CBSE Class 10 Biology Control And Coordination Set D

Question. Which of the following is a plant hormone ?
(a) Insulin
(b) Thyroxin
(c) Oestrogen
(d) Cytokinin
Answer : (d) Cytokinin.

Question. The gap between two neurons is called a :
(a) dendrite
(b) synapse
(c) axon
(d) impulse
Answer : (b) synapse.

Question. The brain is responsible for :
(a) thinking
(b) regulating the heart beat
(c) balancing the body
(d) all of the above
Answer : (d) all of the above.

Question. Which part of the brain maintains posture and equilibrium of the body ?
Answer : Cerebellum.

Question. How do we detect the smell of an agarbatti (incense stick) ?
Answer : When the agarbatti burns, it produces vapours having a characteristic pleasant smell. This smell is detected by the olfactory receptors present inside our nose. The action of smell of agarbatti on receptors sets off chemical reactions which generate electrical impulses. The sensory neurons carry these electrical impulses to the sensory area of forebrain (called cerebrum). This makes us detect the smell of burning agarbatti.

Question. What is the role of the brain in reflex action ?
Answer : Reflex action takes place in the spinal cord. Only the information that a reflex action has taken place goes on to reach the brain.

Question. What are plant hormones ?
Answer : The organic chemical substances produced in plants which control growth, development and responses in plants, are called plant hormones. The examples of plant hormones are : Auxins, Gibberellins, Cytokinins and Abscisic acid.

Question. How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light ?
Answer : See Sample Problem 3 on page 92 of this book.

Question. Give an example of a plant hormone that promotes growth.
Answer : Auxin.

Question. How do auxins promote the growth of a tendril around a support ?
Answer : See Sample Problem 2 on page 92 of this book.

Question. Design an experiment to demonstrate hydrotropism.
Answer : See page 87 of this book.

Short Questions Answer

Question. What is the difference between a reflex action and walking ?
Answer : Reflex action is a rapid, automatic response to a stimulus which is not under the voluntary control of the brain. It is a kind of involuntary action which involves only the spinal cord. A reflex action is not based on our thinking. On the other hand, walking is a voluntary action which we undertake knowingly. The action of walking involves thinking by the brain. The central nervous system (brain and spinal cord) takes part in the action of walking.

Question. Smita’s father has been advised by a doctor to reduce his sugar intake.
Name the disease he is suffering from and name the hormone whose deficiency is?
Identify the gland that secretes it and mention the function of this hormone.
Explain how the time and amount of secretion of this hormone is regulated in human system.
Answer. He is suffering from diabetes. Deficiency of insulin causes diabetes. Pancreas secretes insulin. Insulin helps in regulating blood sugar. When the sugar level in blood increases, it is detected by the a-cells of the pancreas which responds by producing more insulin. As the blood sugar level falls, insulin secretion is reduced.

Question. (a) How is brain protected from injury and shock?
(b) Name two main parts of hind brain and state the functions of each.
Answer : (a) Brain is covered by a three layered membrane called meninges. In between the layers of meninges and brain, cavity fluid named Cerebro Spinal Fluid (CSF) is filled. The hard skull covers the meninges. Thus Meninges, CSF and Skull protects our brain for a certain extent.
(b) Two main parts of hind-brain are — Medulla and Cerebellum. Their functions are:
Medulla : Involuntary actions such as blood pressure, salivation and vomiting. 
Cerebellum : It is responsible for precision of voluntary actions and maintaining the posture and balance of the body.

