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BW Abitur Biology Bilingual EN study materials
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BW Abitur Biology Bilingual EN practice questions

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Question 1Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A student examines a cell and finds a rigid structure lying outside the plasma membrane that provides mechanical support and prevents the cell from bursting in a hypotonic medium. Which structure is being described?

  1. The cell wall Correct answer.
  2. The tonoplast
  3. The cytoskeleton
  4. The glycocalyx

Explanation

The cell wall is an extracellular rigid layer, made of cellulose in plants or peptidoglycan in bacteria, that resists turgor pressure and prevents lysis. The cytoskeleton lies inside the cytoplasm, the glycocalyx is a carbohydrate coat on the membrane surface, and the tonoplast is the vacuolar membrane.

Question 2Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement about the fluid mosaic model of the plasma membrane is correct?

  1. Cholesterol is present only in the outer leaflet of plant membranes
  2. The hydrophobic phosphate heads face the aqueous interior of the bilayer
  3. Membrane proteins are embedded in or attached to a fluid phospholipid bilayer Correct answer.
  4. Phospholipids flip frequently between the two leaflets without enzymatic help

Explanation

The fluid mosaic model describes proteins dispersed within or bound to a laterally fluid phospholipid bilayer. Transverse flipping of phospholipids is rare and requires flippases, the phosphate heads are hydrophilic and face outward toward water, and cholesterol is not restricted to one leaflet in that way.

Question 3Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A researcher adds a drug that blocks ATP synthesis in a cell. Which transport process will be affected first?

  1. The sodium-potassium pump moving ions against their gradient Correct answer.
  2. Facilitated diffusion of glucose through a channel
  3. Simple diffusion of oxygen across the bilayer
  4. Osmosis of water through aquaporins

Explanation

The sodium-potassium pump is primary active transport and directly hydrolyses ATP, so blocking ATP synthesis stops it immediately. Simple diffusion, osmosis and facilitated diffusion are passive and driven by concentration gradients rather than by ATP.

Question 4Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

In an experiment, a plant cell is placed in a strongly hypertonic sucrose solution. What is the most likely observable result?

  1. The cell swells and bursts because of turgor
  2. The cell wall dissolves and the membrane ruptures
  3. The protoplast shrinks away from the cell wall (plasmolysis) Correct answer.
  4. Water enters the vacuole and turgor pressure rises

Explanation

In a hypertonic solution the external water potential is lower than inside the cell, so water leaves the vacuole and cytoplasm. The protoplast shrinks and detaches from the rigid wall, a condition called plasmolysis. The wall itself remains intact.

Question 5Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which property of enzymes explains why a small amount of enzyme can convert a large amount of substrate?

  1. Enzymes are catalysts and are regenerated unchanged after each reaction cycle Correct answer.
  2. Enzymes are consumed stoichiometrically in the reaction
  3. Enzymes increase the free energy released by the reaction
  4. Enzymes lower the activation energy permanently for one substrate molecule

Explanation

Enzymes are catalysts: they are not consumed and return to their original state after each turnover, so one molecule can act repeatedly. They lower activation energy but do not alter the overall free energy change of the reaction.

Question 6Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A competitive inhibitor is added to an enzyme assay. Which change in kinetics is expected when substrate concentration is increased sufficiently?

  1. Both Vmax and Km fall proportionally
  2. Vmax is unchanged and the apparent Km increases Correct answer.
  3. Vmax falls and Km stays unchanged
  4. Neither Vmax nor Km can be restored by substrate

Explanation

A competitive inhibitor binds the active site reversibly, so high substrate concentrations can outcompete it. Vmax is therefore unchanged, but more substrate is needed to reach half-maximal rate, so the apparent Km increases.

Question 7Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which structural feature of the mitochondrion directly supports its role in oxidative phosphorylation?

