Carbohydrates and Lipids: Structure, Function, and Biological Roles

Protein Structure

Levels of Protein Structure

Proteins are complex molecules with hierarchical structures that determine their function. The four levels of protein structure are:

• Primary Structure: The linear sequence of amino acids in a polypeptide, held together by peptide bonds.

• Secondary Structure: Local folding into α-helices and β-pleated sheets, stabilized by hydrogen bonds between backbone atoms.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide, stabilized by interactions among R groups (side chains), including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

• Quaternary Structure: The association of multiple polypeptide chains (subunits) into a functional protein complex.

Example: Hemoglobin is a protein with quaternary structure, composed of four polypeptide subunits.

The 20 Major Amino Acids

Amino acids are the building blocks of proteins. They differ in their R groups, which can be nonpolar, polar, or charged (acidic or basic). The properties of the R group affect the amino acid's behavior in water and its role in protein structure.

• Charged R groups: Hydrophilic, interact favorably with water, and often participate in ionic bonds.

• Nonpolar R groups: Hydrophobic, tend to cluster away from water.

Carbohydrates: Structure and Function

Introduction to Carbohydrates

Carbohydrates are essential biomolecules involved in energy storage, structural support, and cellular identity. They are composed of carbon, hydrogen, and oxygen, typically with the formula (CH2O)n.

• Monosaccharides: Simple sugars, monomers of carbohydrates (e.g., glucose, ribose).

• Oligosaccharides: Short chains of monosaccharides.

• Polysaccharides: Long chains of monosaccharides (complex carbohydrates).

Structural Variations Among Monosaccharides

Monosaccharides vary in several ways, leading to diverse structures and functions:

• Location of the carbonyl group: At the end (aldose) or within the carbon chain (ketose).

• Number of carbon atoms: Trioses (3C), pentoses (5C), hexoses (6C), etc.

• Spatial arrangement of atoms: Different arrangements of hydroxyl groups.

• Linear and ring forms: Sugars can exist in both forms, with ring forms predominating in aqueous solutions.

Example: Glucose and galactose differ only in the spatial arrangement of one hydroxyl group, but this difference leads to distinct properties.

Polysaccharides: Structure and Types

Polysaccharides are polymers of monosaccharides linked by glycosidic bonds. Their structure determines their function:

• Starch: Storage polysaccharide in plants, composed of α-glucose monomers. Exists as amylose (unbranched) and amylopectin (branched).

• Glycogen: Storage polysaccharide in animals, highly branched and similar to amylopectin.

• Cellulose: Structural polysaccharide in plants, composed of β-glucose monomers. Forms straight, rigid fibers due to β-1,4-glycosidic linkages.

• Chitin: Structural polysaccharide in fungi and arthropods, contains N-acetylglucosamine (NAG) monomers.

• Peptidoglycan: Structural polysaccharide in bacterial cell walls, composed of alternating NAG and NAM monomers cross-linked by peptides.

Comparison of Major Polysaccharides

Polysaccharide	Monomer	Linkage	Function
Starch	α-glucose	α-1,4 and α-1,6	Energy storage in plants
Glycogen	α-glucose	α-1,4 and α-1,6 (more branched)	Energy storage in animals
Cellulose	β-glucose	β-1,4	Structural support in plants
Chitin	NAG	β-1,4	Structural support in fungi and arthropods
Peptidoglycan	NAG and NAM	β-1,4 (cross-linked by peptides)	Structural support in bacteria

Functional Roles of Carbohydrates

• Energy Storage: Starch and glycogen are hydrolyzed by enzymes (amylase, phosphorylase) to release glucose for cellular respiration.

• Structural Support: Cellulose, chitin, and peptidoglycan provide rigidity to cell walls and exoskeletons. Their β-glycosidic linkages are resistant to hydrolysis.

• Cellular Identity: Glycoproteins and glycolipids on cell surfaces are involved in cell recognition and signaling.

Example: Dietary fiber consists of indigestible structural carbohydrates, important for digestive health.

Lipids: Structure and Function

Introduction to Lipids

Lipids are hydrophobic, carbon-containing molecules that are insoluble in water due to their high proportion of nonpolar C–C and C–H bonds. They serve as energy storage, structural components, and signaling molecules.

Major Types of Lipids

• Fats (Triglycerides): Composed of three fatty acids linked to glycerol by ester bonds. Primary function is long-term energy storage.

• Steroids: Characterized by a four-ring structure. Examples include cholesterol (membrane component) and hormones (estrogen, testosterone).

• Phospholipids: Consist of a glycerol backbone, two fatty acid tails, and a phosphate group. Major component of cell membranes, forming bilayers due to their amphipathic nature (hydrophilic head, hydrophobic tails).

Fatty Acids and Bond Saturation

• Saturated fatty acids: Only single bonds between carbons; straight chains; solid at room temperature.

• Unsaturated fatty acids: One or more double bonds; kinks in the chain; liquid at room temperature.

• Polyunsaturated fatty acids: Multiple double bonds; even more fluid at room temperature.

Example: Butter (saturated fat) is solid at room temperature, while olive oil (unsaturated fat) is liquid.

Functions of Lipids

• Energy storage: Fats store more energy per gram than carbohydrates due to their reduced, nonpolar bonds.

• Pigments: Some lipids capture or respond to sunlight (e.g., carotenoids).

• Cell signaling: Steroid hormones act as signaling molecules.

• Waterproofing: Waxes and other lipids form protective coatings.

• Vitamins: Some vitamins (A, D, E, K) are lipid-soluble and essential for cellular processes.

Comparison: Fats vs. Carbohydrates for Energy Storage

Fats store more energy than carbohydrates because they have more C–H bonds, which release more energy upon oxidation.

Molecule	Energy Storage	Structure
Carbohydrate	Short-term	Hydrophilic, easily mobilized
Fat	Long-term	Hydrophobic, compact storage

Additional info: The notes also briefly cover the role of enzymes (amylase, phosphorylase) in carbohydrate metabolism and the importance of glycoproteins/glycolipids in cell identity, which are key concepts in cellular biology.