Cellular Metabolism, Genetic Information Flow, and Membrane Transport: Study Notes for Anatomy & Physiology

Cellular Metabolism

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which ATP is synthesized using energy derived from the transfer of electrons in the electron transport chain (ETC) within mitochondria. This process is essential for aerobic energy production in cells.

• Definition: The production of ATP from ADP and inorganic phosphate, powered by the movement of protons (H+) across the inner mitochondrial membrane.

• Mechanism: High-energy electrons from NADH and FADH2 are transferred through protein complexes in the ETC, pumping protons into the intermembrane space, creating a proton gradient.

• ATP Synthase: Protons flow back into the mitochondrial matrix via ATP synthase, driving the phosphorylation of ADP to ATP.

• Equation: $\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

• Example: Each glucose molecule can yield up to 34 ATP molecules via oxidative phosphorylation.

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons from NADH and FADH2 to oxygen, the final electron acceptor.

• Key Steps:

  1. Electrons from NADH and FADH2 enter the ETC.

  2. Energy released pumps protons into the intermembrane space.

  3. Proton gradient drives ATP synthesis.

  4. Oxygen combines with electrons and protons to form water.

• Equation: $2\,\text{H}^+ + 2\,\text{e}^- + \frac{1}{2}\,\text{O}_2 \rightarrow \text{H}_2\text{O}$

• Example: The ETC is the final stage of aerobic respiration.

Anaerobic Glycolysis

Anaerobic glycolysis is the metabolic pathway that converts glucose to pyruvate, generating ATP without the use of oxygen. When oxygen is limited, pyruvate is converted to lactic acid.

• Definition: The breakdown of glucose to produce ATP in the absence of oxygen.

• Key Steps:

  1. Glucose is converted to pyruvate, producing 2 ATP and 2 NADH.

  2. Pyruvate is reduced to lactic acid by lactate dehydrogenase, regenerating NAD+.

• Equation: $\text{Glucose} \rightarrow 2\,\text{Lactic acid} + 2\,\text{ATP}$

• Example: Muscle cells during intense exercise rely on anaerobic glycolysis.

Phosphagens in Skeletal Muscle

Phosphagens, such as creatine phosphate, serve as rapid sources of high-energy phosphate to regenerate ATP during short bursts of intense activity in skeletal muscle.

• Definition: High-energy compounds that store and transfer phosphate groups to ADP to quickly replenish ATP.

• Creatine Phosphate Reaction: $\text{Creatine phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• Role: Supplements ATP supply during the initial seconds of muscle contraction.

• Example: Sprinting or weightlifting utilizes phosphagen stores.

Excess Postexercise Oxygen Consumption (EPOC)

After exercise, oxygen consumption remains elevated to restore metabolic conditions, replenish ATP and phosphagen stores, and remove lactic acid.

• Definition: The increased rate of oxygen intake following strenuous activity.

• Key Point: EPOC represents the difference between theoretical oxygen demand and actual postexercise oxygen uptake.

• Example: Heavy breathing after running is due to EPOC.

Genetic Information Flow

Genes and Regulation

Genes are regions of DNA that encode instructions for protein synthesis. Gene expression can be constitutive (always active) or regulated (induced or repressed).

• Gene: A segment of DNA that codes for a specific protein or RNA molecule.

• Constitutive Genes: Continuously expressed in cells.

• Regulated Genes: Expression is controlled in response to cellular signals.

Transcription

Transcription is the process by which DNA is used as a template to synthesize messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

• Definition: Synthesis of RNA from a DNA template.

• Types of RNA:

  • mRNA: Carries genetic code from DNA to ribosomes.

  • tRNA: Transfers amino acids during protein synthesis.

  • rRNA: Forms the core of ribosomes and catalyzes peptide bond formation.

• Equation: $\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{RNA}$

Translation

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

• Definition: Assembly of amino acids into a polypeptide chain based on mRNA sequence.

• Steps:

  1. Initiation: Ribosome binds to mRNA.

  2. Elongation: tRNA brings amino acids, peptide bonds form.

  3. Termination: Ribosome reaches stop codon, releases polypeptide.

• Post-translational Modification: Newly synthesized proteins may be folded, cleaved, or chemically modified for proper function.

Membrane Transport and Osmotic Balance

Fluid Compartments and Homeostasis

The body contains two main fluid compartments: intracellular fluid (ICF) within cells and extracellular fluid (ECF) outside cells. Homeostasis maintains equilibrium between these compartments.

• Osmotic Equilibrium: Water moves freely to balance solute concentrations.

• Chemical Disequilibrium: Solute concentrations differ between compartments.

• Electrical Disequilibrium: Charge differences exist across cell membranes.

• Other Fluids: Lymph, cerebrospinal, synovial, and serous fluids.

Diffusion

Diffusion is the passive movement of molecules from regions of higher concentration to lower concentration until equilibrium is reached.

• Definition: Movement of solutes down their concentration gradient.

• Key Point: Diffusion stops when concentration difference (AC) is zero.

• Example: Oxygen diffuses from blood into tissues.

Osmolarity and Osmolality

Osmolarity and osmolality measure the concentration of osmotically active particles in a solution, important for understanding water movement across membranes.

• Osmolarity: Number of osmotically active solutes per liter ($\text{osmol/L}$).

• Osmolality: Number of osmotically active solutes per kilogram ($\text{osmol/kg}$).

• Equation: $\text{Osmolarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

Tonicity and Cell Volume Regulation

Tonicity describes how a solution affects cell volume, depending on the concentration of nonpenetrating solutes.

• Isotonic: No net water movement; cell volume remains unchanged.

• Hypotonic: Water enters cell; cell swells and may lyse.

• Hypertonic: Water leaves cell; cell shrinks.

• Key Point: Osmolarity alone does not predict cell shape; permeability of solutes must be considered.

Table: Comparison of Solution Types and Effects on Cells

Solution Type	Osmolarity	Tonicity	Effect on Cell
Isosmotic	Equal to cell	Isotonic	No change
Hyposmotic	Lower than cell	Hypotonic	Cell swells
Hyperosmotic	Higher than cell	Hypertonic	Cell shrinks

Cell Membrane and Water Movement

The cell membrane regulates water and solute movement, maintaining homeostasis. Cells cannot actively pump water; they control water permeability and solute transport.

• Key Point: Water moves passively; solute transport is regulated by membrane proteins.

• Example: Aquaporins increase water permeability.

Additional info: Academic context was added to clarify mechanisms, definitions, and physiological relevance for each topic.