Metabolism for Middle School Science

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Metabolism is the set of chemical reactions that happen inside living organisms to keep them alive and working. These reactions help organisms break down food for energy, build new cells, and carry out all the processes they need to survive, like moving, growing, and healing. Every living thing has a metabolism, although it can work very differently depending on the type of organism.

How Metabolism Works in Different Organisms

Humans: In humans, metabolism turns the food we eat, like bread, fruits, and meat, into energy that powers our muscles, brain, and organs. The process includes digestion, breaking food into smaller parts like glucose, and then using oxygen to help release energy from that glucose in a process called cellular respiration.

Plants: Plants have a unique kind of metabolism. They don’t eat food like animals. Instead, they make their own food using photosynthesis—a process that uses sunlight, carbon dioxide, and water to create glucose. The plant then uses that glucose in cellular respiration, just like animals do, to power its growth and repair.

Fungi (like mushrooms): Fungi don’t photosynthesize or eat like animals. They absorb nutrients from other organisms by releasing special chemicals that break down dead material around them. Their metabolism allows them to digest food outside their bodies and then take in the nutrients they need.

Bacteria: Bacteria have all kinds of metabolisms. Some break down sugars just like we do, but others can survive on things like sulfur, iron, or even oil! Some bacteria don’t need oxygen at all. Their diverse metabolism is one reason bacteria can live in extreme places, from hot springs to deep ocean vents.

Jellyfish: Jellyfish have a much simpler metabolism than mammals. They absorb oxygen directly from the water through their skin and use it to break down food in their gut cavity. Because they don’t have brains or blood, their metabolism is less complex, but it still gives them the energy to move and sting prey.

Cellular Respiration

Cellular respiration is the process that cells use to turn food into energy. Just like a car burns fuel to run, your cells "burn" food to get the energy they need to work. This happens in almost all living things, including animals, plants, fungi, and even many tiny organisms like bacteria.

Here's how it works: Cells take in glucose (a type of sugar from food) and oxygen (from the air you breathe). Inside the cell, these two things go through a chemical reaction that produces energy, along with carbon dioxide and water as waste. The energy made is called ATP, and cells use it to grow, move, repair themselves, and do all the things that keep you alive!

Glucose + Oxygen → Energy (ATP) + Carbon Dioxide + Water

Photosynthesis

Photosynthesis is the process that plants use to make their own food. Instead of eating like animals, plants take sunlight, carbon dioxide from the air, and water from the ground to create a kind of sugar called glucose, which gives them energy. This process happens mostly in the leaves, inside special parts of the plant cells called chloroplasts that contain a green pigment called chlorophyll.

During photosynthesis, plants also make oxygen as a waste product—and that’s the oxygen we breathe!

Sunlight + Carbon Dioxide + Water → Glucose (Sugar) + Oxygen

Metabolism is what keeps all living things alive and active. Whether it’s a plant making its own food through photosynthesis or an animal breaking down food for energy, every organism depends on metabolism to grow, move, and survive. Metabolism also connects all living things through the food chain and food webs. Plants use sunlight to make energy, herbivores eat plants, carnivores eat other animals, and decomposers break everything down. Each step involves metabolism. Without it, the energy needed to power life could never move through ecosystems. In this way, metabolism is not just something that happens inside living things; it’s the driving force behind life on Earth.