Question 1

In microbial growth kinetics, the primary reason for the lag phase is best described as cells undergoing physiological acclimatization before rapid division. Which option states this most accurately?

Accepted Answer

Introduction / Context: The lag phase precedes exponential (log) growth in batch culture and reflects a period of physiological adjustment rather than net population increase. Understanding its cause helps optimize inoculum preparation, medium design, and process start-up in bioprocess engineering. Given Data / Assumptions: Batch culture of bacteria or yeast in fresh medium. Cells transferred from a different physiological state (e.g., stationary phase or another medium). No significant cell death assumed; net count is approximately constant. Concept / Approach: Lag phase duration is governed by how quickly cells can repair stress damage, synthesize missing enzymes/transporters, rehydrate, and adjust internal pools to the new carbon, nitrogen, and ion conditions. When inoculum is large and actively growing, lag is short; when inoculum is stressed or nutrient-shifted, lag is longer. Step-by-Step Solution: 1) Identify the controlling factor: metabolic reprogramming and enzyme induction are needed before division resumes.2) Recognize that temperature stress can lengthen lag, but is not its defining universal cause.3) Distinguish between substrate preference and the real issue: lack of immediate enzymatic capacity to utilize the available substrate efficiently.4) Conclude: acclimatization to the new environment and substrate is the core reason for the lag phase. Verification / Alternative check: Inoculating log-phase cells into identical medium often yields minimal lag; a strong carbon-source shift (e.g., glucose to lactose) introduces enzyme-induction delays (e.g., lac operon), producing a pronounced lag. Why Other Options Are Wrong: Higher growth temperature (option B) is a stressor that may impair growth but does not capture the fundamental physiological adaptation process.Substrate disliking (option C) is not a scientific description; utilization depends on inducible systems.Variation in substrate concentration alone (option D) affects rates post-acclimatization but does not by itself explain lag. Common Pitfalls: Confusing lag with death phase, assuming lag is purely due to low inoculum density, or attributing lag solely to nutrient transport rather than full metabolic induction. Final Answer: Acclimatization to the new environment and substrate

Question 2

In industrial fermentations, why is a relatively large seed volume (about 10–40% of working fermenter volume) often used?

Accepted Answer

Introduction / Context: Scale-up lines use seed trains to prepare cultures with the right physiology for production vessels. Choosing an appropriate inoculum ratio affects start-up dynamics, risk of contamination, by-product profiles, and time-to-harvest economics. Given Data / Assumptions: Well-conditioned seed at late exponential phase. Production fermenter inoculated at 10–40% v/v depending on organism and process. Objective: minimize non-productive time and variability. Concept / Approach: A larger seed volume introduces a high initial viable-cell concentration with enzymes already induced for the intended substrate. This shortens or essentially eliminates lag, rapidly establishes desired metabolism, and suppresses contaminants by competitive advantage, all of which shorten the overall batch duration to reach target titer. Step-by-Step Solution: 1) Higher starting cell density increases initial volumetric rate (dX/dt) at the same specific growth rate.2) Pre-adapted physiology reduces time needed for enzyme induction in the production medium.3) Fast dominance lowers contamination risk and stabilizes pH/DO control sooner.4) Net effect: decreased overall fermentation time to reach the desired endpoint. Verification / Alternative check: Process histories show that increasing inoculum ratio from 5% to, say, 20% frequently cuts several hours from batch time while preserving yield; diminishing returns appear beyond organism-specific optima. Why Other Options Are Wrong: Option A contradicts observed kinetics; larger inoculum shortens, not lengthens, batches.Option B: yield may improve indirectly, but the primary rationale is time reduction and robustness.Option D is opposite of process goals and is not a consequence of larger seed volume. Common Pitfalls: Ignoring oxygen transfer constraints at high inoculum, or assuming that larger inoculum automatically increases final titer irrespective of substrate or product inhibition. Final Answer: To decrease the overall fermentation time by reducing lag and quickly establishing target physiology

Question 3

In an airlift bioreactor, what is the correct name for the region where air (or gas) is sparged to drive circulation?

