Metabolism Revised

Overview of Cellular Metabolism

Definition of Metabolism

  • Definition: All chemical reactions that occur in an organism.

Key Terms in Metabolism

  • Catabolism:

    • Definition: Breakdown of organic substrates within the body.

  • Anabolism:

    • Definition: Synthesis of new organic molecules within the body.

  • Cellular metabolism:

    • Definition: Chemical reactions occurring specifically within cells.

Metabolism (Continued)

Metabolic Turnover

  • Definition: The process of continual breakdown and replacement of all cellular organic components, except DNA.

  • Sources for Building Blocks:

    • Obtained from:

    • Catabolic reactions.

    • Absorption of organic molecules from surrounding interstitial fluids.

  • Both processes contribute to a nutrient pool, which provides accessible organic substrates for cellular functions.

Cellular Catabolism

  • Aerobic metabolism occurs in mitochondria.

    • Energy Capture: 40% of energy captured.

    • Used for: Conversion of ADP to ATP.

    • Energy Loss: 60% of energy escapes as heat, which helps to warm the cell interior and surrounding tissue.

The Nutrient Pool

Definition of Nutrient Pool

  • Nutrient Pool: A source for organic substrates for both catabolism and anabolism.

Importance of Anabolism in Cells

  • Requires for:

    • Replacing membranes, organelles, enzymes, and structural proteins.

Importance of Catabolism in Cells

  • Requires for:

    • Converting substrates to a two-carbon molecule, utilized by mitochondria to produce ATP.

Mobilization of Metabolic Reserves

Mobilization Process

  • Reserves are mobilized when nutrient absorption across the digestive tract is insufficient to maintain levels.

  • Different cells and their functions during mobilization:

    • Liver Cells: Break down triglycerides and glycogen to release fatty acids and glucose.

    • Adipocytes: Break down triglycerides to release fatty acids.

    • Skeletal Muscle Cells: Break down contractile proteins to release amino acids.

Restoration of Metabolic Reserves

  • Occurs when nutrient absorption exceeds immediate nutrient needs.

    • Liver Cells: Store triglycerides and glycogen.

    • Adipocytes: Convert excess fatty acids to triglycerides.

    • Skeletal Muscles: Build glycogen reserves and utilize amino acids to increase myofibril counts.

Utilization of Resources

  • Continuous absorption and catabolism of glucose in most tissues, vital for cellular functions.

  • Neural Tissue: Must have a continuous glucose supply; during starvation, other tissues shift to utilizing fatty acids or amino acids to conserve glucose for neural tissue.

Review of Digestion

Definition of Digestion

  • Digestion: The breakdown of physical structure and chemical bonds of organic compounds, allowing absorption in the digestive tract and distribution via the bloodstream for energy and restocking of the intracellular nutrient pool.

Steps in the Process of Digestion

  1. Oral Cavity:

    • Saliva dissolves organic molecules.

    • Mechanical processing by teeth and tongue disrupts structures.

    • Begins chemical digestion of carbohydrates and lipids.

  2. Stomach:

    • Further physical and chemical breakdown by stomach acid and enzymes, beginning protein digestion.

  3. Duodenum:

    • Buffers from pancreas and liver moderate pH of chyme.

    • Digestive enzymes catalyze catabolism of all organic molecules needed by cells.

  4. Jejunum and Ileum:

    • Site of most nutrient absorption, with nutrients delivered to the liver via hepatic portal vein for necessary absorption.

  5. Large Intestine:

    • Water reabsorbed.

    • Bacterial action generates organic nutrients and vitamins.

    • Indigestible material and wastes are excreted as feces.

Vitamins

Nutrition and Vitamins

  • Nutrition: Absorption of nutrients from food.

  • Vitamins: Organic compounds required in very small quantities for essential metabolic activities.

Classes of Vitamins

  1. Fat-soluble Vitamins:

    • Vitamins A, D3, E, K.

  2. Water-soluble Vitamins:

    • Contains B vitamins and C.

Fat-Soluble Vitamins

  • Fat-Soluble Vitamins: Absorbed from the digestive tract with lipid contents of micelles.

    • Sources:

    • Vitamins A, E, K from vegetables.

    • Vitamin D3 synthesized in skin via sunlight exposure.

    • Vitamin K produced by intestinal bacteria.

  • Storage: Stored in lipid deposits, allowing large bodily reserves (metabolic activities can continue for months without dietary sources).

