Biology of Toxins

Sample First Midterm Exam Questions - KEY

1. Which of the following is(are) likely explanation(s) for why organisms produce molecules that are toxic to other organisms?

a.  To protect themselves from predators.

b.  To help them capture prey for food.

c.  The molecules may be waste products that just happen to be toxic to other organisms.

d.  All of the above.

e.  None of the above

2.Which of the following is not directly involved in gene expression?

a.  RNA polymerase

b.  mRNA

c.  Ribosomes

d.  Exocytosis

3. Distribution of a toxin throughout the body is accomplished primarily by the:

a.  liver.

b.  skeletal muscles.

c.  circulatory system.

d.  kidneys.

e.  lungs

4. For a toxin that’s absorbed from the digestive system, which of the body fluid compartments will the toxin have the most difficulty entering, and why?

It would have most difficulty entering the cerebrospinal fluid (CSF) compartment, because the blood-brain barrier is so impermeable to most substances.

5. Graph the following data and estimate the LD50 for each of the two toxins:

% of Individuals Showing Response
Dose
( mg/kg ) /

Toxin 1

/

Toxin 2

0 / 0 / 0
10 / 0 / 0
20 / 2 / 0
30 / 12 / 18
40 / 30 / 75
50 / 65 / 94
60 / 85 / 99
70 / 95 / 100
80 / 98 / 100

What is the NOAEL (No Observable Adverse Effect Lever) for Toxin 2? Note: I want numerical values, not a definition. Which toxin would you say is more toxic, or ‘stronger’? BRIEFLY justify your answer.

The NOAEL for Toxin 2 is 0–20+ mg kg-1.

Toxin 2 would be considered stronger because its LD50 ( @ 37 mg kg-1 ) is less that the LD50 for Toxin 1 ( @ 45 mg kg-1. ).

6. Among-individual and among-species variation in susaceptibility to a particular toxin is due to:

a.  differences in the strength of the toxin

b.  genetic differences among individuals

c.  differences in the way we measure susceptibility

d.  This is a bogus question….there is no variation in susceptibility to toxins.

7. Lipophilic toxicant molecules are often more toxic than hydrophilic molecules because:

a.  they can pass through cell membranes more readily than hydrophilic molecules.

b.  they are usually enzymes.

c.  they are produced by bacteria, rather than by plants.

d.  they’re impossible to biotransform.

e.  All of the above.

f.  None of the above.

8. In lecture, I told you that available evidence suggests that toxins are costly to produce. Briefly describe one of those lines of evidence that I mentioned in lecture, and tell me why it supports the hypothesis that toxins are costly to produce.

Bacteria that are resistant to antibiotics are out-competed by bacteria that are not resistant, when growing in the absence of antibiotics.

Many venomous snake bites are ‘dry’ – no venom is injected.

The wild ginger-banana slug story – when wild ginger produces palatability-reducing compounds, it suffers reduced growth and seed-production rates.

9. Be very specific with your response: the primary objective of biotransformation of xenobiotics is to:

Increase the water solubility of the xenobiotic and make them easier to excrete.

10. (15 pts. each) Answer TWO (2) of the following:

·  Construct a graph that clearly illustrates the presence of susceptible and resistant individuals in a population.

This is straight out of your notes.

Use this graph as the basis for a discussion of how resistance to a toxin can evolve.

Individuals showing evidence of an adverse effect at low doses of a toxin are said to be susceptible, while those requiring high doses of a toxin are said to be resistant. Interindividual differences in susceptibility are largely due to genetic differences, so one's susceptibility/resistance can be passed to one's descendents. If a population of organisms is exposed to a relatively low dose of a toxin, the most susceptible individuals will be less apt to survive and reproduce.Thus, the subsequent generation will be composed of progeny of the more resistant individuals and will, therefore, tend to be more resistant to the toxin. Over time, the resistant genotypes will become more and more common, and the species as a whole will become more resistant.

·  Define and discuss biotransformation. Where in the body does most biotransformation occur? Define bioactivation, and describe an example.

This is straight out of your notes. The example could be one of the following:

·  Second-generation antihistamines

·  Tylenol

·  Milk sickness (tremetone)

·  Discuss excretion of toxins. What do you see as benefits and drawbacks for the various avenues by which toxins are cleared from the body?

This is straight out of your notes.