AP Biology CH.5 Exam

Selective Permeability

The property of a biological membrane that allows some substances to cross more easily than others, maintaining cell chemical discrimination.[cite: 1]

Fluid Mosaic Model

Model describing the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.[cite: 1]

Amphipathic Molecule

A molecule possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, such as phospholipids and most membrane proteins.[cite: 1]

Phospholipid Bilayer Structure

The hydrophobic fatty acid tails face inward away from water, while the hydrophilic phosphate heads face outward toward aqueous solutions.[cite: 1]

Membrane Fluidity & Temperature

As temperature drops, membranes switch from a fluid to a solid state depending on lipid composition; unsaturated tails increase fluidity, saturated tails increase viscosity.[cite: 1]

Role of Cholesterol in Animal Membranes

Acts as a temperature buffer: restrains phospholipid movement at high temperatures to reduce fluidity and hinders close packing at low temperatures to prevent solidification.[cite: 1]

Integral Proteins

Membrane proteins that penetrate the hydrophobic interior of the lipid bilayer; most are transmembrane proteins spanning the entire membrane.[cite: 1]

Peripheral Proteins

Membrane proteins loosely bound to the surface of the membrane or to exposed parts of integral proteins, not embedded in the lipid bilayer.[cite: 1]

Six Major Functions of Membrane Proteins

1. Transport 2. Enzymatic activity 3. Signal transduction 4. Cell-cell recognition 5. Intercellular joining 6. Attachment to the cytoskeleton and ECM.[cite: 1]

Glycolipids & Glycoproteins

Carbohydrates covalently bonded to lipids or proteins on the extracellular surface; function as cell identification tags in cell-cell recognition.

Membrane Sidedness

The asymmetric distribution of proteins, lipids, and carbohydrates across the membrane is determined as the membrane is built by the ER and Golgi apparatus.[cite: 1]

Molecules crossing lipid bilayer directly

Nonpolar (hydrophobic) molecules like O2, CO2, and lipids dissolve in the lipid bilayer and cross easily without assistance.[cite: 1]

Molecules impeded by lipid bilayer

Polar (hydrophilic) molecules like glucose, water, and charged ions (Na+, K+, H+) pass through very slowly or cannot cross the hydrophobic core without transport proteins.[cite: 1]

Transport Proteins

Transmembrane proteins (channel or carrier proteins) that allow specific hydrophilic substances to pass through the lipid bilayer without contacting the hydrophobic core.[cite: 1]

Aquaporins

Specific channel proteins that facilitate the rapid single-file diffusion of water molecules across cell membranes.[cite: 1]

Carrier Proteins

Transport proteins that hold onto specific passenger molecules and change shape to shuttle them across the membrane.[cite: 1]

Diffusion

The spontaneous movement of particles from an area of higher concentration to an area of lower concentration down a concentration gradient without energy input.[cite: 1]

Passive Transport

The movement of a substance across a biological membrane down its concentration gradient that requires no energy expenditure by the cell.[cite: 1]

Osmosis

The diffusion of free water across a selectively permeable membrane from a region of lower solute concentration (higher free water) to higher solute concentration (lower free water).[cite: 1]

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water, depending on its concentration of nonpenetrating solutes.[cite: 1]

Isotonic Solution

A solution with equal solute concentration to the cell; no net movement of water occurs, leaving animal cells stable and plant cells flaccid.[cite: 1]

Hypertonic Solution

A solution with higher solute concentration than the cell; causes animal cells to shrivel and plant cells to plasmolyze.[cite: 1]

Hypotonic Solution

A solution with lower solute concentration than the cell; causes animal cells to swell and lyse, but keeps plant cells turgid (healthy).[cite: 1]

Osmoregulation

The control of solute concentrations and water balance in organisms lacking rigid cell walls (e.g., Paramecium using a contractile vacuole).[cite: 1]

Plasmolysis

A phenomenon in plant/walled cells in a hypertonic environment where the plasma membrane pulls away from the cell wall as water leaves, causing wilting/death.[cite: 1]