Question. State the functions of plant hormones. Name four different types of plant hormones.
Answer : Plant hormones help to coordinate growth, development and responses in environment.
Four different types of plant hormones are – Auxins, Gibberellins, Cytokinins, Ethylene and Abscisic acid.
• Auxins control the tropic (growth related) movements of the plants in response to light, gravity, touch etc by increasing the size of cells. Under the influence of auxins, the plant stem bends towards unidirectional light where as the roots bend away from it.
• Gibberellins stimulate stem elongation and leaf expansion. Its application causes stem elongation in small plants such as cabbage. Spraying gibberellins on sugarcane plant increases the stem size and hence the yield.
• Cytokinins are produced in regions of the plant body where rapid cell division occur, such as root tips, developing shoot buds, young fruits and seeds. Cytokinins promote growth by stimulating cell division. They also help in production of new leaves and chloroplasts in leaves.
• Ethylene causes ripening of the fruits.
• Abscisic acid inhibits (i.e., slows down) the growth in different parts of the plant body. It also inhibits germination of seeds. It increases the tolerance of plant to different kinds of stresses such as temperature changes. So, it is also called the stress hormone in plants. It also causes the drying and falling of older leaves, flowers and fruits.

Question. (a) Draw the structure of neuron and label cell body and axon.
(b) Name the part of neuron:
(i) where information is acquired
(ii) through which information travels as an electrical impulse.
Answer : (a) A nerve cell (Neuron)

Question. What happens at the synapse between two neurons ?
Answer : The synapse is a microscopic gap between any two adjacent neurons. Electrical impulses (or nerve impulses) carrying messages pass over the synapse when going from one neuron to another. Actually, synapse between two neurons acts as a one-way valve which allows electrical impulses to pass in one direction only. This happens as follows : When an electrical impulse coming from the receptor reaches the end of the axon of sensory neuron, then the electrical impulse releases tiny amount of a chemical substance (called neurotransmitter substance) into the synapse (or gap) between two adjacent neurons. This chemical substance crosses the synapse (or gap) and starts a similar electrical impulse in the dendrite of the next neuron. In this way, the electrical impulses (or messages) are passed from one neuron to the next across the synapse.

Question. Draw neat diagram of human brain and label on it the following parts :
(i) Midbrain (ii) Pituitary gland
Answer :

Question. How does the chemical coordination take place in animals ?
Answer : The chemical coordination in animals takes place through the action of chemicals called ‘hormones’. Hormones are the chemicals which are made inside the animal body in very small amounts by certain glands. Hormones are released directly into the blood. They are carried by the blood circulatory system to other parts of the body. Hormones travel all over the body but affect only particular organs at particular places. The organs which they affect are called target organs The hormones control and coordinate several functions of the animal body such as growth, development, metabolism, behaviour and secondary sexual characteristics, etc.

Question. Why is the use of iodised salt advisable ?
Answer : Iodine is necessary for the thyroid gland to make thyroxine hormone. Thyroxine hormone regulates the metabolism of carbohydrates, fats and proteins so as to produce the best balance for growth. If there is deficiency of iodine in our diet, the formation of thyroxine hormone will be reduced and lead to a disease called goitre. One of the symptoms of goitre disease is that the neck of the person appears to be swollen. Iodised salt contains appropriate amounts of iodine compounds. Iodised salt can provide all the iodine needed by the thyroid gland to make sufficient thyroxine hormone for the body. So, if we take iodised salt, there can be no deficiency of iodine (or thyroxine) in the body and hence goitre disease can be prevented.

Question. List in tabular form three differences between nervous system and Endocrine system.
Answer : 

Question. How does our body respond when adrenaline is secreted into the blood ?
Answer : The adrenaline hormone prepares our body to function at maximum efficiency during emergency situations like danger, anger, excitement, etc. This happens as follows : When we are faced with a dangerous situation (like being chased by a ferocious dog), then our nervous system stimulates the adrenal glands to secrete more adrenaline hormone into our blood. This adrenaline hormone increases our ‘heart beats’, ‘breathing rate’, ‘blood flow into muscles’ and causes liver ‘to put more stored glucose into our blood’. All these actions of adrenaline hormone produce a lot of energy in our body very, very quickly. And this energy helps us to cope up with emergency situations (like running away very fast from a ferocious dog or fighting an enemy, etc.).