  1. A single membrane enclosing hydrolytic enzymes
  2. A double membrane with the inner membrane folded into cristae Correct answer.
  3. A continuous membrane shared with the endoplasmic reticulum
  4. Ribosome-free stroma containing starch grains

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, and folding into cristae increases its surface area. The outer membrane is smooth, and the compartmentalisation allows a proton gradient to be maintained.

Question 8Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A protein destined for secretion is synthesised and follows the endomembrane route. Which sequence of compartments is correct?

  1. Rough ER, Golgi apparatus, secretory vesicle, plasma membrane Correct answer.
  2. Golgi apparatus, rough ER, secretory vesicle, plasma membrane
  3. Smooth ER, lysosome, Golgi apparatus, plasma membrane
  4. Cytosol, nucleus, Golgi apparatus, plasma membrane

Explanation

Secretory proteins are synthesised on ribosomes of the rough ER, packaged into transport vesicles, modified in the Golgi apparatus, and then carried in secretory vesicles that fuse with the plasma membrane. The order reflects the direction of vesicular traffic.

Question 9Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which function is carried out by the smooth endoplasmic reticulum rather than the rough endoplasmic reticulum?

  1. Synthesis of proteins with a signal peptide
  2. Detoxification of lipophilic drugs and synthesis of lipids Correct answer.
  3. Assembly of ribosomal subunits
  4. Modification of N-linked glycans on secretory proteins

Explanation

The smooth ER lacks bound ribosomes and is specialised for lipid and steroid synthesis, detoxification and calcium storage. Protein synthesis and glycan modification occur at the rough ER, and ribosomal subunits assemble in the nucleolus.

Question 10Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

The Golgi apparatus modifies proteins and sorts them for different destinations. Which feature best explains this sorting capacity?

  1. A stack of flattened cisternae with distinct cis and trans faces Correct answer.
  2. A network of tubules continuous with the nuclear envelope
  3. A single cisterna with one uniform lumen
  4. A double membrane enclosing oxidative enzymes

Explanation

The Golgi is a polar stack of flattened cisternae with a cis (entry) and trans (exit) face, and enzymes differ between cisternae. This compartmentalisation allows sequential modification and the addition of different targeting signals.

Question 11Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Lysosomes are described as the cell's digestive organelles. Which property is essential for their function?

  1. A neutral lumen with high ATP concentration
  2. A double membrane containing chlorophyll
  3. An acidic lumen maintained by proton pumps Correct answer.
  4. A lumen continuous with the cytosol

Explanation

Lysosomal hydrolases have acidic pH optima, and proton pumps in the lysosomal membrane keep the lumen acidic. This acidic environment both activates the enzymes and protects the cytosol if leakage occurs.

Question 12Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A cell biologist treats a cell with a drug that depolymerises microtubules. Which process is most directly impaired?

  1. Movement of chromosomes along the mitotic spindle Correct answer.
  2. Transport of protons across the inner mitochondrial membrane
  3. Formation of the cleavage furrow during cytokinesis
  4. Maintenance of desmosome adhesion between cells

Explanation

The mitotic spindle is built from microtubules, and chromosome movement depends on their assembly and disassembly. The cleavage furrow depends on actin microfilaments, desmosomes on intermediate filaments, and proton transport on membrane proteins.

Question 13Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement correctly distinguishes prokaryotic from eukaryotic cells?

  1. Prokaryotes contain mitochondria but no plasma membrane
  2. Prokaryotes have a nucleus but no ribosomes
  3. Prokaryotes have ribosomes but no nuclear envelope Correct answer.
  4. Eukaryotes lack membrane-bound organelles

Explanation

Prokaryotes possess 70S ribosomes but lack a membrane-bound nucleus, so their DNA lies in a nucleoid region. Eukaryotes have a true nucleus and membrane-bound organelles, and every cell has a plasma membrane.

Question 14Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A membrane protein moves sodium ions down their electrochemical gradient while simultaneously moving glucose against its gradient into the cell. How is this transport best classified?

