how cells obtain energy from food

How Cells Obtain Energy from Food

overview of how cells convert energy in glucose into ATP through glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation

ATP (adenosine triphosphate)

the cell’s primary energy currency; releases energy when its terminal phosphate bond is hydrolyzed to form ADP and Pi

Energetic Coupling

the use of energy released from exergonic reactions to drive endergonic reactions that require energy input

Exergonic Reaction

a reaction that releases free energy and can occur spontaneously

Endergonic Reaction

a reaction that requires an input of energy and is not spontaneous

ATP Cycle

continuous regeneration of ATP from ADP and Pi using energy from cellular respiration and other energy-releasing processes

Cellular Respiration

the process by which cells harvest energy from organic molecules to produce ATP, usually using oxygen as the final electron acceptor

Mitochondrion

double-membrane organelle where pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation occur in eukaryotic cells

Photosynthesis

process by which light energy is converted into chemical energy stored in organic molecules; provides glucose and oxygen used in respiration

Oxidation

loss of electrons from a molecule, atom, or ion

Reduction

gain of electrons by a molecule, atom, or ion

NAD+

electron carrier that accepts high-energy electrons and hydrogen ions to become NADH

NADH

reduced electron carrier that stores high-energy electrons for ATP production during oxidative phosphorylation

NADPH

electron carrier primarily used in anabolic pathways and photosynthesis to provide reducing power

Activated Carrier

energy-storage molecule such as ATP, NADH, or NADPH that transfers energy between metabolic reactions

Glycolysis

ten-step metabolic pathway that breaks one glucose molecule into two pyruvate molecules, producing ATP and NADH

Purpose of Glycolysis

to extract energy from glucose and generate pyruvate, ATP, and NADH

Location of Glycolysis

cytosol of both prokaryotic and eukaryotic cells

Who Performs Glycolysis

virtually all organisms including bacteria, plants, fungi, protists, and animals

Energy Investment Phase of Glycolysis

first stage of glycolysis in which 2 ATP are consumed to phosphorylate and activate glucose

Energy Payoff Phase of Glycolysis

second stage of glycolysis in which ATP and NADH are produced as glucose fragments are oxidized

Phosphofructokinase (PFK)

major regulatory enzyme of glycolysis that catalyzes formation of fructose-1,6-bisphosphate; stimulated by AMP and inhibited by ATP and citrate

Glyceraldehyde-3-Phosphate (G3P)

three-carbon intermediate produced during glycolysis that is oxidized to generate NADH

Dehydrogenase Enzyme

enzyme that removes electrons and hydrogen from a substrate, often transferring them to NAD+

Substrate-Level Phosphorylation

formation of ATP by direct transfer of a phosphate group from a substrate molecule to ADP

Phosphoenolpyruvate (PEP)

high-energy glycolytic intermediate that donates a phosphate group to ADP to form ATP

Net Yield of Glycolysis

2 ATP, 2 NADH, and 2 pyruvate molecules per glucose molecule

Pyruvate

three-carbon end product of glycolysis and key branch point in metabolism

Fermentation

anaerobic process that regenerates NAD+ from NADH, allowing glycolysis to continue in the absence of oxygen

Lactic Acid Fermentation

fermentation pathway that converts pyruvate to lactate; common in muscle cells and some bacteria

Alcoholic Fermentation

fermentation pathway that converts pyruvate to ethanol and carbon dioxide; common in yeast and some plant cells

Pyruvate Oxidation

process that converts pyruvate into acetyl-CoA, producing NADH and CO2 before entry into the citric acid cycle

Location of Pyruvate Oxidation

mitochondrial matrix in eukaryotes

Pyruvate Dehydrogenase Complex

large multi-enzyme complex that catalyzes pyruvate oxidation and links glycolysis to the citric acid cycle

Acetyl-CoA

two-carbon molecule attached to coenzyme A that enters the citric acid cycle and serves as a key metabolic intermediate

Coenzyme A (CoA)

nucleotide-derived coenzyme that carries acetyl groups in metabolic pathways

Citric Acid Cycle (Krebs Cycle/TCA Cycle)

cyclic pathway that oxidizes acetyl-CoA to CO2 while generating NADH, FADH2, and ATP (or GTP)

