BIO 109 Unit 3: Cell Structure, Function, and Membrane Transport

Practice vocabulary flashcards defining essential terminology, cellular structures, organelles, and membrane transport concepts from Units 3 & 4 (Chapters 4 & 5).

Cell Theory

A foundational biological principle consisting of three ideas: all living things are composed of cells, the cell is the basic unit of life (proposed by Schleiden and Schwann in the late 1830s), and all cells arise from existing cells (added by Virchow in the 1850s).

Magnification

A measure of how much larger an image appears compared with the actual physical size of the specimen.

Resolution

The capacity of an optical system to distinguish two adjacent points as separate entities, determining the sharpness and clarity of an image.

Light Microscope

An instrument using visible light and glass lenses that typically magnifies up to approximately $400\times$ with a resolution limit around $200\,\text{nm}$, capable of observing live or stained specimens.

Electron Microscope

A high-magnification instrument that uses a beam of electrons instead of light; includes the transmission electron microscope (TEM) for internal ultrastructure and the scanning electron microscope (SEM) for three-dimensional surface architecture.

Surface Area-to-Volume Ratio

A geometric relationship explaining why cells must remain small; as cell dimensions increase, internal volume expands faster than membrane surface area, decreasing the ratio and limiting exchange efficiency.

Plasma Membrane

A selectively permeable phospholipid bilayer embedded with proteins and exterior carbohydrates that encases the cell and regulates what enters and exits.

Cytoplasm

The entire internal cellular region situated between the plasma membrane and the nuclear envelope, comprising organelles, cytoskeletal filaments, and fluid.

Cytosol

The gel-like fluid portion of the cytoplasm, consisting of $70\%$ to $80\%$ water along with dissolved salts, sugars, amino acids, and proteins, where diverse metabolic reactions occur.

Ribosome

A non-membrane-bound cellular structure assembled in the nucleolus from rRNA and protein that translates mRNA blueprints to synthesize polypeptide chains.

Nucleus

The eukaryotic membrane-bound organelle that protects hereditary genomic DNA in the form of chromatin and directs ribosome and protein synthesis.

Nuclear Envelope

A double phospholipid bilayer barrier enclosing the nucleoplasm that is structurally continuous with the endoplasmic reticulum and perforated by nuclear pores.

Nucleolus

A condensed, dark-staining substructure within the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled.

Chromatin

The unwound complex of nuclear DNA wrapped around histone proteins, which condenses into distinct visible chromosomes prior to cell division.

Mitochondrion

A double-membrane organelle responsible for cellular respiration, producing ATP from glucose and oxygen while releasing $\text{CO}_2$; contains its own circular DNA and ribosomes.

Cristae

The convoluted infoldings of the inner mitochondrial membrane that expand the surface area available for ATP-generating biochemical complexes.

Mitochondrial Matrix

The innermost compartment enclosed by the inner mitochondrial membrane, housing metabolic enzymes, mitochondrial ribosomes, and circular DNA.

Rough Endoplasmic Reticulum (RER)

A network of folded membranous sacs studded with ribosomes on its cytosolic face; synthesizes, modifies, and folds proteins meant for membranes or secretion.

Smooth Endoplasmic Reticulum (SER)

A tubular, ribosome-free membranous network involved in synthesizing lipids, phospholipids, and steroid hormones, detoxifying poisons, and sequestering $\text{Ca}^{2+}$ ions.

Golgi Apparatus

A series of flattened membranous cisternae that modifies, sorts, tags, and packages proteins and lipids received at its cis face and shipped out from its trans face.

Lysosome

A specialized animal cell digestive vesicle maintained at an acidic pH, containing hydrolytic enzymes that digest macromolecules, engulfed pathogens, and aged organelles.

Peroxisome

A single-membrane organelle containing enzymes that carry out oxidation reactions to degrade fatty acids and amino acids, producing toxic $\text{H}_2\text{O}_2$ that is quickly reduced to $\text{H}_2\text{O}$ and $\text{O}_2$.

