Biochemistry Honor

Science & Health

Requirements

  1. Define the following terms:
    • Carbohydrate
    • Lipid
    • Fatty acid
    • Protein
    • Peptide
    • Enzyme
    • Amino acid
    • Nucleic acid
    • Hydrophilic and hydrophobic
    • Triglyceride
    • Monosaccharide

    Answer: 1) Carbohydrate: a biomolecule formed by carbon, hydrogen, and oxygen that serves as the main source of energy (sugars and starches). 2) Lipid: a nonpolar biomolecule (fats and oils) that stores energy, makes up cell membranes, and insulates the organism. 3) Fatty acid: a chain of carbon and hydrogen ending in a carboxyl group (-COOH); it is the basic unit that forms lipids, and can be saturated or unsaturated. 4) Protein: a macromolecule formed by one or more chains of amino acids, with structural, transport, defense, and catalysis functions. 5) Peptide: a short chain of amino acids joined by peptide bonds (a small protein). 6) Enzyme: a protein that acts as a biological catalyst, accelerating chemical reactions without being consumed. 7) Amino acid: the monomer of proteins, composed of an amino group (-NH2), a carboxyl group (-COOH), and a characteristic radical. 8) Nucleic acid: a macromolecule that stores and transmits genetic information (DNA and RNA), formed by nucleotides. 9) Hydrophilic and hydrophobic: hydrophilic is what has an affinity for water and dissolves in it; hydrophobic is what repels water and does not dissolve (like fats). 10) Triglyceride: a lipid formed by a molecule of glycerol bound to three fatty acids; it is the main form of fat reserve. 11) Monosaccharide: a simple sugar, the smallest unit of carbohydrate (e.g.: glucose, fructose). — These are the basic blocks of biomolecules; saturated/unsaturated fatty acids define the type of fat; enzymes such as amylase accelerate biological reactions a thousandfold; human DNA has 3 billion base pairs according to the Human Genome Project (2003); standard definitions from Lehninger's biochemistry used throughout the world, in force.

  2. What is the importance of water in organisms? What are the main physical and chemical characteristics of the water molecule?

    Answer: Water is a universal solvent, transports nutrients, regulates body temperature, takes part in chemical reactions, and maintains cell volume. It is 60-70% of the human body. Characteristics: an H₂O molecule with angular geometry, strong polarity, formation of hydrogen bonds, high specific heat, high surface tension, and a liquid state at Earth's ambient temperature. — Polarity comes from the difference in electronegativity between O and H; hydrogen bonds explain the high boiling point (100°C); 0.5 L of water is lost per day through respiration; the book "Biochemistry" by Lehninger dedicates an entire chapter to water as the basis of all life — a principle applied in medicine, biology, and pharmacy worldwide, currently in force.

  3. What does metabolism mean?

    Answer: Metabolism is the set of all chemical reactions that occur in the cells and the body to sustain life. It includes catabolism (the breakdown of molecules to generate energy, e.g., digestion of glucose) and anabolism (the synthesis of molecules, e.g., building proteins). It is what allows us to breathe, grow, move, reproduce, and maintain temperature. — The word comes from the Greek metabolē (change); basal metabolism is the minimum expenditure at rest (about 1,500 kcal/day for an adult); enzymes catalyze practically all reactions; ATP is the universal energy currency — a principle discovered by Lipmann (Nobel 1953) and applied throughout all modern molecular biology in the world.

  4. Biochemically, why do we feel hunger?

    Answer: We feel hungry when blood glucose levels drop (hypoglycemia), the stomach contracts and releases the hormone ghrelin, which stimulates the hypothalamus. The brain interprets these signals and sends the impulse to seek food. After eating, hormones such as leptin and insulin signal satiety, restoring the body's energy balance. — Ghrelin was discovered in 1999 by Kojima and Kangawa; leptin was discovered in 1994 by Friedman; the gut-brain axis involves the vagus nerve and more than 20 hormones; the mechanisms of hunger and satiety are the target of modern anti-obesity medications such as semaglutide (Ozempic) — the basis of modern endocrinology worldwide, currently in force.

  5. Explain how the glycolysis pathway occurs. What is its importance for metabolism?

    Answer: Glycolysis is the pathway that breaks down glucose (6 carbons) into two molecules of pyruvate (3 carbons) in the cell's cytoplasm. It occurs in 10 enzymatic steps, generates a net 2 ATP and 2 NADH. It is essential because it produces energy quickly, feeds the Krebs cycle, and provides energy for tissues such as red blood cells and the human brain. — Glycolysis was elucidated by Embden, Meyerhof, and Parnas (1930-40) and bears the name EMP; it occurs in all living organisms (evolutionary universality); each step is catalyzed by a specific enzyme; pyruvate can enter the Krebs cycle (aerobic) or become lactate (anaerobic) — the basis of modern human and animal biochemistry worldwide.

