CBSE Class 10 Science Control and coordination Notes

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Control and coordination is the process by which the parts of living organisms work together as a single unit. This Class 10 Science notes page covers the nervous system of humans and animals, nerve cells and nerve impulses, the types of neurons, the reflex action and reflex arc, the human brain and its parts, the tropic and nastic movements and plant hormones in plants, and the endocrine glands and hormones in humans.

Topics Covered

1. Introduction and homeostasis
2. Nervous system and nerve cells
3. Types of nerve cells and nerve impulse
4. Pathways of nerves
5. Nervous system in humans and reflex action
6. Human brain
7. Coordination in plants (tropic and nastic movements, plant hormones)
8. Hormones in animals and the endocrine system

INTRODUCTION

As the complexity of individuals, plants or animals increases, the different cells and organs become separated from each other by greater distances. Thus it becomes necessary to have a system by which the different parts of the organism can function as a single unit. This is possible only if the different parts can coordinate with each other and carry out a particular function.

To carry out a simple function, such as picking up an object from the ground, there has to be coordination of the eyes, hands, legs and the vertebral column. The eyes focus on the object, the hands pick it up and grasp it, the legs bend and so does the backbone (vertebral column). All these actions have to be coordinated in such a manner that they follow a particular sequence and the action is completed. A similar mechanism is also needed for the internal functions of the body.

Individuals also have to adjust to the changing conditions around them and vary their responses. At the same time, the internal conditions of the body should be maintained constant. This is called homeostasis. Homeostasis is derived from 'homeo' meaning same and 'stasis' meaning standing still. The internal conditions of the body are maintained at a constant by controlling the physiology of the organism. Just as in animals, plants also have to control and coordinate their various functions.

NERVOUS SYSTEM

The nervous system of an animal is composed of:
i) Specialised cells called neurons or nerve cells, which can detect, receive and transmit different kinds of stimuli.
ii) The nerve fibres, which are bundles of extended processes of nerve cells.

NERVE CELLS

Nerve cells or neurons are the structural and functional units of the nervous system. Billions of nerve cells make up our brain.

A nerve cell is a microscopic structure consisting of three major parts, namely the cell body, dendrites and axon.

Cell body:
It is the cell structure, irregular in shape or polyhedral in structure. It is also called the cyton. The cell body contains cytoplasm with typical cell organelles, and certain granular bodies called Nissl granules.

Dendrites:
Dendrites or dendrons are shorter fibres which branch repeatedly and project out of the cell body. Dendrites transmit electrical impulses towards the cyton.

Axon:
One of the fibres arising from the cell body is very long, with a branched distal end. It is called the axon.

The distal branches terminate as bulb-like structures called synaptic knobs, which are filled with chemicals called neurotransmitters. The axon contains axoplasm inside and is covered by a membrane called the neurilemma. The neurilemma encloses the axon except at the branched distal ends.

In some neurons, called myelinated neurons, an additional white fatty fibre called the myelin sheath covers the neurilemma. The myelin sheath is not continuous over the neurilemma. The gaps left by the myelin sheath on the axon are called nodes of Ranvier. Over the myelin sheath are found certain cells called Schwann cells.

TYPES OF NERVE CELLS

a) Myelinated or medullated or white neurons:
When the axon is enclosed by the white fatty myelin cover, it is called a myelinated, medullated or white neuron. These neurons form the white matter of the brain.

b) Non-myelinated or non-medullated or grey neurons:
This neuron is not enclosed by a myelin sheath, so it appears greyish in colour. The axon is covered by only the neurilemma and Schwann cells. These neurons form the cerebral cortex, which is the grey matter of the brain.

c) Unipolar neurons:
The embryonic nervous tissue contains unipolar neurons. A unipolar neuron has a nerve cell body with a single process or fibre, which acts both as the axon and the dendron.

d) Bipolar neurons:
The sensory hair cells of the sense organs, such as the rods and cones of the retina, are made up of bipolar neurons. Each bipolar neuron has a cell body and two processes at its ends, one acting as the axon and the other as the dendron.

e) Multipolar neurons:
The cerebral cortex contains multipolar neurons. Each multipolar neuron has a cell body with many dendrites and an axon.

