Energy is needed for all living things to survive. We need energy to get nutrients, move nutrients around our bodies, get rid of wastes, and maintain a balance between our bodies and the environment. But where does all of that energy come from?
Ultimately, we all “eat the sun.” If you are a plant, you use the sun directly. If you are a primary consumer, then you use the energy that the plants get from the sun. If you eat a primary consumer, then you are still eating energy that’s essentially coming from the sun! Without photosynthesis, there would be very little usable energy on Earth. So, here’s what happens in the chloroplasts of plants:
Photosynthesis: 6CO2 + 6H2O + sunlight → 6O2 + C6H12O6
This means that carbon dioxide, water and sunlight are all combined to make oxygen and glucose (a type of sugar). The energy from the sun is stored in the chemical bonds of glucose, which can then be used by the plant or by anything else that eats it.
There are some producers that cannot get sunlight because they are deep in the ocean, where the sun does not reach. However, like many forms of life, they have found a way to survive. These organisms gather around vents in the sea floor that spit out hydrogen sulfide. They then combine the hydrogen sulfide with carbon dioxide and oxygen to make formaldehyde, sulfur and water:
Chemosynthesis: CO2 + O2 + 4H2S → CH2O + 4S + 3H2O
The hydrogen sulfide that comes from the vents in the sea floor contains energy; this energy comes directly from inside the Earth. Since the Earth came from the sun, this energy, too, is ultimately from the sun! In the case of chemosynthesis, it gets stored as formaldehyde, which humans use to preserve dead organisms.
Once either photosynthesis or chemosynthesis has trapped energy from the sun, the sugar (or formaldehyde) that gets produced can then be moved around. Just like we have power plants to produce electricity and electrical lines to move that electricity around, producers use sugars to move energy around. All living things then need to free up that energy and use it, to do things like move around, eat, drink, and reproduce. If there’s oxygen available, then a living thing will do respiration:
Respiration: 6O2 + C6H12O6 → 6CO2 + 6H2O + energy
In respiration, oxygen is combined with glucose to make carbon dioxide, water and energy. Inside of us, our cells are constantly doing respiration, which is why we need to breathe in oxygen and we then give off carbon dioxide. This is what happens most of the time in organisms’ mitochondria.
Some of the time, however, when oxygen is unavailable, fermentation happens. Most famously, this happens in yeast, bacteria and in our muscle cells. When our muscle cells cannot get oxygen because we are working very hard, then they do fermentation instead. In our muscles, this produces lactic acid which causes a burning sensation: if you “feel the burn”, then your muscles are not getting enough oxygen:
Fermentation: C6H12O6 → C2H5OH + CO2 + energy
In yeast and some bacteria, fermentation takes glucose and produces ethanol (a type of alcohol), carbon dioxide and energy. Different types of fermentation produce bread, beer, cheese, yogurt, sour cream, and wine!