Biochemistry for Health Sciences
Master the molecular foundations of life: protein structure, enzyme kinetics, metabolic pathways, nucleic acids, lipid chemistry, and acid-base buffering — all contextualized to clinical nursing and allied health practice.
Proteins — Structure and Function
From amino acid sequence to biological activity
Proteins are the workhorses of the cell. Assembled from 20 amino acids linked by peptide bonds, each protein's function is determined entirely by the sequence and folding of its amino acid chain. Understanding four levels of protein structure explains how a single amino acid substitution can cause catastrophic disease.
Four Levels of Protein Structure
Denaturation and Clinical Consequences
Denaturation is the loss of a protein's 3D structure without breaking its primary peptide bonds. The amino acid sequence remains intact, but the functional shape is destroyed. Clinical triggers: high fever (>42°C/108°F) breaks hydrogen bonds maintaining tertiary structure; extreme pH shifts disrupt ionic bonds; heavy metals (lead, mercury) bind sulfhydryl groups on cysteine, preventing disulfide bonds. Burns cause irreversible protein denaturation in skin collagen and intracellular enzymes — this is why burn wounds do not heal normally and why pasteurization kills microbes by denaturing their enzymes.
Essential Amino Acids (must eat)
His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val — mnemonic: "PVT TIM HaLL" (Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys). *Arg is conditionally essential in growth/illness.
Protein Functions at a Glance
Structural: collagen (tendons, skin), keratin (hair, nails). Transport: hemoglobin (O₂), albumin (drugs, fatty acids, bilirubin). Enzymatic: amylase, lipase. Hormonal: insulin, glucagon. Immune: immunoglobulins (antibodies). Contractile: actin, myosin (muscle).
Proteins — Self-Check
1/4A patient's blood pH drops to 7.20 due to severe sepsis. Which protein structural feature is MOST directly disrupted?
Allied Health Relevance — Proteins
Nursing: Monitor protein status (albumin, pre-albumin) as markers of nutritional status; low albumin alters drug binding and increases free drug toxicity. Respiratory Therapy: Surfactant is a phospholipoprotein — neonatal respiratory distress syndrome results from insufficient surfactant protein. MLT/Medical Lab Tech: Serum protein electrophoresis (SPEP) identifies abnormal immunoglobulin bands (multiple myeloma, monoclonal gammopathy). Paramedic/EMS: Recognize protein denaturation context in hyperthermia and severe burns; protect airway early in inhalation injury before edema occludes it.
Enzymes — Catalysts of Life
Activation energy, kinetics, and diagnostic markers
Enzymes are biological catalysts — mostly proteins — that accelerate metabolic reactions by lowering activation energy without being consumed in the process. The human body contains over 25,000 distinct enzymes. Each is specific for one substrate or reaction type, and their activity is tightly regulated.
Lock-and-Key vs. Induced Fit
Lock-and-key: Active site is rigid, perfectly complementary to substrate shape. Induced fit (modern model): Active site is flexible — substrate binding triggers conformational change that optimizes the fit, bringing catalytic residues into position. Most enzymes use induced fit, which explains how substrate binding increases catalytic efficiency.
Factors Affecting Enzyme Activity
pH: Pepsin (pH 2, stomach), salivary amylase (pH 7, mouth), trypsin (pH 8, duodenum). Temperature: Optimal ~37°C; fever increases rate briefly, but >40°C risks denaturation. Substrate concentration: Activity increases until all active sites are saturated (Vmax). Inhibitors: Competitive or non-competitive.
Competitive vs. Non-Competitive Inhibition
Competitive inhibitors resemble the substrate and bind the active site reversibly. They increase apparent Km (reduce apparent affinity) but Vmax is unchanged if substrate concentration is raised. Example: statins mimic HMG-CoA to competitively inhibit HMG-CoA reductase (cholesterol synthesis). Non-competitive inhibitors bind an allosteric site (not the active site), changing enzyme shape. They decrease Vmax without affecting Km — cannot be overcome by adding more substrate. Example: heavy metals binding enzyme SH groups.