[Figure: Structure of neuron and types. Unipolar neuron (Axon), Bipolar neuron (Axon), Multipolar neuron with Dendrite, Nucleus and Axon labelled]

Synapse:
The dendrites and the synaptic knobs of the axons of neighbouring neurons are in physical contact with one another without fusing. This point of contact between neighbouring nerve cells is called a synapse.

NERVE IMPULSE

The conduction of stimuli by the nerve cells is called a nerve impulse. The dendrites receive the stimuli from the receptor (sense organ) and conduct them as electrical impulses to the axon through the cyton. At the synapse, the synaptic knobs release chemical substances called neurotransmitters, which convert the electrical impulse into a chemical impulse and pass it to the neighbouring neuron.

TYPES OF NEURON

1. Sensory neuron: These neurons receive signals from a sense organ.
2. Motor neuron: These neurons send signals to a muscle or a gland.
3. Association neuron: These neurons relay the signals between the sensory neuron and the motor neuron.

PATHWAYS: FROM STIMULUS TO RESPONSE

In the holding stick activity you observed that there is coordination between the eye and the finger. Different pathways are taken by nerves to bring about this coordinated activity. On the basis of the pathways followed, nerves are classified mainly into three different types.

Afferent neurons:
Afferent (ferrying towards) nerves carry messages towards the central nervous system (spinal cord or brain) from the nerve endings on the muscles and sense organs that sense the change in surroundings. These are called stimulus detectors. They are also called sensory nerves.

Efferent neurons:
Efferent (ferrying away) nerves carry messages from the central nervous system to the parts that carry out the response, called the effectors (nerve endings). They are also called motor nerves.

Association neurons:
Association nerves link together the afferent and efferent nerves.

NERVOUS SYSTEM IN HUMANS

The nervous system can be divided into two major regions: the central nervous system and the peripheral nervous system. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else. The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the CNS is what is inside these two cavities and the PNS is outside them, because some elements of the PNS are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery, meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal.

The nervous system can also be divided into parts mostly on the basis of a functional difference in responses.

Somatic nervous system (SNS):
The somatic nervous system is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision. If a friend jumps out from behind a corner and yells "Boo!", you will be startled and you might scream or leap back. You did not decide to do that, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (unconscious) as a person learns motor skills, referred to as "habit learning" or "procedural memory".

Autonomic nervous system (ANS):
The autonomic nervous system is responsible for the involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can come from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down, which is a homeostatic mechanism. But when you are nervous, you might also start sweating. That is not homeostatic; it is the physiological response to an emotional state.

Enteric nervous system (ENS):
The enteric nervous system is responsible for controlling the smooth muscle and glandular tissue in the digestive system. It is a large part of the PNS and is not dependent on the CNS. It is sometimes considered to be a part of the autonomic system, because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion. There are some differences between the two, but there is a good bit of overlap.

[Figure: Divisions of the nervous system in the human body. Brain (CNS): perception and processing of sensory stimuli (somatic/autonomic), execution of voluntary motor responses (somatic), regulation of homeostatic mechanisms (autonomic). Spinal cord (CNS): initiation of reflexes from ventral horn (somatic) and lateral horn (autonomic) grey matter, pathways for sensory and motor functions between periphery and brain. Nerves (PNS): fibres of sensory and motor neurons (somatic/autonomic). Ganglia (PNS): reception of sensory stimuli by dorsal root and cranial ganglia (somatic/autonomic), relay of visceral motor responses by autonomic ganglia (autonomic). Digestive tract (ENS): the enteric nervous system, located in the digestive tract, is responsible for autonomous functions and can operate independently of the brain and spinal cord]

MUSCULAR MOVEMENTS AND NERVOUS CONTROL

Muscle tissues have special filaments called actin and myosin. When a muscle receives a nerve signal, a series of events is triggered in the muscle. Calcium ions enter the muscle cells. This results in the actin and myosin filaments sliding towards each other, and that is how a muscle contracts. The contraction of a muscle brings about movement in the related organ.

