Table of Contents
Overview
Polysaccharides are carbohydrates made from many monosaccharide units linked together. While monosaccharides and disaccharides are often soluble and sweet, polysaccharides are typically large, less soluble, and used mainly for storage or structural purposes.
In this chapter, we focus on:
- How polysaccharides are built
- Important storage polysaccharides (starch, glycogen)
- Important structural polysaccharides (cellulose, chitin)
- How structure relates to function
Basic carbohydrate concepts and the idea of glycosidic bonds are assumed from previous sections.
General Features of Polysaccharides
Degree of polymerization
Polysaccharides consist of many monosaccharide units (often hundreds to tens of thousands). The number of units is called the degree of polymerization (DP). There is no strict boundary, but:
- Oligosaccharides: usually up to about 10–20 sugar units
- Polysaccharides: more than about 20 units (often far more)
Homopolysaccharides vs. heteropolysaccharides
- Homopolysaccharides (homoglycans):
Built from just one kind of monosaccharide. - Example: starch, glycogen, cellulose (all mainly glucose)
- Heteropolysaccharides (heteroglycans):
Built from two or more different monosaccharides. - Example: many extracellular matrix polysaccharides in animals (e.g., hyaluronic acid)
In this beginner chapter, we focus mainly on common homopolysaccharides.
Linear vs. branched
- Linear polysaccharides:
Monosaccharides form one long, unbranched chain. - Branched polysaccharides:
Side chains branch off the main chain at regular or irregular intervals.
Branching strongly affects:
- Solubility
- How quickly enzymes can break the polymer down
- How compactly energy can be stored
Storage Polysaccharides
Storage polysaccharides are energy reserves: organisms build them when energy is plentiful and break them down when energy is needed.
Starch – storage polysaccharide in plants
Starch is the main carbohydrate reserve in plants. It is stored in plastids (often amyloplasts) as starch grains, especially in seeds, tubers, and roots.
Chemically, starch is a mixture of two glucose polymers:
Amylose
- Structure:
- Mostly linear chain of D-glucose units
- Linked by $$\alpha(1 \rightarrow 4)$$ glycosidic bonds
- Shape:
- Tends to form a helical (coil-like) structure in water
- Properties:
- Less branched = fewer ends for enzymes = digested more slowly
- Contributes to forming gels (thickening agents in cooking)
Amylopectin
- Structure:
- Main chain of glucose units with $$\alpha(1 \rightarrow 4)$$ bonds
- Branches via $$\alpha(1 \rightarrow 6)$$ bonds
- Branching points approximately every 24–30 glucose units (typical value; varies)
- Shape:
- Highly branched, tree-like molecule
- Properties:
- More branch ends = more sites where enzymes can act
- Broken down more quickly than pure amylose
Functional consequences
- Plants:
- Can pack large amounts of glucose in an osmotically “cheap” form (few large molecules instead of many small ones)
- Animals (including humans):
- Digest starch with enzymes like amylase
- Starch is a major source of dietary energy from plant foods (e.g., potatoes, grains, rice, corn)
Glycogen – storage polysaccharide in animals and fungi
Glycogen is the main carbohydrate storage form in animals and many fungi.
Structure
- Built from D-glucose units
- Main chain: $$\alpha(1 \rightarrow 4)$$ bonds
- Branches: $$\alpha(1 \rightarrow 6)$$ bonds
- Even more highly branched than amylopectin:
- Branch point roughly every 8–12 glucose units (much more frequent)
Location and role in animals
- Stored as granules in the cytoplasm, especially in:
- Liver cells: glycogen for maintaining blood glucose levels
- Muscle cells: glycogen for local energy supply during muscle work
- Frequent branching:
- Provides many chain ends for rapid addition or removal of glucose units
- Allows quick mobilization of energy when needed (e.g., exercise, stress)
Comparison: starch vs. glycogen
- Both:
- Made of glucose
- Use $$\alpha(1 \rightarrow 4)$$ and $$\alpha(1 \rightarrow 6)$$ bonds
- Serve as energy stores
- Differences:
- Glycogen is more highly branched than amylopectin
- Glycogen is the main energy reserve in animals; starch in plants
- Glycogen must be mobilized rapidly; plant starch often can be mobilized more slowly
Structural Polysaccharides
Structural polysaccharides provide rigidity and shape rather than serving primarily as energy reserves. Their structures make them tough and resistant to most enzymes.