Accepted Answer

Introduction / Context: Airlift bioreactors use gas injection to create density differences between two communicating columns, driving circulation without mechanical agitation. Knowing the named regions is essential for scale-up and mass transfer discussions. Given Data / Assumptions: Typical internal- or external-loop airlift configuration. Gas sparged into one section to induce upward flow. Liquid circulates through riser and downcomer via hydrostatic head difference. Concept / Approach: The section receiving gas forms the riser: gas–liquid dispersion reduces bulk density, causing upward flow. After disengagement at the top (gas released in the headspace within the disengagement zone), degassed liquid returns via the downcomer, completing the loop and providing shear characteristics suitable for shear-sensitive cultures. Step-by-Step Solution: 1) Identify gas-injected zone: that is the riser.2) Recognize the degassed return path: the downcomer.3) Note the headspace separation region at the top: the disengagement zone.4) Therefore, the air sparging region is correctly termed the air riser. Verification / Alternative check: Flow visualization and pressure measurements show lower gas holdup and higher liquid density in the downcomer versus the aerated riser, producing the circulation driving head. Why Other Options Are Wrong: Downcomer: opposite section, where degassed liquid descends.Disengagement zone: where bubbles separate from liquid at the top, not where they are introduced.None of these: incorrect because riser is the standard term. Common Pitfalls: Confusing riser/downcomer due to schematic orientation or assuming the tallest section is always the riser regardless of sparging position. Final Answer: Air riser (riser section)

Question 4

For ideal tracer studies, the residence time distribution (RTD) of a vessel reflects macromixing and flow patterns. Which factor does RTD NOT depend upon?

Accepted Answer

Introduction / Context: Residence time distribution (RTD) characterizes how long fluid elements spend inside a reactor and is measured using non-reactive tracers. RTD is central to diagnosing deviations from ideal plug flow or well-mixed behavior at the macroscopic level. Given Data / Assumptions: Non-reactive tracer experiments under steady flow. Focus on macromixing and hydrodynamic residence times, not on molecular-scale events. Geometric and hydraulic features can shape flow patterns appreciably. Concept / Approach: RTD captures macroscopic dispersion and by-pass/recirculation zones. Micromixing refers to molecular-level mixing inside eddies or fluid packets and affects reaction outcomes where kinetics compete with diffusion, but it does not alter the time a packet spends inside the vessel as measured by a passive, non-reactive tracer. Step-by-Step Solution: 1) Identify macroscopic determinants: inlet/outlet placement, baffles, volume, aspect ratio, and flow rate sculpt the E(t) curve.2) Recognize that RTD is measured by tracking tracer concentration at the outlet versus time; it integrates path-length distributions, not internal molecular-scale mixing states.3) Therefore, RTD is independent of micromixing but depends on vessel geometry and hydrodynamics (volume, height, width). Verification / Alternative check: Two reactors with identical geometry and flow rates but different impeller micro-scales can show similar RTDs if macropatterns are unchanged; however, selectivity in fast reactions may differ due to micromixing. Why Other Options Are Wrong: Volume, height, and width all affect flow regime, dead zones, and circulation loops and thus shape RTD. Common Pitfalls: Conflating micromixing with macromixing; assuming better micromixing shortens RTD peak width—RTD does not quantify molecular-scale mixing quality. Final Answer: Micromixing (molecular-scale mixing within fluid elements)

Question 5

In bioprocess terminology, the 'concentration of the growth-limiting substrate' refers specifically to which of the following?

Accepted Answer

Introduction / Context: Chemostats and fed-batches often operate under a defined growth-limiting substrate so that specific growth rate mu can be controlled. Clear definition avoids confusion when multiple nutrients are present but only one is limiting at steady state. Given Data / Assumptions: Single dominant limitation is intended (e.g., carbon, nitrogen, phosphate, or oxygen). Other nutrients are supplied in non-limiting excess. We are concerned with the rate-limiting nutrient that determines mu via Monod kinetics. Concept / Approach: The growth-limiting substrate is the specific nutrient whose concentration S appears in mu = mu_max * S / (Ks + S), thereby throttling growth. Identifying it correctly allows precise feed control and avoids simultaneous multi-nutrient limitations that complicate interpretation. Step-by-Step Solution: 1) Determine which nutrient is below its saturation threshold (relative to Ks) under operating conditions.2) Define that nutrient’s concentration as the growth-limiting substrate concentration.3) Recognize that other nutrients (including trace elements) may be important but are not limiting if supplied in excess. Verification / Alternative check: In chemostat steady state, residual S* satisfies mu = D = mu_max * S* / (Ks + S*); changes in feed S_f shift S*, confirming which nutrient limits. Why Other Options Are Wrong: Trace elements as a group (option A) may or may not limit; the definition is not collective but specific.Bulk sugar concentration (option C) is not necessarily limiting unless shown to control mu.Dissolved oxygen (option D) can be limiting in aerobic systems but is not universally the growth-limiting substrate. Common Pitfalls: Assuming carbon is always limiting; overlooking shifts in limitation after process changes; ignoring interaction with maintenance energy. Final Answer: The concentration of the particular substrate that controls the cell growth rate

