5.2 Cellular Respiration: Glycolysis, Krebs Cycle, and Electron Transport Chain
3 min read•august 7, 2024
Cellular respiration is the powerhouse of energy production in cells. It's a complex process that breaks down glucose to create , the energy currency of life. This process involves three main stages: , the , and the .
These stages work together to maximize energy extraction from glucose. Glycolysis happens in the , while the Krebs cycle and electron transport chain occur in . Understanding this process is key to grasping how cells power life.
Glycolysis and Fermentation
Glycolysis Overview
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- Glycolysis first step in cellular respiration where glucose is broken down into two pyruvate molecules
- Occurs in the cytoplasm of the cell
- Does not require oxygen (anaerobic process)
- Produces a net gain of 2 ATP and 2 molecules per glucose molecule
- Consists of two phases:
- Energy investment phase: Uses 2 ATP to phosphorylate glucose
- Energy payoff phase: Produces 4 ATP and 2 NADH, resulting in a net gain of 2 ATP and 2 NADH
Pyruvate and Fermentation
- Pyruvate end product of glycolysis
- Can enter the Krebs cycle for further oxidation if oxygen is present
- Converted to lactate or ethanol through fermentation in the absence of oxygen
- cellular respiration that occurs without oxygen, such as fermentation
- Fermentation process that regenerates NAD+ from NADH in the absence of oxygen
- Allows glycolysis to continue by providing NAD+ for the energy payoff phase
- Two main types of fermentation:
- fermentation: Pyruvate is reduced to lactate (occurs in animal muscle cells during intense exercise)
- Alcoholic fermentation: Pyruvate is decarboxylated to acetaldehyde and then reduced to ethanol (occurs in yeast and some bacteria)
Krebs Cycle and Electron Transport Chain
Krebs Cycle (Citric Acid Cycle)
- Krebs cycle second stage of cellular respiration, occurs in the matrix of the mitochondria
- Acetyl-CoA (produced from pyruvate) enters the Krebs cycle by combining with oxaloacetate to form citrate
- Series of redox reactions that generate high-energy molecules (3 NADH, 1 , and 1 GTP/ATP) per acetyl-CoA
- is released as a byproduct
- Regenerates oxaloacetate to continue the cycle
Electron Transport Chain and Oxidative Phosphorylation
- Electron transport chain (ETC) final stage of cellular respiration, occurs in the
- NADH and FADH2 (produced in glycolysis and Krebs cycle) donate electrons to the ETC
- Electrons are passed through a series of protein complexes, releasing energy used to pump protons (H+) into the intermembrane space
- Creates an electrochemical gradient (proton gradient) across the inner mitochondrial membrane
- production of ATP using the energy from the proton gradient
- Protons flow back into the matrix through ATP synthase, driving the synthesis of ATP
- Process is called chemiosmosis: Movement of ions across a semipermeable membrane down their electrochemical gradient
- ETC and oxidative phosphorylation are highly efficient, producing around 34 ATP per glucose molecule
Cellular Respiration Overview
Aerobic Respiration
- cellular respiration that requires oxygen, includes glycolysis, Krebs cycle, and electron transport chain
- Occurs primarily in the mitochondria, the powerhouses of the cell
- Mitochondria have a double membrane structure, with the inner membrane folded into cristae to increase surface area for the ETC
- Most efficient form of cellular respiration, producing around 38 ATP per glucose molecule
- Glycolysis: 2 ATP
- Krebs cycle: 2 ATP (directly) and high-energy molecules (NADH and FADH2)
- Electron transport chain: Around 34 ATP (from oxidative phosphorylation)
Anaerobic Respiration and Efficiency
- Anaerobic respiration cellular respiration that occurs without oxygen, such as fermentation
- Includes glycolysis followed by either lactic acid or alcoholic fermentation
- Produces only 2 ATP per glucose molecule (from glycolysis)
- Comparison of aerobic and anaerobic respiration:
- Aerobic respiration is much more efficient (38 ATP vs. 2 ATP per glucose)
- Anaerobic respiration is faster and can provide energy in the absence of oxygen (such as during intense exercise)
- Aerobic respiration produces CO2 and H2O as byproducts, while anaerobic respiration produces lactate or ethanol
Key Terms to Review (22)
Aerobic respiration: Aerobic respiration is the process by which cells convert glucose and oxygen into energy, producing carbon dioxide and water as byproducts. This process is essential for the generation of ATP, the energy currency of the cell, and involves multiple stages including glycolysis, the Krebs cycle, and the electron transport chain. It is crucial for organisms that rely on oxygen for their energy needs.
