Lecture: Energy, Enzymes & Metabolism
Lecture script · about 15–20 minutes spoken · Week 5
Opening
Think back to the last time you had a fever. Your body ran only a few degrees warmer than its usual 37 °C, and yet you felt weak and foggy. A few degrees sounds tiny. By the end of this lecture, you'll have one way to read why it might matter.
These ideas show up well beyond this course. A nursing student watching a fever climb on a hospital ward is watching them. So is a home cook watching an egg turn solid in a hot pan, and so is an athlete feeling a sprint burn. Each one is seeing energy being spent and the proteins that spend it.
Here's where we are. In Week 1 you learned to read a controlled experiment. Week 2 gave you water and the pH scale. Week 3 gave you proteins and the idea that a protein's shape is its job. Week 4 toured the cell and met a pump that spends ATP. In this lecture I'll put those pieces to work. We'll define energy and the two laws it obeys. We'll sort the cell's chemistry into building and breaking. We'll meet ATP, then the enzymes that make reactions possible, and we'll finish with the potato experiment from your lab. Next week, in Week 6, we follow sugar through the pathways that recharge ATP.
1. What energy is
Picture an archer drawing a bow and holding it still. Nothing is moving. Yet you can feel that the bow is loaded with something. Let the string go, and the arrow flies. That something is energy.
Here's the term. Energy is the capacity to do work, such as moving something, building something, or warming something. It comes in two forms. Kinetic energy is the energy of motion, including heat, which is the motion of molecules. Potential energy is stored energy that can do work later, such as the energy in a stretched bowstring or in the chemical bonds of food.
So the drawn bow holds potential energy, and the flying arrow has kinetic energy. A sprinter crossing the line is all kinetic. The sandwich in your bag is potential, stored in the bonds of its sugars and fats. For a cell, that last one matters most. Chemical bonds are the cell's savings account.
Pause and try this. A granola bar sits in your backpack. Is its energy kinetic or potential?
Think about whether anything in that bar is moving.
Here's the answer. It's potential energy, because the energy is stored in the chemical bonds of the bar's sugars and fats, waiting to be used.
2. The two laws of thermodynamics
Have you noticed that a phone gets warm while it charges? Not all of the electricity ends up in the battery. Some of it turns into heat. Every energy change in the universe works like that, and two laws describe it.
The first law of thermodynamics is the rule that energy can be transferred from place to place and transformed from one form to another, but is never created or destroyed. The second law of thermodynamics is the rule that every energy transfer or transformation loses some energy as heat that can no longer do work, which raises the entropy of the surroundings. Entropy is a measure of disorder. When heat spreads out, it gets less useful, and entropy rises.
Put the two laws together and you get the first rule of this week. A cell captures and transfers energy. It never makes energy. When you eat a sandwich and climb the stairs, your cells move energy out of the sandwich's bonds, into ATP, and then into the motion of your legs. At every step, some of it leaks away as heat. Nothing was created.
Pause and try this. Someone says, "My cells make energy when I exercise." Which law does that break, and how would you fix the sentence?
Take a moment with the word "make."
Here's the answer. It breaks the first law, because energy is never created. A better sentence is, "My cells capture energy from food and transfer it."
3. Building and breaking: metabolism
Think about a set of toy building bricks. You can pull a model apart into single bricks, or snap single bricks together into something new. A cell does both, all day, with molecules.
Metabolism is the whole set of chemical reactions in a cell or organism, both those that break molecules down and those that build them up. A catabolic reaction breaks a larger molecule into smaller ones and releases energy, as in digesting food. An anabolic reaction builds a larger molecule from smaller ones and requires an input of energy, as in building a protein.
Two more words name the same split from the energy side. Exergonic describes a reaction that releases energy overall. Endergonic describes a reaction that takes in energy overall. Here's a hook. Ex is for exit: energy leaves. End is for enter: energy goes in. So catabolic reactions are exergonic, and anabolic reactions are endergonic. You met both directions in Week 3. Hydrolysis breaks a polymer, so it's catabolic. Dehydration synthesis builds one, so it's anabolic.
