Why do trees need sunlight? An environmental scientist explains photosynthesis

The reason trees need sunlight is the same reason their leaves are green. Scottb211/Flickr, CC BY

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Why do trees need sunlight? – Tillman, age 9, Asheville, North Carolina

Trees need sunlight for the same reason you need food. The energy from the Sun’s rays is a crucial ingredient in how plants make their own food that helps them power all their cells. Since trees don’t harvest or hunt food, they have to produce their own. The way they make their food is a unique and important chemical process called photosynthesis.

Chlorophyll is what makes leaves green.
Kristian Peters-Fabelfroh/Wikimedia, CC BY-SA

What is photosynthesis?

The cells in plants and all other living things have microscopic components called organelles. One type of organelle in plant cells is the chloroplast, and it contains the pigment chlorophyll, which is what makes leaves green. When chlorophyll receives sunlight, it starts the photosynthesis reaction.

The name photosynthesis comes from the ancient Greek words “photo,” which means light, and “synthesis,” which means to make. During this food-making process, plants take carbon dioxide from the air and water from the ground, and with the energy from sunlight, make glucose. Glucose is a very simple type of sugar. Because it is a simple compound, it is simple to make.

Most of the time, photosynthesis occurs in leaves, and leaves take in sunlight to make food. There are some special plants, though, that actually absorb sunlight on their stems. Some of these include cactuses like the balloon-shaped golden barrel cactus, the spiky Munz’s Cholla and the paddle-shaped prickly pear. Some plants even have roots that can photosynthesize, like the rare palm Cryosophila albida.

Sunlight gives plants the energy to turn water and carbon dioxide into carbohydrates – the food their cells need to live and grow.
At09kg/Wikimedia, CC BY-SA

Photosynthesis is billions of years old

Photosynthesis evolved more than 3.5 billion years ago. Initially, only single-celled organisms, kind of like today’s algae, could make sugar this way. Oxygen is a waste product from the photosynthesis process, and over time, these single-celled organisms released enough oxygen to change the Earth’s atmosphere. Ultimately, we and all other animals needed this to happen to be able to live and breathe.

Over time, aquatic plants developed, and gradually plants moved to land around 500 million years ago to better access their most vital resource: sunlight. Plants eventually got taller by around 350 million years ago. This is when the first tree evolved, which grew up to 150 feet tall. These trees looked like the evergreen trees we see today – sort of like pines, firs and spruce. And about 125 million years ago, trees that looked like the maples, oaks and beech trees we see today shared the landscape when dinosaurs ruled the Earth.

Not just good for plants

The Sun provides energy for the Earth. However, we humans are not capable of taking in the sun directly and using it to power our bodies. So how do we make use of the Sun’s energy? Plants do it for us.

Plants take in that energy and make food for us and other animals to eat and oxygen for us to breathe. We wouldn’t exist without plants and photosynthesis.

Like the ancient tiny single-celled organisms from 3.5 billion years ago, some microorganisms today use photosynthesis. Specifically, the algae that you might see living on top of lakes and the ocean do. Chlorophyll is why algae is green.

There are aquatic plants that use sunlight to grow. They typically make use of less sunlight because sunlight does not travel well through water.

Some plants can do photosynthesis underwater, where there is less sunlight.
Chesapeake Bay Program/Flickr, CC BY-NC

In addition, there are a very few animals that can photosynthesize. The pea aphid uses pigment to harvest sunlight to make energy. The Oriental hornet uses a pigment in its exoskeleton to make energy from sunlight. The emerald-green sea slug eats algae and then incorporates chlorophyll from the algae into its body to photosynthesize. Because of this strategy, the sea slug can go nine months without eating.

So the answer to this question – why do trees need sunlight – is to make their food. And thanks to trees and other plants turning sunlight into their food, most of the rest of the living things on Earth get to eat, too!

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Rebekah Stein does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.