Question. Why are some patients of diabetes treated by giving injections of insulin ?
Answer : Insulin is a hormone which is produced and secreted by a gland called pancreas. The function of insulin hormone is to lower the blood sugar level. Deficiency of insulin hormone in the body raises the blood sugar level causing a disease known as diabetes. The high sugar level in the blood of a diabetic person can lead to many harmful effects. Some persons having diabetes are treated by giving injections of insulin because addition of insulin hormone to blood lowers the blood sugar level.

Question. What is the function of receptors in our body ? Think of situations where receptors do not work properly. What problems are likely to arise ?
Answer : Receptors are the special cells present in our sense organs Receptors detect all the information from our environment and feed it to the nervous system. For example, gustatory receptors present in our tongue detect taste of our food whereas olfactory receptors present in our nose detect smell. Now, if the gustatory receptors of our tongue do not work properly, we will not be able to know the taste of different types of foods (whether sweet, salty, sour or bitter, etc.). And if the olfactory receptors present in our nose do not work properly, we will not be able to smell things (like the flavour of a food or the fragrance of a perfume).

Question. What is ‘hydrotropism’? Describe an experiment to demonstrate ‘hydrotropism’.
Answer. ‘Hydrotropism’ is the directional growth of a plant part in response to water. For example, roots show hydrotropism as they grow towards water in the soil and are positively hydrotropic.
An experiment to demonstrate hydrotropism is as follows:

• A porous pot filled with water is taken and inserted in a tub filled with dry sand.
• A freshly germinated pea seedling is sowed in the sand.
• As water is not available in sand, the root growing will bend towards the porous pot filled with water.
• A hydrotropic curvature of the root is observed as it grows towards water.
• This bending of root shows the movement in response towards water.

Question. Draw the structure of a neuron and explain its function.
Answer : A neuron consists of three parts : cell body, dendrites and axon (see Figure on next page). The cell body of a neuron is like a typical animal cell which contains cytoplasm and a nucleus. A number of long and thin fibres stretch out from the cell body of a neuron. They are called nerve fibres. The shorter fibres on the cell body of a neuron are called dendrites. The longest fibre on the cell body of a neuron is called axon. The axon has an insulating and protective sheath (or cover) of myelin around it. The function of neurons is to carry messages over long distances in the body of a person quickly. The messages which neurons transmit in nervous system are in the form of electrical impulses (also called nerve impulses). Actually, neurons make the whole nervous system work efficiently. For example, the sensory neurons transmit impulses from the sensory cells or receptors (present in sense organs) towards the central nervous system (spinal cord and brain). And the motor neurons transmit impulses from the central nervous system towards the muscle cells (or effectors) for taking appropriate action.

Question. How does phototropism occur in plants ?
Answer : The bending of a plant stem towards light is an example of phototropism. The plant stem responds to light and bends towards it due to the action of ‘auxin hormone’. This happens as follows : The auxin hormone is present in the tip of the stem of the growing plant. The auxin hormone prefers to stay in shade, away from light. So, when sunlight falls on the stem from one side, auxin hormone gets concentrated on the side of the stem which is away from light. Auxin promotes growth. So, due to more auxin, the shady side of stem (which is away from light) grows to be longer than the side of stem which is facing light, and makes the stem bend towards light. For example, in Figure given here, the left side A of the stem (which is away from light) has more auxin hormone and grows faster than right side B which is towards light. Since side A of stem becomes longer than side B, the stem bends towards right side (in the direction of light).

Question. Which signals will get disrupted in case of a spinal cord injury ?
Answer : Reflex actions and involuntary actions will get disrupted in case of a spinal cord injury.

56. List the components of reflex arc in correct sequence. State in brief the role of brain in reflex action.
Answer. The reflex arc pathway is shown in the flow chart as follows:
The Reflex arc does not involve brain. It minimises the overloading of brain.

Question. How does chemical coordination occur in plants ?
Answer : The plants do not have nervous system but they can still sense things. The plants can sense the presence of stimuli such as light, gravity, chemicals, water, and touch, etc., and respond to them by the action of hormones. Thus, the plants coordinate their behaviour against environmental changes by using organic chemicals called hormones. This is called ‘chemical coordination’. The hormones in plants coordinate their behaviour by affecting the growth of a part of the plant resulting in the movement of that plant part in response to a stimulus. For example, when sunlight falls on a shoot from one side, the auxin hormone causes the shady side of shoot to grow faster, making the shoot bend towards sunlight.

