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

Comprehensive vocabulary flashcards reviewing cell structure, compartmentalization, organelle functions, surface area-to-volume ratio, membrane permeability, passive/active transport mechanisms, and water potential calculations for AP Biology.

Amphipathic

The property of having both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region, characteristic of phospholipids and membrane integral proteins.

Fluid Mosaic Model

A model that describes the cell membrane as a mosaic of various protein molecules drifting laterally in a fluid bilayer of phospholipids held together by weak hydrophobic interactions.

Cholesterol (Membrane Fluidity Buffer)

A membrane steroid that regulates membrane fluidity by hindering phospholipid movement at high temperatures and preventing the tight packing of phospholipids at low temperatures.

Integral Proteins

Proteins permanently embedded into the lipid bilayer, typically transmembrane proteins that span the membrane and possess amphipathic characteristics.

Peripheral Proteins

Proteins not embedded in the lipid bilayer but loosely bonded to the interior or exterior surface of the plasma membrane.

Glycolipids

Membrane carbohydrates covalently bonded to lipids that serve as cell markers crucial for cell-to-cell recognition.

Glycoproteins

Membrane carbohydrates covalently bonded to proteins, representing the most abundant carbohydrate marker involved in cell-to-cell recognition.

Selective Permeability

The property of cellular membranes that allows them to regulate which substances can cross into or out of the cell.

Plasmodesmata

Cytosol-filled channels traversing the cell walls of adjacent plant cells that connect the chemical environments of neighboring cells.

Prokaryotic Cells

Cells lacking a membrane-enclosed nucleus and membrane-bound organelles, characterized by DNA concentrated in a nucleoid region (found in Domains Bacteria and Archaea).

Eukaryotic Cells

Complex cells containing a membrane-bound nucleus housing genetic material and membrane-bound organelles (found in protists, fungi, animals, and plants).

Cellular Compartmentalization

The segregation of internal cellular spaces into distinct membrane-bound organelles, which increases surface area for reactions and prevents conflicting metabolic pathways from interfering.

Nucleolus

A prominent, dense structure inside the nucleus where ribosomal RNA ($\text{rRNA}$) is synthesized and assembled with proteins into large and small ribosomal subunits.

Ribosome

A non-membranous cellular complex composed of ribosomal RNA ($\text{rRNA}$) and proteins that translates messenger RNA ($\text{mRNA}$) into polypeptide chains.

Free Ribosomes

Ribosomes suspended freely in the cytosol that primarily synthesize proteins designed to function within the cytosol itself.

Bound Ribosomes

Ribosomes attached to the endoplasmic reticulum or nuclear envelope that synthesize proteins destined for insertion into membranes, packaging into organelles, or secretion via transport vesicles.

Rough Endoplasmic Reticulum (Rough ER)

A network of membranous sacs studded with bound ribosomes that synthesizes membranes and compartmentalizes proteins destined for secretion away from the cytosol.

Smooth Endoplasmic Reticulum (Smooth ER)

A portion of the endoplasmic reticulum lacking ribosomes that synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons.

Golgi Complex

An organelle consisting of flattened membranous sacs called cisternae with directionality (cis receiving face, trans shipping face) that modifies, sorts, tags, and packages proteins received from the ER.

Lysosomes

Membranous sacs of hydrolytic enzymes found in animal cells that digest macromolecules and recycle the cell's own damaged organelles via autophagy.

Autophagy

A lysosomal recycling process in which a damaged organelle or cytosol region becomes surrounded by a double membrane and is broken down by hydrolytic enzymes for cell renewal.

Peroxisomes

Specialized metabolic compartments bounded by a single membrane that catalyze oxidation reactions producing hydrogen peroxide ($H_2O_2$), which is subsequently broken down into water.

Central Vacuole

A large, fluid-filled organelle found in mature plant cells that stores water and inorganic ions, exerting pressure against the cell wall to maintain turgor pressure.

Endosymbiont Theory

The evolutionary theory stating that mitochondria and chloroplasts originated when an early eukaryotic ancestral cell engulfed a photosynthetic or aerobic prokaryote, evolving into a unified organism.

Mitochondria

Double-membraned energy organelles that serve as the site of cellular respiration and ATP generation, containing smooth outer membranes, folded inner cristae, and a metabolic matrix.

Cristae

The numerous inner foldings of the mitochondrial membrane that divide the organelle into two compartments and substantially increase surface area for cellular respiration.

Chloroplasts

Double-membraned photosynthetic organelles containing the green pigment chlorophyll, flattened thylakoid sacs stacked into grana, and an internal fluid called stroma.

Thylakoids

Interconnected flattened membranous sacs inside the chloroplast that stack into grana and serve as the site of light-dependent photosynthetic reactions.

Stroma

The fluid-filled space outside the thylakoids within the chloroplast double membrane, housing chloroplast DNA, ribosomes, and the enzymes that drive the Calvin cycle.

