Which word describes metabolic pathways that build complex molecules from simpler ones and require an input of energy?
Anabolic
✓
Catabolic
Exergonic
Hydrolytic
Correct Answer
Anabolic
Anabolic pathways, such as building proteins from amino acids, assemble larger molecules and consume energy. Catabolic pathways do the opposite, breaking large molecules down and releasing energy. Exergonic describes reactions that release free energy, which fits breakdown rather than building. Hydrolytic describes reactions that split molecules by adding water.
Category: Energy and Thermodynamics
Question 2
Which statement expresses the first law of thermodynamics?
Every energy transfer loses some energy as heat
Energy can change form but is never created or destroyed
✓
The disorder of the universe decreases over time
Energy is created whenever chemical bonds form
Correct Answer
Energy can change form but is never created or destroyed
The first law states that the total amount of energy in the universe is constant, so energy can only be transferred or transformed. Losing some energy as heat in every transfer is the second law of thermodynamics. Disorder, or entropy, tends to increase, not decrease. Forming chemical bonds transfers or transforms energy that already exists; it does not create new energy.
Category: Energy and Thermodynamics
Question 3
In mammals, the liver combines ammonia from broken-down amino acids with carbon dioxide to make which nitrogen waste?
Lactic acid
Glycerol
Urea
✓
Pyruvate
Correct Answer
Urea
When amino acids are used for energy, their amino groups are removed and become ammonia, which the mammalian liver converts to urea for removal in urine. Lactic acid is the product of lactic acid fermentation and contains no nitrogen. Glycerol is the backbone of triglycerides. Pyruvate is the three-carbon end product of glycolysis.
Category: Connected Pathways
Question 4
A chemical reaction with a negative change in free energy, which releases energy and occurs spontaneously, is described as what?
Endergonic
Anabolic
Nonspontaneous
Exergonic
✓
Correct Answer
Exergonic
Exergonic reactions have a negative change in free energy, meaning their products hold less free energy than their reactants, so energy is released. Endergonic reactions have a positive change and need an energy input. Anabolic pathways build molecules and generally require energy. Nonspontaneous describes endergonic reactions, the opposite case.
Category: Energy and Thermodynamics
Question 5
How does an enzyme speed up a chemical reaction?
It lowers the activation energy
✓
It makes the free energy change more negative
It raises the temperature of the cell
It supplies extra ATP to the reactants
Correct Answer
It lowers the activation energy
An enzyme lowers the activation energy, the energy needed to start the reaction, so the reaction proceeds faster at body temperature. Making the free energy change more negative is impossible for an enzyme, which leaves the overall energy difference between reactants and products unchanged. Enzymes do not heat the cell. Enzymes also do not supply ATP; they work by binding substrates at the active site.
Category: Enzymes
Question 6
Which model proposes that an enzyme changes shape slightly when its substrate binds, creating a tighter fit?
Lock-and-key model
Induced-fit model
✓
Fluid mosaic model
Feedback model
Correct Answer
Induced-fit model
The induced-fit model holds that substrate binding causes a small change in the enzyme's shape that improves the match and promotes the reaction. The lock-and-key model is the older idea that the enzyme and substrate fit together rigidly with no shape change. The fluid mosaic model describes the structure of cell membranes. Feedback describes how pathway products regulate enzymes, not how substrates fit.
Category: Enzymes
Question 7
A molecule that resembles the substrate and blocks the enzyme by occupying its active site is called what?
Noncompetitive inhibitor
Allosteric activator
Competitive inhibitor
✓
Coenzyme
Correct Answer
Competitive inhibitor
A competitive inhibitor competes with the substrate for the same active site, so less substrate can bind. A noncompetitive inhibitor binds elsewhere on the enzyme and changes the active site's shape. An allosteric activator binds away from the active site and increases the enzyme's affinity for its substrate. A coenzyme is an organic helper that assists the enzyme rather than blocking it.