REFLEX ACTION

A reflex action is a special case of involuntary movement in voluntary organs. When a voluntary organ is in the vicinity of a sudden danger, it is immediately pulled away from the danger to save itself. For example, when your hand touches a very hot electric iron, you move your hand away in a jerk. All of this happens in a flash, and your hand is saved from the imminent injury. This is an example of a reflex action.

Reflex arc:
The path through which nerve signals travel in a reflex action is called the reflex arc. The receptor is the organ which comes into the danger zone. The sensory neurons pick up signals from the receptor and send them to the relay neuron. The relay neuron is present in the spinal cord. The spinal cord sends signals to the effector via the motor neuron. The effector comes into action and moves the receptor away from the danger.

[Figure: Reflex action of the hand touching a hot pan. Receptors (heat and pain) in the skin, sensory neuron, relay neuron in the spinal cord (CNS), message to brain, motor neuron and effector (muscle in arm)]

[Figure: Flow chart of the reflex arc: Stimulus → Receptor → Sensory neuron → Interneuron in spinal cord → Motor neuron → Effector → Response]

The reflex arc passes through the spinal cord, and the signals involved in a reflex action do not travel up to the brain. This is important because sending signals to the brain would take more time. Although every action is ultimately controlled by the brain, a reflex action is mainly controlled at the level of the spinal cord.

HUMAN BRAIN

The human brain is a highly complex organ, mainly composed of nervous tissue. The tissues are highly folded to accommodate a larger surface area in less space. The brain is covered by a three-layered system of membranes called meninges. Cerebrospinal fluid (CSF) is filled between the meninges. The CSF provides a cushion to the brain against mechanical shocks. Furthermore, the brain is housed inside the skull for optimum protection.

The human brain can be divided into three regions, namely the forebrain, midbrain and hindbrain.

Parts of the human brain:
Forebrain: It is composed of the cerebrum, thalamus and hypothalamus.
Midbrain: It is composed of the reticular formation.
Hindbrain: It is composed of the cerebellum, pons and medulla oblongata.

[Figure: Flow chart of the human brain. Brain divides into Forebrain (Cerebrum, Thalamus, Hypothalamus), Mid-brain (Reticular formation) and Hindbrain (Medulla, Pons, Cerebellum)]

Cerebrum:
The cerebrum is the largest part of the human brain. It is divided into two hemispheres, called the cerebral hemispheres.
Functions of the cerebrum:
- It controls voluntary motor actions.
- It is the site of sensory perceptions, such as tactile and auditory perceptions.
- It is the seat of learning and memory.

Hypothalamus:
The hypothalamus lies at the base of the cerebrum. It controls the sleep and wake cycle (circadian rhythm) of the body. It also controls the urges for eating and drinking.

Cerebellum:
The cerebellum lies below the cerebrum and at the back of the whole structure. It coordinates the motor functions. When you are riding a bicycle, the perfect coordination between your pedalling and steering is achieved by the cerebellum.

Medulla:
The medulla forms the brain stem along with the pons. It lies at the base of the brain and continues into the spinal cord. The medulla controls various involuntary functions, such as heartbeat and respiration.

COORDINATION IN PLANTS

Unlike animals, plants do not have a nervous system. Plants use chemical means for control and coordination. Many plant hormones are responsible for various kinds of movements in plants. Movements in plants can be divided into two main types: tropic movement and nastic movement.

Tropic movement:
The movements which are in a particular direction in relation to the stimulus are called tropic movements. Tropic movements happen as a result of the growth of a plant part in a particular direction. There are four types of tropic movements: geotropic, phototropic, hydrotropic and thigmotropic.

- Geotropic movement: The growth of a plant part in response to gravity is called geotropic movement. Roots usually show positive geotropic movement, that is, they grow in the direction of gravity. Stems usually show negative geotropic movement.
- Phototropic movement: The growth of a plant part in response to light is called phototropic movement. Stems usually show positive phototropic movement, while roots usually show negative phototropic movement. If a plant is kept in a container in which no sunlight reaches, but a hole in the container allows some sunlight in, the stem finally grows in the direction of the sunlight. This happens because of a higher rate of cell elongation in the part of the stem which is away from the sunlight. As a result, the stem bends towards the light. The increased growth is caused by increased secretion of the plant hormone auxin in the part which is away from the sunlight.
- Hydrotropic movement: When roots grow in the soil, they usually grow towards the nearest source of water. This shows positive hydrotropic movement.
- Thigmotropic movement: The growth of a plant part in response to touch is called thigmotropic movement. Such movements are seen in the tendrils of climbers. The tendril grows in a way so that it can coil around a support. The differential rate of growth in different parts of the tendril happens due to the action of auxin.

