Nutrition & Cellular Respiration Review

Nutrients & Nutrition

• Nutrition (Definition)

  • Science that studies how living organisms obtain and utilize food substances for growth, maintenance, reproduction, and health.
  • Integrates biochemistry, physiology, and behavioral science.

• Nutrients (Definition)

  • Chemical substances in food required for energy, growth, maintenance, and regulation of bodily processes.
  • Classified as essential (must be obtained from diet) or non-essential (can be synthesized by the body).

Classification of Nutrients

1. Macronutrients

(Needed in gram quantities; supply energy or serve as bulk structural material)

• Carbohydrates
  • Main quick energy source (1\,\text{g} \approx 4\,\text{kcal}).
  • Stored as glycogen in liver & skeletal muscle or converted to fat via lipogenesis.
  • Structural roles in nucleic acids (ribose, deoxyribose) & glycoproteins.
• Lipids
  • Concentrated energy store (1\,\text{g} \approx 9\,\text{kcal}).
  • Triglycerides: energy storage; phospholipids/cholesterol: membrane integrity; steroids: hormones & bile salts.
  • Stored unlimitedly in adipocytes; mobilized by lipolysis/β-oxidation.
• Proteins
  • Provide 1\,\text{g} \approx 4\,\text{kcal} but primarily structural/functional: enzymes, contractile fibers, transporters, antibodies, some hormones.
  • Excess amino acids deaminated ➜ keto-acids ➜ ATP or lipogenesis; nitrogen excreted as urea.
• Water (sometimes listed)
  • Solvent, temperature regulation, transport medium; \approx 60\% body weight.

2. Micronutrients

(Needed in mg or µg amounts; no caloric value but vital for metabolism)

• Vitamins (organic)
  • Fat-soluble: A, D, E, K (stored; risk of toxicity).
  • Water-soluble: B-complex, C (limited storage; excreted in urine).
  • Coenzymes (e.g., B_3 ➜ NAD+, B_2 ➜ FAD), antioxidants (C, E), vision (A), calcium homeostasis (D), clotting (K).
• Minerals (inorganic)
  • Major (>100 mg/d): Ca, P, Mg, Na, K, Cl, S – skeletal structure, electrolyte balance, ATP formation.
  • Trace (≤100 mg/d): Fe, Zn, Cu, I, Se, Mn, Cr, Mo, Co – enzyme cofactors, oxygen transport, thyroid hormone synthesis.

Calories & Energy Balance

• Calorie (nutritional kilocalorie): Energy required to raise 1\,\text{kg} of water by 1^{\circ}\text{C}; symbolized as kcal.
• Energy balance: \text{Energy\,in} - \text{Energy\,out} = \Delta \text{Body Stores}.
• Basal metabolic rate (BMR) + physical activity + thermic effect of food determine expenditure.

Major Nutrient Biomolecules

1. Carbohydrates

   • Digested to monosaccharides (mainly glucose).
   • Catabolism: Glycolysis ➜ Acetyl-CoA ➜ Citric Acid Cycle ➜ Electron Transport System (ETS) ➜ 30!–!32\,\text{ATP} per glucose (\text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2 \rightarrow 6\,\text{CO}2 + 6\,\text{H}2\text{O} + \text{ATP}).
   • Storage: Glycogenesis; mobilization: Glycogenolysis.

2. Lipids

   • Triglycerides ➜ glycerol + 3 fatty acids.
   • Glycerol ➜ glyceraldehyde-3-phosphate (enters glycolysis).
   • Fatty acids ➜ β-oxidation ➜ multiple acetyl-CoA units enter citric cycle.
   • Excess acetyl-CoA in starvation ➜ ketone bodies.

3. Proteins

   • Proteolysis ➜ amino acids.
   • Transamination/deamination ➜ keto-acids (pyruvate, acetyl-CoA, citric intermediates) + ammonia (converted to urea).
   • Used for gluconeogenesis, ATP, or lipogenesis if energy needs met.

Cellular Respiration—Sequence Review

1. Glycolysis (cytosol)
   • Glucose (6C) ➜ 2 pyruvate (3C), net 2\,\text{ATP}, 2\,\text{NADH}.
2. Intermediate Stage (mitochondrial matrix)
   • Pyruvate ➜ Acetyl-CoA + \text{CO}_2, produces 2\,\text{NADH} / glucose.
3. Citric Acid Cycle (matrix)
   • Acetyl-CoA + OAA ➜ citrate ➜ cycle; per turn: 3\,\text{NADH}, 1\,\text{FADH}2, 1\,\text{ATP}, 2\,\text{CO}2.
4. Electron Transport System (inner membrane)
   • NADH/FADH_2 donate electrons; proton gradient drives ATP synthase ➜ \approx 26\,\text{ATP}.

Entry Points for Nutrients into Pathway

• Glucose ➜ directly starts glycolysis.
• Triglyceride breakdown
  • Glycerol ➜ glycolysis (as G3P).
  • Fatty acids ➜ β-oxidation producing Acetyl-CoA ➜ citric cycle.
• Amino acids ➜ after transamination, enter as pyruvate, acetyl-CoA, or various citric intermediates (α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate).

Metabolic Flexibility & Interconversion Advantages

• Maintains ATP production when one nutrient type is scarce.
• Permits storage of surplus energy in optimal form (e.g., fat is energy-dense).
• Allows glucose sparing (brain priority) by shifting to fatty acid oxidation during fasting.
• Supports gluconeogenesis for strict glucose-dependent tissues (RBCs, renal medulla).

Fate of Excess Nutrients

• Excess Carbohydrates

  • Short-term: glycogen stores (limited to \sim500\,\text{g}).
  • Beyond capacity: converted to fatty acids ➜ triglycerides (lipogenesis) stored in adipose tissue.

• Excess Lipids

  • Directly added to adipocyte triglyceride pool.
  • Adipose mass expands via hypertrophy (size) and hyperplasia (number) if overnutrition persists.

• Excess Amino Acids

  • Not stored as protein; deaminated ➜ acetyl-CoA/ketoglutarate etc. ➜ fat synthesis or ATP.

Practical & Clinical Connections

• Body Mass Index (BMI) & chronic disease risk correlate with sustained positive energy balance.
• Essential fatty acids (linoleic, α-linolenic) & essential amino acids (9 in adults) must be supplied by diet.
• Vitamin D deficiency ➜ rickets/osteomalacia; B_12 malabsorption ➜ pernicious anemia; iron deficiency ➜ microcytic anemia.
• High saturated fat/cholesterol intake ⟶ atherosclerosis; trans fats elevate LDL & reduce HDL.

Ethical & Philosophical Considerations

• Sustainable nutrition emphasizes plant-based diets to lower environmental impact.
• Food equity: ensuring access to micronutrient-rich foods in low-income populations.
• Ethical debates over animal-derived nutrient sources vs. plant or cultured alternatives.