Metabolism and Energy

Anabolic and Catabolic Pathways

Metabolic pathways are series of chemical reactions in a cell, managed by enzymes, that build up or break down molecules.

• Anabolic pathways: Build complex molecules from simpler ones (e.g., synthesis of proteins from amino acids). These processes require energy input.

• Catabolic pathways: Break down complex molecules into simpler ones (e.g., cellular respiration). These processes release energy.

• Example: Cellular respiration is catabolic; photosynthesis is anabolic.

First and Second Laws of Thermodynamics

Thermodynamics governs energy flow in biological systems.

• First Law (Conservation of Energy): Energy cannot be created or destroyed, only transformed. Application: Energy from sunlight is converted to chemical energy in photosynthesis.

• Second Law (Entropy): Every energy transfer increases the entropy (disorder) of the universe. Application: Some energy is lost as heat in metabolic processes.

• Ultimate Source of Energy: The sun provides energy for most life on Earth.

• Ultimate Fate of Energy: Energy is eventually lost as heat to the environment.

ATP and Energy Coupling

Adenosine triphosphate (ATP) is the cell's main energy currency.

• ATP couples exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

• Hydrolysis of ATP releases energy:

• This energy is used to drive cellular work (e.g., muscle contraction, active transport).

Enzymes: Structure, Function, and Regulation

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Enzymes are usually proteins with a specific active site for substrate binding.

• Factors affecting enzyme activity: temperature, pH, substrate concentration, and inhibitors.

• Inhibitors:

  • Competitive inhibitors bind to the active site.

  • Noncompetitive inhibitors bind elsewhere, changing enzyme shape.

• Enzyme activity graphs show optimal conditions and effects of inhibitors.

Photosynthesis and Cellular Respiration

Locations, Reactants, and Products

• Photosynthesis occurs in chloroplasts (plants, algae). Reactants: CO2, H2O, light energy Products: Glucose (C6H12O6), O2

• Cellular respiration occurs in mitochondria (eukaryotes). Reactants: Glucose, O2 Products: CO2, H2O, ATP

• The two processes are complementary: photosynthesis stores energy, respiration releases it.

Stages of Photosynthesis

• Light Reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP, NADPH); produce O2.

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

Stages of Cellular Respiration

• Glycolysis: Cytoplasm; glucose → pyruvate; produces ATP and NADH.

• Pyruvate oxidation and Citric Acid Cycle: Mitochondrial matrix; completes glucose breakdown; produces CO2, ATP, NADH, FADH2.

• Electron Transport Chain and Oxidative Phosphorylation: Inner mitochondrial membrane; uses NADH/FADH2 to make ATP; O2 is final electron acceptor, forming H2O.

DNA Structure and Replication

Structure of DNA

• DNA is a double helix composed of two antiparallel strands of nucleotides.

• Each nucleotide: phosphate group, deoxyribose sugar, nitrogenous base (A, T, C, G).

• Bases pair via hydrogen bonds: A-T, C-G.

DNA Replication

• Replication is semiconservative: each new DNA has one old and one new strand.

• Key enzymes:

  • Helicase: unwinds DNA

  • DNA polymerase: synthesizes new DNA

  • Primase: adds RNA primer

  • Ligase: joins Okazaki fragments

• Leading strand: synthesized continuously; Lagging strand: synthesized in fragments (Okazaki fragments).

• Nucleotides are added to the 3' end of the growing strand.

DNA Packaging

• DNA wraps around histone proteins to form nucleosomes.

• Nucleosomes coil to form chromatin; further condensed into chromosomes during cell division.

Gene Expression: Transcription and Translation

Transcription

• Process of copying DNA into RNA (mRNA).

• Occurs in the nucleus (eukaryotes).

• Key enzyme: RNA polymerase.

• Template strand is used to synthesize complementary mRNA.

• Nucleotides are added to the 3' end of the RNA.

Translation

• Process of synthesizing proteins from mRNA template.

• Occurs in the cytoplasm at ribosomes.

• Key molecules:

  • mRNA: carries genetic code

  • rRNA: forms ribosome structure

  • tRNA: brings amino acids to ribosome

• Codons (three-base sequences) specify amino acids.

• Use the codon table to determine amino acid sequence.

Mutations

• Changes in DNA sequence; can be substitutions, insertions, deletions.

• Potential impacts: silent, missense, nonsense, or frameshift mutations.

Cell Division and the Cell Cycle

Functions of Cell Division

• Growth, repair, reproduction (asexual in single-celled organisms).

Phases of the Cell Cycle

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

• M phase: Mitosis (nuclear division) and cytokinesis (cytoplasm division).

• Cell cycle is controlled by checkpoints (e.g., G1, G2, M).

Mitosis vs. Meiosis

• Mitosis: Produces two identical diploid cells; for growth and repair.

• Meiosis: Produces four non-identical haploid cells (gametes); for sexual reproduction.

• Key events:

  • Mitosis: prophase, metaphase, anaphase, telophase

  • Meiosis I: homologous chromosomes separate

  • Meiosis II: sister chromatids separate

• Plant vs. Animal Mitosis: Plant cells form a cell plate; animal cells form a cleavage furrow.

Diploid vs. Haploid

• Diploid (2n): Two sets of chromosomes (somatic cells).

• Haploid (n): One set of chromosomes (gametes).

Human Life Cycle

• Alternates between diploid (zygote, somatic cells) and haploid (gametes) stages.

• Fertilization restores diploid number; meiosis produces haploid gametes.

Table: Comparison of Mitosis and Meiosis

Additional info: Academic context and definitions have been expanded for clarity and completeness.