Nastic movement:
The movements which do not depend on the direction of the stimulus are called nastic movements. For example, when someone touches the leaves of the mimosa (touch-me-not plant), the leaves droop. The drooping is independent of the direction from which the leaves are touched. Such movements usually happen because of changing water balance in the cells. When the leaves of mimosa are touched, the cells in the leaves lose water and become flaccid, resulting in the drooping of the leaves.

Some plant hormones:
Auxin, gibberellins and cytokinin promote growth in plant parts. Abscisic acid inhibits growth in a particular plant part.

HORMONES IN ANIMALS

The hormones in animals are produced by the endocrine glands, and they also play an important role in control and coordination. Hormones are chemical substances that control and coordinate the activities of living organisms and also their growth. The term hormone was introduced by Bayliss and Starling. Animal hormones do not bring about directional growth depending on environmental cues, but promote controlled growth in various areas to maintain the body design.

The various endocrine glands in humans are the hypothalamus, pineal gland, pituitary gland, thyroid gland, parathyroid glands, thymus, pancreas, adrenal glands, ovary (in females) and testis (in males).

Features and functions of hormones:
1. They are secreted by the endocrine glands and are called 'chemical messengers'.
2. They are poured directly into the bloodstream, as endocrine glands have no ducts of their own (ductless glands).
3. They usually have their effect at sites different from the sites where they are made. They act on specific areas called target organs.
4. Hormones coordinate body activities and growth.
5. They are released in minute quantities.

ENDOCRINE SYSTEM

The endocrine system is composed of several endocrine glands. A ductless gland is called an endocrine gland. An endocrine gland secretes its product directly into the bloodstream. Hormones are produced in the endocrine glands, and a hormone is mainly composed of protein.

Hormones assist the nervous system in control and coordination. Nerves do not reach every nook and corner of the body, and hence hormones are needed to affect control and coordination in those parts. Moreover, unlike nervous control, hormonal control is somewhat slower.

The different types of endocrine glands present in our body are the pituitary gland, pineal gland, hypothalamus, thyroid, parathyroid, thymus, adrenal gland, pancreas, testes and ovary.

The pituitary gland:
- It is a pea-shaped gland located at the base of the brain.
- It is considered to be the master gland, as it secretes many hormones to regulate the organs as well as the other glands.
- Different hormones secreted by this gland include growth hormone, TSH, FSH, LH, ACTH, MSH, vasopressin and oxytocin.

The hypothalamus:
- It is a neuro-endocrine part of the brain.
- It links the nervous system and the endocrine system through the pituitary gland.
- Different hormones secreted by this gland include TRH, GnRH, GHRH, CRH, somatostatin and dopamine.

The thyroid gland:
- It is located in the neck, ventral to the larynx.
- It is one of the largest endocrine glands.
- The principal hormones produced by this gland are triiodothyronine and thyroxine.
- Thyroxine is a hormone that regulates the metabolism of carbohydrates, proteins and fats in the body.
- Hyposecretion of thyroxine leads to cretinism in children and myxoedema in adults.
- Hypersecretion of thyroxine leads to exophthalmic goitre in adults.
- Goitre is caused by a deficiency of iodine in food. Iodine is essential for the synthesis of thyroxine.

Parathyroid glands:
- These are two pairs of small, oval-shaped glands embedded on the dorsal surface of the thyroid gland in the neck.
- They secrete parathormone. Parathormone helps in the regulation of calcium and phosphate ions in the bones and blood.
- Hyposecretion leads to parathyroid tetany, and hypersecretion causes osteoporosis.

