Unit 6: Bioenergetics — Interactive Notes
Definition: Bioenergetics is the study of how energy flows through living systems — exploring how cells store and expend energy. Photosynthesis acts as an energy-capturing process, respiration as an energy-releasing process.
★ ATP was discovered in 1929 by Karl Lohmann. In 1941, Nobel laureate Fritz Lipmann proposed that ATP is the main energy-transfer molecule in the cell.
ATP-ADP Cycle: The covalent bonds between phosphates are high-energy bonds. Breaking one releases ~7.3 kcal/mole: ATP + H₂O → ADP + Pi + energy. In some cases, ADP is further broken to AMP + Pi. Cells make ATP during energy-releasing processes and break it during energy-consuming processes — ATP transfers energy between metabolic reactions.
Definition: Photosynthesis involves the use of light energy absorbed and converted into chemical energy by photosynthetic pigments — energy-poor CO₂ is reduced to energy-rich carbohydrates.
Equation: 6CO₂ + 12H₂O + Light → C₆H₁₂O₆ + 6O₂ + 6H₂O
★ Compensation Point: At dawn/dusk, low light intensity means photosynthesis and respiration rates are equal for a short time — no net gas exchange between leaves and atmosphere. At noon, photosynthesis rate exceeds respiration, resulting in net O₂ release and CO₂ uptake.
Role of Light: Provides the energy to drive reactions transforming simple molecules into complex organic compounds — absorbed by chlorophyll, converted to chemical energy stored as C-H bond energy.
Role of CO₂: Acts as the carbon source for sugars — reduced using ATP and NADPH. Enters leaves through stomata. ★ ~10% of photosynthesis is terrestrial; aquatic organisms use dissolved CO₂/bicarbonates/carbonates, land organisms use atmospheric CO₂.
Water is the source of hydrogen for CO₂ reduction. Oxygen released comes from water.
Van Neil's contribution: In the 1930s, Van Neil hypothesized that plants split water as a hydrogen source, releasing oxygen as a by-product (based on photosynthetic bacteria studies) — confirmed by scientists in the 1940s.
Using isotopic tracer (¹⁸O), scientists prepared heavy-oxygen water (H₂¹⁸O) and heavy-oxygen CO₂ (C¹⁸O₂), giving each to separate plant groups. Group 1 (H₂¹⁸O + normal CO₂) produced ¹⁸O₂; Group 2 (normal H₂O + C¹⁸O₂) produced normal ¹⁶O₂ — proving water, not CO₂, is the source of released oxygen.
Photosynthetic pigments are present in thylakoid membranes and capture light energy. Light behaves as photons; when a pigment absorbs a photon, its electrons move to a higher energy level (excited state). Short-wavelength (blue) photons have higher energy than long-wavelength (red) photons.
Chlorophyll: a lipid molecule. Chlorophyll a, b, c, d are found in plants/algae; others (bacteriochlorophylls) in photosynthetic bacteria.
Head: porphyrin ring (4 pyrrole rings held by central Mg atom) — absorbs light. Chlorophyll-a has CH₃ at the 2nd pyrrole ring; chlorophyll-b has CHO at the same spot.
Tail: long hydrocarbon chain, anchors the molecule in the thylakoid membrane.
Light absorption: Chlorophyll mainly absorbs violet-blue and orange-red wavelengths; green is least absorbed (transmitted/reflected — giving plants their green colour).
Accessory pigments: all pigments other than chlorophyll-a that gather light — chlorophyll-b, carotenoids (in plants), phycobilins (red algae/cyanobacteria). Energy transfer order: Carotenoids → Chlorophyll b → Chlorophyll a.
Action spectrum: shows the effectiveness of different light wavelengths in photosynthesis — measured by illuminating a plant with different colours and measuring O₂ emitted. First made by German biologist T.W. Engelmann in 1883, using Spirogyra — maximum photosynthesis (and O₂) occurred with blue and red light.
Absorption Spectrum: shows wavelengths absorbed by a pigment — chlorophylls absorb maximum blue light (430nm) and red light (670nm). The action spectrum of photosynthesis coincides with the absorption spectrum of photosynthetic pigments.
| Feature | Chlorophyll-a | Chlorophyll-b |
|---|---|---|
| Absorbance | 430 nm | 455 nm |
| Colour | Blue-Green | Yellow-Green |
| Role | Primary pigment | Accessory pigment |
Definition: For efficient absorption and utilization of solar energy, photosynthetic pigments are organized into clusters called photosystems, embedded in thylakoid membranes.
Components: Each photosystem has a light-gathering antenna complex (many pigment molecules capturing light, passing excitation energy as high-energy electrons) and a reaction centre (one or more chlorophyll-a molecules passing high-energy electrons to a primary electron acceptor, which passes them to the electron transport chain).
