What exactly is energy?

What exactly is energy?


DEAR DOCTOR :
We use the word “energy” a lot. But what exactly is it?
DEAR READER:
We work — our hearts beat, our brains think, our muscles move — because our cells work. Our cells work because they can generate energy. They can generate it because the blood brings them fuel they convert into energy. Let’s call this “physical” energy.
Why is energy important? Without it, you wouldn’t be able to breathe, get up out of your chair or read this column. I emphasize particularly that last point.
Where does the fuel for your cells come from? The food you eat is digested and absorbed into the bloodstream, which delivers it to cells throughout the body. A major fuel is the sugar glucose, which is produced by the digestion of the carbohydrates you eat. Oxygen from your lungs is another fuel that travels to your cells in the bloodstream.
Inside your cells, tiny structures called mitochondria convert fuel food nutrients into the chemical adenosine triphosphate (ATP). ATP is the body’s primary source of energy for everything from walking to thinking. (Below, I’ve put an illustration showing how food and oxygen become energy.)
How food and oxygen become energy

The food you eat is digested and absorbed into the bloodstream, which delivers it to cells throughout the body. Oxygen from your lungs also travels to your cells, where tiny structures called mitochondria use it to convert the food nutrients into a chemical called adenosine triphosphate (ATP). ATP provides energy for everything from walking to thinking.
Your body stores only a small amount of ATP, but it makes more as quickly as it’s needed. When demand increases — when you are exercising, for example — your body must churn out more. To do this, it taps into sugar (glucose) stored in muscles and the liver, as well as fats stored around the body.
Your body’s ability to create ATP determines your capacity for physical exertion. The reverse is also true: Your physical fitness influences how well you can make ATP. Insufficient sleep may also reduce your ability to make ATP. So you can see that diet, exercise and sleep all play crucial roles in generating energy.
When we talk about the energy we have, we’re often talking about what I call “mental” energy. Energy is also about your mind and emotions. When you’re mentally energetic, you’re curious and alert — you’re “on.” But mental energy is more complicated than physical energy. In fact, we really don’t know much about it.
On one level, mental energy probably is about energy production in cells — specifically, brain cells. But mental energy also reflects our emotional state and how interested we are in something. When you’re really absorbed in an activity, you feel more energetic than when you are only half-interested in it.
Does this result from inadequate energy in brain cells? We don’t know, and it’s a very hard thing to study — the ability of brain cells to make energy in a living human being who either feels very energetic or who feels he or she lacks energy.
It’s valuable to try to understand what the chemistry of mental energy is inside the cells of our brain. From that research may come more fundamental ways of feeling more