BIOLOGY 3 Marks Question

Along a myelinated nerve fibre, the conduction of impulse is called saltatory conduction. This is so because the ionic changes and consequent depolarisation taking place only at the nodes of Ranvier free from myelin sheath leading to the jumping of action potential from one node to the next. 


Figure: Saltatory conduction.

SA Node has a resting potential of approx. -55 to-60 mV. It is a pacemaker. Cardiac impulses originate in it and atrial contraction proceeds ventricular contraction. This has the highest degree of self-contraction that is approx. 70-75 times per minute. Cardiac impulses reach A.V. Node app. 0.03 second after their origin from SA Node.

Muscles exhibit various special properties, some of them are as follows:
i. Contractibility: The cells of muscle can be shortened considerably and return to the original relaxed state.
ii. Excitability: It is due to the energy stored in the electrical potential difference across the plasma membrane.
iii. Muscles also possess properties of extensibility and elasticity (because of proteins actin and myosin).

Various types of auxins are used in agriculture for economic gain. Some of the synthetic auxins are used as a herbicide. 2, 4 - dichloro phenoxy acetic acid (2, 4-D) is a dicotyledonous weed killer. 2,4-D is used to kill broad-leaved weeds. It is used to induce flowering in pineapple and litchi. 


The chemical formula of 2,4-D

Lipids: They are macro biomolecules. They are organic compounds; insoluble in water but soluble in organic solvents such as chloroform, benzene, and petroleum, ether, etc. They have long chains of aliphatic hydrocarbons or a benzene ring. Oil, butter, ghee, waxes, natural rubber, and cholesterol are either lipids or rich in lipids. Fats form an important structural component of cell membranes, pigments such as carotene and lycopene, menthol, hormones, and vitamins A, E and K. Chemically, fats are compounds of C, H, and O.

i. When the body cavity is absent, the animal is called acoelomate, e.g. platyhelminthes. In some animals; mesoderm is present as scattered pouches between ectoderm and endoderm. Such animals are called pseudocoelomate, e.g. aschelminthes.
ii. The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. The dorsal nerve cord is a hollow cord dorsal to the notochord. It is formed from a part of the ectoderm that rolls, forming the hollow tube. The notochord is replaced by a vertebral column in higher vertebrates. The nerve cord develops into the central nervous system.
iii. A polyp is a sessile and cylindrical form whereas medusa is umbrella-shaped and free-living. Polyps produce medusae asexually and medusae from the polyps sexually (e.g. Obelia). Examples of polyps are Hydra and Adamasia whereas of medusa is Aurelia.

i. Red algae: The pigment phycoerythrin in Rhodophyceae gives it the unique red colour and hence the name red algae.
Brown algae: The pigment fucoxanthin in phaeophyceae gives it the unique brown colour and hence the name brown algae.
ii. In liverworts there is no protonema stage, while in moss the life cycle begins with the protonema stage.
iii. Majority of pteridophytes are homosporous, while some of them are heterosporous. Selaginella and salvinia are heterosporous. Basically the presence of heterospory in certain pteridophytes is a precursor of seed habits of higher plants, like gymnosperms and angiosperms.