  1. Receptor-mediated endocytosis
  2. Primary active transport
  3. Secondary active transport (cotransport) Correct answer.
  4. Simple diffusion

Explanation

The energy for moving glucose against its gradient comes indirectly from the sodium gradient, not directly from ATP hydrolysis at the transporter. This coupling of a downhill ion movement to an uphill solute movement is secondary active transport, here a symport.

Question 15Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Aquaporins greatly increase the rate of water movement across a membrane. Which statement about aquaporins is correct?

  1. They transport water only together with sodium ions
  2. They are lipid-soluble carriers that flip between leaflets
  3. They hydrolyse ATP to pump water against its gradient
  4. They form channels that allow passive water flow along the water potential gradient Correct answer.

Explanation

Aquaporins are channel proteins that permit rapid, passive movement of water down its water potential gradient without energy input. They do not hydrolyse ATP and are not carriers that flip between membrane leaflets.

Question 16Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which observation best supports the endosymbiotic theory for the origin of mitochondria?

  1. Mitochondria synthesise all their own proteins independently
  2. Mitochondria contain their own circular DNA and 70S ribosomes Correct answer.
  3. Mitochondria divide only together with the Golgi apparatus
  4. Mitochondria are surrounded by a single membrane

Explanation

Circular DNA and 70S ribosomes resemble bacterial features and support a prokaryotic ancestry. The double membrane, partial dependence on nuclear genes and independent division are also consistent with this theory.

Question 17Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

In a chloroplast, where does the light-independent fixation of carbon dioxide take place?

  1. In the intermembrane space
  2. In the thylakoid lumen
  3. On the outer chloroplast membrane
  4. In the stroma Correct answer.

Explanation

The Calvin cycle enzymes, including Rubisco, are located in the stroma, the fluid surrounding the thylakoids. The thylakoid membranes host the light reactions, and the intermembrane space lies between the two envelope membranes.

Question 18Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A student compares a plant cell and an animal cell of similar volume. Which combination of structures is found in the plant cell but not in the animal cell?

  1. Mitochondria, ribosomes and a plasma membrane
  2. Lysosomes, cytoskeleton and peroxisomes
  3. Chloroplasts, a large central vacuole and a cellulose cell wall Correct answer.
  4. A nucleus, Golgi apparatus and endoplasmic reticulum

Explanation

Plant cells typically contain chloroplasts, a large central vacuole bounded by a tonoplast, and a cellulose cell wall. Mitochondria, ribosomes, nucleus, Golgi, ER, cytoskeleton and peroxisomes occur in both cell types.

Question 19Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A reaction has an activation energy of 80 kJ/mol without an enzyme. In the presence of the enzyme the activation energy falls to 50 kJ/mol. Which statement correctly describes the effect on the reaction?

  1. The overall free energy change of the reaction becomes more negative
  2. The enzyme supplies 30 kJ/mol of energy directly to the substrates
  3. The equilibrium constant increases because the products become more stable
  4. The reaction rate increases because more molecules can overcome the lower energy barrier Correct answer.

Explanation

Lowering the activation energy means a larger fraction of molecules possess sufficient kinetic energy to react at a given temperature, so the rate increases. The enzyme does not change the free energy difference between substrates and products, so the equilibrium position is unaffected.

Question 20Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which factor would most likely denature a globular enzyme and abolish its catalytic activity?

  1. An increase in the volume of the reaction vessel
  2. A shift in pH far from the enzyme's optimum Correct answer.
  3. The presence of a buffer maintaining pH 7
  4. A small rise in substrate concentration

Explanation

Extreme pH alters the ionisation of amino acid side chains and disrupts ionic and hydrogen bonds, unfolding the active site. Substrate concentration, vessel volume and buffering do not denature the protein.

Question 21Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A cell is placed in a solution whose solute concentration equals that of the cytoplasm, but the solute can freely cross the membrane. What happens over time?

