Question

Starch and cellulose both are polymers of glucose that are synthesized by plants. What do plants use starch for? What do they use cellulose for? Why is starch digestible by humans, but cellulose is not digestible?

Short answer

Plants use starch as a primary energy storage molecule, produced during photosynthesis and stored in chloroplasts. They use cellulose for structural support, as it is the primary component in plant cell walls. Starch is digestible by humans due to the presence of amylase enzymes in our bodies that break down alpha-glucose bonds. However, we lack enzymes to break down cellulose's beta-glucose bonds, making it indigestible and acting as dietary fiber for us.

Step-by-step solution

Introduction to starch and cellulose

Starch and cellulose are both complex carbohydrates (polymers) made up of glucose units. Their structural organization is different, which results in varying functions and properties when it comes to their usage in plants and digestibility by humans.

Use of starch in plants

Plants use starch as a primary energy storage molecule. They store starch in structures known as plastids, particularly in chloroplasts - the site of photosynthesis. Starch is produced during photosynthesis when plants convert sunlight, water, and carbon dioxide into glucose molecules. When plants need energy, they break down the stored starch into glucose units, which are then used to fuel cellular respiration and various metabolic processes.

Use of cellulose in plants

Cellulose, on the other hand, serves as a structural component in plants. It is the primary component of plant cell walls, providing strength and rigidity to the cells. Cellulose's function as a structural molecule is critical for the overall support and shape of plants, preventing them from collapsing under their own weight.

Why humans can digest starch but not cellulose

Starch is digestible by humans because our bodies produce enzymes called amylases, which can break down the alpha-glucose bonds in starch molecules into simpler sugars that our bodies can absorb and use. Amylase enzyme is found in our saliva and the pancreas, facilitating the digestion of starch in the mouth and the small intestine, respectively. In contrast, cellulose is composed of beta-glucose bonds that our bodies lack the necessary enzymes to break down. Cellulose cannot be cleaved by human amylases, making it indigestible for us. However, some microorganisms present in the digestive tracts of herbivores can break down cellulose, enabling these animals to derive energy from plant materials that are inaccessible to humans. This is the reason why cellulose acts as a dietary fiber or roughage for humans, which is not digested and helps in maintaining a healthy digestive system.

Key concepts

Polymers of Glucose

Glucose polymers are long chains of glucose molecules bonded together, forming complex carbohydrates. Two of the most significant glucose polymers are starch and cellulose, both synthesized by plants. These polymers are constructed from hundreds to thousands of glucose units, and their arrangements determine their functions.
Starch has a branched structure composed of alpha-glucose units. This structure allows it to be compact and easily broken down by organisms that consume plants. On the other hand, cellulose is structured with beta-glucose units, forming linear and rigid fibers. This arrangement gives plants their structural strength, making cellulose harder to digest for many organisms, including humans.
The distinction between these glucose polymers is not just in structure but in function. Understanding the arrangement of glucose units in starch and cellulose is crucial for exploring why some organisms can digest these polymers while others cannot.

Energy Storage in Plants

Plants cleverly use starch as a form of stored energy, essentially serving as the plant's energy bank. During photosynthesis, plants convert sunlight into chemical energy, producing glucose as a straightforward sugar. They link these glucose molecules together into starch for long-term energy storage.
Starch accumulates mainly in chloroplasts and amyloplasts, where it is stored until the plant needs it. These stored glucose units can be broken down when the plant requires energy, such as during the night or when sunlight is scarce.

• Starch acts as a reserve; it is easily accessible and can be mobilized quickly.
• It efficiently stores glucose without impacting the osmotic balance within plant cells.
• Starch's breakdown provides a continual supply of glucose for various cellular processes.

Plants strategically use starch to ensure they have a reliable energy source during times when photosynthesis doesn't occur, exemplifying an essential survival mechanism.

Digestive Enzymes

Humans rely on digestive enzymes to break down food into smaller substances that can be absorbed. A crucial enzyme in carbohydrate digestion is amylase, which specifically targets the alpha-glucose bonds found in starch.
Amylase is produced in the salivary glands and the pancreas, serving different roles in digestion:

• Salivary amylase starts breaking down starch during chewing, beginning the digestive process in the mouth.
• Pancreatic amylase continues this process in the small intestine, breaking starch into small sugars like maltose.

In contrast, cellulose, with its beta-glucose bonds, is resistant to human digestive enzymes. Our bodies lack the enzyme necessary to break these bonds, which is why cellulose passes through the human digestive system mostly intact. Instead of being a source of energy, cellulose provides a valuable dietary fiber, assisting in maintaining digestive health by adding bulk to the stool and promoting regular bowel movements.