BIOLOGY 3 Marks Question

It is called so because it can be compared to the transmission of a message through a telephone cable. It travels very fast just like an electric current. Though its speed is not as much fast as that of electric current through a wire. Thus the passage of a stimulus in a nerve is an electrical process.

It is the hardening of arteries and arterioles due to the thickening of the fibres tissue and the consequent loss of elasticity. It is often referred to as atherosclerosis. This mainly affects the vessels, which are mainly responsible for supplying blood to the heart muscle. It seems to occur due to deposition of calcium, fat cholesterol, and fibrous tissues, making the lumen of arteries narrower

Ribs in Humans: There are 12 pairs of ribs in humans. They support the sides of the thoracic cavity. They fall into 3 groups.
i. True ribs: Upper 7 pairs of ribs attached directly to the sternum.
ii. False ribs (Vertebrochondral ribs): Next three pairs of ribs joined to the ribs above. They are 8th, 9th and 10th ribs.
iii. Floating ribs: Lower two pairs of ribs free in front (11th and 12th) are called floating ribs.
Thoracic vertebrae, ribs as well as sternum form rib cage. Vertebral and sternal are two parts of the rib.

Main features of class reptilia are:
i. They are the first truly terrestrial vertebrates.
ii. They have dry skin covered with scales.
iii. They are tetrapods and pentadactyl.
iv. The hands and feet have claws.
v. Lungs are well-developed and reptiles never breathe by gills.
vi. There is vo larval stage in its development.
vii. The ventricle of the heart is partially divided into two by an incomplete septum but this division is complete in the crocodiles.

Main characteristics of pteridophytes:
i. Fern plant is a sporophyte.
ii. It is divided into root, stem, and leaves.
iii. They have vascular tissues xylem and phloem.
iv. Sporangia are formed in the leaves. The sporangia bear spores which are haploid.
v. Spores germinate and form gametophyte which is known as prothallus.
vi. Prothallus possesses sex organs archegonia and antheridia.
vii. They do not form seeds.
viii. Alternation of generation is an important feature.
ix. Examples are Ferns, Dryopteris (a garden fern), Pteris and Nephrolepis, etc.