Starch vs. Glycogen: Structure & Function

In this lesson, we’ll discuss starch and glycogen, their relationship to glucose, and how they are used by both plants and animals differently to survive. Then, take the quiz to test what you learned.

Just a Bit on Carbohydrates

If I say the word ‘starch’ to you, what do you think of? Most people probably think of food like a potato. Some might think of laundry, like a starched shirt. How about ‘glycogen?’ Does that word stir up any images? My guess is not as many as starch (if any at all). Well, in this lesson, we will hopefully add to your visual library as we explore the structure and function of both starch and glycogen.

It’s worthwhile to take a moment and discuss carbohydrates as a group. The term carbohydrate comes from ‘carbon’ and ‘water’ (as in to hydrate you). The main function of carbohydrates in animals is to provide energy for cellular work. Plants are a little more complicated – they also use carbohydrates for building structures and storage.

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Glucose is the simplest carbohydrate. It a single glucose molecule, also called a monosaccharide (‘mono-‘ means ‘single’; ‘-saccharide’ means ‘sugar’). All carbohydrates are made from attaching glucose molecules together through a chemical reaction called dehydration synthesis (sometimes referred to as condensation). Dehydration synthesis works by removing a water molecule (two hydrogens and one oxygen) from two monosaccharides, thus forming a bond between them.

Starch and glycogen are known as complex carbohydrates and are called polysaccharides (‘poly-‘ means ‘many’). A good analogy for these structures is to think of a train. You have a bunch of train cars all linked together. Each train car would be a monosaccharide and the whole train with all the cars together would be the polysaccharide.

Starch in Plants

Starch is a complex carbohydrate made by plants to store energy. They make energy through photosynthesis, which produces a lot of glucose. Plants use the glucose to do cellular work, but they also take the excess glucose that is not used right away and build starch from it.

Starch has a fairly rigid structure and can actually be molded into a variety of shapes. An application of this can take place at your local laundry or at home. Laundry starch is made from plant material and can be applied to fabrics to ‘stiffen’ them up to look crisp and wrinkle free. This was popular in the early part of the last century, but you still see starched shirts every once in a while.

In temperate climates, plants use the stored starch to survive when sunlight isn’t available for photosynthesis. Kind of like a squirrel who finds some nuts and can either eat them for an immediate pick-me-up, or store them somewhere to use for food during the winter.

In the summer, when sunlight is abundant, plants send excess glucose from the leaves through the stems down to the roots. The glucose is attached through dehydration synthesis and converted into starch within the roots. The plants then slowly break the starch apart a little bit at a time releasing enough glucose so that the cells can survive through the winter.

Starch in Animals

Animals eat starch as a primary source of energy. Carbohydrates provide the bulk of an animal’s energy source to do cellular work. Remember the potato that was mentioned in the introduction? Potatoes are very high in starch and are a great source of complex carbohydrates in animal diets.

Starch is broken down through a process called hydrolysis into glucose for use in the cells. Hydrolysis is the opposite reaction to dehydration and uses water (and enzymes) to break starch apart, thereby freeing up the individual glucose molecules. Animals do not store starch. They use as much of the glucose as they can from the starch they eat but then convert the rest into either glycogen or fat.

Glycogen

Glycogen is a complex carbohydrate found only animals. It has the same function as starch has in plants – it’s stored for later use. Excess starch is converted into glycogen in the liver and the muscles. Structurally, glycogen is similar to starch but is more branched and can contain hundreds of thousands of glucose molecules all linked together through dehydration synthesis.

If an animal exceeds its need for glycogen, any remaining glucose in the body is then converted into fat for long term energy. Glycogen is often released and converted back into glucose during exercise to provide the cells with additional energy.

Lesson Summary

Starch and glycogen are both complex carbohydrates (polysaccharides) built from interconnected glucose molecules that have been attached in a long, branched chain. Both of these molecules are essential for supporting life.

To recap:

  • Plants make starch from excess glucose through dehydration synthesis
  • Animals take in starch by eating plant material
  • The starch is broken down into glucose through hydrolysis
  • Excess glucose is converted to glycogen in the liver or muscles
  • When glycogen needs are met, excess glucose is instead converted to fat
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