Question 6

Growth curve nuances: the brief transitional period after lag but before full exponential growth, with gradually increasing division rate, is commonly called the accelerated growth phase. It is often included as part of which phase?

Accepted Answer

Introduction / Context: Standard batch growth curves are divided into lag, exponential (log), stationary, and death phases. Some texts further describe transitional subphases that refine the dynamics of entry into exponential growth. Recognizing how these are grouped improves interpretation of sampling data and model fitting. Given Data / Assumptions: Cells adapt during lag, then accelerate division as machinery ramps up. Accelerated growth shows an increasing specific growth rate approaching mu_max. Taxonomy of phases varies slightly between references. Concept / Approach: The accelerated growth phase bridges lag and steady log growth. Because division rate is rising and the system is on its way to the constant-mu regime, many authors include it within the exponential growth phase, particularly when plotting on semi-log axes where curvature straightens toward a line. Step-by-Step Solution: 1) Identify the hallmark: mu increases with time after adaptation completes.2) Compare to lag (very low mu) and true log (mu approximately constant at mu_max).3) Since the process is converging to log behavior, categorize it under the exponential phase. Verification / Alternative check: Process sampling often reveals rising specific oxygen uptake rate (qO2) and substrate uptake just before linear semi-log plotting begins—consistent with accelerated growth being grouped with log phase. Why Other Options Are Wrong: Lag: dominated by adaptation, not rising division rate per se.Stationary/Death: occur after nutrients limit or toxins accumulate, not before log growth. Common Pitfalls: Overinterpreting phase boundaries as sharp; they are operational and can overlap depending on measurement frequency. Final Answer: Exponential (log) growth phase

Question 7

Limitations of the exponential growth model: which choice correctly captures the model’s shortcomings in real cultures?

Accepted Answer

Introduction / Context: The exponential growth model assumes constant specific growth rate and unlimited resources. While mathematically convenient, it is applicable only over a restricted interval in real bioprocesses. Recognizing its limitations helps motivate logistic, Monod-based, or structured models. Given Data / Assumptions: Batch culture with finite nutrients and accumulating by-products. Exponential model: X(t) = X0 * e^(mu * t) with mu constant. Question focuses on limitations, not strengths. Concept / Approach: Real systems depart from constant-mu behavior as soon as substrate becomes limiting, oxygen transfer caps, pH drifts, or inhibitors accumulate. The exponential model does not incorporate any of these constraints. It is a local approximation during the log phase and cannot predict onset of stationary or death phases. Step-by-Step Solution: 1) Evaluate option A: true—model applies during log phase only.2) Evaluate option B: true—model neglects slowing factors (resource and inhibition effects).3) Evaluate option C: false—exponential is not linear; it is exponential in time.4) Therefore, the best single choice summarizing the limitations is Both (a) and (b). Verification / Alternative check: Fitting exponential to early data works, but parameters drift as conditions change. Logistic or Monod models capture saturation or resource limitation better. Why Other Options Are Wrong: Option C contradicts the definition of the exponential model.Option D is correct; hence choices A or B alone are incomplete. Common Pitfalls: Believing exponential applies until nutrients are exhausted; in practice, mu declines earlier due to oxygen transfer, pH, and toxic by-products. Final Answer: Both (a) and (b)

Question 8

Optical density (OD) behavior in early growth: why can the measured OD sometimes drop during the lag phase in a spectrophotometer even though viable cells remain?

