Cellular Processes and Metabolism Vocabulary

Course Logistics, Quizzes, and Exam Schedule

• Quiz Format and Protocol:

  • Quizzes begin with an individual quiz component.

  • Once the individual quiz is submitted, students must not take out notes or start looking up answers. Study materials must remain put away while waiting for others to finish.

  • After all individual quizzes are turned in, students are organized into pairs (or groups of three in the case of an odd student count) to complete a group quiz.

  • The group quiz contains the exact same questions as the individual quiz.

  • Students may converse and work together to figure out the answers on the group quiz, but notes and outside resources remain prohibited.

  • Grading Policy: If a student scores $50\%$ or higher on their individual quiz, their final quiz score utilizes a beneficial calculation taking the average of the individual and group quiz scores if that average is higher than the individual score alone.

• Course Schedule and Timeline:

  • Exam 1: Scheduled in two weeks, covering Chapters 1 through 5.

  • Current Week Schedule:

    • Today: Chapter 4 (Cellular Processes).

    • Tomorrow (Tuesday): Laboratory exercise.

    • Wednesday: Chapter 5 lecture.

  • Following Week Schedule:

    • Tuesday: Laboratory exercise.

    • Wednesday: Exam over Chapters 1 through 5.

Laboratory Exercises and Food Label Project

• Laboratory Focus:

  • Students will experimentally determine whether specific chemical reactions are exothermic or endothermic (exergonic vs. endergonic).

• Food Label Analysis Assignment:

  • Students will work in pairs.

  • Assignment Requirement: Each individual must bring in one food item or food label (e.g., candy, healthy food option, or unhealthy food option). Backup food labels collected over previous years will be available if a student forgets to bring one.

  • Activity Objectives:

    • Analyze where calories originate in the food item.

    • Identify the specific vitamins and minerals present on the food label.

    • Conduct research to determine the physiological function and necessity of each identified vitamin and mineral in the human body.

Foundations of Nutrition, Metabolism, and Reaction Types

• Nutritional Categories and Physiological Uses:

  • Nutrients: Supply raw energy, calories, and cellular building blocks.

  • Vitamins: Essential organic compounds used predominantly as precursors to synthesize coenzymes.

  • Proteins: Polypeptide chains composed of amino acid monomers. Ingested dietary proteins are hydrolyzed into amino acids, which are then reassembled to construct endogenous cellular proteins.

  • Carbohydrates: Primary nutritional source utilized for cellular energy production.

  • Minerals: Essential inorganic elements found on the periodic table (e.g., sodium, potassium) required for basic physiological processes.

• Metabolism Definitions:

  • Metabolism: The sum total of all chemical reactions taking place within the body or inside individual cells.

  • Catabolism: The breakdown phase of metabolism involving reactions that cleave complex molecules into simpler sub-components (e.g., digestion of food).

  • Anabolism: The building phase of metabolism involving synthetic reactions that assemble complex molecules from simpler precursors (e.g., muscle tissue construction via anabolic steroids).

• Key Chemical Reaction Types:

  • Dehydration Reactions: Anabolic processes that assemble larger molecules by removing a molecule of water ($H_2O$). Examples include joining monosaccharides into disaccharides or polysaccharides, and linking amino acids into polypeptide chains.

  • Hydrolysis Reactions: Catabolic processes that break down complex molecules into smaller units through the addition of water ($H_2O$). In this term, "hydro" means water and "lysis" means to split or break. Hydrolysis is the primary mechanism used by the digestive system to break down proteins, starches, and fats.

  • Redox Reactions (Oxidation-Reduction): Reactions involving the gain, loss, or transfer of electrons ($e^-$) between molecules. Electrons carry chemical energy, and their movement is facilitated by coenzymes.

Enzymes, Substrates, and Coenzymes

• Enzymatic Functions:

  • Definition: Biological catalysts—predominantly proteins—that accelerate cellular chemical reactions.

  • Activation Energy: Enzymes speed up reactions by lowering the required activation energy—the initial energy burst or "spark" needed to start a chemical reaction (analogous to lighting a match to start a campfire).

  • Biological Necessity: Without enzymes, cellular chemical reactions that take $10\,\text{minutes}$ to occur would result in cellular death within $9\,\text{minutes}$.

• Substrates and Nomenclature:

  • Substrates: The specific molecules, atoms, or compounds upon which an enzyme acts (whether breaking them apart or joining them together).

  • Naming Conventions: Enzyme names are typically derived from their specific substrate and end with the suffix "-ase".