Cofactors and Coenzymes
Many enzymes require non-protein helpers. Cofactors (inorganic minerals): Mg²⁺ (kinases, DNA polymerase), Zn²⁺ (carbonic anhydrase, alcohol dehydrogenase), Fe²⁺ (cytochromes), Cu²⁺ (lysyl oxidase). Coenzymes (organic, often derived from vitamins): NAD⁺ (from niacin/B3, accepts electrons in redox reactions), FAD (from riboflavin/B2), coenzyme A (from pantothenic acid/B5, carries acyl groups), pyridoxal phosphate (from B6, used in aminotransferases).
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Why Enzyme Lab Values Diagnose Organ Damage
Enzymes are normally confined inside healthy cells. When cells are damaged or die, intracellular enzymes leak into the bloodstream in measurable quantities. Each organ has a characteristic enzyme signature: elevated AST and ALT signal hepatocellular injury (AST:ALT ratio >2 suggests alcoholic hepatitis); troponin I and troponin T are highly cardiac-specific — even small elevations confirm myocardial infarction; amylase rises within hours of acute pancreatitis but is non-specific (also elevated in parotiditis); lipase is more pancreas-specific and stays elevated longer; CK-MB rises 4-6 hours post-MI; LDH is elevated in hemolysis, liver disease, and PE.
Match the Enzyme to Its Diagnostic Significance
Terms
Definitions
Allied Health Relevance — Enzymes
Nursing: Monitor enzyme trends (troponin serial draws q3-6h), interpret LFTs before hepatotoxic drug administration (acetaminophen, statins, isoniazid). Respiratory Therapy: Alpha-1 antitrypsin is a serine protease inhibitor — deficiency allows neutrophil elastase to destroy alveolar walls (emphysema without smoking history). MLT: Perform enzyme assays with precise temperature and pH control in the analyzer to ensure accurate results; reference ranges are method-dependent. Paramedic/EMS: Recognize that field troponin point-of-care testing is now standard; elevated troponin with chest pain triggers cath lab activation protocol.
Carbohydrates — Structure and Energy
Glucose as the body's primary fuel
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen (CH₂O)ₙ. They are the body's preferred fuel source, supplying 4 kcal/gram. The brain and red blood cells are obligate glucose users under normal conditions — they cannot directly oxidize fats or proteins for fuel.
Classification of Carbohydrates
Glucose Regulation — The Feedback Loop
Blood glucose regulation is a classic negative feedback loop. After a meal, glucose rises → pancreatic beta cells secrete insulin → insulin binds receptors on muscle and adipose cells → GLUT4 transporters migrate to the cell membrane → glucose enters cells → blood glucose falls back to 70-100 mg/dL. During fasting, glucose falls → pancreatic alpha cells secrete glucagon → glucagon activates liver glycogen phosphorylase → glycogenolysis releases glucose → blood glucose rises. Failure of insulin signaling (type 2 diabetes) or insulin production (type 1 diabetes) breaks this loop, causing chronic hyperglycemia with multi-organ damage.
Allied Health Relevance — Carbohydrates
Nursing: Monitor blood glucose q4-6h in diabetic patients; recognize Somogyi effect (nocturnal hypoglycemia → rebound hyperglycemia) vs. dawn phenomenon (early-morning hyperglycemia from growth hormone/cortisol); calculate carbohydrate content for insulin-to-carb ratio dosing. Respiratory Therapy: Excess carbohydrate feeding increases CO₂ production (respiratory quotient of carbohydrates = 1.0 vs fat = 0.7) — wean ventilated patients cautiously if overfeeding carbohydrates. MLT: HbA1c measures non-enzymatic glycosylation of hemoglobin over 90-120 days (RBC lifespan), reflecting average blood glucose — falsely low in hemolytic anemia. Paramedic/EMS: Oral glucose (D50W) is first-line for severe hypoglycemia when IV access is unavailable; dextrose concentration matters — D50W is hypertonic and causes vein irritation.
Lipids — Fats, Membranes, and Hormones
Energy storage, cell structure, and signaling
Lipids are a diverse group of hydrophobic biomolecules. They serve as long-term energy stores (9 kcal/gram — more than twice carbohydrates), form the structural backbone of all cell membranes, and serve as precursors to steroid hormones, bile acids, and fat-soluble vitamins. Understanding lipid biochemistry is essential for interpreting lipid panels and cardiovascular risk.
