AP Biology · Lesson 1

Responses to the environment and energy flow

Know Know it

Organisms respond to their environment in ways that help them survive and reproduce. Behavior can be innate or learned. A taxis is movement toward or away from a stimulus; a kinesis is a change in speed or turning with no set direction. Animals communicate with visual, sound, touch and chemical signals; a pheromone is a chemical signal to members of the same species. Plants respond too: phototropism bends a shoot toward light, and photoperiodism times flowering by the length of the night.

Endotherms, such as birds and mammals, keep a steady body temperature mostly with heat from their own metabolism, so they need much more food than ectotherms of the same size. Ectotherms, such as lizards and most fish, take their body temperature mostly from their surroundings and control it by behavior. Among endotherms, small animals use more energy per gram of body mass, because they have more surface per unit of volume and lose heat faster.

Energy enters most ecosystems as sunlight captured by producers. Gross primary productivity is all the energy producers capture; net primary productivity is what is left after their own respiration, the energy stored in new biomass that consumers can eat. Energy then moves up trophic levels: primary consumers, secondary consumers and so on. Only a small share, roughly 10 percent though it varies, passes from one level to the next. The rest is lost as heat from respiration, or is never eaten or never digested. So there is far less energy, and usually far less biomass, at the top of a food chain.

Energy flows through an ecosystem in one direction, but matter cycles. Decomposers break down dead matter and return nutrients to the soil. In the carbon cycle, photosynthesis takes carbon dioxide from the air and respiration and burning return it. In the nitrogen cycle, certain bacteria carry out nitrogen fixation, turning nitrogen gas into ammonia that plants can use. Phosphorus has no gas phase and cycles mainly through rock, soil and water.

Trap Energy is lost, matter is recycled

Energy that reaches a trophic level is not passed on whole. Most of it leaves as heat and cannot be reused, which is why energy must keep coming in from the sun. Matter such as carbon and nitrogen is different: decomposers return it, and it is used again.

Worked example Easy

In a fictional grassland, the plants store 40,000 kilocalories per square meter per year in new biomass. If about 10 percent of the energy passes from each trophic level to the next, about how much reaches the secondary consumers?

  1. 4,000 kilocalories
  2. 400 kilocaloriesAnswer
  3. 40 kilocalories
  4. 20,000 kilocalories

How to solve it

  1. Plants to primary consumers: 10 percent of 40,000 = 4,000 kilocalories.
  2. Primary to secondary consumers: 10 percent of 4,000 = 400 kilocalories.
  3. So about 400 kilocalories reach the secondary consumers.

Why each choice is right or wrong

  • A. Incorrect. 4,000 kilocalories is about what reaches the primary consumers, one step up from the plants.
  • B. Correct. 10 percent of 40,000 is 4,000 for primary consumers, and 10 percent of 4,000 is 400 for secondary consumers.
  • C. Incorrect. 40 kilocalories is about what reaches a third level of consumers, one step too far up the chain.
  • D. Incorrect. 20,000 kilocalories is half the plant energy; far less than half passes up even one level.

Explain Explain it

Task verbs: explain, predict

To explain a response to the environment, name the response, then say how it helps the organism survive or reproduce in that setting. For temperature questions, name whether the animal is an endotherm or an ectotherm and tie its oxygen or food use to heat production.

To explain a pattern in a food chain, follow the energy: what share reaches the next level, and where the rest goes. Fewer kilocalories at the top means a level can support fewer, or smaller, organisms.

Worked example Medium

A student measured oxygen use by a fictional mouse and a fictional lizard of similar mass, each resting in a chamber, at two air temperatures. The mouse used 2.6 mL of oxygen per gram per hour at 10 degrees C and 1.4 at 30 degrees C. The lizard used 0.05 mL per gram per hour at 10 degrees C and 0.20 at 30 degrees C.

Which statement best explains the mouse’s results?

  1. The mouse is an ectotherm, so its metabolism speeds up as its body cools
  2. Cold air holds more oxygen, so any animal takes in more of it at low temperature
  3. The mouse is an endotherm and burns more fuel to make heat in the coldAnswer
  4. The mouse moved less in the cold, so it needed more oxygen to stay still

How to solve it

  1. The mouse used more oxygen in the cold; the lizard used less.
  2. An animal that uses more oxygen in the cold is burning fuel to make heat: an endotherm.
  3. The lizard’s metabolism slows as its body cools, the pattern of an ectotherm.

Why each choice is right or wrong

  • A. Incorrect. An ectotherm’s metabolism slows as it cools, which is the pattern the lizard shows, not the mouse.
  • B. Incorrect. The lizard used less oxygen in the cold, so cold air alone does not make animals use more oxygen.
  • C. Correct. An endotherm makes its own body heat, so colder air means more respiration, and more oxygen, to hold its temperature.
  • D. Incorrect. Staying still uses less energy, not more, so less movement cannot explain the rise in oxygen use.

Explain Your turn

A fictional lake food chain has four trophic levels: algae, tiny grazing animals, small fish and large fish. Explain why the lake supports far fewer kilograms of large fish than of algae, using what happens to energy between trophic levels.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Says that only a small share of the energy at one level (roughly 10 percent) ends up in the next level
  • Says that most of the energy is lost at each step, mainly as heat from respiration, or is not eaten or digested
  • Explains that after several steps little energy is left for the large fish, so the lake can support only a small mass of them

Model answer

At each step up the chain, only about 10 percent of the energy stored at one trophic level ends up stored in the next. Most of the rest is used by the organisms for their own respiration and lost as heat, and some is in parts that are never eaten or digested. After three such steps, from algae to grazers to small fish to large fish, only a tiny share of the algae’s energy is left, so the lake can support only a small mass of large fish.

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