Bioenergetics notes

MDCAT Biology

Bioenergetics explains how organisms obtain, transform, store and use energy. These notes cover ATP, cellular respiration, glycolysis, link reaction, Krebs cycle, electron transport chain, anaerobic respiration and the related stages of photosynthesis included in this topic.

Energy and ATP

Cells require energy for active transport, movement, growth, synthesis of molecules and maintenance of cellular organisation. Most usable cellular energy is transferred through ATP, or adenosine triphosphate.

ATP contains adenine, ribose and three phosphate groups. Energy is released when its terminal phosphate bond is hydrolysed. The reaction is ATP + H2O → ADP + Pi, and approximately 7 kcal of energy is released per mole of ATP in the convention used in FSc biology.

ATP is the immediate energy currency of the cell.
ADP means adenosine diphosphate, while Pi means inorganic phosphate.
ATP hydrolysis produces ADP, Pi and approximately 7 kcal of energy.
ATP is continuously formed from ADP and Pi by phosphorylation.
Substrate-level phosphorylation forms ATP directly by transferring a phosphate group to ADP.
Oxidative phosphorylation forms ATP using energy released through an electron transport chain.

Cellular Respiration: Overview

Cellular respiration is the controlled oxidation of food molecules to release energy. Glucose is commonly used as the respiratory substrate. In aerobic respiration, oxygen is the final electron acceptor and the products are carbon dioxide, water and ATP.

Aerobic respiration occurs in stages. Glycolysis occurs in the cytoplasm. The link reaction, Krebs cycle and electron transport chain occur in mitochondria in eukaryotic cells.

Overall aerobic respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
For every glucose molecule completely respired, 6 carbon dioxide molecules are released.
Glycolysis occurs in the cytoplasm and does not directly require oxygen.
The link reaction occurs in the mitochondrial matrix.
The Krebs cycle occurs in the mitochondrial matrix.
The electron transport chain is located on the inner mitochondrial membrane.
Aerobic respiration produces much more ATP than anaerobic respiration.

Glycolysis

Glycolysis is the first stage of respiration. It is the splitting of one six-carbon glucose molecule into two three-carbon pyruvate molecules. It takes place in the cytoplasm and can occur in the absence of oxygen.

Glycolysis has an energy investment phase and an energy payoff phase. ATP is first used to activate glucose. Later, ATP and reduced NAD are produced. The net yield is obtained by subtracting the ATP used from the ATP formed.

One glucose molecule forms two molecules of pyruvate.
Glycolysis uses 2 ATP in its early steps.
Glycolysis produces 4 ATP in total, giving a net gain of 2 ATP.
Glycolysis produces 2 reduced NAD molecules, often written as NADH or NADH2 in FSc texts.
Glycolysis does not release carbon dioxide.
Glycolysis does not require mitochondria.
Pyruvate enters the mitochondrion when aerobic respiration continues.

Link Reaction

Before entering the Krebs cycle, each pyruvate molecule is converted into acetyl-CoA. This step is called the link reaction because it links glycolysis with the Krebs cycle.

During the link reaction, pyruvate is decarboxylated and oxidised. One carbon atom is removed as carbon dioxide, hydrogen is transferred to NAD, and the remaining two-carbon acetyl group combines with coenzyme A.

Pyruvate is a three-carbon compound.
Pyruvate is decarboxylated into a two-carbon acetyl group and CO2.
The acetyl group combines with coenzyme A to form acetyl-CoA.
For one glucose molecule, two pyruvate molecules undergo the link reaction.
The link reaction produces 2 CO2 per glucose molecule.
The link reaction produces 2 reduced NAD molecules per glucose molecule.
No ATP is formed directly during the link reaction.

Krebs Cycle or TCA Cycle

The Krebs cycle is also called the citric acid cycle or tricarboxylic acid cycle. Acetyl-CoA combines with a four-carbon acceptor molecule to form a six-carbon citrate molecule. Through a series of enzyme-controlled reactions, the original acceptor molecule is regenerated.

The cycle includes oxidation, decarboxylation and substrate-level phosphorylation. Reduced NAD and reduced FAD carry high-energy electrons to the electron transport chain. Succinate is oxidised to fumarate by succinate dehydrogenase, and FAD is reduced to FADH2 in this step.

Krebs cycle is also known as the TCA cycle because citric acid has three carboxyl groups.
The cycle occurs in the mitochondrial matrix.
One turn of the cycle occurs for each acetyl-CoA molecule.
Two turns occur for each glucose molecule.
For each acetyl-CoA, the cycle produces 2 CO2, 3 reduced NAD, 1 reduced FAD and 1 ATP or equivalent GTP.
For each glucose molecule, the cycle produces 4 CO2, 6 reduced NAD, 2 reduced FAD and 2 ATP or equivalent GTP.
Oxidation of succinate produces FADH2.
Malonate is a competitive inhibitor of succinate dehydrogenase because it resembles succinate.

Electron Transport Chain and Chemiosmosis

The reduced coenzymes NADH and FADH2 transfer high-energy electrons to the electron transport chain. The chain consists of electron carriers embedded in the inner mitochondrial membrane. As electrons pass along the carriers, energy is released and used to pump hydrogen ions across the membrane.

This creates a hydrogen ion concentration gradient. Hydrogen ions flow back through ATP synthase. The energy of this movement drives the formation of ATP from ADP and Pi. Oxygen accepts the electrons at the end of the chain and combines with hydrogen ions to form water.