Accepted Answer

Introduction / Context: Optical density at 600 nm (OD600) is a turbidity proxy for biomass in suspension. However, OD depends on light scattering, which is sensitive not only to particle count but also to particle size, shape, and refractive index contrast with the medium. Understanding this helps interpret misleading early-culture readings. Given Data / Assumptions: Lag phase after inoculation into fresh medium. No large changes in viable cell count yet. Cells may undergo physiological water uptake and membrane adjustments. Concept / Approach: Light scattering intensity is proportional to a complex function of particle size and refractive index difference between cells and the medium. If cells transiently swell with water during adaptation, their internal refractive index may approach that of the medium, lowering scattering efficiency per cell. Consequently, measured OD can decrease slightly even if total cell mass or count has not decreased. Step-by-Step Solution: 1) During acclimatization, osmotic shifts drive water influx, increasing cell size but diluting intracellular solute concentration.2) Reduced refractive index contrast (n_cell − n_medium) reduces scattering at 600 nm.3) Spectrophotometer records lower OD despite similar viable counts.4) As metabolism ramps and composition normalizes, OD rises and tracks cell division in the log phase. Verification / Alternative check: Parallel plating (CFU) or flow cytometry often shows stable or rising viability while OD transiently dips. Adding osmolytes or adjusting medium ionic strength can modulate the magnitude of the OD artifact. Why Other Options Are Wrong: Option A: growth without division typically increases OD due to greater mass and cross-section.Option B and D: macromolecule accumulation increases scattering, not decreases it, and is not characteristic of lag-phase physiology. Common Pitfalls: Assuming any OD drop means cell death; relying solely on OD without corroborating viability assays; ignoring the role of refractive index in scattering-based measurements. Final Answer: Cells swell osmotically with water, altering refractive index contrast and scattering efficiency

Question 9

Process capability: for which types of bioprocesses can a stirred-tank fermenter (STF) be appropriately employed under standard configurations?

Accepted Answer

Introduction / Context: Stirred-tank fermenters are the workhorse of bioprocessing due to flexible control of mixing, mass transfer, and temperature. Properly configured, they support both oxidative and fermentative metabolisms across diverse organisms. Given Data / Assumptions: Availability of impellers, baffles, spargers, and headspace control. Gas handling systems can deliver air/oxygen for aerobic runs or nitrogen/CO2 purges for anaerobic runs. Appropriate shear management for sensitive mammalian or plant cells via low-shear impellers. Concept / Approach: By adjusting aeration rate, oxygen transfer coefficient (kLa), agitation, and gas composition, the STF can operate in fully aerobic regimes or strictly anaerobic conditions. For shear-sensitive cultures, low-shear impellers and microcarriers enable animal/plant cell processes, while high-kLa setups suit fast-growing microbes. Step-by-Step Solution: 1) Aerobic mode: supply air or O2-enriched gas; control DO via PID loops and agitation to meet OUR.2) Anaerobic mode: strip O2 with N2/CO2, ensure seals and feeds are O2-free, and monitor redox potential.3) Tailor impeller and baffle design to balance mixing and shear for the specific cell type.4) Therefore, STFs can be used for both aerobic and anaerobic fermentations with proper configuration. Verification / Alternative check: Industrial practice spans aerobic antibiotic production (e.g., Streptomyces) to anaerobic ethanol fermentations (e.g., Saccharomyces), all in stirred tanks scaled from liters to hundreds of cubic meters. Why Other Options Are Wrong: Option A or B alone understate the versatility of STFs.Option D is overly narrow and excludes widely successful applications. Common Pitfalls: Assuming a single impeller type suffices for all cells; neglecting oxygen ingress during anaerobic operations; overlooking foam control and CO2 stripping in aerobic runs. Final Answer: Both (a) and (b)

Question 10

Bubble column versus airlift bioreactor (same working volume): which comparative statements are generally true about bubbles, oxygen transfer, and interfacial area in bioprocess operation?

