Sugar In Trees

Nov 10, 2020

In this month's installment of Field Notes Scott Bowe of Kemp Station discusses sugar in trees, how Mother Nature makes trees and plant with simple sugars.

When I mention sugar in trees, you might think about the sugar maple and the sweet syrup we produce from its sap every spring.  But what you may not realize is that sugar is the fundamental building block of all trees and plants. Too much sugar in your diet is a bad thing.  Sugar in a tree is a good thing and is the foundation of life on earth.

Let’s start out by grouping all life on earth into two categories.  There are producers and there are consumers.  Trees and other plants are producers since they can make their own food using the sun through the process of photosynthesis.  People and other animals are consumers since the depend upon plants or other animals for food.  I’ve been out in the sun all day long and gotten very hungry.  As a consumer, I can’t make my own food using the sun.

So how do trees use the sun to make food?  They take light energy from the sun and convert it into chemical energy in the form of carbohydrates and sugars.  The process begins with water gathered by the tree’s root system, carbon dioxide from the air, and light energy from the sun.  Green chlorophyll pigments found mainly in the tree’s leaves absorb energy from the sun, then use this energy to break apart and recombine the basic building blocks of water and carbon dioxide.  Six carbon dioxide molecules and six water molecules are used to produce one sugar molecule, which is called glucose, and six oxygen gas molecules.  For animals like us, glucose is the most important source of energy for cellular respiration and the by-product, oxygen, is very important for all life on earth.

Let’s focus on the glucose sugar molecule – If the tree uses it as its primary building block, why doesn’t a branch taste sweet if we lick it? Because the tree uses more chemistry to recombine the glucose into other building blocks.  Imagine if wood tasted sweet, that the glucose was readily available for us to consume, then popsicle sticks would taste as sweet as the popsicle.  You would hurry up and eat the popsicle so that you could get to the tasty stick inside!  Trees recombine glucose into another carbohydrate called cellulose.  Cellulose is a long polymer of glucose molecules linked together.  First the glucose molecules link together to form pairs, then these pairs link together to form chains more than 10,000 pairs long.  These long cellulose polymers combine with other polymers and lignin to form the cell walls, the actual wood itself, in the tree.

If the wood in a tree is made from cellulose, and cellulose is made from glucose, then why can’t we eat wood for our energy needs?  The simple answer is that humans and most animals cannot digest cellulose.  The glucose molecules that make up cellulose chemically bond together, then these long strands of cellulose chemically bond as parallel strands.  These cross-linked structure makes the glucose very difficult to remove and use for energy.  If we eat wood, it will pass through our system undigested.  That is the purpose of fiber in our diets, it will keep you regular, but it does not provide any food value since it can’t be digested.

There are a few types organisms, including fungi, certain bacteria, and certain protozoa that can digest wood.  These organisms have a special enzyme called cellulase that can break cellulose back into individual glucose molecules. Even termites, which are notorious consumers of wood in our homes in the south, depend upon a symbiotic relationship with bacteria and protozoa to digest cellulose.  In ruminant animals like cows and sheep, cellulase is produced by symbiotic bacteria, which break down the cellulose in the hay and plants that they consume.

We certainly don’t like organisms like fungi when they cause decay in our backyard deck or roof, but they are absolutely necessary.  Without consumers like fungi, there would be no mechanism to recycle fallen leaves and woody debris back into the ecosystem.  Our forests would be choked with wood and leaves. A friend and colleague at the University of Wisconsin has pointed out that an ecosystem only needs producers like plants and consumers like fungi to function.  People and other animals exist in the middle.

Trees uses some amazing chemistry.  With a little help from the sun, they make their own food, a simple sugar called glucose.  Then they combine the glucose in such a way that very few organism can digest it. What an amazing strategy for survival. Wisconsin’s native trees can grow and live for many decades and the products that we produce from those trees can be used for many decades more.  An amazing material indeed.