Microfilaments

The thinnest cytoskeletal components, composed of actin protein polymers, functioning in maintaining cell shape, muscular contraction, and cellular motility.

Intermediate Filaments

Rope-like cytoskeletal protein cables of intermediate diameter that provide tensile strength, anchor organelles, and maintain structural cell shape.

Microtubules

The thickest cytoskeletal fibers, constructed as hollow cylinders of polymerized tubulin that establish internal tracks for ATP-powered motor proteins and form division spindles.

Centrosome

The primary eukaryotic microtubule-organizing center near the nucleus containing a pair of centrioles arranged at perpendicular right angles.

Centriole

A cylindrical cellular structure composed of nine triplets of microtubules that duplicates before division and organizes the basal body of cilia and flagella.

$9 + 2$ Pattern

The characteristic arrangement of microtubules inside eukaryotic cilia and flagella, consisting of a ring of nine fused doublets encircling two unfused central single microtubules.

Basal Body

A structure derived from a centriole that anchors the base of each eukaryotic cilium or flagellum within the cytoplasm.

Cell Wall

A rigid extracellular structural boundary found outside the plasma membrane of plants (cellulose), fungi (chitin), and bacteria (peptidoglycan).

Middle Lamella

A pectin-rich adhesive layer situated between adjacent plant cell walls that cements neighboring plant cells together.

Central Vacuole

A large fluid-filled plant organelle that stores water, ions, and pigments while maintaining osmotic turgor pressure against the surrounding cell wall.

Turgor Pressure

The outward osmotic pressure exerted by fluid inside the plant central vacuole pushing the plasma membrane firmly against the rigid cell wall.

Plastids

A family of double-membrane organelles in plants and algae, including chloroplasts, pigment-storing chromoplasts, starch-storing amyloplasts, and lipid-storing elaioplasts.

Chloroplast

A photosynthetic double-membrane plastid containing thylakoids and chlorophyll that captures radiant light energy to synthesize carbohydrates.

Thylakoids

Disc-like, membranous internal sacs organized into stacks called grana inside chloroplasts where light-dependent reactions of photosynthesis occur.

Stroma

The aqueous fluid compartment surrounding thylakoid stacks inside a chloroplast where enzymes synthesize sugars from $\text{CO}_2$.

Nucleoid

The irregularly shaped, non-membrane-enclosed central cytoplasmic region of a prokaryote where the circular chromosome is located.

Peptidoglycan

A complex polymer composed of cross-linked sugars and amino acids forming the primary structural framework of bacterial cell walls.

Endosymbiotic Theory

The scientific explanation, championed by Lynn Margulis (Lynn Sagan) in 1967, stating that mitochondria and plastids evolved from free-living aerobic and photosynthetic bacteria engulfed by an ancestral host.

Extracellular Matrix (ECM)

A complex network of fibrous collagen proteins, proteoglycans, and adhesive glycoproteins secreted by animal cells to provide structural support and intercellular signaling.

Plasmodesmata

Microscopic plasma membrane-lined channels traversing adjacent plant cell walls that permit direct cytoplasmic transport of water, ions, and chemical messengers.

Tight Junctions

Specialized animal cell connections that create a watertight, leak-proof belt between adjacent plasma membranes, preventing extracellular fluid transit.

Desmosomes

Localized, mechanical spot welds that tether animal cells together and link to intermediate filaments, providing high tensile strength in stretchable tissues.

Gap Junctions

Protein complexes assembled from connexin subunits forming intercellular pores between neighboring animal cells for direct electrical and chemical passage.

Fluid Mosaic Model

A structural model of cell membranes formulated by S. J. Singer and G. L. Nicolson in 1972 depicting a fluid phospholipid bilayer populated with laterally drifting proteins, cholesterol, and carbohydrates.

Amphipathic

Describing a chemical molecule that possesses both a hydrophilic, water-attracted region and a hydrophobic, water-repelling region, such as a phospholipid.

