Biology: Atomic Structure, Chemical Bonds, and Cell Membranes

Proton

Positively charged subatomic particle located in the nucleus of an atom.

Neutron

Neutrally charged subatomic particle located in the nucleus of an atom.

Electron

Negatively charged subatomic particle found in orbitals around the nucleus.

Ionic bond

Bond formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.

Covalent bond

Bond formed by the sharing of one or more pairs of electrons between atoms.

Ionic vs covalent

Ionic bonds transfer electrons and form ions; covalent bonds share electrons and form molecules.

Molecule

Two or more atoms chemically bonded together, can be same or different elements.

Compound

A substance composed of two or more different elements chemically bonded in fixed proportions.

Hydrogen bond

Weak attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

Solubility

The ability of a substance (solute) to dissolve in a solvent, forming a homogeneous solution.

Dissociation

The separation of an ionic compound into its constituent ions when dissolved in water.

Electrolyte

A substance that dissociates into ions in solution and conducts an electric current.

Nonelectrolyte

A substance that dissolves in water without dissociating into ions and does not conduct electricity.

Dehydration synthesis

Reaction that joins monomers by removing a water molecule to form a covalent bond.

Hydrolysis

Reaction that breaks covalent bonds in polymers by adding water, yielding smaller subunits.

Carbohydrates

Biomolecules of C, H, O; include sugars and starches; function in energy storage and structural support.

Lipids

Hydrophobic biomolecules (fats, phospholipids, steroids); function in energy storage, membrane structure, and signaling.

Proteins

Polymers of amino acids folded into specific 3D structures; function as enzymes, structural components, transporters, and regulators.

Nucleic acids

Polymers of nucleotides (DNA, RNA); function in storing and transferring genetic information and energy (ATP).

DNA

Double-stranded deoxyribonucleic acid helix that stores the hereditary blueprint of an organism.

RNA

Single-stranded ribonucleic acid involved in protein synthesis and gene regulation.

ATP

Adenosine triphosphate; the primary energy currency of the cell, fuels biochemical reactions.

General parts of a cell

Plasma membrane, cytoplasm (cytosol and organelles), nucleus (in eukaryotes), and cytoskeleton.

Plasma membrane

Phospholipid bilayer with embedded proteins and cholesterol; regulates passage of materials and cell signaling.

Membrane potential

Electrical potential difference across the plasma membrane due to uneven distribution of ions.

Phospholipids

Amphipathic lipids with hydrophilic heads and hydrophobic tails that form the membrane bilayer.

Cholesterol

Steroid lipid interspersed in the bilayer that modulates fluidity and stability of the membrane.

Glycolipids

Lipids with attached carbohydrate chains on the extracellular surface for cell recognition and stability.

Fluid mosaic model

Description of the membrane as a fluid lipid bilayer with proteins that move laterally like a mosaic.

Membrane proteins

Integral and peripheral proteins serving as transporters, receptors, enzymes, and structural anchors.

Transport proteins

Integral proteins that facilitate the movement of specific ions or molecules across the membrane.

Selective permeability

The property of the plasma membrane to allow certain substances to cross while excluding others.

Simple diffusion

Passive movement of solutes down their concentration gradient directly through the lipid bilayer.

Facilitated diffusion

Passive transport of solutes down their gradient via specific carrier or channel proteins without energy.

Active transport

Energy-dependent movement of solutes against their concentration gradient via transport proteins.

Diffusion

Net movement of molecules from regions of higher concentration to regions of lower concentration.

Osmosis

Diffusion of water across a selectively permeable membrane toward higher solute concentration.

Osmotic pressure

Pressure required to halt the net movement of water during osmosis.

Hypotonic solution

Solution with lower solute concentration than the cell's cytosol; water enters cells, causing swelling.

Isotonic solution

Solution with equal solute concentration to the cell's cytosol; no net water movement, cell shape remains constant.

Hypertonic solution

Solution with higher solute concentration than the cell's cytosol; water leaves cells, causing shrinkage.

Mediated transport

Movement of substances across the membrane via specific carrier or channel proteins; can be passive or active.