Types: PS-I — has P700 chlorophyll-a, absorbs best at 700nm. PS-II — has P680 chlorophyll-a, absorbs best at 680nm (named in order of discovery).
When water molecules split, they are oxidized (lose electrons/H⁺, yield oxygen); meanwhile CO₂ is reduced to sugar (gains electrons/H⁺). Oxidation and reduction go hand in hand.
6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O (reduction of CO₂; oxidation of H₂O)
It is a complex metabolic pathway: light-dependent reactions occur on thylakoid membranes of grana; light-independent reactions occur in the stroma.
Key events: (1) absorption of light energy, (2) excitation of electrons, (3) formation of ATP and NADPH. The formation of ATP is called photophosphorylation — non-cyclic or cyclic.
The zigzag electron path through PS-II, PS-I and electron transport chains is the Z-scheme.
Under certain conditions, photoexcited electrons of PS-I cycle back from the primary electron acceptor to P700 chlorophyll via the electron transport chain — using only PS-I. No NADPH produced, no O₂ released, but ATP is generated. Occurs when the Calvin cycle slows and NADPH accumulates.
Chemiosmosis is the mechanism by which thylakoid membranes couple redox reactions (of the electron transport chain) with ATP synthesis.
The electron transport chains in mitochondria and chloroplasts generate ATP by this same mechanism.
Light-independent reactions occur in the stroma, using carbon (CO₂), energy (ATP), and hydrogen (NADPH) to build sugar molecules. Also called dark reactions since they can occur with or without light, as long as ATP/NADPH are available. Discovered by Melvin Calvin (Nobel Prize 1961).
Phase I — Carbon Fixation: Rubisco combines 3 CO₂ with 3 ribulose bisphosphate (RuBP) to form 6 molecules of 3-phosphoglyceric acid (3-PGA).
Phase II — Reduction: 1,3-biphosphoglyceric acid is reduced to G3P using NADPH (phosphate groups detached). 6 G3P are produced; 1 leaves the cycle to make glucose.
Phase III — Regeneration: 5 G3P molecules are converted into 3 RuP, then phosphorylated (using 3 ATP) back into 3 RuBP to continue the cycle.
Definition: Cellular respiration is the universal process to get usable energy from breaking down complex carbon-containing compounds.
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Glucose has many C-H bonds (stored energy); CO₂ and H₂O have none. Glucose is the universal respiratory fuel.
Types: 1. Anaerobic respiration (fermentation, no oxygen) 2. Aerobic respiration (with oxygen). Both start with glycolysis (glucose → 2 pyruvic acid).
Glucose is not completely oxidized — yields only 2 ATP per glucose (~2% of its energy, equivalent to 14.6 kcal).
a. Alcoholic Fermentation (bacteria, yeast): pyruvic acid → alcohol (C₂H₅OH) + CO₂.
b. Lactic Acid Fermentation (muscle cells, some bacteria): pyruvic acid → lactic acid (C₃H₆O₃), no oxygen needed. Occurs during extreme physical activity when oxygen can't be delivered fast enough.
★ When life evolved, free O₂ was unavailable — only anaerobic respiration was possible. With the evolution of photosynthesis, O₂ accumulated, enabling aerobic respiration.
Mechanism: Complete glucose breakdown occurs only in aerobic respiration, releasing all C-H bond energy.
4 Stages: 1. Glycolysis (cytosol, no O₂ needed) 2. Pyruvic acid oxidation 3. Krebs cycle 4. Electron transport chain + chemiosmosis (all within mitochondria, O₂ essential).
Definition: Glycolysis is the breakdown of glucose into two molecules of pyruvic acid — occurs in both aerobic and anaerobic respiration, in the cytosol (no oxygen needed). Enzymes, ATP, and NAD⁺ are essential.
Glucose → glucose 6-phosphate (uses 1 ATP) → fructose 6-phosphate (isomerization) → fructose 1,6-bisphosphate (uses 1 ATP) → splits into 2 molecules of G3P (via DHAP).
Each G3P → 1,3-BPGA (NAD⁺→NADH) → 3-PGA (produces 1 ATP) → 2-PGA → PEP (dehydration) → Pyruvic acid (produces 1 ATP). Net result: 2 ATP + 2 NADH per glucose (4 ATP produced − 2 ATP invested).
Pyruvic acid cannot directly enter the Krebs cycle:
Worked out by Sir Hans Krebs — also called the citric acid cycle (after the 6-carbon citric acid formed first). All steps occur in mitochondria.
