Food is fuel for the body, meaning we eat food for energy. But what is energy? Even more importantly, how do we move energy around the body to get work done?

Why this is important…Any work done in the body requires energy, which comes from the food we eat. We need to understand how energy gets transferred so we can understand how food fuels the body’s work.

Foundations

Energy is what powers us through every day. No organism can live without it. Plants get energy from sunlight and animals get theirs from food.

Energy is the ability to do work. That may seem like a really vague definition, but energy is surprisingly difficult to define. It is clearly part of the natural world, but it is not a physical object.

Energy is stored in chemical bonds of molecules. When bonds are broken, energy is released. Therefore, chemical reactions power our lives. Scientists have described basic rules to how energy moves around.

Key terms

Energy – the ability to do work

calorie – one way to measure energy content based on the heat released when it is broken down. Important note: food calories in the US are reported in “Calories,” with a capital C, which equal 1000 calories (small c) or one kilocalorie.

ATP – energy-carrying molecule in cells that temporarily stores energy and then transfers it to structures in the cell to do work; the acronym stands for Adenosine Tri Phosphate

Metabolism – the entire collection of chemical reactions that take place in the body; often, this emphasizes the transfer of energy (verb: metabolize)

Anabolic (adj.) – reactions that construct larger molecules from smaller ones

Catabolic (adj.) – reactions that break larger molecules into smaller ones

Endergonic (adj.)– reactions that require energy input

Exergonic (adj.) - reactions that release energy

Details

Energy is measured in calories. At least, that is the way we measure the amount of energy in food. You may also know of other measures of energy like the watt, joule or horsepower.

Food energy must be broken up and distributed to use on a cellular scale. We cannot feed a cell a peanut butter sandwich. Not only is it is too large, but it contains about 300,000 calories (don’t worry, that’s still just 300 Calories) – far too much for a single cell to use. Each cell only needs a fraction of a calorie to operate, but we have about 30 trillion cells so we need to break up food to distribute to each of them.

How are food calories divided up so small? The biggest change comes from digestion, which breaks the food into molecule-sized pieces, like sugars, amino acids or fatty acids. These molecules can be carried to the cell through the bloodstream. Within the cell, these molecules are broken apart further and the energy is transferred to many ATP molecules. Each ATP holds a little of the energy (0.0000000000012 calories) from the high-energy glucose or fatty acid molecule.

There are formal rules that describe how energy can be moved around – the Laws of Thermodynamics. These rules apply to all energy and matter. For living systems, we only need two:

First law of thermodynamics - energy cannot be created or destroyed, but it can be transferred from one system (or object) to another.
Second law of thermodynamics - when energy is transferred, some of the useful energy is wasted

VIDEO 5a: Energy

After watching the video, you should be able to:

describe what energy is and the forms it takes
describe the laws of thermodynamics and give examples
apply these concepts to chemical reactions and determine which are endergonic and exergonic
construct or interpret an energy graph for a chemical reaction

The main message from these laws is this: we can move energy from one molecule to another, but some energy is wasted each time. The wasted energy in living systems is usually produces heat, which cannot be used to do work. There can be other forms of wasted energy (like scattered molecules) but usually energy is lost as heat. Since our muscles and nerves use lots and lots of energy, that means they also generate a lot of heat, which keeps us warm. Figure 5-2 illustrates other examples of energy transfer.

Stop & Think #1: Use the laws of thermodynamics to explain why we get warmer when we exercise.

Two illustrations of energy transfer. 1. wheat plants capture light energy, which is stored in carbohydrates (bread) which powers muscle movement (jumping). 2. Wind turbines capture mechanical wind energy and convert it to electricity (power lines) which lights a light bulb.

FIGURE 5-2. Laws of Thermodynamics. Biological and physical examples of energy transfer. ("Wheat field" by Maxdrobot is in the Public Domain || “Bread” by J. Buegge is in the Public Domain || "Testing the limit of jumping" by Killy95 is licensed under CC BY-SA 4.0 || "Wind farm on Columbia Hills" by Walter Siegmund is licensed under CC BY-SA 3.0 || "Power lines and clouds" by Neotigen is licensed under CC BY-SA 4.0 || "Light bulb" by Ch Maheswara Raju is licensed under CC BY-SA 4.0)

If energy cannot be created, then where does our energy come from? The answer is the sun! Almost all life on earth is energized by the sun. Some organisms –plants, algae and some bacteria – are able to capture light energy and transfer that into biomolecules like sugars. These become food for other organisms.

Stop & Think #2: Narrate or draw the path that energy takes to get from the sun to your cells.

Chemical reactions store and release energy from molecules. In a simplified view, we can think of cells as miniature factories that perform thousands of chemical reactions. Each chemical reaction rearranges molecules by breaking and reforming chemical bonds. Those chemical bonds store energy. Since chemical bonds are formed by electron sharing, it is the movement of electrons that moves energy around the body. (Read this paragraph again – these concepts are very important to understand fully.)

Chemical reactions are usually written as an equation, with the inputs (reactants) shown to the left of the arrow and outputs (products) shown to the right.

Photosynthesis: 6 CO₂+ 6 H₂O ⟶ C₆H₁₂O₆+ 6 O₂

Cellular respiration: C₆H₁₂O₆+ 6 O₂ ⟶ 6 CO₂+ 6 H₂O

The summary equations above accurately summarize molecular inputs and outputs, both oversimplify the processes. Neither of these happen in just one step (as implied by the arrow) and nothing is mentioned about the transfer of energy. Chemical equations are just a starting point for understanding important processes.

In living systems, chemical reactions fall into two main categories – anabolic or catabolic. Each has its own purpose. The digestive system performs lots of catabolic reactions – larger molecules are broken down into smaller ones. At the same time, it performs some anabolic reactions, where smaller molecules are assembled into larger ones, such as when glands construct enzymes. Table 5-1 summarizes more about these two broad categories of chemical reactions.

TABLE 5-1. Anabolic and catabolic reactions. Summary of their characteristics in living systems.

Anabolic	Catabolic
Assemble larger molecules from smaller molecules	Break larger molecules into smaller molecules
Require energy input (endergonic)	Release energy (exergonic)
Dehydration synthesis is one example of this type of reaction	Hydrolysis is one example of this type of reaction
Other examples: photosynthesis, growing wood, building muscle, growing hair	Other examples: digestion, cellular respiration, burning wood

Stop & Think #3: Think of other examples of chemical reactions you experience in life. Classify each as anabolic or catabolic.

Thousands of chemical reactions work together to support life. This network of reactions is referred to as metabolism. Very often, one chemical reaction supports another. For example, energy released from cellular respiration reactions is used to power other chemical reactions that require energy, like moving chromosomes around a cell.

Bottom line: we need a continual input of energy just to carry out life’s basic functions.

Learning objectives

Explain the fundamental concepts of energy transfer as it relates to biological processes.
Apply the laws of thermodynamics to real examples.
Classify chemical reactions as anabolic or catabolic (or endergonic or exergonic) based on the inputs and outputs.

Stop & Think Questions Explained

Exercise requires us to use more energy, generating more ATP. With more chemical reactions happening, more heat is given off.
Sunlight energy is stored in sugars by plants (by photosynthesis). We eat the plants, extracting sugars and transporting them to our cells. Our cells break down sugars, releasing energy and transferring it into ATP.
Catabolic reactions break things down and release energy, such as, burning gasoline in a car, rotting food, or burning a candle. Anabolic reactions build larger things and require energy, such as, baking a cake, putting together a puzzle, or gaining weight.