Plant Biology - Respiration - Quick Guides | Biology

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Study GuidePlant Biology–Respiration1. Glycolysis1.1What Is Glycolysis?Glycolysis literally means“sugar splitting”•Glyco= sugar•Lysis= breakingGlucose + 2NAD⁺ + 2ADP + 2Pi→ 2Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂OIn glycolysis,one 6-carbon glucose moleculeis broken down intotwo 3-carbon molecules calledpyruvate.This happens through aseries of enzyme-controlled reactions.As glucose is broken down, its energy is releasedgraduallyand stored in:•ATP(the cell’s energy currency)•NADH(a high-energy electron carrier)Note:Pyruvic acid quickly loses an H⁺ion to formpyruvate. Because they exist together in cells,textbooks often use the namespyruvic acidandpyruvateinterchangeably.1.2Phase 1: Preparatory Phase (Energy-Using Steps)This phaseuses energyto get glycolysis started.Step 1: Energy Investment Begins•OneATPmolecule donates a phosphate group to glucose.•Glucose becomesglucose-6-phosphate.•This “energizes” the glucose and traps it inside the cell.Step 2: Second ATP Is Used•The molecule is rearranged.•Asecond ATPadds another phosphate group.

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Study Guide•The energized 6-carbon sugar now splits intotwo 3-carbon molecules:oDihydroxyacetone phosphate (DHAP)oGlyceraldehyde-3-phosphate (PGAL)At this point,2 ATP molecules have been used.Phase 2: Payoff Phase (Energy-Releasing Steps)Now the cell startsearning energy back.Step 3: Making Two PGAL Molecules•DHAP is converted into PGAL.•From here on,two PGAL moleculesmove through the same steps.Step 4: Oxidation and NADH Formation•PGAL isoxidized(hydrogen atoms and electrons are removed).•NAD⁺gains these electronsand becomesNADH.•This stores energy for later use.Step 5: Adding Phosphate Groups•Aninorganic phosphate (Pi)is added to PGAL.•The new compound formed is1,3-bisphosphoglycerate.•Its phosphate bonds contain high energy.Step 6: First ATP Is Made (Substrate-Level Phosphorylation)•One phosphate group is transferred toADP, formingATP.•This process is calledsubstrate-level phosphorylation.•Because there aretwo PGAL molecules,2 ATP moleculesare made here.So far:•2 ATP used•2 ATP made•Energy balance =zero

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Study GuideStep 7: Rearrangement and Water Removal•The molecule is rearranged.•Awater molecule is removed.•This forms a very high-energy compound calledphosphoenolpyruvate (PEP).Step 8: Second ATP Is Made•The final phosphate group is transferred to ADP.•Another ATPis formed from each PEP.•Since there are two molecules,2 more ATPsare produced.1.3Final Products of GlycolysisForeach glucose molecule, glycolysis produces:•2 pyruvate molecules•4 ATP molecules (gross)•2 NADH moleculesNet Energy Gain•2 ATP were used•4 ATP were produced•Net gain = 2 ATPAbout80% of glucose’s energyremains in the pyruvate molecules.TheNADHmolecules later help generate more ATP in the mitochondria(Each NADH can helpproduce about3 ATPwhen re-oxidized).Important Points to Remember•Glycolysis takes place in thecytosol.•Every step is controlled by aspecific enzyme.•Oxygen is NOT required.•Glycolysis is ananaerobic process.

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Study Guide2. Aerobic Respiration2.1Introduction to the Krebs CycleWhen oxygen is available, the process of respiration continues after glycolysis.The pyruvate formed during glycolysis does not stop there. Instead, it enters theKrebs cycle, alsoknown as thecitric acid cycle.The Krebs cycle is a key part ofaerobic respiration. It helps release energy from food in a controlledand step-by-step way.2.2Where Does the Krebs Cycle Occur?The Krebs cycle takes place inside themitochondriaof the cell.This is different from glycolysis, which happens in thecytoplasm.Keeping these processes in different parts of the cell helps the cell work more efficiently.2.3Role of the MitochondriaMitochondria are known as thepowerhouses of the cell.Each mitochondrion has:•An outer membrane•An inner membranethat is folded many timesThese folds are calledcristae.They increase the surface area, allowing more chemical reactions to happen at the same time.The inner space, called thematrix, contains enzymes and other substances needed for the Krebscycle.The electron carriers used in the next stage of respiration are located on the cristae.

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Study Guide2.4Preparation of Pyruvate (Link Reaction)Before the Krebs cycle begins, pyruvate must be prepared.•Each glucose molecule formstwo pyruvate moleculesduring glycolysis.•These pyruvate molecules move into themitochondrial matrix.Inside the matrix:•Each 3-carbon pyruvate is converted into a2-carbon acetyl group•Two molecules of carbon dioxide (CO₂)are released•Two molecules of NADHare formed•Each acetyl group joins withcoenzyme Ato formacetyl-CoANow the Krebs cycle can begin.Steps of the Krebs Cycle (Cyclical Reactions)1.Acetyl-CoA enters the cycleoThe 2-carbon acetyl group combines with a 4-carbon compound calledoxaloaceticacidoThis forms a 6-carbon compound calledcitric acid2.Series of enzyme-controlled reactionsoCitric acid goes throughseven stepsoDuring these steps:•Two carbon atoms are released as CO₂•Energy is captured in the form ofNADH, FADH₂, and ATP3.Regeneration of oxaloacetic acidoAt the end of the cycle, oxaloacetic acid is re-formedoThis allows the cycle to start againTwo complete turns of the Krebs cycle are neededto fully break down one glucose molecule(which has six carbons).

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