Allosteric Regulation: Allosteric regulation refers to the process by which an enzyme's activity is modulated by the binding of an effector molecule at a site other than the enzyme's active site. This binding induces a conformational change in the enzyme, affecting its ability to catalyze reactions. This regulation is crucial in metabolic pathways as it allows for fine-tuning of enzyme activity in response to cellular conditions and the needs of the organism.
Anaerobic Respiration: Anaerobic respiration is a metabolic process that occurs in the absence of oxygen, allowing cells to convert glucose into energy. This process is crucial for organisms that either lack access to oxygen or cannot utilize it for energy production. Anaerobic respiration generates less energy compared to aerobic respiration and results in byproducts like lactic acid or ethanol, depending on the organism and conditions.
ATP: ATP, or adenosine triphosphate, is the primary energy currency of cells, acting as a crucial molecule that stores and transfers energy for various biochemical processes. It plays a vital role in energy coupling, where the energy released from the breakdown of ATP is used to drive endergonic reactions, essential for cellular functions like metabolism and muscle contraction. The production and utilization of ATP are integral to processes such as glycolysis, the Krebs cycle, and the electron transport chain, making it a central player in cellular respiration and energy metabolism.
Carbon dioxide: Carbon dioxide (CO₂) is a colorless, odorless gas that is produced by the respiration of animals and plants, as well as through the combustion of fossil fuels. It plays a critical role in cellular respiration, where it is generated as a byproduct during the breakdown of glucose, and it is also a key component in photosynthesis, where plants use CO₂ to produce glucose and oxygen. Additionally, carbon dioxide has significant implications for climate change and global warming.
Citrate synthase: Citrate synthase is an enzyme that catalyzes the first step of the citric acid cycle (Krebs cycle) by combining acetyl-CoA and oxaloacetate to form citrate. This reaction is crucial for cellular respiration, linking glycolysis and the Krebs cycle, which are both essential for energy production in cells.
Cytoplasm: Cytoplasm is the jelly-like substance found within a cell, excluding the nucleus, that contains all organelles and cellular components. It plays a crucial role in maintaining cell structure and facilitating metabolic processes by providing a medium for biochemical reactions to occur. This environment is vital for cellular respiration and other cellular activities, making it essential for both prokaryotic and eukaryotic cells.
Electron Transport Chain: The electron transport chain (ETC) is a series of protein complexes and other molecules located in the inner mitochondrial membrane that play a crucial role in cellular respiration by transferring electrons from electron donors to electron acceptors. This process is essential for ATP production as it creates a proton gradient that drives the synthesis of ATP through oxidative phosphorylation. The ETC also connects to key metabolic pathways, enhancing the overall energy yield of cellular processes.
Fadh2: FADH2, or flavin adenine dinucleotide in its reduced form, is a crucial coenzyme that plays an essential role in cellular respiration. It is primarily involved in the Krebs Cycle, where it serves as an electron carrier, helping to transport high-energy electrons to the electron transport chain. FADH2 contributes to ATP production by donating electrons, which ultimately leads to the synthesis of ATP through oxidative phosphorylation.
Feedback inhibition: Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme involved in its production, thereby controlling the pathway's activity. This process helps maintain homeostasis by preventing the overproduction of substances and ensuring that resources are used efficiently. It is a crucial aspect of metabolic regulation that allows cells to adapt to changing conditions and demands for energy and materials.
Glycolysis: Glycolysis is the metabolic process that breaks down glucose into pyruvate, generating small amounts of energy in the form of ATP and NADH. This process occurs in the cytoplasm of the cell and serves as the first step in both aerobic and anaerobic respiration, linking energy production to cellular activities.
