Bio93 Quiz 3

What is the equation relating free energy change to enthalpy and entropy?

∆G = ∆H – T∆S

What sign of ∆G indicates a spontaneous (exergonic) reaction?

Negative ∆G

What's the difference between exergonic and endergonic reactions?

Exergonic = net release of free energy, spontaneous. Endergonic = absorbs free energy, nonspontaneous.

What is energy coupling?

Using an exergonic process (like ATP hydrolysis) to drive an endergonic one.

What three parts make up ATP?

Ribose (sugar), adenine (nitrogenous base), and three phosphate groups.

How is ATP "renewed" after hydrolysis?

A phosphate group is added back to ADP, powered by energy from catabolic reactions.

What is activation energy (EA)?

The initial energy needed to start a chemical reaction (break existing bonds).

How do enzymes speed up reactions?

They lower the activation energy (EA) barrier — they do NOT change ∆G.

What is the active site?

The region of the enzyme where the substrate binds.

What's the difference between competitive and noncompetitive inhibitors?

Competitive inhibitors bind the active site, competing with substrate. Noncompetitive inhibitors bind elsewhere, changing enzyme shape and reducing active site effectiveness.

What is feedback inhibition?

The end product of a metabolic pathway shuts down (inhibits) an earlier enzyme in that pathway.

If Glucose → Fructose-1,6-bisphosphate has a net ∆G = –3.4 kcal/mol, is this reaction spontaneous?

Yes — negative ∆G means it's spontaneous (exergonic).

The first step (Glucose → Glucose-6-phosphate) has ∆G = +3.35 kcal/mol. Is this exergonic or endergonic?

Endergonic (positive ∆G, requires energy input, nonspontaneous on its own).

ATP hydrolysis has ∆G = –7.4 kcal/mol. What is the net ∆G when this is coupled to Glucose → Glucose-6-phosphate (∆G = +3.35 kcal/mol)?

–4.05 kcal/mol (+3.35 + (–7.4) = –4.05), making the coupled reaction spontaneous overall.

What is a redox reaction?

A chemical reaction that transfers electrons between reactants (oxidation-reduction).

Define oxidation.

A substance loses electrons (is oxidized).

Define reduction.

A substance gains electrons (is reduced).

What is the reducing agent?

The electron donor.

What is the oxidizing agent?

The electron acceptor.

During cellular respiration, what happens to glucose and O2?

Glucose is oxidized (loses electrons)

O2 is reduced (gains electrons), forming H2O.

The oxidation of glucose to CO2 involves:

A loss of electrons (with their hydrogens) — glucose is oxidized.

What role does NAD+ play in cellular respiration?

It's a coenzyme/electron acceptor (oxidizing agent) — electrons from organic compounds are transferred to it, forming NADH.

What does NADH represent?

Stored energy (from captured electrons) that is later used to help synthesize ATP.

What are the three stages of harvesting energy from glucose?

1) Glycolysis, 2) Citric acid (Krebs) cycle, 3) Oxidative phosphorylation.

Where does glycolysis occur in the cell?

The cytosol (cytoplasm).

Where does the citric acid cycle occur?

The mitochondrial matrix.

Where does oxidative phosphorylation (electron transport chain + chemiosmosis) occur?

The inner mitochondrial membrane.

What enters glycolysis?

One molecule of glucose (a 6-carbon sugar).

What are the products of glycolysis (per glucose)?

2 pyruvate molecules, a net gain of 2 ATP, and 2 NADH.

Why is the ATP yield from glycolysis called "net" 2 ATP?

Because 2 ATP are invested early on and 4 ATP are produced later — 4 produced – 2 invested = net 2 ATP.

How many net ATP would be made for 3 glucose molecules during glycolysis?

6 (2 net ATP per glucose × 3 glucose = 6 ATP).

Does glycolysis require oxygen?

No — glycolysis can occur with or without O2.

What happens to pyruvate before the citric acid cycle can begin?

It is converted to acetyl Coenzyme A (acetyl CoA), linking glycolysis to the citric acid cycle.

What carries out the conversion of pyruvate to acetyl CoA?

A multienzyme complex that catalyzes three reactions.

What byproduct is released when pyruvate is converted to acetyl CoA?

CO2 (one carbon is released as CO2).

What enters the citric acid cycle?

Acetyl CoA (derived from pyruvate).

What does the citric acid cycle complete?

The breakdown of pyruvate-derived fuel all the way to CO2.

What are the products of ONE turn of the citric acid cycle?

1 ATP, 3 NADH, and 1 FADH2 (plus CO2 released as a byproduct).

Since each glucose produces 2 pyruvate (and thus 2 acetyl CoA), how many turns of the citric acid cycle occur per glucose?

2 turns — so totals per glucose are 2 ATP, 6 NADH, and 2 FADH2 from this stage.

Who is the citric acid cycle named after?

Hans Krebs (hence "Krebs cycle").

What gas is released as a byproduct of the citric acid cycle?

CO2.

What enters oxidative phosphorylation?

Electrons carried by NADH and FADH2 (from glycolysis and the citric acid cycle).

What do NADH and FADH2 do at the electron transport chain (ETC)?

They donate electrons to protein complexes in the chain, which then pump protons (H+) across the membrane.

How do NADH and FADH2 drive ATP synthesis?

They donate electrons to complexes that then pump protons across the membrane (creating a proton gradient that powers ATP synthase).

What is the final electron acceptor in the electron transport chain?

O2 — it is reduced to form H2O.

What drives ATP synthesis in oxidative phosphorylation?