  1. The membrane becomes impermeable to the solute
  2. Solute enters the cell, water follows and the cell may swell Correct answer.
  3. The cell shrinks because water leaves the cell
  4. The cell remains unchanged because the solution is isotonic

Explanation

If the solute permeates the membrane, it diffuses into the cell down its concentration gradient, raising the internal solute concentration. Water then follows osmotically, so the cell may swell despite the initial isotonicity.

Question 22Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement about the nuclear pore complex is correct?

  1. It regulates selective transport of macromolecules such as mRNA and proteins Correct answer.
  2. It is a lipid bilayer that fuses the inner and outer nuclear membranes
  3. It permits free passage of ribosomal subunits but blocks mRNA
  4. It actively pumps DNA out of the nucleus during translation

Explanation

Nuclear pore complexes are large protein channels that control nucleocytoplasmic transport, allowing mRNA and ribosomal subunits out and proteins such as transcription factors in, usually with transport signals. They are not lipid bilayers and do not transport DNA.

Question 23Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A scientist wants to follow the path of a newly synthesised membrane protein to the plasma membrane. Which technique is best suited to trace this route?

  1. Radioactive labelling of amino acids combined with autoradiography over time Correct answer.
  2. Determining the osmotic potential of the cell
  3. Measuring oxygen consumption of isolated mitochondria
  4. Staining lipids with iodine solution

Explanation

Pulse-chase labelling with radioactive amino acids, followed by autoradiography at intervals, reveals the sequential appearance of the protein in ER, Golgi and secretory vesicles. The other methods measure unrelated properties.

Question 24Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement about the relationship between surface area and volume in cells is correct?

  1. As a cell grows, its volume increases faster than its surface area, limiting exchange Correct answer.
  2. Surface area and volume increase in direct proportion at all sizes
  3. Surface area is irrelevant to the rate of diffusion into a cell
  4. As a cell grows, its surface area increases faster than its volume

Explanation

For a sphere, surface area scales with the square of the radius while volume scales with the cube. Larger cells therefore have relatively less membrane area per unit volume, which limits diffusional exchange and is one reason cells remain small or become compartmentalised.

Question 25Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A researcher measures the rate of an enzyme-catalysed reaction at increasing substrate concentrations and finds that the rate plateaus. What explains the plateau?

  1. All active sites are occupied, so the enzyme is saturated Correct answer.
  2. The substrate has been completely degraded and none remains
  3. The activation energy has increased to its maximum value
  4. The enzyme has been irreversibly denatured by the substrate

Explanation

At saturation every enzyme molecule is working at maximum turnover, so adding more substrate cannot increase the rate further. This plateau defines Vmax, which depends on enzyme concentration and catalytic constant.

Question 26Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A student compares the lipid composition of the inner mitochondrial membrane with that of the plasma membrane. The inner mitochondrial membrane contains a high proportion of the phospholipid cardiolipin and lacks cholesterol. Which statement best explains the functional significance of this composition?

  1. Cardiolipin acts as a proton channel that dissipates the electrochemical gradient.
  2. Cardiolipin stabilises the respiratory chain supercomplexes and increases the proton permeability barrier. Correct answer.
  3. The absence of cholesterol allows the inner membrane to fuse more readily with lysosomes.
  4. Cholesterol is replaced by cardiolipin to increase membrane fluidity at low temperatures.

Explanation

Cardiolipin is a dimeric phospholipid found almost exclusively in the inner mitochondrial membrane and bacterial membranes. Its unique structure allows it to interact with respiratory chain complexes, stabilising supercomplexes and contributing to the tight proton seal required for oxidative phosphorylation. Cholesterol, which modulates fluidity in the plasma membrane, is largely absent from the inner mitochondrial membrane, but this absence is not primarily for fusion or proton leak.

Question 27Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement correctly describes the role of the signal recognition particle (SRP) in the synthesis of a secretory protein?

