Cellular Respiration Overview

Overview of Cellular Respiration

• Cellular respiration is a series of catabolic reactions
• These reactions convert the energy in fuel molecules into energy in ATP.
• Sugar molecules like glucose are broken down during cellular respiration
• The breakdown happens in the presence of oxygen to produce carbon dioxide and water.
• Cellular respiration releases energy because the potential energy of the reactants is greater than that of the products.
• ATP is generated in two ways during cellular respiration: substrate-level phosphorylation and oxidative phosphorylation.
• Cellular respiration is an oxidation-reduction reaction.
• Electrons are transferred from one molecule to another in oxidation-reduction reactions
• Oxidation is the loss of electrons
• Reduction is the gain of electrons.
• Electron carriers transfer electrons to an electron transport chain
• This chain harnesses the energy of these electrons to generate ATP.
• Cellular respiration includes four stages: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

Glycolysis

• Glycolysis is the partial oxidation of glucose, resulting in the production of pyruvate, ATP, and reduced electron carriers.
• Glycolysis occurs in the cytoplasm
• Glucose is oxidized to pyruvate in a series of 10 reactions
• Glycolysis consists of preparatory, cleavage, and payoff phases.
• For each molecule of glucose broken down during glycolysis, the net gain is two molecules of ATP and two molecules of NADH.

Pyruvate Oxidation

• Pyruvate is oxidized to acetyl-CoA, connecting glycolysis to the citric acid cycle.
• Pyruvate oxidation transpires in the mitochondrial matrix
• The conversion of pyruvate to acetyl-CoA produces one molecule of NADH and one molecule of carbon dioxide.

The Citric Acid Cycle

• The citric acid cycle results in the complete oxidation of fuel molecules and the generation of ATP and reduced electron carriers.
• The citric acid cycle transpires in the mitochondrial matrix.
• The acetyl group of acetyl-CoA is completely oxidized in the citric acid cycle.
• It is a cycle because the acetyl group of acetyl-CoA combines with oxaloacetate
• A series of reactions regenerates oxaloacetate.
• A complete turn of the citric acid cycle yields one molecule of GTP (converted to ATP), three molecules of NADH, and one molecule of FADH2.
• Citric acid cycle intermediates are starting points for the synthesis of many different organic molecules.

Oxidative Phosphorylation

• Oxidative phosphorylation involves the transfer of electrons along an electron transport chain to oxygen, harnessing energy to pump protons and synthesize ATP.
• NADH and FADH2 donate electrons to the electron transport chain
• The chain is located in the internal mitochondrial membrane.
• Electrons move from one redox couple to the next in the electron transport chain.
• The transfer of electrons is coupled with the movement of protons across the inner mitochondrial membrane into the intermembrane space.
• A proton electrochemical gradient results from the buildup of protons in the intermembrane space
• This stores potential energy.
• The movement of protons back into the mitochondrial matrix through the Fo subunit of ATP synthase is coupled with the formation of ATP
• The reaction is catalyzed by the F₁ subunit of ATP synthase.
• Complete oxidation of one glucose molecule by the four stages nets approximately 32 molecules of ATP.

Anaerobic Metabolism

• Glucose can be broken down in the absence of oxygen by fermentation, producing a modest amount of ATP.
• Pyruvate, the end product of glycolysis, is processed differently in the presence and absence of oxygen.
• Pyruvate enters one of several fermentation pathways in the absence of oxygen.
• Pyruvate is reduced to lactic acid in lactic acid fermentation.
• In ethanol fermentation, pyruvate is converted to acetaldehyde, which is reduced to ethanol.