  • Lactose and Lactase Example: Lactose is a dairy disaccharide composed of galactose and glucose. The specific enzyme produced in the digestive tract that breaks down lactose is named lactase.

• Coenzymes:

  • Definition: Helper molecules (either inorganic ions or larger organic molecules) required by enzymes to perform catalytic functions.

  • Vitamin Connection: Many coenzymes are synthesized directly from dietary vitamins, particularly B vitamins.

  • Key Coenzyme Examples:

    • $\text{NAD}^+$ (Nicotinamide Adenine Dinucleotide)

    • $\text{FAD}$ (Flavin Adenine Dinucleotide)

    • $\text{Coenzyme A}$ ($\text{CoA}$)

Cellular Respiration Overview and Energy Currency

• Adenosine Triphosphate ($\text{ATP}$):

  • The universal refined energy currency ("gasoline") of the cell.

  • Unrefined dietary energy sources (analogous to crude oil) include carbohydrates, lipids/fats, and proteins, which are refined by cellular pathways into $\text{ATP}$.

• Respiration Pathways:

  • Aerobic Pathways: Processes that strictly require molecular oxygen ($O_2$) to support long-term energy production.

  • Anaerobic Pathways: Processes that operate in the absence of molecular oxygen ($O_2$) to supply rapid, short-term energy bursts.

• Cellular Respiration Process:

  • The process of combining sugar (monosaccharide glucose, $C_6H_{12}O_6$) with oxygen ($O_2$) to produce $\text{ATP}$, releasing carbon dioxide ($CO_2$) and water ($H_2O$) as metabolic byproducts.

  • Coupling with Pulmonary Respiration: Inhalation supplies the molecular oxygen ($O_2$) required by mitochondrial pathways, while exhalation removes the metabolic waste carbon dioxide ($CO_2$) generated during cellular oxidation reactions.

Glycolysis

• Location: Cytoplasm.

• Oxygen Requirement: Anaerobic (functions without $O_2$).

• Definition: The initial step of cellular respiration in which a 6-carbon glucose molecule ($C_6H_{12}O_6$) is cleaved into two 3-carbon molecules.

• Reaction Steps: A 10-step biochemical sequence utilizing roughly 10 distinct enzymes.

• Summary Equation Inputs:

  • $1\,\text{Glucose}$ ($C_6H_{12}O_6$

  • $2\,\text{NAD}^+$

  • Initial energy investment: $2\,\text{ATP}$

• Summary Equation Outputs:

  • $2\,\text{Pyruvate}$ (or pyruvic acid; 3-carbon molecules)

  • $2\,\text{NADH}$

  • $2\,\text{ATP}$ net yield ($4\,\text{ATP}$ synthesized minus $2\,\text{ATP}$ invested)

Pyruvate Pathways: Reduction vs. Oxidation

• Anaerobic Pathway: Pyruvate Reduction (Lactic Acid Fermentation)

  • Condition: Triggered when cellular oxygen ($O_2$) is lacking.

  • Location: Cytoplasm.

  • Reaction:         $\text{Pyruvate} + \text{NADH} \rightarrow \text{Lactate} + \text{NAD}^+$

  • Enzyme: Lactate dehydrogenase.

  • Purpose: Recycles $\text{NAD}^+$ back to glycolysis so the cell can continuously generate $2\,\text{ATP}$ rapidly without oxygen.

  • Context: Occurs in skeletal muscle cells during intense short-term exertion (e.g., a 100-meter sprint). Lactate (lactic acid) represents a metabolic waste product that cannot sustain prolonged effort.

• Aerobic Pathway: Pyruvate Oxidation

  • Condition: Triggered when cellular oxygen ($O_2$) is abundant.

  • Location: Mitochondria.

  • Reaction (per pyruvate):         $\text{Pyruvate} + \text{Coenzyme A} + \text{NAD}^+ \rightarrow \text{Acetyl-CoA} + CO_2 + \text{NADH}$

  • Yield per Glucose Molecule ($2\,\text{Pyruvates}$):

    • $2\,\text{Acetyl-CoA}$

    • $2\,CO_2$

    • $2\,\text{NADH}$

    • $0\,\text{ATP}$ (Pyruvate oxidation is the only one of the four main stages that generates zero $\text{ATP}$)

The Citric Acid Cycle (Krebs Cycle)

• Historical Context: Named after Hans Krebs, who discovered the cyclic sequence.

• Location: Mitochondria.

• Oxygen Requirement: Aerobic.

• Pathway Characteristics: An 8-to-10 step cyclic pathway driven by the acetyl group of Acetyl-CoA.

