CIE Categories Archives: 14. Coordination and Response

14.5) Tropic responses

14.5) Tropic responses


Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water.


Gravi(geo)tropism: is a response in which plant grows towards or away from gravity.

Phototropism: is a response in which a plant grows towards or away from the direction from which light is coming.


There are two main types of tropisms:

  • positive tropisms – the plant grows towards the stimulus
  • negative tropisms – the plant grows away from the stimulus


Seedlings are good material for experiments on sensitivity because their growing roots (radicals) and shoots respond readily to the stimuli of light and gravity.


Advantages of positive phototropism:

  • Leaves exposed to more sunlight and are able to do more photosynthesis,
  • Flowers can be seen by insects for pollination.
  • The plant gets higher for better seed dispersal.


Advantages of positive geotropism:

  • By growing deeply into the soil, the root fixes the plant into the ground firmly,
  • Roots are able to reach more water,
  • Roots have a larger surface area for more diffusion and osmosis.



  • Auxins are a family of plant hormones.
  • They are mostly made in the tips of the growing stems and roots.
  • Diffuse to other parts of the stems or roots.
  • Is unequally distributed in response to light and gravity.

Auxins change the rate of elongation in plant cells, controlling how long they become.


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14.4) Homeostasis

14.4) Homeostasis

Homeostasis: is the maintenance of a constant internal environment.


Homeostasis is the control of internal conditions within set limits:

  • Cells: change composition of blood as they remove nutrients and O2 and add wastes and CO2.
  • Heart: keeps blood pressure constant to deliver oxygen and nutrients around body.
  • Skin: to maintain heat exchange with external environment.
  • Kidneys: regulate water and salt levels (osmoregulation) and the removal of wastes like urea (excretion).
  • Lungs: regulate gas exchange Intestines: supply soluble nutrients and water to blood.
  • Liver: regulates blood solutes and removes toxins.


Negative feedback:

Homeostatic control is achieved using negative feedback mechanisms:

  • if the level of something rises, control systems reduce it again
  • if the level of something falls, control systems raise it again


Regulation of blood sugar:

  • Blood glucose levels are monitored and controlled by the pancreas
  • The pancreas produces and releases different hormones depending on the blood glucose level
  • Insulin is released when blood glucose levels are high – the liver stores excess glucose as glycogen
  • Glucagon is released when blood glucose levels are low – the liver converts stored glycogen into glucose and releases it into the blood



  • Diabetes is a condition in which the blood glucose levels remain too high.
  • It can be treated by injecting insulin.
  • The extra insulin causes the liver to convert glucose into glycogen, which reduces the blood glucose level. There are two types of diabetes – Type 1 and Type 2.


Type 1 diabetes is caused by the lack of insulin:

  • Symptoms: feeling tired, thirsty, frequent urination and weight loss.
  • Treatment: regular exercise, injecting insulin, and monitoring the diet.

Skin structure:

  • The basal layer and the cells above it constitute the epidermis.
  • There a specialised pigment cells in the basal layer and epidermis. These produce a black pigment, melanin, which gives the skin its colour.The more melanin, the darker the skin.
  • The dermis contains connective tissue with hair follicles, sebaceous glands, sweat glands, blood vessels and nerve endings.
  • There is a layer of adipose tissue (fat deposit) beneath the dermis.


Temperature regulation:

The human body is designed to function most efficiently at 37ºC. If you become too hot or too cold, there are ways in which your body temperature can be controlled.

  • Insulation: provided by fatty tissue retains heat. Hairs become erect to trap warm air by contracting erector muscles and vice versa.
  • Vasodilatation: when it is hot, arterioles, which supply blood to the skin surface capillaries, dilate (become wider) to allow more blood near to skin surface to increase heat loss (face redder)
  • Vasoconstriction: when it is cold, arterioles, which supply blood to the skin-surface capillaries, constrict (become smaller) to allow less blood near to skin surface to decrease heat loss
  • Sweating: the water evaporates giving a cooling effect
  • Skin receptors: sense heat and sensory neurons send impulses to the hypothalamus
  • Shivering: muscular activity generates heat
  • Thermoregulatory center: in the hypothalamus, it controls the use of corrective mechanisms (e.g. sweating and shivering).
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14.3) Hormones in humans

14.3) Hormones in humans


Hormone: is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target


The role of adrenaline:

  • As adrenaline circulates around the body it affects a number of organs.
  • Examples of situations in which adrenaline secretion increase are stressful and dangerous situations.



