Lecture — Chapter 8 | 3-3-26
Overall Equation:
$$C_6H_{12}O_6 + 6O_2 \rightleftharpoons 6CO_2 + 6H_2O + ATP \quad \text{(Cellular Respiration)}$$
| Name of Stage | Location in Cell | What is Needed at Beginning of Stage | What is Produced | Flow of Electrons |
|---|---|---|---|---|
| 1. Glycolysis | Cytoplasm | Glucose (C6) | Pyruvate (2) | Released |
| 2. Pyruvate Oxidation | Matrix of mitochondria | Pyruvate | Acetyl Coenzyme A | Released |
| 3. Citric Acid Cycle (Krebs) | Matrix of mitochondria | Acetyl Co-A | Oxaloacetate, Citric Acid, CO₂ | Released |
| 4. Electron Transport Chain | Inner mitochondrial membrane | Membrane proteins, electrons, oxygen | H₂O | Gained |
| 5. Chemiosmosis | Inner membrane space | H⁺ / protons | ATP | — |
Lecture | 3-5-26
Overall Equation:
$$6CO_2 + 6H_2O + \text{Sunlight} \rightleftharpoons 6O_2 + C_6H_{12}O_6$$
| Product | Stage | Location | Requirements |
|---|---|---|---|
| Oxygen, NADPH | Light-Dependent Reactions | Chloroplast — Thylakoid membrane (chlorophyll) | Sunlight, Water, Chlorophyll |
| Sugar | Carbon Fixation / Light-Independent Reactions / Calvin Cycle | Stroma | CO₂ + RuBP (Rubisco enzyme) |
Chloroplast Structure — two layers:
Photosynthesis — the process that converts solar energy into the chemical energy of a carbohydrate.
Autotrophs — photosynthetic organisms (including plants, algae, and cyanobacteria) that produce their own food.
Heterotrophs — consumers that cannot synthesize carbohydrates themselves and must take in preformed organic molecules; they use organic molecules produced by photosynthesis as building blocks for growth and repair and as a source of chemical energy for cellular work.
Stomata — small openings in a leaf through which carbon dioxide in the air enters.
Chloroplasts — organelles that carry out photosynthesis, into which carbon dioxide and water diffuse after entering a leaf.
Stroma (Gk. stroma, "bed, mattress") — the semifluid interior of a chloroplast, surrounded by a double membrane, which contains an enzyme-rich solution where carbon dioxide is first attached to an organic compound and then reduced to a carbohydrate.
Thylakoids (Gk. thylakos, "sack") — flattened sacs formed by a different membrane system within the stroma, which in some places are stacked to form grana; their spaces are all thought to be connected, forming an inner compartment called the thylakoid space.
Grana (sing., granum) — stacked formations of thylakoids found within the stroma of a chloroplast. Chlorophyll — a pigment contained in the thylakoid membrane that is capable of absorbing the solar energy that drives photosynthesis.
Light reactions — the first stage of photosynthesis, named because they occur only when the sun is out; they take place on thylakoids, where chlorophyll absorbs solar energy and converts it to chemical energy (ATP and NADPH) by energizing electrons that move down an electron transport chain.
Calvin cycle reactions — the second stage of photosynthesis, named after Melvin Calvin (who received a 1961 Nobel Prize in Chemistry for discovering them); these enzymatic reactions take place in the stroma of chloroplasts and reduce carbon dioxide to a carbohydrate, and can occur both day and night.
Absorption spectrum — the range of wavelengths of light that the pigments in chloroplasts are capable of absorbing; it is determined by exposing a purified sample to different wavelengths of light in a spectrophotometer, measuring how much light passes through, and plotting the amount absorbed at each wavelength on a graph.
Carotenoids — accessory pigments in photosynthesis that are shades of yellow and orange, capable of absorbing light in the violet-blue-green range; they become noticeable in the fall when chlorophyll breaks down.
Photosystem — a structure within the thylakoid membrane consisting of a pigment complex (molecules of chlorophyll a, chlorophyll b, and carotenoids) and electron acceptor molecules; the pigment complex serves as an "antenna" for gathering solar energy.
Noncyclic pathway — the usual pathway electrons follow during the light reactions, beginning with photosystem II, in which the pigment complex absorbs solar energy that is passed from pigment to pigment until concentrated in the reaction center, causing electrons to become so energized that they escape and move to nearby electron acceptor molecules.
Electron transport chain (ETC) — a series of carriers in the thylakoid membrane that pass energized electrons from one to the other; as electrons move from one carrier to the next, energy is captured and stored in the form of a hydrogen ion (H⁺) gradient, which then flows through ATP synthase complexes to produce ATP that is used by the Calvin cycle reactions to reduce carbon dioxide to a carbohydrate.
ATP synthase complex — an enzyme that has a channel and a protruding ATP synthase, and joins ADP + ⓟ (phosphate) to produce ATP.
Carbon dioxide fixation — the first step of the Calvin cycle, in which a molecule of carbon dioxide from the atmosphere is attached to RuBP (ribulose-1,5-bisphosphate), a 5-carbon molecule, resulting in one 6-carbon molecule that splits into two 3-carbon molecules.
RuBP carboxylase — the enzyme that speeds the carbon dioxide fixation reaction; it is a protein making up about 20–50% of the protein content of chloroplasts, and is unusually slow, processing only a few molecules of substrate per second compared to thousands per second for a typical enzyme, which is why it must be so abundant to keep the Calvin cycle going.
Cellular respiration — an aerobic pathway in which glucose is completely broken down to CO₂ and H₂O, consisting of four phases: glycolysis, the prep reaction, the citric acid cycle, and the passage of electrons along the electron transport chain.
Aerobic — a pathway that requires oxygen (as in cellular respiration).
Anaerobic — a pathway that occurs in the absence of oxygen (as in glycolysis and fermentation).
Fermentation — an anaerobic process involving glycolysis followed by the reduction of pyruvate by NADH either to lactate (in animals) or to alcohol and CO₂ (in yeast), which frees NAD⁺ to accept more hydrogen atoms from glycolysis.
Glycolysis — a series of anaerobic enzymatic reactions in the cytoplasm that breaks down glucose into two molecules of pyruvate, producing a net gain of two ATP via substrate-level ATP synthesis and two NADH.
Substrate-level ATP synthesis — the method of ATP production during glycolysis that provides a net gain of two ATP.
Mitochondrion — the organelle where the prep reaction, citric acid cycle, and electron transport chain take place when oxygen is available.
Prep reaction — the reaction in the mitochondrion where CO₂ is removed from pyruvate, NAD⁺ is reduced to NADH, and CoA receives the remaining acetyl group; occurs twice per glucose molecule.
Citric acid cycle — a cyclical series of reactions in the mitochondrial matrix where complete oxidation produces two CO₂, three NADH, one FADH₂, and one ATP per turn; must turn twice per glucose molecule.
Electron transport chain — located in the cristae of the mitochondria, a chain of carriers that passes electrons from NADH and FADH₂ until they are received by oxygen, which combines with H⁺ to produce water, while energy is captured for ATP production.
Cytochromes — proteins in the electron transport chain that capture energy for use in ATP production. Chemiosmosis — ATP synthesis that occurs when H⁺ flows down an electrochemical gradient through an ATP synthase complex, capturing energy to form ATP from ADP and ⓟ.