Metabolism & Nutrition
Every second you are alive, thousands of chemical reactions are running inside your cells. Some build things up, and some break things down. All of them deal with energy. That whole web of reactions is called metabolism. In this chapter, the food on your plate finally meets the machines inside your cells. You will follow a molecule of glucose, a simple sugar, as it is taken apart to make ATP. You will watch your body switch between storing fuel after a meal and spending it between meals. You will also learn what the six kinds of nutrients do once they are inside you. By the end, a simple question, why do we eat?, will have a clear and satisfying answer.
By the end of this chapter you can
- Define metabolism. Tell anabolism apart from catabolism by which way they go and how they handle energy.
- Explain how ATP stores and hands out energy, and how catabolism is linked to anabolism.
- Follow glucose through the four steps of cellular respiration. Count the ATP made at each step.
- Explain why aerobic respiration needs oxygen, and what the cell does when it runs short.
- Compare the absorptive and postabsorptive states, and the roles of insulin and glucagon.
- Name the six classes of nutrients. Say which ones give energy, and how many calories each one gives.
- Sort the fat-soluble vitamins from the water-soluble ones. Predict which are stored and which you must eat often.
- Describe energy balance and the three parts of the energy you burn each day.
Section 1Anabolism & catabolism
One word for every reaction
Metabolism is the sum of every chemical reaction in the body. A huge number of them run at the same time. To make sense of them, we sort each reaction into one of two directions. Anabolism is the building direction. Small molecules are joined into bigger ones. This is usually done by dehydration synthesis, which means a water molecule is removed to form a bond. Amino acids become proteins. Glucose units become glycogen, a stored form of sugar. Fatty acids and glycerol become fat.
Building costs energy. So anabolic reactions are endergonic, which means they take in energy. They must be paid for. Catabolism is the breaking-down direction. Big molecules are split into smaller ones. This is usually done by hydrolysis, which means water is added to break a bond. Catabolism frees the energy that was locked in those bonds. So these reactions are exergonic, which means they give off energy. They pay out. The two directions work as a team. The energy freed by catabolism is exactly what pays for anabolism.
ATP: the cell's rechargeable battery
The body does not spend food energy directly. Instead, it turns that energy into one kind of money that every cell uses. That money is ATP (adenosine triphosphate). ATP holds its energy in the bonds between its three phosphate groups. Snap off the last phosphate, and you get ADP, a free phosphate, and a burst of energy the cell can use. The cell uses that burst to power muscle movement, nerve signals, and building work.
Catabolism then recharges the battery. It sticks the phosphate back on, turning ADP back into ATP. This constant charge and use is called energy coupling. A reaction that gives off energy is linked to one that needs energy. That way, the energy is handed off instead of lost as heat. You cycle about your whole body weight in ATP every day. That is not because you store that much. It is because each molecule gets recharged thousands of times.
Following the electrons
Many reactions in metabolism are oxidation-reduction (redox) reactions. In these, electrons move from one molecule to another. A molecule that loses electrons is oxidized. A molecule that gains them is reduced. This matters because electrons carry energy. The cell moves them around with shuttle molecules called electron carriers. The main ones are NAD⁺ and FAD. When these carriers pick up electrons, they become NADH and FADH₂. They are loading up on energy to deliver later. Hold on to that idea. In the next section, those loaded carriers are the key to making most of your ATP.
| Feature | Anabolism | Catabolism |
|---|---|---|
| Direction | Builds big molecules from small ones | Breaks big molecules into small ones |
| Reaction type | Dehydration synthesis | Hydrolysis |
| Energy | Needs energy (endergonic); uses ATP | Gives off energy (exergonic); makes ATP |
| Example | Amino acids → protein; glucose → glycogen | Protein → amino acids; glycogen → glucose |
Switch between the building and breaking directions in the interactive below.
- Metabolism is all the reactions in the body. Anabolism builds up (endergonic, uses ATP). Catabolism breaks down (exergonic, makes ATP).
- ATP is the cell's energy money. It breaks into ADP + phosphate to do work, and catabolism recharges it.
- Energy coupling links reactions that give off energy to ones that need it. So the energy is used, not wasted.
- The electron carriers NAD⁺ and FAD carry energy-rich electrons from catabolism to the steps that make ATP.