Cellulose – structural polysaccharide in plants
Cellulose is the most abundant organic polymer on Earth. It is the main component of plant cell walls and provides mechanical strength.
Structure
- Long, unbranched chains of D-glucose
- Linked by $$\beta(1 \rightarrow 4)$$ glycosidic bonds
- The $$\beta$$ configuration forces each glucose to be rotated relative to its neighbor, so chains are straight and can lie parallel.
Multiple cellulose chains align side by side and form:
- Hydrogen bonds between chains
- Bundles called microfibrils
- Microfibrils group further into larger fibrils, giving high tensile strength
Properties and digestion
- Very insoluble, forms strong fibers
- Most animals lack enzymes (cellulases) that cleave $$\beta(1 \rightarrow 4)$$ bonds in cellulose
- Some herbivores (e.g., cows, termites) rely on symbiotic microorganisms in their gut that make cellulases to digest cellulose partially.
Functional significance
- Plants:
- Cell walls resist turgor pressure (internal water pressure)
- Provide structural support, allowing plants to stand upright without a skeleton
- Humans:
- Important as dietary fiber, supporting healthy digestion, even though not a major energy source
Chitin – structural polysaccharide in fungi and animals
Chitin is another important structural polysaccharide, similar to cellulose but with modified sugar units.
Structure
- Polymer of N-acetyl-D-glucosamine (a glucose derivative with an acetylamino group)
- Linked by $$\beta(1 \rightarrow 4)$$ glycosidic bonds
- Forms long, linear chains that associate via hydrogen bonds
- Often embedded in proteins or mineralized, increasing hardness
Where chitin is found
- Arthropod exoskeletons:
- Insects, spiders, crustaceans (e.g., crabs, shrimps)
- Exoskeleton is a composite of chitin and proteins (sometimes with minerals)
- Fungal cell walls:
- Major structural component, instead of cellulose
Properties
- Tough and flexible
- Resistant to many enzymes and chemicals
- Can be partially deacetylated to form chitosan, which is more soluble and used in various applications (e.g., biomedical, water purification)
Other Biologically Important Polysaccharides
Beyond the big, familiar examples, many other polysaccharides play important roles.
Plant storage and structural variants
- Inulin:
- Storage polysaccharide in some plants (e.g., chicory, Jerusalem artichoke)
- Made largely from fructose units
- Used in some foods as a fiber and low-calorie sweetener component
- Hemicelluloses:
- Group of plant cell wall polysaccharides (e.g., xylans, mannans)
- Often branched and built from several different monosaccharides
- Fill spaces between cellulose microfibrils, contributing to wall structure and flexibility
Polysaccharides in animal extracellular matrix
- Many are heteropolysaccharides (discussed in more detail elsewhere), often combined with proteins to form proteoglycans.
- Example: hyaluronic acid
- Long, unbranched polymer of repeating disaccharides (glucuronic acid and N-acetylglucosamine)
- Important in joint fluid, skin, and connective tissues
- Binds water, contributing to lubrication and shock absorption
Structure–Function Relationships in Polysaccharides
The key idea for polysaccharides is that small differences in structure create large differences in function:
- Type of bond (α vs. β):
- $$\alpha(1 \rightarrow 4)$$ (starch, glycogen): helices, easily broken down by common enzymes → good for energy storage
- $$\beta(1 \rightarrow 4)$$ (cellulose, chitin): straight chains, form strong fibers, resistant to digestion → good for structure
- Degree of branching:
- Highly branched (glycogen, amylopectin): many ends for rapid mobilization of glucose
- Unbranched (amylose, cellulose): fewer ends; cellulose specifically forms strong fibers
- Chemical modifications of the sugar units:
- N-acetyl groups (chitin) or acidic groups (hyaluronic acid) change:
- Interactions with water
- Interactions with proteins and ions
- Mechanical properties (hardness, elasticity, gel formation)
Understanding these relationships helps explain why different organisms use different polysaccharides for storage or structure and how these molecules contribute to life’s diversity at the molecular level.