All living things use energy. Anyone who has taken the first day of a biology class knows this. In fact, some scientists see life itself as just a transfer of energy with some temporary reallocation to form large chemical structures.
In the beginning, organisms absorbed energy from deep sea vents or from photons from the sun, the original energy source for our planet. We call these organisms autotrophs. Over time, some organisms discovered a new niche: consuming other organisms to steal the energy stored in their chemical structure. Organisms that eat other organisms are called heterotrophs. We’re going to focus on this group because it’s our group.
Gathering energy is important. To illustrate just how important it is, we’ll look back to when you were just starting to divide from a single cell in your mother’s womb. You started out by dividing outward, forming a larger ball of cells that eventually rearranges to form a cavity in the middle. One side of the hollow ball folds inward to form a dimple. That dimple will become your anus. The dimple deepens through the hollow ball to form a tube. The exit hole of that tube will become your mouth. Your digestive system is so important that it is the very first part of you to form!
Over time, that tube will become much longer and fold over itself many times to form your stomach and intestines. You are essentially a tube for food to go in and waste to come out. Everything else has developed as a way to keep you alive for long enough to eat again.
Let’s follow the journey of a single piece of food as it travels through our digestive tract. Food enters through your mouth where enzymes in the saliva and the dense calcium of your teeth begin to break down the food’s chemical structure. Not all organisms use calcium in their teeth; some use their teeth to dig through rock or eat solid coral. Those organisms could bite straight through your bones, which are also made of calcium since teeth are bones.
From the mouth, the food is swallowed down the gullet and it travels down the esophagus. Much like your pulsating blood vessels, the esophagus also pulses to push food downward toward the stomach. The stomach is a section of the tube that has expanded to collect eaten food and has large folds to allow for more possible expansion. Within the stomach are cells that secrete hydrochloric acid, a compound with a low pH making it very reactive to substances that interact with it. Your stomach does contract a bit to condense the food, but the acid does most of the chemical breakdown.
Eating is an incredibly vital function which means eating the wrong thing could be deadly. When we detect foul food, the muscles of the stomach and esophagus are triggered to send the food back up. You know this as vomiting and it’s such a basic function that your body often tries it when you don’t feel good but don’t know why. People mostly vomit from being sick or ingesting poison. Expelling the contents of the stomach is a good way to fix both those problems.
Digested food leaves the stomach through a small opening that allows little bursts of material out at a time. A compound with a higher pH, called bile, is then introduced from storage in the gallbladder or directly from the liver where it is produced. The difference in pH helps to neutralize the acid leaving from the stomach. The neutralized food now traverses the 6 meter (almost 20 ft) length of your small intestine where proteins and sugars are absorbed into the transport system.
Lipids cannot directly pass into the bloodstream but must be reorganized into groups of three, called triglycerides, and coated with a protein before they can pass through the epithelium. Lipids will enter the bloodstream after passing through the lymphatic system.
Other parts of our food cannot be digested, like fiber. We cannot break down fiber but the bacteria living in our intestine can. These bacteria symbiotically break down fiber and release methane gas as a byproduct. When enough gas builds up, it has to release the pressure in the form of a fart.
The small intestine absorbs nutrients from the food passing through it and passes the rest onto the large intestine. The large intestine has adapted to reabsorb water from the passing material and compact the rest into solid waste, which is then removed through the anus. You’ve heard the phrase “everybody poops” before, but now you know how and why.
Life is nothing if not resourceful; wasting energy is a trait that would, understandably, be wiped from the gene pool very early on. When you eat, you gain energy that your cells need to function properly. If you eat more than you need to properly function, the extra nutrients are stored in larger compounds to be used for later. You know of this process as storing body fat, and it’s an advantageous adaptation for when our ancestors did not know the next time they would eat.
Your communication system also plays a role in digestion as it sends signals for the process to begin. You have neurons that have specifically adapted to signal when nutrient levels are low: you know this feeling as hunger. Similarly, thirst indicates you that you’re lacking water. When you’re hungry, neurons will signal your stomach to begin producing acid and churning in preparation of food. This why your stomach sometimes hurts or growls.
That hunger signal can also be synapsed to an internal clock if you stick to a schedule. If you eat lunch at noon every day, your body will start preparing for food to arrive at noon every day. However, if enough time passes while hungry, you will become habituated to it and might even forget that you’re hungry in the first place. This is an advantageous adaptation because focusing on your hunger means you are distracted from something else, like searching for more food.
Managing energy doesn’t just involve gaining energy but also resting to conserve the energy you’ve already gained. The primary method of resting is sleep and it conserves energy by literally turning off your conscious access to your muscles and sensory organs. Sleep was also just plain opportunistic since hunting at night was difficult. Our ancestors might as well conserve their energy for when they could see better.
The most complex organisms developed a way to maintain a specific internal temperature to best suit the processes of the organism. We call this development homeostasis and it’s present in most mammals and some birds. Homeostasis is life’s way of using energy to keep a consistent environment within the organism.
You probably already know about our ways of keeping the same temperature using negative feedback loops. When you get too hot, you sweat. The energy can then leave your body as it is transferred to break the liquid bonds and evaporate your sweat. When you get too cold, you shiver. The vibrations of your muscles causes friction which transfers the kinetic energy into heat energy. Homeostasis refers to much more than temperature, but those are the most accessible examples.
You are the cream of the crop, the pinnacle of evolutionary development. We got to this point because our ancestors were able to efficiently allocate their resources to maintaining the best internal environment. Our ancestors survived the great extinctions that wiped out the reptiles, who ruled the planet before us, because of their ability to manage their energy.
It’s easy to forget your adaptations living in a society where food is assured and the temperature of your environment can be set with a flick of a dial. You are alive because you can gather and manage energy and what you put into your body plays a direct role in how you function. It took millions of years to develop your adaptations, remember not to take them for granted.