Major Classes of Lipids
Saturated/Trans Fat Inflammation Mechanism
Saturated and trans fats promote inflammation through multiple mechanisms: they increase LDL cholesterol while decreasing HDL; oxidized LDL triggers macrophage recruitment and foam cell formation in arterial walls; trans fats also directly promote endothelial dysfunction by inhibiting prostacyclin (a vasodilator and platelet inhibitor). Additionally, saturated fats activate toll-like receptor 4 (TLR-4) on macrophages, directly triggering the innate immune inflammatory cascade — this connects dietary fat to systemic inflammation, insulin resistance, and atherosclerosis progression.
Allied Health Relevance — Lipids
Nursing: Interpret lipid panels (LDL goal <100 mg/dL high-risk, <70 mg/dL very high-risk; triglycerides <150 mg/dL; HDL >40 men/>50 women); educate patients that dietary cholesterol has less impact than saturated fat on LDL; monitor patients on statins for myopathy (elevated CK). Respiratory Therapy: Pulmonary surfactant is ~90% phospholipid (DPPC); lecithin-to-sphingomyelin (L/S) ratio >2.0 in amniotic fluid indicates lung maturity. MLT: Lipemia (turbid sample) interferes with spectrophotometric assays — specimen rejection criteria apply. Paramedic/EMS: Recognize pancreatitis (severe triglyceridemia >1000 mg/dL is a trigger) in abdominal pain presentations; ketone smell ('fruity breath') in DKA is acetone volatilization.
Nucleic Acids — DNA and RNA
The molecular basis of heredity and protein synthesis
Nucleic acids — DNA and RNA — store, transmit, and express genetic information. DNA is the permanent archive; RNA is the working copy used to build proteins. Understanding nucleic acid chemistry illuminates genetics, pharmacology (drug targets on DNA/RNA), and the molecular basis of genetic disease.
DNA Structure
Double helix: two antiparallel strands wound around each other. Bases pair specifically: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds). G–C pairs are more stable (more H-bonds), explaining why G–C rich regions denature at higher temperatures. The antiparallel orientation (one strand 5'→3', its complement 3'→5') is essential for replication and transcription. The major groove presents base pair information to DNA-binding proteins (transcription factors, restriction enzymes).
RNA Types
mRNA (messenger): single-stranded transcript of one gene; carries codon sequence to ribosome. tRNA (transfer): cloverleaf structure; anticodon loop base-pairs with mRNA codon; 3' end carries specific amino acid. rRNA (ribosomal): structural and catalytic component of ribosomes (ribozyme activity). siRNA/miRNA (regulatory): small RNAs that silence gene expression post-transcriptionally — important in cancer and gene therapy.
The Central Dogma
Transcription occurs in the nucleus; translation occurs on ribosomes in the cytoplasm (or rough ER for secreted proteins). Reverse transcription (RNA → DNA) occurs in retroviruses (HIV) — the basis for antiretroviral drugs that inhibit reverse transcriptase.
Mutations and Clinical Consequences
Silent mutation: codon change does not alter amino acid (due to genetic code degeneracy). Missense mutation: single base change → different amino acid (sickle cell: GAG → GTG, glutamate → valine). Nonsense mutation: base change → premature stop codon → truncated, usually non-functional protein (many BRCA1 mutations). Frameshift mutation: insertion or deletion of bases not in multiples of 3 → shifts reading frame → garbled protein downstream (Duchenne muscular dystrophy, many inherited diseases). Codons: 64 triplets encode 20 amino acids — the code is degenerate (multiple codons per amino acid) but not ambiguous (each codon specifies only one amino acid). Start codon: AUG (methionine). Stop codons: UAA, UAG, UGA.
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Nucleic Acids — Self-Check
1/4Sickle cell disease results from a single base change in the beta-globin gene. This type of mutation is called:
Allied Health Relevance — Nucleic Acids
Nursing: Understand pharmacogenomics (CYP2D6 and CYP2C19 genetic variants alter drug metabolism — affects codeine, warfarin dosing); counsel patients on genetic testing and BRCA results. Respiratory Therapy: CFTR mutation (cystic fibrosis) disrupts chloride channel protein — thick mucus, recurrent infections; new CFTR modulators (ivacaftor, elexacaftor) correct the misfolded protein. MLT: PCR (polymerase chain reaction) amplifies DNA for SARS-CoV-2 detection, genetic disease diagnosis, and cancer mutation profiling. Paramedic/EMS: Understand that cell death in ischemia/trauma destroys nuclear DNA — released cell-free DNA is an emerging biomarker of injury severity being studied in trauma protocols.
