Control and coordination / The Regulators of Life – Summary
• Control is systematic regulation of various activities. Coordination is the orderly execution of the processes.
• Homeostasis, in biological terms means that the body is maintaining internal equilibrium to adjust
itself internally and physiologically, in response to the external environmental changes.
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• A system of control and coordination is essential in living organisms so that the different body parts can function as a single unit to maintain homeostasis as well as respond to various stimuli.
• In animals, the nervous system and hormonal system are responsible for control and coordination.
The system that controls and coordinates all the activities of various body parts in response to external and internal stimuli by the conduction of nerve impulses is called nervous system. The nervous system is made up of highly specialized tissue called nervous tissue. It consists of –
(1) Neurons or Nerve cells (2) Nerve fibres (3) Neuroglia
• Neurons are specialized cells of the nervous system. They use electrical and chemical signals for transferring information.
• Receptors are specialized tips of the nerve fibres that collect the information to be conducted by the nerves.
• Nerve impulses travel in the following manner from one neuron to the next :
Dendrites → Cyton → Axon → Terminal arborization → Synapse → Dendrite of next neuron
• Chemicals released from axon tip of one neuron, cross the synapse or neuromuscular junction to reach the next cell (neuron or muscle fibre).
• Nerve impulses from many neurons interact to carry out the complex process of thinking.
• The nervous system has three parts:
(a) Central nervous system (CNS)
(b) Peripheral nervous system (PNS)
(c) Autonomic nervous system (ANS)
• Reflex actions, voluntary actions and involuntary actions are the various types of responses shown by the nervous system.
• Central nervous system comprises of the brain and spinal cord. Spinal cord controls the reflex actions and conducts messages between different parts of the body and brain.
• Reflex action is an automatic, rapid and immediate reaction to a stimulus and is below the level of consciousness. No thinking is involved in reflex action.
• Reflex arc is the neural pathway that mediates a reflex action.
• Pathway of reflex arc :
Receptor → Sensory neuron → Centre (CNS) → Motor neuron → Effector
• The sensory neurons of reflex arcs synapse in the CNS i.e. spinal cord which then activates the spinal motor neurons without delay to execute a quick action, especially in case of emergencies. The brain also receives the information while the reflex action occurs.
• The three main parts of the brain are forebrain, midbrain and hindbrain.
• The largest part of the brain is the forebrain. It is the main thinking region. It is made up of cerebrum, hypothalamus and thalamus. Cerebellum, pons and medulla constitute the hindbrain.
• Cerebrum is the largest part of the brain whereas the cerebellum is the second largest part.
• Brain is protected by a bony box called cranium, within which are present three layers of fluid-filled
membranes called meninges for absorbing shock.
• Peripheral nervous system consists of cranial nerves arising from the brain and spinal nerves arising from the spinal cord. It assists in transmitting information between central nervous system and rest of the body.
• The sense organs detect changes in surroundings and pass this information to the central nervous
system, which after processing the information, acts through the muscles.
• The movements of muscle tissues are brought about by the contraction and relaxation of the contractile proteins in response to nerve impulses.
• Plants lack nervous and muscular system.
• Plants respond to stimuli by showing two types of movements – growth independent movement and growth dependent movement.
• Growth independent movements are usually quicker than growth dependent ones, and involve the use of electrochemical signals by the plant. To achieve this movement, the plant cells change shape by
altering their water content.
• Growth dependent movements or tropic movements are slow, occurring either towards or away
from the stimulus. It is brought by the protoplasm of the cell.
• Tropic movements are shown in response to environmental factors such as light, gravity, water and chemicals. The common tropisms for a plant are –
(a) Phototropism – movement due to light stimulus. e.g. stems show positive phototropism and roots show negative phototropism.
(b) Hydrotropism – movement due to water stimulus. e.g.- roots grow towards water.
(c) Chemotropism – movement due to chemical stimulus. e.g.- growth of pollen tubes towards ovules.
(d) Geotropism – movement due to gravity stimulus.
(e) Thigmotropism – growth response of a plant to physical contact (touch) e.g. tendrils of pea plant.
Plant roots are positively geotropic and negatively phototropic whereas plant shoots are usually
negatively geotropic and positively phototropic.
• In addition to electrochemical signals, plants and animals use hormones for control and coordination.
• Important plant hormones are auxin, gibberellin, cytokinin, abscisic acid and ethylene.
Plant hormone Function
Auxin Cell elongation
Cytokinin Cell division
Gibberellin Growth of stem
Abscisic acid Inhibits growth
Ethylene Ripening of fruits
• Auxin causes the bending of plant stem towards light as well as the curling of plant tendrils around a support.
• Animal hormones do not bring about directional growth depending on environmental cues, but promote controlled growth in various areas to maintain the body design.
• In case of flight or fight reaction to an emergency situation, adrenal glands release adrenaline into blood which acts on heart and other tissues. It causes faster heart beat to supply more oxygen to muscles and speeds up the conversion of glycogen into glucose. It reduces blood supply to digestive system and skin and diverts blood to skeletal muscles. This increases breathing rate also.
• Deficiency of iodine causes goiter whereas deficiency of growth hormone and insulin causes dwarfism and diabetes respectively.