Unit 3: Cellular Respiration - Glycolysis, Citric Acid Cycle, and Oxidative Phosphorylation

Practice flashcards covering key vocabulary, enzyme reactions, and pathways across glycolysis, the citric acid cycle, and oxidative phosphorylation.

Metabolites

The building block units that food must be broken down into.

Three stages of harvesting energy from glucose

1. Glycolysis (breaks glucose into two pyruvate), 2. Citric acid cycle (completes the breakdown of glucose), 3. Oxidative phosphorylation (makes most of the ATP).

Mitochondria

Organelle where most of the energy from food molecules is harvested.

Enzymes (in food breakdown)

Proteins required to catalyze each individual reaction in the ordered series of reactions that break down food.

Digestion (Stage 1)

First stage of food breakdown; happens mostly outside the cell and in intracellular lysosomes.

Lysosomes

Intracellular compartments where some digestion (Stage 1) takes place.

Stage 2 of food breakdown

Glycolysis: starts in the cytosol, splits one glucose into two pyruvate, makes ATP and NADH; pyruvate is then converted to acetyl CoA in the mitochondrial matrix.

Other sugars in glycolysis

Sugars besides glucose can feed into the glycolysis pathway after being converted to an intermediate.

Pyruvate transport into mitochondria

Pyruvate must be transported across two membranes (outer and inner mitochondrial) to reach the matrix.

Stage 3 of food breakdown

Occurs entirely in mitochondria; begins with the citric acid cycle in the matrix and ends with oxidative phosphorylation on the inner membrane.

Result of complete oxidation of food

ATP and NADH (energy and electrons for biosynthesis) plus $CO_2$ and $H_2O$ (waste products).

Two ways ATP is produced

1. Substrate-level phosphorylation, 2. Oxidative phosphorylation.

Substrate-level phosphorylation

ATP formed directly by transferring a phosphate during a reaction step; happens at glycolysis steps 7 and 10 and once in the citric acid cycle.

Glycolysis

Ten-step reaction sequence in the cytosol that converts one glucose into two pyruvate, producing ATP and NADH.

Pyruvate

Three-carbon compound; the end product of glycolysis (two per glucose).

Energy use and production in glycolysis

Two ATP used at the start; two NADH and four ATP produced by the end.

Three phases of glycolysis

Energy investment (steps 1-3), cleavage of one 6C molecule into two 3C molecules (steps 4-5), energy generation (steps 6-10).

Energy investment phase

Glycolysis steps 1-3; uses 2 ATP and produces fructose 1,6-bisphosphate.

Cleavage phase

Glycolysis steps 4-5; one 6C molecule is split into two 3C molecules.

Energy generation phase

Glycolysis steps 6-10; produces NADH and ATP and ends with pyruvate.

Glycolysis step 1

Phosphorylation of glucose; uses ATP.

Glycolysis step 2

Isomerization; carbonyl group moves from C1 to C2, so the sugar goes from an aldose to a ketose.

Aldose

Sugar with its carbonyl group on C1.

Ketose

Sugar with its carbonyl group on C2.

Glycolysis step 3

Phosphorylation of fructose 6-phosphate to make fructose 1,6-bisphosphate; uses ATP.

Fructose 1,6-bisphosphate

Product of glycolysis step 3; the end of the energy investment phase.

Glycolysis step 4

Cleavage of the 6C sugar into two 3C molecules.

Glycolysis step 5

Isomerization of dihydroxyacetone phosphate into glyceraldehyde 3-phosphate.

Dihydroxyacetone phosphate

3C molecule from step 4 that is isomerized into glyceraldehyde 3-phosphate in step 5.

Glycolysis step 6

Oxidation of two molecules of glyceraldehyde 3-phosphate; makes the activated carrier NADH.

Glycolysis step 7

Substrate-level phosphorylation to generate ATP (1,3-bisphosphoglycerate to 3-phosphoglycerate).

Phosphoglycerate kinase

Enzyme of glycolysis step 7; makes ATP while converting 1,3-bisphosphoglycerate to 3-phosphoglycerate.

Glycolysis step 8

Shift of a phosphate group by a mutase.

