Skills you’ll gain in this topic:
- Describe the stages of cellular respiration: glycolysis, Krebs cycle, and oxidative phosphorylation.
- Explain how cells convert glucose into ATP, CO2, and water.
- Contrast aerobic respiration with anaerobic processes like fermentation.
- Relate the electron transport chain to ATP production.
- Predict the effects of cellular respiration on energy availability.
Glucose Breakdown and ATP Production
Cellular Respiration is a chemical process with the following equation: C6H12O6 + O2 → H2O + CO2. All organisms, including those capable of photosynthesis, go through the process of cellular respiration. The overall reaction breaks down a carbohydrate, most frequently modeled by glucose, and converts the energy stored in that molecule into the most basic cellular energy, ATP. Respiration is almost the complete opposite of photosynthesis. So if you understood photosynthesis, understanding respiration should be relatively easy.
Cellular Respiration is broken down into three major steps which are dependent on one another: glycolysis* the Krebs cycle, and the electron transport chain. While glycolysis takes place in the cytoplasm of the cell, the Krebs cycle and the electron transport chain take place inside of the mitochondria.
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Glycolysis
Glycolysis is the most evolutionarily conserved process in cellular respiration. The process takes place in all living organisms in almost the exact same way. Fundamentally, glycolysis involves breaking down glucose, which possesses 6 carbons, into two 3-carbon molecules of pyruvate.
In the process, a small amount of energy is released due to the breaking of bonds. This is captured as 2 molecules of ATP. Similarly, the breaking of bonds releases a few electrons that are picked up by electron carriers, NADH. These electrons will be dropped off to the electron transport chain later.
Before pyruvate can continue on into the mitochondria to enter the Krebs cycle, pyruvate oxidation takes place. Oxidation is the loss of electrons. In this process, pyruvate becomes a 2-carbon molecule called acetyl CoA. A molecule of carbon dioxide is released from each pyruvate molecule that is oxidized.
Krebs Cycle
The Krebs Cycle takes place in the mitochondria. In this cycle, similarly to the Calvin Cycle, a number of enzymes process a number of reactions that… you DON’T need to know about! (unless you go to medical school one day… good luck!)
The moral of the story is that a number of highly specific enzymes break down acetyl CoA in reactions that create a number of electrons and a little bit of energy. The process results in the creation of a lot of electron carriers (around 8) such as NADH and FADH2. These electron carriers will allow a lot of ATP production in the electron transport chain. 2 ATP are also produced in the Krebs Cycle.
Electron Transport Chain
The electron transport chain is where the majority of ATP is produced. The chain works in the same way as the electron transport chain in photosynthesis. A concentration gradient is formed, and ATP synthase is responsible for creating ATP.
When hydrogen ions are dropped off by electron carriers to the electron transport chain, the hydrogen ion is pumped across the plasma membrane to form a high concentration gradient of hydrogen ions. These will be used by ATP synthase.
The electron travels through the electron transport chain on a number of electronegative proteins. It eventually ends up binding with oxygen, the final electron acceptor. When oxygen accepts the electron, it forms a bond with hydrogen ions and water is created.
The concentration gradient of hydrogen travels through ATP synthase, in the same way as it does in photosynthesis, the kinetic energy is used to phosphorylate ADP into ATP. This process is called chemiosmosis, as ions are moving down their concentration gradient. This process produces somewhere between 30 and 40 ATP molecules. Don’t worry, you don’t need to know specific numbers! Just know that a TON more ATP is produced through this process than through either glycolysis or the Krebs cycle.
Another important aspect of the electron transport chain is the recycling of electron carriers. This takes place when they drop off their electron and can then be refilled in glycolysis or the Krebs cycle. If these carriers were not emptied, the cycle would not be able to continue.
Fermentation
In organisms without access to oxygen, anaerobic respiration takes place. This happens in a number of bacteria, and in other organisms when oxygen is being used up faster than it can be inhaled (think crazy workout).
Without oxygen, the Krebs cycle and electron transport chain cannot take place, because there is no final electron acceptor. Instead, electron carriers must be recycled elsewhere. This happens through the process of fermentation.
Organisms find other molecules to drop off their electrons. Some examples include creating lactic acid, ethanol, and carbon dioxide. This is how beer and wine are fermented by various bacteria and yeast. In humans, our body produces lactic acid when oxygen is in short supply, such as in a tough workout. This can create sore muscles the next day.