Question. What is the need for a system of control and coordination in an organism ?
Answer : An organism needs a system of control and coordination for its survival in this world. The system of control and coordination is necessary in plants as well as animals (including human beings). (i) The plants need carbon dioxide, water and sunlight. It is due to the presence of a hormonal system of control and coordination in plants that the stomata in their leaves open to allow in carbon dioxide gas, the roots bend towards water and the shoots grow towards sunlight. It is also due to control and coordination that tendrils in plants having weak stems make them climb on to neighbouring supports. (ii) In human beings, the system of control and coordination is needed for all our actions, thinking and behaviour. The human nervous system receives information from the surroundings, processes it and then responds accordingly. Our heart beats, breathing, reading, writing, cycling, dancing and various reflex actions are all because of the nervous system. The hormonal system (endocrine system) helps in controlling and coordinating activities like metabolism, development, reproduction and preparing our body to cope up with grave situations.

Question. How are involuntary actions and reflex actions different from each other ?
Answer : Involuntary actions are those which we cannot control even if we want to. There is no stimulus involved in the involuntary actions. They take place on their own. For example, our heart beats all the time without our thinking about it. So, the beating of heart is a purely involuntary action. Involuntary actions are regulated by the brain. The reflex actions are also a kind of involuntary actions but they take place in response to a stimulus. For example, the decrease in the size of the pupil of our eye on stepping out in bright light is a reflex action which takes place in response to a stimulus ‘light’. Reflex actions are usually regulated by the spinal cord.

Question. Compare and contrast nervous and hormonal mechanisms for control and coordination in animals.
Answer : Nervous system: Nervous system is defined as “ the system which receives stimulus, transmits it to the other parts of the body and shows corresponding effects”.
The nervous system regulates and integrates internal activities by coordinating the process of various parts of the body.
It produces sensations like vision, hearing, taste, smell and pain with the help of sense organs attached to them.
Nervous system helps to react to external environmental changes.
Hormonal system: Hormones pass through the target cell's plasma membrane and bind to a receptor protein in the cytoplasm.
Secondary messengers are produced as a result of a chain of events that this binding starts.
The cell's physiological state is changed by a sequence of molecular interactions brought on by these secondary messengers.
Signal transduction is another name for this procedure.

Question. What is the difference between the manner in which movement takes place in a sensitive plant and the movement in your legs ?
Answer : Movement in plants - Movement in sensitive plants is classified as tropic or nastic movement and is an instinctive response to external or internal stimuli.
It does not require any specific tissues for information transmission.
There are no specific proteins involved in the movement of the sensitive plant.
Movement in legs - Leg movement is controlled by the neurological system.
Leg mobility is aided by a sophisticated neural system network that includes the CNS and PNS.
Actin, myosin, and other proteins aid in the contraction and relaxation of leg muscles to facilitate leg movement.

Question. What is synapse? In a neuron cell how is an electrical impulse created and what is the role of synapse in this context?
Answer : Synapse is the junction between two adjustment neuron or nerve cells, i.e. between axon ending of one and the dendrite of the next. Transmission of Nerve Impulse. The information acquired at the end of the dendritic tip of a neuron sets off a chemical reaction which creates an electrical impulse. This impulse travels from the dendrite to the cyton along the axon to its end. At the end of the axon, the electrical impulse sets off the release of some chemicals, which cross the synapse and start a similar electrical impulse in a dendrite of the next neuron. In this way nerve impulses travel in the body. Synapse helps in transmitting impulses from one neuron to another.