Mutase

Enzyme that shifts a phosphate group to a new position in the same molecule (glycolysis step 8).

Glycolysis step 9

A high-energy enol phosphate linkage is created.

Glycolysis step 10

Substrate-level phosphorylation to generate ATP and release pyruvate.

Steps where glycolysis spends ATP

Steps 1 and 3 (2 ATP total).

Steps where glycolysis forms ATP

Steps 7 and 10 (4 ATP total).

Step where glycolysis makes NADH

Step 6 (2 NADH total).

Net result of glycolysis

One glucose becomes two pyruvate, with a net gain of 2 ATP and 2 NADH.

Energy left after glycolysis

Most of the energy is still in the bonds of pyruvate and NADH; later steps harvest it.

Terminal electron acceptor

Molecule waiting at the end of the electron transport chain to accept electrons; oxygen in aerobic respiration.

NADH (role in respiration)

Activated carrier that shuttles electrons to the electron transport chain, where their energy is harvested to make ATP.

What happens without oxygen

Electrons have nowhere to go, the electron transport chain shuts down, NADH is not converted to $NAD^+$, and ATP production stops.

Fermentation

Glucose oxidation in the absence of a terminal electron acceptor; regenerates $NAD^+$ so glycolysis keeps running and some ATP is still made.

Aerobic respiration

Complete oxidation of glucose in the presence of oxygen; oxygen is the terminal electron acceptor and is reduced to water.

Anaerobic respiration

Complete oxidation of glucose in the absence of oxygen, using acceptors like sulfate, nitrate, sulfur, protons, or ferric ions; used by certain bacteria and archaea.

Respiration in prokaryotes

Electron transport chain complexes assemble in the plasma membrane.

Pyruvate when no $O_2$ is present

Goes to fermentation in the cytosol, making ethanol, lactate, or other products.

Pyruvate when $O_2$ is present

Enters the mitochondrion, becomes acetyl CoA, and goes through the citric acid cycle (aerobic cellular respiration).

Delta G ($\Delta G$)

The free energy change of a reaction.

Energetically favorable reaction

Reaction with negative $\Delta G$ ($< 0$); can occur spontaneously and increases the disorder of the universe.

Energetically unfavorable reaction

Reaction with positive $\Delta G$ ($> 0$); can occur only if coupled to a second, energetically favorable reaction.

Reaction coupling

Using an energetically favorable reaction to drive an energetically unfavorable one.

Glycolytic enzymes (energy coupling)

Couple oxidation to energy storage in activated carriers; favorable reactions drive the unfavorable formation of NADH and ATP.

Phosphate bond hydrolysis energies

Differences in the hydrolysis energies of different phosphate bonds allow ATP to be formed by substrate-level phosphorylation.

Five stages of cellular respiration

1. Glycolysis, 2. Pyruvate oxidation, 3. Citric acid cycle, 4. Electron transport, 5. ATP synthesis.

Pyruvate oxidation

Conversion of pyruvate into acetyl CoA and $CO_2$ in the mitochondrial matrix.

Pyruvate dehydrogenase complex

Enzyme complex that converts pyruvate into acetyl CoA and $CO_2$.

Activated carriers

Molecules that store energy or chemical groups in an easily transferred form (examples: ATP, NADH, $FADH_2$, acetyl CoA).

Acetyl coenzyme A (acetyl CoA)

Activated carrier whose acetyl group is readily transferred to other molecules; participates in protein, carbohydrate, and lipid metabolism.

Other organic molecules and acetyl CoA

Other organic molecules can be converted to acetyl CoA in the mitochondrial matrix.

Fate of amino acids

Proteins are broken into amino acids, which are 1. used to build new proteins or 2. broken down for energy if in excess.

Amino acid catabolism

Free amino acids are converted into intermediates of mainstream catabolism in as few steps as possible.

Products of amino acid catabolism

Pyruvate, acetyl CoA, or a few key citric acid cycle intermediates.

Triacylglycerol

The form in which fats are stored.

Lipases

Enzymes that break fats down into glycerol and fatty acids.