Pause and try this. A muscle cell joins amino acids into a new protein. Is that catabolic or anabolic, and exergonic or endergonic?
Ask yourself which way the energy flows.
Here's the answer. It's anabolic and endergonic, because the cell is building a bigger molecule, and building costs energy.
4. ATP, the energy currency
Think about a phone battery. You charge it at night and spend it all day. The battery isn't the power plant, and it isn't the app. It carries the charge to where it's needed. In a cell, that job belongs to ATP.
ATP is adenosine triphosphate, the cell's energy currency: adenosine carrying a chain of three phosphates. Picture adenosine at one end, then three phosphate groups in a row, like three linked train cars. When the cell spends ATP, it unhooks the outer car. That releases energy the cell can use. What's left is ADP, adenosine diphosphate, the spent form, carrying two phosphates. In words, ATP becomes ADP plus a phosphate plus energy.
Then the cell recharges. It uses energy captured from food to hook a phosphate back onto ADP, and it has ATP again. So ATP runs in a cycle, not a one-way street. One glucose recharges roughly 30 ATP, and you'll see how in Weeks 6 and 7. Here's a trap to avoid. ATP and DNA both contain a base and a sugar, so students blur them. ATP is the energy currency. DNA is the instruction set. They're never the same molecule.
Pause and try this. A molecule carries two phosphates. Is it charged or spent?
Count the train cars.
Here's the answer. It's spent, because two phosphates is ADP. Three phosphates is ATP, the charged form.
5. How an enzyme works
Imagine pushing a shopping cart to a parking lot that sits lower than the store. The trip is downhill overall. But there's a small hill in the way, and you have to push the cart up it first. Many reactions in a cell are like that cart. They'd release energy overall, but they barely start on their own.
That hill has a name. Activation energy is the energy that must be put in to get a reaction started, pictured as a hill. An enzyme is a protein that speeds up a reaction by lowering its activation energy, without changing where the reaction starts or ends; it comes out of the reaction unchanged and is reused. In the cart story, the enzyme digs a lower pass through the hill. The store and the lot stay exactly where they were.
Every enzyme has a partner and a pocket. The substrate is the molecule an enzyme acts on. The active site is the pocket on an enzyme where the substrate binds and the reaction takes place. Because the pocket has one shape, each enzyme does one job. Lactase works on lactose, the sugar in milk. Catalase splits hydrogen peroxide into water and oxygen gas, and that oxygen is the foam in your lab. When the job is done, the catalase lets go, unchanged, and grabs the next molecule.
Pause and try this. A friend says enzymes get used up, like fuel. What would you say back?
Think about what happens to catalase after one reaction.
Here's the answer. An enzyme comes out of each reaction unchanged and is reused, so it's more like a tool than a fuel.
6. Two models of the active site
Think about a house key and a winter glove. A key either fits the lock or it doesn't, and nothing bends. A glove is only roughly hand-shaped, and it closes around your hand as you pull it on. Biologists use both pictures for enzymes.
The lock-and-key model is a classroom model in which a rigid active site fits one substrate the way a lock fits one key. The induced fit model is a classroom model in which the active site changes shape slightly as the substrate binds, like a glove closing on a hand. Induced fit refines lock-and-key, and it's the picture biologists prefer now. Both are models, simplifications you can hold in one hand, not photographs of every real enzyme. Lock-and-key explains why an enzyme is choosy. Induced fit keeps that and adds that the grip tightens as the substrate arrives.
Pause and try this. You learn that an active site changes shape a little when its substrate binds. Which model does that fit?
Remember which object bends.
Here's the answer. It fits the induced fit model, because only that model lets the active site change shape, like the glove.
7. What changes an enzyme's rate
Picture an oven that's too cold to bake bread, and another that burns it. Somewhere in between is the temperature that works best. Enzymes have that kind of sweet spot. An enzyme's optimum is the temperature or pH at which it works fastest.
Start with temperature. As a human enzyme warms toward about 37 °C, it speeds up. Past that optimum, the rate crashes to zero, because the heat denatures the enzyme and destroys its active site. A cooked egg doesn't un-cook. Cold is different. Cold only slows an enzyme, and when it warms up again, it works as before. So cold slows, and heat destroys.
