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Understanding the relationship between structure and function of macromolecules
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exploring carbs and lipids and how its structure affects it function
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SBI4U Case StudiesJO
Carbs Case Study
1. Venn Diagram Comparing Starch, Cellulose, and Glycogen
Includes similarities and differences between starch, cellulose, and glycogen, incorporating molecular structure,
metabolic implications, and sources:
● Starch: Plants store excess glucose as starch, found in roots and seeds. It consists of glucose monomers
linked by α-1,4 glycosidic bonds in unbranched chains (amylose) and α-1,6 glycosidic bonds at branch
points (amylopectin). This branched, helical structure allows easy access for digestive enzymes to break
it down, making starch digestible by humans.
● Cellulose: Cellulose, the most abundant natural biopolymer, provides structural support in plant cell
walls. Like starch, it is composed of glucose monomers, but they are joined by β-1,4 glycosidic bonds.
Every other glucose monomer in cellulose is flipped over, forming long, straight chains. These chains
pack tightly, giving cellulose rigidity and high tensile strength, which is crucial for plant cells. However,
the β-1,4 linkage makes cellulose indigestible by humans.
● Glycogen: Animals store excess glucose as glycogen, mainly in the liver and muscle cells. It is a highly
branched molecule composed of glucose monomers linked by α-1,4 glycosidic bonds in the main chains
and α-1,6 glycosidic bonds at the branch points. This extensive branching facilitates the rapid
breakdown of glycogen into glucose when needed for energy. Glycogenolysis is the process of glycogen
breakdown to release glucose.
Key Similarities:
● All three are polysaccharides composed of glucose monomers.
Key Differences:
● Type of glycosidic bonds: Starch and glycogen use α-1,4 and α-1,6 linkages, while cellulose has β-1,4
linkages.
● Branching: Glycogen is highly branched, starch is moderately branched (amylopectin), and cellulose is
unbranched.
● Digestibility: Humans can digest starch and glycogen, but not cellulose.
● Function: Starch and glycogen serve as energy storage molecules in plants and animals, respectively,
while cellulose provides structural support in plants.
2. The Calculation of Food Calories and Dietary Fiber
On food packages, dietary fibre is listed under 'carbohydrate'. However, the potential energy of dietary fiber
is not included when calculating food calories. Here's why:
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● Indigestible by Humans: Dietary fiber, primarily composed of cellulose, is a complex carbohydrate
that the human body cannot digest. This is because humans lack the enzyme cellulase, which is
necessary to break down the β-1,4 glycosidic bonds present in cellulose.
● Lack of Energy Extraction: As a result of this inability to break down cellulose, humans cannot extract
energy from dietary fiber. Although fiber is a glucose polymer, the human body cannot access this
glucose for energy.
● Calories Reflect Usable Energy: Food calories represent the amount of energy the body can obtain
from digesting and metabolising the food. Since dietary fiber is not digested and metabolised in the
same way as other carbohydrates, its potential energy is not factored into calorie calculations.
Therefore, despite being listed under 'carbohydrate', dietary fiber does not contribute to the calorie content listed
on food labels. This is a prime example of how the structure of a molecule directly affects its function. The
specific β-1,4 glycosidic bonds in cellulose make it indigestible by humans, distinguishing it from other
carbohydrates like starch and glycogen, which the body readily uses for energy.
3. Glucose Acquisition and Utilisation in Animal Cells
When animal cells need glucose in between feedings, they obtain it from glycogen stored in the liver and
muscles. This stored glycogen is a readily available energy source that can be broken down into glucose when
needed.
For this glucose to be used by the mitochondria for ATP production, a multi-step process must occur:
1. Glycogenolysis: When blood glucose levels decrease, glycogen stored in the liver and muscles is broken
down into glucose in a process called glycogenolysis.
2. Glycolysis: Once glucose enters the cell, it undergoes glycolysis, a metabolic pathway that breaks down
glucose into two molecules of pyruvate. This process occurs in the cytoplasm and generates a small
amount of ATP.
3. Krebs Cycle (Citric Acid Cycle): The pyruvate molecules are transported into the mitochondria, where
they are converted into Acetyl CoA. Acetyl CoA then enters the Krebs cycle, a series of chemical
reactions that further break down the glucose molecule, producing NADH and FADH2, molecules that
carry high-energy electrons.
4. Oxidative Phosphorylation (Electron Transport Chain): The NADH and FADH2 generated in the
Krebs cycle deliver their electrons to the electron transport chain, located on the inner mitochondrial
membrane. As electrons move down the chain, energy is released and used to pump protons across the
membrane, creating an electrochemical gradient. This gradient drives the synthesis of ATP through a
process called chemiosmosis.
In summary, animal cells bridge the gap between meals by using stored glycogen as a glucose source. This
glucose is then broken down systematically through glycolysis, the Krebs cycle, and oxidative
phosphorylation to generate ATP, the primary energy currency of cells.
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