Question. (a) Explain how auxins help in bending of plant stem towards light.
(b) State the objective of the experiment for which experimental set-up is shown in the given diagram.
Answer : (a) In plant shoots, the role of auxin is to cause a positive phototropism, i.e. to grow the plant towards the light. When light is incident on a plant from one direction, it causes the auxins to redistribute towards the shaded side of the plant. One function of auxin is to cause cell elongation.
The redistribution causes the cells on the shaded side to elongate more than those on the side with the light shining on them. This causes the shoot to bend towards the light.
(b) The objective of the experiment is to show phototropic movement of plant.

Question. Write one example each of the following tropic movements :
(i) Positive phototropism (ii) Negative phototropism
(iii) Positive geotropism (iv) Negative geotropism
(v) Hydrotropism (vi) Chemotropism
Answer : (i) Positive phototropism: shoots growing towards light.
(ii) Negative phototropism: roots growing away from light towards ground.
(iii) Positive geotropism: growth of roots towards earth due to the pull of the earth.
(iv) Negative geotropism: shoots growing away from the earth.
(v) Hydrotropism: roots growing towards the source of water.
(vi) Chemotropism: growth of pollen tubes towards the ovules.

Question. What causes a tendril to encircle or coil around the object in contact with it is? Explain the process involved.
Answer : When a tendril comes in contact with any support, the part of the tendril in contact with the object does not grow as rapidly as the part away of the tendril away from the object. This cause the tendril to circle around the object and thus, cling to it.

Question. Name any three endocrine glands in human body and briefly write the function of each of them.
Answer : Three endocrine glands with their function in human body are as follows:
• Thyroid gland : It secretes a hormone called thyroxine which regulates the metabolism of carbohydrates, fats and proteins in the body and so provide the best balance for nutrients and mental ability.
• Adernal gland : It secretes two hormones—adrenalin and corticoid hormones regulate blood pressure, heartbeat, breathing rate and carbohydrate metabolism.
• Pancreas: It secretes two hormones—insulin and glucagon. Insulin hormone lowers the blood glucose level. Glucagon hormone increases the blood glucose level.

Question. What are ‘hormones’? State one function of each of the following hormones:
(i) Thyroxine (ii) Insulin
Answer : Hormones are the chemical substances which coordinate and control the activities of living organisms and also their growth. The term hormone was introduced by Bayliss and Starling.
(i) Function of Thyroxine: This hormone regulates the metabolism of carbohydrates and fats.
(ii) Function of insulin: This hormone helps in regulating sugar level in the blood.

Question. (a) Explain any three directional movements in plants.
(b) How brain and spinal cord are protected in human ?
(c) Name the master gland present in the brain. 
Answer : (a) Stimuli is responsible for the movement of the plant parts towards or away from it. This movement is called as Tropic Movement.
Phototropism: movement of plant towards or away from the light. Geotropism: movement of plant parts towards the earth or away from it. Hydrotropism: movement of plant parts towards or away from any source of water.
(b) Both the brain and the spinal cord are protected by bone: the brain by the bones of the skull and the spinal cord is protected by a set of ring-shaped bones called vertebrae. They are both cushioned by layers of membranes called meninges as well as a special fluid called cerebrospinal fluid. This fluid helps to protect the nerve tissue to keep it healthy, and remove waste products.
(c) Pituitary gland present in the brain is known as the master gland.

Question. Which part of the brain controls involuntary actions? Write the function of any two regions of it.
Answer : Hind-brain controls the involuntary actions. Cerebellum controls the coordination of body movement and posture. Medulla oblongata regulates center for swallowing, coughing, sneezing and vomiting.

Question. What is chemotropism? Give one example. Name any two plant hormones and mention
their functions.
Answer : Chemotropism is the movement of a part of the plant in response to a chemical stimulus. It can be positive chemotropism or negative chemotropism. Example: The growth of pollen tube towards a chemical which is produced by an ovule during the process of fertilisation in a flower. 
Two plant hormones with their functions are as follows:
Auxins promote cell elongation, root formation, cell division, respiration and other physiological processes like protein synthesis, etc.
Gibberellins stimulate stem elongation, seed germination and flowering.

