In this lesson
- Video lesson (coming soon)
- Lesson notes
- Worked examples
- Big ideas
- NGSS alignment
Take it with you
- Lesson slides (PDF)
- Practice worksheet (PDF)
- Answer key (PDF)
Essential Question
How do plants and animals respond to changes in their surroundings?
A plant bends toward a window, and a lizard slides into the shade. Both are responding to a change in their surroundings. In this lesson, you’ll learn how organisms respond to stimuli, how tropisms change plant growth, how animal behavior can act as a feedback loop, and how to investigate a response.
Video lesson coming soon. In the meantime, read through the lesson below.
Stimulus and Response
- A stimulus is a change in the surroundings
- A response is how the organism reacts
- Many responses help the organism survive
- Some responses, like closing stomata or a lizard moving into shade, are negative feedback that keeps conditions in a range
A stimulus is a change in an organism’s surroundings or inside the organism, such as light, temperature, water, or touch. The organism detects the change and makes a response. Responses help organisms stay alive and keep conditions in the right range. Plants and animals both respond, but plants usually respond by changing how they grow, while animals often respond by moving.
Plant Tropisms
- A tropism is growth toward or away from a stimulus
- Phototropism: shoots grow toward light
- Gravitropism: roots grow down and shoots grow up
- Thigmotropism: growth in response to touch
A tropism is a plant’s growth response toward or away from a stimulus. In phototropism, shoots grow toward light, which helps leaves capture more light. In gravitropism, roots grow downward, toward water and minerals in the soil, and shoots grow upward, even when a seedling is lying on its side. In thigmotropism, a plant such as a vine grows in response to touch and coils around a support. Tropisms are slow because they happen through growth.
Plants Respond to Water and Light
- Stomata are pores in the leaf surface
- Each stoma is opened and closed by two guard cells
- Open stomata let CO2 in and water vapor out
- Closed stomata save water but limit photosynthesis
Leaves have tiny pores called stomata, each opened and closed by a pair of guard cells. When there is light and plenty of water, the stomata open, which lets in the carbon dioxide needed for photosynthesis and lets out water vapor. When a plant is short of water, the stomata close, which cuts down water loss. This response has a cost, because less carbon dioxide can enter, so photosynthesis slows. Closing the stomata is a feedback response that helps the plant keep its water balance.
Behavior as a Response
- Animals often respond by moving
- A lizard warms up in the sun and cools off in the shade
- This behavior keeps body temperature in a range
- It is negative feedback
Animals respond to many stimuli through behavior. A lizard cannot generate much body heat of its own, so it uses its surroundings. When it is too cool, it moves onto a sunny rock, and when it is too warm, it moves into the shade. Each response reverses the change that caused it, so this is negative feedback carried out through behavior, and it helps keep the lizard’s body temperature in a range. The stomata loop in the figure works the same way inside a plant.
Investigating a Response
- Change one variable (the independent variable)
- Measure the response (the dependent variable)
- Keep other conditions the same
- Repeat trials and use the mean
To find out how an organism responds to a stimulus, you plan an investigation. In a choice chamber, one half is damp and the other is dry, so the independent variable is the moisture. The dependent variable is the number of pill bugs on each side after a fixed time. You keep other conditions, such as temperature, light, and the number of animals, the same. Repeating the trial several times and using the mean makes the result more reliable. A result such as 15 of 20 on the damp side suggests a response, but you need repeated trials to be confident.
Worked Examples
Try each problem on your own first, then open the solution to check your work.
Example 1: Choice Chamber Data
In a choice chamber, 20 pill bugs were placed in the middle, and the number on the damp side was counted after 10 minutes in five trials (sample data). Find the mean number on the damp side and the mean percent of the pill bugs on the damp side.
| Trial | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Pill bugs on the damp side (of 20) | 14 | 16 | 15 | 17 | 13 |
Show the solution
- Mean: (14 + 16 + 15 + 17 + 13) / 5 = 75 / 5 = 15.0 pill bugs.
- Percent: 15.0 / 20 x 100 = 75%.
- Interpretation: on average, 15 of the 20 were on the damp side and 5 were on the dry side, so the pill bugs tended to move to the damp side.
- Answer: A mean of 15.0 pill bugs, which is 75%, on the damp side.
Example 2: Light and Bending
Seedlings were grown with light from one side or from above, and the bending was measured each day (sample data). Write a claim, evidence, and reasoning about the effect of one-sided light.
| Day | Light from one side (degrees) | Light from above (degrees) |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 12 | 1 |
| 2 | 28 | 0 |
| 3 | 41 | 2 |
| 4 | 52 | 1 |
Show the solution
- Claim: seedlings bend toward a light that comes from one side.
- Evidence: with light from one side the bending reached 52 degrees by day 4, but with light from above it stayed between 0 and 2 degrees.
- Reasoning: the only difference between the groups was the direction of the light, so the direction of the light caused the bending. This is phototropism, and it helps the shoot capture more light.
- Answer: Seedlings bent toward one-sided light, reaching 52 degrees by day 4.
Example 3: Predict the Tropism
A seedling is laid on its side in a dark box so that light and touch do not matter. Use the model to predict how the root and the shoot will grow.
Show the solution
- Stimulus: with no light, the only stimulus that matters is gravity, so this is gravitropism.
- Root: the root will curve downward.
- Shoot: the shoot will curve upward.
- Answer: The root grows downward and the shoot grows upward (gravitropism).
Practice
Name the type of response in each example, calculate the mean and percent from choice chamber data, read data on a phototropism experiment, and plan an investigation. Try it on your own first, then check your work with the answer key.
Lesson Slides
Big Ideas
- A stimulus is a change, and a response is how the organism reacts
- Tropisms are plant growth responses: phototropism, gravitropism, and thigmotropism
- Stomata close in dry conditions, saving water but limiting photosynthesis
- Behavior can act as negative feedback
- An investigation changes one variable, measures a response, and repeats trials
Back to the essential question: organisms detect changes and respond to them. Plants respond by growing toward or away from a stimulus and by opening and closing stomata, and animals often respond with behavior. Many responses are negative feedback loops that help keep conditions in a range. Next, you’ll move on to how cells divide.
NGSS Alignment
This lesson builds toward these Next Generation Science Standards (NGSS) performance expectations:
- HS-LS1-3: Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.
Disciplinary core ideas: LS1.A Structure and Function