SSLC, Biology, Chapter 2. Windows of knowledge notes

Expand the given table by adding sense organs and their receptors.

Answer:

How are the eyes protected? Discuss and fill in the blanks.

Answer:

Structure of eye

How is the entry of light in the eye regulated?

Iris and the aperture at its centre called pupil regulate the amount of light falling on the eyes.

The size of the pupil is regulated by the action of circular muscles and radial muscles. When the radial muscles contract in dim light, the size of the pupil increases. When the circular muscles contract in intense light, the size of the pupil decreases. Thus the amount of light falling on the lens is regulated according to the intensity of light.

What are the peculiarities of image formed by the lens of the eye?

Real

Inverted

Diminished

While viewing distant and nearby objects

Power of accommodation of eye?

The ability of the eye to adjust the focal length of the lens by changing its curvature in accordance to the distance of the object from the eye and form the image on the retina is called the power of accommodation of the eye.

Which are the photoreceptors in the retina?

Rod cells and cone cells

Rod cells

Rod cells are more in number than cone cells. Rod cells contain the visual pigment called rhodopsin. This pigment is formed from a protein named opsin and retinal which is a derivative of Vitamin A. Since they are activated even in dim light, we are able to see objects in dim light. These cells cannot detect colour.

Cone cells

Cone cells contain a pigment called photopsin. This is also called iodopsin. This pigment is also composed of opsin and retinal. There are three types of cone cells in our eyes, which help us to detect three primary colours of light – red, green and blue. This diversity is due to the difference in amino acids in the opsin molecule. So, cone cells provide us with colour vision.

Chemistry of vision

In the presence of light, the pigments present in photoreceptors, dissociate into retinal and opsin. This chemical change leads to the formation of impulses. These impulses are transmitted to the cerebrum through optic nerves and this enables vision.

Complete the flowchart related to the sense of sight given below.

Two images of the same object are formed in our two eyes. But we see only one image of the object. How does this happen?

This is possible due to the binocular vision. The images from two sides of the same object are formed in the left and right eye. When these two images combine as a result of the activities of the brain, a three dimensional image of the object is formed. This is called binocular vision.

Defects and diseases of eye

Night blindness

The retinal, a part of the visual pigment, is derived from Vitamin A. The deficiency of Vitamin A results in the low production of retinal. This in turn prevents the synthesis of rhodopsin. In this condition, objects cannot be seen clearly in dim light and this disease is called night blindness.

Xerophthalmia

If there is a prolonged deficiency of Vitamin A, the conjunctiva and cornea become dry and opaque. This causes xerophthalmia and leads ultimately to blindness.

Colour Blindness

The retina contains cone cells which can detect red, green and blue colours. Some persons cannot distinguish green and red colours due to the defect of cone cells. This condition is called colour blindness.

Glaucoma

Aqueous humor is the fluid which nourishes the tissues in the eyes. If the reabsorption of aqueous humor does not occur, it causes an increase in the pressure inside the eyes. This causes damage to the retina and the photoreceptor cells and ultimately leads to blindness. This defect is called glaucoma. It can be rectified by laser surgery.

Cataract

It is a condition in which the lens of the eyes become opaque resulting in blindness. This can be rectified by replacing the lens with an artificial one, through surgery.

Conjunctivitis

This is an infection of the conjunctiva. The causative organisms may be bacteria, virus etc. This disease is transmitted through contact and can be prevented by maintaining personal hygiene.

Mention a few suggestions to maintain the health of our eyes.

Avoid falling of bright source of light directly to the eyes.

Don’t look at the sun.

Avoid reading under dim light.

Do not watch TV screen, computer screen, mobile screen etc. continuously

Frequently wash our eyes.

Include vitamin A contained items in our daily food.

EAR

How is hearing possible???

Cochlea is a coiled tube like a snail shell. It consists of three chambers. Specialized sensory hair cells which are present in the basilar membrane that separates the middle and lower chambers, function as auditory receptors. The basilar membrane and sensory hair cells together constitute the Organ of Corti.

The sound waves which pass through the external ear vibrate the tympanum.This vibration of the tympanum is transmitted to the ear ossicles which causes the vibration of the membrane in the oval window. This vibration further causes the movement of the fluid inside the cochlea. As a result, the sensory hair cells of the Organ of Corti of the cochlea are stimulated and impulses are generated. These impulses reach the cerebrum through the auditory nerve and hearing is effected.

Ear and Body Balancing

Body balance is maintained in accordance with the movement of the head. Movements of the head bring about the movement of the endolymph present inside the vestibule and the semicircular canals. This causes movement of the sensory hair cells and generates impulses. These impulses are transmitted by the vestibular nerves to the cerebellum, and the equilibrium of the body is maintained.

Tongue

Chemoreceptors seen inside the mouth and tongue help us to detect taste. These are seen mainly on the surface of the tongue. The projected structures seen on the surface of the tongue are called papillae. The parts seen on the papillae that detect taste are the taste buds. We have taste buds that are stimulated by tastes like sweet, salt, sour, bitter, umami etc.

Each taste bud has chemoreceptors that help to detect different tastes. Substances responsible for taste dissolve in saliva, stimulate the chemoreceptors and generate impulses. These impulses reach the brain through the respective nerves and we experience taste.

Nose

Stages of sensing smell

¢ Aromatic particles diffuse in the air and enter the nostrils.

¢ These aromatic particles dissolve in the mucus inside the nostrils.

¢ The Olfactory receptor get stimulated and impulses are formed.

¢ These impulses are transmitted through the nerves to the brain.

¢ Brain helps in the perception of smell.

What all receptors are seen in the skin to receive stimuli?

Heat, cold, touch, pressure and pain are felt by our skin. When these receptors are stimulated, impulses reach cerebrum for its perception.

Receptors in Various Organisms

Planaria

Eye spot - to detect light.

Shark

Lateral line - There are receptors in the lateral line on either side of the body which help to detect the change in the balance of body.

Snake

Jacobson’s organ - The aromatic particles that stick on the tongue of the snake reach Jacobson's organ seen on the roof of the mouth cavity. The olfactory receptors seen there get stimulated then.

Housefly

Ommatidia - Formed of thousands of small eyes. The eye of an insect consists of a cluster of photoreceptors called ommatidia.

Let us assess

1. Select the right answer from the following:

While viewing nearby objects.

(a) ciliary muscles relax

(b) curvature of lens decreases

(d) focal length increases

Ans: ciliary muscles contract

2. Identify the odd one and write down the common feature of

the others.

Malleus, Eustachian tube, Stapes, Incus