Biology Key Concepts: CHNOPS, Cell Membranes, Enzymes, Photosynthesis, Cell Cycle, DNA Replication, and Protein Synthesis

CHNOPS

The six most common elements in living organisms (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur).

Hydrogen Bond

A weak attraction between a hydrogen atom (with a partial positive charge) and a negatively charged atom (like Oxygen or Nitrogen).

Hydrophobic Interactions

The clumping of nonpolar molecules (like lipids) to avoid water.

Cohesion & Adhesion

Water sticking to itself (cohesion) versus water sticking to other charged surfaces (adhesion).

Dehydration Synthesis

A reaction that joins monomers into polymers by removing a water molecule.

Hydrolysis

The process of breaking down polymers into monomers by adding water.

Amphipathic

A molecule (like a phospholipid) that has both a hydrophilic (water-loving) and hydrophobic (water-fearing) region.

Activation Energy

The energy required to start a chemical reaction; enzymes lower this.

Active Site

The specific region on an enzyme where the substrate binds.

Allosteric Site

A site on an enzyme other than the active site where a non-competitive inhibitor binds, changing the enzyme's shape.

Catabolic vs. Anabolic

Catabolic reactions break down molecules (releasing energy); anabolic reactions build them up (consuming energy).

Competitive Inhibition

When a molecule mimics the substrate and competes for the active site.

Denaturation

The unfolding of a protein (enzyme) due to extreme pH or temperature, rendering it non-functional.

Vmax

The maximum reaction rate when an enzyme is saturated with substrate.

Endosymbiont Theory

The theory that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

Selectively Permeable

The property of the plasma membrane that allows some substances to pass while blocking others.

Fluid Mosaic Model

The description of the cell membrane as a flexible layer of lipid molecules with embedded proteins.

Facilitated Diffusion

Passive transport of molecules across a membrane through a specific protein carrier or channel.

Active Transport

Moving substances against their concentration gradient (low to high) using ATP.

Water Potential ($\Psi$)

A measure of the tendency of water to move from one area to another; calculated as $\Psi = \Psi_p + \Psi_s$.

Hypertonic / Hypotonic / Isotonic

Terms describing relative solute concentrations that determine the direction of osmosis.

Chemiosmosis

The movement of ions (H+) across a semipermeable membrane, down their electrochemical gradient, to power ATP synthesis.

Oxidative Phosphorylation

The production of ATP using energy derived from the redox reactions of an electron transport chain.

Photophosphorylation

The process of generating ATP from ADP and phosphate using the energy of sunlight during photosynthesis.

Calvin Cycle

The light-independent reactions of photosynthesis where carbon fixation occurs to produce glucose.

Glycolysis

The anaerobic breakdown of glucose into pyruvate in the cytosol.

Krebs Cycle (Citric Acid Cycle)

The stage of cellular respiration that oxidizes pyruvate derivatives to $CO_2$.

Surface Area to Volume Ratio

The constraint that limits cell size; a high ratio is necessary for efficient material exchange.

Cyclins & CDKs

Regulatory proteins that control the progression of the cell cycle at various checkpoints.

Mitosis vs. Meiosis

Mitosis creates two identical diploid ($2n$) clones; meiosis creates four genetically unique haploid ($1n$) gametes.

Crossing Over

The exchange of genetic material between homologous chromosomes during Prophase I of meiosis, increasing genetic diversity.

Law of Independent Assortment

Mendel's law stating that alleles for different traits are distributed to gametes independently.

Linked Genes

Genes located close together on the same chromosome that tend to be inherited together.

Nondisjunction

The failure of chromosomes to separate properly during meiosis, leading to aneuploidy (abnormal chromosome number).

Antiparallel

The arrangement in DNA where one strand runs 5' to 3' and the other runs 3' to 5'.

Semiconservative Replication

DNA replication where each new double helix consists of one old 'parental' strand and one new strand.

Okazaki Fragments

Short segments of DNA synthesized discontinuously on the lagging strand.

Transcription

The process of copying a DNA sequence into mRNA.

Introns vs. Exons

Introns are non-coding sequences 'cut out' during mRNA editing; exons are the coding sequences 'spliced' together.

Translation

The process where ribosomes use the mRNA code to assemble a polypeptide chain (protein).