Energy and Metabolism exam 3

Energy and Metabolism

Introduction

Energy metabolism is a central concept in nutrition, describing how the body obtains and uses energy from food. This topic covers the nutrients involved in energy metabolism, the chemical reactions that occur in cells, and how the body manages energy during periods of feasting and fasting.

Types of Chemical Reactions in the Body

Overview of Energy and Metabolism

• Energy: Exists in various forms—heat, mechanical, electrical, and chemical.

• Photosynthesis: The process by which green plants use the sun’s energy to make carbohydrates.

• Fuel: Compounds such as glucose, fatty acids, and amino acids that cells use for energy.

• Metabolism: The sum total of all chemical reactions in living cells. Energy metabolism refers specifically to reactions by which the body obtains and expends energy from food.

Cells as Sites of Metabolic Reactions

Cellular Structures and Functions

• Cell membrane: Controls the movement of substances in and out of the cell.

• Nucleus: Contains genetic material (DNA) and directs cellular activities.

• Mitochondria: Known as the "powerhouse" of the cell, where most energy-yielding reactions occur.

• Cytoplasm: Site of many metabolic pathways.

Metabolic Pathways

Definition and Types

A metabolic pathway is a series of reactions in which the products of one reaction are used as reactants for the next.

• Catabolic pathway: Energy is released by the breakdown of complex molecules to simpler compounds; overall $\Delta G$ is negative.

• Anabolic pathway: Energy is used to build complicated molecules from simpler ones; overall $\Delta G$ is positive.

Anabolic vs. Catabolic Reactions

Comparison and Examples

• Anabolic reactions: Synthesize molecules such as glycogen, triglycerides, and proteins. Require energy input.

• Catabolic reactions: Break down molecules such as glycogen, triglycerides, and proteins, releasing energy.

Reaction Type	Process	Energy Change
Anabolic	Building molecules	Requires energy
Catabolic	Breaking down molecules	Releases energy

Transfer of Energy in Reactions

ATP: The Energy Currency

• Adenosine Triphosphate (ATP): High-energy compound that powers cellular activities.

• Contains three phosphate groups; hydrolysis of ATP releases energy.

• Coupled reactions: Energy produced by one reaction is consumed by another.

ATP Hydrolysis Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

Regeneration of ATP

ATP/ADP Cycle

• ATP is continually hydrolyzed and resynthesized in cells.

• ATP synthesis from ADP and $P_i$ is an endergonic reaction, using energy from food breakdown.

• Approximately 10 million ATP molecules are hydrolyzed and resynthesized each second in a typical cell.

ATP Synthesis Equation:

$\text{ADP} + \text{P}_i + \text{Energy} \rightarrow \text{ATP}$

The Capture and Release of Energy by ATP

Mechanism

• Energy is released when a high-energy phosphate bond in ATP is broken.

• Energy is required to recharge ATP by attaching a phosphate group to ADP.

• ATP acts as a rechargeable battery, cycling between ATP (charged) and ADP (used).

The Helpers in Metabolic Reactions: Enzymes and Coenzymes

Enzymes

• Enzymes: Biological catalysts that accelerate metabolic reactions by lowering activation energy.

• Enzymes are not consumed in the reaction and are released unchanged.

Coenzymes

• Coenzymes: Organic molecules that associate with enzymes and are essential for their function.

• Without coenzymes, some enzymes cannot function.

Role of Enzymes in Biological Reactions

Activation Energy and Reaction Pathways

• Enzymes reduce activation energy by inducing the transition state.

• They do not alter the overall free energy change ($\Delta G$) of the reaction.

• Each enzyme is specific to a particular substrate or group of related molecules (enzyme specificity).

Quiz Yourself: Key Concepts

Matching Terms

Statement	Term
Uses energy to build compounds	Anabolism
Most metabolically active body cells	Liver
How the body obtains and uses energy from food	Energy metabolism
Breaks compounds and releases energy	Catabolism

Fill in the Blanks

• Carbohydrates are broken down during digestion to glucose.

• Fats (triglycerides) are broken down to form glycerol and fatty acids.

• Proteins are broken down to amino acids.

• These are examples of catabolic reactions.

Simplified Overview of Energy Yielding Pathways

Main Steps

• All energy-yielding nutrients—protein, carbohydrate, and fat—can be broken down to acetyl CoA.

• Acetyl CoA enters the TCA cycle (also known as the Krebs cycle).

• Hydrogen atoms are released and carried by coenzymes to the electron transport chain.

• ATP is synthesized.

• Hydrogen atoms react with oxygen to produce water.

Feasting and Fasting

Feasting: Excess Energy

• Feasting results in excess energy, favoring fat formation.

• Excess protein, carbohydrates, or fat can all contribute to body fat stores.

• A balanced mixture of energy sources is recommended for daily activities.

Transition from Feasting to Fasting

• After a meal, glucose and fat are used as needed; the rest is stored.

• During fasting (voluntary or involuntary), the body draws on stores of fat and glycogen for energy.

Fasting: Inadequate Energy

Metabolic Changes

• Glucose and fatty acids are broken down into acetyl CoA, which enters the energy pathway.

• After several hours of fasting, liver glycogen depletes, causing blood glucose levels to drop.

• Gluconeogenesis: The process of making glucose from noncarbohydrate sources (e.g., amino acids).

Adaptation: Creating an Alternative Fuel

Making Glucose and Using Fat

• The nervous system and red blood cells require glucose.

• Amino acids can yield pyruvate, which is used to make glucose.

• Body proteins are broken down to supply amino acids for gluconeogenesis.

• During prolonged fasting, the body produces ketone bodies from fat to fuel the brain, slowing protein breakdown and inducing appetite loss (ketosis).

Adaptation: Conserving Energy

Energy Conservation During Starvation

• Hormonal changes reduce energy output.

• Fasting supports weight loss but is not the best option for fat loss.

• Symptoms of starvation include physical (wasting, slowed heart rate, lowered body temperature, organ failure) and psychological (depression, anxiety, food-related dreams) effects.

Lecture Summary

• Identified the nutrients involved in energy metabolism and the high-energy compound (ATP) that captures energy released during breakdown.

• Summarized the main steps in the energy metabolism of glucose, glycerol, fatty acids, and amino acids.

• Explained how excess or inadequate intake of energy-yielding nutrients affects body fat and metabolism.