A. - Sign 2. Choose a word from the box at the bottom of the page to fill in the gaps in the
sentences below. You can use words once, twice or not at all.
In animals, oxygen
into cells across cell membranes to be used in
Carbon dioxide
out of cells.
In plants, carbon dioxide diffuses into cells to be used in
Water enters the roots of plants by
Water moves into cells through
permeable membranes, which allow small molecules, such as water, to
pass through, but not large molecules.
Plants use a process called
to move minerals such as nitrates into
root cells. This requires
diffuses
photosynthesis
osmosis
active
transport
energy
partially
respiration
Boardworks Ltd 2011

Which of the following statements are fundamental to the clonal-selection theory of how the adaptive immune system functions? I) Each lymphocyte has a unique membrane receptor that recognizes one antigen. II) When the lymphocyte binds an antigen, it is activated and begins dividing to form many identical copies of itself. III) Cloned lymphocytes have slight differences and are selected by the spleen for removal if they do not bind an antigen. IV) Cloned cells descend from an activated lymphocyte and persist even after the pathogen is eliminated.

A. only I and III

B. only II and IV

C. only I, II, and IV

D. only II, III, and IV

C. only I, II, and IV

Explanation:

Lymphocytes have specific membrane receptors. These receptors serve as binding sites for specific antigens. Binding of the antigen to these surface receptors triggers the propagation of lymphocytes which in turn form effector cells and the memory cells.  

For example, the activated B lymphocytes divide into plasma cells and memory cells. The plasma cells divide and form a group of antibodies all of which have receptors for the antigen recognized by the parent B lymphocyte. The memory cells live in the body and begin the immune response when the same antigen ever enters. T lymphocytes also divide and form effector cells (active helper T cells and cytotoxic cells) and memory T cells.