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The coenzyme nicotinamide adenine dinucleotide which is an electron carrier in redox reactions. The reaction involves the transfer of a proton and two electrons.

Oxidative phosphorylation

The coupling of the oxidation of NADH to the production of ATP.


The diffusion of protons across a membrane which drives the synthesis of ATP (in mitochondria and chloroplasts in eukaryotic cells).

ATP synthase

A membrane protein that uses the potential energy of the H+ gradient to drive ATP synthesis. It is a molecular motor composed of two parts: the F₀ unit that is a transmembrane H+ channel and the F₁ unit which is a rotating, six-sub-unit ring.

Cellular respiration

Catabolic pathway in which a reduced molecule, the carbohydrate glucose, is oxidized, often all the way to CO₂.

carbohydrate + 6O₂ → 6CO₂+ 6 H₂O + chemical energy


6CO₂+ 6 H₂O + light energy → carbohydrate + 6O₂


The conversion of the six-carbon monosaccharide glucose into two three-carbon molecules of pyruvate. Involves ten enzyme catalyzed reactions and takes place in the cytosol. Consumes 2 ATP and results in 4 ATP. Final products of glycolysis are:

2 molecules of pyruvate
2 molecules of ATP (net)
2 molecules of NADH

Pyruvate oxidation

Two three-carbon molecules of pyruvate are oxidized to two two-carbon molecules of acetyl CoA and two molecules of CO₂. This process links glycolysis to the citric acid cycle.

Citric acid cycle

Two two-carbon molecules of acetyl CoA are oxidized to four molecules of CO₂.

Acetyl Coenzyme A (acetyl CoA)

Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. Acetyl-CoA is produced during the second step of aerobic cellular respiration, pyruvate decarboxylation, which occurs in the matrix of the mitochondria. Acetyl-CoA then enters the citric acid cycle.

Pyruvate dehydrogenase complex

A group of three enzymes that decarboxylates pyruvate, creating an acetyl group and carbon dioxide. The acetyl group is then attached to coenzyme A to produce acetyl-CoA, a substrate in the Krebs cycle. In the process, NAD+ is reduced to NADH. The pyruvate dehydrogenase complex is the second stage of cellular respiration.


A four-carbon molecule that binds with the two-carbon acetyl unit of acetyl-CoA to form citric acid in the first step of the Krebs cycle.

Respiratory chain

The terminal reactions of cellular respiration, in which electrons are passed from NAD or FAD, through a series of intermediate carriers, to molecular oxygen, with the concomitant production of ATP.

Electron transport

Part of Oxidative Phosphorylation, Electrons from reduced coenzymes (NADH & FADH2) are passed through. The terminal electron acceptor is O2, forming H2O. Energy from electron transfer creates PROTON GRADIENT across cell membrane.


Generation of glucose from non-sugar carbon substrates like pyruvate, lactate, glycerol, and amino acids The vast majority takes place in the liver and, to a smaller extent, in the cortex of kidney. This process occurs during periods of fasting, starvation, or intense exercise and is highly endergonic.

Carbon-fixation reaction

Reduction reactions of photosynthesis in which carbon from carbon dioxide becomes incorporated into organic molecules, leading to the production of carbohydrate. These reactions do not use light directly.

Chlorophyll a

It absorbs most energy from wavelengths of violet-blue and orange-red light.

Chlorophyll b

Its color is yellow, and it primarily absorbs blue light


Absorbs blue and blue-green wavelengths and appear to be a deep yellow.


Accessory pigment that absorbs yellow-green, yellow and orange light, found in red algae

Reaction center

Complex of proteins associated with two special chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, one of the chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.


A cluster of chlorophyll and other pigment molecules in the thylakoid membranes of chloroplasts that harvest light energy for the light reactions of photosynthesis

Photosystem I

One of two light-harvesting units of a chloroplast's thylakoid membrane; it uses the P700 reaction-center chlorophyll. It absorbs light energy at 700 nm and passes an excited electron to NADP+ reducing it to NADPH.

Photosystem II

One of two light-harvesting units of a chloroplast's thylakoid membrane; it uses the P680 reaction-center chlorophyll. It absorbs light energy at 680 nm and produces ATP and oxidizes water molecules.

Cyclic electron transport

In photosynthesis, the cyclic flow of electrons through Photosystem I; ATP is formed by chemiosmosis, but no photolysis of water occurs, and O2 and NADPH are not produced.

Calvin cycle

The second of two major stages in photosynthesis (following the light reactions), involving atmospheric CO2 fixation and reduction of the fixed carbon into carbohydrate.


Ribulose carboxylase, the enzyme that catalyzes the first step of the Calvin cycle (the addition of CO2 to RuBP, or ribulose bisphosphate).

Inorganic phosphate (often abbreviated Pi)

Hydrogen phosphate (HPO₄²-)

Substrate-level phosphorylation

The formation of ATP by directly transferring a phosphate group to ADP from an intermediate substrate in catabolism.

Oxidative phosphorylation

ATP formation in the mitochondrion, associated with flow of electrons through the respiratory chain.


An uncoupling protein found in the mitochondria of brown adipose tissue (BAT). It is used to generate heat by non-shivering thermogenesis. Non-shivering thermogenesis is the primary means of heat generation in hibernating mammals and in human infants.

carbohydrate + 6 O2 → 6 CO2 + 6 H2O + chemical energy

Formula of cellular respiration

6 CO2 + 6 H2O + light energy → 6 O2 + carbohydrate

Formula of photosynthesis

Acetyl Coenzyme A

The starting point for the citric cycle.


Used for the reduction of 3-phosphoglycerate (3PG) in photosynthesis.

In glycolysis, one molecule of glucose containing 6 carbons is converted to two molecules of ________ containing 3 carbons each. This reaction also yields two molecules of _______ and two molecules of _______.

Pyruvate; ATP; NADH

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