AP Biology: Cell Structure, Function, Membrane Transport, and Cell Size

Vocabulary practice flashcards covering cell organelles, membrane structure, surface area-to-volume ratio calculations, transport mechanisms, tonicity, and the endosymbiotic theory.

Subcellular Components and Organelles

Membrane-bound compartments or specialized macromolecular complexes in eukaryotic cells (or prokaryotic structural features) that compartmentalize cellular processes to allow incompatible reactions to occur simultaneously.

Ribosome

A non-membrane-bound structure composed of large and small subunits made of ribosomal RNA ($rRNA$) and proteins that synthesizes proteins by translating messenger RNA ($mRNA$).

Ribosomal RNA ($rRNA$)

The RNA molecules that serve as structural and catalytic components of ribosomes.

Messenger RNA ($mRNA$)

The RNA type that carries the genetic code from DNA to the ribosome for polypeptide synthesis.

Transfer RNA ($tRNA$)

The RNA type that delivers specific amino acids to the ribosome based on the $mRNA$ codon sequence.

Rough Endoplasmic Reticulum (Rough ER)

A network of membranous tubules and flattened sacs (cisternae) studded with ribosomes on its cytoplasmic surface; synthesizes, modifies, and packages proteins into transport vesicles.

Smooth Endoplasmic Reticulum (Smooth ER)

A network of interconnected membranous tubules lacking ribosomes; functions in lipid, phospholipid, and steroid hormone synthesis, carbohydrate metabolism, and drug/poison detoxification.

Golgi Apparatus

A series of flattened, stacked membrane sacs (cisternae) with cis (receiving) and trans (shipping) faces that modifies, sorts, and packages proteins and lipids received from the ER.

Mitochondria

A double-membraned organelle featuring a smooth outer membrane, a folded inner membrane (cristae), and a fluid-filled matrix; site of the Krebs cycle and oxidative phosphorylation to produce ATP.

Lysosome

A membrane-bound sac containing hydrolytic enzymes operating best in acidic conditions; performs intracellular digestion, recycling of cellular materials (autophagy), and programmed cell death (apoptosis).

Autophagy

The process by which lysosomes engulf and digest old or dysfunctional cellular organelles so that organic building blocks can be reused.

Apoptosis

Programmed cell death occurring when lysosomal contents are intentionally leaked under regulated conditions to cleanly dismantle and destroy the cell.

Food Vacuoles

Membrane-bound vesicles formed by phagocytosis to temporarily store and digest ingested nutrients.

Central Vacuole

A large plant organelle that stores water, ions, and waste, and creates internal hydrostatic pressure to maintain plant cell turgor pressure.

Contractile Vacuole

A specialized organelle found in freshwater protists that pumps excess water out of the cell to maintain osmotic balance.

Chloroplast

A double-membraned plant and algal organelle enclosing stroma and thylakoid stacks (grana); captures solar energy via photosynthesis to convert carbon dioxide and water into glucose.

Cristae

Infoldings of the inner mitochondrial membrane that maximize surface area to house electron transport chain complexes and ATP synthase enzymes.

Thylakoids

Coin-like, stacked membranous sacs in chloroplasts containing embedded chlorophyll; their lumen encloses a compartment to establish a proton ($H^+$) gradient for ATP synthesis.

Grana

Stacked columns of coin-like thylakoid membranes located inside the chloroplast.

Stroma

The fluid-filled space within the chloroplast enclosing thylakoid networks; the site of carbon fixation (Calvin-Benson cycle).

Surface Area-to-Volume ($SA:V$) Ratio

The mathematical relationship between a cell's surface area and interior volume; as cell size increases, volume grows cubically ($r^3$) while surface area grows quadratically ($r^2$), decreasing diffusion efficiency.

