NCERT Solutions for Class 11 Biology Chapter 21

Q.1 Briefly describe the structure of the following:
(a) Brain
(b) Eye
(c) Ear

Ans-

(a) Brain: Brain is the main coordinating centre of nervous system in all vertebrates and most invertebrates. It is most complex organ of the body. Brain consists of neurons, glial cells and blood vessels. Brain is made up of three main parts:

• Forebrain
• Midbrain
• Hindbrain

Following diagram shows the structure and location of various parts of human brain.

Forebrain: Forebrain consists of cerebrum, thalamus, and hypothalamus.

• The cerebrum or cortex is the largest part of the human brain and is a highly folded structure. This is also known as grey matter due to the presence of a large number of neuron cell bodies giving it a greyish appearance. Cerebrum is divided into two halves (hemispheres) by a deep furrow. Two hemispheres are joined to each other with nerve fibres, known as the corpus callosum. The cerebrum is further divided into four lobes: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. The motor areas, sensory areas and large regions that are neither sensory nor motor in function are known as association areas. These areas are involved in complex functions like inter-sensory associations, memory and communication. The inner part of cerebral hemisphere consists of fibres which are covered with myelin sheath and thus; gives an opaque white appearance, hence it is called white matter.
• Thalamus is a major coordination centre for sensory and motor signalling. It is involved in functions like thoughts and actions. It is covered by cerebrum.
• Below thalamus, hypothalamus is present which controls body temperature, hunger and thirst.

Midbrain: The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain. Four round swellings or lobes constitute the dorsal portion of the midbrain and are known as corpora quadrigemina. It consists of the tectum and tegmentum. It is associated with vision, hearing, motor control, sleep/wake, alertness and temperature regulation.

Hindbrain: The hindbrain is made of the cerebellum, pons and medulla. Cerebellum is a highly convoluted structure and thus; provides extra space for a large number of neurons. Like cerebrum, cerebellum is also divided into two hemispheres. Cerebellum is associated with regulation and coordination of movement, posture, and balance. Pons is made up of fibre tracts interconnecting various regions of the brain. The medulla is connected to the spinal cord and contains regulatory centres for respiration, cardiovascular reflexes and gastric secretions.

(b) Eye: Eyes are organs for the perception of light and colours. The pair of eyes are located in sockets of the skull known as orbits. They are nearly spherical in shape. The wall of the eyeball consists of the following three layers: the outer-most sclera, middle choroid and inner-most retina.

Sclera: It is the outer most layer and consists of dense connective tissues. The anterior portion of this layer is called cornea.

Choroid: it is the middle layer and contains blood vessels. It is bluish in appearance. It thickens at the anterior portion of the eye and forms a ciliary body that holds the lens with the help of other ligaments. The ciliary body further extends to form iris, which gives characteristic colour to the eyes. There is an aperture in the front part of the eyes, called pupil. Pupil is surrounded by iris. Muscles of iris regulate the diameter of pupil.

Retina: Retina is the innermost layer and it contains three layers of cells – from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. Photoreceptor cells are of two types-rods and cones.

Eye is divided into two chambers-

Aqueous chamber: It is the space between the cornea and the lens, and contains a thin watery fluid called aqueous humour.

Vitreous chamber: It is the space between the lens and the retina and is filled with a transparent gel, called the vitreous humour. The place where blood vessels enter and optic nerves leave the eyeball is called blind spot because no photoreceptors are present in this region, so no image is formed at this place. There is another place on the retina where only cones are present and it is fovea. Fovea is the region of highest visual resolution.

(c) Ear: Ears are organ of hearing and maintaining balance. Anatomically an ear is divided into three parts- outer, middle and inner ear.

• Outer ear: Outer ear consists of the pinna and external auditory meatus (canal). External auditory meatus extends up to the tympanic membrane (eardrum). Pinna and meatus contain sebaceous glands.
• Middle ear: Middle ear contains three small bones (ossicles) called malleus, incus and stapes. They are attached to each other like a chain. Malleus is attached to tympanic membrane and stapes is connected to inner ear. Middle ear is connected to pharynx by a tube called Eustachian tube. Eustachian tube helps in balancing air pressure at both sides of tympanic membrane.
• Inner ear: Inner ear is a fluid-filled structure called labyrinth. Labyrinth is divided into two parts- bony and membranous labyrinth. The bony labyrinth is filled with perilymph, while membranous labyrinth is filled with endolymph. Membranous labyrinth is further divided into following parts:

1. Vestibular apparatus: It is a sac-like structure and is composed of three semi-circular canals and otolith organ (otolith consists of utricule and saccule). Base of the canals are swollen and is called ampulla (ampulla contains crista ampullaris). Otolith contains a projecting ridge called macula. Macula and crista are responsible for maintaining posture and the balance of the body.
2. Cochlea: Cochlea is a coiled structure and extension of succulus. It contains organ of Corti which acts as the auditory receptor. It is the main organ for hearing.

