Campbell Biology Chapter 8 Review: An Introduction to Metabolism

8.1 Metabolism and the laws of energy

pp. 142–145

On the AP exam? Yes

Topic 3.3 asks you to explain why every living thing needs a steady energy input and how it stays organized without breaking the laws of thermodynamics; energy lost as heat at each step of a food chain comes back in Topic 8.2.

In the course: Topic 3.3 Cellular Energy, Topic 8.2 Energy Flow Through Ecosystems (notes, videos and more questions)

Key points

Metabolism is the whole set of chemical reactions in an organism. Most reactions are linked in pathways, where each enzyme makes a product that the next enzyme uses as its starting material.
Catabolic pathways take large molecules apart and release energy; cellular respiration breaking down glucose is the big one. Anabolic pathways spend energy to build large molecules, such as proteins from amino acids.
Energy is the ability to make change happen. Kinetic energy is energy of motion, and heat is the kinetic energy of randomly moving molecules. Potential energy is stored by position or structure, and chemical energy is the potential energy held in a molecule's arrangement of atoms.
The first law of thermodynamics says energy can move or change form, but it's never created or destroyed. Your muscles don't make energy; they convert chemical energy from food into motion and heat.
The second law says that every time energy is transferred or converted, some becomes heat that can't do useful work, so the universe's total entropy (disorder) keeps rising.
Cells and organisms are open systems: matter and energy flow in (as food or light) and flow back out (as heat, CO₂ and other small wastes). They can build and keep a lot of order inside because they add even more disorder to their surroundings, so life obeys the second law.
That's why life needs a constant energy supply. If the energy coming in can't keep up with what's lost, a cell's order falls apart and it dies.

Key terms (13)

metabolism: All the chemical reactions in an organism, taken together. It's how a living thing manages its matter and its energy.
metabolic pathway: A chain of reactions, each sped up by its own enzyme, in which one step's product becomes the next step's reactant.
catabolic pathway: A pathway that breaks bigger molecules into smaller ones and releases energy, such as cellular respiration.
anabolic pathway: A pathway that uses energy to build bigger, more complex molecules from small ones, such as making glycogen from glucose.
energy: The ability to make something change. In a cell, that means moving things, building molecules or pumping substances across membranes.
kinetic energy: Energy of motion, like a swimming sperm cell or molecules colliding in a solution.
thermal energy (heat): Kinetic energy from the random jiggling of atoms and molecules. Heat flows from warmer places to cooler ones.
potential energy: Stored energy that comes from where something is or how it's built, like a boulder perched on a cliff or ions crowded on one side of a membrane.
chemical energy: Potential energy stored in how a molecule's atoms and electrons are arranged. It's released when the molecule reacts to form more stable products.
first law of thermodynamics: Energy can be moved or turned into a different form, but it can't be made from nothing or destroyed. Also called conservation of energy.
second law of thermodynamics: Every energy transfer or conversion loses some energy as unusable heat, so the total disorder of the universe goes up.
entropy: How disordered or random a collection of matter and energy is. The more spread out and mixed up matter and energy are, the higher the entropy.
open system: A system that exchanges both matter and energy with its surroundings. Every cell and organism is one.

Check yourself: 8.1 Metabolism and the laws of energy

4 questions on 8.1 Metabolism and the laws of energy. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of the following is an example of an anabolic process?

Question 2 of 4

A sports drink's label says, "Our drink creates energy for your muscles." Which revision makes the claim consistent with the first law of thermodynamics?

Question 3 of 4

A hummingbird sips sugary nectar and then hovers in front of a flower, beating its wings about 50 times per second. Which best describes the energy changes involved?

Question 4 of 4

A classmate argues that a chick embryo growing inside an egg breaks the second law of thermodynamics, because its body becomes far more organized than the yolk and egg white it develops from. Which observation best refutes this claim?

0 of 4 answered

8.2 Free energy: which reactions can run on their own

pp. 146–149

On the AP exam? Yes

Topic 3.3 tests energy-releasing versus energy-requiring reactions and why living cells never settle at equilibrium, and Topic 3.2 says the amounts of substrate and product affect how a reaction proceeds; the Gibbs free energy equation itself is excluded from the exam.