  1. It adds a mannose-rich oligosaccharide to the protein in the ER lumen.
  2. It transports the newly synthesised protein from the ER to the Golgi apparatus.
  3. It cleaves the signal peptide from the protein once the protein enters the ER lumen.
  4. It binds to the signal peptide and pauses translation until the ribosome attaches to the ER. Correct answer.

Explanation

The SRP recognises the hydrophobic signal peptide as it emerges from the ribosome, binds to it, and temporarily arrests translation. This pause allows the SRP–ribosome complex to bind to the SRP receptor on the rough ER membrane, after which translation resumes and the protein is translocated into the ER. Cleavage of the signal peptide is carried out by signal peptidase, and glycosylation is performed by oligosaccharyltransferase.

Question 28Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

A cell is treated with a drug that increases the fluidity of the plasma membrane by inserting into the lipid bilayer. Which immediate consequence is most likely for the function of a transmembrane carrier protein?

  1. The protein will be unable to bind ATP because ATP is hydrophobic.
  2. The protein may undergo a conformational change that alters its substrate specificity. Correct answer.
  3. The protein will be permanently locked in an open conformation.
  4. The protein will be immediately degraded by proteasomes in the cytoplasm.

Explanation

Membrane fluidity affects the packing of lipids around integral membrane proteins. Increased fluidity can allow the protein to change shape more readily, which may alter its conformation and thus its substrate specificity or transport activity. Proteasomal degradation and ATP binding are not immediate consequences of altered lipid fluidity, and permanent locking in an open conformation is not a general effect.

Question 29Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which structural feature of the chloroplast directly enables the light-dependent reactions to generate a proton gradient?

  1. The outer chloroplast membrane is freely permeable to small ions.
  2. The thylakoid membrane is impermeable to protons and contains ATP synthase complexes. Correct answer.
  3. The stroma contains high concentrations of soluble electron carriers.
  4. The thylakoid lumen has a high pH due to the accumulation of hydroxide ions.

Explanation

The thylakoid membrane houses the photosystems and the cytochrome b6f complex, which pump protons from the stroma into the thylakoid lumen during electron transport. Because the membrane is largely impermeable to protons, a gradient builds up, and ATP synthase uses this gradient to produce ATP. The stroma contains enzymes for the Calvin cycle, and the outer membrane's permeability does not create the gradient.

Question 30Zellbiologie / Cell biology (structure and function of cells, membranes, transport, enzymes, compartmentalisation)

Which statement best explains why enzymes lower the activation energy of a reaction?

  1. They increase the kinetic energy of substrate molecules by raising the temperature.
  2. They change the equilibrium constant of the reaction to favour product formation.
  3. They provide a hydrophobic environment that excludes water from the active site.
  4. They stabilise the transition state by providing favourable interactions in the active site. Correct answer.

Explanation

Enzymes lower activation energy primarily by binding and stabilising the transition state of the reaction, which reduces the energy barrier. This is achieved through complementary interactions in the active site that are optimal for the transition state rather than the substrate. Enzymes do not raise temperature, alter the equilibrium constant, or rely solely on hydrophobic exclusion to achieve catalysis.

Question 31Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

In the light-dependent reactions of photosynthesis, which pair of reactants is split and rearranged to release molecular oxygen?

  1. Two carbon dioxide molecules
  2. One glucose and six oxygen molecules
  3. Two water molecules Correct answer.
  4. Two NADPH molecules

Explanation

Photosystem II contains the oxygen-evolving complex, which oxidises water to supply electrons to the reaction centre. Two water molecules are split, releasing four electrons, four protons and one O2 molecule. Carbon dioxide is fixed later in the Calvin cycle and is not the source of released oxygen.

Question 32Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

A suspension of isolated chloroplasts is illuminated in the presence of an artificial electron acceptor that removes electrons from photosystem I. Which immediate consequence is expected?