The adrenal glands:
- These are located above the kidneys and are hence called suprarenal glands.
- The two regions of the adrenal gland are the adrenal cortex and the adrenal medulla.
- The adrenal cortex secretes hormones like cortisol, aldosterone and androgens.
- The adrenal medulla secretes hormones like adrenaline and noradrenaline.
Adrenaline is also called the "hormone of fight or flight", or the emergency hormone. It prepares the body to face an emergency condition of physical stress, such as danger, anger and excitement.

The pancreas:
- It is located just below the stomach, within the curve of the duodenum. It is both exocrine and endocrine in function.
- It secretes hormones such as insulin, glucagon, somatostatin and pancreatic polypeptide.
- Insulin regulates the sugar level in our blood. Insufficient insulin raises the sugar level in our blood, which causes a disease called diabetes mellitus.

Gonads:
Two types of gonads are present in human beings: female gonads and male gonads.

Female gonads:
- A pair of ovaries forms the gonads in females. The ovaries are the female sex organs, which lie one on either side of the abdominal cavity.
- The ovaries produce two hormones, oestrogen and progesterone.
- Oestrogen controls the changes that occur during puberty, such as a feminine voice, soft skin and development of the mammary glands.
- Progesterone controls the uterine changes in the menstrual cycle and helps in the maintenance of pregnancy.

Male gonads:
- A pair of testes forms the gonads in males. The testes are the male sex organs, located in the scrotum, outside the abdomen.
- The testes produce the hormone testosterone.
- Testosterone controls the changes that occur during puberty, such as a deeper voice, development of the penis, and facial and body hair.

The pineal gland:
- It is located near the centre of the brain, dorsal to the diencephalon.
- It produces the hormone melatonin.
- Melatonin affects reproductive development, the modulation of wake and sleep patterns, and seasonal functions.

The thymus gland:
- It is located in front of the heart, in the upper part of the sternum.
- It produces the hormone thymosine.
- It helps in the maturation of T-lymphocytes.

INTEXT QUESTIONS (PAGE 119)

Question 1: What is the difference between a reflex action and walking?

Answer:
A reflex action is a rapid, automatic response to a stimulus. It does not involve any thinking. For example, we close our eyes immediately when a bright light is focused on them. Walking, on the other hand, is a voluntary action. It is under our conscious control.

Teacher's Note:
a) A reflex is fast and does not need the brain to think about it.
b) Walking is a voluntary action, which we decide and control.

Question 2: What happens at the synapse between two neurons?

Answer:
A very small gap occurs between the last portion of the axon of one neuron and the dendron of the other neuron. This gap is known as a synapse. It acts as a one-way valve to transmit impulses in one direction only. This one-way transfer of impulses occurs because the chemicals are produced on only one side of the neuron, that is, the axon's side. From the axon, the impulses travel across the synapse to the dendron of the other neuron.

Teacher's Note:
a) The synapse is the gap between two neurons, not a part of a single neuron.
b) Neurotransmitters carry the impulse across the synapse as chemical messengers.

Question 3: Which part of the brain maintains posture and equilibrium of the body?

Answer: The cerebellum, a part of the hindbrain, is responsible for maintaining posture and equilibrium of the body.

Teacher's Note:
a) The cerebellum lies in the hindbrain, along with the pons and medulla oblongata.
b) The cerebellum also coordinates the motor functions, such as balance while riding a bicycle.

Question 4: How do we detect the smell of an agarbatti (incense stick)?

Answer:
The thinking part of our brain is the forebrain. It has separate areas that are specialised for hearing, smelling, sight, taste, touch and so on. The forebrain also has regions that collect information or impulses from the various receptors. When the smell of an incense stick reaches us, our forebrain detects it. Then the forebrain interprets it by putting it together with the information received from other receptors, and with the information already stored in the brain.

Teacher's Note:
a) Smell is detected by receptors in the nose, and the signal is interpreted in the forebrain.
b) Memory of earlier smells helps the brain identify the incense stick.

Question 5: What is the role of the brain in reflex action?

Answer:
Reflex actions are sudden responses, which do not involve any thinking. For example, when we touch a hot object, we withdraw our hand immediately without thinking, as thinking may take time, which would be enough to get us burnt.