In the course: Topic 3.3 Cellular Energy, Topic 3.2 Environmental Impacts on Enzyme Function (notes, videos and more questions)

Key points

Free energy (G) is the part of a system's total energy that cells can actually harness for work under their conditions. ΔG is the change in free energy: the final state minus the starting state.
A process with a negative ΔG gives off free energy and can happen without any energy being supplied. Biologists call it spontaneous, but that means 'energetically favorable', not 'fast'.
Exergonic reactions release free energy (ΔG < 0). Endergonic reactions absorb free energy (ΔG > 0) and go only if energy is supplied. If a reaction is exergonic one way, its reverse is endergonic by the same amount.
Higher free energy means less stable. Systems tend to change toward lower free energy, and equilibrium is the lowest point: nothing changes overall, so a system at equilibrium can't do any work.
Bonds don't hold energy that pops out when they break: breaking any bond costs energy. A reaction releases energy overall when the new bonds in the products are more stable, leaving the products with less free energy.
A living cell is never at equilibrium. New reactants keep arriving, products are used by the next step or removed, and that keeps reactions running and doing work. If a cell's reactions all settled at equilibrium, it could do no work at all, which means it would be dead.
The book shows how ΔG depends on heat content and disorder (ΔG = ΔH − TΔS). You don't need that equation for AP; just know what a negative or positive ΔG means.

Key terms (9)

free energy (G): The part of a system's total energy that can be harnessed for work under cell conditions. Changes that lower it can happen on their own.
ΔG (free-energy change): Free energy at the end of a process minus free energy at the start. Negative means energy was released; positive means energy had to be put in.
spontaneous process: A change that can occur without an outside energy input because it lowers free energy. It might be very fast or extremely slow.
nonspontaneous process: A change that occurs only if energy is supplied; left alone, it would tend to run in the opposite direction.
exergonic reaction: A reaction that releases free energy (ΔG < 0). Its products hold less free energy than its reactants.
endergonic reaction: A reaction that absorbs free energy (ΔG > 0). Its products hold more free energy than its reactants, so it needs an energy source.
chemical equilibrium: The state where a reversible reaction's forward and reverse rates are equal, so amounts stop changing. Free energy is at its lowest, and no work can be done.
stability: How unlikely a system is to change on its own. The lower the free energy, the more stable the system.
enthalpy (H): Roughly, the total energy a system contains. It appears in the free energy equation, which the AP exam leaves out.

Check yourself: 8.2 Free energy: which reactions can run on their own

4 questions on 8.2 Free energy: which reactions can run on their own. Pick an answer to see if you got it, and why.

Question 1 of 4

Which statement correctly describes an endergonic reaction?

Question 2 of 4

Under the conditions inside a cell, the reaction X → Y has ΔG = −15 kJ/mol. Under the same conditions, what is true of the reverse reaction, Y → X?

Question 3 of 4

Which of the following changes results in a decrease in free energy?

Question 4 of 4

In a test tube, the reversible reaction F ⇌ G comes to equilibrium when the concentration of G is about 4 times that of F. In living liver cells, the concentration of G is kept at only about 1/20 that of F. Which conclusion is best supported?

0 of 4 answered

8.3 ATP: the cell's energy go-between

pp. 149–151

On the AP exam? Yes

Energy coupling and ATP are core to Topic 3.3, ATP-powered pumps are Topic 2.8, and making ATP is covered in Topics 3.4 and 3.5; you won't need to memorize ATP's ΔG value.