Q.2 Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
(c) Choroid and retina

Ans-

(a)

(b)

(c)

Q.3 Explain the following processes:
(a) Polarisation of the membrane of a nerve fibre
(b) Depolarisation of the membrane of a nerve fibre
(c) Conduction of a nerve impulse along a nerve fibre
(d) Transmission of a nerve impulse across a chemical synapse

Ans-

(a) Polarisation of the membrane of a nerve fibre: Polarisation of the membrane of neuron occurs during resting phase when there is no conduction of nerve impulse. There are different types of ion channels present on the neural membranes. During the resting phase, membrane is relatively more permeable for potassium ions (K⁺) and nearly impermeable to sodium ions (Na⁺). The membrane is also impermeable to negatively charged proteins present inside the cell. As a result, concentration of sodium ions is higher on the outer side of membrane as compared to the inner side. These gradients across the resting membranes are maintained by the active transport of ions by the sodium-potassium pump which transports three Na⁺ outwards for two K⁺ into the cell. The outer side of the membrane becomes positively charged and the inner side becomes negatively charged due to a difference in the concentration of ions. This is known as polarisation of the membrane of a nerve fibre.

(b) Depolarisation of the membrane of a nerve fibre: When there is a nerve impulse at the site of a polarised membrane, permeability of membrane towards sodium ion changes. The membrane at the site of impulse becomes freely permeable to sodium ions. This causes a rapid influx of sodium ions resulting in increased sodium ion concentration at the inner side of the membrane. The inner side of the membrane becomes positively charged and the outer side becomes negatively charged. This reversal in polarity of the membrane is called depolarisation.

(c) Conduction of a nerve impulse along a nerve fibre: Conduction of nerve impulse is carried out by depolarisation of plasma membrane of nerve cell. Whenever there is a nerve impulse, it opens the sodium ion channel present on the membrane of nerve fibres resulting in the rapid influx of sodium ions to the inside of the nerve fibre. This generates a positive charge at the inner side of the membrane and a negative charge on outer side. This potential difference is called action potential and the state of nerve fibre is said to be in depolarised state. Just ahead of this point, the polarity of the axon membrane is opposite i.e. the outer side is positive and inner side is negative. This results in flowing of current from site of action potential to further down the axon. On the outer surface, current flows in reverse order completing the circuit. The polarity at the site of origin of impulse is reversed and action potential is generated further ahead. As the nerve impulse moves forward, the membrane becomes depolarised that causes the nerve impulse to travel forward.

(d) Transmission of a nerve impulse across a chemical synapse: Junction between two neurons is called synapse. Nerve impulse travels from one neuron to another through these synapses. At a chemical synapse, the membranes of the two neurons (pre-synaptic and post-synaptic neurons) are separated by a synaptic cleft, which is filled with a fluid. The nerve impulse is transmitted from presynaptic neuron to post-synaptic neuron with the help of neuro-transmitters in the following manner:

• Axon terminal of a presynaptic neuron contains neurotransmitters in vesicles.
• When there is nerve impulse, vesicles filled with neurotransmitters fuse with the membrane and release neurotransmitters in the synaptic cleft.
• At synaptic cleft, neurotransmitters bind to their specific receptors present on the membrane of the post-synaptic neurons.
• This opens ion channels in post-synaptic neurons resulting in the generation of potential difference.
• The potential difference leads to the transmission of nerve impulse which may be excitatory or inhibitory from pre-synaptic neurons to post-synaptic neurons.

Q.4 Draw labelled diagrams of the following:
(a) Neuron
(b) Brain
(c) Eye
(d) Ear

Ans-

(a) Neuron:

(b) Brain:

(c) Eye

(d) Ear

Q.5 Write short notes on the following:
(a) Neural coordination
(b) Forebrain
(c) Midbrain
(d) Hindbrain
(e) Retina
(f) Ear ossicles
(g) Cochlea
(h) Organ of Corti
(i) Synapse

Ans-

(a) Neural coordination: Coordination by the neural system is done through electric impulses that send signals to target organs/tissues to act coordinately with other organs or tissues. Neural coordination is performed mainly by brain and neurons in the body. Brain receives the stimulus with the help of nerves and sends signals in the form of an electric impulse to the effector organs.

(b) Forebrain: Forebrain consists of the cerebrum, thalamus, and hypothalamus.