The electron transport chain explains the mechanism of ATP synthesis.
The inner mitochondrial membrane contains the electron carriers and ATP synthase.
NADH and FADH2 donate electrons and hydrogen to the chain.
Oxygen is the final electron acceptor in aerobic respiration.
Water is formed at the end of the electron transport chain.
The proton gradient provides the force for ATP synthesis.
Oxidative phosphorylation is ATP formation linked with electron transport.
A drug that reduces mitochondrial activity in kidney nephrons can reduce ATP production, impairing active glucose reabsorption and causing increased glucose in urine.

Anaerobic Respiration

Anaerobic respiration releases energy without using oxygen as the final electron acceptor. It allows glycolysis to continue by regenerating NAD from reduced NAD. It produces much less ATP than aerobic respiration because the Krebs cycle and mitochondrial electron transport chain do not continue in the usual way.

In yeast and plants, pyruvate is converted into ethanol and carbon dioxide. In animal muscle cells, pyruvate is converted into lactic acid. Anaerobic respiration may occur when oxygen supply is insufficient.

Anaerobic respiration can be performed by yeast, some bacteria and animal muscle cells.
In yeast: glucose → ethanol + carbon dioxide + energy.
In muscle cells: glucose → lactic acid + energy.
The net ATP yield of anaerobic respiration is 2 ATP per glucose molecule.
Anaerobic respiration regenerates NAD so glycolysis can continue.
Ethanol fermentation produces carbon dioxide, while lactic acid fermentation does not.
Anaerobic respiration is less efficient than aerobic respiration.

Photosynthesis and Light Reactions

Photosynthesis converts light energy into chemical energy. It occurs in chloroplasts of green plant cells. Chlorophyll absorbs light energy, which is used to form ATP and reduced NADP, commonly written as NADPH2 in FSc texts.

The light-dependent reactions occur on the thylakoid membranes. Thylakoids are arranged in stacks called grana. Photosystem I and photosystem II are present in the grana, along with electron carriers.

The products of photosynthesis are glucose and oxygen.
Chlorophylls are embedded in thylakoid membranes.
Photosystem I and photosystem II are found in the grana of chloroplasts.
Light energy is converted into chemical energy through the formation of ATP and NADPH2.
Photolysis is the splitting of water using light energy.
Photolysis provides electrons and hydrogen ions and releases oxygen.
The electron carrier plastoquinone, abbreviated Pq, occurs in the photosynthetic electron transport chain.
Carotenoids are yellow to orange-red pigments.
Carotenoids help absorb additional light and protect chlorophyll from damage by intense light.

Calvin Cycle or Light-Independent Reactions

The light-independent reactions occur in the stroma of the chloroplast. They are also called the Calvin cycle. These reactions do not use light directly, but they depend on ATP and NADPH2 made during the light-dependent reactions.

Carbon dioxide is absorbed mainly by mesophyll cells and combines with a five-carbon acceptor. The resulting compounds are reduced to form energy-rich carbohydrates. ATP and NADPH2 are converted into ADP + Pi and NADP+ during this process.

The dark reaction or light-independent reaction takes place in the stroma.
The light-independent phase involves the formation of energy-rich carbohydrates.
The first stable three-carbon product is 3-phosphoglyceric acid, also called 3-PGA.
ATP supplies energy and NADPH2 supplies hydrogen and reducing power.
ATP is converted into ADP + Pi, while NADPH2 is converted into NADP+.
Six molecules of G3P are formed for the net production of one glucose molecule.
Only one of the six G3P molecules leaves the Calvin cycle; the other five help regenerate the carbon dioxide acceptor.
Glucose is formed from carbohydrate products of the Calvin cycle.

Key terms

Bioenergetics: The study of energy transformations in living organisms.
ATP: Adenosine triphosphate, the main immediate energy carrier of cells.
Glycolysis: The cytoplasmic breakdown of one glucose molecule into two pyruvate molecules.
Pyruvate: The three-carbon end product of glycolysis.
Link reaction: The mitochondrial conversion of pyruvate into acetyl-CoA before the Krebs cycle.
Acetyl-CoA: A two-carbon acetyl group attached to coenzyme A that enters the Krebs cycle.
Krebs cycle: A cyclic series of reactions that oxidises acetyl-CoA and produces reduced coenzymes.
TCA cycle: Another name for the Krebs cycle, referring to tricarboxylic acid or citric acid.
Succinate dehydrogenase: The enzyme that oxidises succinate to fumarate and reduces FAD to FADH2.
Malonate: A competitive inhibitor of succinate dehydrogenase.
Electron transport chain: A series of electron carriers that transfers electrons and helps produce ATP.
Oxidative phosphorylation: ATP formation powered by electron transport and a proton gradient.
Anaerobic respiration: Energy release without oxygen as the final electron acceptor.
Chloroplast: The organelle in which photosynthesis occurs in green plant cells.
Thylakoid membrane: The chloroplast membrane containing chlorophyll, photosystems and electron carriers.
Grana: Stacks of thylakoids inside chloroplasts.
Stroma: The fluid region of a chloroplast where the Calvin cycle occurs.
Calvin cycle: The light-independent pathway that uses CO2, ATP and NADPH2 to form carbohydrates.
3-phosphoglyceric acid: The first stable three-carbon product of the Calvin cycle.
Carotenoids: Accessory pigments that absorb light and protect chlorophyll from intense light.

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