In the course: Topic 3.3 Cellular Energy, Topic 2.8 Mechanisms of Transport, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Cells do three broad kinds of work: chemical work (building molecules that won't form on their own), transport work (pumping substances against their gradients) and mechanical work (moving cilia, muscles or chromosomes).
Energy coupling means using an energy-releasing process to drive an energy-requiring one. In most cells, ATP is the link between the two.
ATP is adenine and the sugar ribose attached to three phosphate groups in a row. Using water to split off the last phosphate (hydrolysis) gives ADP plus inorganic phosphate (Pᵢ) and releases free energy.
ATP's phosphate bonds aren't extra strong. Its three phosphates all carry negative charges that push against each other, so ATP plus water has more free energy than ADP plus Pᵢ.
ATP rarely just releases heat. Usually an enzyme moves its phosphate onto another molecule (phosphorylation), making that molecule less stable and more reactive, or ATP binding and hydrolysis changes a protein's shape, as in pumps and motor proteins.
Coupling works only if the combined ΔG is negative: ATP hydrolysis has to release more energy than the uphill reaction needs.
ATP is recycled constantly. Catabolism, especially cellular respiration, supplies the energy to put a phosphate back on ADP, and plants can also use light energy. Making ATP is itself an uphill (endergonic) reaction.

Key terms (12)

ATP (adenosine triphosphate): A nucleotide made of adenine, the sugar ribose and three phosphate groups. It's the main energy carrier cells spend on their work.
ADP (adenosine diphosphate): What's left after ATP loses its last phosphate. Adding a phosphate back turns it into ATP again.
inorganic phosphate (Pᵢ): A free phosphate group, such as the one released when ATP is hydrolyzed.
ATP hydrolysis: Using water to split the last phosphate off ATP, giving ADP and Pᵢ. It releases free energy, so it's exergonic.
energy coupling: Pairing an energy-releasing process with an energy-requiring one so the first can drive the second.
phosphorylation: Adding a phosphate group to a molecule. When ATP supplies the phosphate, the molecule that receives it becomes more reactive or changes shape.
phosphorylated intermediate: A molecule that has just picked up a phosphate from ATP. It's less stable than before, which lets an uphill reaction go forward.
ATP cycle: The nonstop loop of spending ATP on cellular work and rebuilding it from ADP and Pᵢ with energy from catabolism (or from light, in photosynthesis).
chemical work: Driving reactions that wouldn't go on their own, such as linking monomers into polymers.
transport work: Pumping substances across a membrane in the direction they wouldn't move by themselves.
mechanical work: Physical movement, like a muscle contracting, a cilium beating or chromosomes being pulled apart.
motor protein: A protein that uses repeated rounds of ATP binding and hydrolysis to change shape and move along the cytoskeleton, often hauling cargo.

Check yourself: 8.3 ATP: the cell's energy go-between

4 questions on 8.3 ATP: the cell's energy go-between. Pick an answer to see if you got it, and why.

Question 1 of 4

Which pairing of a cellular activity with the type of work it represents is correct?

Question 2 of 4

Which statement best explains why hydrolysis of ATP's last phosphate group releases free energy?

Question 3 of 4Calculator allowed

Under standard lab conditions, hydrolyzing ATP releases about 30.5 kJ/mol. Inside a working cell, where ATP is kept far more concentrated than ADP, it releases about 50 kJ/mol. An enzyme couples one ATP hydrolysis to a reaction with ΔG = +40 kJ/mol. Which statement about the coupled process is correct?

Question 4 of 4

In one enzyme-catalyzed synthesis, ATP first moves its last phosphate onto reactant R, making R–P. R–P then reacts with molecule S to form the product, and the phosphate is released. What is the most important role of R–P in this process?

0 of 4 answered

8.4 How enzymes speed up reactions

pp. 152–157

On the AP exam? Yes

This is the heart of Topics 3.1 and 3.2: activation energy, active sites, temperature, pH, substrate amount and the two kinds of inhibitors. The course description doesn't spell out cofactors or the specific ways active sites lower activation energy, so treat those as background.

In the course: Topic 3.1 Enzymes, Topic 3.2 Environmental Impacts on Enzyme Function, Topic 6.7 Mutations (notes, videos and more questions)

