Which transport system in plants is made of living cells?

Since life evolved in the sea, most invertebrate organisms are osmoconformers. This means they are:

Most frogs have external fertilization. What are the disadvantages of this?

Which type of cells can communicate via electrical impulses in animals?

How can plants regulate molecules that come through the apoplastic route  in roots, before going into the xylem?

The following is a diagram of the circulatory system in mammals. What number represents a vein?

For oxygen exchange between respiratory and circulatory system to take place, partial pressure of O2 should be_________________ in pulmonary artery than in alveoli. 

Which type of cells can communicate via electrical impulses in animals?

In this diagram of countercurrent gas exchange, the following is TRUE:

The antigen-binding sites of an antibody molecule are formed from the molecule’s variable regions. Why do we say these regions are called variable?