  1. ATP synthesis by photophosphorylation decreases Correct answer.
  2. Oxygen evolution stops because water becomes the acceptor
  3. The rate of water splitting increases sharply
  4. Carbon fixation becomes independent of light

Explanation

Photophosphorylation depends on the light-driven electron transport chain building a proton gradient across the thylakoid membrane. If electrons are diverted out of the chain at photosystem I, proton pumping and ATP synthesis decline. Water splitting is upstream of photosystem I and is not directly stimulated by a downstream electron sink.

Question 33Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which statement best describes the role of rubisco in the Calvin cycle?

  1. It attaches CO2 to ribulose-1,5-bisphosphate, forming two molecules of 3-phosphoglycerate Correct answer.
  2. It converts pyruvate into acetyl-CoA before the cycle begins
  3. It phosphorylates ADP to ATP in the thylakoid membrane
  4. It transfers electrons from NADPH to the electron transport chain

Explanation

Rubisco is the enzyme that catalyses carbon fixation, condensing CO2 with the five-carbon acceptor ribulose-1,5-bisphosphate. The unstable six-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate. This is the entry point of carbon into the Calvin cycle.

Question 34Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

A plant is transferred to an atmosphere containing only about 0.04 percent oxygen but normal CO2. Which effect on the Calvin cycle is most likely?

  1. Photorespiration is suppressed, so less fixed carbon is lost Correct answer.
  2. ATP production in the thylakoids stops completely
  3. Rubisco oxygenase activity rises and more carbon is lost as glycolate
  4. Stomata close permanently and CO2 uptake ceases

Explanation

Rubisco can bind oxygen instead of CO2, initiating photorespiration and wasting fixed carbon. Lowering the oxygen concentration shifts the competition in favour of carboxylation. The result is reduced photorespiratory loss and improved net carbon gain.

Question 35Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which statement correctly distinguishes cyclic from non-cyclic photophosphorylation?

  1. Only cyclic photophosphorylation involves photosystem II
  2. Only cyclic photophosphorylation produces ATP without generating NADPH Correct answer.
  3. Only non-cyclic photophosphorylation uses a proton gradient
  4. Only non-cyclic photophosphorylation releases oxygen from water

Explanation

In cyclic photophosphorylation electrons from photosystem I are returned to the chain rather than passed to NADP+, so only ATP is made. Non-cyclic photophosphorylation uses both photosystems and produces ATP, NADPH and oxygen. Both routes rely on a proton gradient.

Question 36Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which molecule is the immediate final electron acceptor at the end of the respiratory electron transport chain in aerobic mitochondria?

  1. ATP synthase
  2. Pyruvate
  3. NAD+
  4. Molecular oxygen Correct answer.

Explanation

Complex IV transfers electrons to molecular oxygen, which is reduced to water. This keeps the chain running and maintains the proton gradient. Without oxygen the chain backs up and NADH cannot be reoxidised.

Question 37Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

In the chemiosmotic model, what directly drives ATP synthesis in mitochondria?

  1. The flow of protons down their electrochemical gradient through ATP synthase Correct answer.
  2. The oxidation of NADH at complex I
  3. The direct transfer of a phosphate group from a substrate to ADP
  4. The reduction of oxygen to water at complex IV

Explanation

Electron transport pumps protons into the intermembrane space, creating an electrochemical gradient. Protons return through the F0 portion of ATP synthase, and this flow drives rotation and ATP formation. Substrate-level phosphorylation is a separate mechanism.

Question 38Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which of the following statements about the net ATP yield of glycolysis is correct?

  1. No ATP is produced because all energy remains in pyruvate
  2. Four ATP are consumed and two are produced, giving a net loss of two ATP per glucose
  3. One ATP is produced per glucose molecule
  4. Two ATP are consumed and four are produced, giving a net gain of two ATP per glucose Correct answer.

