MCAT Bio/Biochem Section

Metabolism

-Key Idea: Carbohydrate, protein, fats & lipids are broken down for ATP production and linked by Acetyl CoA

-Fed state (Insulin): ↑ Energy Store by ↑ Glycolysis, glycogenesis, fatty acid synthesis 

-Fasting state (Glucagon): ↑ Energy Release by ↑ Gluconeogenesis, glycogenolysis, β-oxidation 

-Glycolysis: Breaks down glucose to make ATP

-Gluconeogenesis: Makes glucose during fasting

-Glycogenesis: Stores glucose as glycogen

-Glycogenolysis: Releases glucose from glycogen

-Citric Acid Cycle: Produces NADH/FADH₂ for ETC

-ETC: Produces most ATP

-β-Oxidation: Breaks down fatty acids into acetyl-CoA

-Lipogenesis/ Fatty Acid Synthesis: Stores excess energy as fat

-Urea Cycle: Removes toxic ammonia

Glycolysis Pathway:

• Net: 1 Glucose + 2 ADP + 2 Pi + 2 NAD⁺ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

• Occurs in the cytoplasm

Gluconeogenesis Pathway:

Citric Acid Cycle Pathway:

• Net: 1 Acetyl-CoA + 3 NAD⁺ + FAD + ADP (or GDP) + Pi + 2 H₂O → 2 CO₂ + 3 NADH + FADH₂ + ATP (or GTP) + CoA-SH + 3 H⁺

• Pathway: 1. Oxaloacetate + Acetyl-CoA —(Citrate synthase)→ 2. Citrate ⇌ (Aconitase) ⇌ 3. Isocitrate —(Isocitrate dehydrogenase)→ 4. α-Ketoglutarate —(α-Ketoglutarate dehydrogenase)→ 5. Succinyl-CoA ⇌ (Succinyl-CoA synthetase) ⇌ 6. Succinate ⇌ (Succinate dehydrogenase) ⇌ 7. Fumarate ⇌ (Fumarase) ⇌ 8. Malate ⇌ (Malate dehydrogenase) ⇌ 9. Oxaloacetate

• Citrate synthase: Inhibitors: ATP, NADH, succinyl-CoA, citrate; Activator: ADP 

• Isocitrate dehydrogenase: Inhibitors: ATP, NADH; Activator: ADP and NAD+ 

• α-Ketoglutarate dehydrogenase: Inhibitors: ATP, NADH, succinyl-CoA; Activator: ADP 

• Eukaryotes: occurs in mitochondrial matrix; prokaryotes: occurs in cytoplasm

Oxidative Phosphorylation:

-Uses NADH and FADH₂ to produce most ATP.

-Pathway:

Complex I: NADH → NAD⁺ (electrons enter ETC; pumps H⁺)

Complex II: FADH₂ → FAD (electrons enter ETC; does not pump H⁺)

Complex III: Transfers electrons to Cyt c; pumps H⁺

Complex IV: Transfers electrons to O₂ → H₂O; pumps H⁺

Complex V (ATP synthase): H⁺ gradient → ATP

-No O₂: ETC stops →  ↓ ATP production → NADH accumulates.

-Complex II inhibition: FADH₂ cannot donate electrons = ↓ ATP production.

-ATP synthase inhibition: H⁺ gradient builds up = ↓ ATP production

-Uncoupling: H⁺ gradient dissipates → ↓ ATP, ↑ heat.

ATP Yield from 1 Glucose:

-Ratio: 1 NADH = 2.5 ATP, 1 FADH2 = 1.5 ATP

-Glycolysis: 2 ATP + 2 NADH = 7 ATP

-Pyruvate Dehydrogenase: 2 NADH = 5 ATP

-Citric Acid Cycle: 2 ATP (GTP) + 6 NADH + 2 FADH₂ = 20 ATP

-Total = 32 ATP/glucose

Fatty Acid Synthesis (FAS/Lipogenesis):

-Cytoplasm converts acetyl-CoA into fatty acid using NADPH for energy storage.

-Acetyl-CoA carboxylase (Rate-limiting enzyme): Activated by insulin (fed-state) = ↑ FAS; inhibited by glucagon (fasting) = ↓ FAS

β-Oxidation:

-Breaks down fatty acids into Acetyl-CoA, NADH, and FADH₂ for ATP production in the mitochondrial matrix.

