Skip to main content
Biology LibreTexts

13.2: Introduction

  • Page ID
  • Part 1: Carbohydrates in Food

    When you eat food, you are acquiring nutrients that you need to maintain homeostasis--to keep your internal conditions regulated to the specific parameters required for your survival and function. From these foods, you might get vitamins that your body cannot synthesize itself, amino acids needed to build proteins, and, in anything that is food in the scientific sense, carbohydrates that you can break down to use as energy.

    Carbohydrates can be simple sugars--monosaccharides like glucose--to complex starches, often made of long chains of those same monosaccharides bonded together to form a much larger molecule called a polysaccharide. Some of these polysaccharides have bonds that our bodies have enzymes to break down, such as Amylose. Amylose is a starch composed of glucose monomers attached together by a glycosidic bond, forming a relatively straight line. In your mouth, you produce an enzyme called amylase that breaks the glycosidic bond, eventually turning the long chain of amylose into many molecules of glucose. Amylase functions best at pH 6.7-7.0 [connect to maintenance of homeostasis]. Other polysaccharides are bonded together in ways that we do not have enzymes for, like cellulose, the primary component of plant cell walls. This is why plants contain “dietary fiber,” which passes right through your system and “keeps you regular.”

    Part 2: Converting Carbohydrates into Usable Energy

    A molecule of glucose stores about 3,000 kJ of chemical energy. Chemical energy is the potential energy stored in the bonds that hold atoms within a molecule together. Everytime you break one of these bonds, that energy is released. Molecules of glucose are relatively stable, so this energy is safely stored inside the molecule. Because of this, we can transport glucose, dissolved in our blood, to areas in our body where it is needed or to our liver, where it can be stored for later in the form of glycogen. Plants can do the same through phloem cells, transporting glucose to areas of the plant where it can be stored long term as starch, such as the roots.

    Because the glucose molecule is stable, the energy is trapped inside it. To access this energy, the glucose molecule must be broken apart. You can make this happen by applying large amounts of energy, such as heat from a fire--this is what happens when you burn wood or paper. These materials are plant-based, composed primarily of plant cell walls containing large amounts of cellulose. As we saw above, this cellulose is composed of long chains of glucose molecules. The wood itself doesn’t spontaneously catch fire, but when you apply enough energy, such as by igniting a small portion of it with a lighter or a match, you can cause some of those molecules to break apart. This releases energy in the form of light and heat, which can then provide the energy to break more molecules apart. This process is called combustion.

    What was required to make this reaction happen? Carbohydrates, oxygen, and an initial energy input. A similar process happens inside most eukaryotic organisms. However, to avoid catching on fire, we break down the glucose molecule piece by piece, releasing small amounts of energy at a time, and using that energy to build molecules of something called ATP (adenosine triphosphate).

    Unlike glucose, ATP is an unstable molecule. It has three negatively charged phosphate groups, all attached end-to-end. The negative charges on these phosphate groups repel each other, much like the two negative sides of different magnets. As the number of negative charges increases, the repulsion increases exponentially. Because of this, the third phosphate group holds a large amount of energy in its bond to the second. This phosphate group can be “donated” to a reaction (called phosphorylation), breaking the bond and releasing the stored chemical energy to power the reaction. The ATP loses its third phosphate group and becomes ADP (adenosine diphosphate), a relatively low-energy molecule. Our cells use ATP to power most of the work done in our bodies.

    Key Point

    While heterotrophic organisms like animals and fungi must consume other organisms to obtain carbohydrates, plants and other autotrophs synthesize their own carbohydrates. However, just like heterotrophs, autotrophic organisms still need to do cellular respiration to access the energy stored in those carbohydrates.

    Contributors and Attributions