The sensory nerves that detect the heat are connected to the nerves that move the muscles of the hand. Such a connection, which detects the signal from the nerves (input) and responds to it quickly (output), is called a reflex arc. The reflex arcs, which are the connections between the input and output nerves, meet in a bundle in the spinal cord.

Reflex arcs are formed in the spinal cord, and the information (input) reaches the brain. The brain is only aware of the signal and the response that has taken place. However, the brain has no role to play in the creation of the response.

Teacher's Note:
a) The reflex arc works through the spinal cord, so the brain is not needed to create the response.
b) The brain only becomes aware of the reflex after it has happened.

INTEXT QUESTIONS (PAGE 122)

Question 1: What are plant hormones?

Answer:
Plant hormones, or phytohormones, are naturally occurring organic substances. They are synthesised in one part of the plant body, in minute quantities, and are translocated to other parts when required. The five major types of phytohormones are auxins, gibberellins, cytokinins, abscisic acid and ethylene.

Teacher's Note:
a) Plant hormones are made in small amounts, so they must be carried to the parts where they act.
b) Remember the five major types: auxins, gibberellins, cytokinins, abscisic acid and ethylene.

Question 2: How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light?

Answer:
The movement of the leaves of the sensitive plant, Mimosa pudica or "touch me not", occurs in response to touch or contact stimuli. This movement is independent of growth. The movement of a shoot towards light is known as phototropism. This type of movement is directional and is dependent on growth.

Teacher's Note:
a) The leaf movement of Mimosa is nastic, while the shoot movement is tropic.
b) Phototropism depends on growth in a particular direction, but the leaf movement does not.

Question 3: Give an example of a plant hormone that promotes growth.

Answer: Auxin is an example of a growth-promoting plant hormone.

Teacher's Note:
a) Gibberellins and cytokinins are also growth-promoting hormones.
b) Abscisic acid is a growth inhibitor, so it is not an example here.

Question 4: How do auxins promote the growth of a tendril around a support?

Answer:
Auxin is synthesised at the shoot tip. It helps the cells to grow longer. When a tendril comes in contact with a support, auxin stimulates faster growth of the cells on the opposite side, so that the tendril forms a coil around the support. This makes the tendrils appear like a watch spring.

Teacher's Note:
a) Auxin promotes cell elongation, which causes the uneven growth that makes the coil.
b) The coiling is an example of thigmotropic movement.

Question 5: Design an experiment to demonstrate hydrotropism.

Answer:
1. Take two small beakers and label them A and B.
2. Fill beaker A with water.
3. Make a cylindrical roll from filter paper and keep it as a bridge between beaker A and beaker B.
4. Attach a few germinating seeds in the middle of the filter paper bridge.
5. Cover the entire set-up with a transparent plastic container so that the moisture is retained.
6. Observation: The roots of the germinating seeds grow towards beaker A.
7. Conclusion: This experiment demonstrates the phenomenon of hydrotropism.

[Figure: Two beakers A and B joined by a cylindrical filter paper bridge, with germinating seeds on the bridge, covered by a transparent plastic container]

Teacher's Note:
a) Only the beaker with water is the source of water, so the roots grow towards it.
b) Keep the set-up covered so that the moisture is retained during the experiment.

INTEXT QUESTIONS (PAGE 125)

Question 1: How does chemical coordination take place in animals?

Answer:
Chemical coordination takes place in animals with the help of hormones. A hormone is the chemical messenger that regulates the physiological processes in living organisms. It is secreted by glands. The regulation of physiological processes, and the control and coordination by hormones, come under the endocrine system. The nervous system along with the endocrine system controls and coordinates the physiological processes in our body.

Teacher's Note:
a) Hormones are chemical messengers carried in the blood.
b) The nervous and endocrine systems work together for control and coordination.

Question 2: Why is the use of iodised salt advisable?

Answer:
Iodine stimulates the thyroid gland to produce the hormone thyroxine. Thyroxine regulates carbohydrate, fat and protein metabolism in our body. Deficiency of iodine results in the enlargement of the thyroid gland. This can lead to goitre, a disease characterised by a swollen neck. Therefore, iodised salt is advised for the normal functioning of the thyroid gland.

