AP Biology Unit 2: Cell Structure, Function, and Transport Vocabulary

A comprehensive vocabulary review of cell structures, organelle functions, surface area-to-volume constraints, membrane transport mechanisms, tonicity, osmoregulation, and water potential formulas.

Plasma Membrane

A selective lipid bilayer boundary containing embedded proteins that separates the cell cytoplasm from its nonliving surrounding environment.

Ribosome

An organelle composed of ribosomal RNA (rRNA) and protein that synthesizes proteins according to mRNA sequences.

Rough Endoplasmic Reticulum

A region of the endoplasmic reticulum studded with bound ribosomes that produces proteins packaged into transport vesicles for export and synthesizes membrane phospholipids.

Smooth Endoplasmic Reticulum

A region of the endoplasmic reticulum involved in lipid synthesis, drug and poison detoxification in the liver, and calcium ion ($\text{Ca}^{2+}$) storage for muscle contraction.

Golgi Apparatus

A membrane-bound cell organelle consisting of flattened sacs (cisternae) that functions in the correct folding, chemical modification, and packaging of newly synthesized proteins.

Lysosome

A membrane-enclosed sac containing hydrolytic enzymes used to digest macromolecules, clean up damaged organelles, and execute programmed cell death.

Apoptosis

Programmed cell death characterized by cell shrinkage, chromatin condensation, membrane blebbing, and nuclear collapse following the release of hydrolytic enzymes.

Tay-Sachs Disease

A genetic disorder caused by the lack of a functional lysosomal hydrolytic enzyme needed to break down fatty substances, leading to toxic lipid accumulation in the brain and spinal cord.

Vacuole

A membrane-bound sac that performs diverse storage functions; in plant cells, a large central vacuole stores water and nutrients while maintaining turgor pressure.

Endomembrane System

A network of cellular membranes including the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane that regulates protein trafficking and performs metabolic functions.

Surface Area-to-Volume Ratio

The mathematical relationship ($\text{SA/V}$) that constrains cell size; smaller cells maintain higher ratios for more efficient material and energy exchange.

Amphipathic Molecule

A molecule, such as a phospholipid, that possesses both a hydrophilic (water-attracting) region and a hydrophobic (water-repelling) region.

Fluid Mosaic Model

A model describing the plasma membrane structure as a fluid phospholipid bilayer embedded with a mosaic of functional proteins, lipids, and carbohydrates.

Cholesterol

A steroid in animal cell membranes that regulates fluidity by restraining phospholipid movement at high temperatures ($37^\circ\text{C}$) and preventing tight packing at cool temperatures.

Integral Proteins

Membrane proteins that penetrate the hydrophobic core of the lipid bilayer, often spanning the entire membrane as transmembrane proteins containing nonpolar $\alpha$-helices.

Peripheral Proteins

Membrane proteins that are loosely bound to the exterior or interior surface of the phospholipid bilayer rather than embedded in its hydrophobic core.

Aquaporin

A specialized transmembrane channel protein that facilitates the rapid diffusion of water molecules across the cell membrane.

Peter Agre and Roderick MacKinnon

Scientists awarded the 2003 Nobel Prize in Chemistry for discovering water channels (aquaporins) and conducting structural and mechanistic studies on ion channels, respectively.

Carrier Protein

A transport protein that selectively binds a specific solute (e.g., glucose) and undergoes conformational changes to shuttle the molecule across the plasma membrane.

Selective Permeability

The property of biological membranes that allows small nonpolar molecules ($\text{N}_2$, $\text{O}_2$, $\text{CO}_2$) to cross freely while restricting large polar molecules and charged ions without transport proteins.

Hypertonic Solution

An environment with a higher solute concentration (and lower water potential) compared to the inside of a cell, causing water to flow out and cells to shrink or crenate.

Hypotonic Solution

An environment with a lower solute concentration (and higher water potential) compared to the inside of a cell, causing water to enter and cells to swell or lyse.

Isotonic Solution

An environment with an equal solute concentration relative to the cell cytoplasm, resulting in no net movement of water.

Plasmolysis

The shrinking of a plant cell's cytoplasm away from its cell wall when placed in a hypertonic environment due to osmotic water loss.

Turgor Pressure

The pressure exerted by the fluid-filled central vacuole pushing the plasma membrane against the rigid cell wall in a plant cell placed in a hypotonic environment.

Contractile Vacuole

A specialized organelle in freshwater unicellular organisms like Paramecium that continuously pumps excess water out of the cell to prevent bursting in a hypotonic environment.

Osmosis

The passive net diffusion of free water molecules across a selectively permeable membrane from an area of high water potential (low solute concentration) to low water potential (high solute concentration).

Water Potential ($\Psi$)

A measure of the tendency of water molecules to move across a membrane by osmosis, calculated as $\Psi = \Psi_P + \Psi_S$ and measured in bars or megapascals ($\text{MPa}$).

Solute Potential ($\Psi_S$)

The component of water potential that decreases (becomes more negative) as solute concentration increases, calculated using the equation $\Psi_S = -iCRT$.

Ionization Constant ($i$)

The number of distinct particles a solute molecule forms when dissolved in water, used in solute potential calculations (e.g., $i = 1$ for sucrose, $i = 2$ for $\text{NaCl}$).

Transpiration

The evaporation of water from plant leaves through open stomata, creating a negative water potential gradient that pulls water upward from roots through the stem.