Question

examine the types of polysaccharides shown below. cellulose is a structural carbohydrate used by plants but is not able to be broken down by animals, compared to starch (plant molecule) and glycogen (animal molecule). why can some carbohydrates be utilized by animals for energy and not others? multiple choice 1 point
(a) molecular structure (linear (cellulose)): parallel chains form intermolecular hydrogen bonds, resulting in tight folds.
(b) branched (starch): branching limits the number of hydrogen bonds that can form in starch molecules, making starch less compact than cellulose.
(c) highly branched (glycogen): the high amount of branching in glycogen makes its bonds more compact than starch.
(d) polysaccharides in cells: layers of cells as seen in the stained electron micrograph. one label cell with first starlight. within these potato cells, starch droplets have a granular shape. the dark circles in this electron micrograph are glycogen deposits in a monkey liver cell.

Explanation:

To solve this, we analyze the structural and functional differences of polysaccharides:

Step 1: Recall Polysaccharide Structures

• Cellulose: Linear chains with strong hydrogen bonds between parallel strands, forming rigid structures (e.g., plant cell walls).
• Starch (amylose/amylopectin): Amylose is linear; amylopectin is branched. Less compact than cellulose.
• Glycogen: Highly branched, very compact (energy storage in animals).

Step 2: Enzyme Specificity

Animals have enzymes (e.g., amylase) to break down starch (α - 1,4 and α - 1,6 glycosidic bonds) and glycogen (α - 1,4 and α - 1,6 bonds). However, animals lack the enzyme (cellulase) to break the β - 1,4 glycosidic bonds in cellulose.

Step 3: Relate Structure to Function

• Cellulose: Rigid, structural (plants) → animals can’t digest it (no cellulase).
• Starch/Glycogen: Less rigid, branched (energy storage) → animals can digest them (have enzymes for their bonds).

The key reason is enzyme specificity: animals lack the enzyme to break cellulose’s β - 1,4 bonds, but have enzymes for starch/glycogen’s α - bonds.

Answer:

Animals lack the enzyme (cellulase) to break the β - 1,4 glycosidic bonds in cellulose (structural polysaccharide), but have enzymes (e.g., amylase) to break the α - 1,4/α - 1,6 bonds in starch (plant energy) and glycogen (animal energy), allowing their use for energy.