Biological Macromolecules: Structure and Function in Anatomy & Physiology

Biological Macromolecules

Introduction to Biological Molecules

Biological molecules are essential for the structure and function of living organisms. Their unique three-dimensional shapes determine their specific roles in the body, and molecular recognition is often based on these shapes.

• Organic compounds have properties defined by their carbon skeleton and attached atoms called functional groups.

• On a molecular scale, many biological molecules are large and complex, known as macromolecules.

• Examples of macromolecules include DNA and carbohydrates.

Categories of Biological Macromolecules

Cells contain four major categories of biological macromolecules, each with distinct structures and functions:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Carbohydrates

Overview and Functions

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They serve as energy sources and provide structural support in cells.

• Include small sugar molecules (e.g., in soft drinks) and long starch molecules (e.g., in pasta and potatoes).

• Key functions: energy storage and structural support.

Types of Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Composed of two monosaccharides (e.g., sucrose, lactose).

• Oligosaccharides: Short chains of monosaccharide units.

• Polysaccharides: Long chains of sugar units (e.g., starch, cellulose, glycogen).

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and serve as the main fuel for cellular work.

• Glucose: Found in sports drinks.

• Fructose: Found in fruit.

• Honey contains both glucose and fructose.

Disaccharides

Disaccharides are formed by joining two monosaccharides through a dehydration reaction.

• Lactose: Found in milk.

• Sucrose: Common table sugar, extracted from sugar cane and sugar beets.

Disaccharides are important dietary sugars. The average American consumes about 64 kg of sugar per year.

Polysaccharides

Polysaccharides are complex carbohydrates consisting of long chains of monosaccharide units.

• Starch: Energy storage in plants (found in potatoes and grains).

• Glycogen: Energy storage in animals; structurally similar to starch.

• Cellulose: Provides structural support in plant cell walls; most abundant organic compound on Earth and a major component of wood and dietary fiber.

Most animals cannot digest cellulose, but grazing animals can due to symbiotic prokaryotes in their digestive tracts.

Lipids

Overview and Functions

Lipids are hydrophobic molecules that perform essential functions such as energy storage, cushioning, and insulation in the human body.

• Insoluble in water

• Major types: triglycerides, phospholipids, steroids

Types of Lipids

• Triglycerides: Composed of glycerol and three fatty acids; primary energy storage molecules.

• Fatty acids: Can be saturated (maximum number of hydrogen atoms) or unsaturated (fewer hydrogen atoms due to double bonds).

• Phospholipids: Major component of cell membranes; consist of a glycerol backbone, two fatty acids, and a phosphate group.

• Steroids: Carbon-based ring structures; include cholesterol, which is used to make hormones such as estrogen and testosterone.

Triglycerides

Triglycerides are the main form of stored energy in animals. Saturated fats (e.g., butter) are common in animal products, while unsaturated fats (e.g., corn oil) are found in plants.

Phospholipids

Phospholipids form the structural basis of cell membranes, creating a bilayer that separates the cell from its environment.

Steroids

Steroids have a structure distinct from other lipids, consisting of four fused carbon rings. Cholesterol is the precursor for steroid hormones.

• Anabolic steroids: Synthetic variants of testosterone; can enhance muscle growth but pose health risks.

Cholesterol and Lipoproteins

Cholesterol is transported in the blood by lipoproteins. High levels of LDL ("bad cholesterol") can lead to atherosclerotic plaques and heart disease, while HDL ("good cholesterol") helps recycle cholesterol in the liver.

Lipoprotein Type	Function	Health Impact
LDL	Delivers cholesterol to cells	High levels increase risk of heart disease
HDL	Removes excess cholesterol, recycles in liver	High levels are protective

Consumption of saturated fatty acids increases LDL levels.

Proteins

Structure and Function

Proteins are polymers made from 20 different amino acids. Their structure determines their function in the body.

• Amino acids: Each has a central carbon atom bonded to four groups, including a variable side chain (R group).

• 9 amino acids are essential and must be obtained from food.

• Proteins have diverse functions: enzymatic catalysis, transport, storage, mechanical support, immune protection, nerve impulse transmission, and regulation of growth and differentiation.

Levels of Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Orientation in space (e.g., alpha helix, beta sheet).

• Tertiary structure: Three-dimensional shape formed by interactions such as disulfide and hydrogen bonds; creates polar and nonpolar regions.

• Quaternary structure: Association of two or more tertiary protein chains.

A change in the primary structure (e.g., substitution of one amino acid in hemoglobin) can cause diseases such as sickle-cell anemia.

Protein Denaturation

Protein shape is sensitive to environmental conditions. Unfavorable temperature or pH can cause denaturation, resulting in loss of function.

Enzyme Function

Enzymes are proteins that catalyze chemical reactions. Their activity depends on:

• Temperature

• pH

• Ion concentration

• Presence of inhibitors

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information and provide instructions for protein synthesis.

• Nucleotides: Building blocks consisting of a phosphate group, a sugar, and a nitrogenous base.

• DNA: Double helix structure; sugar is deoxyribose; bases are adenine, thymine, cytosine, and guanine.

• RNA: Single-stranded; sugar is ribose; bases are adenine, uracil, cytosine, and guanine.

Base Pairing

• DNA: Adenine pairs with thymine, cytosine pairs with guanine.

• RNA: Adenine pairs with uracil, cytosine pairs with guanine.

ATP (Adenosine Triphosphate)

ATP is a nucleotide that serves as the primary energy carrier in cells.

• Structure: Adenine base, ribose sugar, three phosphate groups.

• Function: Provides energy for cellular processes.

ATP hydrolysis equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Additional info: The notes have been expanded to include definitions, examples, and academic context for each macromolecule class, as well as a summary table for lipoproteins.