Why do plants grow toward light?

Imagine you put a plant in a room with only one window. You leave it there for several days. When you come back, what do you think the plant will look like?

Why do you think that happens?

The plant can't walk toward the window. It doesn't have eyes. So how does it "know" where the light is coming from? That's today's mystery.

Before we investigate, think about what a plant actually needs light for.

Find one plant — outside, in your house, at school, or in a classroom. Spend about 2 minutes simply observing it before answering the questions below. Don't worry about getting the "right" answer — scientists begin by observing what is actually happening. Your job is to collect evidence: look closely at the stem, leaves, and light around the plant.

We usually think of light as something that lets us see.

But light does much more than that — light is a form of electromagnetic radiation. The basic idea is simple: light can travel through space and carry energy from one place to another.

The light from the Sun travels through space, reaches Earth, and provides energy that plants can capture. Sunlight shining on a leaf isn't just making the leaf brighter — it is delivering energy.

Plants use that energy during photosynthesis to build energy-rich molecules — molecules that are later used by the plant, and ultimately by other organisms that eat the plant.

But here's something interesting: not all light is the same. The light we see as different colors is made up of electromagnetic radiation with different wavelengths.

Imagine watching waves move across the surface of water.

The distance from one wave peak to the next is called the wavelength.

Light can behave like a wave too — except the wavelengths of visible light are incredibly small. Wavelength is usually measured in nanometers (nm). To put that into perspective, 1 millimeter is about the thickness of a penny. One nanometer is one millionth of a millimeter — 1 mm = 1,000,000 nm.

🌈 Different wavelengths, different colors. Visible light contains a range of wavelengths that our eyes perceive as different colors:

Blue light has a shorter wavelength than red light. Red light has a longer wavelength than blue light.

🌈 Quick check — look at the rainbow above.

We know light carries energy. But does the amount of energy depend on the wavelength? Yes.

Light travels as individual particles called photons. A shorter-wavelength photon carries more energy than a longer-wavelength one.

So within visible light: blue light (shorter wavelength) carries more energy per photon, while red light (longer wavelength) carries less.

When a plant absorbs light, it isn't simply absorbing "color" — it is absorbing energy carried by photons of particular wavelengths.

🌿 Chlorophyll absorbs light especially strongly in the blue and red parts of the spectrum. It absorbs green light much less strongly.

So when sunlight reaches a leaf, some wavelengths are absorbed, and some are reflected or transmitted. The wavelengths that reach our eyes influence the color we see.

We now know:

• Plants need light energy for photosynthesis.
• Light comes in different wavelengths.
• Shorter-wavelength light carries more energy per photon.
• Plants contain pigments that absorb certain wavelengths.
• Plants have specialized light-sensing molecules that let them respond to light.
• Plants can respond to the direction of light.

But we still haven't answered the original question: 🌱 how does a plant actually grow toward the light?

The growth of a plant in response to the direction of light is called phototropism. Photo = light, tropism = directional growth response.

Many plant shoots show positive phototropism, meaning they grow toward a light source.

Plants use chemical signals called hormones to control growth. One important plant hormone is called auxin.

When light comes mainly from one side of a plant shoot, specialized light-sensing molecules help the plant detect the direction of the light. The plant then changes how auxin is distributed within the shoot.

In a typical shoot showing positive phototropism, more auxin accumulates on the shaded side, and the cells on that side elongate more — so: more growth on one side → unequal growth → stem bends → plant grows toward the light.

Let's put all of the evidence together. Why does a plant grow toward light?

1. Plants need light energy for photosynthesis.
2. Light contains different wavelengths and carries energy.
3. Plants contain pigments such as chlorophyll that absorb particular wavelengths of light.
4. Plants also have specialized light-sensing molecules that can detect the direction of light.
5. When light comes mainly from one side, the plant changes its growth response.
6. Auxin becomes redistributed toward the shaded side of the shoot.
7. Cells on that side elongate more.
8. Unequal growth causes the stem to bend toward the light.

This directional growth response is called:

Imagine this experiment.

We've spent the last part of the mission figuring out how plants respond to light. Before we turn this into code, put the steps in the correct order.

You've just learned that plants can bend toward light because the two sides of a stem can grow at different rates. Now we're going to use a tiny piece of code to model what happens to the two sides of a plant over several days.

A variable is a name that stores information. For example:

height = 5

This tells the computer: "Remember that height is 5." We can give different variables different values:

left_side = 5
right_side = 8

Now the computer remembers left_side = 5 and right_side = 8. In our plant model, we'll use variables to keep track of how much each side of the stem has grown.

Sometimes we want a computer to repeat the same action many times. Instead of writing the same line over and over…

print("Day 1")
print("Day 2")
print("Day 3")
print("Day 4")
print("Day 5")

…we can use a for loop. A for loop tells the computer: "Repeat these instructions a certain number of times." For example:

for day in range(5):
    print(day)

This tells the computer to repeat the instructions 5 times. The loop goes through 0, 1, 2, 3, 4 — so the computer is doing the same thing once for each day. In our plant model, we'll use a for loop to represent 5 days of growth.

Now let's put the ideas together. Imagine a plant receiving light from the right side. Because of the way auxin affects the shoot, the shaded side of the stem grows more than the side receiving the light. Our code will represent that difference.

left_side = 1
right_side = 1

for day in range(5):
    left_side = left_side + 2
    right_side = right_side + 0

print("Left side:", left_side)
print("Right side:", right_side)

Before you run it — let's read it line by line.

Before you run it, what numbers do you predict?

Output will appear here.

Now return to our original plant. Remember: light is coming from the right. The left side of the stem grew more. The right side grew less.

Complete the sentence: Plants grow toward light because...