Teacher's Note:
a) Goitre is caused by lack of iodine in the diet.
b) Iodine is needed to make thyroxine, so iodised salt helps the thyroid gland work normally.

Question 3: How does our body respond when adrenaline is secreted into the blood?

Answer:
Adrenaline is a hormone secreted by the adrenal glands in case of any danger, emergency or stress. It is secreted directly into the blood and is transported to different parts of the body.

When secreted in large amounts, it speeds up the heartbeat and hence supplies more oxygen to the muscles. The breathing rate also increases due to contractions of the diaphragm and rib muscles. It also increases the blood pressure. All these responses enable the body to deal with any stress or emergency.

Teacher's Note:
a) Adrenaline is called the hormone of fight or flight.
b) The heartbeat, breathing rate and blood pressure all rise during an emergency.

Question 4: Why are some patients of diabetes treated by giving injections of insulin?

Answer:
Diabetes is a disease in which the level of sugar in the blood is too high. Insulin, a hormone secreted by the pancreas, helps in regulating the blood sugar level. This is the reason why diabetic patients are treated by giving injections of insulin.

Teacher's Note:
a) Insulin lowers the blood sugar level.
b) Insulin is a protein, so it is given by injection and not as a tablet.

EXERCISE QUESTIONS (PAGE 125, 126)

Question 1: Which of the following is a plant hormone?
(a) Insulin
(b) Thyroxin
(c) Oestrogen
(d) Cytokinin

Answer: (d) Cytokinin

1. Insulin, thyroxin and oestrogen are hormones of animals, secreted by endocrine glands.
2. Cytokinin is one of the five major types of plant hormones.
3. Hence, the correct option is (d).

Teacher's Note:
a) Plant hormones are called phytohormones.
b) Auxin, gibberellins, cytokinin, abscisic acid and ethylene are the five plant hormones.

Question 2: The gap between two neurons is called a
(a) dendrite.
(b) synapse.
(c) axon.
(d) impulse.

Answer: (b) synapse

1. The gap between the axon of one neuron and the dendron of the next neuron is called a synapse.
2. A dendrite and an axon are parts of a single neuron, and an impulse is a signal, not a gap.
3. Hence, the correct option is (b).

Teacher's Note:
a) Neurotransmitters cross the synapse to carry the impulse.
b) Do not confuse the synapse with a dendrite, which receives impulses.

Question 3: 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

1. The forebrain is responsible for thinking.
2. The medulla regulates the heart beat.
3. The cerebellum balances the body.
4. Hence, the brain is responsible for all of these, so the correct option is (d).

Teacher's Note:
a) Each function belongs to a different part of the brain.
b) When more than one option is true, "all of the above" is correct.

Question 4: 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 sensory structures (organs, tissues or cells) present all over the body. The receptors are grouped together in the eye or ear, and scattered in the skin.

Functions of receptors:
(i) They sense external stimuli such as heat or pain.
(ii) They also trigger an impulse in the sensory neuron, which sends the message to the spinal cord.

When the receptors are damaged, the external stimuli transferring signals to the brain are not felt. For example, if the receptors are damaged and we accidentally touch a hot object, our hand might get burnt, as the damaged receptors cannot perceive the external stimuli of heat and pain.

Teacher's Note:
a) Receptors work as the first step of every reflex and every sensation.
b) Damage to receptors, such as in the skin, can cause serious injury because pain is not felt.

Question 5: Draw the structure of a neuron and explain its function.

Answer:
Neurons are the functional units of the nervous system. The three main parts of a neuron are the axon, the dendrite and the cell body.

[Figure: Structure of a neuron with the dendrite, cell body (cyton with nucleus), and axon labelled]

Functions of the three parts of a neuron:
Axon: It conducts messages away from the cell body.
Dendrite: It receives information from the axon of another cell and conducts the messages towards the cell body.
Cell body: It contains the nucleus, mitochondria and other organelles. It is mainly concerned with the maintenance and growth of the neuron.

