Chapter 17: Nutrition and Metabolism – Study Guide

Nutrition and Metabolism

Introduction to Nutrition and Metabolism

Nutrition and metabolism are fundamental to maintaining homeostasis and supporting cellular functions. Cells break down organic molecules to obtain energy, primarily generating ATP in mitochondria. Essential body chemicals include oxygen, water, vitamins, mineral ions, and organic substrates.

• Oxygen: Required for oxidation of food molecules to yield ATP.

• Water: Necessary for nearly all metabolic reactions.

• Nutrients: Include vitamins, minerals, and organic substrates.

Metabolism

Metabolism encompasses all chemical reactions occurring in the body at any given time. It includes cellular metabolism, which provides energy for homeostasis and essential functions such as metabolic turnover, growth, cell division, secretion, contraction, and action potential propagation.

• Metabolic turnover: Replacement of cell’s organic components.

• Nutrient pool: Source of substrates for catabolism and anabolism.

• Catabolism: Breakdown of organic substrates into smaller molecules, releasing energy for ATP synthesis.

• Anabolism: Synthesis of new organic molecules from smaller ones, consuming ATP.

Functions of Organic Compounds

• Structural maintenance and repairs

• Support growth

• Produce secretions

• Store nutrient reserves

Key organic compounds:

• Glycogen: Branched chain of glucose, main storage carbohydrate in liver and muscle.

• Triglycerides: Main storage lipids, composed of fatty acids.

• Proteins: Most abundant organic components, vital for cellular functions.

Carbohydrate Metabolism

Overview

Carbohydrate metabolism involves glycolysis, ATP production, and gluconeogenesis. Cellular respiration is a three-step process that generates ATP by breaking down carbohydrates, mainly glucose.

• Glucose is the preferred energy source for most cells; RBCs and neurons rely exclusively on glucose.

• B-vitamins are essential for these reactions.

Cellular Respiration Steps

1. Glycolysis: Occurs in cytosol, splits glucose into smaller molecules, generates 2 ATP, anaerobic.

2. TCA Cycle (Krebs Cycle): Occurs in mitochondria, removes hydrogen atoms, generates 1 ATP per cycle (2 cycles per glucose).

3. Oxidative Phosphorylation and Electron Transport System (ETS): Occurs in mitochondria, requires oxygen, produces >90% of ATP.

Overall equation:

$\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

ATP Yield:

• 2 ATP from glycolysis

• 2 ATP from TCA cycles

• 3 ATP from NADH (glycolysis)

• 25 ATP from ETS

• Total: 30–32 ATP per glucose molecule

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors such as lactic acid, glycerol, and amino acids. Surplus glucose is stored as glycogen in liver and muscle.

Lipid Metabolism

Lipid Catabolism (Lipolysis)

Lipid metabolism includes lipolysis, beta-oxidation, and transport/distribution of lipids. Triglycerides are the most abundant lipids, broken down into glycerol and fatty acids.

• Hydrolysis: Splits triglycerides into glycerol and three fatty acids.

• Glycerol: Converted to pyruvic acid, enters TCA cycle.

• Fatty acids: Converted to acetyl-CoA via beta-oxidation.

Beta-Oxidation

Beta-oxidation breaks fatty acids into 2-carbon fragments, generating acetyl-CoA and NADH. Each 2-carbon fragment yields:

• 2 ATP from acetyl-CoA (TCA cycle)

• 5 ATP from NADH

Breakdown of one 18-carbon fatty acid yields about 120 ATP. Lipids provide more energy per gram than carbohydrates or proteins.

Lipid Synthesis and Transport

• Most organic substrates can be converted to acetyl-CoA for lipid synthesis.

• Glycerol is synthesized from dihydroxyacetone phosphate (glycolysis intermediate).

• Nonessential fatty acids and steroids are synthesized from acetyl-CoA.

Lipoproteins: Lipid-protein complexes for transport. Five classes:

Class	Main Function
Chylomicrons	Transport dietary lipids from intestines
VLDLs	Transport triglycerides from liver
IDLs	Intermediate between VLDL and LDL
LDLs	"Bad cholesterol"; delivers cholesterol to tissues
HDLs	"Good cholesterol"; removes excess cholesterol

Protein Metabolism

Protein Catabolism

Protein catabolism involves transamination and deamination. Proteins are broken down when other energy sources are inadequate, with amino acids entering the TCA cycle at various points.

• Transamination: Transfers amino group to keto acid, forming new amino acid.

• Deamination: Removes amino group, generating ammonium ion (toxic).

Protein catabolism is less efficient and produces toxic by-products. Proteins are essential for structure and function, so catabolism is minimized.

Protein Synthesis

• Body synthesizes about half of required amino acids.

• Essential amino acids: Must be obtained from diet (8 not synthesized, 2 insufficiently synthesized).

• Nonessential amino acids: Made by the body as needed.

