Tag: biology

The electrical potential difference between outside and inside of a nerve fibre before excitation (polarized nerve fibre) is known as resting potential whose value is $-70$ mV.

The second type of water-current system found also in calcareous sponges, is at first characterized by choanocytes that surround fingerlike projections of the sponge wall. So the correct option is ' sycon'.

Nerve net found in many cnidarians is a part of more advanced nervous systems. Cytoplasmic processes join the nerve cells (neurons) of nerve nets. In cnidarians, the neurons are joined to epithelial receptors and to contractile cells. In vertebrates, nerve nets may be found around blood vessels and the alimentary tract. The nerve net is the simplest form of a nervous system found in multicellular organisms. Unlike, central nervous systems where neurons are typically grouped together, neurons found in nerve nets are found spread apart. This nervous system allows cnidarians to respond to physical contact. They can detect food and other chemicals in a rudimentary way. While the nerve net allows the organism to respond to its environment, it does not serve as a means by which the organism can detect the source of the stimulus. For this reason, simple animals with nerve nets, such as Hydra, will typically produce the same motor output in response to contact with a stimulus regardless of the point of contact. 

Human circulatory system, also called the blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated. It is essentially a network of tubes that supplies to all living cells essential nutrients, O$ _2$ and other essential substances. Also, the waste or harmful substances produced are removed continuously for the healthy functioning of tissues.

Every organ in our body needs the involvement of circulating fluid. Ex - The digestive system digests & absorb nutrients which are needed to be transported to every cell in the body.

Unicellular organisms also require transporting mechanism in order to transport nutrients, enzymes, waste products etc. The transport mechanism in unicellular organisms is simpler than multicellular organisms. Unicellular organisms can transport substances in and out of the body by diffusion and osmosis.

The human transport system is a system of tubes with a pump and valves to ensure one-way blood flow. We need a transport system to deliver oxygen, nutrients, hormones and other substances to all our body cells and take away waste products from them. 

A) The heart is not membranous.