Hexokinase: Hexokinase is an enzyme that plays a crucial role in the first step of glycolysis, facilitating the conversion of glucose into glucose-6-phosphate. This reaction is important because it traps glucose within the cell and prepares it for further breakdown during cellular respiration, making hexokinase essential for energy production and metabolism.
Inner mitochondrial membrane: The inner mitochondrial membrane is a highly folded membrane located within the mitochondria that plays a crucial role in cellular respiration by housing the components of the electron transport chain. Its extensive surface area is essential for maximizing ATP production through oxidative phosphorylation. Additionally, this membrane is selectively permeable, allowing only specific molecules to pass through, which helps maintain the mitochondrial environment necessary for energy production.
Krebs Cycle: The Krebs Cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA. This cycle plays a crucial role in cellular respiration, linking glycolysis to the electron transport chain by producing electron carriers that are vital for ATP production. By processing pyruvate derived from glucose, the Krebs Cycle not only helps in energy production but also produces carbon dioxide as a waste product, which is expelled from the body.
Lactic Acid: Lactic acid is a three-carbon organic acid produced during anaerobic respiration when glucose is broken down without enough oxygen. It plays a crucial role in energy production, especially during intense exercise when oxygen levels are low, helping to regenerate NAD+ for glycolysis. Its accumulation in muscles can lead to fatigue and discomfort, but it's also used by the body as a fuel source when converted back into glucose or used directly in energy metabolism.
Mitochondria: Mitochondria are membrane-bound organelles found in eukaryotic cells, often referred to as the 'powerhouses of the cell' because they produce adenosine triphosphate (ATP), the main energy currency of cells. They play a vital role in cellular respiration, converting biochemical energy from nutrients into ATP through various metabolic pathways, including glycolysis, the Krebs cycle, and the electron transport chain. Their unique structure and functions also link them to cell theory and the classification of cells into eukaryotic and prokaryotic types.
NADH: NADH, or nicotinamide adenine dinucleotide (reduced form), is a crucial coenzyme involved in metabolic processes. It plays a significant role in cellular respiration by acting as an electron carrier, transferring electrons from one reaction to another, particularly during glycolysis and the Krebs cycle. This energy-rich molecule is essential for generating ATP, the primary energy currency of the cell, during the electron transport chain.
Oxidation-reduction reactions: Oxidation-reduction reactions, commonly known as redox reactions, are chemical processes where the transfer of electrons occurs between substances. In these reactions, one substance is oxidized by losing electrons, while another is reduced by gaining those electrons, allowing energy to be harnessed for biological functions. This electron transfer is crucial for cellular processes such as energy production and metabolism.
Oxidative Phosphorylation: Oxidative phosphorylation is the final stage of cellular respiration where ATP is generated using the energy derived from the transfer of electrons through the electron transport chain and the subsequent pumping of protons across the inner mitochondrial membrane. This process connects to energy coupling by providing ATP, which powers various cellular activities. It relies heavily on the preceding steps of glycolysis and the Krebs cycle, which produce the electron carriers that feed into this mechanism.
Pyruvate Kinase: Pyruvate kinase is an essential enzyme in the glycolytic pathway that catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate, while transferring a phosphate group to ADP to form ATP. This reaction represents one of the key regulatory steps in glycolysis, making it crucial for cellular energy production. Pyruvate kinase's activity is influenced by various factors, including allosteric effectors and covalent modifications, linking it to broader metabolic processes like gluconeogenesis and energy balance in the cell.
Substrate-level phosphorylation: Substrate-level phosphorylation is a metabolic process that directly generates ATP from ADP and an inorganic phosphate group during specific biochemical reactions. This mechanism occurs in both glycolysis and the Krebs cycle, where high-energy substrates donate a phosphate group to ADP, forming ATP without the involvement of the electron transport chain or chemiosmosis. It contrasts with oxidative phosphorylation, which relies on electron transport and chemiosmosis to produce ATP.
Water: Water is a simple chemical compound (H₂O) essential for all known forms of life. It acts as a solvent, regulates temperature, and facilitates various biological processes. Its unique properties, like high specific heat and cohesive behavior, make it vital for functions such as nutrient transport, cellular respiration, and maintaining homeostasis in organisms.