Category: Enzymes
Question 8
Organic helper molecules that assist enzymes, many of them derived from vitamins, are called what?
Substrates
Inhibitors
Products
Coenzymes
✓
Correct Answer
Coenzymes
Coenzymes are organic molecules that help enzymes carry out their reactions, and many come from vitamins in the diet. Substrates are the reactants an enzyme acts on. Inhibitors reduce enzyme activity rather than helping it. Products are the molecules a reaction makes.
Category: Enzymes
Question 9
When the end product of a metabolic pathway slows its own production by inhibiting an enzyme earlier in the pathway, the process is called what?
Feedback inhibition
✓
Denaturation
Induced fit
Substrate activation
Correct Answer
Feedback inhibition
In feedback inhibition, a pathway's product shuts down an upstream enzyme, so the cell stops making more of the product once enough has built up. Denaturation is the loss of an enzyme's shape from heat or extreme pH. Induced fit is the shape change that occurs when a substrate binds. Substrate activation does not describe a product limiting its own pathway.
Category: Enzymes
Question 10
Where in a eukaryotic cell does glycolysis take place?
In the mitochondrial matrix
In the cytoplasm
✓
In the inner mitochondrial membrane
In the nucleus
Correct Answer
In the cytoplasm
Glycolysis occurs in the cytoplasm of all eukaryotic cells and most prokaryotic cells, outside any organelle. The mitochondrial matrix is where pyruvate is converted to acetyl CoA and the citric acid cycle runs. The inner mitochondrial membrane holds the electron transport chain. The nucleus stores DNA and is not a site of glucose breakdown.
Category: Glycolysis
Question 11
Glycolysis splits one molecule of glucose into which products?
Two molecules of acetyl CoA
Six molecules of carbon dioxide
Two molecules of pyruvate
✓
Two molecules of lactic acid
Correct Answer
Two molecules of pyruvate
Glycolysis breaks the six-carbon glucose into two three-carbon pyruvate molecules. Acetyl CoA is made later, in the mitochondrion, when each pyruvate loses a carbon dioxide. Six carbon dioxide molecules are released only after the later stages of respiration are complete. Lactic acid forms from pyruvate only during fermentation.
Category: Glycolysis
Question 12
What is the net gain of ATP from glycolysis of one glucose molecule?
4 ATP
6 ATP
36 ATP
2 ATP
✓
Correct Answer
2 ATP
Glycolysis spends 2 ATP in its first half and makes 4 ATP in its second half, for a net gain of 2. Four ATP is the total made before subtracting the 2 invested. Six ATP does not match any stage of glycolysis. Thirty-six ATP is in the range often quoted for complete aerobic respiration, not for glycolysis alone.
Category: Glycolysis
Question 13
Before entering the citric acid cycle, each pyruvate loses one carbon, and its remaining two-carbon acetyl group is attached to a carrier, forming which molecule?
Acetyl CoA
✓
Oxaloacetate
Lactate
Citrate
Correct Answer
Acetyl CoA
In the mitochondrial matrix, pyruvate loses one carbon as carbon dioxide, and the remaining two-carbon acetyl group is attached to coenzyme A to form acetyl CoA. Oxaloacetate is the four-carbon molecule that accepts the acetyl group within the cycle. Lactate is formed from pyruvate during fermentation, not before the cycle. Citrate forms later, inside the cycle, when acetyl CoA hands its acetyl group to oxaloacetate.
Category: Citric Acid Cycle
Question 14
Each turn of the citric acid cycle produces which set of energy-rich molecules?
1 NADH, 3 FADH2 and 1 ATP
3 NADH, 1 FADH2 and 1 ATP
✓
2 NADH, 2 FADH2 and 2 ATP
3 NADH, 1 FADH2 and 3 ATP
Correct Answer
3 NADH, 1 FADH2 and 1 ATP
One turn of the citric acid cycle yields three NADH, one FADH2 and one ATP or an equivalent molecule, and releases two carbon dioxide. The set with three FADH2 reverses the two electron carriers. The set with two of each does not match the cycle. The set with three ATP overstates how much ATP the cycle makes directly.
