all around body (widespread)
Nervous system Endocrine system
2.84 – Understand that the central nervous system consists of the brain and spinal cord
and is linked to sense organs by nerves.
– The central nervous system (CNS) consists of the brain and spinal cord.
– The central nervous system is linked to sense organs by nerves.
2.85 – Understand that stimulation of receptors in the sense organs sends electrical
impulses along neurones into and out of the central nervous system, resulting in rapid
responses.
– The stimulation of receptors in the sense organs sends electrical impulses along
neurones into and out of the central nervous system, resulting in rapid responses.
Nervous Communication Hormonal Communication
Fast Slow
Relies on an electrical impulse to deliver response Relies on hormones (chemical messengers) to
deliver response
Travel along neurones Travels in blood
Response is short-lived Response is long lasting
Targets individual cells (localised) Targets all around body (widespread)
Nervous system Endocrine system
Part Function
Cell Body (Soma)
To receive synaptic information and transfer it to other cells via the axon. Also
completes a variety of biochemical processes to keep the neurone from
functioning properly.
Nucleus To control the neurone and what it does. Contains the genetic information of the
cell.
Axon Generates nerve impulses and transmit them away from the cell body towards
other cells.
Myelin Sheath A fatty substance that insulates the neurone and prevent ‘short circuits’ with other
neurones.
Dendrons /
Dendrites
Tree-like extensions at the cell body that increases the surface area of the cell
body. Covered in synapses and receive information from other neurones.
Axon Terminal
(Motor End Plates)
End of an axon of a neurone. The electrical impulses reach the axon terminal
and the terminal buds and stimulates the vesicle sacs that release
neurotransmitters into the synapse which takes the impulse to the next neurone.
Synapse A gap that provides a connection between neurones that allow electrical
impulses to travel between them.
2.86 – Describe the structure and functioning of a simple reflex arc illustrated by the
withdrawal of a finger from a hot object.
– Receptor detects the hot object.
– An electrical impulse is generated in the sensory (afferent) neurone.
– The impulse travel across synapses to the central nervous system.
– The relay neurones in the spinal cord transfer the impulse from sensory neurones to
motor (efferent) neurones.
– The motor neurones travel to the effector muscle, resulting in a rapid contraction.
2.87 – Describe the structure and function of the eye as a receptor.
– The eye is a receptor of light.
– The eye has receptor cells in its retina, which converts the light into electrical
impulses that are sent to the brain.
Part Description / Function
Sclera White part of eye.
Cornea A transparent layer in front of the sclera which lets light into the eye.
Iris A coloured ring of tissue behind the cornea.
Pupil A hole in the middle of the iris that lets light through.
Choroid
A dark layer underneath the sclera. It is dark because it contains many blood
vessels and pigment cells.
The pigments stop light from being reflected around the inside of the eye, similar to a
camera. Cameras are painted matt black on the inside so that stray light does not
bounce around and fog the image.
Retina Light-sensitive layer where light energy is converted into electrical energy of nerve
impulses. Contains cells called rods and cones.
Rods Cells that work well in dim light, but are unable to distinguish between colours.
Cones Cells that work well in bright light. There are 3 types of cones which responds to the
different wavelengths / colours of light (red, green, blue).
Circular
Muscles Located in the iris; forms a ring shape.
Radial Muscles Located in the iris; lie like the spokes of a wheel.
Blind Spot The place where an image cannot be formed as there are no rods or cones in this
area. This is where the optic nerve leaves the eye.
2.88 – Understand the function of the eye in focusing near and distant objects, and
in responding to changes in light intensity.
– Accommodation: The focusing of near and distant objects of the eye.
Object Suspensory
Ligaments Ciliary Muscles Shape of Lens Amount of
Refraction
Near Slacken Taut Rounded Lots
Distant Taut Slacken Flat Small
– Pupillary reflex: The changes in the size of the pupil by muscles in the iris to receive
different amounts of light.
Light Intensity Iris Pupil Circular
Muscles
Radial
Muscles Size of Pupil
Low Open Dilate Relax Contract Large
High Close Constrict Contract Relax Small
2.89 – Describe the role of the skin in temperature regulation, with reference to
sweating, vasoconstriction and vasodilation.
– Changes in body temperature are detected by the hypothalamus in the brain.
– Vasoconstriction: The narrowing of blood vessels. Blood is diverted away from the
surface of the skin to conserve heat.
• Occurs when body temperature decreases.
• Hair erector muscles contract to make hairs stand up, trapping a layer of insulated
air.
– Vasodilation: The widening of blood vessels. Blood is taken close to the surface of
the skin so it is radiated out.
• Occurs when body temperature increases.
• Hair erector muscles relax to make hairs lie flat and because no layer of air is
trapped, heat can radiate out.
– When body temperature is too high, glands under the skin secrete sweat, increasing
heat loss by evaporation.
2.90 – Understand the sources, roles and effects of the following hormones: ADH,
adrenaline, insulin, testosterone, progesterone and oestrogen.
Gland Hormone Function
Pituitary
Gland ADH Affects the permeability of the collecting duct to control water levels.
Adrenal
Gland Adrenaline Fight or flight’ response; diverts blood from skin into muscles and increases
heartbeat.
Pancreas Insulin Converts glucose into glycogen.
Ovaries Oestrogen
Progesterone
Makes ovum; secondary sexual characteristics of females.
Regulates menstrual cycle.
Testis Testosterone Makes sperm; secondary sexual characteristics of males