Facilitated Diffusion

Passive transport of specific polar molecules or ions across a membrane down their concentration gradient aided by channel or carrier proteins.[cite: 1]

Gated Ion Channels

Channel proteins that transport ions and open or close in response to specific stimuli (electrical signals or chemical ligands).[cite: 1]

Active Transport

The movement of a solute across a membrane against its concentration gradient (low to high concentration), requiring energy expenditure (usually ATP) and carrier proteins.[cite: 1]

Sodium-Potassium Pump

An active transport carrier protein in animal cells that uses ATP to pump 3 Na+ ions out of the cell for every 2 K+ ions pumped into the cell.[cite: 1]

Membrane Potential

The voltage across a cell membrane created by an unequal distribution of anions and cations, making the inside of the cell negative relative to the outside.[cite: 1]

Electrochemical Gradient

The combined chemical force (concentration gradient) and electrical force (voltage) driving the diffusion of ions across a membrane.[cite: 1]

Electrogenic Pump

A transport protein that generates voltage across a membrane while pumping ions (e.g., sodium-potassium pump in animals, proton pump in plants/bacteria).[cite: 1]

Proton Pump

The main electrogenic pump in plants, fungi, and bacteria that actively transports H+ ions out of the cell, storing energy as voltage and chemical gradients.[cite: 1]

Cotransport

A mechanism where a transport protein couples the "downhill" diffusion of a solute to the "uphill" transport of a second substance against its concentration gradient (e.g., H+/sucrose cotransporter).[cite: 1]

Exocytosis

The secretion of biological molecules by the fusion of transport vesicles (derived from the Golgi) with the plasma membrane.[cite: 1]

Endocytosis

The cellular uptake of bulk materials and large particles by forming new vesicles inward from the plasma membrane.[cite: 1]

Phagocytosis

A type of endocytosis ("cellular eating") where a cell engulfs large particles or organisms by extending pseudopodia around them into a food vacuole.[cite: 1]

Pinocytosis

A type of endocytosis ("cellular drinking") where a cell continually takes in droplets of extracellular fluid into tiny coated vesicles to acquire dissolved solutes.[cite: 1]

Receptor-Mediated Endocytosis

A highly specific type of endocytosis where receptor proteins in coated pits bind to specific ligands (e.g., LDL cholesterol particles) to trigger vesicle formation.[cite: 1]

Familial Hypercholesterolemia

A genetic disorder caused by defective or missing LDL receptors, preventing cholesterol uptake via endocytosis and causing cholesterol buildup in the blood.[cite: 1]

Paracrine Signaling

A form of local cell signaling in animals where a cell secretes local regulators (like growth factors) that influence nearby target cells simultaneously.[cite: 1]

Synaptic Signaling

A specialized local signaling in the nervous system where a nerve cell releases neurotransmitter molecules across a synapse to trigger a target cell.[cite: 1]

Endocrine (Hormonal) Signaling

Long-distance cell signaling in animals where specialized cells release hormones into body fluids (blood) to reach distant target cells.[cite: 1]

Three Stages of Cell Signaling

1. Reception (detecting signal molecule) 2. Transduction (converting signal via pathway) 3. Response (cellular activity).[cite: 1]

Ligand

A molecule that specifically binds to another molecule (receptor protein), usually causing the receptor to change shape and initiate signaling.[cite: 1]

G Protein-Coupled Receptors (GPCRs)

Cell-surface transmembrane receptors that work with a G protein using GTP to relay signals inside the cell.[cite: 1]

Ligand-Gated Ion Channel

A cell membrane receptor containing a gate that opens or closes when a specific ligand binds, allowing or blocking ion flow down its concentration gradient.[cite: 1]

Intracellular Receptors

Receptors located in the cytoplasm or nucleus that bind hydrophobic or tiny signaling molecules (e.g., steroid hormones like aldosterone, nitric oxide) that easily diffuse through the membrane.[cite: 1]

Transcription Factors

Specialized proteins that control which genes are turned on and transcribed into mRNA in response to a signal (e.g., activated aldosterone receptor).[cite: 1]

