AP Biology Chapters 5 and 6 Study Guide Flashcards

Flashcard set covering key concepts from Chapters 5 and 6 of AP Biology In Focus 3rd Edition, including membrane structure, transport, cell signaling, bioenergetics, enzyme regulation, and metabolic control.

What does it mean that phospholipids and most other membrane constituents are amphipathic molecules?

Amphipathic molecules possess both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

How does the fluid mosaic model explain experimental findings about cell membrane structure?

It depicts the membrane as a fluid structure with a mosaic of various proteins embedded in or attached to a double layer of phospholipids, accounting for hydrophobic protein interactions, variable protein composition across membranes, and protein particle distributions visible in freeze-fracture electron micrographs.

How do temperature and membrane composition influence membrane fluidity?

As temperature decreases, membranes transition to a solid state. Unsaturated hydrocarbon tails with kinks prevent tight packing, keeping the membrane fluid at lower temperatures, whereas saturated tails pack tightly together, decreasing fluidity.

How does cholesterol resist changes in membrane fluidity as temperatures change?

At high temperatures, cholesterol restrains the movement of phospholipids to reduce membrane fluidity; at lower temperatures, it hinders close packing of phospholipids to prevent membrane solidification.

What is the difference between peripheral and integral membrane proteins?

Integral proteins penetrate the hydrophobic interior of the lipid bilayer (often as transmembrane proteins), whereas peripheral proteins are not embedded in the lipid bilayer at all and are loosely bound to the surface of the membrane.

What are the 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 extracellular matrix (ECM)

What role do membrane carbohydrates play in cell-cell recognition?

Membrane carbohydrates act as cell-surface identification markers (forming glycolipids or glycoproteins) that allow cells to distinguish one type of neighboring cell from another.

How do hydrophobic molecules cross cell membranes?

Hydrophobic (nonpolar) molecules dissolve in the lipid bilayer of the membrane and cross it rapidly without requiring transport proteins.

How do channel proteins differ from carrier proteins?

Channel proteins provide a hydrophilic continuous channel through which specific molecules or ions cross the membrane, whereas carrier proteins bind to specific molecules and undergo a conformational shape change to move them across.

How do aquaporins facilitate the passage of water through membranes?

Aquaporins are specialized channel proteins that allow water molecules to pass single file through the membrane, greatly accelerating the rate of osmotic water movement.

What is diffusion, and why is it a passive and spontaneous process?

Diffusion is the movement of particles of any substance so that they spread out into available space down a concentration gradient; it is passive and spontaneous because it occurs without energy input ($\Delta G < 0$).

Why does a concentration gradient of a substance across a membrane represent potential energy?

A concentration gradient represents potential energy because the unequal distribution of molecules drives their net movement down the gradient toward equilibrium.

How do hypertonic, hypotonic, and isotonic solutions affect cell contents?

In a hypertonic solution, a cell loses water to its environment and shrivels; in a hypotonic solution, water enters the cell, causing it to swell and potentially lyse; in an isotonic solution, there is no net movement of water across the cell membrane.

What is osmosis, and how is the direction of water movement predicted?

Osmosis is the diffusion of free water across a selectively permeable membrane; water moves from a region of lower solute concentration (higher free water concentration) to a region of higher solute concentration (lower free water concentration).

How do living cells with and without cell walls regulate water balance?

Cells without walls rely on osmoregulation (e.g., contractile vacuoles or ion transport) to balance water gain/loss; cells with walls use turgor pressure in hypotonic environments to maintain rigidity and undergo plasmolysis in hypertonic environments.

How do transport proteins facilitate diffusion?

Transport proteins provide passage across the hydrophobic core of the plasma membrane for polar or charged molecules down their concentration gradients without consuming cellular energy.

What are the distinctions between osmosis, facilitated diffusion, and active transport?

Osmosis is the passive diffusion of water; facilitated diffusion is the passive movement of solutes down their concentration gradient using transport proteins; active transport pumps solutes against their concentration gradient using cellular energy (typically ATP).

What two forces combine to produce an electrochemical gradient?

1. A chemical force (the ion's concentration gradient)
2. An electrical force (the effect of the membrane potential on the ion's movement)

How does an electrogenic pump create voltage across a membrane, and what are two examples?

An electrogenic pump generates membrane potential by transferring net electrical charge across the membrane. Examples include the sodium-potassium pump ($Na^+/K^+$ ATPase) in animal cells and the proton pump ($H^+$ pump) in plants, fungi, and bacteria.

What is the process of cotransport?

Cotransport is a mechanism in which a transport protein couples the downhill diffusion of one solute to the uphill transport of a second substance against its own concentration gradient.

How are large molecules transported across a cell membrane?

Large molecules such as proteins and polysaccharides cross the membrane in bulk packaged inside vesicles.

How does exocytosis differ from receptor-mediated endocytosis?

Exocytosis exports materials by fusing intracellular vesicles with the plasma membrane; receptor-mediated endocytosis imports specific bulk substances by using membrane receptor proteins that bind target ligands and pinch inward to form coated vesicles.

