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Metabolic Pathways. Biology for Majors. Catabolic Reactions. Catabolic reactions break a larger molecule into smaller pieces.
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Metabolic Pathways Biology for Majors
Catabolic Reactions Catabolic reactions break a larger molecule into smaller pieces. Energy contained in the bonds of glucose is released in small bursts, and some of it can be captured in the form of adenosine triphosphate (ATP), a small molecule that is used to power reactions in the cell. Much of the energy from glucose is still lost as heat, but enough is captured to keep the metabolism of the cell running.
Redox Reactions Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions), and they play a central role in the metabolism of a cell. In a redox reaction, one of the reacting molecules loses electrons and is said to be oxidized, while another reacting molecule gains electrons (the ones lost by the first molecule) and is said to be reduced. The atom or molecule that donates electrons is called the reducing agent, because its donation of electrons allows another molecule to become reduced. The atom or molecule that accepts the electrons is known as the oxidizing agent, because its acceptance of electrons allows the other molecule to become oxidized.
Redox Reactions Involving Carbon Some redox reactions change the amount of electron density on a particular atom by altering how it shares electrons in covalent bonds. Below, carbon has lost electron density (because oxygen is now hogging its electrons), while oxygen has gained electron density (because it can now hog electrons shared with other elements). It’s thus reasonable to say that carbon was oxidized during this reaction, while oxygen was reduced.
Energy in Redox Reactions In redox reactions, energy is released when an electron loses potential energy as a result of the transfer. Electrons have more potential energy when they are associated with less electronegative atoms (such as C or H), and less potential energy when they are associated with a more electronegative atom (such as O).
Electron Carriers Coenzymes serve as oxidizing agents, accepting a pair of electrons, along with one or more protons, to switch to their reduced forms. NAD+ (left) accepts two electrons and one H+ to become NADH (right). FAD is the other coenzyme in cellular respiration.
ATP ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. The negative charges on the phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these bonds are broken. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced.
ATP in Substrate-level Phosphorylation Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. Below is a diagram of substrate-level phosphorylation.
ATP in Chemiosmosis and Oxidative Phosphorylation 90% of the ATP generated during glucose catabolism is derived from chemiosmosis, which takes place in mitochondria (right) within a eukaryotic cell or the plasma membrane of a prokaryotic cell. This process uses oxygen, and is known as oxidative phosphorylation.
Oxidative Phosphorylation Electrons from glucose are transferred to small molecules known as electron carriers. The electron carriers take the electrons to a group of proteins in the inner membrane of the mitochondrion, called the electron transport chain. As electrons move through the electron transport chain, they go from a higher to a lower energy level and are ultimately passed to oxygen (forming water). Energy released in the electron transport chain is captured as a proton gradient, which powers production of ATP by a membrane protein called ATP synthase. Since this process requires oxygen, it is known as aerobic respiration.
Photosynthesis The energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules. Photosynthesis powers 99% of Earth’s ecosystems.
Photoautotrophs Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds via photosynthesis. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. Heterotrophssuch as animals rely on the sugars produced by autotrophs for their energy needs.
Structures of Photosynthesis: Anatomy of a Leaf In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.
Chloroplast Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain.
Light Energy The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.
Electromagnetic Spectrum Electromagnetic radiation from the sun exists at different wavelengths, each of which has its own characteristic energy.
Pigments and Light Absorption Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments have a narrow range of energy levels that they can absorb, as shown below.
Pigments and Light Absorption II (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Each has (d) a unique absorbance spectrum.
Stages of Photosynthesis Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2.
Light-dependent Reactions The pigments absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions.
Photosystems A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced.
Photosystems I and II In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain.
PSII and PSI The photosystem II (PSII) reaction center and the photosystem I (PSI). Note the electron transport chain.
Light-Independent Reactions After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. This occurs during the light-independent reactions of photosynthesis, also known as the Calvin cycle.
Cellular Respiration: Glycolysis Glycolysis is the first pathway used in the breakdown of glucose to extract energy. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.
Glycolysis Above are the reactants and products of glycolysis.
First Half of Glycolysis The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.
Second Half of Glycolysis The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.
Pyruvate Oxidation In the presence of oxygen, pyruvate (the product of glycolysis) is transformed into an acetyl group attached to a carrier molecule of coenzyme A. In this process a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked up by NAD+, and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.
The Citric Acid Cycle The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. The electrons temporarily stored in molecules of NADH and FADH2 are used to generate ATP in a subsequent pathway. One molecule of either GTP or ATP is produced by substrate-level phosphorylation on each turn of the cycle. There is no comparison of the cyclic pathway with a linear one.
Electron Transport Chain The electron transport chain is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water.
ATP Synthase ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi).
Oxidative Phosphorylation In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP.
Anaerobic Cellular Respiration Without oxygen NADH must be reoxidized to NAD+ for reuse as an electron carrier for the glycolytic pathway to continue. Some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD+ from NADH are collectively referred to as fermentation. In contrast, some living systems like the bacteria at right use an inorganic molecule as a final electron acceptor.
Lactic Acid Fermentation Lactic acid fermentation is common in muscle cells that have run out of oxygen. This is also performed by the bacteria that ferment milk to yogurt.
Alcohol Fermentation Some organisms such as yeast perform alcohol fermentation in low oxygen conditions. This provides the “bubbles” (and ethanol) in alcoholic beverages as well as bread.
Metabolism and the Carbon Cycle Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide. These two processes play an important role in the carbon cycle.
Practice Question Photosynthesis powers 99% of the life on Earth. What powers the other 1%?
Quick Review • What role does movement of electrons play in energy exchanges in cells? • What are the basic components and steps of photosynthesis? • What are the reactants and products of cellular respiration? • Where do these reactions occur in a cell? • What are the basic components and steps of fermentation?