Phosphorylation Cascade

A sequence of signal transduction events where a series of protein kinases sequentially phosphorylate and activate the next protein in the pathway.[cite: 1]

Protein Kinase

An enzyme that transfers phosphate groups from ATP to a protein (phosphorylation), usually activating the protein in a signaling pathway.[cite: 1]

Protein Phosphatase

An enzyme that rapidly removes phosphate groups from proteins (dephosphorylation), inactivating protein kinases to turn off signaling pathways.[cite: 1]

Second Messengers

Small, nonprotein, water-soluble molecules or ions (e.g., cyclic AMP, Ca2+) that rapidly spread throughout the cytoplasm by diffusion to pass along a signal.[cite: 1]

Cyclic AMP (cAMP)

A common second messenger synthesized from ATP by adenylyl cyclase in response to extracellular signals like epinephrine, which activates protein kinase A.[cite: 1]

Fluid Mosaic Model

The model describing the cell membrane as a fluid collage of proteins bobbing in a dynamic lipid bilayer[cite: 2].

Primary Fluid Fabric

The phospholipid bilayer forms the primary fluid fabric of the boundary, internally hydrophobic and externally hydrophilic[cite: 2].

Membrane Carbohydrate Tags

Glycoproteins and glycolipids are short carbohydrate chains (under 15 sugar units) acting as cellular identification tags[cite: 2].

Cytoskeleton Anchors

Internal anchors (microfilaments, microtubules, intermediate filaments) holding certain membrane proteins in place and providing structural support[cite: 2].

Phospholipid Hydrophilic Head

Polar region attracted to the aqueous environment of the cytosol and extracellular fluid[cite: 2].

Phospholipid Hydrophobic Tails

Two nonpolar hydrocarbon chains that actively repel water[cite: 2].

Bilayer Dimension

Phospholipids naturally align into a stable 8nm-thick dual layer to maximize head contact with water and shelter tails[cite: 2].

Lipid Structures in Water

Phospholipids can organize into a liposome (double-layer sphere), micelle (single-layer sphere), or bilayer sheet[cite: 2].

Saturated Fatty Acid Tails

Straight hydrocarbon tails pack tightly, increasing membrane viscosity and causing solidification at lower temperatures[cite: 2].

Unsaturated Fatty Acid Tails

Kinks caused by double bonds prevent tight packing, maintaining membrane fluidity in extreme cold (e.g., cold-water fish)[cite: 2].

Cholesterol Fluidity Buffer

Wedged between lipids in animal cells; at 37°C+ it restrains movement to prevent excessive fluidity, while at low temps it prevents tight packing to hinder solidification[cite: 2].

Transmembrane Protein Hydrophobic Core

20-30 nonpolar amino acids coiled into alpha-helices, anchoring the protein in the lipid interior[cite: 2].

Transmembrane Protein Extremities

Hydrophilic extremities include the N-terminus exposed to extracellular fluid and the C-terminus exposed to the cytoplasm[cite: 2].

Membrane Protein Function: Transport

Channels and pumps act like doorways to control what enters or leaves the cell[cite: 2].

Membrane Protein Function: Enzymatic Activity

Proteins built into the membrane that help catalyze sequential metabolic pathways[cite: 2].

Membrane Protein Function: Signal Transduction

Receptor binding sites fit specific chemical messengers to send messages inside the cell[cite: 2].

Membrane Protein Function: Cell-Cell Recognition

Glycoprotein ID tags that send messages out and allow cells to recognize each other[cite: 2].

Membrane Protein Function: Intercellular Joining

Proteins stick together to form gap junctions or tight junctions for strong communication[cite: 2].

Simple Diffusion

Passive transport mode where nonpolar hydrophobic molecules (fats, O2, CO2) pass down their concentration gradient (High to Low) without ATP[cite: 2].

Facilitated Diffusion

Passive transport mode using protein channels or carriers to move polar/hydrophilic cargo (glucose, water) down their gradient (High to Low) without ATP[cite: 2].

