Chapter 9: Cellular Respiration and Fermentation

Merged flashcards from Chapter 9 of Pearson's Campbell Biology, Twelfth Edition.

Cellular respiration

The process by which plant and animal cells break down organic molecules within mitochondria

• Uses O₂ and organic molecules to make ATP with waste CO₂ and H₂O

• Includes aerobic and anaerobic respiration

Photosynthesis

The use of light, CO₂, and H₂O to make organic molecules and O₂

Catabolic pathways

Chains of exergonic reactions that release stored energy by breaking down complex molecules using electron transfers from food molecules

• Only linked to work by ATP as they do not power work

Fermentation

A partial degradation of sugars that occurs without oxygen; is a type of anaerobic respiration

Aerobic respiration

Process that utilizes oxygen and organic molecules to yield ATP

• Represented by the equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

• Energy takes the form of ATP and heat

Electron transfer

The transfer of electrons during chemical reactions to release stored energy in organic molecules for ATP synthesis

Redox reactions (reduction-oxidation reactions)

Chemical reactions that transfer electrons between reactants

• Some electrons are shared via covalent bonds in these

• Oxygen atoms attract electrons and do not share them equally; this still constitutes one of these

Oxidation

The loss of electrons from a substance in a redox reaction

Reduction

The addition of electrons to a substance in a redox reaction

• Refers to the positive charge

Reducing agent

The electron donor in a redox reaction

• Reduces the electron acceptor

• Seen with food molecules in cellular respiration due to their high abundance of hydrogen

Oxidizing agent

The electron acceptor in a redox reaction

• Oxidizes the electron donor

• Seen with oxygen during cellular respiration

Electronegativity

The attraction of electrons toward an atom; higher levels of this attract more electrons

• Electrons lose potential energy when shifting to atoms with higher levels of this, releasing kinetic energy for ATP synthesis

Nicotinamide adenine dinucleotide (NAD+)

A coenzyme that functions as an electron carrier and oxidizing agent during cellular respiration

NADH

The reduced form of NAD+, representing stored energy tapped to synthesize ATP

• Passes electrons to the electron transport chain through a series of redox reactions, releasing a small amount of energy

Electron transport chain

A series of molecules built into the inner membrane of the mitochondria to break the fall of electrons to oxygen in several energy-releasing steps

Oxygen

The final electron acceptor in the electron transport chain that captures electrons and hydrogen nuclei to form H₂O

• Yields energy through the attraction of electrons in redox reactions

Glycolysis

The first step of cellular respiration, breaking down one molecule of glucose into two molecules of pyruvate

Citric acid cycle

The second step of cellular respiration with pyruvate oxidation, completing the breakdown of glucose to CO₂

Oxidative phosphorylation

The closing of cellular respiration and the electron transport chain to facilitate synthesis of 90% of the cell’s ATP

• Powered by redox reactions

Substrate-level phosphorylation

The formation of some ATP in glycolysis and the citric acid cycle after an enzyme transfers a phosphate group directly from a substrate to ADP

Glycolysis

The breakdown of one glucose molecule to two molecules of pyruvate that occurs in the cytoplasm

• Creates two net ATP molecules by substrate-level phosphorylation

  • Done through an energy investment and energy payoff phase

• Does not release any CO₂, occuring whether or not O₂ is present

Energy investment phase

The first phase of glycolysis, splitting glucose into two three-carbon sugar molecules (G3P) using 2 molecules of ATP

Glyceraldehyde 3-phosphate (G3P)

The three-carbon sugar molecule that glucose is initially broken into two of in the energy investment phase using 2 molecules of ATP

Energy payoff phase

The second phase of glycolysis

• Synthesizes 4 molecules of ATP

• Reduces 2 NAD+ to NADH

• Oxidizes the small sugars to create 2 pyruvate and 2 H₂O

Pyruvate

Molecules created as a product of glycolysis in the first step of cellular respiration to be passed into processes for pyruvate oxidation and the citric acid cycle

Pyruvate oxidation

The removal of electrons from a pyruvate molecule before entering the citric acid cycle with most of the energy from glucose remaining

• Happens within a mitochondrion if oxygen is present, or within the cytosol for aerobic prokaryotes

Coenzyme A

The coenzyme that pyruvate is joined to to form acetyl CoA after pyruvate oxidation

Acetyl coenzyme A (Acetyl CoA)

The substance pyruvate is converted to before entering the citric acid cycle

Pyruvate dehydrogenase

An enzyme that breaks down pyruvate through the catalyzation of three reactions

• Oxidation of pyruvate’s carboxyl group, releasing the first CO₂ of cellular respiration

• Reducation of NAD+ to NADH

• Combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA

Citric acid cycle (Krebs cycle)

Cycle that oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH₂ per turn as well as 2 CO₂ as waste

• Runs twice per glucose molecule consumed due to 2 pyruvate being present after glycolysis