Metabolism — ATP Generation
Glycolysis, Krebs cycle, and oxidative phosphorylation
Metabolism is the sum of all chemical reactions in a living organism. Anabolism (building up) requires energy; catabolism (breaking down) releases energy. The currency of cellular energy is ATP (adenosine triphosphate). Understanding ATP production explains why ischemia kills cells within minutes and why metabolic poisons are lethally effective.
Major Metabolic Pathways
Why Ischemia Causes Rapid Cell Death
Aerobic respiration generates approximately 36-38 ATP per glucose molecule. Anaerobic glycolysis generates only 2 ATP per glucose. When a coronary artery is blocked, cardiac myocytes lose their O₂ supply within seconds. Oxidative phosphorylation halts. The cell is immediately dependent on glycolysis for energy — an 18-fold reduction in ATP yield. Within 4-6 minutes, ATP depletion causes Na⁺/K⁺-ATPase failure, ion gradient collapse, cell swelling, calcium influx, and irreversible cell death. This is the molecular basis for why time is muscle in myocardial infarction, and why door-to-balloon time targets <90 minutes.
Lactic Acidosis — Mechanism and Clinical Significance
During anaerobic metabolism, pyruvate cannot enter the Krebs cycle (no O₂ to regenerate NAD⁺). Lactate dehydrogenase converts pyruvate → lactate, regenerating NAD⁺ to allow glycolysis to continue briefly. Lactate accumulates and dissociates (lactic acid ⇌ H⁺ + lactate⁻), lowering blood pH. In clinical practice, elevated serum lactate (>2 mmol/L) is an early marker of tissue hypoperfusion — even before blood pressure falls. Serial lactate measurements guide resuscitation in septic shock. Metformin blocks hepatic lactate clearance (gluconeogenesis) and is contraindicated in renal failure, where lactate accumulation causes metformin-associated lactic acidosis (MALA).
Allied Health Relevance — Metabolism
Nursing: Monitor serum lactate in sepsis (lactate clearance is a resuscitation target — goal >10% decrease per 2 hours); recognize signs of tissue hypoperfusion (cool skin, altered mental status, oliguria) before hemodynamic collapse; manage DKA with insulin + IV fluids + electrolyte replacement. Respiratory Therapy: Mechanical ventilation can be adjusted to compensate for metabolic acidosis by increasing minute ventilation (blowing off CO₂); monitor RQ (respiratory quotient) to guide nutritional support in ventilated patients. MLT: Lactic acid assay requires immediate sample processing on ice (RBCs continue glycolysis ex vivo, falsely elevating lactate); plasma fluoride/oxalate tubes inhibit this. Paramedic/EMS: High-flow O₂ in carbon monoxide poisoning competitively displaces CO from cytochrome oxidase and hemoglobin; HBO therapy for severe poisoning shifts oxyhemoglobin dissociation curve.
Vitamins and Coenzymes
Essential micronutrients and their biochemical roles
Vitamins are organic micronutrients required in small amounts for metabolic reactions. They cannot be synthesized in adequate quantities by the body and must be obtained from diet or supplementation. Vitamins are divided into water-soluble (B complex, C) and fat-soluble (A, D, E, K) groups — a distinction that determines storage, toxicity risk, and clinical significance.
Water-Soluble Vitamins (B Complex + C)
Not stored in significant quantities; excess excreted in urine; toxicity rare. B1 (Thiamine): coenzyme for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase; deficiency → Wernicke's encephalopathy (ataxia, ophthalmoplegia, confusion) in alcoholics, or beriberi (wet: heart failure, dry: neuropathy). B12 (Cobalamin): DNA synthesis, myelin integrity; deficiency → megaloblastic anemia + subacute combined degeneration (posterior and lateral spinal cord). Folate (B9): one-carbon transfers for purine/thymidylate synthesis; deficiency → neural tube defects (spina bifida) — supplement before and during early pregnancy. C (Ascorbic acid): cofactor for prolyl hydroxylase (collagen synthesis); antioxidant; deficiency → scurvy (bleeding gums, poor wound healing, perifollicular hemorrhage).