Key points

An enzyme is a catalyst: it speeds up a reaction and comes out unchanged, so one enzyme can be used thousands of times. Most enzymes are proteins; a few catalysts are RNA.
Even a downhill reaction must first climb an energy hill, the activation energy (Eₐ), to reach a strained, unstable transition state in which bonds can break. Enzymes lower that hill but leave ΔG alone: a reaction that's uphill without the enzyme is still uphill with it.
A substrate binds the enzyme's active site only if their shapes and charges match. As it binds, the enzyme shifts to grip it more tightly (induced fit), forming an enzyme–substrate complex.
Inside the active site, an enzyme can line reactants up so they meet the right way, bend them toward the transition state, supply just the right chemical surroundings, or even take part in the reaction for a moment, and it always ends up unchanged.
More substrate raises the rate until every active site is busy (saturation). Warming speeds reactions up to the optimum, but too much heat or a pH outside the optimal range breaks the hydrogen and ionic bonds holding the enzyme's shape and can denature it, sometimes reversibly.
Competitive inhibitors resemble the substrate and block the active site; plenty of extra substrate can outcompete them. Noncompetitive inhibitors bind elsewhere and change the enzyme's shape, so extra substrate doesn't help. An inhibitor that forms a covalent bond with the enzyme usually can't come off, so that enzyme molecule stays blocked.
Many enzymes need a non-protein helper called a cofactor, such as a metal ion; an organic cofactor, often made from a vitamin, is a coenzyme. New enzymes evolve when mutations change amino acids and natural selection favors versions with useful activity.

Key terms (15)

enzyme: A biological catalyst, usually a protein, that speeds up a particular reaction and isn't used up, so it works again and again.
activation energy (Eₐ): The energy reactants must absorb before their bonds can break and the reaction can get going, like a hill even a downhill reaction must climb first.
transition state: The unstable, high-energy arrangement reactants pass through at the peak of the activation energy hill.
substrate: The molecule (or molecules) an enzyme acts on.
active site: The pocket or groove on an enzyme where the substrate binds and the reaction takes place. Its shape and charges decide what fits.
enzyme–substrate complex: The enzyme and its substrate bound together, just before the substrate is changed into product.
induced fit: The slight shape change an enzyme makes as its substrate binds, which tightens its hold and lines up the groups that do the chemistry.
saturation: When there's so much substrate that every active site is occupied. Adding more substrate then can't raise the rate.
optimal temperature: The temperature at which an enzyme works fastest. Above it, the enzyme begins to lose its working shape.
optimal pH: The pH at which an enzyme works best. It usually matches where the enzyme works, so an enzyme from an acidic place can have a low optimum.
denaturation: Loss of a protein's working 3D shape when heat, pH or chemicals disrupt the weak bonds holding it. It can sometimes be reversed.
cofactor: A non-protein helper some enzymes need in order to work, such as a magnesium or manganese ion.
coenzyme: A cofactor that's an organic molecule. Many are built from vitamins, which is one reason you need vitamins in your diet.
competitive inhibitor: A molecule that looks enough like the substrate to block the active site. Adding a lot of substrate can outcompete it.
noncompetitive inhibitor: A molecule that binds somewhere other than the active site and changes the enzyme's shape, so it works less well no matter how much substrate is around.

Check yourself: 8.4 How enzymes speed up reactions

4 questions on 8.4 How enzymes speed up reactions. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

An energy diagram for an uncatalyzed reaction shows the reactants at 60 kJ/mol, the peak of the curve at 140 kJ/mol and the products at 25 kJ/mol. When an enzyme is added, the peak drops to 90 kJ/mol. For the catalyzed reaction, what are the activation energy and ΔG?

Question 2 of 4

An enzyme from a soil bacterium breaks down compound X but has no effect on compound Y, which contains exactly the same atoms bonded in a mirror-image 3D arrangement. What is the best explanation?

Question 3 of 4

A fixed amount of an enzyme was mixed with different concentrations of its substrate, and the initial reaction rate was measured (invented data). Substrate (mM) | Initial rate (µmol/min) 0.5 | 12 1 | 21 2 | 33 4 | 40 8 | 42 16 | 42 Which statement best explains why the rate hardly changes between 8 mM and 16 mM?

Question 4 of 4

Researchers measured the activity of a digestive enzyme from a cold-water fish at different temperatures (invented data, as % of the highest activity). Temperature (°C) | Activity (%) 5 | 40 15 | 72 25 | 100 35 | 46 45 | 3 Which statement best explains the drop in activity from 25 °C to 45 °C?

