BIOLOGY 3 Marks Question

SA-Node. It is a pacemaker located at the right wall of right auricle/atrium. Its full name is a sinoatrial node.
AV-Node. It is a pacesetter located at the junction of auricle and ventricle. Its full name is an auriculoventricular node.
Functions of SA-Node:
i. Cardiac impulse originates in it.
ii. It determines the rate of heartbeat.
Function of AV-Node:
i. The Cardiac impulse passes to the AV bundle through the AV Node.
ii. Its branches called purkinje fibres conduct impulses to ventricles.

Red and White Muscles. Muscle contains a red coloured oxygen storing pigment called myoglobin. Myoglobin content is high in some of the muscles which gives a reddish appearance. Such muscles are called the Red fibres. These muscles also contain plenty of mitochondria which can utilize the large amount of oxygen stored in them for ATP production. These muscles, therefore, can also be called aerobic muscles.
On the other hand, some of the muscles possess very less quantity of myoglobin and therefore, appear pale or whitish. These are the White fibres. Number of mitochondria are also few in them, but the amount of sarcoplasmic reticulum is high. They depend on anaerobic process for energy.

In animals, once a particular type of tissue is formed it cannot change to any other tissue. Moreover, growth in all the organs stop after a certain age of the animal. On the contrary, growth in the plant is indeterminate because plants retain the capacity for unlimited growth throughout their life. A senescent part is usually replaced in plants. Differentiated cells undergo dedifferentiation in plants if a need arises.
Sponges are exceptions in the animal kingdom. A sponge shows an open type of growth.

a. In open circulatory system, blood flows in spaces called sinuses and cells and tissues are directly bathed in blood. In a closed circulatory system, blood flows through a network of vessels. Arthropods and molluscs have open circulatory system, while annelids and vertebrates have closed circulatory systems.
b. In oviparity; animals lay eggs and young ones hatch from them after some time, e.g. reptiles, birds and some mammals. In viviparity; animals give birth to young ones, e.g. scoliodon and most of the mammals.
c. When the young ones are different from adult animals; this case is called indirect development, e.g. frog, cockroach, silkworm, etc. When the young ones resemble the adult animal; this case is called direct development e.g. birds, mammals, reptiles, etc.