Accepted Answer

Introduction / Context: When scaling aerobic bioprocesses, engineers often choose between bubble columns and airlift bioreactors of comparable size. This question asks you to compare three performance-linked features—bubble size, oxygen transfer rate, and gas–liquid interfacial area—and decide whether they trend favorably in airlift units relative to bubble columns. Given Data / Assumptions: Same working volume and similar gas flow rates for both reactors. Airlift design promotes internal circulation (riser–downcomer) and reduced backmixing compared to simple bubble columns. Microbial or cell cultures require oxygen; mass transfer is captured via kLa and interfacial area a. Concept / Approach: Oxygen transfer depends on kLa, where L is liquid-side coefficient and a is interfacial area. Airlift reactors often generate smaller, more uniformly distributed bubbles and sustain higher liquid circulation velocities, raising gas holdup and a. Improved circulation and lower coalescence typically increase volumetric mass transfer (kLa) at a given gas flow, especially at larger scales. Step-by-Step Solution: 1) Airlift geometry sets up a density difference (gassed riser vs ungassed downcomer) that drives circulation.2) Circulation reduces bubble coalescence and improves dispersion → smaller mean bubble diameter.3) Smaller bubbles increase a (surface area per volume) and residence time, boosting gas holdup.4) The combined effect increases kLa and therefore oxygen transfer rate (OTR) relative to many bubble columns of similar size. Verification / Alternative check: Pilot and industrial reports frequently measure higher gas holdup and kLa in external- and internal-loop airlifts at comparable gas inputs, particularly beneficial for shear-sensitive cultures requiring good O2 supply without impellers. Why Other Options Are Wrong: Choosing any single statement ignores the coupled improvements (bubble size → interfacial area → OTR). “None of the above” contradicts widely observed airlift advantages. Common Pitfalls: Assuming every airlift always outperforms every bubble column; geometry, sparger type, and operating regime matter. The comparison here is general/typical for well-designed systems. Final Answer: All of the above

Question 11

Large-scale airlift bioreactors: why is the air riser commonly located outside the draft tube (external-loop designs) in industrial units?

Accepted Answer

Introduction / Context: Airlift bioreactors rely on density differences between riser and downcomer to drive circulation. Industrial-scale variants often employ external-loop configurations with the aerated segment (riser) placed outside a draft tube or external to the main column. The question asks why this placement is beneficial. Given Data / Assumptions: Scale-up introduces constraints on gas distribution, heat removal, and hydrodynamics. We are comparing internal vs external riser placement under similar production conditions. Target cultures can be shear-sensitive and oxygen-demanding. Concept / Approach: External-loop airlifts allow engineers to tailor cross-sectional areas of riser and downcomer to tune circulation velocity, bubble residence time, and gas holdup. Better loop hydraulics improve both oxygen transfer (kLa) and overall heat removal (via jackets or external exchangers). Sparger configuration can be optimized without interfering with internals. Step-by-Step Solution: 1) Positioning the riser externally simplifies routing of cooling surfaces and improves temperature control → better heat transfer.2) Flow path control raises circulation velocity and gas residence, aiding oxygen transfer.3) External riser geometry provides space for robust, maintainable spargers (even if compact rings are used), while preserving stable hydrodynamics. Verification / Alternative check: Case studies show that external-loop designs can match or exceed stirred-tank OTR at lower specific energy inputs, especially valuable for mammalian or fungal cultures. Why Other Options Are Wrong: Each individual reason is valid; picking any single one underestimates the multi-factor benefit. “None” is incompatible with routine design practice. Common Pitfalls: Assuming oxygen transfer is controlled only by sparger size; loop hydraulics and heat management are equally important. Final Answer: All of these

Question 12

Bioprocess supply chains: why is applying Just-In-Time (JIT) manufacturing particularly important in biotechnology production environments?