• Has eight steps, each catalyzed by a specific enzyme, to join acetyl CoA and oxaloacetate to form citrate and then decompose it back to oxaloacetate for a cycle

• NADH and FADH₂ carry electrons to the electron transport chain

Oxaloacetate

The molecule that acetyl CoA is joined with to form citrate and is eventually decomposed back to after the citric acid cycle

Citrate

The molecule that results when joining acetyl CoA and oxaloacetate before decomposing back into oxaloacetate

NADH and FADH₂

The two electron carriers produced by the citric acid cycle to carry electrons to the electron transport chain

NADH and FADH₂

The two electron carriers produced during glycolysis and the citric acid cycle that account for most of the extracted energy from glucose

• These donate electrons to the electron transport chain, powering ATP synthesis

Inner mitochondrial membrane

The location of the molecules of the electron transport chain in eukaryotic cells, folded into cristae for greater surface area

• Mostly comprised of proteins as part of a multi-protein complex that accepts electrons

Plasma membrane

The location of the electron transport chain in prokaryotic cells

Cytochromes

Proteins with heme groups containing an iron atom

• Serves as one of the carrier molecules in the electron transport chain

Electron transport chain

The chain of molecules that passes on electrons from NADH and FADH₂ through proteins to gradually release free energy towards oxygen molecules

• Serves to make energy to pump H⁺ from the mitochondrial matrix to the intermembrane space to catalyze ATP synthesis

• Can also accept and release H⁺ to maintain the H⁺ gradient and couple reactions to ATP synthesis

ATP synthase

The protein pump that H⁺ moves across after being powered by electrons to move into the intermembrane space

• The movement of H⁺ down the concentration gradient onto this protein’s binding sites in a rotor causes a spin that catalyzes ADP phosphorylation

Chemiosmosis

The use of energy in a H⁺ gradient to drive cellular work

• Seen in oxidative phosphorylation where H⁺ atoms are moved into the intermembrane space by electrons then moved into an ATP synthase rotor, catalyzing phosphorylation

Proton-motive force

An H⁺ gradient with the capacity to do work

32

The approximate number of ATP molecules created as a result of cellular respiration, representing about 34% of the energy in a glucose molecule

• The rest is lost as heat

Aerobic respiration

The most common type of cellular respiration that involves the use of oxygen to pull electrons down the electron transport chain

Anaerobic respiration

Respiration wihtout oxygen that uses an electron transport chain with an electron acceptor other than oxygen

• Sulfate ions may serve the role of acceptor in some organisms, making H₂S instead

Glycolysis

Process that oxidizes glucose to pyruvate without the involvement of O₂ or an electron transport chain

• Produces 2 net ATP by substate-level phosphorylation regardless of O₂ presence

• Most widespread catabolic pathway on Earth that functions in both fermentation and cellular respiration

NAD⁺

The oxidizing agent that accepts electrons during glycolysis

• Regenerated from NADH by transferring electrons to the electron transport chain under aerobic conditions

• Anaerobic conditions require fermentation to regenerate this

Fermentation

An extension of glycolysis that oxidizes NADH by transferring electrons to pyruvate or its derivatives, includes:

• Alcohol fermentation

• Lactic acid fermentation

Differs from cellular respiration as electrons are not transferred to the electron transport chain and does not produce nearly as much ATP

Alcohol fermentation

The conversion of pyruvate to ethanol by:

• Releasing CO₂ from pyruvate

• Producing NAD⁺ and ethanol

Used in brewing, winemaking, and baking

Lactic acid fermentation

The reduction of pyruvate directly by NADH to form lactate and NAD⁺

• Does not release CO₂

• Used to make cheese and yogurt with fungi and bacteria

Lactate

A substance that was thought to only have been produced by human muscle cells with a lack of oxygen

• Actually is produced even under aerobic conditions, thus disqualifying it from being classified as fermentation

Catabolic pathways

Chains of catabolic reactions that funnel electrons from many kinds of organic molecules into cellular respiration

• Seen in glycolysis’ use of many carbohydrates

Deamination

The digestion of proteins used for fuel by breaking them down into their amino acid groups

• Nitrogenous waste is excreted as ammonia (NH₃), urea, or other products

Nitrogenous waste

Produced as proteins are digested to amino acid groups in the forms of ammonia (NH₃), urea, or other products

Glycerol

The substance fats are digested to for glycolysis, broken down in a process known as beta oxidation that produces twice as much ATP as the same mass of carbohydrates

Anabolic pathways

Chains of anabolic reactions to build macromolecules from small molecules in food or from cellular respiration

• Seen in protein synthesis from amino acid

Feedback inhibition

The most common mechanism for metabolic control that dictates the level of energy production based on need

• Higher ATP demands leads to more respiration

• Controlls catabolism by regulating enzymatic activity at strategic points in the pathway