Question. What is a reflex action? Describe the steps involved in a reflex action.
Answer : Reflex action: Jt is defined as an unconscious, automatic and involuntary response of effectors,
i.e. muscles and glands, to a stimulus, which is monitored through the spinal cord.
Mechanism of reflex action : It involves the following steps:
• Receptor organ like skin perceives the stimulus and activates a sensory nerve impulse.
• Sensory organ carries message in the form of sensory impulse to the spinal cord.
• The spinal cord acts as modulator : The neurons of spinal cord transmit the sensory nerve impulse to motor neuron.
• Motor never conducts these impulses to the effectors like leg muscles which responds by pulling back the organ away from the harmful stimulus.

Question. Define ‘hormones’. Name the hormone secreted by thyroid. Write its function. Why is the use of iodised salt advised to us?
Answer : Hormones are the chemical substances which coordinate and control the activities of living organisms and also their growth. The hormone secreted by thyroid is thyroxine. Its function is to regulate the metabolism of carbohydrates, fats and proteins in the body so as to provide the best balance for growth. The use of iodised salt is advised to us because iodine is necessary for the thyroid gland to produce thyroxine hormone. Thyroxine regulates carbohydrate, protein and fat metabolism which is required for growth. Deficiency of iodine in the body causes disorder like swollen neck and disease called goitre

Question. A young boy met an accident while riding a bike. Petrolling police found the boy and brought him to a hospital for treatment. After the accident, the boy lost his memory completely. Left side of his body also suffered from paralysis.
(a) What values were shown by police persons?
(b) Which injured body part can cause paralysis?
(c) What precaution should one take while riding a two-wheeler?
Answer : (a) The values showed by the police persons were truthful to duty and humanity.
(b) Injury to brain or spinal cord can cause paralysis.
(c) Riders must wear helmets while riding a two-wheeler. One must drive in a controlled speed to avoid accident.

Question. State the functions of any three of the structural and functional unit of nervous system.
Answer : The structural and functional unit of nervous system, i.e. neuron with their functions are as
• Cell body: Stimulus received from dendrite is changed into impulse in the cyton.
• Dendrites: They receive sensation or stimulus, which may be physical or chemical.
• Axon: It conducts impulse away from the cell body.

Question. What is the function of receptors in our body? Think of situation where receptors do not work properly. What problems are likely to arise?
Answer : Receptors are present in our all parts of the body for example in skin, eye, nose tongue etc. They detect the signals and then send them to brain in the form of electrical signals. If these receptors are damaged then it they will not detect the input which leads to the harm for our body in dangerous situation.

Question. (a) Name the hormone which is released into the blood when its sugar level rises. Explain the need of Chemical communication in multicellular organisms the organ which produces this hormone and its effect on blood sugar level. Also mention the digestive enzymes secreted by this organ with one function of each.
(b) Explain the need of Chemical communication in multicellular organisms.
Answer : (a) Glucose is needed by cells for respiration. It is important that the concentration of glucose in the blood is maintained at a constant level. Insulin is a hormone produced by the a-cells that regulates glucose levels in the blood.
In order for multicellular organisms to function properly, their cells must communicate. For instance, your muscles must contract when your brain sends a message to contract. 
Pancreas produces insulin and p-cells which increase glucose in blood. It also – produces digestive enzyme (pancreatic amylase).
(b) Cell-to-cell signaling is a critical component of coordinating cellular activities. Through this communication, messages are carried from signaling cells to receiving cells, also known as target cells. This signaling occurs with proteins and other types of signaling molecules. Other things which happens in our body due to cell communication are – growth and development, cellular reproduction, tissue repair, sensing pain, etc.