Teacher's Note:
a) Remember the direction of flow: dendrite to cell body, then along the axon.
b) A labelled diagram should show the cell body, the dendrites and the long axon.

Question 6: How does phototropism occur in plants?

Answer:
The growth movement in plants in response to a light stimulus is known as phototropism. The shoots show positive phototropism and the roots show negative phototropism. This means that the shoots bend towards the source of light, whereas the roots bend away from the light source.

Some examples of phototropism are as follows:
(a) The flower head of a sunflower is positively phototropic, and hence it moves from east to west along with the Sun.
(b) The ovary stalk of groundnut is positively phototropic before fertilisation and becomes negatively phototropic after fertilisation, so that the fruit is formed underground.

Teacher's Note:
a) Shoots are positively phototropic and roots are negatively phototropic.
b) The groundnut example shows that the same plant part can change its response over time.

Question 7: Which signals will get disrupted in case of a spinal cord injury?

Answer:
The reflex arc connections between the input and output nerves meet in a bundle in the spinal cord. In fact, nerves from all over the body meet in a bundle in the spinal cord on their way to the brain. In case of any injury to the spinal cord, the signals coming from the nerves as well as the signals coming to the receptors will be disrupted.

Teacher's Note:
a) The spinal cord is the relay point for reflexes as well as the route to the brain.
b) Because signals are blocked, reflexes and sensations below the injury may be affected.

Question 8: How does chemical coordination occur in plants?

Answer:
In animals, control and coordination occur with the help of the nervous system. However, plants do not have a nervous system. Plants respond to stimuli by showing movements. The growth, development and responses to the environment in plants are controlled and coordinated by a special class of chemical substances known as hormones.

These hormones are produced in one part of the plant body and are translocated to other needy parts. For example, a hormone produced in the roots is translocated to other parts when required. The five major types of phytohormones are auxins, gibberellins, cytokinins, abscisic acid and ethylene. These phytohormones are either growth promoters (such as auxins, gibberellins, cytokinins and ethylene) or growth inhibitors (such as abscisic acid).

Teacher's Note:
a) Plants use hormones, not nerves, for control and coordination.
b) Ethylene is also a growth promoter in plants.

Question 9: What is the need for a system of control and coordination in an organism?

Answer:
Coordination is the working together of the various integrated body systems to maintain the body functions in response to changes in the body. All the movements that occur in response to stimuli are carefully coordinated and controlled. In animals, control and coordination are provided by the nervous and muscular systems. The nervous system sends messages to and away from the brain, and the spinal cord plays an important role in relaying these messages.

In the absence of this system of control and coordination, the body will not be able to function properly. For example, when we accidentally touch a hot utensil, we immediately withdraw our hand. In the absence of nerve transmission, we would not withdraw our hand and may get burnt.

Teacher's Note:
a) Control and coordination keep the body working as one unit.
b) The hot utensil example shows why a fast response is needed.

Question 10: How are involuntary actions and reflex actions different from each other?

Answer:
Involuntary actions cannot be consciously controlled. For example, we cannot consciously control the movement of food in the alimentary canal. These actions are, however, directly under the control of the brain.

On the other hand, reflex actions, such as closing the eyes immediately when a bright light is focused on them, show a sudden response and do not involve any thinking. This means that, unlike involuntary actions, reflex actions are not under the control of the brain.

Teacher's Note:
a) Involuntary actions are controlled by the brain, while reflex actions are controlled by the spinal cord.
b) Both are not under our conscious control, but only reflexes are sudden responses.

Question 11: Compare and contrast nervous and hormonal mechanisms for control and coordination in animals.

Answer:

Comparison of nervous and hormonal mechanisms

Teacher's Note:
a) Nerves give quick, short-lived responses, while hormones give slow, long-lasting ones.
b) Use the same four points when writing a comparison, so that each row matches.

Question 12: What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs?

Answer:

Comparison of movement in a sensitive plant and in our legs

Teacher's Note:
a) The sensitive plant moves without a nervous system, by changes in water in its cells.
b) Leg movement is voluntary and controlled by the brain through the nervous system.

EFFERENT NEURON:
Efferent (or ferrying away) wh