NCERT Class X Science Class: Chapter –7. Control and Coordination – Part-10

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Question 49:

Name various plant hormones. Also give their physiological effects on plant growth and development.


1] Auxin - it promotes plant growth and fruit growth

2] gibberellins- it helps in fruit growth

3] Cytokinin -its function is same as gibberellins

4] Abscisic acid- it inhibits plant growth and is also responsible for the falling of leaves fruits and flowers

Question 50:

What are reflex actions? Give two examples. Explain a reflex arc.


A reflex action is defined as a spontaneous, automatic and mechanical response to a stimuli without the will of an individual. In such actions there is no involvement of the brain. All reflex actions are conveyed through the spinal cord by a path called reflex arc.

The reflex action travels in the following sequence:

Image Figure shown the reflex actions

Image Figure Shown the Reflex Actions

Image Figure shown the reflex actions

The reflex arc constitutes the following components:

  • A receptor to perceive the stimulus.

  • A sensory or afferent nerve which carries the message from the receptor to the spinal cord.

  • The neurons of spinal cord transmit the impulse from afferent neurons to efferent neurons.

  • The motor or efferent nerve carries messages from spinal cord to the muscles (effectors) that show the response.

Some examples of reflex actions are:

  • Blinking of eyes when a foreign particle gets in them

  • Sneezing if an unwanted particle enters the nose

  • Watering of mouth at the sight or smell of good food

  • Withdrawal of foot while walking if a nail comes in the way and pricks the foot

  • Immediate withdrawal of hand of a person if some hot thing touches it

Question 51:

“Nervous and hormonal systems together perform the function of control and coordination in human beings.” Justify the statement.


Nervous system serves to pass the sensory information from receptor to effector organs and thereby generate response. The afferent/sensory neurons transmit the sensory information from sensory receptors to spinal cord or brain. The interneurons integrate the incoming information and signal is relayed to motor/efferent neurons. The efferent/motor pathway transmits the signal from motor neurons to the effector organs (muscles and glands etc.)

Image control and coordination in human beings

Image Control and Coordination in Human Beings

Image control and coordination in human beings

The effector organs generate response in terms of secretion of either any activity or secretion of hormones. It can be said that secretion of hormones is under regulation of nervous system. Hormonal system can regulate wise range of target tissues as compared to nervous system. Hence, together, nervous system and hormonal system serve the function of control and coordination.

Question 52:

How does chemical coordination take place in animals?


The endocrine system is involved in the chemical coordination. Here stimulated cells release a chemical compound (called hormones), which diffuse all around the original cell. The hormones are released into the blood which carry them to different parts of the body. When this hormone reaches specific target cells, which have special signals to detect this compound, these target cells would be able to recognize the information and even transmit it. The hormones used by multicellular organisms for control and coordination show a great deal of diversity in order to enable them to carry out various functions. The advantage of chemical signals is that it would reach all cells of the body and does not rely on specialized structures like nervous connections.

Question 53:

Why is the flow of signals in a synapse from axonal end of one neuron to dendritic end of another neuron but not the reverse?


When an electrical signal reaches the axonal end of a neuron, it releases a chemical substance. This chemical diffuses towards the dendrite end of next neuron where it generates an electrical impulse or signal. Hence, the electrical signal is converted into a chemical signal at the axonal end. Since these chemicals are absent at the dendrite end of the neuron the electrical signal, cannot be converted into chemical signal.

Image of one neuron to dendritic end of another neuron

Image of One Neuron to Dendritic End of Another Neuron

Image of one neuron to dendritic end of another neuron

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