Category: Citric Acid Cycle
Question 15
In eukaryotic cells, where does the citric acid cycle occur?
Intermembrane space
Cytosol
Mitochondrial matrix
✓
Outer mitochondrial membrane
Correct Answer
Mitochondrial matrix
The citric acid cycle runs in the matrix, the innermost compartment of the mitochondrion. The intermembrane space is where hydrogen ions are pumped by the electron transport chain. The cytosol outside the mitochondria is where glycolysis occurs. The outer mitochondrial membrane surrounds the organelle but does not hold the cycle's enzymes.
Category: Citric Acid Cycle
Question 16
What is the final electron acceptor at the end of the electron transport chain in aerobic respiration?
Water
NAD+
Carbon dioxide
Oxygen
✓
Correct Answer
Oxygen
Oxygen accepts the electrons at the end of the chain and combines with hydrogen ions to form water, which keeps electrons flowing. Water is the product formed, not the acceptor. NAD+ picks up electrons earlier, in glycolysis and the citric acid cycle, and delivers them to the chain as NADH. Carbon dioxide is a waste product released in the citric acid cycle and does not accept electrons from the chain.
Category: Electron Transport and ATP Synthesis
Question 17
During oxidative phosphorylation, hydrogen ions flow back into the matrix through which protein, driving the production of ATP?
ATP synthase
✓
Cytochrome c
Coenzyme A
Lactate dehydrogenase
Correct Answer
ATP synthase
ATP synthase is an integral protein of the inner mitochondrial membrane that uses the flow of hydrogen ions down their gradient to join ADP and phosphate into ATP. Cytochrome c is a mobile carrier that passes electrons along the chain. Coenzyme A carries acetyl groups into the citric acid cycle. Lactate dehydrogenase converts pyruvate to lactic acid during fermentation.
Category: Electron Transport and ATP Synthesis
Question 18
As electrons pass along the electron transport chain, hydrogen ions are pumped in which direction?
From the intermembrane space into the matrix
From the matrix into the intermembrane space
✓
From the cytosol into the matrix
From the matrix out into the cytosol
Correct Answer
From the matrix into the intermembrane space
The complexes of the chain use energy from electrons to pump hydrogen ions out of the matrix into the intermembrane space, building a steep gradient. Movement from the intermembrane space into the matrix is the return flow through ATP synthase, not the pumping step. Pumping between the cytosol and the matrix does not describe the chain. The chain's pumping is confined to the two sides of the inner membrane, not the cytosol.
Category: Electron Transport and ATP Synthesis
Question 19
Yeast carrying out alcohol fermentation convert pyruvate into which products?
Lactic acid only
Acetyl CoA and water
Ethanol and carbon dioxide
✓
Ethanol and oxygen
Correct Answer
Ethanol and carbon dioxide
In alcohol fermentation, pyruvate releases carbon dioxide and becomes acetaldehyde, which accepts electrons from NADH to become ethanol. Lactic acid is the product of lactic acid fermentation in muscles and some bacteria. Acetyl CoA forms only when pyruvate enters the mitochondrion during aerobic respiration. Oxygen is not produced by fermentation, which occurs without it.
Category: Fermentation
Question 20
Which molecule does fermentation regenerate so that glycolysis can keep producing ATP?
ATP
Glucose
FADH2
NAD+
✓
Correct Answer
NAD+
Glycolysis needs NAD+ to accept electrons, and without an electron transport chain to recycle NADH, fermentation passes those electrons to an organic molecule and regenerates NAD+. ATP is made by glycolysis itself, not by the fermentation steps. Glucose is the fuel consumed, not something fermentation rebuilds. FADH2 is an electron carrier made in the citric acid cycle and plays no role in fermentation.
Category: Fermentation