Unit 2 · Cell Structure and Function

Ribosome ● (core concept) — A non-membrane subcellular structure made of ribosomal RNA (rRNA) and protein, found in cells in all forms of life and reflecting the common ancestry in all known life. Ribosomes synthesize proteins according to messenger RNA (mRNA) sequences.

Endomembrane system ● (core concept) — A group of membrane-bound organelles and subcellular components — endoplasmic reticulum, Golgi complex, lysosomes, vacuoles, transport vesicles, nuclear envelope, and plasma membrane — that work together to modify, package, and transport polysaccharides, lipids, and proteins.

Rough ER ● (core concept) — Endoplasmic reticulum studded with membrane-bound ribosomes. It compartmentalizes the cell and carries out protein synthesis.

Smooth ER ● (core concept) — Endoplasmic reticulum lacking ribosomes. Its functions include detoxification of cells and lipid synthesis.

Golgi complex ● (core concept) — A membrane-bound stack of flattened sacs that correctly folds and chemically modifies newly synthesized cellular products and packages proteins for trafficking.

Mitochondrion ● (core concept) — A double-membrane organelle that houses the metabolic reactions of aerobic cellular respiration. Its smooth outer membrane and highly folded inner membrane create compartments that enable efficient ATP synthesis.

Lysosome ● (core concept) — A membrane-enclosed sac containing hydrolytic enzymes that digest material. Lysosomes also play a role in programmed cell death (apoptosis).

Apoptosis ● (core concept) — Programmed cell death, a controlled process in which lysosomes play a role.

Vacuole ● (core concept) — A membrane-bound sac with varied roles. In plant cells, a large central vacuole maintains turgor pressure through water and nutrient storage; in animal cells, vacuoles are smaller, more numerous, and store cellular materials.

Turgor pressure ● (core concept) — The pressure exerted by stored water against the plant cell wall, maintained by the large central vacuole.

Chloroplast ● (core concept) — A double-membrane organelle found in plants and photosynthetic algae; the site of photosynthesis.

Nuclear envelope ● (core concept) — The double membrane surrounding the nucleus; part of the endomembrane system.

Transport vesicle ● (core concept) — A small membrane sac that carries polysaccharides, lipids, or proteins between components of the endomembrane system.

Glycosylation ○ — Chemical modification of proteins (e.g., adding sugars) that takes place in the Golgi complex and determines protein function or targeting. Listed as an illustrative (suggested, not required) example in the CED.

Surface area-to-volume ratio ● (core concept) — The ratio of a cell's surface area to its volume. Smaller cells have a higher surface area-to-volume ratio and exchange materials with the environment more efficiently; as cells grow, the ratio falls and demand for internal resources rises.

Metabolic rate and body size ● (core concept) — Typically, the smaller a multicellular organism, the higher its metabolic rate per unit body mass. Larger organisms have lower surface area-to-volume ratios and exchange heat with the environment more slowly.

Phospholipid bilayer ● (core concept) — The plasma membrane's foundation: phospholipids arranged so their polar hydrophilic phosphate heads face the aqueous environments inside and outside the cell, while their nonpolar hydrophobic fatty acid tails face each other in the membrane interior.

Fluid mosaic model ● (core concept) — The model of the plasma membrane as a phospholipid framework embedded with proteins, steroids (such as cholesterol in vertebrates), glycoproteins, and glycolipids — all able to move laterally within the membrane.

Glycoprotein ● (core concept) — A membrane protein with attached carbohydrate chains; one of the mobile components of the fluid mosaic model.

Glycolipid ● (core concept) — A membrane lipid with attached carbohydrate chains; one of the mobile components of the fluid mosaic model.

Embedded protein ● (core concept) — A protein embedded in the cell membrane that may be hydrophilic (with charged and polar side groups), hydrophobic (with nonpolar side groups), or both. Hydrophilic regions of the protein are either inside the interior of the protein or exposed to the cytosol (cytoplasm); hydrophobic regions of the protein surface interact with the fatty acids in the interior of the membrane.

