Carbohydrates, Lipids and Proteins
1. Major food biomolecules
Carbohydrates, proteins, and lipids are three major classes of organic biomolecules present in foods and are important for energy supply, structure, metabolism, and physiological function.
2. Carbohydrates
Carbohydrates are polyhydroxy aldehydes, ketones, or compounds that yield such substances on hydrolysis. They include monosaccharides, oligosaccharides, and polysaccharides.
3. D-Glucose
D-Glucose is the most abundant naturally occurring aldose monosaccharide and serves as an important metabolic fuel in humans.
4. Polysaccharides
Polysaccharides are polymers containing monosaccharide or monosaccharide-derivative units joined mainly through glycosidic bonds. Homopolysaccharides contain predominantly one type of monosaccharide unit, whereas heteropolysaccharides contain more than one type.
5. Examples of polysaccharides
Important homopolysaccharides include starch, glycogen, cellulose, and inulin. Many structural and extracellular polysaccharides are heteropolysaccharides containing different monosaccharide residues.
6. Lipids
Lipids are a diverse group of hydrophobic or amphipathic compounds. They function as energy stores, membrane components, precursors of signaling molecules, and carriers of fat-soluble vitamins.
7. Essential fatty acids
Linoleic acid (LA) and alpha-linolenic acid (ALA) are essential fatty acids because humans cannot synthesize sufficient amounts of their parent n-6 and n-3 fatty-acid structures and therefore require dietary sources.
8. Essential fatty-acid deficiency
Severe deficiency of essential fatty acids can produce abnormalities of the skin and other physiological systems. A condition historically described as phrynoderma may occur in association with nutritional deficiencies, but it is not specific to essential fatty-acid deficiency alone.
9. Cholesterol
Cholesterol is an important structural component of animal cell membranes and serves as a precursor for bile acids, steroid hormones, and vitamin D.
10. Steroids
Steroids contain a characteristic four-ring steroid nucleus consisting of three six-membered rings and one five-membered ring, commonly referred to as the cyclopentanoperhydrophenanthrene nucleus.
11. Collagen
Collagen is the most abundant protein family in mammals and is a major structural component of connective tissues. It is particularly rich in glycine, proline, and hydroxyproline.
12. Hydroxylysine in collagen
Collagen contains hydroxylysine residues in addition to hydroxyproline. Hydroxylation of selected amino-acid residues is important for collagen structure and cross-linking.
13. Proteins
Proteins are polymers of amino acids linked by peptide bonds. Most naturally occurring protein amino acids in humans are L-α-amino acids, although specialized amino acids and post-translational modifications also occur.
14. Secondary structure of proteins
The major regular secondary structures of proteins are the α-helix and β-sheet. These structures are stabilized mainly by hydrogen bonding between peptide backbone groups.
15. Alpha-helix
The α-helix is a helical arrangement of a polypeptide chain stabilized by hydrogen bonds between backbone carbonyl and amide groups.
16. Beta-sheet
A β-sheet consists of extended polypeptide segments arranged next to one another. The strands may form parallel or antiparallel arrangements and are stabilized by hydrogen bonding.
Enzymes and Vitamins
17. Enzymes
Enzymes are biological catalysts that accelerate biochemical reactions without being consumed in the overall reaction. Most enzymes are proteins, although some catalytic RNA molecules also occur.
18. Major enzyme classes
The traditional six major enzyme classes are oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. The current EC classification also recognizes translocases as a seventh class.
19. Oxidoreductases
Oxidoreductases catalyze oxidation-reduction reactions involving transfer of electrons or hydrogen atoms between substrates.
20. Hydrolases
Hydrolases catalyze bond cleavage through the addition of water. Examples include proteases, lipases, and many glycosidases.
21. Vitamins
Vitamins are organic micronutrients required in small amounts for normal physiological and metabolic functions. They are classified according to solubility as fat-soluble vitamins and water-soluble vitamins.
22. Fat-soluble vitamins
The fat-soluble vitamins are A, D, E, and K. They can be stored in body tissues to varying degrees and have different metabolic functions.
23. Water-soluble vitamins
The water-soluble vitamins include vitamin C and the B-group vitamins. The B-group includes thiamine, riboflavin, niacin, pantothenic acid, vitamin B6, biotin, folate, and vitamin B12.
24. Vitamin D
Vitamin D is metabolized to biologically active forms, with 1,25-dihydroxyvitamin D (calcitriol) being the principal hormonally active form. It contributes to regulation of calcium and phosphate metabolism.
Digestion and Carbohydrate Metabolism
25. Digestion
Digestion is the mechanical and enzymatic breakdown of food components into smaller molecules that can be absorbed and utilized by the body.
26. Carbohydrate digestion
Carbohydrate digestion begins in the mouth through the action of salivary α-amylase. Most enzymatic digestion occurs in the small intestine through pancreatic amylase and intestinal brush-border enzymes.
27. Lactose intolerance
Lactose intolerance results from inadequate digestion of lactose because of insufficient lactase activity in the small intestine. Undigested lactose reaches the colon, where microbial fermentation can produce gastrointestinal symptoms.
