Photosynthesis and Respiration

Every leaf you see is doing something amazing. It is taking sunlight, water, and a gas from the air, and turning all of that into food. The food is a sugar called glucose. The leaf also gives off the oxygen that you and every other animal need to stay alive. That food-making job is called PHOTOSYNTHESIS. The opposite job — using that food to release energy — is called RESPIRATION. These two processes are the engine room of life on Earth.

Photosynthesis — Plants Make Their Own Food

Photosynthesis happens inside tiny green parts of plant cells called CHLOROPLASTS. Chloroplasts contain a green pigment named chlorophyll, which is what makes leaves look green. Chlorophyll catches sunlight and uses its energy to power a chemical reaction. The plant pulls in water (H2O) from the soil through its roots and breathes in carbon dioxide (CO2) from the air through tiny holes in its leaves. Using light energy, the plant rearranges those raw ingredients into glucose (a sugar — its food) and oxygen (O2 — the gas it releases back into the air).

1. INPUTS: sunlight, water (H2O), and carbon dioxide (CO2).
2. WHERE: chloroplasts in the leaves (the green parts).
3. OUTPUTS: glucose (food the plant uses or stores) and oxygen (released to the air).
4. WHO: plants, algae, and some bacteria. Animals do NOT photosynthesize.

Respiration — Releasing Energy from Food

Once the plant has made glucose, what does it do with it? The same thing every other living thing does — it BREAKS IT DOWN to release energy. That breakdown is called respiration. Respiration happens inside tiny structures called MITOCHONDRIA, which are found in nearly every cell of nearly every living thing. The cell takes glucose and oxygen and combines them in a controlled chemical reaction. The result is energy (stored in a molecule called ATP) plus waste products: water and carbon dioxide. Yes — every time you breathe out, you are sending the carbon dioxide from respiration back into the air.

1. INPUTS: glucose (food) and oxygen (O2).
2. WHERE: mitochondria in cells (almost all living things).
3. OUTPUTS: energy (ATP) plus carbon dioxide and water.
4. WHO: plants AND animals. Plants do both photosynthesis and respiration.

Two Equations You Should Recognize

You do not need to memorize these like a chemist, but you should be able to read them. Photosynthesis: 6 CO2 + 6 H2O + light energy -> C6H12O6 (glucose) + 6 O2. Respiration runs the other way: C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + energy. Notice how the two equations look like mirror images. The outputs of photosynthesis (glucose and oxygen) are exactly the inputs of respiration. The outputs of respiration (CO2 and water) are exactly the inputs of photosynthesis. The processes feed each other.

How They Balance Each Other on Earth

Plants take in CO2 and release O2. Animals (and plants too, at night and inside their cells) take in O2 and release CO2. This back-and-forth keeps the atmosphere balanced. Without photosynthesis, we would run out of oxygen — and we would have nothing to eat, because nearly all food on Earth traces back to plants. Without respiration, no living thing could move, grow, or even keep its heart beating. The two processes together are the reason Earth supports life.