Question. What is meant by reflex-action ? With the help of a labelled diagram trace the sequence of events which occur when we touch a hot object.
Answer : Stimulus: In the example above, the stimulus is the contact with the hot pot. This contact causes a nerve impulse that will travel to the spinal cord via the sensory neurons.
Sensory neurons: These neurons carry the nerve impulse to the spinal cord. Similar to the interneuron and motor neuron, sensory neurons receive incoming impulses at the dendrites. The impulses move away from the cell body along the axon to the synaptic terminal where the impulse is sent to the next interneuron with the help of a neurotransmitter (acetylcholine).
Interneurons: The interneuron is also known as relay neuron. These neurons are fully contained in the central nervous system. The interneuron serves as the connection between the sensory neurons and the motor neurons. Synapse: The synapse is a tiny space between two neurons. When an impulse gets to the end of one neuron and has to be sent down to the next neuron, the synapse acts as a bridge. The signal arrives at the end of one neuron (close to the synapse) as an electrical signal, crosses the synapse as a chemical signal (with the help of a neurotransmitter known as acetylcholine released by the synaptic vesicles at the synaptic terminal) and continues as an electrical signal in the next neuron.
Motor neurons: These neurons send nerve impulses away from the central nervous system to effector organs or muscle fiber in our example above. This causes the muscle fiber to contract, resulting in you snatching your hand away from the hot pot.

Response: To respond to the stimulus of the reflex arc, the muscle needs to contract to pull the hand quickly away from the hot pot. For this to happen, the impulse travels to the synaptic terminal of the motor neuron. Synaptic vesicles at the synaptic . terminal will then release acetylcholine which will cross the synapse and bind to the receptors on the muscle fibers to trigger the muscle contraction known as the ‘response’.

Question. State the structural and the functional unit of the nervous system. Draw its neat labelled diagram and write two functions.
Answer : The structural and functional unit of the nervous system is the neuron.

The functions of the neuron are:
(a) Passing of information takes place-
(i) By electric impulse (inside the neuron) and
(ii) In the form of chemicals (At synapse)
(b) Reflex action- spontaneous, involuntary and automatic response to a stimulus to protect us from harmful situations. For example, on touching a hot object unknowingly we instantly withdraw our hand.

Question. What is a reflex arc? Draw a neat labelled diagram of the components in a reflex arc. Why do impulses flow only in one direction in a reflex arc?
Answer : Reflex arc is the pathway taken by the nerve impulses and responses in a reflex action,
i.e. from the receptor organs like skin to the spinal cord and from the spinal cord to the effector organs like muscles.

Impulses flow only in one direction in a reflex arc, because each synapse in the reflex arc allows impulses to cross it in a single direction.

Question. Suggest six reflex actions of the body. Explain how the reflex arc is the same in all of them.
Answer : Six reflex actions of the body are:
(i) When we see a speeding car moving towards us, we move aside.
(ii) We withdraw our hands on being pricked by a pin.
(iii) We withdraw our hands on touching very hot substance.
(iv) We close our eyes on seeing direct sun or extremely bright source of light.
(v) We close our eyes on hearing a loud noise.
(vi) We shiver on feeling cold.
Reflex arc in all the above cases is same because in all the cases, the stimulus is received by sense organs. Then this information is carried to spinal cord through sensory nerves. Thus, information from spinal cord is sent to the effectors such as muscles via motor neurons.

Question. (a)

(i) Name the parts labelled A and B in the neuron drawn above.
(ii) Which part acquires the information in the neuron?
(iii) Through which part does the information travel?
(iv) In what form does this information travel?
(v) Where is the impulse converted into a chemical signal for onward transmission?
(b) Name the hormone secreted by thyroid. What is its function? Why is the use of iodised salt advisable.
Answer : (a) (i) A – Dendrite, B – Axon
(ii) The information in the neuron is acquired at the end of the dendrite tip.
(iii) The information travels from the dendrite to the cell body and then along the axon to its end.
(iv) The information travels in the form of an impulse.
(v) The impulse is converted into a chemical signal at the end of the axon.
(b) Thyroxine hormone is secreted by thyroid.
Function of Thyroxine hormone
It regulates carbohydrate, protein and fat metabolism in the body so as to provide the best balance for growth. The use of iodised salt is advisable because iodine is essential for the synthesis of thyroxin. In case of iodine deficiency in our diet, there is a possibility one can suffer from goitre, which is a disease of swollen neck.: