Table of Contents
Overview of Carbohydrates
Carbohydrates are a large, diverse group of biological molecules built mainly from carbon, hydrogen, and oxygen. They are sometimes called “sugars” or “saccharides,” though not all carbohydrates taste sweet. They serve several major functions:
- Primary energy source (e.g., glucose)
- Short‑ and long‑term energy storage (e.g., glycogen, starch)
- Structural components (e.g., cellulose, chitin)
- Parts of nucleic acids and other important molecules (e.g., ribose in RNA)
- Recognition “labels” on cell surfaces (e.g., in blood groups)
Their general empirical formula for simple sugars is often close to:
$$
\mathrm{C}_n\mathrm{H}_{2n}\mathrm{O}_n
$$
For example, glucose has the formula $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$.
Carbohydrates are traditionally divided into:
- Monosaccharides (“one sugar unit”)
- Disaccharides (“two sugar units”)
- Oligosaccharides (a few sugar units, typically 3–10)
- Polysaccharides (many sugar units, sometimes thousands)
Monosaccharides, disaccharides, and polysaccharides have their own sections, so this chapter concentrates on what unites them and what makes carbohydrates as a class distinctive.
Chemical Features of Carbohydrates
Basic Building Block: The Monosaccharide Unit
All larger carbohydrates are built from monosaccharides. Each monosaccharide:
- Contains several hydroxyl groups (
–OH) - Contains either:
- An aldehyde group (
–CHO) → aldose - Or a ketone group (often
–CO–in the second position) → ketose - Has a carbon backbone, usually 3–7 carbon atoms long
Because of the many –OH groups, carbohydrates are:
- Generally polar
- Usually water‑soluble (at least the smaller ones)
- Reactive in specific ways (e.g., can form rings, form bonds with each other)
Linear and Ring Forms
In aqueous solution, most monosaccharides with 5 or 6 carbons exist mainly as ring structures. The carbonyl group (aldehyde or ketone) reacts with one of the internal –OH groups to form a ring.
- A 6‑membered ring (5 carbons + 1 oxygen) is called a pyranose form.
- A 5‑membered ring (4 carbons + 1 oxygen) is called a furanose form.
The same sugar can exist in different ring forms and in an open‑chain form that are in equilibrium. The ring form introduces a special carbon atom:
- Anomeric carbon: the former carbonyl carbon that becomes a new stereocenter in the ring.
- Two configurations at this carbon are called α (alpha) and β (beta) anomers.
The α/β form of the anomeric carbon is biologically important, because:
- It influences how sugars bond to each other.
- It affects digestibility and enzyme recognition (e.g., β‑1,4 bonds in cellulose vs. α‑1,4 bonds in starch).
Stereochemistry and Isomers
Carbohydrates are rich in stereoisomers (molecules with the same formula and sequence of atoms but different 3D arrangement).
Key points:
- Many monosaccharides have multiple chiral centers (carbons with four different groups), giving rise to many stereoisomers.
- Monosaccharides are usually classified as D‑ or L‑forms based on the configuration at one reference carbon (a historical convention).
- In biological systems, D‑sugars (like D‑glucose) are far more common.
Different stereoisomers can have:
- Very different biological effects (e.g., one is used as fuel, the other is not).
- Different taste, digestibility, and roles in metabolism.
Enzymes that process sugars are often highly specific to one stereoisomer.
Formation of Larger Carbohydrates
Glycosidic Bonds
Monosaccharides link together via glycosidic bonds to form di‑, oligo‑, and polysaccharides.
A glycosidic bond:
- Is formed between the anomeric carbon of one sugar and a hydroxyl group on another sugar (or another molecule like a base in nucleotides).
- Is a type of covalent bond created in a condensation (dehydration) reaction, where a molecule of water is released.
- Is broken by hydrolysis, where water is added back to split the bond.
Glycosidic bonds are named by:
- The position of the carbons involved (e.g., 1→4, 1→6).
- The configuration at the anomeric carbon (α or β).
Examples:
- α‑1,4‑glycosidic bond: between the anomeric carbon 1 of one sugar (in α‑form) and carbon 4 of another.
- β‑1,4‑glycosidic bond: same positions, but anomeric carbon in β‑form.
This small structural difference (α vs. β) has big functional consequences, especially in polysaccharides.
Condensation and Hydrolysis
When two monosaccharides join:
$$
\text{monosaccharide}_1 + \text{monosaccharide}_2 \rightarrow \text{disaccharide} + \mathrm{H_2O}
$$
This condensation:
- Requires energy and enzymes in cells.
- Is often specific: particular enzymes form particular bonds (e.g., sucrose synthase, glycogen synthase).
The reverse hydrolysis reaction:
$$
\text{disaccharide} + \mathrm{H_2O} \rightarrow \text{monosaccharide}_1 + \text{monosaccharide}_2
$$
- Releases the original monosaccharides (or their derivatives).
- Is catalyzed by other specific enzymes (e.g., sucrase, lactase, amylases).
In living organisms, these reactions are tightly regulated and coupled to energy metabolism.
Classification Beyond Mono/Di/Poly
Beyond the simple count of sugar units, carbohydrates can be classified by:
Size and Complexity
- Monosaccharides: single sugar units (covered in detail elsewhere).
- Disaccharides: two units (sucrose, lactose, maltose).
- Oligosaccharides (3–10 units):
- Often attached to proteins or lipids on cell surfaces.
- Play roles in cell recognition, immune response, and signaling.
- Polysaccharides (dozens to thousands of units):
- Energy storage polysaccharides (e.g., starch, glycogen).
- Structural polysaccharides (e.g., cellulose, chitin).
Composition
- Homopolysaccharides:
- Built from one type of monosaccharide only.
- Examples: starch, glycogen, cellulose (all made of glucose units).
- Heteropolysaccharides:
- Built from two or more different monosaccharides.
- Common in connective tissues and on cell surfaces.
Branching
Polysaccharides can be:
- Unbranched (linear): each monosaccharide connects to two others (except at the ends).
- Branched:
- Have side chains at certain positions (e.g., α‑1,6 branches).
- Branching level affects:
- How compact the molecule is.
- How quickly enzymes can add/remove sugar units.
- How easily the polymer dissolves or interacts with water.
Functional Roles of Carbohydrates
Carbohydrates as Energy Sources and Stores
Carbohydrates are central to energy metabolism:
- Many organisms use glucose as a primary fuel.
- Carbohydrates can be oxidized to release energy:
$$
\mathrm{C_6H_{12}O_6} + 6\ \mathrm{O_2} \rightarrow 6\ \mathrm{CO_2} + 6\ \mathrm{H_2O} + \text{energy}
$$ - Excess dietary carbohydrates are:
- Stored as glycogen (in animals and fungi) or starch (in plants).
- Converted to other storage forms when necessary (e.g., fats).
Key properties making carbohydrates suitable as energy stores:
- Can be rapidly mobilized by enzymes.
- Can be stored in large amounts (e.g., starch granules in plant cells, glycogen granules in animal cells).
- Their breakdown is closely linked to ATP production.
Structural Roles
Certain polysaccharides are adapted for mechanical strength and rigidity:
- They form fibers or networks that resist physical and chemical breakdown.
- They often have:
- Long unbranched chains
- Hydrogen bonding between chains
- Specific glycosidic bonds (e.g., β‑1,4) that produce straight, rigid molecules
These structural roles:
- Support plant cells, tissues, and whole organisms.
- Protect invertebrates and fungi through rigid exoskeletons or cell walls.
- Can be integrated with proteins and other molecules to form complex extracellular matrices.
Recognition, Signaling, and Protection
Short chains of sugars (oligosaccharides), often attached to proteins or lipids, act as molecular “tags”:
- Glycoproteins:
- Proteins with one or more carbohydrate chains attached.
- Important in:
- Cell–cell recognition
- Hormone and receptor interactions
- Immune system recognition
- Glycolipids:
- Lipids with carbohydrate chains.
- Present in cell membranes, especially in animal cells.
- Contribute to:
- Blood group antigens (e.g., A, B, O antigens)
- Recognition of “self” vs. “non-self”
- Viral or bacterial attachment sites.
Carbohydrates also:
- Form protective mucus and gels (e.g., in the respiratory and digestive tracts).
- Contribute to the viscosity and lubrication of bodily fluids (through certain heteropolysaccharides).
Precursor and Component Functions
Carbohydrates serve as building blocks or precursors for:
- Nucleotides and nucleic acids:
- Ribose (in RNA) and deoxyribose (in DNA) are pentose sugars.
- Metabolic intermediates:
- Many intermediates in central metabolic pathways are carbohydrate derivatives.
- Other biomolecules:
- Certain amino acids and lipids can be synthesized from carbohydrate precursors.
- Various secondary metabolites in plants and microorganisms derive from sugar backbones.
Physical and Chemical Properties Relevant to Biology
Solubility and Osmotic Effects
Due to their multiple –OH groups, most simple carbohydrates:
- Are highly soluble in water.
- Have strong effects on osmotic pressure:
- High concentrations of small sugars in cells attract water.
- Organisms must regulate carbohydrate concentrations to maintain osmotic balance and cell volume.
In some organisms:
- Sugars and sugar alcohols (e.g., sorbitol) act as osmoprotectants, helping cells cope with:
- Drought
- High salt
- Freezing
Reducing and Non‑Reducing Sugars
Carbohydrates can be classified as reducing or non‑reducing based on their ability to act as mild reducing agents in chemical tests.
