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Plant Biology - Respiration

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    Study Guide Plant Biology – Respiration 1. Glycolysis 1.1 What Is Glycolysis? Glycolysis literally means “sugar splitting” • Glyco = sugar • Lysis = breaking Glucose + 2NAD⁺ + 2ADP + 2P i → 2Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O In glycolysis, one 6 - carbon glucose molecule is broken down into two 3 - carbon molecules called pyruvate . This happens through a series of enzyme - controlled reactions . As glucose is broken down, its energy is released gradually and stored in: • ATP (the cell’s energy currency) • NADH (a high - energy electron carrier) Note: Pyruvic acid quickly loses an H ⁺ ion to form pyruvate . Because they exist together in cells, textbooks often use the names pyruvic acid and pyruvate interchangeably. 1.2 Phase 1: Preparatory Phase (Energy - Using Steps) This phase uses energy to get glycolysis started. Step 1: Energy Investment Begins • One ATP molecule donates a phosphate group to glucose. • Glucose becomes glucose - 6 - phosphate . • This “energizes” the glucose and traps it inside the cell. Step 2: Second ATP Is Used • The molecule is rearranged. • A second ATP adds another phosphate group.

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    Study Guide • The energized 6 - carbon sugar now splits into two 3 - carbon molecules : o Dihydroxyacetone phosphate (DHAP) o Glyceraldehyde - 3 - phosphate (PGAL) At this point, 2 ATP molecules have been used . Phase 2: Payoff Phase (Energy - Releasing Steps) Now the cell starts earning energy back . Step 3: Making Two PGAL Molecules • DHAP is converted into PGAL . • From here on, two PGAL molecules move through the same steps. Step 4: Oxidation and NADH Formation • PGAL is oxidized (hydrogen atoms and electrons are removed). • NAD ⁺ gains these electrons and becomes NADH . • This stores energy for later use. Step 5: Adding Phosphate Groups • An inorganic phosphate (Pi) is added to PGAL. • The new compound formed is 1,3 - bisphosphoglycerate . • Its phosphate bonds contain high energy. Step 6: First ATP Is Made (Substrate - Level Phosphorylation) • One phosphate group is transferred to ADP , forming ATP . • This process is called substrate - level phosphorylation . • Because there are two PGAL molecules , 2 ATP molecules are made here. So far: • 2 ATP used • 2 ATP made • Energy balance = zero

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    Study Guide Step 7: Rearrangement and Water Removal • The molecule is rearranged. • A water molecule is removed . • This forms a very high - energy compound called phosphoenolpyruvate (PEP) . Step 8: Second ATP Is Made • The final phosphate group is transferred to ADP. • Another ATP is formed from each PEP. • Since there are two molecules, 2 more ATPs are produced. 1.3 Final Products of Glycolysis For each glucose molecule , glycolysis produces: • 2 pyruvate molecules • 4 ATP molecules (gross) • 2 NADH molecules Net Energy Gain • 2 ATP were used • 4 ATP were produced • Net gain = 2 ATP About 80% of glucose’s energy remains in the pyruvate molecules. The NADH molecules later help generate more ATP in the mitochondria (Each NADH can help produce about 3 ATP when re - oxidized). Important Points to Remember • Glycolysis takes place in the cytosol . • Every step is controlled by a specific enzyme . • Oxygen is NOT required . • Glycolysis is an anaerobic process .

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    Study Guide 2. Aerobic Respiration 2.1 Introduction to the Krebs Cycle When oxygen is available, the process of respiration continues after glycolysis. The pyruvate formed during glycolysis does not stop there. Instead, it enters the Krebs cycle , also known as the citric acid cycle . The Krebs cycle is a key part of aerobic respiration . It helps release energy from food in a controlled and step - by - step way. 2.2 Where Does the Krebs Cycle Occur? The Krebs cycle takes place inside the mitochondria of the cell. This is different from glycolysis, which happens in the cytoplasm . Keeping these processes in different parts of the cell helps the cell work more efficiently. 2.3 Role of the Mitochondria Mitochondria are known as the powerhouses of the cell . Each mitochondrion has: • An outer membrane • An inner membrane that is folded many times These folds are called cristae . They increase the surface area, allowing more chemical reactions to happen at the same time. The inner space, called the matrix , contains enzymes and other substances needed for the Krebs cycle. The electron carriers used in the next stage of respiration are located on the cristae.

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    Study Guide 2.4 Preparation of Pyruvate (Link Reaction) Before the Krebs cycle begins, pyruvate must be prepared. • Each glucose molecule forms two pyruvate molecules during glycolysis. • These pyruvate molecules move into the mitochondrial matrix . Inside the matrix: • Each 3 - carbon pyruvate is converted into a 2 - carbon acetyl group • Two molecules of carbon dioxide (CO ₂ ) are released • Two molecules of NADH are formed • Each acetyl group joins with coenzyme A to form acetyl - CoA Now the Krebs cycle can begin. Steps of the Krebs Cycle (Cyclical Reactions) 1. Acetyl - CoA enters the cycle o The 2 - carbon acetyl group combines with a 4 - carbon compound called oxaloacetic acid o This forms a 6 - carbon compound called citric acid 2. Series of enzyme - controlled reactions o Citric acid goes through seven steps o During these steps: • Two carbon atoms are released as CO ₂ • Energy is captured in the form of NADH, FADH ₂ , and ATP 3. Regeneration of oxaloacetic acid o At the end of the cycle, oxaloacetic acid is re - formed o This allows the cycle to start again Two complete turns of the Krebs cycle are needed to fully break down one glucose molecule (which has six carbons).

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