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Unit 3: Cellular Energetics
Enzyme Structure and Function
Enzymes as Catalysts: Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering the activation energy required to initiate the reaction. This allows reactions to proceed faster without the enzyme being consumed in the process.
Active Site and Enzyme-Substrate Complex: The active site is the specific region of the enzyme where the substrate binds to form an enzyme-substrate complex. This complex undergoes a conformational change in the enzyme, known as the induced fit model, which enhances the enzyme's ability to catalyze the reaction.
Allosteric Sites and Feedback Inhibition: Many enzymes contain allosteric sites, where molecules can bind to regulate enzyme activity. Allosteric regulation can either activate or inhibit enzyme function. Feedback inhibition is a common regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme early in the pathway to prevent overproduction of the product.
Enzyme Catalysis and Metabolic Pathways
Catabolic vs. Anabolic Pathways:
Catabolic pathways break down molecules, releasing energy. An example is cellular respiration, where glucose is broken down to release energy.
Anabolic pathways build complex molecules from simpler ones, requiring energy. An example is protein synthesis, where amino acids are joined to form proteins.
Lowering Activation Energy: Enzymes lower the activation energy required for a reaction, which allows reactions to proceed at a faster rate.
Forms of Energy
Types of Energy in Cells:
Kinetic energy is energy of motion, such as in muscle contractions.
Thermal energy is heat energy, often released during biochemical reactions.
Potential energy is stored energy, such as the chemical energy in molecular bonds.
Thermodynamics and Entropy:
The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed or transferred.
The Second Law of Thermodynamics states that energy transformations increase entropy (disorder) in the system, often releasing heat.
Spontaneous vs. Nonspontaneous Processes:
Spontaneous reactions occur without external energy input and move systems toward stability.
Nonspontaneous reactions require an input of energy to proceed.
Enzyme Regulation and Environmental Impacts
Factors Influencing Enzyme Activity: Temperature, pH, and substrate concentration affect enzyme function. Each enzyme works optimally at a specific temperature and pH. Deviations from these conditions can lead to enzyme denaturation, disrupting the enzyme's structure and function.
Inhibitors:
Competitive inhibitors bind to the enzyme's active site, preventing substrate binding.
Noncompetitive inhibitors bind to a different site on the enzyme, causing a change in the enzyme's shape that reduces its ability to bind the substrate.
Cellular Energy and ATP
Types of Cellular Work:
Chemical work: Synthesizing complex molecules like proteins.
Transport work: Moving ions or molecules across membranes.
Mechanical work: Performing physical tasks like muscle contraction.
ATP – The Cell’s Energy Currency: ATP (adenosine triphosphate) provides energy for cellular activities. Its high-energy phosphate bonds release energy when broken. ATP is regenerated from ADP (adenosine diphosphate) in energy-producing reactions.
Photosynthesis
Role of Photosynthesis: Photosynthesis converts light energy into chemical energy in the form of glucose, releasing oxygen as a byproduct. This process occurs in chloroplasts, where chlorophyll captures light energy.
1. Light Reactions (Light-dependent Reactions):
Location: Thylakoid membranes of the chloroplast.
Purpose: Capture light energy and convert it into chemical energy in the form of ATP and NADPH, which are then used in the Calvin Cycle.
Key Steps in Light Reactions:
Photon Absorption: Chlorophyll molecules in the Photosystem II (PSII) absorb light energy (photons), exciting electrons in the chlorophyll.
Water Splitting (Photolysis): The excited electrons are passed through the electron transport chain (ETC). To replace these lost electrons, water molecules are split by the enzyme water-splitting complex, releasing oxygen (O₂) as a byproduct.
Electron Transport Chain (ETC): The excited electrons move through a series of proteins in the thylakoid membrane, losing energy as they travel. This energy is used to pump protons (H⁺) into the thylakoid lumen, creating a proton gradient.
ATP Synthesis (Chemiosmosis): The proton gradient created by the ETC drives ATP synthase to convert ADP and inorganic phosphate (Pi) into ATP as protons flow back into the stroma through the ATP synthase complex.
NADPH Formation: Electrons from the ETC eventually reach Photosystem I (PSI), where they are re-excited by light energy. These high-energy electrons are transferred to NADP⁺, forming NADPH (the reduced form of NADP+).
Products of Light Reactions:
Oxygen (O₂) as a byproduct from the splitting of water.
ATP for energy.
NADPH as a reducing agent for the Calvin Cycle.
2. Calvin Cycle (Light-independent Reactions):
Location: Stroma of the chloroplast.
Purpose: Use ATP and NADPH produced in the light reactions to convert carbon dioxide (CO₂) into organic compounds like glucose.
Key Stages of the Calvin Cycle:
a. Carbon Fixation:
Process: CO₂ from the atmosphere is fixed into an organic molecule. This is facilitated by the enzyme RuBisCO, which attaches CO₂ to a 5-carbon molecule called ribulose bisphosphate (RuBP), producing an unstable 6-carbon compound.
The 6-carbon compound immediately splits into two 3-carbon molecules of 3-phosphoglycerate (3-PGA).
b. Reduction:
ATP and NADPH Usage: The 3-PGA molecules are then converted into G3P (Glyceraldehyde-3-phosphate), a 3-carbon sugar, through a series of reactions.
First, ATP is used to phosphorylate 3-PGA, forming 1,3-bisphosphoglycerate (1,3-BPG).
Next, NADPH donates electrons to reduce 1,3-BPG to G3P.
For every three CO₂ molecules that enter the cycle, six G3P molecules are produced. However, only one G3P molecule exits the cycle to be used in the synthesis of glucose, while the remaining five G3P molecules are recycled to regenerate RuBP.
c. Regeneration of RuBP:
Recycling of G3P: To complete the cycle and ensure its continuity, five molecules of G3P are used to regenerate three molecules of RuBP. This requires ATP.
RuBP Regeneration: The regeneration of RuBP is a multi-step process where G3P molecules are rearranged and combined to form RuBP, which can then accept CO₂ to start the cycle again.
Overall Output of the Calvin Cycle:
G3P: The main product, which can be used to form glucose and other carbohydrates.
RuBP: Regenerated to continue the cycle.
ATP and NADPH: Used and recycled from the light reactions.
Overall Summary of Photosynthesis:
Light Reactions capture sunlight to generate ATP and NADPH, which store chemical energy.
The Calvin Cycle uses the ATP and NADPH from the light reactions to fix carbon dioxide into organic molecules, producing glucose precursors like G3P.
Cellular Respiration
Equation and Overview: Cellular respiration breaks down glucose to release energy:
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP
This process occurs in three main stages.
Stages of Cellular Respiration:
Glycolysis: Glucose is split into two pyruvate molecules in the cytoplasm, generating ATP and NADH.
Krebs Cycle: Acetyl-CoA is broken down in the mitochondrial matrix to produce CO₂, ATP, NADH, and FADH₂.
Electron Transport Chain (ETC) and Chemiosmosis: Electrons from NADH and FADH₂ are passed along the ETC in the inner mitochondrial membrane, creating a proton gradient. ATP Synthase uses this gradient to produce ATP via oxidative phosphorylation.
Anaerobic Respiration and Fermentation
Anaerobic Pathways: In the absence of oxygen, cells can produce ATP through fermentation, which is less efficient than aerobic respiration.
Lactic Acid Fermentation: In muscle cells, pyruvate is converted to lactate, causing muscle fatigue but producing some ATP.
Alcoholic Fermentation: Yeast cells convert pyruvate into ethanol and CO₂, which is used in brewing and baking.