Primary Active Transport

Protein pumps move specific ions (Na+, K+, H+) against their gradient (Low to High) requiring ATP hydrolysis[cite: 2].

Channel Proteins

Provide a dedicated hydrophilic tunnel through the membrane for specific solutes[cite: 2].

Aquaporin Scale

A single aquaporin channel protein allows up to 3 billion water molecules to pass through per second[cite: 2].

Carrier Proteins

Subtle shape-shifting shuttles that move specific molecules across the boundary[cite: 2].

Glucose Carrier Specificity

Red blood cell glucose transporters move glucose 50,000x faster than simple diffusion, while completely rejecting structural isomers like fructose[cite: 2].

Sodium-Potassium Pump Membrane Voltage

An electrogenic pump that expends ATP energy to maintain steep ion concentration gradients and a cell voltage of -50 to -200 mV[cite: 2].

Na+/K+ Pump Steps 1-3

3 Na+ ions bind from the inside; ATP phosphorylates the pump; protein shape changes and 3 Na+ are ejected outside[cite: 2].

Na+/K+ Pump Steps 4-6

2 K+ ions bind from the outside; phosphate group drops off; protein shape reverts and 2 K+ are released inside[cite: 2].

Na+/K+ Pump Net Charge

Result of every "crank" is a net transfer of one positive charge outward, generating electrical potential energy[cite: 2].

Secondary Active Transport (Cotransport)

A 3-step process coupling downhill diffusion of an ion to the uphill transport of a target nutrient[cite: 2].

Cotransport Step 1: The Uphill Pump

An ATP-driven proton pump concentrates H+ outside the cell, creating an electrochemical reservoir of potential energy[cite: 2].

Cotransport Step 2: The Downhill Flow

H+ ions diffuse back down their electrochemical gradient through a specific cotransporter protein[cite: 2].

Cotransport Step 3: Coupled Transport

The kinetic energy of returning H+ is harnessed to drag a target nutrient (sucrose or glucose) into the cell against its gradient[cite: 2].

Exocytosis

Bulk export process where transport vesicles from the Golgi apparatus fuse with the plasma membrane to secrete massive biological molecules (insulin, neurotransmitters)[cite: 2].

Phagocytosis

"Cellular eating": bulk import where the cell engulfs large particles or organisms into food vacuoles using pseudopodia[cite: 2].

Pinocytosis

"Cellular drinking": bulk import where the cell gulps droplets of extracellular fluid into tiny coated vesicles[cite: 2].

Receptor-Mediated Endocytosis

"Selective cellular drinking": bulk import triggered when specific ligands (e.g., LDL cholesterol capture) bind to receptors in coated pits[cite: 2].

Osmosis Definition

The passive diffusion of free water across a selectively permeable membrane to balance solute concentrations[cite: 2].

Hypotonic Effect on Animal vs Plant Cells

Lower solute outside; water rushes IN; animal cells undergo lysis (burst), plant cells become turgid (healthy, firm)

Isotonic Effect on Animal vs Plant Cells

Equal solute; net zero water movement; animal cells remain stable/normal, plant cells become flaccid (limp)

Hypertonic Effect on Animal vs Plant Cells

Higher solute outside; water rushes OUT; animal cells shrivel, plant cells plasmolyze (membrane pulls from wall, lethal)[cite: 2].

Paracrine Signaling

Local regulator range where secreted molecules (like growth factors) act only on nearby target cells in the immediate vicinity

Synaptic Signaling

Local range where electrical signals traveling down an axon trigger neurotransmitter release across a narrow synapse to a target cell

Hormonal/Endocrine Signaling

Long-range signaling where specialized cells secrete hormones into body fluids (bloodstream or plant sap) to reach distant target cells with matching receptors.

Cell Signaling Stage 1: Reception

The target cell detects a chemical signal when a ligand (hormone, growth factor, neurotransmitter) binds to a complementary receptor protein, causing a shape change

Cell Signaling Stage 2: Transduction

A cascade of molecular interactions where relay protein molecules reshape to functional forms (often via phosphorylation by protein kinases) to convert the signal