AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-3)
Unit 8 · Topic 8.3
8.3 Endocrine Disruptors
Endocrine disruptors are chemicals that interfere with hormones, the signals that control growth, development and reproduction. Even tiny amounts can have effects, especially on developing animals, and examples include DDT, some industrial chemicals and pharmaceuticals in wastewater, plus the debated weedkiller atrazine.
Key terms
- endocrine system
- hormone
- endocrine disruptor
- atrazine
- birth defects
The endocrine system
Your endocrine system is the set of glands, such as the thyroid, adrenal glands, ovaries and testes, that release hormones into your blood. Hormones are chemical messengers. They attach to receptor molecules on target cells, like a key fitting a lock, and tell those cells what to do: grow, mature, make eggs or sperm, or control metabolism. Hormones work at very low concentrations, so small disruptions can matter a lot.
How endocrine disruptors work
- Mimicking a hormone: the chemical fits the receptor and triggers a response when the body didn't call for one. Many disruptors mimic estrogen, a female sex hormone.
- Blocking a hormone: the chemical sits in the receptor without activating it, so the real hormone can't work.
- Changing hormone levels: the chemical alters how the body makes, moves or breaks down its own hormones.
Examples and effects
DDT, an insecticide, and its breakdown products interfere with hormone systems and with eggshell formation in birds (topic 8.8). PCBs, once used in electrical equipment, disrupt thyroid and sex hormones. Some plastic additives, such as bisphenol A (BPA) and phthalates, are studied as possible disruptors. Synthetic estrogen from birth control pills passes through people into sewage; treatment plants don't remove all of it, and it ends up in rivers.
Documented effects in wildlife include male fish downstream of sewage outfalls developing eggs in their testes (intersex fish), reproductive failure in birds, and developmental problems. In people, exposure during pregnancy and early childhood is of most concern, and endocrine disruptors have been linked to birth defects, reproductive problems and developmental problems.
Atrazine is a widely used weedkiller. Some lab studies linked it to feminized male frogs, but other studies did not find the same effect, and scientists and regulators still disagree about how much harm it causes at the levels found in streams. The European Union stopped approving it in 2004, mainly over groundwater contamination, while it is still used in the United States.
Why they're hard to regulate
Traditional toxicity testing assumes higher doses cause bigger effects. Hormones work at extremely low levels, so endocrine disruptors can have effects at doses far below those that cause obvious poisoning, and timing matters: exposure during early development can cause permanent changes that wouldn't happen in an adult. That makes it harder to set a single safe level.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Designing an investigation
Biologists suspect that a sewage treatment plant's discharge is causing male fish in a river to develop female traits. Describe an investigation: give a hypothesis, the independent and dependent variables, and one control.
Show the solutionHide the solution
- Step 1: Hypothesis: if male fish live downstream of the treatment plant's outfall, then a higher percentage will show female traits than males upstream, because the discharge contains estrogen-like chemicals.
- Step 2: Independent variable (what differs between groups): location, upstream versus downstream of the outfall (or the concentration of estrogenic chemicals in the water).
- Step 3: Dependent variable (what you measure): the percentage of male fish with female traits, such as eggs in the testes or blood levels of a yolk protein normally made only by females.
- Step 4: Controls (kept the same): same species, same age and size of fish, same time of year, and similar water temperature at both sites. The upstream site serves as the control group.
Answer: Compare male fish upstream (control) and downstream of the outfall, holding species, age, season and temperature constant, and measure the percentage showing female traits.
Common mistakes
- Saying endocrine disruptors only matter at high doses. They can act at very low doses, especially during development.
- Stating atrazine's harm to wildlife as settled fact. Its effects are still debated; describe it as suspected or linked in some studies.
- Confusing endocrine disruption with biomagnification. They are different ideas, though some chemicals, like DDT, do both.
On the exam
- Expect questions asking how an endocrine disruptor affects an organism. Explain the mechanism (mimics or blocks a hormone) and the result (reproductive or developmental harm).
- This topic often appears in 'design an investigation' questions. Practice naming the hypothesis, variables and controls for a field study.
Connected topics
Videos
Check yourself
4 questions on 8.3 Endocrine Disruptors. Pick an answer to see if you got it, and why.
Downstream of a wastewater treatment plant, biologists find male fish with immature eggs in their testes. Which explanation is best supported?
| Herbicide concentration (µg/L) | Genetic males with ovarian tissue (%) |
|---|---|
| 0 | 2 |
| 0.1 | 8 |
| 1 | 18 |
| 10 | 24 |
Hypothetical lab study: tadpoles raised in groups of 50 with the same food, light and temperature
What is the purpose of the group raised at 0 µg/L?
Which conclusion is best supported by the results?
Some endocrine disruptors cause effects at concentrations of less than one part per billion. Which statement best explains why such tiny amounts can matter?
0 of 4 answered