What is the yeast mating signal pathway, and what evidence suggests its early evolutionary origin?

Yeast cells identify potential mates by releasing signaling molecules ($\alpha$ and $a$ factors) that bind to G-protein-coupled receptors on opposite cells; the universal similarities between these pathways in unicellular organisms and animal signaling suggest they evolved before multicellular life.

What is paracrine signaling, and what is an example?

Paracrine signaling is local signaling where a cell secretes signal molecules into extracellular fluid that act on nearby target cells; an example is growth factor signaling stimulating neighboring cells to grow and divide.

How do plant and animal hormones travel to target cells?

Animal hormones travel through the circulatory system (bloodstream) to reach target cells; plant hormones move through vascular tissue (phloem/xylem), pass directly through cells, or diffuse through the air as gas.

What are the three stages of cell signaling?

1. Reception: Detection of a signaling molecule coming from outside the cell.
2. Transduction: Conversion of the signal to a form that can bring about a cellular response.
3. Response: The specific cellular reaction or activity triggered by the transduced signal.

How does a ligand-receptor interaction initiate a signal-transduction system?

The binding of a ligand (signaling molecule) to a specific receptor site induces a conformational shape change in the receptor protein, activating it to interact with other downstream signaling molecules.

Where may signal receptors be located in target cells?

Receptors are located either on the plasma membrane (for water-soluble signaling molecules) or inside the cytosol/nucleus (for hydrophobic or small signaling molecules that pass through the lipid bilayer).

How do G-protein-linked receptors, tyrosine-kinase receptors, and ligand-gated ion channels compare?

G-protein-linked receptors activate intracellular G proteins bound to GTP; tyrosine-kinase receptors catalyze the transfer of phosphate groups from ATP to tyrosine residues and can activate multiple signaling pathways at once; ligand-gated ion channels open or close in response to ligand binding to allow ion flow.

What are two advantages of using a multistep pathway in signal transduction?

1. Signal amplification: Increasing the geometric yield of response molecules at each step.
2. Fine-tuning and coordination: Providing multiple control points for regulating cellular responses.

How can an original signaling molecule produce a cellular response without entering the target cell?

It binds to a surface receptor, triggering a relay cascade of intracellular protein conformational changes and second messengers that transmit the message inside without the ligand crossing the membrane.

How does phosphorylation propagate signal information in target cells?

Protein kinases transfer phosphate groups from ATP to specific target proteins (phosphorylation cascade), inducing conformational changes that convert inactive proteins into active forms.

Why may a single cell require hundreds of different protein kinases?

Because each protein kinase specifically targets and regulates distinct protein substrates across diverse metabolic and cellular signaling pathways.

How do protein phosphatases turn off signal-transduction pathways?

Protein phosphatases catalyze dephosphorylation, rapidly removing phosphate groups from active proteins to render them inactive and shut down the pathway when the signal is removed.

What is a second messenger, and what is its role in signaling pathways?

A second messenger is a small, nonprotein, water-soluble molecule or ion (such as $cAMP$ or $Ca^{2+}$) that rapidly diffuses throughout the cytoplasm to relay signals from activated receptors to internal target proteins.

How is cyclic AMP formed, and how does it propagate signal information?

Adenylyl cyclase converts ATP to cyclic AMP ($cAMP$) upon activation by a G protein; $cAMP$ then activates protein kinase A (PKA), which phosphorylates downstream target proteins.

How does the cholera bacterium disrupt G-protein signaling pathways to produce cholera symptoms?

Vibrio cholerae secretes a toxin that chemically modifies a G protein so it cannot hydrolyze GTP to GDP, locking it in an active state; this continuously stimulates adenylyl cyclase to make $cAMP$, driving intestinal cells to secrete high amounts of salts and water into the gut.

How is the cytosolic concentration of $Ca^{2+}$ altered, and how is it involved in signal transduction?

Active transport pumps maintain low cytosolic $Ca^{2+}$ concentrations relative to extracellular fluid and ER; signaling pathways trigger $IP_3$ production to open $IP_3$-gated calcium channels on the ER, releasing $Ca^{2+}$ into the cytosol where it binds to relay proteins.

How is signal information transduced into cellular responses in the cytoplasm versus the nucleus?

In the cytoplasm, signals regulate the activity of existing proteins (e.g., opening ion channels or activating metabolic enzymes); in the nucleus, signaling pathways activate or deactivate transcription factors to regulate gene expression and synthesis of specific proteins.

How is signal amplification accomplished in target cells?

Signal amplification occurs in enzyme cascades where each active enzyme molecule catalyzes multiple substrate conversions, exponentially increasing the number of active products at each successive step.

Why may different types of cells respond differently to the same signal molecule?

Different cell types express different sets of receptor proteins, intracellular relay proteins, and target effector proteins.

How do scaffolding proteins help coordinate a cell's response to incoming signals?