The main takeaway about fermentation is that cells MUST recycle their electron carriers in order to continue to reuse them to produce ATP. They will find another molecule to drop their electrons off on. Secondly, during anaerobic respiration, glycolysis, alone, is producing ATP. This means that ATP production is MUCH lower than in aerobic respiration.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
adenosine triphosphate | The primary energy currency of cells that powers cellular functions. |
ADP | Adenosine diphosphate; a molecule that is phosphorylated to form ATP during oxidative phosphorylation. |
aerobic cellular respiration | The metabolic pathway that uses oxygen as the terminal electron acceptor to generate ATP from biological macromolecules. |
ATP synthase | A membrane-bound enzyme that uses the proton gradient to drive the synthesis of ATP from ADP and inorganic phosphate. |
biological macromolecules | Large organic molecules such as carbohydrates, lipids, and proteins that store chemical energy used in cellular respiration. |
carbon dioxide | A gas released during the Krebs cycle as organic molecules are oxidized. |
cellular respiration | The metabolic process by which cells break down biological macromolecules to release energy and synthesize ATP. |
chemiosmosis | The process by which the flow of protons across a membrane through ATP synthase drives ATP synthesis. |
decoupling | The separation of oxidative phosphorylation from electron transport, resulting in heat generation instead of ATP synthesis. |
electrochemical gradient | The combined effect of the concentration gradient and electrical potential difference across a membrane that influences ion movement. |
electron acceptor | A molecule that receives electrons during a redox reaction; oxygen is the terminal electron acceptor in aerobic respiration. |
electron transport chain | A series of protein complexes in membranes that transfer electrons and establish an electrochemical gradient to generate ATP during photosynthesis and cellular respiration. |
endothermic organisms | Organisms that generate and regulate their own body heat through metabolic processes. |
enzyme-catalyzed reactions | Chemical reactions in cells that are accelerated by enzymes, which act as biological catalysts. |
eukaryotes | Organisms whose cells contain a membrane-bound nucleus and other membrane-bound organelles. |
FAD | A coenzyme that accepts electrons during the Krebs cycle, forming FADH₂. |
FADH₂ | Flavin adenine dinucleotide (reduced form); an electron carrier that delivers electrons to the electron transport chain. |
fermentation | An anaerobic metabolic process that regenerates ATP and NAD+ without using the electron transport chain or oxygen. |
glucose | A six-carbon sugar whose energy is released through cellular respiration to power cellular functions. |
glycolysis | A biochemical pathway in the cytosol that breaks down glucose and releases energy to form ATP, NADH, and pyruvate. |
heat | Thermal energy generated when oxidative phosphorylation is uncoupled from electron transport in cellular respiration. |
inner mitochondrial membrane | The innermost membrane of the mitochondrion that contains the electron transport chain and is the site of ATP synthesis. |
inorganic phosphate | A free phosphate group (Pi) that is added to ADP to form ATP during ATP synthesis. |
intermembrane space | The region between the inner and outer mitochondrial membranes where protons accumulate during the electron transport chain. |
Krebs cycle | A biochemical cycle in the mitochondrial matrix that oxidizes pyruvate, releases CO₂, generates ATP, and transfers electrons via NAD⁺ and FAD. |
lactic acid | An organic molecule produced during fermentation in the absence of oxygen. |
mitochondria | Membrane-bound organelles in eukaryotic cells that are the primary site of aerobic cellular respiration and ATP synthesis. |
mitochondrial matrix | The innermost compartment of the mitochondrion where the Krebs cycle occurs. |
mitochondrion | An organelle where pyruvate is oxidized and ATP is generated through the Krebs cycle and electron transport chain. |
NAD⁺ | A coenzyme that accepts electrons during glycolysis and the Krebs cycle, forming NADH. |
NADH | Nicotinamide adenine dinucleotide (reduced form); an electron carrier that delivers electrons to the electron transport chain. |
oxidation | The process of losing electrons, which occurs when pyruvate and other molecules are broken down in the Krebs cycle. |
oxidation-reduction reactions | Chemical reactions involving the transfer of electrons between molecules, where one molecule is oxidized and another is reduced. |
oxidative phosphorylation | The synthesis of ATP coupled to electron transport in the electron transport chain during aerobic cellular respiration. |
oxygen | An element that is a prevalent component of biological molecules and is found in carbohydrates, lipids, proteins, and nucleic acids. |
plasma membrane | The selectively permeable membrane that surrounds the cell, composed of phospholipids, proteins, and other molecules that regulate what enters and exits the cell. |
prokaryotes | Single-celled organisms without a membrane-bound nucleus, such as bacteria and archaea. |
proton gradient | A difference in proton concentration across a membrane, with higher concentration on one side than the other. |
pyruvate | A three-carbon molecule produced from glycolysis that is transported to the mitochondrion for further oxidation. |