Evolution — BIOLOGY STD 12 Science — Question

Describe in detail about food chain and food web existing in an ecosystem.

1. Draw the embryo sac of a flowering plant and label:

1. Central cell
2. Chalazal end of the embryo sac
3. Synergids.

2. Name the cell that develops into the embryo sac and explain how this cell leads to the formation of embryo sac. Also mention the role played by the various cells of the embryo sac.

(a) Taking an example of a small pond, explain how the four components of an ecosystem function as a unit.
(b) Name the type of food chain that exists in a pond.

What are the major causes of species losses in a geographical region?

While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.

Pistil of a flower does not accept pollen from any plant other than from its own kind. How does it happen? Explain.

Answer the following questions:
Draw a schematic diagram of a human sperm and label the cellular components. Give the functions of any three parts.

Describe the post-zygotic events leading to implantation and placenta formation in humans. Mention any two functions of placenta.

OR

1. Briefly explain the events of fertilisation and implantation in an adult human female.
2. Comment on the role of placenta as an endocrine gland.

What is biogas? Explain its production with the help of a diagram.

Draw a picture of viral replication. Write the full name of AIDS pathogen. How does hybridization occur? Explain the symptoms of AIDS in human body.