Essential Amino Acids	Nonessential Amino Acids
Isoleucine, Leucine, Lysine, Threonine, Tryptophan, Phenylalanine, Valine, Methionine, Arginine*, Histidine*	Made by the body
*Arginine and Histidine are insufficiently synthesized

Nucleic Acid Metabolism

RNA Catabolism

Nucleic acid catabolism involves RNA, not DNA. RNA is disassembled to nucleotides; ribose, cytosine, and uracil can be catabolized and enter TCA cycle. Adenine and guanine are deaminized to uric acid for excretion. Excess uric acid can cause gout.

Nucleic Acid Synthesis

• DNA: Replication prior to cell division.

• RNA: Transcription at the start of protein synthesis.

Nutrition

Overview

Adequate nutrition is essential for preventing deficiency disorders and maintaining homeostasis. Nutrition is the absorption of nutrients from food, and requirements vary for each nutrient.

• Balanced diet provides substrates for energy, essential amino acids and fatty acids, minerals, vitamins, and water.

Minerals, Vitamins, and Water

• Minerals: Not synthesized by the body; essential for osmotic balance, physiological processes, and as cofactors.

• Vitamins: Only small quantities synthesized; essential for metabolic reactions.

• Water: Needed to replace losses via urine, feces, and evaporation.

Fat-Soluble Vitamins

Vitamin	Main Function
Vitamin A	Component of visual pigment retinal; deficiency causes night-blindness
Vitamin D	Converted to calcitriol; increases calcium and phosphorus absorption
Vitamin E	Stabilizes intracellular membranes
Vitamin K	Required for synthesis of blood clotting factors; counteracts warfarin

Water-Soluble Vitamins

• Include Vitamin C and B-vitamins

• Components of coenzymes for cellular respiration and metabolism

• Rapidly exchanged between digestive tract and blood; excess excreted in urine

Diet and Disease

• Water requirement: ~2500 mL/day

• Typical U.S. diet: Excess sodium, calories, lipids

• Poor diet contributes to obesity, heart disease, atherosclerosis, hypertension, diabetes

Metabolic Rate and Thermoregulation

Metabolic Rate

Metabolic rate is the average caloric expenditure, reflecting the sum of all anabolic and catabolic processes. It changes with activity and is measured in calories per hour, day, or per unit body weight.

• Basal Metabolic Rate (BMR): Minimum resting energy expenditure, regulated by thyroid gland.

• BMR measured by monitoring respiratory activity; energy utilization proportional to oxygen consumption.

Energy Content of Food:

Nutrient	Calories per gram
Lipids	9.46 Cal/g
Carbohydrates	4.18 Cal/g
Proteins	4.32 Cal/g

If energy intake exceeds demand, excess is stored as triglycerides. If expenditure exceeds intake, reserves are used and weight is lost.

Thermoregulation

Thermoregulation balances heat production and loss, maintaining body temperature within a narrow range for optimal enzyme function.

• Heat production: Energy not captured as ATP is released as heat.

• Heat transfer mechanisms: Radiation, conduction, convection, evaporation.

Mechanism	Description	Contribution to Heat Loss
Radiation	Infrared energy loss	~50%
Conduction	Direct contact	Minimal
Convection	Heat loss to air	~15%
Evaporation	Water loss from skin/lungs	~20%

• Insensible water loss: 20–25 mL/hour from skin and alveoli.

• Sensible perspiration: From sweat glands, varies with activity.

Regulation of Heat Gain and Loss

• Coordinated by heat-gain and heat-loss centers in hypothalamus.

• Heat-loss center stimulates vasodilation, sweating, increased respiration.

• Heat-gain center stimulates vasoconstriction, shunting blood to deep veins, shivering, and hormone release.

Shivering thermogenesis: Increases muscle tone and heat production (up to 400%).

Nonshivering thermogenesis: Hormonal increase in metabolic activity and heat production (10–15%).

Nutrition and Aging

Caloric needs decline with age due to reduced metabolic rates, body mass, activity, and exercise tolerance. Calcium needs increase. After age 50, caloric requirements decrease by 10% per decade.

Key Terms

• Nutrient: Substance required for growth, maintenance, and repair.

• Cellular Respiration: Process of generating ATP from organic molecules.

• Lipolysis: Breakdown of lipids.

• Vitamin: Organic compound required in small amounts for metabolism.

• Lipogenesis: Synthesis of lipids.

• Ketosis: Accumulation of ketone bodies from fatty acid breakdown.

• Metabolism: All chemical reactions in the body.

• Essential Fatty Acid: Fatty acid that must be obtained from diet.

• Ketoacidosis: Dangerous acidification of blood due to excess ketone bodies.

• Glycolysis: Anaerobic breakdown of glucose.

• Basal Metabolic Rate: Minimum energy expenditure at rest.

• Gout: Disorder caused by uric acid accumulation.

• Glycogen: Storage form of glucose.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

• Uric Acid: Waste product from nucleic acid metabolism.

Additional info: Some explanations and tables were expanded for clarity and completeness.