Explanation

Glycolysis uses two ATP in the investment phase to phosphorylate glucose and fructose-6-phosphate. The payoff phase generates four ATP by substrate-level phosphorylation. The net yield is therefore two ATP, along with two NADH and two pyruvate.

Question 39Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

In the link reaction, pyruvate is converted to acetyl-CoA. Which combination of changes occurs?

  1. Decarboxylation and oxidation, with NAD+ reduced to NADH Correct answer.
  2. Carboxylation and reduction, with NADH oxidised to NAD+
  3. Hydration and cleavage, with FADH2 produced
  4. Phosphorylation and isomerisation, with ATP consumed

Explanation

Pyruvate dehydrogenase removes a carboxyl group as CO2 and oxidises the remaining two-carbon unit. The electrons reduce NAD+ to NADH, and coenzyme A is attached. The product acetyl-CoA then enters the citric acid cycle.

Question 40Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Per turn of the citric acid cycle, which products are formed directly?

  1. Four NADH and four CO2
  2. Three NADH, one FADH2, one GTP or ATP, and two CO2 Correct answer.
  3. Two NADH, two FADH2, and one CO2
  4. One NADH, one FADH2, and one ATP only

Explanation

One turn of the cycle oxidises an acetyl group and releases two CO2. The oxidations reduce three NAD+ and one FAD, and one GTP or ATP is formed by substrate-level phosphorylation. These carriers then feed the electron transport chain.

Question 41Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

During strenuous exercise, a muscle cell regenerates NAD+ by reducing pyruvate to lactate. What is the main purpose of this reaction?

  1. To keep glycolysis running by maintaining the NAD+ supply Correct answer.
  2. To increase the proton gradient across the inner membrane
  3. To produce large amounts of ATP directly from lactate
  4. To supply the citric acid cycle with extra acetyl-CoA

Explanation

Glycolysis requires a continuous supply of NAD+ to oxidise glyceraldehyde-3-phosphate. Under anaerobic conditions the respiratory chain cannot reoxidise NADH, so lactate formation regenerates NAD+. This allows glycolysis, and its modest ATP yield, to continue.

Question 42Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which statement about the effect of temperature on an enzyme-catalysed reaction is correct?

  1. The rate is unaffected by temperature as long as substrate is present
  2. The rate falls steadily as temperature rises because collisions become less frequent
  3. The rate rises with temperature until denaturation of the enzyme causes a sharp decline Correct answer.
  4. The rate rises indefinitely because activation energy keeps decreasing

Explanation

Higher temperature increases kinetic energy and collision frequency, so the rate rises. Beyond an optimum, non-covalent bonds break and the active site loses its shape, causing denaturation. The rate then falls sharply rather than continuing to rise.

Question 43Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

An enzyme is assayed at increasing substrate concentrations. Which change is observed as the substrate concentration approaches saturation?

  1. The enzyme's affinity for substrate falls to zero
  2. The reaction rate increases without limit in direct proportion to substrate
  3. The Michaelis constant increases as substrate is added
  4. The reaction rate approaches a maximum because active sites become occupied Correct answer.

Explanation

At low substrate concentration the rate rises steeply, but as active sites become saturated the rate levels off at Vmax. The Michaelis constant Km is a property of the enzyme-substrate pair under given conditions and does not rise with substrate concentration. Saturation reflects limited enzyme, not loss of affinity.

Question 44Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

A competitive inhibitor is added to an enzyme assay. Which kinetic change is expected?

  1. Vmax decreases while Km is unchanged
  2. Both Km and Vmax decrease
  3. Both Km and Vmax increase
  4. The apparent Km increases while Vmax is unchanged Correct answer.

Explanation

A competitive inhibitor binds the active site reversibly and can be outcompeted by more substrate. This raises the apparent Km because more substrate is needed for half-maximal rate. Because enough substrate can displace the inhibitor, Vmax is unchanged.

Question 45Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

A non-competitive inhibitor binds an enzyme at a site distinct from the active site. Why does Vmax fall?