Section 2Cellular respiration
Burning glucose, carefully
Cellular respiration is the careful, step-by-step breakdown of glucose to make ATP. Think of it as a slow, safe burn. The cell does not free all of glucose's energy at once in a harmful flash. Instead, it takes the energy away in small steps and saves it in ATP as it goes. The process has four stages in two places. The first stage happens in the cytoplasm, the fluid that fills the cell. The last three happen inside the mitochondrion, the cell's power plant. With enough oxygen, one glucose molecule can make up to about 30 to 32 ATP. Let's follow it stage by stage.
The four stages
1. Glycolysis happens in the cytoplasm and needs no oxygen. One glucose, which has six carbons, is split into two pyruvate molecules. Each pyruvate has three carbons. The cell spends two ATP to start and gets four back. That is a net gain of 2 ATP, plus 2 NADH.
2. Pyruvate oxidation is also called the transition reaction. Each pyruvate moves into the matrix, the inner space of the mitochondrion. There, one carbon is cut off and leaves as CO₂. The rest is joined to a carrier to form acetyl CoA. This step makes more NADH.
3. The citric acid cycle (Krebs cycle) also happens in the matrix. Acetyl CoA is fed into a loop. The loop frees the rest of the carbons as CO₂. It makes a little ATP directly. Most important, it loads up many NADH and FADH₂ carriers.
4. Oxidative phosphorylation happens on the inner membrane of the mitochondrion. This is where the big payoff comes. The loaded carriers drop their electrons into the electron transport chain. The chain uses that energy to pump H⁺ ions across the membrane, building up a crowd on one side. The ions then rush back through an enzyme that spins like a water wheel. As it spins, it snaps phosphates onto ADP. This step alone makes most of the ATP.
| Stage | Location | Needs O₂? | Main products |
|---|---|---|---|
| Glycolysis | Cytoplasm | No | 2 pyruvate, net 2 ATP, 2 NADH |
| Pyruvate oxidation | Mitochondrial matrix | Yes (aerobic) | Acetyl CoA, CO₂, NADH |
| Citric acid cycle | Mitochondrial matrix | Yes (aerobic) | CO₂, 2 ATP, NADH, FADH₂ |
| Oxidative phosphorylation | Inner mitochondrial membrane | Yes | About 26–28 ATP, H₂O |
Why oxygen, and what happens without it
Here is the big point of the whole process. Oxygen is the final electron acceptor. At the very end of the electron transport chain, the used-up electrons and some H⁺ ions are handed to O₂. This forms H₂O, water. That is the water you make from breathing. If no oxygen is there to take them, the chain backs up like a clogged drain. The carriers stay loaded, and stages 2 through 4 grind to a stop.
When that happens, the cell falls back on anaerobic fermentation, which means making energy without oxygen. It keeps glycolysis going by turning pyruvate into lactic acid. This step frees up the NAD⁺ that glycolysis needs. It gives a fast trickle of 2 ATP with no oxygen. That helps a sprinting muscle. But it is very wasteful, and lactic acid builds up. That is part of why hard exercise burns.
Step through all four stages in the interactive. Watch the ATP add up.
- Cellular respiration is the careful breakdown of glucose. It makes up to about 30–32 ATP.
- Four stages: glycolysis (cytoplasm) → pyruvate oxidation → citric acid cycle (matrix) → oxidative phosphorylation (inner membrane).
- Oxidative phosphorylation makes most of the ATP. O₂ is the final electron acceptor, and it forms water.
- Without oxygen, cells use anaerobic fermentation. It makes only 2 ATP per glucose, and lactic acid is left over.
Section 3Metabolic states: fed vs fasting
Two modes: store or spend
Your fuel supply is not steady. It comes in bursts at mealtimes, then runs low for hours. To cope, the body runs in two metabolic states. The absorptive (fed) state is the few hours after a meal. Nutrients pour in from the gut, and blood sugar rises. The job now is to store the extra. Extra glucose is packed into glycogen. This is called glycogenesis. Extra fuel is also turned into fat, which is called lipogenesis. Amino acids are used to build protein.
The postabsorptive (fasting) state takes over once the meal is fully taken in. This happens overnight, between meals, or during a fast. Now the job flips to releasing stored fuel to keep blood sugar steady. Glycogen is broken back down into glucose, which is called glycogenolysis. Fat is broken down, which is called lipolysis. The liver even makes brand-new glucose from things that are not carbs, like amino acids. This is called gluconeogenesis, which means "making new glucose."
