Ch 23: Metabolism and Energy Balance – Study Guide and Review Notes

Ch 23: Metabolism and Energy Balance

Overview

This chapter explores the integrated actions and regulation of metabolism and energy balance in the human body. It covers the biochemical pathways, hormonal regulation, and physiological responses that maintain homeostasis in response to feeding, fasting, and energy demands.

Energy Balance and Metabolism

Energy Balance Equation

• Energy balance is the relationship between energy intake (food) and energy expenditure (work and heat).

• The equation for energy balance is:

$\text{Energy intake} = \text{Energy output (work + heat)}$

• Positive energy balance leads to weight gain; negative energy balance leads to weight loss.

Types of Work

• Transport work: Moving molecules across membranes (e.g., ion pumps).

• Mechanical work: Muscle contraction, movement of cilia, etc.

• Chemical work: Synthesis of macromolecules, cellular growth, and repair.

Respiratory Quotient (RQ)

• RQ is the ratio of CO2 produced to O2 consumed during metabolism.

• It varies with the type of macronutrient oxidized:

  • Lipids: lowest RQ

  • Carbohydrates: highest RQ

  • Proteins: intermediate RQ

Regulation of Feeding and Satiety

Hypothalamic Feeding Center

• The hypothalamus integrates signals to regulate hunger and satiety.

• Key signals include neural, hormonal, and nutrient-related factors.

Hormones and Signals Affecting Appetite

Stimulus/Chemical	Location of Production	Effect on Feeding/Satiety	Additional Effect
Ghrelin	Stomach	Stimulates hunger	Increases food intake
CCK	Small intestine	Promotes satiety	Slows gastric emptying
GLP-1	Intestine	Promotes satiety	Stimulates insulin secretion
Eating & Chewing	Oral/nervous system	Promotes satiety	Behavioral feedback
Light, smell, taste	Sensory organs	Stimulates hunger	Triggers feeding behavior
Leptin	Adipose tissue	Promotes satiety	Regulates long-term energy balance

Additional info: Table entries inferred from standard physiology sources.

Metabolism of Organic Macromolecules

Major Pathways

Process	Starting Molecule	Ending Molecule	Characteristic Enzyme	Location of Catabolism	Location of Storage
Glycolysis	Glucose	Pyruvate	Hexokinase, PFK	Cytoplasm	Blood, muscle, liver
Glycogenolysis	Glycogen	Glucose	Glycogen phosphorylase	Liver, muscle	Liver, muscle
Glycogenesis	Glucose	Glycogen	Glycogen synthase	Liver, muscle	Liver, muscle
Lipolysis	Triglycerides	Fatty acids, glycerol	Lipase	Adipose tissue	Adipose tissue
Lipogenesis	Acetyl-CoA	Fatty acids	Fatty acid synthase	Liver, adipose	Adipose tissue
Protein synthesis	Amino acids	Proteins	Ribosomes	All cells	Muscle, tissues

Additional info: Table completed with standard metabolic pathways.

Hormonal Regulation of Metabolism

Key Hormones

Location of Production	Insulin	Glucagon
Pancreatic beta cells	Yes	No
Pancreatic alpha cells	No	Yes

• Insulin: Promotes glucose uptake and storage; lowers blood glucose.

• Glucagon: Stimulates glycogen breakdown and gluconeogenesis; raises blood glucose.

Hormonal Effects on Metabolic Processes

Metabolic Process	Glucagon	Epinephrine	Cortisol	Growth Hormone
Glycogenolysis	✔	✔	✔	✔
Gluconeogenesis	✔	✔	✔	✔
Inhibition of glucose uptake		✔	✔	✔

Additional info: Table entries inferred from standard hormone actions.

Metabolic States

Absorptive vs. Postabsorptive State

• Absorptive state: Occurs during and shortly after eating; nutrients are absorbed and stored.

• Postabsorptive state: Occurs when the gut is empty; energy is supplied by breakdown of body stores.

Body Mass and Health Assessment

BMI (Body Mass Index)

• Definition: $\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

• Use: Screening tool for overweight and obesity.

• Limitations: Does not distinguish between muscle and fat mass.

Cholesterol and Lipoproteins

LDL vs. HDL

• LDL (Low-Density Lipoprotein): Delivers cholesterol to tissues; high levels associated with atherosclerosis.

• HDL (High-Density Lipoprotein): Removes cholesterol from tissues; protective against heart disease.

Diabetes Mellitus

Types and Effects

Effect	Diabetes Mellitus	Diabetes Insipidus
Polyuria	Yes	Yes
Polydipsia	Yes	Yes
Glucosuria	Yes	No
Blood glucose	High	Normal

• Type 1 Diabetes: Autoimmune destruction of beta cells; insulin deficiency.

• Type 2 Diabetes: Insulin resistance; often associated with obesity.

Thermoregulation

Mechanisms of Heat Transfer

Used for Heat Loss	Radiation	Conduction	Convection	Evaporation
Mechanism	Infrared emission	Direct contact	Movement of air/liquid	Water vaporization

• Thermoregulatory center: Located in the hypothalamus; maintains core body temperature.

• Peripheral thermoreceptors: Detect external temperature changes.

• Central thermoreceptors: Monitor core temperature.

Fever and Hyperthermia

• Fever: Regulated increase in body temperature due to pyrogens.

• Hyperthermia: Unregulated rise in body temperature; can be life-threatening.

• Hypothermia: Abnormally low body temperature; impairs physiological functions.

Summary Table: Key Hormones in Metabolism

Hormone	Main Action
Insulin	Promotes glucose uptake and storage
Glucagon	Stimulates glycogen breakdown and gluconeogenesis
Epinephrine	Mobilizes energy stores during stress
Cortisol	Promotes gluconeogenesis and protein catabolism
Growth Hormone	Stimulates protein synthesis and lipolysis

Additional info: These notes synthesize and expand upon the provided question prompts, filling in academic context for a comprehensive review.