Fatty acyl CoA

Form fatty acids are converted to before they are oxidized.

Beta oxidation

Cycle of four enzymes that oxidizes fatty acyl CoA, removing two carbons per turn until the fatty acid is completely degraded.

Products of one turn of beta oxidation

One $FADH_2$, one NADH, and one acetyl CoA.

Citric acid cycle

8-step cycle in the mitochondrial matrix where acetyl CoA is completely oxidized to $CO_2$.

Source of oxygen in $CO_2$ from the citric acid cycle

Comes from the splitting of water.

One round of the citric acid cycle involves

Entry of two carbons as acetyl CoA, release of two $CO_2$, and regeneration of oxaloacetate.

Citrate

Six-carbon molecule formed when acetyl CoA (2C) joins oxaloacetate (4C).

Oxaloacetate

The original 4C acceptor of the citric acid cycle; regenerated at the end of each turn.

Decarboxylation in the citric acid cycle

Release of $CO_2$ at steps 3 and 4, balancing the two carbons that enter as acetyl CoA.

Oxidation steps (five)

Four in the citric acid cycle itself and one when pyruvate is converted to acetyl CoA; electrons go to activated carriers each time.

Steps where $NAD^+$ picks up electrons

Pyruvate oxidation, and citric acid cycle steps 3, 4, and 8 (making NADH).

Step where FAD picks up electrons

Citric acid cycle step 6 (making $FADH_2$).

Substrate-level phosphorylation in the citric acid cycle

ATP is generated at one point in the cycle, with GTP as an intermediate in animal cells.

$$FADH_2$$

Reduced form of FAD; FAD accepts two hydrogen atoms along with their electrons.

Totals from one glucose after the citric acid cycle

All carbons released as $CO_2$; 4 ATP, 10 NADH, 2 $FADH_2$.

Sources of the 4 ATP per glucose

2 from glycolysis, 2 from two rounds of the citric acid cycle.

Sources of the 10 NADH per glucose

2 from glycolysis, 2 from forming two acetyl CoA, 6 from two rounds of the citric acid cycle.

Yield of one 18C fatty acid tail

9 rounds of beta oxidation give 9 $FADH_2$, 9 NADH, 9 acetyl CoA; the 9 acetyl CoA through the citric acid cycle give 9 ATP, 27 NADH, 9 $FADH_2$.

Biosynthetic pathways

Many begin with glycolysis or the citric acid cycle; their products serve as precursors for many of the cell's macromolecules.

Oxidative phosphorylation

Electron-transport-driven ATP synthesis that completes the catabolism of food molecules and generates the bulk of the cell's ATP.

Regulation of metabolism

Catabolic and anabolic reactions are organized and regulated; glycolysis and the citric acid cycle are only a small fraction of a cell's reactions.

Enzymes that use pyruvate as a substrate

Pyruvate dehydrogenase, lactate dehydrogenase, and alanine transaminase.

Phosphofructokinase

Key glycolysis enzyme; glucose metabolism is controlled through feedback inhibition of it.

Feedback inhibition

Control in which a pathway's product inhibits an earlier enzyme in that pathway.

Gluconeogenesis

Glucose synthesis by a set of enzyme-catalyzed reactions; specific enzymes bypass the essentially irreversible steps of glycolysis.

Why gluconeogenesis matters

Keeps blood glucose steady for organs with a constant need for glucose: brain, red blood cells, kidneys.

Food storage reservoirs

Special reservoirs where cells store food molecules to prepare for periods of need.

Glycogen

Form in which animal cells store glucose.

Adipocytes

Fat cells that store fat droplets.

Food storage in plant cells

Plants store both starch and fats in their chloroplasts.

Stages of ATP generation (ATP each)

Glycolysis (2 ATP), citric acid cycle (2 ATP), oxidative phosphorylation (30 ATP).

Membrane-based mechanism

How cells get most of their energy: establish a proton ($H^+$) gradient across a membrane, then use its stored energy to drive ATP synthesis.

Stage one of the membrane-based mechanism

Establish the proton gradient: proton pumps move $H^+$ across the membrane, powered by electron transfer; protons come from water.