Next, pH. Most human enzymes work best near neutral. Pepsin is the one to remember: a protein-digesting enzyme that works in the acidic stomach contents, roughly pH 1.5 to 3.5. Last, substrate. More substrate speeds the reaction, until every active site is busy. That point is saturation. Think of a checkout line. Once every register has a customer, a longer line doesn't move faster.
Pause and try this. An enzyme is boiled and then cooled back down. Does it work again?
Think about the egg.
Here's the answer. No. Boiling denatured it and destroyed the active site, and cooling doesn't rebuild that shape.
8. Helpers and brakes
Think about a cordless drill. It won't run until you snap in the battery pack. The drill is the tool, but it needs a helper part. Many enzymes are like that drill.
A cofactor is a non-protein helper an enzyme needs in order to work, such as an iron or magnesium ion. A coenzyme is an organic helper molecule an enzyme needs, often made from a vitamin.
Cells also need brakes. Enzymes often work in chains, where one enzyme's product feeds the next enzyme. Feedback inhibition is a control in which the end product of a chain of reactions slows an earlier enzyme in that chain. It works like a home thermostat. When the room is warm enough, the heat turns down. When the room cools, it comes back on. So when the end product piles up, it slows the chain, and when the cell uses it up, the chain speeds up again. That's why a pathway is never stuck at full speed.
Pause and try this. In feedback inhibition, which enzyme does the end product slow: the last one, or an earlier one?
Picture the thermostat and the furnace.
Here's the answer. It slows an earlier enzyme in its own chain, so the product controls its own supply from upstream.
9. The catalase experiment
Now let's read your lab's experiment from start to finish. The numbers are an illustrative set built for this course, not real data. Catalase from potato splits hydrogen peroxide, and the oxygen makes foam. Taller foam means a faster reaction. The question is how temperature changes that speed. The potato was tested cold at about 5 °C, at room temperature at about 22 °C, at body temperature at about 37 °C, and after boiling at about 100 °C, cooled before testing. Each condition had three trials, and the foam was measured after 60 seconds.
The cold trials were 1, 2, and 3 centimeters. Add them for 6, divide by 3, and the average is 2. Room temperature gave 4, 5, and 6. That adds to 15, and 15 divided by 3 is 5. Body temperature gave 9, 10, and 11. That's 30, divided by 3, for an average of 10, the peak. The boiled trials were 0, 0, and 0, so that average is 0.
Now compare. 10 divided by 2 is 5, so body temperature made 5 times the cold foam. 10 divided by 5 is 2, so it made 2 times the room foam. Each condition spread over at most 2 centimeters. The shape is the story. The averages rise from 2 to 5 to 10, then crash to 0. The independent variable is temperature. The dependent variable is foam height. The evidence supports the hypothesis, and it doesn't prove it.
Pause and try this. The cold sample made a little foam, and the boiled sample made none. Which one was slowed, and which was denatured?
Think about which one could recover.
Here's the answer. The cold sample was slowed, and it would recover when warmed. The boiled sample was denatured, so its active site was destroyed and it read 0.
Closing
Let me pull it together in order. Energy is the capacity to do work, kinetic when it moves and potential when it's stored. The first law says energy is never created or destroyed, and the second says every transfer loses some as heat. So a cell captures and transfers energy and never makes it. Catabolic reactions break down and release energy, and anabolic reactions build and cost it. ATP, with three phosphates, is the charged currency, and ADP, with two, is the spent form. An enzyme lowers the hill and is reused. Lock-and-key and induced fit are both models. Rate rises to an optimum and crashes when heat denatures the enzyme. Cofactors help, and feedback inhibition brakes. The catalase averages, 2, 5, 10, and 0, show the rise and the crash.
Next in this module come the slide deck, the Readings and Resources page, the lecture tutorial, the practice set, and the quiz. Then, in Week 6, Cellular Respiration, we'll follow one glucose through the pathways that recharge ATP.
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