• Yield per Single Turn (per Acetyl-CoA molecule):

  • $3\,\text{NADH}$

  • $1\,\text{FADH}_2$

  • $2\,CO_2$

  • $1\,\text{ATP}$

• Yield per Glucose Molecule ($2\,\text{Acetyl-CoA}$ molecules / two turns):

  • $6\,\text{NADH}$

  • $2\,\text{FADH}_2$

  • $4\,CO_2$

  • $2\,\text{ATP}$

• Metabolic Excretion: The carbon dioxide produced during the Citric Acid Cycle and Pyruvate Oxidation constitutes the waste $CO_2$ that must be exhaled by the body.

The Electron Transport System and Oxidative Phosphorylation

• Location: Inner membrane of the mitochondria.

• Oxygen Requirement: Aerobic (requires molecular oxygen, $O_2$).

• Carrier Molecules: Electron carriers (NADH and $\text{FADH}_2$) generated in preceding steps deliver high-energy electrons. Every molecule of $\text{NADH}$ and $\text{FADH}_2$ drops off exactly $2\,\text{electrons}$.

• Proton Pumping Mechanism:

  • Membrane-bound protein complexes pass electrons along a chain (analogous to a bucket brigade passing buckets of water).

  • As electrons pass down the chain, protein complexes actively pump hydrogen ions ($H^+$) out of the mitochondrial matrix into the intermembrane space.

  • Elevator Analogy: Hydrogen ions become tightly packed in the intermembrane space (like cramming 25 to 30 people into an elevator built for 20), establishing a steep electrochemical concentration gradient driving $H^+$ back into the matrix.

• ATP Synthesis:

  • ATP Synthase: A dumbbell-shaped enzyme complex embedded in the membrane.

  • As $H^+$ ions flow back down their concentration gradient through ATP synthase, the enzyme catalyzes the phosphorylation of ADP:         $\text{ADP} + \text{Phosphate} \rightarrow \text{ATP}$

  • Oxidative Phosphorylation: This overall process of driving ATP synthesis via the electron transport chain and oxygen is termed oxidative phosphorylation.

• Terminal Electron Acceptor:

  • Oxygen ($O_2$) accepts the depleted electrons at the end of the transport chain and combines with hydrogen ions ($H^+$) to form water ($H_2O$).

• Energy Yield: Generates approximately $28\,\text{ATP}$ per glucose molecule and regenerates free $\text{NAD}^+$ and $\text{FAD}$ to return to earlier pathways.

Metabolic Integration: The Role of Acetyl-CoA

• Central Metabolic Hub: Acetyl-CoA serves as a critical junction connecting carbohydrate, lipid, and protein metabolism.

• Lipid Catabolism: Fatty acids from lipid breakdown are chopped into 2-carbon units to yield Acetyl-CoA molecules (generating $\text{NADH}$ and $\text{FADH}_2$ in the process), which then enter the Citric Acid Cycle.

• Lipogenesis (Fat Synthesis): Excess carbohydrates are processed into Acetyl-CoA via glycolysis and pyruvate oxidation. When ATP demands are satisfied, surplus Acetyl-CoA molecules are linked together to synthesize fatty acids, which are subsequently stored as body fat.

Protein Synthesis: Transcription and Translation

• Genetic Material and Genes:

  • DNA (Deoxyribonucleic Acid): Located inside the cell nucleus; contains the master structural plans for approximately 100,000 distinct cellular proteins.

  • Gene: A specific segment of DNA that contains the explicit chemical instructions (recipe) for constructing a single protein.

  • Cookbook Analogy: DNA represents an entire heirloom cookbook containing thousands of recipes; a gene represents one specific recipe inside that cookbook.

• Transcription:

  • Location: Nucleus.

  • Process: Direct synthesis of a complementary RNA strand from a single DNA gene sequence (making a portable recipe copy from the master cookbook).

• Translation:

  • Location: Cytoplasm at the ribosome.

  • Process: Converting the nucleotide code of RNA into an amino acid sequence to assemble a functional protein (converting from nucleic acid language to amino acid language).

• Functional Types of RNA:

  • Messenger RNA (mRNA): The linear RNA copy of the gene recipe that travels from the nucleus to the cytoplasm to dictate amino acid sequence assembly (e.g., instructions for building myosin or lactate dehydrogenase).

  • Ribosomal RNA (rRNA): Structural and enzymatic RNA synthesized in the nucleolus that combines with proteins to construct ribosomes.

  • Transfer RNA (tRNA): Supplier molecules that carry individual amino acids from the cytoplasm to the ribosome, matching them to the specific codons dictated by the mRNA instructions.