- A reducing sugar has a free or easily formed anomeric carbon that can revert to an open‑chain form with an aldehyde or certain ketone groups.
- Many simple monosaccharides are reducing sugars.
- Some disaccharides are reducing if they have a free anomeric carbon.
- Non‑reducing sugars:
- Have their anomeric carbons tied up in glycosidic bonds.
- Cannot easily form the open‑chain reactive carbonyl form.
This distinction is:
- Used in laboratory tests (e.g., to detect the presence of certain sugars).
- Biologically relevant because reactive carbonyl groups can:
- Participate in unwanted reactions with proteins and other molecules.
- Contribute to aging processes and some disease mechanisms when sugar concentrations are chronically high.
Interaction with Water and Hydrogen Bonding
Carbohydrates readily form hydrogen bonds with:
- Water
- Each other
- Proteins, lipids, and nucleic acids
Consequences:
- Hydration shells: water molecules bound around carbohydrate chains influence:
- Solubility
- Viscosity
- Shape and flexibility of the molecule
- Structural materials: hydrogen bonding between carbohydrate chains enhances mechanical strength (e.g., in cellulose microfibrils).
- Recognition processes: specific 3D structures of sugar chains are stabilized by these interactions and recognized by complementary binding sites on proteins.
Biological Diversity of Carbohydrate Structures
Enormous Combinatorial Variety
Proteins are built from ~20 amino acids, and nucleic acids from 4 bases, but carbohydrates can:
- Vary in sugar type (glucose, galactose, mannose, etc.).
- Vary in linkage type (α vs. β; 1→2, 1→3, 1→4, 1→6).
- Vary in branching pattern and length.
- Include modified sugars (e.g., with amino groups, sulfates, acetyl groups).
Even with a small number of different monosaccharides, there are:
- Huge numbers of possible oligosaccharide sequences.
- Many possible three‑dimensional arrangements.
This structural diversity allows carbohydrates to:
- Encode detailed recognition information on cell surfaces (often more complex and variable than proteins and nucleic acids in this regard).
- Evolve rapidly in response to pathogens or environmental pressures.
- Provide specific binding sites for bacteria, viruses, and immune system proteins.
Species and Tissue Specificity
Different species, and even different tissues within the same organism, display:
- Distinct patterns of glycosylation (attachment of sugars to proteins and lipids).
- Characteristic sets of carbohydrate structures on cell surfaces.
This has many implications:
- Immune recognition and transplant compatibility.
- Pathogen host range (which species a pathogen can infect).
- Developmental processes, where changing carbohydrate patterns help guide:
- Cell adhesion
- Migration
- Differentiation
Nutritional and Medical Aspects (Overview)
Dietary Roles of Carbohydrates
In many diets, carbohydrates are:
- The major source of energy.
- Present as:
- Simple sugars (e.g., in fruits, honey, table sugar).
- Starches (e.g., in grains, potatoes, legumes).
- Indigestible fibers (e.g., cellulose, certain other polysaccharides).
The body:
- Breaks down digestible carbohydrates into monosaccharides.
- Uses these for energy or converts them to storage forms.
- Depends on enzymes that recognize specific glycosidic bonds.
Some carbohydrates (often called dietary fiber) are not digested by human enzymes but:
- Are fermented by gut microbes.
- Influence intestinal health, blood sugar regulation, and cholesterol metabolism.
Medical Relevance
Carbohydrates and their metabolism are involved in many medical contexts, such as:
- Disorders of carbohydrate metabolism (e.g., problems in regulating blood sugar).
- Enzyme deficiencies that prevent normal digestion of specific disaccharides.
- Altered carbohydrate structures on cells during:
- Inflammation
- Cancer
- Infection
In medicine and biotechnology, carbohydrates are:
- Targets for vaccines and drugs (e.g., bacterial capsule polysaccharides).
- Used as diagnostic markers (e.g., specific carbohydrate antigens).
- Modified or mimicked to interfere with pathogen attachment or immune evasion.
Summary
Carbohydrates are a fundamental class of macromolecules and their building blocks, characterized by:
- A backbone of carbon atoms bearing multiple hydroxyl groups and a carbonyl group.
- A tendency to form ring structures with distinct α/β configurations.
- The ability to form glycosidic bonds, giving rise to disaccharides, oligosaccharides, and polysaccharides.
- Enormous structural diversity through variations in:
- Sugar type
- Linkage position
- α/β configuration
- Branching and modifications
This chemical versatility underlies their many biological roles:
- Rapidly available energy and long‑term energy storage
- Structural support in cell walls and exoskeletons
- Recognition and signaling on cell surfaces
- Components and precursors of many other important biomolecules
Subsequent sections on monosaccharides, disaccharides, and polysaccharides examine representative examples and specific functions in more detail.