Microtubules

Hollow rods composed of tubulin dimers growing from centrosomes that shape the cell, guide motor protein organelle movements, separate chromosomes during division, and form cilia and flagella.

Microfilaments

Thin, solid cytoskeletal rods constructed from actin that bear tension, maintain cell shape, interact with myosin for muscle contraction, and form the cleavage furrow during animal cell division.

Intermediate Filaments

Permanent, tension-bearing fibrous proteins that anchor the nucleus and other organelles while providing structural reinforcement by forming the nuclear lamina.

Surface Area-to-Volume Ratio ($\text{SA:V}$)

A geometric ratio that dictates cellular efficiency; as cell size increases, volume outpaces surface area, decreasing exchange rates of resources, wastes, and thermal energy.

Passive Transport

The spontaneous movement of molecules across a biological membrane down a concentration or electrochemical gradient without the consumption of cellular energy ($ATP$).

Diffusion

The spontaneous movement of solute particles from a region of higher concentration to a region of lower concentration driven by constant thermal molecular motion.

Osmosis

The net diffusion of free water across a selectively permeable membrane down its concentration gradient or from an area of low solute concentration to high solute concentration.

Facilitated Diffusion

The passive transmembrane movement of hydrophilic molecules, ions, or water down their concentration gradient mediated by specific channel or carrier proteins.

Aquaporins

Specialized channel proteins that span the plasma membrane to facilitate the rapid diffusion of water molecules into and out of cells.

Carrier Proteins

Transmembrane transport proteins that undergo alternating conformational (shape) changes to move specific solute molecules across the membrane down or against gradients.

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water, depending on the relative concentration of nonpenetrating solutes.

Osmoregulation

The physiological processes by which cells and organisms regulate solute concentrations and balance water gain and loss.

Isotonic Solution

An extracellular environment with a nonpenetrating solute concentration equal to that inside the cell, resulting in no net diffusion of water.

Hypertonic Solution

An extracellular solution with a higher solute concentration than the cytoplasm, causing water to flow out of the cell and leading to shriveling in animal cells or plasmolysis in plant cells.

Hypotonic Solution

An extracellular solution with a lower solute concentration than the cytoplasm, causing water to rush into the cell, resulting in lysis (cytolysis) of animal cells and optimal turgor pressure in plant cells.

Plasmolysis

A condition in walled cells placed in a hypertonic medium where the central vacuole loses water, causing the plasma membrane to pull away from the rigid cell wall.

Water Potential ($\Psi$)

The physical property predicting the direction of water flow based on solute concentration and physical pressure, defined by the formula $\Psi = \Psi_s + \Psi_p$.

Solute Potential ($\Psi_s$)

The component of water potential determined by the concentration of dissolved solutes, calculated via $\Psi_s = -iCRT$; pure water has a solute potential of $0\,\text{MPa}$, and adding solute makes it negative.

Pressure Potential ($\Psi_p$)

The physical pressure exerted on a solution relative to atmospheric pressure, possessing a value of $0\,\text{MPa}$ (or $0\,\text{bars}$) when open to the air.

Active Transport

The movement of a solute across a cellular membrane against its concentration or electrochemical gradient, requiring energy expenditure typically supplied by $ATP$.

Membrane Potential

The electrical voltage difference across a membrane caused by an unequal distribution of cations and anions, resulting in a cytosol that is relatively negative compared to the extracellular fluid.

Electrogenic Pump

A transport protein that generates voltage across a membrane while pumping ions, thereby storing potential energy in the resulting electrochemical gradient.

Sodium-Potassium Pump

An electrogenic pump in animal cells that uses $ATP$ phosphorylation to actively pump $3\,\text{Na}^+$ out of the cell for every $2\,\text{K}^+$ pumped in, yielding a net $+1$ charge to the extracellular fluid.

Proton Pump

An electrogenic integral membrane protein used by plants, fungi, and bacteria that actively pumps $H^+$ out of the cell to establish a steep proton gradient.

Cotransport

A transport mechanism in which an electrogenic pump indirectly drives the active transport of a solute by pairing its uphill movement with the downhill diffusion of an ion (such as $H^+$).

Exocytosis

A form of active bulk transport where intracellular transport vesicles fuse with the plasma membrane to secrete bulk contents into the extracellular fluid.

Endocytosis

An active bulk transport mechanism in which the plasma membrane invaginates and pinches inward to form vesicles, capturing extracellular matter into the cell.

Phagocytosis

A specific form of endocytosis ('cellular eating') in which a cell projects pseudopodia to engulf large particles or whole organisms into a food vacuole for lysosomal digestion.

Pinocytosis

A nonspecific form of endocytosis ('cellular drinking') where the cell continually gulps extracellular fluid and dissolved solutes into small, protein-coated vesicles.

Receptor-Mediated Endocytosis

A highly specific bulk transport mechanism where extracellular ligand binding to plasma membrane receptors triggers the clustering of ligands and the formation of an inward coated vesicle.