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14.2) Sense organs

14.2) Sense organs

Sense organs: are groups of sensory cells responding to specific stimuli, such as light, sound, touch, temperature and chemicals.

Pupil reflex:

  • The pupil of the eye is the dark round area in the centre of it. It is surrounded by a coloured ring structure called the iris.
  • The pupil and ciliary muscle together play a big role in protecting the eye from damage by limiting the amount of light entering the eye. If too much light fall on the retina, the rods and cones get damaged. The iris and pupil change their size to smiddle that happening.
  • The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside.
  • In bright light circular muscles contract to make the pupil smaller.
  • In dim light radial muscles contract to stretch the pupil outwards making it wider.
  • The circular and radial muscle act antagonistically, when one contracts the other relaxes.


    • The millions of light-sensitive cells in the retina are of two kinds, the rods and the cones.
    • Rods play an important part in night vision.
    • Thought to be three types of cone cells, each responds best to red, green, or blue.
    • If all three types are equally stimulated we get the sensation of white.

  • The central part of the retina is called the fovea.
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14.1) Nervous control in humans

14.1) Nervous control in humans


Co-ordination is the way all the organs and systems of the body are made to work efficiently together.


A nerve impulse is an electrical signal that passes along nerve cells called neurons.


The human nervous system consists of:

  • the central nervous system (CNS) – the brain and spinal cord
  • the peripheral nervous system – nerve cells that carry information to or from the CNS


Nerves cells:

Nerve cells are also called neurons. They are adapted to carry electrical impulses from one place to another:

  • The axon is an extended cymiddlelasm thread along which electrical impulses travel.
  • Axons are coated by a layer of myelin called myelin sheath, this is an electrically insulating layer which is essential for the proper functioning of the nervous system.
  • Dendrite’s function is to pick up electrical impulses from other cells.
  • Motor end plate passes the electrical impulses from the neurone to the muscle fibres.


Sensory Neurones: carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.

Motor Neurone: Transmits electrical impulses from the Central nervous system to the effectors.

Relay Neurone: Relay neurones are located in the CNS. Their job is to pass electrical impulses from the sensory neurone onto the motor neurone, so it acts like a diversion.


The reflex arc:

A reflex action is the means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors. (muscles and glands)

A well-known reflex is the knee-jerk reflex.

  1. Receptor in the skin detects a stimulus (the change in temperature).
  2. Sensory neurone sends impulses to relay neurone.
  3. Motor neurone sends impulses to effector.
  4. Effector produces a response (muscle contracts to move hand away).



Voluntary and involuntary actions:

The reflex arc is a reflex action. Reflex means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some impulses are carried by other neurons to the brain, and some impulses are passed onto the motor neurone to the effector muscle and the response takes place. The electrical impulses going to your brain are much slower that the ones going to the effector muscle directly. This is why the reflex action takes place before you realise it, it is uncontrollable.

Reflex actions are said to be involuntary actions. Involuntary actions start at the sense organ heading to the effector. They are extremely quick.

Voluntary actions are the ones that you make the choice to do. Like picking up a bag from the floor for example. Your brain sends electrical impulses to the effector muscles ordering them to contract so you could pick the bag up. Voluntary actions are slower than involuntary actions and they start at the brain.



Synapse: is a junction between two neurones.

  • When an impulse arrives at the synapse, vesicles in the cytoplasm release a tiny amount of the neurotransmitter
  • It rapidly diffuses across the gap (aka synaptic cleft) and binds with neurotransmitter receptor molecules in the membrane of the neuron on the other side of the synapse.
  • This then sets off an impulse in the neurone.
  • Sometimes several impulses have to arrive at the synapse before enough transmitter substance is released to cause an impulse to be fired off in the next eurone.
  • Synapses control the direction of impulses because neurotransmitter substances are only synthesised on one side of the synapse, while receptor molecules are only present on the other side.
  • They slow down the speed of nerve impulses slightly because of the time taken for the chemical to diffuse across the synaptic gap.
  • Many drugs produce their effects by interacting with receptor molecules at synapses.

Heroin, stimulates receptor molecules in synapses in the brain, triggering the release of dopamine (a neurotransmitter), which gives a short-lived ‘high’.

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