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Flashcards · AP® Biology · Unit 3

Unit 3: Cellular Energetics flashcards

36 cards on topics 3.1 to 3.5. Pick a topic or study the whole unit.

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3.1 Enzymes

How do enzymes speed up reactions?
Enzymes are protein catalysts that lower the activation energy (Eₐ), the energy needed to start a reaction, so many more encounters lead to products. They are not used up and are reused after each reaction.
What do enzymes not change about a reaction?
They do not change ΔG, do not supply energy and do not change the amount of product finally made. On an energy profile, the start and end levels are the same; only the hill is lower.
What determines enzyme specificity?
The active site is a pocket lined by R groups with a particular shape and pattern of charge and polarity. A substrate binds only if it is compatible in both shape and charge, forming the enzyme–substrate complex.
Why does the rate of an enzyme reaction fall over time?
The substrate is being used up; the enzyme is not. E appears on both sides of E + S → ES → E + P.
How do you find the initial rate from a product–time graph?
The rate is the gradient, not the final height: rate = change in amount ÷ time, measured early before substrate falls much. 18 cm³ of gas in 60 s gives 0.30 cm³ s⁻¹.
How do you find a relative rate when timing an end point?
Relative rate = 1 ÷ time, for example the time taken for a colour change.
Why include a boiled-enzyme tube in an enzyme experiment?
As a negative control: it shows whether the effect needs an intact, heat-sensitive protein, ruling out other substances in the extract. A positive control is one where an effect is expected.

3.2 Environmental Impacts on Enzyme Function

What is enzyme denaturation?
Loss of the working 3D shape, so the enzyme can no longer catalyse. Weak interactions (hydrogen bonds, ionic and hydrophobic interactions) are disrupted by heat, pH or chemicals; peptide bonds are not broken. Enzymes are not "killed".
How does temperature affect enzyme activity?
Below the optimum, rate rises because collisions are more frequent and energetic. Above it, hydrogen bonds in the enzyme break, the active site changes shape and the rate falls steeply. Cold slows enzymes without denaturing them.
How does pH affect enzyme activity?
pH changes the charges on R groups, disrupting the fold and the active site's charge pattern, so rate falls away from the optimum. Each enzyme has its own optimum pH.
Why does an enzyme reaction rate level off at high substrate concentration?
The enzyme is saturated: all active sites are busy. The enzyme is not used up.
How do competitive and noncompetitive inhibitors differ?
Competitive inhibitors bind the active site; the rate recovers at high substrate as substrate outcompetes them. Noncompetitive inhibitors bind an allosteric site, change the active site and lower the rate at all substrate levels.
How do substrate and product concentrations affect an enzyme reaction?
Their relative concentrations set the direction and efficiency of the net reaction. As product builds up the reverse reaction becomes more frequent; removing product keeps the forward reaction going.
How do you express an inhibited rate as a percentage of the control?
Inhibited rate ÷ uninhibited rate × 100. 3.0 ÷ 12.0 × 100 = 25%. For a percentage change, divide by the old value: (new − old) ÷ old × 100.

3.3 Cellular Energy

Why do living systems need a continuous energy input?
Life is highly ordered, and keeping that order costs energy all the time. Energy comes from light (photosynthesis) or food molecules (respiration); a large, lasting deficit leads to loss of order and death.
How do organisms stay ordered without breaking the second law of thermodynamics?
They are open systems: every energy conversion releases heat, and they release heat, CO₂ and H₂O that increase the disorder of the surroundings by more than their own order increases.
What is energy coupling?
Energy-releasing (exergonic) processes drive energy-requiring (endergonic) ones, usually through ATP + H₂O → ADP + Pᵢ. Energy is released by the whole hydrolysis reaction, not by breaking a bond.
How do you decide whether coupling to ATP makes a reaction proceed?
Overall change = energy required (+) + energy released (−), using about −30 kJ mol⁻¹ per ATP; it must be negative. +12 − 30 = −18 kJ mol⁻¹. For +75 kJ mol⁻¹, 3 ATP are needed (whole numbers only).
Why are metabolic pathways sequential?
The product of one step is the reactant of the next, so energy comes out in small, controllable amounts (the total is the same as burning). If a step is blocked, its substrate builds up and later products fall.
Why are glycolysis and ATP synthase evidence of common ancestry?
Glycolysis and ATP synthesis by a membrane ATP synthase are found in Bacteria, Archaea and Eukarya, which points to shared ancestry, not just that all cells need energy.

