Ask Dr. Universe: The lowdown on photosynthesis

Dear Dr. Universe,

How do flowers use sun light and water to grow?

— Jackie, Virginia

When I was a kitten, my family measured how tall I was. They marked it on the wall. It was amazing to see how much I grew.

I talked about how plants grow with Helmut Kirchhoff. He’s a scientist at Washington State University. He studies plants and biochemistry.

He told me plants grow by making new cells. To make a new cell, an existing cell splits into two. That’s called cell division. Then, the new cells grow bigger. That’s called cell growth. So, a plant can make its stem or roots longer by making new cells in those places. When it’s time to bloom, a plant can make new types of cells that form flowers.

Plants need the right ingredients to do cell division and cell growth.

“We’re all made of sugars, proteins, oils and special molecules where our genetic information is coded like DNA or RNA,” Kirchhoff said. “These are the main building blocks all life is made from — and to make them you need energy.”

Plants get energy from sunlight, water and nutrients from the soil, and a gas from the air called carbon dioxide. The process is called photosynthesis.

Here’s how it works. Everything in the universe is made of elements. Water is made of the elements hydrogen and oxygen. Carbon dioxide is made of the elements carbon and oxygen.

Every element is made of teeny parts called protons, neutrons and electrons. The important thing to know is that electrons get excited by light. Scientists call this absorption.

For photosynthesis, plants pull in the things they need — water, nutrients and carbon dioxide. Then a green pigment in their leaves called chlorophyll collects light from the sun. The light excites electrons in the chlorophyll. This excited chlorophyll carries the energy from the sun.

That energy has the power to split water into oxygen and electrons. Then the excited electrons can jump to the carbon dioxide to make a sugar called glucose. It also makes a small electric current in the plant. That changes light energy into chemical energy called adenosine triphosphate or ATP for short.

“ATP is the energy currency that all living cells use — from the simplest bacteria to us,” Kirchhoff said.

Have you ever played a video game with an energy bar? When the energy bar is full, your character can farm crops or fight enemies. When the energy bar runs out, your character might slow way down or fall asleep.

Remember how cell division and cell growth use up lots of energy? ATP is the energy living things use to do those things. Photosynthesis is how a plant fills up its energy bar.

The sugar made during photosynthesis is how the plant stores energy for later. Like at night when the sun isn’t shining. When it needs energy, it can break down the sugar and tap into the energy stored there.

Humans like you and cats like me grow by doing cell division and cell growth, too. But animals can’t do photosynthesis to make energy. That’s why we eat plants like veggies and fruits. When you eat a big bowl of spinach, you give your body sugars, proteins and oils from the spinach plant. Your cells can break down the sugar into energy your body can use.

You could say it’s a sweet system.

Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at Dr.Universe@wsu.edu, on her website at askDrUniverse.wsu.edu, via Twitter at @AskDrUniverse or at facebook.com/AskDrUniverse.