• The cerebrum or cortex is the largest part of the human brain and is highly folded structure. The cerebrum is divided into two halves (hemispheres) by a deep furrow. Two hemispheres are joined to each other with nerve fibres known as the corpus callosum. The cerebrum is further divided into four lobes: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. The cerebrum is involved in complex functions like intersensory associations, memory and communication.
• Thalamus is a major coordination centre for sensory and motor signalling. It is involved in functions like thoughts and actions. It is covered by the cerebrum.
• Below thalamus, the hypothalamus is present which controls body temperature, hunger and thirst.

(c) Midbrain: It is located between the thalamus of forebrain and pons of the hindbrain. Midbrain consists of the tectum and tegmentum. It is associated with vision, hearing, motor control, sleep/wake, alertness and temperature regulation.

(d) Hindbrain: The hindbrain consists of the cerebellum, pons and medulla oblongata. Like cerebrum, cerebellum is also divided into two hemispheres and is a highly convoluted structure. Cerebellum is associated with regulation and coordination of movement, posture, and balance. Pons connects two hemispheres of the cerebellum. The medulla oblongata is the posterior part of the brain. It is associated with the maintenance of balance and posture of body.

(e) Retina: Retina is the innermost layer of the eyeball and it contains three layers of cells from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. Photoreceptor cells are of two types- rods and cones. Cone cells contain iodopsin pigment that is highly sensitive to bright light. Cone cells are involved in the perception of bright light and colour. Rod cells of retina contain pigment rhodopsin which is highly sensitive to dim light.

(f) Ear ossicles: The middle ear contains three very small bones called ossicles. They are as follows.

• Malleus: Malleus is attached to the eardrum (tympanic membrane) at one side and to the incus at another side.
• Incus: Incus is connected with stapes.
• Stapes: It is attached to the internal ear.

They together transmit sound waves from the external ear to the internal ear.

(g) Cochlea: Cochlea is a coiled structure and extension of the succubus. It contains the organ of Corti which acts as an auditory receptor. It is the main organ for hearing. Cochlea forms three chambers- upper scala vestibule, middle scala media and lower scala tympani.

(h) Organ of Corti: Organ of Corti is the part of cochlea, the internal ear. It is located in cochlear duct between the scala vestibuli and the scala tympani. It is found in mammals only. It contains hair cells and auditory receptors. It is the main site for hearing.

(i) Synapse: In a nervous system, synapse is a gap between the two neurons through which nerve impulse gets transferred from one neuron to another. There are two types of synapse-

(a) If the two neurons are in very close proximity, nerve impulse is transferred directly from one neuron to another like the transfer of nerve impulse within a single neuron. This kind of synapse is called electrical synapse.

(b) If the two neurons are separated by a synaptic cleft, then transfer of nerve impulse is mediated by neurotransmitters and the synapse is called as chemical synapse.

Q.6 Give a brief account of:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing

Ans-

(a) Mechanism of synaptic transmission: Synaptic transmission is the transfer of nerve impulse from one neuron to another. Synaptic transmission is of two types:

Direct transmission, like at electrical synapse

• At electrical synapse, the membranes of pre- and post-synaptical neurons have hardly any gap (synaptic cleft is absent).
• Thus electrical current or the nerve impulse is transferred directly from one neuron to another.
• This type of transmission is very fast.

Indirect transmission mediated by neurotransmitters, like at chemical synapse

• At chemical synapse, axon of first neuron (presynaptic neuron) releases neurotransmitters (Acetylcholine) in synaptic cleft (synaptic cleft is a fluid filled gap between axon of first neuron and dendrite of the second neuron).
• Neurotransmitters then activate the ion channels present on the dendrite of next neuron (called as post-synaptic neuron)
• This causes the depolarisation resulting in transmission of nerve impulse.
• This type of transmission is slower as compared to electric transmission.

(b) Mechanism of vision: Retina in the eye balls is the site of light and colour perception. Retina contains photoreceptor cells- rod cells and cone cells.

• Photoreceptor cells contain light sensitive photopigments that are composed of opsin (a protein) and retinal (an aldehyde of vitamin A).
• As the light falls on retina passing through pupil, it causes dissociation of retinal from opsin.
• This causes the change in conformation of opsin resulting in change in membrane permeability in photoreceptor cells and generation of action potential.
• These action potentials (impulses) are transmitted to the visual cortex area of the brain by the optic nerves,
• The neural impulses are analysed and image formed on retina is perceived.