Diffusion of H+ (protons) down their gradient through ATP synthase (chemiosmosis).

What are the products of oxidative phosphorylation?

The majority of the cell's ATP (~most ATP synthesis of the whole process) and H2O (from O2 + electrons + H+).

What happens to electrons as they move down the electron transport chain?

They "fall" and drop in free energy, releasing energy used to pump protons.

Why is fermentation needed?

Without O2, the electron transport chain stops working, so glycolysis couples with fermentation to keep regenerating NAD+ so glycolysis can continue producing ATP.

What enters alcohol fermentation?

Pyruvate (from glycolysis).

What are the products of alcohol fermentation?

Ethanol and CO2 (CO2 released first, then ethanol forms).

What is alcohol fermentation used for industrially?

Brewing, winemaking, and baking.

What enters lactic acid fermentation?

Pyruvate.

What is the product of lactic acid fermentation?

Lactate (lactic acid) — no CO2 is released.

When do human muscle cells use lactic acid fermentation?

When O2 is scarce, to keep generating ATP.

What foods are made using lactic acid fermentation?

Cheese and yogurt.

How do proteins feed into cellular respiration?

They're digested into amino acids

the amino groups can feed into glycolysis or the citric acid cycle.

How do fats feed into cellular respiration?

They're digested into glycerol (feeds glycolysis) and fatty acids (broken down by beta oxidation into acetyl CoA).

Why does a gram of fat yield more ATP than a gram of carbohydrate?

Fat is more reduced/energy-dense — an oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate.

How is cellular respiration regulated?

Mainly through feedback inhibition of enzymes at key points

when ATP is high, respiration slows, and when ATP drops, respiration speeds up.

What do chloroplasts split, and what happens to the products?

They split H2O into hydrogen and oxygen

electrons from hydrogen are incorporated into sugar molecules, and oxygen is released as a byproduct.

What pigment gives leaves their green color, and where is it located?

Chlorophyll, located within chloroplasts.

Where in the leaf are most chloroplasts found?

The mesophyll (interior tissue of the leaf)

each mesophyll cell contains 30–40 chloroplasts.

How do CO2 and O2 enter/exit the leaf?

Through microscopic pores called stomata.

What are thylakoids, and where is chlorophyll located within the chloroplast?

Thylakoids are connected sacs inside the chloroplast (stacked into grana)

chlorophyll is located in the thylakoid membranes.

What is the stroma?

The dense fluid interior of the chloroplast (surrounding the thylakoids), where the Calvin cycle occurs.

Why do leaves appear green?

Chlorophyll reflects and transmits green light (absorbs other wavelengths).

What is the main photosynthetic pigment?

Chlorophyll a.

What do accessory pigments do?

Broaden the spectrum of light used for photosynthesis (e.g., chlorophyll b) and protect against excess light (carotenoids).

What is a photosystem made of?

A reaction-center complex surrounded by light-harvesting complexes.

What happens to excited electrons in the reaction center?

They are passed to a primary electron acceptor, which becomes reduced — this is the first step of the light reactions.

What enters the light reactions (linear electron flow)?

Light energy, H2O, and NADP+ (plus ADP + Pi).

What are the products of linear electron flow?

ATP, NADPH, and O2 (as a byproduct).

Which photosystem functions first in linear electron flow, and what wavelength does it absorb best?

Photosystem II (PS II)

absorbs best at 680 nm (reaction center called P680).

What happens when H2O is split at Photosystem II?

Electrons are transferred from H2O's hydrogen atoms to P680+ (reducing it back to P680)

O2 is released as a byproduct.

What drives ATP synthesis in the light reactions?

Energy released as electrons move down the electron transport chain (from PS II to PS I) creates a proton gradient across the thylakoid membrane

diffusion of H+ through ATP synthase drives ATP synthesis.

What is the reaction-center pigment of Photosystem I called, and what happens there?

P700 — light energy excites it, and it loses an electron to a primary electron acceptor.

Where do electrons from Photosystem I ultimately go?

Down an electron transport chain to ferredoxin (Fd), then to NADP+, reducing it to NADPH.

What are the electrons in NADPH used for?

They power the Calvin cycle (reactions that build sugar).

How does chlorophyll harness light energy?

Electrons of pigment molecules are excited by photons and jump from one light-harvesting complex to the next, ultimately reaching the primary electron acceptor.

What enters cyclic electron flow?

Light energy — using only Photosystem I.

What are the products of cyclic electron flow?

ATP only (no NADPH, no O2 released).

Why is cyclic electron flow useful to the cell?

It generates surplus ATP to satisfy the higher ATP demand of the Calvin cycle (which needs more ATP than NADPH).

What enters the Calvin cycle?

CO2, ATP, and NADPH (the products of the light reactions).

What is the product of the Calvin cycle, and where does carbon "leave" the cycle?

Glyceraldehyde 3-phosphate (G3P) — a sugar.

How many turns of the Calvin cycle are needed to make a net gain of 1 G3P, and how many CO2 molecules are fixed?

3 turns of the cycle, fixing 3 molecules of CO2.

What are the three phases of the Calvin cycle?

1) Carbon fixation (catalyzed by rubisco), 2) Reduction, 3) Regeneration of the CO2 acceptor (RuBP).

What enzyme catalyzes carbon fixation?

Rubisco.

Where in the chloroplast does the Calvin cycle take place?

The stroma.

How are cellular respiration and photosynthesis complementary processes?

Photosynthesis generates O2 and organic molecules (sugar), which are used as inputs for cellular respiration

cellular respiration releases CO2 and H2O, which are inputs for photosynthesis.