Glycolysis

What is glycolysis?

The breakdown of glucose into two pyruvate molecules, producing ATP and NADH.

What is the main purpose of glycolysis?

To break down glucose and capture energy as ATP and NADH.

How many steps are in glycolysis?

10 steps.

What are the two phases of glycolysis?

The preparatory phase and the payoff phase.

What happens during the preparatory phase?

Glucose is broken down into two glyceraldehyde 3-phosphate molecules, using ATP.

How many ATP molecules are used during the preparatory phase?

2 ATP.

How many NADH molecules are produced during the preparatory phase?

0 NADH.

What happens during the payoff phase?

Glyceraldehyde 3-phosphate is converted into pyruvate, producing ATP and NADH.

What is the net energy yield of glycolysis from one glucose molecule?

2 pyruvate, 2 ATP, and 2 NADH.

What is the net free energy change of glycolysis?

−85 kJ/mol.

Why is glycolysis essentially irreversible overall?

Its large negative free energy change drives the process forward.

Why is glycolysis considered catabolism?

It breaks down molecules and releases usable energy.

What is true about the nine glycolytic intermediates between glucose and pyruvate?

Each intermediate is phosphorylated.

Why are phosphorylated intermediates important in glycolysis?

They help keep sugars inside the cell and allow the cell to use energy from phosphoryl groups.

Why can't phosphorylated sugars easily leave the cell?

The plasma membrane lacks transporters for phosphorylated sugars.

What types of groups often activate metabolites before breakdown?

High-energy groups, often phosphoryl groups.

How do ATP and NADH help track energy changes?

The amounts produced or used help track energy storage during reactions.

What are the three types of reactions in the preparatory phase?

Two phosphorylations, two isomerizations, and one retro-aldol reaction.

What does an isomerization reaction do?

It rearranges a molecule into a different isomer.

What does the retro-aldol reaction do in the preparatory phase?

It cleaves the bond between carbons 3 and 4, splitting the six-carbon sugar into two three-carbon molecules.

Why is glucose phosphorylated at carbon 6 in the first step?

It adds a charged group that helps trap glucose inside the cell.

Why does isomerization happen before phosphorylation at carbon 1?

It allows the ring to open, which is needed for the later reaction.

Which glycolysis steps generate ATP or NADH in the payoff pathway?

Steps 6, 7, and 10.

How does glucose oxidation help produce ATP?

Energy released from oxidation is captured in ATP and NADH.

Why is glycolysis important for cells without oxygen?

It provides ATP when cells cannot use aerobic respiration.

What carbohydrates besides glucose can feed into glycolysis?

Glycogen, starch, disaccharides, and other hexoses.

What are feeder pathways?

Pathways that convert other carbohydrates into intermediates that enter glycolysis.

Why is it useful for different sugars to enter the same pathway?

It simplifies metabolic regulation and reduces the number of enzymes the cell needs to make.

What is lactose intolerance?

An inability to digest lactose because lactase is absent or decreases with age.

What two monosaccharides make up lactose?

Glucose and galactose.

What can happen when enzymes in carbohydrate feeder pathways are defective?

Medical problems can occur because carbohydrates cannot be properly processed.

What are the two main fates of pyruvate?

Aerobic oxidation to produce large amounts of ATP or anaerobic fermentation to lactate or ethanol.

What determines the fate of pyruvate?

Whether enough oxygen is available.

What is hypoxia?

A condition in which the oxygen supply to tissues is severely limited.

What are the two types of anaerobic fermentation?

Ethanol fermentation in yeast and lactate fermentation in mammals.

What is the main purpose of fermentation?

To regenerate NAD⁺ so glycolysis can continue producing ATP.

What happens during ethanol fermentation?

Pyruvate is converted into ethanol and CO₂, while NAD⁺ is regenerated.

What are the products of ethanol fermentation?

Ethanol, CO₂, and NAD⁺.

What is ethanol fermentation used for?

Making beer, wine, and bread.

What happens during lactate fermentation?

Electrons from NADH are transferred to pyruvate, forming lactate and regenerating NAD⁺.

Why do hard-working muscles use lactate fermentation?

Oxygen cannot be supplied fast enough to meet their energy demands, so glycolysis becomes a major ATP source.

What is the Warburg effect?

Cancer cells carry out glycolysis at a much higher rate than normal cells, even when oxygen is available.

Why do many tumor cells rely heavily on glycolysis?

They may have insufficient oxygen because their capillary network has not developed enough to supply them.

How can glucose uptake help locate tumors?

Tumors often take up glucose at higher rates than normal tissues, helping identify their locations.

What is the active functional part of thiamine pyrophosphate?

The thiazole ring.

In the alcohol dehydrogenase reaction, where does the hydride from NADH attack?

The carbon of the carbonyl group.

What is the main purpose of the pentose phosphate pathway?