Scaffolding proteins are large relay proteins that anchor multiple signaling enzymes together in physical proximity, improving transduction efficiency and speed.

What are the roles of catabolic and anabolic pathways in cellular metabolism?

Catabolic pathways release energy by breaking down complex molecules into simpler compounds; anabolic pathways consume energy to synthesize complex molecules from simpler building blocks.

How do kinetic energy and potential energy differ?

Kinetic energy is the energy associated with the relative motion of objects; potential energy is stored energy that matter possesses because of its location or spatial arrangement.

How does an isolated system differ from an open system, and why is an organism an open system?

An isolated system cannot exchange energy or matter with its surroundings, whereas an open system can exchange both energy and matter; organisms are open systems because they absorb chemical energy/nutrients and release heat/waste into the environment.

What are the First and Second Laws of Thermodynamics?

First Law: Energy can be transferred and transformed, but it cannot be created or destroyed. Second Law: Every energy transfer or transformation increases the entropy (disorder) of the universe.

Why do highly ordered living organisms not violate the second law of thermodynamics?

Organisms maintain low internal entropy by expending free energy, but they release heat and metabolic waste, resulting in a net increase in the total entropy of the universe.

What is the equation for free-energy change, and what does each component represent?

The equation is \n\Delta G = \Delta H - T \Delta S\n where \n\Delta G\n is the change in free energy, \n\Delta H\n is the change in total enthalpy, \nT\n is absolute temperature in Kelvin ($K$), and \n\Delta S\n is the change in entropy.

How do exergonic and endergonic reactions differ in terms of free energy change?

Exergonic reactions release free energy, have a negative change in free energy ($\Delta G < 0$), and occur spontaneously; endergonic reactions absorb free energy, have a positive change in free energy ($\Delta G > 0$), and are non-spontaneous.

Why is metabolic disequilibrium one of the defining features of life?

At metabolic equilibrium, \n\Delta G = 0\n and a cell can perform no work, leading to cell death; living cells prevent equilibrium by continually supplying substrates and exporting waste products.

What are the three main kinds of cellular work, and how do cells obtain energy to do them?

The three kinds of work are chemical work, transport work, and mechanical work. Cells obtain energy to drive these endergonic processes through energy coupling, using exergonic ATP hydrolysis.

What is the structure of ATP, and to which major macromolecule class does it belong?

ATP consists of the nitrogenous base adenine, the sugar ribose, and three attached phosphate groups; it belongs to the nucleic acid (nucleotide) class.

How does ATP perform cellular work?

ATP transfers a phosphate group to another molecule via phosphorylation, creating a reactive phosphorylated intermediate, or undergoes hydrolysis to change protein shapes and binding affinities.

What is the function of enzymes in biological systems?

Enzymes act as biological catalysts that speed up metabolic reactions by lowering the activation energy ($E_a$) barrier without being consumed by the reaction.

Why is an investment of activation energy necessary to initiate a spontaneous reaction?

Activation energy ($E_a$) is required to push reactant molecules uphill to an unstable transition state where chemical bonds can be broken and reformed.

How does enzyme structure determine enzyme specificity?

The precise three-dimensional amino acid structure of the enzyme's active site specifically complements the shape and chemical environment of its target substrate.

What is the induced-fit model of enzyme function?

It states that binding of substrate induces a slight conformational change in the enzyme active site, wrapping around the substrate to bind it securely and facilitate catalysis.

By what mechanisms do enzymes lower activation energy?

1. Orienting substrates precisely
2. Straining substrate chemical bonds
3. Providing a favorable microenvironment (such as localized pH)
4. Directly participating in covalent bonding with the substrate

How does substrate concentration affect the rate of an enzyme-catalyzed reaction?

As substrate concentration increases, the reaction rate increases until all enzyme active sites are occupied (saturated), at which point the reaction reaches maximum speed ($V_{max}$).

How do temperature, pH, cofactors, and inhibitors affect enzyme activity?

Temperature and pH optimize enzyme rates up to specific thresholds beyond which extreme values denature the enzyme; cofactors assist enzymatic catalytic function; competitive inhibitors block active sites, whereas noncompetitive inhibitors bind allosteric sites to alter enzyme shape.

How do allosteric regulators inhibit or stimulate enzyme activity?

Allosteric regulators bind to a distinct regulatory site separate from the active site, inducing conformational changes that stabilize either the active or inactive form of the enzyme.

How does the binding of oxygen to hemoglobin illustrate cooperativity?

The binding of one oxygen ($O_2$) molecule to one subunit increases the oxygen affinity of the remaining subunits, amplifying the protein's overall response to changes in oxygen concentration.

How does feedback inhibition prevent a cell from wasting chemical resources?

The final product of a metabolic pathway binds allosterically to and inhibits an enzyme involved earlier in the pathway, halting further synthesis when the product is abundant.

How does the localization of enzymes within a cell help order metabolism?

Enzymes are organized into structural membrane complexes or compartmentalized within specific organelles (such as mitochondria), arranging sequential metabolic steps efficiently.