Two hormones run the switch
Two hormones from the pancreas mostly decide which state you are in. They work against each other. Insulin is the storage hormone. It rises after a meal and moves glucose into cells. It turns on all the building pathways. Glucagon is the releasing hormone. It rises during fasting and turns on the pathways that free stored fuel. Think of them as a seesaw. High insulin means store. High glucagon means spend. One organ is the hub for all of this: the liver. It stores glycogen after meals and sends out glucose between them. This keeps blood sugar in its narrow, healthy range all day and night.
Protecting the brain
Why go to all this trouble? Because the brain runs almost only on glucose. It can't store its own fuel, so blood sugar must never crash. During a long fast, the body uses glucose sparing. Most tissues switch to burning fatty acids. That saves the scarce glucose for the brain. In long starvation, the liver turns fat into ketone bodies. These are a backup fuel the brain can partly use. It is a clever trick. But ketones are acidic, and a large buildup can make the blood dangerously acidic.
| Feature | Absorptive (fed) | Postabsorptive (fasting) |
|---|---|---|
| When | Hours after a meal | Between meals, overnight, fasting |
| Goal | Store extra fuel | Release stored fuel |
| Main hormone | Insulin | Glucagon |
| Key processes | Glycogenesis, lipogenesis, making protein | Glycogenolysis, lipolysis, gluconeogenesis, ketogenesis (making ketones) |
| Blood sugar trend | Rising → stored | Falling → protected |
Flip between the fed and fasting states in the interactive below.
- The absorptive state stores extra fuel (glycogenesis, lipogenesis). The postabsorptive state releases it (glycogenolysis, gluconeogenesis, lipolysis).
- Insulin drives storage, and glucagon drives release. They act like a seesaw, and the liver is the center.
- The brain needs glucose. So during fasting, the body protects blood sugar by glucose sparing. In a long fast, it also makes ketone bodies.
Section 4The six nutrient classes
Three that fuel you, three that don't
Everything worth eating falls into six classes of nutrients. Three of them give you energy you can use. You eat them in large amounts, so they are called the macronutrients ("macro" means large). They are carbohydrates, lipids (fats), and proteins. The other three give no energy at all. But you need them in smaller amounts. They are vitamins, minerals, and water.
"No energy" does not mean "you can skip it." Without water or tiny amounts of certain minerals, you would die long before you starved. Each class has a job. Carbs are the body's favorite quick fuel. Fats are the most packed energy store, and they build every cell's outer membrane. Proteins mostly build and run things, like enzymes, muscle, and antibodies. The body burns protein for energy only as a last choice.
How much energy is in food?
The energy in food is measured in kilocalories. On food labels, these are written as "Calories" with a capital C. Only the three macronutrients have calories, and they are not equal. Carbs and protein each give about 4 kcal per gram. Fat has more than twice that, about 9 kcal per gram. That is why fatty foods are so high in calories. It is also why the body picks fat for its long-term energy store. Alcohol is not a nutrient, but it also has energy, about 7 kcal per gram.
Essential nutrients: the ones you must eat
An essential nutrient is one your body can't make, or can't make enough of. So it must come from food. These include about nine essential amino acids and a couple of essential fatty acids. They also include all the vitamins and minerals. This is why protein quality matters. A complete protein has all the essential amino acids. Meat, eggs, dairy, and soy are examples. Most single plant foods are incomplete. But if you combine plants that fill each other's gaps, like beans with rice, you get the full set.
| Nutrient class | Gives energy? | Main role | Energy value |
|---|---|---|---|
| Carbohydrates | Yes | Favorite quick fuel | 4 kcal/g |
| Proteins | Yes | Building, enzymes, defense; last-choice fuel | 4 kcal/g |
| Lipids (fats) | Yes | Packed energy store; cell membranes; hormones | 9 kcal/g |
| Vitamins | No | Coenzymes (enzyme helpers) and controllers | None |
| Minerals | No | Ions, bone, enzyme helpers (Na⁺, Ca²⁺, Fe) | None |
| Water | No | Dissolves things, carries things, controls body heat | None |
Tap each nutrient class in the interactive to see its role.
- There are six nutrient classes. Carbohydrates, lipids, proteins give energy. Vitamins, minerals, water give no energy, but you still need them.
- Calories: carbs and protein give 4 kcal/g. Fat gives 9 kcal/g.
- Essential nutrients must come from food. They include some amino acids and fatty acids, and all vitamins and minerals.
- Complete proteins have all the essential amino acids. You can combine plant foods to get the same result.