Acetyl-CoA + oxaloacetic acid (4C) → citric acid (6C) → oxidative decarboxylation → α-ketoglutaric acid (5C, releases CO₂, NAD⁺→NADH) → succinyl-CoA (4C, releases CO₂, NAD⁺→NADH) → succinic acid (produces 1 ATP) → fumaric acid (FAD→FADH₂) → malic acid (water added) → oxidized back to oxaloacetic acid (NAD⁺→NADH), regenerating the cycle.
Per turn: 3 NADH, 1 FADH₂, 1 ATP (substrate-level), 2 CO₂ released.
Electrons are transferred from NADH and FADH₂ to a series of carriers, and finally to oxygen, forming water. This transfer results in oxidation/reduction reactions along the chain.
Pathway: NADH oxidized → coenzyme Q → cytochrome b → cytochrome c → cytochrome a complex → oxygen (forms H₂O with H⁺).
Chemiosmosis: couples these redox reactions with ATP synthesis, built in the inner mitochondrial membrane.
As redox reactions occur along the electron transport chain, released energy actively transports H⁺ ions from the matrix to the inter-membrane space, creating a gradient. H⁺ ions diffuse back through ATP synthase, driving ATP synthesis.
Oxidation of 1 NADH produces 3 ATP; oxidation of 1 FADH₂ produces 2 ATP. Two hydrogen ions and two electrons are taken up by an oxygen atom to form water.
Cells generate ATP by two ways: chemiosmotic phosphorylation and substrate-level phosphorylation. In substrate-level phosphorylation, an enzyme directly transfers a phosphate group from an organic substrate to ADP — simpler, no membrane/ETC involved. E.g., in the last step of glycolysis: PEP → pyruvic acid, transferring phosphate to ADP → ATP. Accounts for only a small percentage of total ATP generated.
| Stage | Products | ATP yield |
|---|---|---|
| Glycolysis | 2 ATP (net) + 2 NADH | 2 + 4 (2 ATP per NADH crossing membrane) = 6 |
| Pyruvic acid oxidation | 2 NADH | 6 |
| Krebs cycle | 2 ATP + 6 NADH + 2 FADH2 | 2 + 18 + 4 = 24 |
| Total net yield | 36 ATP |
Each NADH from glycolysis produces 2 ATP instead of 3 (costs 1 ATP to shuttle across the mitochondrial membrane). Aerobic oxidation nets 36 ATP — 18 times more efficient than anaerobic (2 ATP).
Proteins: digested to amino acids; some deaminated and converted to pyruvic acid, acetyl CoA, or Krebs cycle organic acids.
Lipids: excellent fuel (many C-H bonds); hydrolysed into glycerol (→ glyceraldehyde 3-phosphate, enters glycolysis) and fatty acids (→ acetyl CoA, enters Krebs cycle).
Definition: Respiratory activity in green cells in the presence of light, releasing CO₂. Needs oxygen, produces CO₂ and H₂O like aerobic respiration, but does NOT produce ATP.
Mechanism: When CO₂ is low and O₂ is relatively high in leaf cells, rubisco adds O₂ to RuBP instead of CO₂, breaking it into phosphoglycerate + phosphoglycolate (2-C). Phosphoglycolate → glycolate (moves to peroxisome) → glyoxylate (using O₂, produces toxic H₂O₂) → glycine (moves to mitochondrion) → serine (2 glycine molecules) → back to peroxisome (glycerate) → chloroplast (phosphoglycerate, re-enters Calvin cycle).
Disadvantages: C-3 plants lose 25-50% of fixed carbon, reducing yields. Rate increases with temperature (rubisco's oxidative activity rises) — severe in tropical climates above 28°C.
Plants like corn, sugarcane, sorghum run C-4 photosynthesis alongside the Calvin cycle. Mesophyll cells have fewer air spaces; Calvin cycle enzymes are concentrated in bundle-sheath cells (impermeable to CO₂).
CO₂ + PEP (3C) → oxaloacetic acid (4C) — hence "C-4". Oxaloacetic acid → malic acid (NADH) → transported to bundle-sheath cells → broken to pyruvic acid + CO₂. Concentrated CO₂ runs the Calvin cycle instead of photorespiration. Pyruvic acid returns to mesophyll, regenerating PEP (using ATP).
Crassulacean Acid Metabolism (CAM) — performed by succulents like cacti, pineapples in hot climates. Stomata open at night, closed during the day (preventing water loss and CO₂ escape) — reducing photorespiration via high CO₂ concentration. CO₂ for sugar production comes from organic acids made the night before. Like C-4 plants, CAM plants use both C-3 and C-4 pathways, but separated by time (night vs day) rather than by cell type.
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