Fat-Soluble Vitamins (A, D, E, K)
Stored in adipose tissue and liver; toxicity possible with excess (especially A and D). Vitamin K: cofactor for gamma-carboxylation of clotting factors II, VII, IX, X; warfarin blocks K recycling. Vitamin D: synthesized in skin (UV-B → cholecalciferol), hydroxylated in liver (25-OH-D₃) then kidney (1,25-OH₂-D₃, calcitriol — active form); promotes intestinal Ca²⁺ and phosphate absorption; deficiency → rickets (children), osteomalacia (adults). Vitamin A (retinol): 11-cis retinal (photoreceptor pigment for night vision), maintains epithelial integrity; deficiency → night blindness, xerophthalmia, increased infection susceptibility. Vitamin E (tocopherol): lipid-soluble antioxidant protecting membrane PUFA from peroxidation.
Mineral Coenzymes
Zinc (Zn²⁺): cofactor for >300 enzymes including carbonic anhydrase, alcohol dehydrogenase, DNA polymerase; wound healing, immune function, taste/smell. Magnesium (Mg²⁺): required for all ATP-requiring reactions (kinases), DNA replication, neuromuscular function; hypomagnesemia causes refractory hypokalemia (fix Mg first). Iron (Fe): hemoglobin heme group, myoglobin, cytochromes; iron deficiency anemia is the world's most common nutritional deficiency. Copper (Cu): lysyl oxidase cross-links collagen and elastin; ceruloplasmin (copper transport, ferroxidase activity); Menkes disease (copper transporter mutation) causes kinky hair, neurodegeneration.
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Vitamin K and Anticoagulation — The Essential Connection
Vitamin K is required as a cofactor for gamma-glutamyl carboxylase, the enzyme that carboxylates glutamate residues on clotting factors II (prothrombin), VII, IX, and X, and anticoagulant proteins C and S. Carboxylation allows these factors to bind calcium, which anchors them to phospholipid surfaces where clotting cascades occur. Warfarin inhibits vitamin K epoxide reductase (VKOR), blocking vitamin K recycling — this is the mechanism of anticoagulation. Newborn infants have minimal gut bacteria to synthesize vitamin K and low hepatic vitamin K stores, making intramuscular vitamin K injection at birth standard of care to prevent hemorrhagic disease of the newborn.
Allied Health Relevance — Vitamins
Nursing: Administer vitamin K (phytonadione) to reverse warfarin toxicity or newborn hemorrhagic disease prophylaxis; monitor for vitamin D deficiency in elderly, dark-skinned, or institutionalized patients — supplement if 25-OH-D₃ <20 ng/mL; thiamine before dextrose in suspected alcoholics (Wernicke's prevention). Respiratory Therapy: Vitamin C deficiency impairs collagen synthesis — impacts wound healing after chest tube placement; vitamin E may reduce oxidative damage in oxygen therapy (though evidence is mixed). MLT: Prothrombin time (PT/INR) measures vitamin K-dependent factors (II, VII, X); prolonged PT in liver disease or vitamin K deficiency; INR must be interpreted in context of anticoagulant therapy. Paramedic/EMS: Give thiamine (100 mg IV) before D50W in unconscious/obtunded patients who may be alcoholic — glucose load precipitates Wernicke's in thiamine-depleted patients; this is a firm prehospital protocol in many EMS systems.
Acid-Base Chemistry at the Molecular Level
Buffers, bicarbonate, and protein chemistry of pH
Blood pH must be maintained between 7.35 and 7.45. This narrow range is enforced by three interlocking buffer systems. Deviations of even 0.1 pH units cause measurable enzyme dysfunction; deviations of 0.4 units are usually fatal. Understanding the molecular basis of buffering explains every acid-base disorder encountered in clinical practice.
