General Biology I Unit 6 Review: Metabolism

What's Metabolism All About?

• Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
• Includes catabolism (breaking down of molecules to obtain energy) and anabolism (synthesis of compounds needed by the cells)
• Metabolic reactions transform chemical energy from nutrients into energy that can be used for cellular processes, such as muscle contraction, nerve impulse propagation, and synthesis of proteins, fats, and carbohydrates
• Metabolic pathways are series of enzymatic reactions that convert an initial molecule into a final product through multiple steps
• Metabolism is closely linked to nutrition and the availability of nutrients
  • Nutrition provides the necessary building blocks and energy for metabolic processes
  • Nutrients are essential for the proper functioning of metabolic pathways
• Metabolic rate is the speed at which the body expends energy and burns calories
  • Factors influencing metabolic rate include age, sex, body fat, muscle mass, activity level, and genetics

Key Players: Enzymes and ATP

• Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process
  • They lower the activation energy required for reactions to occur
  • Enzymes are highly specific, typically acting on one particular substrate or group of related substrates
• Enzymes have an active site where the substrate binds, leading to the formation of an enzyme-substrate complex
• Cofactors are non-protein molecules that assist enzymes in their function (metal ions, vitamins)
• Coenzymes are organic molecules that work with enzymes to facilitate reactions (NAD+, FAD, Coenzyme A)
• ATP (adenosine triphosphate) is the primary energy currency of the cell
  • It consists of an adenosine molecule bonded to three phosphate groups
  • Energy is released when the high-energy bonds between phosphate groups are broken (hydrolysis)
• ATP is generated through various metabolic pathways, such as cellular respiration and photosynthesis
• ATP powers numerous cellular processes, including muscle contraction, nerve impulse transmission, and the synthesis of complex molecules

Breaking It Down: Catabolism

• Catabolism is the set of metabolic processes that break down complex molecules into simpler ones
• The main purpose of catabolism is to release energy stored in the bonds of larger molecules, which can then be used to power cellular activities
• Catabolic pathways are typically oxidative and often involve the release of energy in the form of ATP
• Carbohydrate catabolism:
  • Glycolysis breaks down glucose into pyruvate, generating ATP and NADH
  • Pyruvate can then enter the citric acid cycle for further oxidation
• Lipid catabolism:
  • Beta-oxidation breaks down fatty acids into acetyl-CoA, which can enter the citric acid cycle
  • Glycerol from triglycerides can be converted into glyceraldehyde-3-phosphate and enter glycolysis
• Protein catabolism:
  • Proteins are broken down into individual amino acids
  • Amino acids can be used to synthesize new proteins or can be deaminated to enter the citric acid cycle as pyruvate or intermediates

Building It Up: Anabolism

• Anabolism is the set of metabolic processes that construct complex molecules from simpler ones
• Anabolic pathways require an input of energy, typically in the form of ATP, to drive the synthesis of larger molecules
• Anabolic pathways are often reductive and involve the formation of new chemical bonds
• Carbohydrate anabolism:
  • Gluconeogenesis is the synthesis of new glucose molecules from non-carbohydrate precursors (amino acids, lactate, glycerol)
  • Glycogenesis is the formation of glycogen (storage polysaccharide) from glucose monomers
• Lipid anabolism:
  • Fatty acid synthesis builds fatty acids from acetyl-CoA units
  • Triglyceride synthesis combines fatty acids and glycerol to form storage lipids
• Protein anabolism:
  • Amino acids are linked together by peptide bonds to form polypeptides
  • Ribosomes translate mRNA into polypeptides, which then fold into functional proteins
• Anabolism is essential for growth, repair, and maintenance of tissues