Section 5Vitamins
Tiny amounts, huge consequences
Vitamins are carbon-based molecules the body needs in very small amounts to run its metabolism. Most act as coenzymes. These are helper molecules that enzymes need in order to work. You need only milligrams or even micrograms a day. Yet if you are short on even one, you can get a specific disease. Some of these diseases can kill.
One simple thing controls how a vitamin behaves: how it dissolves. Some dissolve in fat, and some dissolve in water. That splits all vitamins into two groups. This one fact tells you how each is taken in and whether it is stored. It also tells you whether it can build up to harmful levels.
Fat-soluble: stored, and able to overdose
The fat-soluble vitamins are A, D, E, and K. Remember them as "ADEK." Because they dissolve in fat, they are taken in along with the fat in your food. Then they are stored in the liver and in fat tissue. This storage cuts both ways. You can go a while without them, because you have a reserve. But the body can't easily flush out the extra. So very large doses of A or D can build up to toxic, or poisonous, levels. Each has its own job. A helps you see. D helps balance calcium and build bone. E is an antioxidant that protects cells from damage. K helps blood clot.
Water-soluble: not stored, needed steadily
The water-soluble vitamins are the B-complex and vitamin C. Because they dissolve in water, the body does not stock up on them. The kidneys filter out any extra, and it leaves in urine. That is why very high doses mostly make expensive urine. The downside is that you need a steady daily supply. If you don't get enough, problems show up fairly fast. Most B vitamins are coenzymes in energy pathways, the same ones from Sections 1 and 2.
The classic shortage diseases are worth knowing. Scurvy comes from a lack of vitamin C. Rickets, a softening of the bones, comes from a lack of vitamin D. Beriberi comes from a lack of B₁. Pellagra comes from a lack of B₃. Pernicious anemia comes from a lack of B₁₂.
| Property | Fat-soluble (A, D, E, K) | Water-soluble (B-complex, C) |
|---|---|---|
| Taken in with | Fat in food | Water |
| Stored? | Yes, in the liver and fat | No (B₁₂ is the exception) |
| Extra | Builds up → risk of poisoning | Leaves in urine |
| How often you need it | Less often (you have a reserve) | Often, almost daily |
| Shortage disease example | Rickets (D), night blindness (A) | Scurvy (C), beriberi (B₁) |
Switch between the two vitamin groups in the interactive, and tap each one.
- You need vitamins in tiny amounts. Most act as coenzymes. How they dissolve controls how they behave.
- Fat-soluble (A, D, E, K) vitamins are taken in with fat and stored. They can reach toxic levels.
- Water-soluble (B-complex, C) vitamins are not stored. Extra leaves in urine, so you need a steady daily supply.
- Each shortage has its own disease: scurvy (C), rickets (D), beriberi (B₁), pellagra (B₃), and pernicious anemia (B₁₂).
Section 6Energy balance
A ledger, not a mystery
At its core, body weight follows a simple money-like rule called energy balance. It compares the calories you take in as food with the calories you spend, or put out. When intake equals spending, weight stays steady. This is neutral balance. When intake is more than spending, the extra is stored, mostly as fat, and weight goes up. This is positive balance. When spending is more than intake, the body draws on its stores, and weight goes down. This is negative balance. The rule is simple. Weight is hard to control because biology runs both sides of the ledger. Willpower alone is not the whole story.
Where the calories go
"Energy out" is not just exercise. The total energy you burn has three parts. By far the largest is the basal metabolic rate (BMR). This is the energy your body burns just to stay alive at rest. It keeps your heart beating, your kidneys filtering, and your body warm. BMR makes up about 60 to 75% of what you burn in a day. That surprises most people.
A second part, about 10%, is the thermic effect of food. This is the energy spent digesting and taking in a meal. The last part changes the most. It is physical activity, from fidgeting to running a marathon. BMR is the biggest part. So things that change BMR have a big effect on how easily a person gains or loses weight. These include muscle mass, age, thyroid hormone, and body size.
The body defends its weight
Energy balance is not left to chance. It is controlled in the same way the body keeps other things steady. The hypothalamus, a small part of the brain, is the control center. It reads hormone signals about how much fuel the body has stored. Leptin comes from fat tissue. It signals that stores are full and turns down hunger. Ghrelin comes from the empty stomach. It signals hunger and makes you want to eat.