-Activated by Glucagon (fasting); Inhibited by Insulin (fed state) = no β-oxidation = ↓ ATP production

Urea Cycle:

-Converts toxic ammonia (NH₃) from amino acid metabolism into urea for removal

-Occurs in the liver (mitochondria and cytosol).

-Links to the Citric Acid Cycle via fumarate.

Pentose Phosphate Pathway (do not need to memorize pathways):

-Produces NADPH (fatty acid synthesis & antioxidant defense) and ribose-5-phosphate (DNA/RNA synthesis) in the cytosol.

-Glucose-6-phosphate dehydrogenase (G6PD) deficiency: ↓ NADPH → oxidative stress → hemolytic anemia (destroy red blood cells)

General Hormone Types In Endocrine System:

-Direct hormones: Act directly on target organs.

-Tropic hormones: Stimulate other endocrine glands.

-Peptide hormones: Bind cell surface receptor via second messenger → fast.

-Steroid hormones: Bind intracellular receptor for gene expression → slow.

Anterior Pituitary Hormones:

-Follicle(Fall-la-co) -stimulating (FSH): Follicle development; spermatogenesis (sperm production) (sperm-ma-gen-nah-sis)

-Luteinizing (LH): Stimulates ovulation; testosterone production.

-Adrenocorticotropic (ACTH): Stimulates adrenal cortex to make cortisol

-Thyroid-stimulating (TSH): Produce thyroid hormones.

-Prolactin: Stimulates milk production

-Endorphins: Inhibits perception of pain in the brain.

-Growth hormone: Stimulates bone/muscle growth; lipolysis (breakdown stored fat) (la-paul-la-sis)

-Mmenonic: FLAT PEG 

Hypothalamus Hormones:

-Stored in posterior pituitary

-Oxytocin (oxy-toe-sin): Stimulates contractions during labor; milk production

-Antidiuretic(ADH; Anti-die-rat-tick)/vasopressin: Water reabsorption in kidneys.

Thyroid Hormones:

-Thyroid hormones (T₃, T₄): Increase metabolic activity.

-Calcitonin (cal-suh-toe-in): Decreases blood Ca²⁺ 

-Parathyroid hormone: Increases blood Ca²⁺ 

Adrenal Cortex Hormones:

-Glucocorticoid (glue-co-corticoid) ↑ blood glucose level and ↓ protein synthesis.

-Mineralocorticoid: ↑ sodium and water reabsorption in kidneys.

Adrenal Medulla Hormones:

-Epinephrine (ep-puh-nah-friend), Norepinephrine (nora-puh-nah-friend): Increases blood glucose level, heart rate, and blood pressure.

Pancreas Hormones:

-Glucagon: Converts glycogen to glucose in the liver; ↑ blood glucose.

-Insulin: ↓ blood glucose; ↑ glycogen stores.

-Somatostatin: Inhibit glucagon and insulin.

Pineal (Pin-knee-o) Hormones:

-Melatonin: Regulates sleep–wake cycles.

Testes Hormones:

-Testosterone: Maintains male secondary s*x characteristics; spermatogenesis (sperm production)

Ovary Hormones:

-Estrogen: Maintains female secondary s*x characteristics.

-Progesterone (Pro-gest-ter-wrong): Maintains endometrium to support pregnancy

Muscular System (Skeletal, Smooth, Cardiac):

Skeletal:

-Support and movement, thermoregulation, blood propulsion (move blood back to heart)

-Voluntary/somatic control

-Multinucleated (1 cell has many nuclei)

Smooth:

-Walls of respiratory, reproductive, cardiovascular, and digestive system

-Involuntary/autonomic control

-Uninucleated (1 cell has 1 nucleus)

Cardiac:

-Heart; pumps blood

-Involuntary/autonomic control

-Uni nucleated (1 cell has 1 nucleus)

-Intercalated discs (gap junctions) allow coordinated contractions.

Sarcomere (Functional Unit of Muscle)

-Structure:

Z-line: Boundary/ends 

M-line: Middle

-Bands:

I-band: Actin only.

H-zone: Myosin only.

A-band: Entire length of myosin 

-Muscle contraction: Actin slides towards Myosin, bringing Z-lines closer to the M-line, shortening sarcomere, I-band and H-zone decrease, and A-band stays the same.