Hypovitaminosis and Hypervitaminosis

  • Hypovitaminosis:

    • Vitamin deficiency disease; rarely occurs (except for vitamin D3).

  • Hypervitaminosis:

    • Results when dietary intake exceeds body’s storage, utilization, or excretion capabilities; can be harmful.

Examples of Fat-Soluble Vitamins

  • Vitamin A:

    • Function: Maintains epithelia, required for synthesis of visual pigments, supports immune system, promotes growth and bone remodeling.

    • Sources: Leafy green and yellow vegetables.

    • Recommended Daily Allowance (RDA): 0.7-0.9 mg.

    • Effects of Deficiency: Night blindness, deterioration of epithelial membranes.

    • Effects of Excess: Liver damage, skin paling, CNS effects (e.g., nausea).

  • Vitamin D3:

    • Function: Essential for normal bone growth, intestinal calcium and phosphorus absorption.

    • Sources: Synthesized in skin exposed to sunlight.

    • RDA: 0.005-0.015 mg.

    • Effects of Deficiency: Rickets, skeletal deterioration.

    • Effects of Excess: Calcium deposits disrupting functions.

  • Vitamin E:

    • Function: Prevents breakdown of vitamin A and fatty acids.

    • Sources: Meat, milk, vegetables.

    • RDA: 15 mg.

    • Effects of Deficiency: Suspected anemia.

  • Vitamin K:

    • Function: Essential for liver synthesis of prothrombin and clotting factors.

    • Sources: Vegetables, produced by intestinal bacteria.

    • RDA: 0.09-0.12 mg.

    • Effects of Deficiency: Bleeding disorders.

Water-Soluble Vitamins

  • Properties:

    • Most are components of coenzymes, rapidly exchanged between digestive tract and blood, excess amounts readily excreted in urine.

  • Sources: B vitamins from meat, eggs, dairy products; Vitamin C from citrus fruits.

  • Exceptions: Vitamins B12 and C are exceptions regarding storage and absorption properties.

Examples of Water-Soluble Vitamins

  • Vitamin B₁ (Thiamine):

    • Function: Coenzyme in many pathways.

    • Sources: Milk, meat, bread.

    • RDA: 1.1-1.2 mg.

    • Effects of Deficiency: Beriberi, muscle weakness, cardiovascular problems.

  • Vitamin B₂ (Riboflavin):

    • Function: Part of FAD, involved in various pathways.

    • Sources: Milk, meat, eggs, cheese.

    • RDA: 1.1-1.3 mg.

    • Effects of Deficiency: Epithelial and mucosal deterioration.

  • Vitamin B3 (Niacin):

    • Function: Part of NAD, involved in multiple pathways.

    • Sources: Meat, bread, potatoes.

    • RDA: 14-16 mg.

    • Effects of Deficiency: Pellagra, CNS and GI deterioration.

  • Vitamin B5 (Pantothenic Acid):

    • Function: Coenzyme A in multiple pathways.

    • Sources: Meat, whole grains.

    • RDA: 10 mg.

  • Vitamin B6 (Pyridoxine):

    • Function: Coenzyme in amino acid and lipid metabolism.

    • Sources: Milk, meat.

    • RDA: 1.3-1.7 mg.

  • Vitamin B9 (Folic Acid):

    • Function: Coenzyme in amino acid and nucleic acid metabolism.

    • Sources: Leafy vegetables, some fruits.

    • RDA: 0.2-0.4 mg.

  • Vitamin B12 (Cobalamin):

    • Function: Coenzyme in nucleic acid metabolism.

    • Sources: Milk, meat.

    • RDA: 0.0024 mg.

  • Vitamin C:

    • Function: Coenzyme in many pathways.

    • Sources: Citrus fruits.

    • RDA: 75-90 mg (add 35 mg for smokers).

Nutrition

Balanced Diet

  • Definition: Contains all ingredients needed to maintain homeostasis, such as:

    • Substrates for ATP production.

    • Essential amino acids.

    • Fatty acids.

    • Vitamins.

    • Electrolytes.

    • Water.

Malnutrition

  • Definition: An unhealthy state arising from inadequate or excessive nutrient absorption.

Dietary Guidelines (ChooseMyPlate.gov)

  • Overview: Personalized eating plans based on current Dietary Guidelines for Americans.

  • Color-Coded Groups:

    • Grains (orange)

    • Vegetables (green)

    • Fruits (red)

    • Dairy products (blue)

    • Proteins (purple)

  • Note: Oils are not represented but should be used sparingly.