Surface Area of a Sphere Formula

$$SA = 4\pi r^2$$

Volume of a Sphere Formula

$$V = \frac{4}{3}\pi r^3$$

Surface Area of a Cube Formula

$$SA = 6s^2$$

Volume of a Cube Formula

$$V = s^3$$

Surface Area of a Rectangular Solid Formula

$$SA = 2ls + 2lw + 2hw$$

Volume of a Rectangular Solid Formula

$$V = l \times w \times h$$

Surface Area of a Cylinder Formula

$$SA = 2\pi rh + 2\pi r^2$$

Volume of a Cylinder Formula

$$V = \pi r^2 h$$

Phospholipid

An amphipathic lipid consisting of a hydrophilic (polar) phosphate head and two hydrophobic (nonpolar) fatty acid tails.

Fluid Mosaic Model

The structural representation of biological membranes showing a fluid phospholipid bilayer embedded with a mosaic of mobile proteins, cholesterol, and carbohydrates drifting laterally.

Cholesterol (Membrane Steroid)

A lipid steroid embedded in the membrane that regulates membrane fluidity by preventing it from solidifying at low temperatures or becoming excessively fluid at high temperatures.

Glycoproteins

Membrane proteins bonded to carbohydrate chains that serve as cell-to-cell recognition markers and immune system identification tags.

Glycolipids

Lipids with attached carbohydrate chains that stabilize membrane integrity and act as cellular surface receptors.

Selective Permeability

The membrane property governed by the hydrophobic lipid bilayer that permits small, nonpolar molecules ($O_2$, $CO_2$, $N_2$) to pass freely while repelling ions and large polar molecules.

Plant Cell Wall

A structural layer exterior to the plasma membrane composed primarily of cellulose microfibrils that maintains cell shape and prevents osmotic lysis.

Fungal Cell Wall

A rigid exterior boundary composed primarily of chitin.

Prokaryotic Cell Wall

A bacterial cell boundary composed primarily of peptidoglycan.

Passive Transport

The net movement of molecules down a concentration gradient from high to low concentration without metabolic energy ($\text{ATP}$) input.

Active Transport

The movement of molecules against their concentration gradient (low to high concentration) requiring metabolic energy ($\text{ATP}$) and carrier protein pumps.

Concentration Gradient

A difference in the concentration of a chemical substance across a distance or semipermeable membrane due to the unequal distribution of particles.

Endocytosis

A transport process in which the cell engulfs extracellular material by folding its plasma membrane inward to form a vesicle.

Exocytosis

A transport process where internal vesicles fuse with the plasma membrane to secrete molecules out of the cell.

Aquaporins

Specialized channel proteins spanning the membrane that facilitate the rapid passage of large volumes of water molecules.

$Na^+/K^+$ ATPase

An active transport pump that hydrolyzes $\text{ATP}$ to export $3\,Na^+$ ions and import $2\,K^+$ ions, creating an electrochemical gradient.

Hypotonic Solution

An external solution with a lower solute concentration (higher water potential) than the cell interior, causing net water influx.

Isotonic Solution

An external solution with a solute concentration equal to the cell interior, resulting in zero net movement of water.

Water Potential ($\Psi$)

A measure of the potential energy of water; calculated using $\Psi = \Psi_p + \Psi_s$, where water always flows from higher to lower values.

Solute Potential ($\Psi_s$) Formula

$\Psi_s = -iCRT$, where $i$ is the ionization constant, $C$ is molar concentration, $R = 0.0831\,\text{L}\cdot\text{bars}/(\text{mol}\cdot\text{K})$, and $T = {^{\circ}\text{C}} + 273$.

Osmoregulation

The active physiological regulation of osmotic pressure in bodily fluids to preserve water and electrolyte homeostasis.

Facilitated Diffusion

Passive movement of polar or charged molecules down their concentration gradient through transmembrane channel or carrier proteins without using $\text{ATP}$.

Endosymbiotic Theory

The evolutionary theory stating that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by an ancestral anaerobic eukaryote and formed an obligate symbiotic relationship.

Evidence for Endosymbiosis

Structural and genetic characteristics shared between organelles and bacteria: own circular DNA, 70S ribosomes, independent reproduction via binary fission-like division, and double membranes.