Muscle Contraction Steps:

1. ACh triggers an action potential.

2. Action potential travels along the sarcolemma (sar-co-leh-muh) and T-tubules.

3. Sarcoplasmic reticulum (SR) releases Ca²⁺.

4. Ca²⁺ binds troponin (trop-o-n), moving tropomyosin.

5. Myosin binds actin → muscle contracts.

6. ATP pumps Ca²⁺ back into the sarcoplasmic reticulum (SR).

7. Tropomyosin re-blocks myosin-binding sites.

8. Muscle relaxes.

Digestive System:

1. Oral Cavity/Mouth: Mastication starts digestion 

2. Pharynx: Epiglottis (ep-puh-glaa-tis) prevents food from entering larynx (airway).

3. Esophagus: Peristalsis (pair-ra-stall-sis) moves food to the stomach.

4. Stomach: Acidic environment; begins protein digestion with:

Pepsin: Digests proteins.

Mucous cells: Protect stomach lining.

Chief cells: Secrete pepsinogen.

Parietal cells: Acidifies stomach; activates pepsin.

G cells: Secrete gastrin (↑ acid secretion).

5. Small intestine: Major site of digestion & nutrient absorption.

6. Large intestine: Absorbs H₂O and salts; forms feces.

7. Rectum: Stores feces before elimination.

Mnemonic: Old People Eats Salty Snacks Like Rabbits(OPESSL)

Carbohydrate Digestion Enzymes:

Salivary amylase (am-may-lays): Salivary glands → mouth; starch → maltose.

Pancreatic amylase: Pancreas → small intestine; starch → maltose.

Maltase: Intestinal glands → small intestine; maltose → 2 glucose.

Sucrase: Intestinal glands → small intestine; sucrose → glucose + fructose.

Lactase: Intestinal glands → small intestine; lactose → glucose + galactose.

Mnemonic: Silly People Make Sweet Lemonade (SPMSL)

Protein Digestion Enzymes:

-Pepsin: Gastric glands (chief cells) → stomach; hydrolyzes specific peptide bonds

-Trypsin: Pancreas → small intestine; hydrolyzes specific peptide bonds & chymotrypsinogen → chymotrypsin

-Chymotrypsin: Pancreas → small intestine; hydrolyzes specific peptide bonds

-Carboxypeptidase: Pancreas → small intestine; hydrolyzes terminal peptide bond at C-terminus

-Aminopeptidase: Intestinal glands → small intestine; hydrolyzes terminal peptide bond at N-terminus

-Dipeptidase: Intestinal glands → small intestine; Hydrolyzes part of amino acids

-Enteropeptidase: Intestinal glands → small intestine; trypsinogen → trypsin

Mmenomic: Please Take Chocolate Cakes After Dinner Everyday. (TCCADE)

Excretory System

-Kidneys filters blood, removes wastes, and produces urine 

1. Bowman's capsule: Filters blood to start urine formation.

2. Proximal convoluted tubule (PCT): Bulk reabsorption of glucose, amino acids, salts & H₂O; secretes H⁺, NH₃ & drugs.

3. Descending loop of Henle: Permeable to H₂O only.

4. Ascending loop of Henle: Permeable to salts only.

5. Distal convoluted tubule (DCT): Fine-tunes ions; responds to aldosterone.

6. Collecting duct: Responds to ADH & aldosterone; Reabsorbs H₂O.

7. Renal pelvis: Collects urine.

8. Ureter (yur-uh-tur): Carries urine to the bladder.

9. Bladder: Stores urine.

10. Urethra (yur-re-tah): Releases urine. 

Mmenomic: Big Pandas Do Awesome Dances Carrying Red Umbrellas Back Up.

Kidney Hormones

-Aldosterone: ↑ Na⁺ reabsorption; ↑ K⁺ & H⁺ secretion = ↑ H₂O reabsorption, blood volume & blood pressure.

-Antidiuretic(ADH; Anti-die-rat-tick)/vasopressin: ↑ H₂O reabsorption in collection duct by ↑ its permeability to water.

Circulation (Blood Flow):

-Deoxygenated: Body → Vena cava → Right atrium → Right ventricle → Pulmonary artery → Lungs.