Cholesterol ● (core concept) — A steroid found in the plasma membranes of vertebrate animals; one of the mobile components of the plasma membrane listed in the fluid mosaic model.

Selective permeability ● (core concept) — The plasma membrane's property of allowing some substances to cross while blocking others, resulting from the membrane's hydrophobic interior.

Cell wall ● (core concept) — A structural boundary in Bacteria, Archaea, Fungi, and plants that also acts as a permeability barrier for some substances and protects the cell from osmotic lysis.

Concentration gradient ● (core concept) — A difference in solute concentration across a membrane, created by selective permeability; drives passive transport.

Passive transport ● (core concept) — The net movement of molecules from a region of high concentration to a region of low concentration, without direct input of metabolic energy.

Active transport ● (core concept) — The movement of molecules across a membrane requiring direct input of metabolic energy — often moving molecules from regions of low concentration to regions of high concentration.

Endocytosis ● (core concept) — An energy-requiring process in which a cell takes in large molecules or particulate matter by folding its plasma membrane inward to form vesicles that engulf material from the external environment.

Exocytosis ● (core concept) — An energy-requiring process that moves large substances or large amounts of substances out of the cell.

Facilitated diffusion ● (core concept) — Passive movement of charged ions or large polar molecules across a membrane through channel or transport proteins — moving down the concentration gradient with no energy input.

Channel protein ● (core concept) — A membrane protein that forms a passage allowing charged ions (such as Na+ and K+) to cross the plasma membrane.

Membrane potential ● (core concept) — The electrical charge difference across a cell membrane. Membranes can become polarized by the movement of ions across them.

Aquaporin ● (core concept) — A channel protein that transports large quantities of water across membranes.

Hypotonic ● (core concept) — An external environment with a lower solute concentration (higher water potential) than the cell interior; water moves into the cell by osmosis.

Hypertonic ● (core concept) — An external environment with a higher solute concentration (lower water potential) than the cell interior; water moves out of the cell by osmosis.

Isotonic ● (core concept) — An external environment with a solute concentration equal to that inside the cell; there is no net movement of water.

Osmosis ● (core concept) — The movement of water from regions of high water potential to regions of low water potential — equivalently, from hypotonic toward hypertonic regions.

Water potential ● (core concept) — The tendency of water to move from one area to another; water moves from high to low water potential. Relevant equation: water potential = pressure potential + solute potential.

Osmoregulation ● (core concept) — The maintenance of water balance, allowing organisms to control their internal solute composition and water potential. Water moves from regions of low osmolarity to regions of high osmolarity.

Osmolarity ○ — The solute concentration of a solution. Water moves by osmosis from regions of low osmolarity to regions of high osmolarity.

Electrochemical gradient ● (core concept) — A combined concentration and electrical gradient across a membrane, established and maintained by active transport. Membrane proteins are necessary to build and use it.

Sodium-potassium pump ● (core concept) — A membrane protein that uses ATP to pump sodium (Na+) and potassium (K+) ions against their gradients, helping maintain the membrane potential.

ATPase ● (core concept) — An enzyme that hydrolyzes ATP to supply the energy for active transport of molecules and ions across membranes.

Compartmentalization ● (core concept) — The partitioning of the eukaryotic cell by membranes and membrane-bound organelles, which separates intracellular metabolic processes and specific enzymatic reactions.

Endosymbiosis ● (core concept) — The evolutionary process by which mitochondria and chloroplasts arose from once free-living prokaryotic cells that were engulfed by a host cell.

Prokaryote ● (core concept) — A cell type that typically lacks internal membrane-bound organelles but has internal regions with specialized structures and functions (e.g., Bacteria, Archaea).

Eukaryote ● (core concept) — A cell type that maintains internal membranes partitioning it into specialized regions (organelles).