Here is a handy rule of thumb. A pound of body fat stores about 3,500 kcal. So eating about 500 kcal less than you burn each day takes off close to a pound a week. That holds at least until the body lowers its BMR to fight the loss.
| Balance | Intake vs output | Result |
|---|---|---|
| Neutral | In = out | Weight stays steady |
| Positive | In > out | Extra stored as fat → weight gain |
| Negative | In < out | Stores used up → weight loss |
Tip the balance one way or the other in the interactive, and see what happens.
- Energy balance compares calories in (food) with calories out. Positive balance stores fat. Negative balance uses it up.
- Total energy burned = BMR (about 60–75%, the largest part) + thermic effect of food (about 10%) + physical activity.
- The hypothalamus controls hunger and stores. It uses signals like leptin (full) and ghrelin (hungry).
- About 3,500 kcal equals a pound of fat. But the body defends its weight by changing its BMR.
The chapter in one breath
- Metabolism is every reaction in the body. Anabolism builds and uses energy. Catabolism breaks down and frees energy. ATP is the shared energy money.
- Cellular respiration breaks down glucose in four stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. It makes up to about 32 ATP.
- Oxygen is the final electron acceptor. Without it, cells fall back on anaerobic fermentation. That makes only 2 ATP, plus lactic acid.
- The body switches between an absorptive state (store, insulin) and a postabsorptive state (release, glucagon). The liver keeps blood sugar steady for the brain.
- Food gives six nutrient classes. Carbs, fats, and proteins give energy (4, 9, and 4 kcal/g). Vitamins, minerals, and water give none.
- Vitamins split by how they dissolve. Fat-soluble A, D, E, K are stored and can be toxic. Water-soluble B and C leave in urine and are needed daily.
- Energy balance weighs calories in against calories out. Calories out come from BMR, the thermic effect of food, and activity. The body works hard to defend its weight.
ReferenceWords worth knowing
Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.
- Absorptive (fed) state
- The hours after a meal, when food is being taken in and blood sugar rises. Insulin directs the body to store the extra fuel.
- Anabolism
- The building side of metabolism. Small molecules are joined into bigger ones, which takes energy (endergonic).
- ATP (adenosine triphosphate)
- The cell's energy money. Energy is freed when its last phosphate comes off to form ADP. It is stored again when the phosphate goes back on.
- Basal metabolic rate (BMR)
- The energy the body burns at full rest just to stay alive. It is the largest part of the energy you burn each day.
- Catabolism
- The breaking-down side of metabolism. Big molecules are split into smaller ones, which frees energy (exergonic).
- Cellular respiration
- The careful, step-by-step breakdown of glucose. It saves glucose's energy in ATP.
- Citric acid cycle (Krebs cycle)
- A loop of reactions in the mitochondrion that breaks down acetyl CoA. It gives off CO₂ and loads NADH and FADH₂ for the electron transport chain.
- Electron transport chain
- A row of carriers in the inner membrane of the mitochondrion. It passes electrons along to pump H⁺ ions, which powers most ATP making.
- Energy balance
- How the calories you eat compare with the calories you burn. It decides whether your weight goes up, goes down, or stays steady.
- Essential nutrient
- A nutrient the body can't make enough of, so it must come from food. Examples are certain amino acids and fatty acids, and all vitamins and minerals.
- Fat-soluble vitamin
- A vitamin (A, D, E, or K) taken in with fat and stored in the body. Too much can be toxic.
- Gluconeogenesis
- The liver making new glucose from things that are not carbs, like amino acids. It happens during fasting.
- Glycogenolysis
- Breaking stored glycogen back into glucose to raise blood sugar between meals.
- Glycolysis
- The first stage of respiration, in the cytoplasm. It splits glucose into two pyruvate and nets 2 ATP without oxygen.
- Ketone bodies
- Acidic fuel molecules the liver makes from fat during a long fast. The brain can use them, but they can make the blood too acidic.
- Kilocalorie (Calorie)
- The unit of food energy. Carbs and protein give about 4 kcal/g, and fat gives about 9 kcal/g.
- Metabolism
- The sum of all chemical reactions in the body. It includes anabolism and catabolism.
- Oxidative phosphorylation
- The last stage of respiration, on the inner membrane of the mitochondrion. It uses the electron transport chain and oxygen to make most of the ATP.
- Postabsorptive (fasting) state
- The time between meals, when stored fuel is released to keep blood sugar steady. Glucagon directs it.
- Vitamin
- A carbon-based molecule needed in tiny amounts. It usually acts as a coenzyme in metabolism.
- Water-soluble vitamin
- A vitamin (the B-complex or C) that dissolves in water and is not stored. Extra leaves in urine, so you need it often.