How does loading of most carbohydrates into companion cells at a source occur?
a. by diffusion of water from the xylem
b. by osmosis
c. by active transport
d. by proton pumping
Ans: c. by active transport
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Helper T cells secrete ____, which activate B cells and stimulate their proliferation
a. antigens b. allergens c. antibodies d. interferons e. interleukins
What does the mass number represent?
a. Number of protons and electrons b. Number of protons c. Number of neurons and electrons d. Number of neutrons e. Number of protons and neutrons
The plants and animals now present on acreage
from which the trees were removed ten years earlier represent
a. primary succession. b. a climax forest. c. pioneer species. d. secondary succession. e. species introduction
Why has finding an effective AIDS vaccine been so difficult?
A. Because it is caused by a virus. B. Because they have not yet isolated the virus that causes AIDS. C. Because the virus constantly mutates and so vaccines need to be made for all types. D. It is a sexually transmitted virus. E. The body does not make antibodies against the virus.