3.4 Photosynthesis

What is the overall equation for photosynthesis?
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light energy. Ratio 6 CO₂ : 6 H₂O : 1 glucose : 6 O₂.
Where do the light reactions and Calvin cycle happen, and what do they exchange?
Light reactions: thylakoid membranes (grana); in: light, H₂O, ADP + Pᵢ, NADP⁺; out: O₂, ATP, NADPH. Calvin cycle: stroma; in: CO₂, ATP, NADPH; out: carbohydrate, ADP + Pᵢ, NADP⁺.
Where does the O₂ released in photosynthesis come from?
From water, split to replace the electrons that photosystem II loses when light boosts them. It does not come from CO₂.
What is the electron path in the light reactions?
Light boosts electrons in photosystems II and I; electrons pass PSII → electron transport chain → PSI → NADP⁺, forming NADPH.
How is ATP made in the chloroplast?
ETC redox reactions pump H⁺ into the thylakoid space (high H⁺ inside, low in the stroma). H⁺ flows back through ATP synthase, making ATP: chemiosmosis, called photophosphorylation. The ETC pumps; ATP synthase lets protons flow back.
Why does the Calvin cycle stop soon after light is switched off?
The ATP and NADPH from the light reactions run out. The Calvin cycle does not "run in the dark".
How did photosynthesis evolve?
It first evolved in prokaryotes. Cyanobacteria first oxygenated the atmosphere (sharp rise about 2.4 billion years ago), and chloroplasts descend from cyanobacteria.
How do you calculate glucose and O₂ from CO₂ fixed?
moles = mass ÷ molar mass, with a 6 : 1 CO₂ : glucose ratio. 0.12 mol CO₂ → 0.020 mol glucose × 180.16 = 3.60 g, and 0.12 mol O₂ × 32.00 = 3.84 g.

3.5 Cellular Respiration

What are oxidation and reduction in respiration?
Oxidation is loss of electrons; reduction is gain. NAD⁺ → NADH and FAD → FADH₂ carry electrons to the electron transport chain.
Where do the stages of cellular respiration happen?
Glycolysis: cytosol (net 2 ATP, 2 NADH, 2 pyruvate). Pyruvate oxidation and Krebs cycle: matrix (CO₂, NADH, FADH₂). ETC and ATP synthase: inner mitochondrial membrane (plasma membrane in prokaryotes), making most ATP.
How is ATP made by oxidative phosphorylation?
Electron flow along the ETC pumps H⁺ into the intermembrane space. H⁺ flows back through ATP synthase into the matrix, driving ADP + Pᵢ → ATP (chemiosmosis). The ETC does not make ATP directly.
Where is O₂ used in respiration?
As the final electron acceptor at the end of the ETC, forming water. It is not used in the Krebs cycle.
Which mitochondrial compartment has the lower pH?
The intermembrane space: H⁺ is pumped into it, so it has the higher H⁺ concentration and lower pH; the matrix has the higher pH.
What is fermentation and why is it needed?
Without O₂, NADH reduces pyruvate to lactic acid or ethanol + CO₂, regenerating NAD⁺ so glycolysis can continue, giving only 2 ATP per glucose (from glycolysis). Without it NAD⁺ runs out and glycolysis stops.
What happens if H⁺ leaks across the inner membrane (uncoupling)?
H⁺ bypasses ATP synthase, so the energy becomes heat (brown fat). O₂ use rises, ATP production falls and more heat is released.
How many ATP does aerobic respiration make per glucose?
A model value of about 30–32 (older books 36–38), against 2 by fermentation. Use the figure a question gives.

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