Bicarbonate Buffer System
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. This reaction is catalyzed by carbonic anhydrase (in RBCs, renal tubular cells, gastric mucosa). Normal values: PaCO₂ 35-45 mmHg; HCO₃⁻ 22-26 mEq/L; ratio HCO₃⁻:H₂CO₃ ≈ 20:1 (giving pH 7.4 per Henderson-Hasselbalch). The lungs compensate metabolic disorders in minutes; the kidneys compensate respiratory disorders in 2-5 days.
Protein and Phosphate Buffers
Hemoglobin buffer: accounts for ~60% of non-bicarbonate buffering. Deoxyhemoglobin accepts H⁺ at histidine residues (imidazole pKa ~6.0) — as blood becomes acidic in tissues, hemoglobin releases O₂ and binds H⁺ (Bohr effect). CO₂ forms carbaminohemoglobin directly with protein N-termini. Phosphate buffer (HPO₄²⁻/H₂PO₄⁻, pKa 6.8): minor in blood but major in urine and intracellular fluid — its pKa is ideal for buffering at intracellular pH ~7.0.
Clinical Effects of pH Deviation
Acidosis (pH <7.35): Alters ionization of enzyme active site residues → reduced catalytic activity; systemic vasodilation (peripheral resistance falls → hypotension); hyperkalemia (H⁺ enters cells, K⁺ exits — each 0.1 pH decrease raises serum K⁺ ~0.6 mEq/L); decreased cardiac contractility; right-shift of oxyhemoglobin dissociation curve (O₂ released to tissues). Alkalosis (pH >7.45): Increased albumin binding of Ca²⁺ → decreased ionized calcium → neuromuscular hyperexcitability, tetany, Chvostek's sign, Trousseau's sign, laryngospasm; hypokalemia (K⁺ shifts into cells); left-shift of oxyhemoglobin curve (hemoglobin holds O₂ tighter, less O₂ delivered to tissues).
The Bicarbonate Buffer in Detail
The Henderson-Hasselbalch equation: pH = pKa + log([HCO₃⁻] / [CO₂]). Normal blood maintains a 20:1 ratio of bicarbonate to dissolved CO₂, giving pH 7.4. The lungs can adjust CO₂ within seconds to minutes; the kidneys adjust HCO₃⁻ over hours to days. Hemoglobin contributes ~60% of intracellular buffering — each gram of hemoglobin can carry approximately 0.8 mmol of H⁺. In the tissues, CO₂ from metabolism enters red blood cells, combines with H₂O (catalyzed by carbonic anhydrase) to form H₂CO₃, which dissociates to H⁺ (buffered by hemoglobin) and HCO₃⁻ (exchanged for Cl⁻, the chloride shift). This elegant system transports CO₂ to the lungs while buffering tissue H⁺ production.
Acid-Base Chemistry — Self-Check
1/4A patient with COPD retains CO₂ (PaCO₂ = 58 mmHg). What happens to blood pH?
Allied Health Relevance — Acid-Base
Nursing: Interpret ABGs using the systematic approach: (1) pH acidotic or alkalotic? (2) Is PaCO₂ responsible? (3) Is HCO₃⁻ responsible? (4) Is there compensation? (5) Calculate anion gap for metabolic acidosis (Na − [Cl + HCO₃⁻], normal 8-12 mEq/L; elevated AG suggests organic acid accumulation — lactic acid, ketoacids, toxins). Respiratory Therapy: Ventilator management directly manipulates PaCO₂ — increasing respiratory rate or tidal volume blows off CO₂, raising pH; decreasing ventilation retains CO₂ to compensate metabolic alkalosis. MLT: ABG quality control requires temperature correction (measured at 37°C regardless of patient temp); potassium in ABG samples is unreliable — use separate serum tube; citrate anticoagulant dilutes ABG results by ~10%. Paramedic/EMS: Transcutaneous capnography (ETCO₂) estimates PaCO₂ in real-time; ETCO₂ <35 mmHg in cardiac arrest suggests good chest compressions and/or ROSC approaching; ETCO₂ trend is more predictive than absolute value.
Biochemistry — Comprehensive Review Quiz (10 Questions)
1/10A patient with a high fever (40.5°C/105°F) is at risk for enzyme dysfunction primarily because:
Pre-nursing comprehensive review
1/20Which organelle contains its own DNA and is inherited exclusively from the mother?