(a) Mnemonic: Big Vikings Ride Red Ponies Loudly (BVRRPL)

-Oxygenated: Lungs → Pulmonary vein → Left atrium → Left ventricle → Aorta → Body.

(a) Mnemonic: Little Ponies Love Leaping Around Barns (LPLLAB)

Electrical Conduction

-Pathway: SA node → AV node → Bundle of His → Purkinje fibers

-SA node: Starts heartbeat (pacemaker).

-AV node: Delays signal before ventricles.

-Bundle of His: Carries signal to ventricles.

-Purkinje fibers: Spread signal through ventricles.

Types of Blood Vessels:

Blood flow: Artery → Arteriole  (R-tear-re-o) → Capillary → Venule → Vein (Mnemonic: All Apples Can Visit Vegas)

-Arteries: Thick, elastic wall; carry blood away from the heart.

-Arterioles: Regulate blood flow.

-Capillaries: One-cell thick wall; site of gas, nutrient & waste exchange.

-Venules: Small veins.

-Veins: Thin walls + valves; carry blood to the heart.

Blood

-Plasma (~55%): Liquid portion; transports nutrients, hormones, wastes & proteins.

-Erythrocytes (uh-reth-o-sight) (Red Blood Cells = RBCs, ~45%): Carry O₂ via hemoglobin; no nucleus or mitochondria.

-Leukocytes (loo-ko-sight) (White Big Cells = WBCs, <1%): Immune defense (fight infection)

-Platelets (plate-liss) (<1%): Blood clotting (thromboplastin → thrombin → fibrin → clot).

Blood Types

-Type A: Antigen A; anti-B antibodies; donates to A, AB; receives A, O.

-Type B: Antigen B; anti-A antibodies; donates to B, AB; receives B, O.

-Type AB: Antigens A & B; no antibodies; donates to AB; receives A, B, AB, O (universal recipient).

-Type O: No antigens; anti-A & anti-B antibodies; donates to A, B, AB, O (universal donor); receives O only.

Rh Factor

Rh⁺: Has Rh antigen; no anti-Rh antibodies.

Rh⁻: No Rh antigen; can produce anti-Rh antibodies after Rh⁺ exposure.

Respiratory System

-Airway Pathway: Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles (brang-key-owlz) → Alveoli (Mmenomic: Nice People Love To Breathe Big Air)

-Larynx: Vocal cords; epiglottis prevents food from entering the airway.

-Trachea & Bronchi: Cilia removes debris.

-Bronchioles: Smallest airways 

-Alveoli: Site of O₂/CO₂ exchange ; surfactant prevents collapse.

-Diaphragm: Main muscle of breathing

Pulmonary Circulation

-Pulmonary artery: Carries deoxygenated blood to the lungs.

-Pulmonary vein: Carries oxygenated blood to the heart.

Breathing

-Inhalation: Diaphragm & external intercostal contract → ↑ lung volume → ↓ lung pressure → air enters.

-Exhalation: Muscles relax → ↓ lung volume → ↑ lung pressure → air exits

Lung Volumes

-Tidal volume (TV): Normal breathing

-Inspiratory reserve volume (IRV): Extra inhale.

-Expiratory reserve volume (ERV): Extra exhale

-Residual volume (RV): Air left after max exhale; Prevents alveolar collapse.

-Vital capacity (VC): Maximum air exhale after maximal inhale (TV + IRV + ERV).

Bicarbonate buffer. Given: CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)

• Regulates pH by changing CO₂ levels.

• ↑ CO₂ (hypoventilation) → ↑ H⁺ → ↓ pH (acidic)

• ↓ CO₂ (hyperventilation) → ↓ H⁺ → ↑ pH (basic)

Humoral Immunity (B cells)

-B cells: Mature in bone marrow; produce antibodies.

-Plasma cells: Secrete antibodies.

-Memory B cells: Provide faster response upon re-exposure.

-Opsonization: Antibodies tag pathogens for phagocytosis.

-Agglutination: Antibodies clump pathogens together.

Cell-Mediated Immunity (T cells)

-T cells: Mature in the thymus; attack infected cells.

-Cytotoxic T cells (CD8⁺): Kill infected cells directly.

-Helper T cells (CD4⁺): Activate B cells, T cells & macrophages via cytokines.

-Regulatory (Suppressor) T cells: Suppress immune response.

-Memory T cells: Provide faster response upon re-exposure.