4.3 Metabolism and Energy Production

Energy production and metabolic pathways are how your body converts the food you eat into usable fuel. Without these processes, your cells couldn't power anything, not muscle contractions, not brain activity, not even keeping your body warm.

Metabolism has two sides: breaking molecules down to release energy, and building molecules up using that energy. ATP (adenosine triphosphate) is the molecule that shuttles energy between these two sides. Think of it as the universal energy currency your cells spend on every task they perform.

Energy Production and Metabolic Pathways

Definition of Metabolism

Metabolism refers to all the chemical reactions happening in your body that maintain life. These reactions fall into two broad categories: reactions that extract energy from nutrients, and reactions that use that energy to build and repair tissues.

Energy production specifically refers to the pathways that release energy from carbohydrates, fats, and proteins. That released energy supports vital functions like growth, tissue repair, movement, and thermoregulation (maintaining body temperature).

Catabolism vs. Anabolism

These are the two halves of metabolism, and they work in opposite directions.

Catabolism breaks down complex molecules into simpler ones, releasing energy in the process. Key catabolic pathways include:

• Glycolysis (breaking down glucose)
• Beta-oxidation (breaking down fatty acids)
• Protein degradation (breaking down proteins into amino acids)

Anabolism builds complex molecules from simpler ones, which requires energy input. Key anabolic pathways include:

• Glycogenesis (building glycogen from glucose for storage)
• Lipogenesis (synthesizing fatty acids for fat storage)
• Protein synthesis (assembling amino acids into new proteins)

ATP in Cellular Energy Transfer

ATP is made up of three parts: adenine (a nitrogen base), ribose (a sugar), and three phosphate groups. The energy is stored in the bonds between those phosphate groups.

When your cells need energy, they break the bond on the last phosphate group through a reaction called hydrolysis:

$ATP + H_2O \rightarrow ADP + P_i + energy$

That released energy then powers whatever the cell needs to do, whether that's contracting a muscle fiber or transporting a molecule across a membrane.

Your body regenerates ATP through three mechanisms:

1. Substrate-level phosphorylation — directly transfers a phosphate group to ADP during glycolysis and the citric acid cycle. This is fast but produces relatively little ATP.
2. Oxidative phosphorylation — uses the electron transport chain in mitochondria to produce the majority of your ATP. This is where most of your energy comes from.
3. Photophosphorylation occurs in plants, not humans, so you won't need to focus on it for this course.

Metabolic Pathways of Macronutrients

Each macronutrient follows its own set of pathways, but they all eventually feed into the same central route: the citric acid cycle (also called the Krebs cycle) and the electron transport chain.

Carbohydrate Metabolism

Carbs are your body's preferred fuel source. Glucose metabolism follows these key pathways:

1. Glycolysis — splits one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each), producing a small amount of ATP. This happens in the cytoplasm and doesn't require oxygen.
2. Citric acid cycle — pyruvate is converted to acetyl-CoA, which enters this cycle in the mitochondria. The cycle oxidizes acetyl-CoA and generates electron carriers (NADH and $FADH_2$) that feed into oxidative phosphorylation.
3. Gluconeogenesis — synthesizes new glucose from non-carbohydrate sources (like amino acids or glycerol) when blood sugar is low.
4. Glycogenesis — converts excess glucose into glycogen for storage in the liver and muscles.
5. Glycogenolysis — breaks stored glycogen back into glucose when your body needs quick energy between meals.

Lipid Metabolism

Fats are the most energy-dense macronutrient, providing about 9 kcal per gram compared to 4 kcal per gram for carbs or protein.

1. Lipolysis — breaks stored triglycerides into glycerol and free fatty acids so they can be used for energy.
2. Beta-oxidation — chops fatty acid chains into two-carbon units of acetyl-CoA, which then enter the citric acid cycle. This is why fats yield so much ATP.
3. Ketogenesis — when carbohydrate availability is very low (such as during fasting or very low-carb diets), the liver converts excess acetyl-CoA into ketone bodies, which the brain and other tissues can use as an alternative fuel.
4. Lipogenesis — converts excess glucose or amino acids into fatty acids for long-term energy storage.

Protein Metabolism

Protein is primarily used for building and repairing tissues, but your body can also use amino acids for energy when needed.

1. Transamination — transfers an amino group from one amino acid to a keto acid, creating a new amino acid. This lets your body shuffle amino groups around to make the specific amino acids it needs.
2. Deamination — removes the amino group from an amino acid, producing ammonia (which is toxic) and a carbon skeleton that can enter energy-producing pathways.
3. Urea cycle — converts toxic ammonia into urea in the liver, which is then excreted by the kidneys. This is how your body safely disposes of nitrogen waste from protein metabolism.
4. Protein synthesis — assembles amino acids into new proteins for muscle, enzymes, hormones, and other structures.