Carbohydrates, Lipids, and Proteins: Structure, Function, and Biological Importance

Carbohydrates

Introduction to Carbohydrates

Carbohydrates are a major source of energy in the human diet and are composed of carbon, hydrogen, and oxygen. They are also known as saccharides, meaning "sugars." Carbohydrates are produced by plants through photosynthesis and are oxidized in living cells to provide energy.

• Key Elements: Carbon (C), Hydrogen (H), Oxygen (O)

• Dietary Sources: Bread, potatoes, pasta

Photosynthesis Equation:

$6CO_2 + 6H_2O + \text{energy} \xrightarrow{\text{photosynthesis}} C_6H_{12}O_6 + 6O_2$

Cellular Respiration Equation:

$C_6H_{12}O_6 + 6O_2 \xrightarrow{\text{respiration}} 6CO_2 + 6H_2O + \text{energy}$

Types of Carbohydrates

Carbohydrates are classified based on the number of sugar units:

• Monosaccharides: The simplest carbohydrates (single sugar units)

• Disaccharides: Composed of two monosaccharide units

• Polysaccharides: Composed of many monosaccharide units

Monosaccharides

Monosaccharides contain several hydroxyl groups attached to a chain of three to eight carbon atoms. They are further classified by their functional group and the number of carbon atoms:

• Aldoses: Contain an aldehyde group

• Ketoses: Contain a ketone group

• Classification by Carbon Number: Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C)

Chemical Properties of Monosaccharides

Monosaccharides can be reduced to sugar alcohols, which are used as sweeteners in sugar-free products. For example, D-glucose can be reduced to D-glucitol (sorbitol).

• Sugar Alcohols: D-sorbitol, D-xylitol, D-mannitol

• Applications: Diet drinks, sugarless gum, diabetic products

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined by a glycosidic bond. Common examples include lactose and maltose.

• Lactose: Found in milk; composed of galactose and glucose

• Maltose: Found in cereals and candies; composed of two glucose units

Polysaccharides

Polysaccharides are large molecules formed by joining many monosaccharide units. They serve as energy storage or structural components in living organisms.

• Cellulose: Composed of glucose units connected by β(1→4) glycosidic bonds; structural component in plants

• Starch: Storage form of glucose in plants; consists of amylose (unbranched) and amylopectin (branched)

• Glycogen: Storage form of glucose in animals

Lipids

Introduction to Lipids

Lipids are biomolecules that contain fatty acids or a steroid nucleus. They are soluble in organic solvents but not in water. Lipids are essential for cell membranes, fat-soluble vitamins, and steroid hormones.

• Types of Lipids: Waxes, triacylglycerols, glycerophospholipids, sphingolipids, steroids

• Hydrolyzable Lipids: Contain ester bonds and can be broken down to fatty acids

• Nonhydrolyzable Lipids: Steroids, which do not contain fatty acids

Fatty Acids

Fatty acids are long, unbranched carbon chains with a carboxylic acid group at one end. They are classified as saturated (no double bonds) or unsaturated (one or more double bonds).

• Saturated Fatty Acids: Pack closely together, leading to higher melting points

• Unsaturated Fatty Acids: Have kinks due to double bonds, preventing tight packing

Essential Fatty Acids

Essential fatty acids, such as linoleic acid, linolenic acid, and arachidonic acid, cannot be synthesized by the human body and must be obtained from the diet.

Waxes and Triacylglycerols

Waxes are esters of long-chain fatty acids and long-chain alcohols, providing protective coatings for plants and animals. Triacylglycerols (triglycerides) are the main storage form of fatty acids in the body, formed by esterification of glycerol with three fatty acids.

Chemical Properties of Triacylglycerols

Triacylglycerols can undergo hydrogenation, where double bonds in unsaturated fatty acids react with hydrogen gas to form saturated fatty acids. This process is used to convert vegetable oils into margarine and shortening.

• Hydrogenation: Increases saturation and melting point

• Partial Hydrogenation: Produces soft, semisolid fats

Steroids: Cholesterol, Bile Salts, and Steroid Hormones

Steroids are lipids with a characteristic structure of four fused rings. Cholesterol is the most important steroid in the body, essential for cell membranes, hormone synthesis, and vitamin D production. Excess cholesterol can lead to plaque formation in arteries, increasing the risk of heart attack.

• Cholesterol: Obtained from diet and synthesized in the liver

• Bile Salts: Synthesized from cholesterol, aid in fat digestion

• Steroid Hormones: Chemical messengers derived from cholesterol

Steroid Hormones and Adrenal Corticosteroids

Steroid hormones include sex hormones (testosterone, estrogens, progesterone) and adrenal corticosteroids (aldosterone, cortisone). These hormones regulate metabolism, electrolyte balance, and other physiological functions.

• Adrenal Corticosteroids: Produced by adrenal glands; regulate electrolytes and glucose levels

• Aldosterone: Regulates water and electrolyte balance

• Cortisone: Increases blood glucose and stimulates glycogen synthesis

Proteins and Amino Acids

Introduction to Proteins

Proteins are large biomolecules made from 20 different amino acids. They serve a wide range of functions, including structural support, catalysis, transport, storage, regulation, and protection.

• Structural Proteins: Collagen, keratin

• Contractile Proteins: Myosin, actin

• Transport Proteins: Hemoglobin, lipoproteins

• Storage Proteins: Casein, ferritin

• Hormonal Proteins: Insulin, growth hormone

• Enzymes: Sucrase, trypsin

• Protective Proteins: Immunoglobulins

Class of Protein	Function	Examples
Structural	Provide structural components	Collagen, Keratin
Contractile	Make muscles move	Myosin, Actin
Transport	Carry essential substances	Hemoglobin, Lipoproteins
Storage	Store nutrients	Casein, Ferritin
Hormone	Regulate metabolism and nervous system	Insulin, Growth hormone
Enzyme	Catalyze biochemical reactions	Sucrase, Trypsin
Protection	Immune response	Immunoglobulins

Amino Acids

Amino acids are the building blocks of proteins. Each amino acid contains a central (α) carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group). At physiological pH, amino acids exist as zwitterions, carrying both positive and negative charges but having an overall neutral charge.