(c) Mechanism of hearing

• External ear receives sound waves and transfers them to the tympanic membrane (eardrum).
• Sound waves generate vibrations in tympanic membrane which are transferred to the oval window of internal ear through ear ossicles (malleus, incus and stapes).
• Vibrations are then passed-on to cochlea where they generate waves in lymph.
• The waves in the lymph induce a ripple in the basilar membrane.
• It causes the hair cells to bend and press them against the tectorial membrane.
• As a result, nerve impulses are generated in the associated afferent neuron.
• These are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognised.

Q.7 Answer briefly:
(a) How do you perceive the colour of an object?
(b) Which part of our body helps us in maintaining the body balance?
(c) How does the eye regulate the amount of light that falls on the retina?

Ans-

(a) The retina in the eyeballs is the site of colour perception. Cone cells present in retina recognise colours. There are three types of cone cells which are responsible for the recognition of different wavelength of light (different colours). Cone cells contain light sensitive photo-pigments that are composed of opsin (a protein) and retinal (an aldehyde of vitamin A). As the light falls on cone cells passing through the pupil, it causes dissociation of retinal from opsin. It causes the change in conformation of opsin that results in a change in membrane permeability in photoreceptor cells and generation of an action potential. These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analyzed and coloured image formed on the retina is perceived.

(b) Vestibular apparatus present in the internal ear is responsible for maintaining the body balance. It is a sac-like structure and is composed of three semicircular canals. Canals contain endolymph (a fluid) and sensors (crista ampullar, present at the base of canals, and macula, present in otolith).

(c) Eyes regulate the amount of light falling on the retina with the help of a pupil. The pupil is an aperture at the front of the eyeballs. It is surrounded by iris that is made up of contractile muscles. Iris contracts or expands to constrict or dilate the pupil which in turn regulates the amount of light passing through it.

Q.8 Explain the following:
(a) Role of Na⁺ in the generation of action potential.
(b) Mechanism of generation of light-induced impulse in the retina.
(c) Mechanism through which a sound produces a nerve impulse in the inner ear.

Ans-

(a) The sodium (Na⁺) ion plays an important role in generating and maintaining the potential difference necessary for the conduction of nerve impulse. When the outer side of the membrane of the nerve fibre is positively charged and the inner side is negatively charged, it is in resting state. At this state, there is a high concentration of sodium ions at the outer side of the membrane of nerve fibres. To generate an action potential, polarity should be reversed i.e. outer side should be negative and inner side of the membrane should be positive. When a neuron gets a nerve impulse, sodium ions cross the membrane of nerve fibres through sodium ion channels. This reverses the charge distribution across the membrane. The outer side of membrane becomes negatively charged, while the inner side becomes positively charged. Potential difference generated by this charge redistribution is called an action potential.

(b) Photoreceptor cells contain light-sensitive photo-pigments that are composed of opsin (a protein) and retinal (an aldehyde of vitamin A). As the light falls on retina passing through the pupil, it causes dissociation of retinal from opsin. It changes the conformation of opsin that results in change in membrane permeability of photoreceptor cells and generates an action potential. These impulses are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analyzed and the image formed on retina is perceived.

(c) The external ear receives sound waves and transfers them to the tympanic membrane (eardrum). Sound waves generate vibrations in the tympanic membrane which are transferred to the oval window of the internal ear through ear ossicles (malleus, incus and stapes). Vibrations are then passed on to cochlea where they generate waves in the lymph. The waves in the lymph induce a ripple in the basilar membrane. It causes the hair cells to bend and presses them against the tectorial membrane. As a result, nerve impulses are generated in the associated afferent neuron that is transmitted via auditory nerves to the temporal lobe of the cerebral cortex of the brain, where the impulses are analysed and the sound is recognised.

Q.9 Differentiate between:
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Rods and cones
(d) Thalamus and Hypothalamus
(e) Cerebrum and Cerebellum

Ans-

(a)

(b)

(c)

(d)

(e)

Q.10 Answer the following:
(a) Which part of the ear determines the pitch of a sound?
(b) Which part of the human brain is the most developed?
(c) Which part of our central neural system acts as a master clock?

Ans-

(a) Cochlea determines the pitch of sound.

(b) Forebrain is the most developed part of brain.

(c) Hypothalamus acts as a master clock of body.

Q.11 The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the
(a) fovea
(b) iris
(c) blind spot
(d) optic chaisma

Ans-

(c) blind spot

[Explanation: Region of the retina where optic nerves pass is called blind spot because this region does not contain any photo-receptors.]

Q.12 Distinguish between:
(a) Afferent neurons and efferent neurons
(b) Impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre
(c) Aqueous humor and vitreous humor
(d) Blind spot and yellow spot
(e) Cranial nerves and spinal nerves

Ans-

(a)

(b)

(c)

(d)

(e)