28. Hemoglobin
Hemoglobin is the major oxygen-carrying protein in red blood cells. It transports oxygen from the lungs to tissues and participates in carbon-dioxide and proton transport.
29. Thalassemia
Thalassemias are inherited disorders characterized by reduced synthesis of one or more globin chains of hemoglobin. Major forms include α-thalassemia and β-thalassemia.
30. Heme
Heme is an iron-containing porphyrin prosthetic group found in proteins such as hemoglobin and myoglobin. Heme biosynthesis occurs mainly in erythroid precursor cells and the liver and involves glycine and succinyl-CoA as early precursors.
31. Bilirubin formation
Heme is degraded through biliverdin to bilirubin, a yellow-orange pigment that is transported to the liver, conjugated, and excreted mainly through bile.
32. Gluconeogenesis
Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors such as lactate, glycerol, and glucogenic amino acids. Leucine and lysine are exclusively ketogenic and cannot provide net carbon for glucose synthesis.
33. Pentose phosphate pathway
The pentose phosphate pathway, also called the hexose monophosphate pathway, is an alternative pathway of glucose-6-phosphate metabolism. It generates NADPH and ribose-5-phosphate and does not primarily function as a pathway for ATP production.
34. Glycolysis
Glycolysis is a sequence of reactions in which glucose is converted to pyruvate, generating ATP and NADH. Under anaerobic conditions in some tissues, pyruvate can be converted to lactate.
Lipid and Protein Metabolism
35. Triacylglycerols
Triacylglycerols are esters containing glycerol and three fatty acids. They are the principal storage form of energy in adipose tissue and provide approximately 9 kcal of metabolizable energy per gram of fat.
36. Atherosclerosis
Atherosclerosis is a chronic disease of arteries involving lipid accumulation, inflammation, and formation of atherosclerotic plaques within the arterial wall. It can contribute to coronary artery disease, stroke, and peripheral arterial disease.
37. Amino-acid transamination
Transamination transfers an amino group from an amino acid to an α-keto acid, commonly forming glutamate. Aminotransferases require pyridoxal phosphate (PLP) as a coenzyme.
38. Amino-acid deamination and urea
Nitrogen from amino-acid metabolism can be released as ammonia and subsequently incorporated into urea through the hepatic urea cycle. Urea is a major route for nitrogen excretion in humans.
39. Glucogenic and ketogenic amino acids
Amino acids are classified according to their metabolic products as glucogenic, ketogenic, or both glucogenic and ketogenic. Leucine and lysine are exclusively ketogenic, whereas several amino acids can contribute to both glucose and ketone-body formation.
40. Calcium
Calcium is required for bone and tooth mineralization, muscle contraction, nerve signaling, intracellular signaling, and blood coagulation.
41. Sodium, potassium and chloride
Sodium, potassium, and chloride are major electrolytes involved in fluid balance, membrane potential, osmotic regulation, and acid-base physiology.
42. Fluoride
Fluoride at appropriate concentrations can reduce the risk of dental caries by promoting enamel remineralization and reducing demineralization. Excessive chronic exposure can cause dental or skeletal fluorosis, depending on the dose and duration.
Hormones, Energy and Nutrition
43. Pituitary gland
The pituitary gland is an important endocrine organ that regulates several physiological processes through hormones under the control of the hypothalamus. The term “master gland” is traditional but does not fully describe the integrated control of the endocrine system.
44. Energy values of macronutrients
The commonly used physiological energy values are approximately 4 kcal/g for carbohydrate, 4 kcal/g for protein, and 9 kcal/g for fat. Actual metabolizable energy can vary with food composition and digestibility.
45. Thermic effect of food
The thermic effect of food is the increase in energy expenditure associated with digestion, absorption, metabolism, and storage of nutrients. It is generally highest for protein, lower for carbohydrate, and lowest for fat.
46. Protein requirements
Protein requirements vary with age, body size, physiological state, health status, and activity level. The FAO/WHO/UNU safe protein intake for healthy adults has been reported at approximately 0.83 g/kg body weight/day, rather than applying a universal 1 g/kg value to every adult.
47. Protein-energy malnutrition
Protein-energy malnutrition refers to inadequate intake or utilization of energy and protein and includes conditions such as wasting and stunting. Severe forms historically described as marasmus and kwashiorkor differ in clinical presentation and underlying nutritional circumstances.
48. Transcription
Transcription is the synthesis of RNA using DNA as a template. In simplified terms, transcription involves initiation, elongation, and termination, although the molecular mechanisms differ among organisms and RNA types.
49. Ribosomes
Ribosomes are ribonucleoprotein complexes responsible for translation, the process by which the nucleotide sequence of messenger RNA is used to synthesize proteins.
50. Insulin and inulin
Insulin is a peptide hormone produced by pancreatic β-cells that regulates glucose and nutrient metabolism. Inulin is a fructan polysaccharide composed mainly of fructose units and occurs naturally in foods such as chicory root, Jerusalem artichoke, onions, and garlic.