Think about your last meal. What did you eat? Chicken, beef, bacon, beans, fish, pork, eggs, or cheese? Then you had plenty of protein. Did you eat french fries, chips, corn, noodles, pancakes, bagels, or cereal? Then you probably got plenty of carbohydrates. Did you eat cookies, a snack food, donuts, potato chips, or anything that was greasy, oily, or cooked in butter? Then you probably got plenty of fat (lipids).
These three major nutrients are the most important organic molecules, meaning that they all come from living things! Of course, we use all three of these nutrients in order to live. Proteins are used to build muscle, help your immune system, help brain and nerve function, and assist in almost every biological function in your body! The body uses carbohydrates for the energy to do many things, build cell membranes and cell walls, and to store energy as fat. Aside from being stored energy, fats (lipids) are used by the body to make hormones. Lipids contain much more energy than carbohydrates, which are used immediately by your body. Since carbohydrates are broken down into sugars, this is why people sometimes feel a “sugar high” that ends very suddenly!
It’s important to get a balance of all of these nutrients in your body because they are necessary for growth, homeostasis, reproduction and movement: all characteristics necessary for life. Since all living things use these nutrients, over and over again, how is it that we don’t run out? Where does more protein come from? How do we get carbohydrates and lipids, anyway?
We get proteins from amino acids: remember how the ribosomes put amino acids together to form proteins? Those amino acids, eventually, come from plants. Those plants make amino acids from nitrogen compounds, which they get from bacteria. And the bacteria? The bacteria get nitrogen from – the air! This is the nitrogen cycle.
The main ingredient in urine is urea, which has nitrogen in it. In other words, after we use the nitrogen, we pee it back out! That nitrogen is again used by bacteria and is often put back into the atmosphere. Other times, it is used again by plants, where we eat the nitrogen again and turn it into proteins.
Speaking of urine, it is important to understand how water cycles through the ecosystem. All of us drink water all of the time, but how is it that rivers and lakes continue to flow, even though we are using it? After we use the water, it gets filtered through the ground and returns to rivers, lakes and oceans. We can use the water from Lake Erie (that comes through our pipes), but sometimes that water evaporates – and then falls back down to the ground as precipitation. More importantly, salty water from the ocean evaporates and falls back down to the ground as fresh water that living things can use more easily.
The last major cycle is the carbon cycle. Carbon is the most important part of carbohydrates and lipids, which are critical to your survival. Carbon dioxide is taken up by plants to make stems, roots, fruit, seeds and leaves, which are then eaten by consumers. Consumers, using up those carbohydrates and lipids, give off carbon in the form of carbon dioxide. This cycle works just fine, as long as it’s only between plants and consumers. Unfortunately, humans have started to burn all sorts of fossil fuels which contain fossilized carbon. Once that carbon dioxide is released into the atmosphere, it adds more carbon dioxide than can be taken up by plants! This is how global warming becomes such a big problem and why we need to stop burning so many fossil fuels.