Cell Biology, Organelles, and Biological Macromolecules

Comprehensive review flashcards covering eukaryotic cell organelles, cellular architecture, endosymbiosis, differentiation, and the major classes of biological macromolecules (carbohydrates, lipids, and proteins).

Cell Membrane (Zellmembran)

A fluid bilayer of phospholipids whose hydrophilic heads face the aqueous interior and exterior, and whose hydrophobic tails face each other; contains embedded proteins for transport and signaling, as well as cholesterol that influences fluidity.

Cytosol and Cytoplasm

Cytosol is the aqueous intracellular base fluid where numerous metabolic reactions take place and which serves as a reaction and transport medium; together with the organelles and structures suspended in it, it forms the cytoplasm.

Nucleus (Zellkern)

An organelle enclosed by a double nuclear envelope containing nuclear pores, housing most of the genetic information (DNA) in the form of chromatin and the nucleolus; controls cellular activity by transcribing genes into RNA for protein synthesis.

Nucleolus

A dense, non-membrane-bound subcompartment inside the cell nucleus where ribosomal components are manufactured and assembled into ribosomal subunits before exiting via nuclear pores into the cytoplasm.

Ribosomes

Membrane-less complexes composed of ribosomal RNA (rRNA) and proteins, consisting of a large and a small subunit; they carry out translation and protein biosynthesis by converting mRNA sequences into polypeptide chains.

Free vs. Bound Ribosomes

Free ribosomes floating in the cytosol predominantly produce proteins destined for use within the cytosol, whereas ribosomes bound to the rough endoplasmic reticulum primarily synthesize proteins incorporated into membranes, transported into organelles, or secreted from the cell.

Rough Endoplasmic Reticulum (Raues ER)

A ribosome-studded, branched membrane network continuous with the outer nuclear membrane that synthesizes, folds, and modifies specific proteins, packaging them into transport vesicles as part of the endomembrane system.

Smooth Endoplasmic Reticulum (Glattes ER)

A branched membrane network lacking ribosomes that synthesizes lipids, participates in metabolic detoxification, and stores calcium ions ($\text{Ca}^{2+}$).

Golgi Apparatus

A cellular organelle consisting of stacks of flattened, membrane-enclosed spaces called cisternae; it receives, modifies, sorts, and packages proteins and lipids from vesicles and routes them to destinations such as the cell membrane, lysosomes, or extracellular space.

Vesicles

Small, single-membrane-enclosed spherical compartments that store or transport substances intracellularly and fuse with the cell membrane to export materials (exocytosis) or ingest materials (endocytosis).

Lysosomes

Membrane-bound acidic vesicles primarily found in animal cells containing digestive enzymes; they degrade ingested materials, invading pathogens, and damaged cellular components to recycle molecular building blocks.

Peroxisomes

Small, single-membrane-bound organelles containing specialized enzymes that break down fatty acids and neutralize toxins, producing harmful hydrogen peroxide ($\text{H}_2\text{O}_2$) as an intermediate.

Catalase

An enzyme present in peroxisomes that rapidly breaks down toxic hydrogen peroxide ($\text{H}_2\text{O}_2$) into harmless water ($\text{H}_2\text{O}$) and oxygen ($\text{O}_2$).

Mitochondria

Double-membrane-bound organelles featuring an extensively folded inner membrane, circular DNA, and their own ribosomes; they act as the site of cellular respiration, converting energy from nutrient molecules into usable $ATP$.

Cell Wall (Zellwand)

A rigid protective layer situated outside the plasma membrane in plant cells, composed primarily of cellulose, which confers structural stability, defines cell shape, and prevents lysis from excessive water uptake.

Central Vacuole and Tonoplast

A large, fluid-filled plant compartment surrounded by a single membrane called the tonoplast; filled with cell sap (Zellsaft), it stores water, ions, and pigments, acts in substance degradation, and generates internal turgor pressure to maintain plant rigidity.

Chloroplasts

Double-membrane-bound plant organelles containing circular DNA, their own ribosomes, and internal disc-like membrane sacs called thylakoids; they capture light energy via chlorophyll to synthesize energy-rich organic compounds during photosynthesis.

Thylakoids and Grana

Thylakoids are flattened membrane sacs within chloroplasts where light absorption takes place; stacked groupings of these thylakoid discs are termed grana.

Plasmodesmata (Plasmodesmen)

Microscopic channels traversing plant cell walls that physically connect adjacent plant cells, enabling direct intercellular exchange of substances and cellular communication.

Chromoplasts

Plastids that synthesize and store pigments, imparting bright red, orange, or yellow coloration to plant structures such as flower petals and fruits.

Leukoplasts and Amyloplasts

Colorless plastids specialized in storing nutrient reserves; amyloplasts are a specific type of leukoplast dedicated to storing starch.

Centrosome and Centrioles

The principal microtubule-organizing center of animal cells, typically containing two perpendicular centrioles; it regulates the cytoskeleton and forms the spindle apparatus required for chromosome separation during cell division.

Cilia and Flagella

Motile cellular projections composed of organized bundles of microtubules; cilia sweep fluid or particles across a cell's surface, while flagella propel single cells such as sperm through liquid media.