0 of 4 answered

8.5 Switching enzymes on and off

pp. 158–160

On the AP exam? Yes

Allosteric sites and noncompetitive inhibitors are in Topic 3.2, feedback inhibition is a molecular example of the negative feedback in Topic 4.4, and keeping enzymes in organelles fits Topic 2.9. Cooperativity and the book's drug-screening study won't be tested.

In the course: Topic 3.2 Environmental Impacts on Enzyme Function, Topic 4.4 Feedback, Topic 2.9 Cell Compartmentalization, Topic 6.5 Regulation of Gene Expression (notes, videos and more questions)

Key points

Cells control metabolism in two ways: by changing how much of an enzyme they make (gene regulation, which is slower) and by turning existing enzymes up or down (which is fast).
In allosteric regulation, a molecule binds a regulatory site away from the active site and changes the enzyme's shape. An activator holds the enzyme in its working shape; an inhibitor holds it in an inactive shape. These regulators attach weakly, so they come off when their concentration falls.
Many regulated enzymes are made of several subunits locked together, so one regulator binding at one spot can flip the whole enzyme between its active and inactive forms.
A cell's energy level steers catabolism: lots of ATP slows key enzymes in energy-releasing pathways, while a buildup of ADP speeds them up.
In cooperativity, the substrate itself acts as the activator: once it binds at one subunit, the enzyme's other active sites grab and convert substrate more easily, so activity climbs steeply over a narrow range of substrate.
In feedback inhibition, a pathway's final product binds an enzyme near the start of the pathway and shuts it off. When the product runs low, the inhibitor comes off and the pathway restarts. It's negative feedback at the molecular level, so the cell doesn't spend raw materials and energy on something it already has.
Location matters too. Enzymes for back-to-back steps can be joined into complexes, built into membranes or kept inside specific organelles, which speeds pathways up and keeps competing reactions apart.

Key terms (9)

allosteric regulation: Control of a protein's activity by a molecule that binds at a site other than the active site and changes the protein's shape.
allosteric (regulatory) site: A binding spot on an enzyme, separate from the active site, where activators or inhibitors attach.
allosteric activator: A regulator that holds an enzyme in its working shape, so the enzyme is more active.
allosteric inhibitor: A regulator that holds an enzyme in an inactive shape, so the enzyme is less active.
cooperativity: When substrate binding at one active site makes the enzyme's other active sites work better, so activity rises steeply over a narrow range of substrate.
feedback inhibition: When a pathway's end product turns off an enzyme early in that pathway, so the cell stops making something it already has enough of.
negative feedback: A response that undoes a change, bringing a system back toward where it started.
multienzyme complex: Several enzymes for back-to-back steps of a pathway held together, so each product passes straight to the next enzyme.
compartmentalization: Keeping particular enzymes and reactions in separate places in a cell, such as inside organelles or built into membranes.

Check yourself: 8.5 Switching enzymes on and off

4 questions on 8.5 Switching enzymes on and off. Pick an answer to see if you got it, and why.

Question 1 of 4

A bacterium makes compound P from compound A in four steps, each catalyzed by its own enzyme: A → B → C → D → P (enzymes 1 to 4). When P is plentiful, which form of regulation would save the cell the most resources?

Question 2 of 4

In the pathway A → B → C → D → P, P normally inhibits enzyme 1 by binding to an allosteric site. A mutation changes that site so P can no longer bind, but enzyme 1's active site still works normally. What is the most likely result?

Question 3 of 4

An enzyme near the start of a sugar-breakdown pathway was tested with different amounts of ATP and ADP, with substrate held constant (invented data). ATP (mM) | ADP (mM) | Activity (% of maximum) 5.0 | 0.5 | 12 2.5 | 2.5 | 48 0.5 | 5.0 | 91 5.0 | 5.0 | 70 Which model is best supported by these data?

Question 4 of 4

An enzyme built from four identical subunits was tested at different substrate concentrations (invented data). Substrate (mM) | Rate (µmol/min) 0.5 | 1 1 | 3 2 | 14 3 | 38 4 | 62 6 | 85 8 | 92 The rate rises slowly at first, then steeply, then levels off. Which explanation best fits this pattern?

0 of 4 answered