To produce NADPH for biosynthesis and ribose sugars for nucleotide synthesis.

What are the two main products of the pentose phosphate pathway?

NADPH and pentose phosphates.

What is NADPH used for in cells?

It provides reducing power for fatty acid, sterol, and other compound synthesis.

What is ribose 5-phosphate used to make?

Nucleotides

What are the two phases of the pentose phosphate pathway?

The oxidative phase and the nonoxidative phase.

What does the oxidative phase produce?

NADPH and ribulose 5-phosphate.

How many steps are in the oxidative phase?

Three steps.

How many steps in the oxidative phase produce NADPH?

Two steps.

What molecule starts the pentose phosphate pathway?

Glucose 6-phosphate.

Which enzyme catalyzes the first step of the pentose phosphate pathway?

Glucose 6-phosphate dehydrogenase (G6PDH).

What controls the rate of the pentose phosphate pathway?

The NADP⁺/NADPH ratio.

Which enzyme catalyzes the major controlling step of the pathway?

Glucose 6-phosphate dehydrogenase (G6PDH).

What does lactonase do in the pentose phosphate pathway?

It hydrolyzes a ring structure to form a linear carbohydrate.

What is the purpose of the nonoxidative phase?

To convert excess pentose phosphates into glycolytic intermediates.

Which glycolytic intermediates can be formed in the nonoxidative phase?

Glucose 6-phosphate, fructose 6-phosphate, and glyceraldehyde 3-phosphate.

Which two enzyme types work in the nonoxidative phase?

Transketolases and transaldolases.

When is the nonoxidative phase especially useful?

When cells need more NADPH than pentose phosphates.

What are reactive oxygen species (ROS)?

Highly reactive oxygen-containing molecules that can damage cells.

What are examples of reactive oxygen species?

Hydrogen peroxide, superoxide, and hydroxyl radicals.

How does superoxide dismutase protect cells?

It converts superoxide radicals into hydrogen peroxide and oxygen.

How does NADPH help protect cells from oxidative damage?

It helps keep glutathione in its reduced form, allowing glutathione to neutralize ROS.

What is glutathione's role in protecting cells?

Reduced glutathione helps neutralize reactive oxygen species.

What is G6PDH deficiency?

A genetic condition involving reduced production or activity of glucose 6-phosphate dehydrogenase.

Why can G6PDH deficiency cause red blood cell damage?

Low NADPH reduces the cell's ability to defend against oxidative stress.

Why can quinolones be dangerous for people with G6PDH deficiency?

They can worsen oxidative stress and increase red blood cell destruction (hemolysis).

What is glycogen?

A polymer of glucose molecules used to store glucose in animals.

Why do animals store glucose as glycogen?

To have a readily available source of glucose for energy, especially for the brain and skeletal muscles.

How do animals store more energy overall, as fat or glycogen?

As fat, which stores about 100 times more energy than glycogen.

Why is glycogen useful compared with fat as an energy source?

Glycogen can provide glucose quickly when the body needs it.

Where is glycogen stored inside cells?

In cytosolic granules called β-granules.

What is glycogenin?

A protein that starts new glycogen chains and helps attach the first few glucose molecules.

What is a sugar nucleotide?

An activated sugar attached to a nucleotide that can donate sugar during the formation of larger carbohydrates.

What is the activated glucose donor used to make glycogen?

UDP-glucose.

What is the role of UDP-glucose in glycogen synthesis?

It donates glucose molecules to the growing glycogen chain.

What enzyme converts glucose 1-phosphate into UDP-glucose?

UDP-glucose pyrophosphorylase.

What makes the formation of UDP-glucose strongly favorable?

Pyrophosphate is produced and then hydrolyzed, releasing energy that drives the reaction forward.

What is glycogen synthase?

The enzyme that adds glucose molecules to lengthen glycogen chains.

To which end of glycogen does glycogen synthase add glucose?

The nonreducing end.

What is the role of the glycogen-branching enzyme?

It creates branches in glycogen by forming α(1→6) glycosidic bonds.

What type of glycosidic bond forms the branch points in glycogen?

α(1→6) glycosidic bonds.

What is the first step in starting a new glycogen chain?

Glucose from UDP-glucose forms a glycosidic bond with a tyrosine residue on glycogenin.

Why does glycogen synthesis require a protein primer?

Glycogen synthase needs an existing short glucose chain to extend.

Which molecule is both an intermediate in glycogen synthesis and glycolysis?

Glucose 6-phosphate.

How many free reducing ends does a glycogen granule have?

One.

What is a glycogen storage disease?

A genetic disease caused by a defect in an enzyme involved in glycogen metabolism.

What causes glycogen storage disease type 0?

A defect in glycogen synthase.

What causes glycogen storage disease type IV?

A defect in the glycogen-branching enzyme.