Cytoskeleton (Cytoskelett)

A dynamic intracellular network of protein fibers—microtubules, microfilaments, and intermediate filaments—that provides structural integrity, drives cell motility, facilitates vesicle and organelle transport, and segregates chromosomes during division.

Endosymbiotic Theory (Endosymbiontentheorie)

The evolutionary theory stating that mitochondria and chloroplasts originated from once free-living prokaryotes that were engulfed by an ancestral host cell, avoided digestion, and evolved into permanent symbiotic organelles.

Evidence for the Endosymbiotic Theory

Key observations supporting this theory include: mitochondria and chloroplasts possess their own circular DNA, contain their own ribosomes, replicate independently through binary fission, and are bounded by a double membrane.

Cell Differentiation (Zelldifferenzierung)

The developmental process by which unspecialized cells turn into specialized cell types with distinct shapes, sizes, and biochemical structures to execute specific physiological tasks (e.g., nerve signal conduction vs. muscle contraction).

Differential Gene Expression

The molecular mechanism underlying cell differentiation in which cells harboring identical DNA exhibit different phenotypes because distinct subsets of genes are turned active or inactive, resulting in cell-specific proteomes.

Monosaccharides

Single-unit simple sugars, such as glucose and fructose, that require no enzymatic digestion and are directly utilized in cellular respiration for immediate energy release.

Disaccharides

Carbohydrates composed of two monosaccharide units covalently linked by a glycosidic bond formed via a condensation reaction; examples include sucrose (glucose + fructose), maltose (two glucose units), and lactose (glucose + galactose).

Starch (Stärke)

The primary energy-storage polysaccharide in plants, composed of numerous glucose units linked mainly by $\alpha$-glycosidic bonds; features branched structures that allow compact storage for subsequent breakdown into glucose.

Glycogen (Glykogen)

The primary glucose-storage polysaccharide in animals and fungi, concentrated in the liver and skeletal muscles; structurally similar to starch but much more highly branched, facilitating rapid mobilization of glucose units.

Cellulose

A plant structural polysaccharide consisting of unbranched glucose chains linked by $\beta$-glycosidic bonds that form straight parallel arrays held in stable, insoluble microfibrils; indigestible by humans due to the lack of a suitable cleavage enzyme.

Triglycerides

The major dietary and storage lipids, each formed from one molecule of glycerol and three fatty acid chains joined by three ester bonds via condensation reactions releasing three water molecules ($3\,\text{H}_2\text{O}$).

Condensation Reaction

A chemical synthesis reaction in which two molecular subunits are covalently linked together with the concurrent elimination of a water molecule ($\text{H}_2\text{O}$).

Hydrolysis

A cleavage reaction in which covalent chemical bonds (such as ester, glycosidic, or peptide bonds) are broken through the chemical addition and consumption of a water molecule ($\text{H}_2\text{O}$).

Saturated Fatty Acids

Fatty acids containing no carbon-carbon double bonds ($\text{C}=\text{C}$), resulting in straight hydrocarbon chains that pack tightly together, causing lipids enriched in them to be solid at room temperature.

Unsaturated Fatty Acids

Fatty acids possessing one or more double bonds ($\text{C}=\text{C}$) that introduce kinks in the carbon chain, preventing close packing and making fats rich in them predominantly liquid oils at room temperature.

Fat-Soluble Vitamins

Vitamins A, D, E, and K, which are hydrophobic and require dietary lipids for efficient absorption and transport within the body.

Phospholipids

Amphipathic membrane lipids possessing a polar, hydrophilic phosphate head that interacts with water and two nonpolar, hydrophobic hydrocarbon tails that shield themselves from water by forming a bilayer.

Peptide Bond

A covalent amide bond formed between the carboxyl group ($-\text{COOH}$) of one amino acid and the amino group ($-\text{NH}_2$) of another through a condensation reaction that releases water ($\text{H}_2\text{O}$).

Proteinogenic Amino Acids

The $20$ distinct standard amino acids used in ribosomal protein biosynthesis, all sharing a universal core structure but differing in their unique side chains (R-groups), which determine whether they are polar, nonpolar, or charged.

Primary Structure

The linear, sequence-specific order of amino acids within a polypeptide chain, strictly determined by genetic coding.

Secondary Structure

Localized, regular folding arrangements of the polypeptide backbone, predominantly forming $\alpha$-helices and $\beta$-pleated sheets, stabilized by hydrogen bonding between backbone atoms.

Tertiary Structure

The overall three-dimensional conformation of an individual polypeptide chain, stabilized by interactions among side chains (R-groups), including hydrogen bonds, ionic bonds, hydrophobic interactions, and covalent disulfide bridges.

Quaternary Structure

The structural arrangement and spatial assembly formed when two or more distinct polypeptide subunits associate into a single, biologically functional multimeric protein complex.

Denaturation (Denaturierung)

The disruption and loss of a protein's native three-dimensional spatial conformation caused by high temperatures, extreme pH values, or specific chemicals, leading to the inactivation of its biological function without breaking the primary peptide bonds.