BIOLOGY 3 Marks Question

i. Difference between Grey matter and White matter:

Grey Matter	White matter
It is composed of cell bodies (cytons) of a nerve cell. It is inside the brain but outside the spinal cord	It is composed of axons from the nerves cells. It is inside the brain but outside the spinal cord.

ii. Difference between A stimulus and An impulse:

A stimulus	An impulse
Any change in the environment of an organism that evokes response or impulse.	It is a self-propagating wave electrochemical in nature; negatively travelling along the axon of a nerve cell.

Heart sounds: These are resulted due to sharp closure of heart valves. The first sound 'lub' occurs at the onset of ventricular systole and; is caused due to sharp closure of AV valve and the ejection of blood from the ventricles.
The second heart sound 'dub' occurs at the beginning of ventricular diastole and is caused by sharp closure of the semilunar valves to the aorta and the pulmonary artery.

The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.
The T-wave represents the return of the ventricles from excited to the normal state (repolarization). The end of the T-wave marks the end of systole.
Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heartbeat rate of an individual. Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of great clinical significance.

Gibberellins are useful in agriculture in the following ways:
i. Application of gibberellins increases the length of the stem and increases the yield of sugar in sugarcane.
ii. Gibberellins delay senescence and prevent the premature fruits drop.
iii. It can cause fruits like apple to elongate and improve in shape.

Nucleotides: They are small, complex molecules made of C, H, O, N, P. Each nucleotide consists of 3 units: a nitrogenous base, a pentose sugar, and phosphate groups. The nitrogenous base is a purine (adenine or guanine) or pyrimidine (thymine/cytosine/ uracil). The sugar may be ribose in ribonucleotide or deoxyribonucleotide. Combination of a nitrogenous base and a pentose sugar forms a nucleoside. Nucleotides are mono or di or triphosphates of nucleosides, e.g., Adenosine monophosphate (AMP), Adenosine diphosphate (ADP) and Adenosine triphosphate (ATP).

The body of sponges contains a supporting exoskeleton. It is made up of either spicules or spongin fibres or both. Spongin fibres are protein fibres. They are either highly branched or show anastomosis. Spicules are calcareous or siliceous with hard spine-like structures. Depending on the branching the spicules are
i. monoaxons (single axis).
ii. triaxons (three-axis forming six rays of hexactinal spicules).
iii. tetraxons (four rays or tetra actinal).
iv. polyaxons (several rays).

Demerits of Artificial System:
i. Criteria used, though simple but arbitrary, are based on random morphological characters/traits.
ii. It lacks the natural relationship amongst the organisms.
iii. It fails to give an idea about the origin and evolution of different species.
iv. Many closely allied individuals were classified in separate groups and those with altogether different characters were placed in the same group.