  6. Which human cells are dependent solely on this pathway to obtain energy?

    Answer: Red blood cells (erythrocytes) are the only human cells totally dependent on glycolysis. They have no mitochondria (these were lost during maturation), so they do not perform the Krebs cycle or oxidative phosphorylation. All energy (ATP) comes from anaerobic glycolysis ending in lactate, which is exported and later recycled by the human liver. — Mature red blood cells lose their nucleus and mitochondria to maximize space for hemoglobin; they live about 120 days; the lactate produced goes to the liver and is converted into glucose by the Cori Cycle (Carl and Gerty Cori, Nobel 1947); this adaptation allows maximum oxygen transport without competing with their own O₂-consuming cells.

  7. Which molecule links the glycolysis pathway and the Krebs cycle?

    Answer: The molecule is acetyl-CoA (acetyl-coenzyme A). It is formed by the oxidative decarboxylation of pyruvate (the final product of glycolysis) in the mitochondrial matrix, by the enzyme pyruvate dehydrogenase. Acetyl-CoA enters the Krebs cycle by combining with oxaloacetate to form citrate, initiating the 8 reactions of the aerobic degradation cycle. — Acetyl-CoA is considered the universal metabolic currency — it also comes from the beta-oxidation of fatty acids and the degradation of amino acids; pyruvate dehydrogenase is an enzyme complex with 3 activities; the Krebs cycle was discovered by Hans Krebs in 1937 (Nobel 1953); the biochemical basis of human aerobic cellular respiration, in force.

  8. What is the importance of the Krebs cycle?

    Answer: The Krebs cycle is the central pathway of aerobic metabolism: it oxidizes acetyl-CoA generating 3 NADH, 1 FADH₂, and 1 GTP per turn. This reducing currency feeds the respiratory chain that produces most of the cell's ATP. It also provides intermediates for the synthesis of amino acids, lipids, and neurotransmitters essential to human life. — Hans Krebs described the cycle in 1937 (Nobel 1953); each glucose generates 30-32 ATP in total, with 90% from the cycle + respiratory chain; intermediates such as α-ketoglutarate and oxaloacetate are precursors of non-essential amino acids; it is the heart of aerobic metabolism according to Lehninger's "Biochemistry" chapter 16, in force worldwide.

  9. What are the functions of lipids?

    Answer: Lipids have several functions: (1) energy reserve (1g of fat = 9 kcal vs 4 kcal for carbohydrate); (2) component of cell membranes (phospholipids in a bilayer); (3) thermal insulation (adipose tissue); (4) steroid hormones (testosterone, estrogen, cortisol); (5) absorption of vitamins A, D, E, K and mechanical protection of organs. — Phospholipids form the membrane bilayer described by Singer and Nicolson in 1972 (the fluid mosaic model); cholesterol is the precursor of all steroids; subcutaneous fat helps maintain the body's 36.5°C; the myelin of the nerves is 80% lipid; the book "Biochemistry" by Lehninger dedicates the entire chapter 10 to the functions of human lipids, in force.

  10. Why are lipids insoluble in water?

    Answer: Lipids are insoluble in water because their long hydrocarbon chains (C-H) are nonpolar. Water is polar (it forms hydrogen bonds with polar molecules such as salt or sugar), but it does not interact with the nonpolar chains of lipids. That is why oil and water do not mix — the nonpolar phase repels the polar one, forming two distinct layers. — Polarity depends on the difference in electronegativity — in C-H bonds (close together) there is low polarity; the hydrophobic effect explains the formation of lipid bilayers in membranes; the concept is the basis of surface chemistry and was formulated by Walter Kauzmann in 1959, being applied in modern pharmacology and cell biology worldwide, in force.

  11. Why are lipids, and not glucose, used for energy storage?

    Answer: Lipids store more energy per gram (9 kcal/g) than glucose (4 kcal/g) — more than double. In addition, they are insoluble in water, so they accumulate compactly in adipose tissue without increasing osmotic pressure. Glucose, being polar, would bring along a lot of water, requiring an absurd body volume. That is why lipids are an efficient long-term reserve. — Adipose tissue stores months of energy (~150,000 kcal in an average adult); glycogen (a form of glucose) only lasts 24h and takes up a lot of space because of the water; a normal human would have to weigh 60% more if they stored energy as glycogen, according to Lehninger's "Biochemistry" (ch. 17) — a principle applied in medical physiology, in force.