Cholesterol

A steroid lipid interspersed among hydrophobic tails in animal cell membranes that buffers membrane fluidity by preventing excessive fluidity at warm temperatures and blocking rigid packing at cold temperatures.

Integral Proteins

Membrane proteins embedded within the hydrophobic core of the lipid bilayer, many of which span the entire width of the membrane as transmembrane proteins.

Peripheral Proteins

Membrane proteins bound reversibly to either the exterior or interior surface of the phospholipid bilayer without penetrating the hydrophobic core.

Glycocalyx

A carbohydrate-rich protective outer surface coating formed by glycolipids and glycoproteins that functions in cell recognition, tissue adhesion, and immunological self-identification.

Selective Permeability

A biological membrane property that allows small nonpolar molecules to diffuse freely across the hydrophobic bilayer while barring or restricting charged ions and large polar solutes without specialized transport mechanisms.

Passive Transport

The net movement of chemical solutes down their concentration gradient from high to low concentration without the expenditure of cellular metabolic energy ($ATP$).

Simple Diffusion

The unassisted passive movement of small, nonpolar, or lipid-soluble molecules directly across the phospholipid bilayer driven solely by thermal motion down a gradient.

Facilitated Diffusion

A passive transport process by which polar molecules or charged ions cross the lipid bilayer down their gradient through transmembrane channel or carrier proteins.

Aquaporins

Specialized transmembrane channel proteins that permit rapid, selective water diffusion across biological membranes.

Osmosis

The passive net diffusion of water molecules across a selectively permeable membrane toward the region with a higher concentration of non-penetrating solutes.

Hypertonic Solution

An external solution possessing a higher concentration of non-penetrating solutes relative to the inside of a cell, driving net osmotic water efflux.

Hypotonic Solution

An external solution having a lower concentration of non-penetrating solutes relative to the inside of a cell, resulting in net osmotic water influx.

Isotonic Solution

An external environment exhibiting a solute concentration equivalent to that inside the cell, generating no net osmotic movement of water across the membrane.

Crenation

The shriveling and scalloped contraction of an animal red blood cell resulting from net water loss in a hypertonic environment.

Plasmolysis

The retraction of the plant cell plasma membrane away from its surrounding cell wall caused by significant vacuolar water loss in a hypertonic medium.

Contractile Vacuole

An energy-consuming organelle in freshwater protists that actively accumulates excess cytoplasmic water and expels it outside to prevent osmotic lysis.

Active Transport

The movement of a solute across a cellular membrane against its concentration or electrochemical gradient, requiring an expenditure of metabolic energy.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ Pump)

A primary active transport carrier enzyme, discovered by Jens Skou in 1957, that hydrolyzes one $ATP$ molecule to pump three $\text{Na}^+$ ions out and two $\text{K}^+$ ions into the cell.

Antiporter

An integral membrane transport protein that cotransports two different substances simultaneously in opposite directions across the membrane.

Secondary Active Transport

A transport mechanism in which the downhill diffusion of one solute (such as $\text{Na}^+$) powers the uphill transport of another substance without directly hydrolyzing $ATP$ at the transport site.

Endocytosis

An active bulk transport mechanism in which the plasma membrane invaginates around external cargo to pinch off a vesicle, bringing substances into the cell.

Phagocytosis

A form of bulk endocytosis ('cell eating') wherein a cell uses pseudopodia to engulf large particles or microorganisms into an internal vacuole that subsequently fuses with a lysosome.

Pinocytosis

A non-specific bulk transport process ('cell drinking') where the cell membrane forms small invaginations to ingest droplets of surrounding extracellular fluid and dissolved solutes.

Receptor-Mediated Endocytosis

A highly selective endocytic process whereby target ligands bind matching cell-surface receptors in coated pits, triggering internalization within coated vesicles.

Exocytosis

An energy-requiring bulk export process wherein intracellular secretory vesicles migrate to and fuse with the plasma membrane, releasing their contents into the extracellular environment.