Cellular Respiration: The Main Event

• Cellular respiration is the process by which cells break down nutrients to release energy in the form of ATP
• Occurs in three main stages: glycolysis, citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain)
• Glycolysis:
  • Takes place in the cytosol and does not require oxygen
  • Glucose is split into two pyruvate molecules, generating 2 ATP and 2 NADH
• Citric Acid Cycle:
  • Occurs in the mitochondrial matrix
  • Pyruvate is oxidized to form acetyl-CoA, which then enters the cycle
  • Series of redox reactions generate 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule
• Oxidative Phosphorylation:
  • Electrons from NADH and FADH2 are transferred through the electron transport chain in the inner mitochondrial membrane
  • Proton gradient is created across the membrane, driving ATP synthase to produce ATP (chemiosmosis)
  • Oxygen acts as the final electron acceptor, forming water
• Cellular respiration is highly efficient, generating around 30-32 ATP per glucose molecule in aerobic conditions

Photosynthesis: Nature's Energy Factory

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose or other sugars
• Occurs in two stages: light-dependent reactions and light-independent reactions (Calvin cycle)
• Light-dependent reactions:
  • Take place in the thylakoid membranes of chloroplasts
  • Light energy is absorbed by chlorophyll and other photosynthetic pigments
  • Electrons are excited and transferred through photosystems I and II, generating ATP and NADPH
  • Oxygen is released as a byproduct of water splitting
• Light-independent reactions (Calvin cycle):
  • Occur in the stroma of chloroplasts
  • Use the ATP and NADPH generated in the light-dependent reactions to fix carbon dioxide into organic compounds
  • Key enzyme is RuBisCO (ribulose bisphosphate carboxylase/oxygenase)
  • Glucose and other sugars are synthesized using the fixed carbon compounds
• C3, C4, and CAM photosynthesis are different adaptations for carbon fixation in various environments
• Photosynthesis is crucial for maintaining Earth's oxygen levels and providing energy for nearly all life on the planet

Metabolism Gone Wrong: Disorders and Diseases

• Metabolic disorders are genetic or acquired conditions that disrupt normal metabolic processes
• Inborn errors of metabolism are genetic disorders caused by mutations in genes encoding metabolic enzymes or transporters
  • Examples include phenylketonuria (PKU), maple syrup urine disease, and glycogen storage diseases
  • Often result in the accumulation of toxic intermediates or the deficiency of essential products
• Acquired metabolic disorders can be caused by factors such as poor nutrition, toxins, or other underlying diseases
• Diabetes mellitus is a metabolic disorder characterized by high blood sugar levels due to insufficient insulin production (Type 1) or insulin resistance (Type 2)
  • Can lead to complications such as cardiovascular disease, kidney damage, and nerve damage
• Obesity is a condition characterized by excessive body fat accumulation, often resulting from an imbalance between energy intake and expenditure
  • Increases the risk of various metabolic disorders, including Type 2 diabetes, cardiovascular disease, and certain cancers
• Metabolic syndrome is a cluster of conditions (high blood pressure, high blood sugar, abnormal cholesterol levels, excess abdominal fat) that increase the risk of heart disease, stroke, and diabetes

Real-World Applications and Cool Facts

• Understanding metabolism is crucial for developing targeted therapies for metabolic disorders and diseases
  • Example: enzyme replacement therapy for certain inborn errors of metabolism
• Metabolic engineering involves modifying metabolic pathways to produce desired compounds or optimize cellular processes
  • Applications in biofuel production, drug synthesis, and creation of novel materials
• Athletes and fitness enthusiasts often aim to optimize their metabolism for improved performance and body composition
  • Strategies include proper nutrition, regular exercise, and adequate rest and recovery
• Some organisms have unique metabolic adaptations:
  • Deep-sea hydrothermal vent bacteria can derive energy from inorganic compounds (chemosynthesis)
  • Tardigrades can enter a state of cryptobiosis, drastically reducing their metabolism to survive extreme conditions
• The study of metabolism has a long history, with key contributions from scientists such as Hans Krebs (citric acid cycle), Maud Menten (enzyme kinetics), and Melvin Calvin (photosynthesis)
• Metabolomics is an emerging field that involves the comprehensive analysis of all metabolites in a biological system
  • Offers potential for early disease detection, personalized medicine, and understanding the effects of diet and lifestyle on health