Clinical Module: Metabolic Disorders

Diet and Digestion Disorders

  • Eating Disorders: Psychological issues leading to inadequate or excessive food consumption.

    • Anorexia Nervosa:

    • Description: Form of self-induced starvation, primarily affects adolescent Caucasian females.

    • Characteristics: Patients often 30% below normal weight, convinced of being overweight.

    • Death Rates: Severe cases have a 10-15% mortality rate.

    • Bulimia:

    • Description: Condition where individuals consume 20,000 calories or more, followed by induced vomiting or use of laxatives.

    • Prevalence: More common than anorexia; generally affects adolescent females.

    • Health Risks: Include cumulative damage from stomach acids, electrolyte imbalances, and cardiac arrhythmias.

Obesity

  • Definition: Condition of being more than 20% over ideal weight.

  • Health Risks Associated:

    • Linked to diabetes, hypertension, and hypercholesterolemia.

  • CDC Estimates: Obesity prevalence at 35.7% among U.S adults and ~17% among U.S children.

  • Etiology: Results from a food energy intake greater than energy usage.

Categories of Obesity

  1. Regulatory Obesity:

    • Description: Failure to regulate food intake; most common type.

  2. Metabolic Obesity:

    • Description: Secondary to an underlying malfunction in cell/tissue metabolism; relatively rare.

Elevated Cholesterol Levels

  • Associated Risks: Development of atherosclerosis and coronary artery disease.

  • Recommendations:

    • Cholesterol intake should be <300 mg/day to maintain health.

    • High LDL (low-density lipoprotein) levels may deposit in blood vessels.

    • Low HDL (high-density lipoprotein) levels below 35 mg/dL are also problematic.

Ketoacidosis

  • Definition: Acidification of blood from ketone body production, leading to ketosis.

  • Etiology: Occurs when glucose availability is limited, leading to fatty acid and amino acid catabolism in the liver.

  • Risks: Extreme cases can lead to blood pH below 7.05, possibly causing coma or cardiac arrhythmias; major risk is in uncontrolled diabetes mellitus.

Gout

  • Definition: Condition involving insoluble uric acid crystals formed from purine bases of RNA catabolism.

  • Normal Levels: 2.7-7.4 mg/dL.

  • Pathophysiology: Higher concentrations lead to saturation of body fluids, forming insoluble crystals that accumulate in joints. Most notably causes painful gouty arthritis, typically presenting in the great toe.

Energetics and Thermoregulation

Energetics

  • Definition: The study of energy flow and energy conversion within biological systems.

  • Basal Metabolic Rate (BMR):

    • Definition: The minimum resting energy expenditure for awake and alert individuals.

    • Measurement: Can be assessed by monitoring respiratory activity.

    • Caloric Expenditure: Approximately 4.825 Cal expended per liter of oxygen consumed; average BMR is about 70 Cal/hour (1680 Cal/day).

    • Factors Influencing BMR: Includes size, weight, physical activity level; food intake must be adequate to support these activities.

Thermoregulation

  • Definition: Homeostatic control of body temperature, essential for maintaining enzyme function within an optimal temperature range.

    • Metabolic activities generate heat; 40% of energy is used to form ATP, 60% is released as heat.

Mechanisms of Heat Transfer

  • To maintain body temperature, heat loss must be balanced by heat production. Four primary mechanisms include:

  1. Radiation:

    • Definition: Heat energy transfer via infrared radiation (e.g. from the sun).

    • Significance: Over 50% of body heat loss indoors is via radiation.

  2. Evaporation:

    • Definition: Water shifts from liquid to vapor, absorbing heat (0.58 Cal per gram of water).

    • Types:

      • Insensible perspiration: Continual water loss (~20-25 mL/hour).

      • Sensible perspiration: Can excrete up to 2–4 liters/hour during high temperature.

  3. Convection:

    • Definition: Heat loss due to air movement; warmer air rises away from the body, replaced by cooler air, accounting for ~15% of indoor heat loss.

  4. Conduction:

    • Definition: Direct transfer of energy through physical contact; typically not effective for heat gain/loss.

Physiological Effects of Failure to Control Body Temperature

  • Consequences: Depending on the severity of temperature control failure, consequences range from impaired thermoregulation to potential death if severe.

Responses to High Body Temperature

  • Behavioral strategies include moving to shade or water, vasodilation/shunting of blood to skin, increased sweat production, and increased respiratory heat loss due to deeper breathing.