  12. What is beta oxidation? Why does this pathway have that name?

    Answer: Beta-oxidation is the breakdown of fatty acids into molecules of acetyl-CoA inside the mitochondria, generating energy for the Krebs cycle. The name "beta" comes from the position of the carbon that is attacked: the oxidation occurs on the beta carbon (the second carbon from the carboxyl group -COOH) of the fatty acid chain during each breakdown cycle. — Beta-oxidation was described by Knoop in 1904; each cycle releases 1 acetyl-CoA + 1 NADH + 1 FADH₂; a 16C fatty acid (palmitic) generates 8 acetyl-CoA = 106 total ATP; it occurs in the mitochondrial matrix and is regulated by carnitine (a transporter) — the basis of lipid metabolism according to Lehninger's "Biochemistry" chapter 17, in force.

  13. What are essential and non-essential amino acids?

    Answer: Essential amino acids are those that the human body cannot produce and that must come from the diet — there are 9: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Non-essential are the 11 that the body synthesizes on its own from other compounds: alanine, aspartate, glutamate, etc. — There are 20 proteinogenic amino acids in total; a deficiency of essential ones causes kwashiorkor (severe malnutrition); complete sources: egg, meat, milk, soy; vegetarians combine grains + legumes to obtain all the essential ones; the table proposed by Rose in 1957 — the basis of modern nutrition worldwide, in force today.

  14. What are ketone bodies, where are they produced, and what are the consequences of excessive production of them?

    Answer: Ketone bodies are molecules (acetoacetate, beta-hydroxybutyrate, acetone) produced in the liver from acetyl-CoA when there is an excess of fat being broken down (prolonged fasting, ketogenic diet, diabetes). Excess causes ketosis (fruity bad breath, weight loss) and in severe cases diabetic ketoacidosis — a risk of coma and death if not treated. — Nutritional ketosis is the basis of popular low-carb diets; diabetic ketoacidosis (DKA) has a mortality of 1-5% according to the guidelines of the Brazilian Diabetes Society; it was discovered as a syndrome in 1886 by Dreschfeld; treatment requires insulin, hydration, and potassium correction in an emergency hospital ICU worldwide, in force.

  15. What compounds are formed by the joining of amino acids? What are the main functions of these compounds?

    Answer: Through the joining of amino acids via peptide bonds, peptides (few amino acids) and proteins (many) are formed. Functions: structural (collagen, keratin), enzymatic (catalysis), transport (hemoglobin), defense (antibodies), hormonal (insulin), motor (actin, myosin), membrane receptors, and storage (ferritin). They are essential for everything in the living human body. — There are millions of different proteins; a peptide bond is covalent, between the carboxyl (-COOH) of one amino acid and the amine (-NH₂) of the next, with the release of water; the book "Biochemistry" by Lehninger dedicates chapter 4 to protein structure (primary, secondary, tertiary, quaternary) — the basis of human molecular biology worldwide, in force.

  16. What is the importance of nucleic acids? What is their structure and what are their components?

    Answer: Nucleic acids (DNA and RNA) store, transmit, and express genetic information. Structure: DNA is a double helix, RNA is a single strand. Components: 1) sugar (deoxyribose in DNA, ribose in RNA); 2) phosphate; 3) nitrogenous base (Adenine, Thymine/Uracil, Cytosine, Guanine). Bases pair through hydrogen bonds (A-T/U, C-G). — The DNA double helix was described by Watson, Crick, and Franklin in 1953 (Nobel 1962); the human genome has 3 billion base pairs distributed across 23 pairs of chromosomes; messenger RNA carries the instruction from the DNA to the ribosomes for protein synthesis — the basis of modern molecular biology worldwide, in force today.

  17. Draw a DNA molecule, with 4 nucleotides, naming its components.

    Answer: Draw the double helix structure with two parallel strands connected by paired nitrogenous bases. Each nucleotide has three parts: a nitrogenous base (A, T, G, or C), a deoxyribose sugar, and a phosphate group. Adenine pairs with Thymine (A-T) and Guanine with Cytosine (G-C) through hydrogen bonds. — The structure was discovered by Watson, Crick, Franklin, and Wilkins in 1953. The hydrogen bonds are double (A-T) or triple (G-C). DNA has 3.4nm per complete turn of the helix. Each nucleotide is joined by a phosphodiester bond. The drawing should show the sugar-phosphate backbone and the bases facing the center.