Metabolic Integration

• Metabolic pathways are integrated, allowing control of the energy level of cells.
• Excess glucose molecules are linked together and stored in polymers called glycogen in animals and starch in plants.
• Other monosaccharides from the digestion of dietary carbohydrates are converted into intermediates of glycolysis.
• Fatty acids contained in triacylglycerols are an important form of energy storage in cells
• The breakdown of fatty acids is called β-oxidation.
• Phosphofructokinase-1 controls a key step in glycolysis
• It has many allosteric activators, including ADP and AMP, and allosteric inhibitors, including ATP and citrate.
• Lactic acid fermentation, aerobic respiration, and β-oxidation generates ATP in muscle cells used to power exercise

Overview of Photosynthesis

• Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates.
• Photosynthesis occurs on land and in the sea.
• It is carried out by eukaryotic organisms, such as plants, algae, and by photosynthetic bacteria.
• Photosynthesis consists of two stages: (1) light capture, in which energy from sunlight is used to produce NADPH and ATP, and (2) carbon fixation, in which NADPH and ATP are used to synthesize carbohydrates using CO₂ as a carbon source.
• Water serves as an electron donor in many photosynthetic organisms: water is oxidized, releasing oxygen gas, and carbon dioxide is reduced, forming carbohydrates.
• Photosynthesis takes place in chloroplasts in eukaryotes

Capturing Sunlight into Chemical Forms

• Energy from sunlight is used to produce NADPH and ATP in photosynthesis
• Chlorophyll absorbs visible light and is associated with proteins in photosystems.
• Photosystems absorb light and transfer energy and electrons.
• Antenna chlorophylls transfer absorbed light energy to the reaction center.
• Reaction center chlorophylls transfer electrons to an electron-acceptor molecule, initiating the photosynthetic electron transport chain.
• The photosynthetic electron transport chain consists of a series of electron transfer or redox reactions, taking place within protein complexes and diffusible compounds.
• Water is the electron donor, and NADP+ is the final electron acceptor.
• Photosystem II pulls electrons from water, resulting in the production of oxygen and protons on the lumen side of the membrane.
• Photosystem I passes electrons to ferredoxin, which then reduces NADP+, producing NADPH.
• The movement of electrons through the electron transport chain is coupled with the transfer of protons from the stroma to the lumen.
• The buildup of protons in the lumen drives the production of ATP by photophosphorylation.
• Cyclic electron transport increases ATP production by redirecting electrons from ferredoxin back into the electron transport chain.

The Calvin Cycle

• The Calvin cycle is a three-phase process that uses carbon dioxide to synthesize carbohydrates.
• The three phases of the Calvin cycle are carboxylation, reduction, and regeneration.
• The first phase is the addition of CO₂ to the 5-carbon sugar RuBP.
• This phase is catalyzed by the enzyme rubisco.
• Rubisco is considered the Earth's most abundant protein.
• The second phase is the input of energy captured by sunlight
• It begins with the donation of a phosphate group by ATP, followed by the transfer of electrons from NADPH to produce 3-carbon triose phosphate molecules.
• Some of these triose phosphates are exported from the chloroplast to the cytosol.
• The third phase is the regeneration of RuBP from five 3-carbon triose phosphates.
• Starch formation provides chloroplasts with a way of storing carbohydrates that does not cause water to enter the cell by osmosis.

Photosynthetic Challenges

• Challenges to the efficiency of photosynthesis include excess light energy and the oxygenase activity of rubisco.
• An imbalance between the rate at which NADPH is produced by the photosynthetic electron transport chain and the rate at which NADPH is used by the Calvin cycle can lead to the formation of reactive oxygen species.
• Protection from excess light energy includes antioxidant molecules that neutralize reactive oxygen species and xanthophyll pigments that dissipate excess light energy as heat.
• Rubisco can catalyze a reaction that combines RuBP with oxygen and with carbon dioxide
• When it reacts with oxygen, energy is lost and CO2 is released.
• Rubisco favors carbon dioxide over oxygen, but the cost of this selectivity is reduced speed.
• The synthesis of carbohydrates through the Calvin cycle results in significant energy losses, due in part to photorespiration.
• The maximum theoretical efficiency of photosynthesis is approximately 4% of total incoming solar energy.

The Evolution of Photosynthesis

• The evolution of photosynthesis impacted life on Earth in a profound way
• The ability to use water as an electron donor evolved in cyanobacteria.
• Cyanobacteria evolved two photosystems either by the transfer of genetic material or by gene duplication and divergence.
• Photosynthesis in eukaryotes likely evolved by endosymbiosis.
• All of the oxygen in Earth's atmosphere results from photosynthesis by organisms containing two photosystems.