Primary active transport

Use of ATP directly by transport proteins (pumps) to move ions or molecules against their gradient.

Secondary active transport

Transport driven by the energy stored in an ion's electrochemical gradient established by primary active transport.

Endocrine signaling

Long-distance cell communication via hormones secreted into the bloodstream to reach target cells.

Paracrine signaling

Local cell communication where signaling molecules affect nearby target cells.

Autocrine signaling

Cells respond to signaling molecules that they themselves secrete.

Juxtacrine signaling (contact-dependent)

Direct cell-cell communication via membrane-bound signals requiring physical contact.

Internal receptors

Cytoplasmic or nuclear receptors that bind lipophilic ligands capable of crossing the plasma membrane.

Cell-surface receptors

Transmembrane proteins that bind hydrophilic ligands and transduce signals to the cell interior.

Ligand structure vs mechanism

Ligand polarity determines whether it binds surface receptors (hydrophilic) or internal receptors (lipophilic).

Ligand binding initiation

Ligand binding induces receptor conformational change, triggering an intracellular signaling cascade.

Phosphorylation

Addition of phosphate groups by kinases to activate or deactivate proteins in signaling pathways.

Second messengers

Small intracellular molecules (e.g., cAMP, Ca²⁺, IP₃) that amplify and distribute signals within the cell.

Signal pathways and cell functions

Signaling cascades activate transcription factors and enzymes to regulate gene expression, metabolism, and growth.

Protein kinase C (PKC)

Kinase activated by diacylglycerol and Ca²⁺ that phosphorylates proteins in key signaling pathways.

Cellular hierarchy

Cells form tissues, tissues form organs, organs form organ systems in multicellular organisms.

Skin organization

Epidermis: stratified epithelial tissue; Dermis: connective tissue layer containing collagen and elastin.

Cell theory

All living organisms are composed of cells; cells are the basic unit of life; all cells arise from preexisting cells.

Basic cell attributes

Characteristics include metabolism, growth, reproduction, responsiveness, and homeostasis.

Prokaryotic vs eukaryotic

Prokaryotes lack a nucleus and organelles and are generally smaller; eukaryotes have a nucleus, organelles, and complex cytoskeleton.

Cytoskeletal filaments

Intermediate filaments provide support, microtubules enable transport and division, microfilaments (actin) drive movement.

Motor proteins

Kinesin transports cargo toward the plus end of microtubules; dynein moves cargo toward the minus end.

Desmosomes

Junctions that anchor adjacent cells together, providing mechanical strength.

Hemidesmosomes

Junctions that anchor epithelial cells to the basement membrane.

Gap junctions

Channels (connexons) that allow direct communication and ion exchange between adjacent cells.

Adherens junctions

Junctions that connect actin cytoskeletons of neighboring cells to maintain tissue integrity.

Tight junctions

Seals between cells that prevent the passage of molecules through the intercellular space.

Integrins

Transmembrane receptors that mediate cell-extracellular matrix adhesion and bidirectional signaling.

Selectins

Cell adhesion molecules that bind carbohydrate ligands and mediate leukocyte-endothelial interactions.

microfilaments

Diameter ~7 nm; F-actin double helix of G-actin monomers; distinct plus/minus ends; rapid assembly/disassembly; functions: cell cortex support, lamellipodia/filopodia, muscle contraction (actin-myosin), cytokinesis, vesicle transport (myosin motors)

intermediate filaments

Diameter ~10 nm; rope-like polymers of cell-specific proteins (e.g., keratins, vimentin, neurofilaments; lamins in nucleus); nonpolar, stable filaments; functions: tensile strength, mechanical resilience, desmosome/hemidesmosome anchoring, nuclear envelope support

microtubules

Diameter ~25 nm; hollow tubes of 13 protofilaments from α/β-tubulin heterodimers; plus end (dynamic GTP-cap growth) and minus end (MTOC anchored); dynamic instability (growth/shrinkage); functions: intracellular transport (kinesin toward plus, dynein toward minus), mitotic spindle/chromosome segregation, cilia/flagella structure, organelle positioning