  1. The inhibitor permanently denatures the entire enzyme population
  2. The inhibitor competes directly with substrate for the active site
  3. The substrate can no longer bind at all in every enzyme molecule
  4. Some enzyme molecules are prevented from catalysing the reaction regardless of substrate concentration Correct answer.

Explanation

Binding at an allosteric site changes the enzyme so that catalysis is impaired even when substrate is bound. Because a fraction of enzyme molecules is effectively removed, the maximum rate falls. Increasing substrate cannot fully overcome this, so Vmax remains reduced.

Question 46Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which property of ATP makes it suited to its role as the universal energy currency of cells?

  1. It can cross the inner mitochondrial membrane freely
  2. Its terminal phosphoanhydride bond can be hydrolysed to release usable free energy Correct answer.
  3. It stores energy in stable bonds that are never broken
  4. It contains more carbon atoms than glucose

Explanation

ATP carries energy in its phosphoanhydride bonds, and hydrolysis of the terminal phosphate releases free energy for cellular work. The reaction is readily reversible, allowing ATP to be regenerated. ATP is also small and can be produced and consumed at many cellular sites.

Question 47Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

The reduction of oxygen to water in the respiratory chain is described as the terminal step. Which statement explains why this step is essential for continued ATP synthesis?

  1. It removes electrons from the chain, allowing complexes I to IV to keep pumping protons Correct answer.
  2. It directly phosphorylates ADP at complex IV
  3. It converts NADH into ATP without a proton gradient
  4. It produces CO2 that drives the citric acid cycle

Explanation

If electrons were not removed at the end of the chain, carriers would remain reduced and transport would stop. Oxygen acts as the final electron acceptor, forming water. Continuous electron flow maintains the proton gradient that powers ATP synthase.

Question 48Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

Which statement correctly compares the location of glycolysis and the citric acid cycle in a eukaryotic cell?

  1. Glycolysis occurs in the mitochondrial matrix, while the citric acid cycle occurs in the cytosol
  2. Both occur in the intermembrane space
  3. Both occur in the mitochondrial matrix
  4. Glycolysis occurs in the cytosol, while the citric acid cycle occurs in the mitochondrial matrix Correct answer.

Explanation

Glycolysis enzymes are soluble in the cytosol, where glucose is converted to pyruvate. Pyruvate is then transported into mitochondria, where the citric acid cycle operates in the matrix. The electron transport chain is located on the inner mitochondrial membrane.

Question 49Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

A researcher measures oxygen consumption by mitochondria supplied with succinate, which donates electrons via FADH2 at complex II. Compared with NADH, what is expected?

  1. More protons are pumped because complex II also pumps protons
  2. Fewer protons are pumped per electron pair, so less ATP is formed Correct answer.
  3. The same amount of ATP is formed because all carriers are equivalent
  4. No oxygen is consumed because complex II bypasses the chain

Explanation

Complex II does not pump protons, so electrons entering here bypass the proton-pumping contribution of complex I. Fewer protons are translocated per electron pair and less ATP is generated. Electrons still pass through complexes III and IV, so oxygen is still consumed.

Question 50Stoffwechsel und Energieumsatz / Metabolism and bioenergetics (photosynthesis, cellular respiration, ATP, enzyme kinetics)

In an experiment, chloroplasts are kept in the dark but supplied with an artificial electron donor that feeds electrons into the transport chain. Which outcome is most likely?

  1. ATP may still be formed, but oxygen evolution from water splitting will not occur Correct answer.
  2. Carbon fixation increases because light is no longer required
  3. Neither ATP nor NADPH can be produced under any circumstances
  4. Both ATP synthesis and oxygen evolution proceed normally

Explanation

The artificial donor bypasses photosystem II, so water is not split and oxygen is not released. Electron flow can still drive proton pumping and ATP synthesis. NADPH production may be limited if photosystem I is not reduced, but ATP formation can continue.