Location of Citric Acid Cycle

mitochondrial matrix in eukaryotic cells

Purpose of Citric Acid Cycle

complete oxidation of acetyl groups and production of high-energy electron carriers

Citrate

first product of the citric acid cycle formed from acetyl-CoA and oxaloacetate

Oxaloacetate

four-carbon molecule that combines with acetyl-CoA to begin the citric acid cycle and is regenerated at the end

Isocitrate

citric acid cycle intermediate that undergoes oxidation and decarboxylation

Alpha-Ketoglutarate

five-carbon intermediate produced after isocitrate loses carbon dioxide and is oxidized

Succinyl-CoA

high-energy citric acid cycle intermediate that participates in substrate-level phosphorylation

Succinate

citric acid cycle intermediate oxidized to fumarate while reducing FAD to FADH2

Fumarate

four-carbon intermediate hydrated to form malate

Malate

four-carbon intermediate oxidized to regenerate oxaloacetate and produce NADH

Decarboxylation

removal of a carbon atom from a molecule in the form of CO2

FAD

electron carrier that accepts electrons and hydrogen to become FADH2

FADH2

reduced electron carrier that donates electrons to the electron transport chain and yields ATP

Citric Acid Cycle Yield per Glucose

2 ATP (or GTP), 6 NADH, 2 FADH2, and 4 CO2 because the cycle turns twice per glucose

Electron Transport Chain (ETC)

series of protein complexes in the inner mitochondrial membrane that transfer electrons and pump protons to create a proton gradient

Oxidative Phosphorylation

ATP production driven by electron transport and chemiosmosis

Chemiosmosis

use of energy stored in a proton gradient to drive ATP synthesis

Proton Motive Force (PMF)

electrochemical gradient of protons across a membrane that stores potential energy

ATP Synthase

membrane enzyme that uses proton flow down the gradient to synthesize ATP from ADP and Pi

Complex I

first electron transport chain complex that accepts electrons from NADH and pumps protons across the membrane

Complex II

electron transport chain complex that accepts electrons from FADH2 but does not pump protons

Complex III

electron transport chain complex that transfers electrons and pumps protons

Complex IV (Cytochrome c Oxidase)

final ETC complex that transfers electrons to oxygen, forming water and pumping protons

Cytochrome c

small electron carrier protein that transfers electrons between Complex III and Complex IV

Ubiquinone (Q)

lipid-soluble electron carrier that transports electrons between ETC complexes

Oxygen (O2)

final electron acceptor in aerobic respiration; combines with electrons and protons to form water

Water (H2O)

product formed when oxygen accepts electrons and protons at the end of the electron transport chain

ATP Yield from Cellular Respiration

approximately 30–32 ATP molecules per glucose under ideal conditions

ATP Yield from NADH

approximately 2.5 ATP produced per NADH oxidized

ATP Yield from FADH2

approximately 1.5 ATP produced per FADH2 oxidized

Overall Energy Flow in Respiration

glucose → NADH/FADH2 → electron transport chain → proton motive force → ATP

Feedback Regulation of Respiration

ATP and citrate inhibit glycolysis, while AMP stimulates glycolysis, helping match ATP production to cellular demand

AMP (Adenosine Monophosphate)

indicator of low cellular energy that activates phosphofructokinase and increases glycolysis

Citrate Inhibition

feedback mechanism where high citrate levels signal sufficient energy and slow glycolysis

Uncoupling Protein (Thermogenin/UCP1)

protein in brown fat mitochondria that allows protons to bypass ATP synthase, releasing energy as heat

Brown Adipose Tissue

specialized tissue rich in mitochondria that generates heat through non-shivering thermogenesis

Beta Oxidation

metabolic pathway that breaks down fatty acids into acetyl-CoA, NADH, and FADH2

Apoptosis

programmed cell death in which mitochondria play a regulatory role

Additional Functions of Mitochondria

calcium storage, heat production, fatty acid breakdown, apoptosis regulation, heme synthesis, and participation in ammonia detoxification through the urea cycl