Accepted Answer

Introduction / Context: Just-In-Time (JIT) manufacturing aims to minimize inventory and synchronize production with demand. In biotechnology, sensitivity of materials and the competitive landscape make JIT practices especially impactful. This question probes representative reasons for JIT adoption in bioprocessing. Given Data / Assumptions: Biotech feedstocks (media components, enzymes, labile supplements) often degrade or contaminate if stored long. Products (e.g., certain biologics, probiotics) may have stability constraints. Competition pressures lead firms to control working capital and reduce waste. Concept / Approach: JIT reduces holding times for sensitive inputs and finished goods, lowering spoilage risk and quality drift. It also compresses cash cycles and improves responsiveness to variable demand, aligning production with validated batches and regulatory release timelines. Step-by-Step Solution: 1) Identify perishability: biodegradable inputs and products impose shelf-life risks; JIT minimizes exposure.2) Link quality to time: less inventory time reduces degradation and supports consistent CQAs (critical quality attributes).3) Competitiveness: lower inventory costs and faster turns free capital for R&D and compliance.4) Therefore, all listed reasons jointly motivate JIT in biotech manufacturing. Verification / Alternative check: Industry best practices emphasize cold-chain integrity, just-in-time media prep, and campaign-based manufacturing to manage product stability and cost. Why Other Options Are Wrong: Each standalone reason is valid; choosing one misses the composite rationale. “None” contradicts standard operations strategy in biotech. Common Pitfalls: Equating JIT solely with cost cutting while ignoring quality/stability drivers unique to biologics. Final Answer: All of the above

Question 13

Stirred-tank fermenter geometry: what is the typical height-to-diameter (H/D) ratio range used in tank fermenters for bioprocessing?

Accepted Answer

Introduction / Context: Reactor geometry influences mixing, gas dispersion, power input, and scale-up rules. For stirred-tank fermenters, the height-to-diameter (H/D) ratio is a simple yet powerful design descriptor. This question asks for the typical H/D range used in practice. Given Data / Assumptions: We consider conventional mechanically agitated, baffled tank fermenters. Designs target good gas–liquid contact and manageable shaft lengths. Viscosities and foaming can vary with the organism and medium. Concept / Approach: Most production-scale stirred tanks use H/D between roughly 2 and 3 to balance mixing time, oxygen transfer, and structural practicality. Ratios substantially below 1.5 can impair gas dispersion and circulation; much greater than 3 can cause mechanical issues and non-uniformities unless multiple impellers and special internals are used. Step-by-Step Solution: 1) Recall common design: H/D about 2–3 for many fermenters.2) Compare options: “>1.5” captures this typical range without overstatement, whereas “>3” overshoots and “<1.5” undershoots.3) Therefore, the best general statement is H/D > 1.5. Verification / Alternative check: Vendor specifications for industrial glass-lined or stainless fermenters frequently list H/D near 2–3 for aerobic cultures with multiple impellers. Why Other Options Are Wrong: “<3” is true but too loose and includes unsuitable low ratios; “>3” is atypical; “<1.5” and “≈1” do not reflect standard fermenter practice. Common Pitfalls: Assuming a single “magic” ratio works for all processes; actual choice depends on power number, gas rates, and rheology. Final Answer: >1.5

Question 14

Wastewater bioreactors: which reactor configuration is most commonly used for biological wastewater treatment at scale?

Accepted Answer

Introduction / Context: Biological wastewater treatment relies on microbial communities to remove organic load and nutrients. Reactor selection balances robustness, mixing, and operational simplicity for variable influent. This question asks which configuration is most widely used in practice. Given Data / Assumptions: Municipal plants handle fluctuating flow and composition. Aeration and complete mixing are desirable for stable removal rates. Sludge handling (settling/return) is integral to the process. Concept / Approach: The activated sludge process is essentially a CSTR for the aeration basin, providing near-complete mixing with continuous feed and discharge. It accommodates variable loads and enables process control via sludge age and dissolved oxygen setpoints. Alternative reactors (plug flow, trickle beds, towers) are used in niches but are not the standard default worldwide for municipal applications. Step-by-Step Solution: 1) Identify the mainstream configuration: aeration basins with diffused or mechanical aeration.2) Map to reactor idealization: these basins behave like CSTRs.3) Conclude the common choice: CSTR (activated sludge). Verification / Alternative check: Design manuals and plant surveys show the prevalence of activated sludge CSTRs, with variants like oxidation ditches and extended aeration sharing the well-mixed principle. Why Other Options Are Wrong: Plug flow is used (e.g., in certain nitrification–denitrification layouts) but not universally; trickle beds/biofilters are common for gases or specific industrial effluents; tower fermenters are unrelated to municipal wastewater. Common Pitfalls: Equating anaerobic biofilters or MBBR/IFAS variants with the dominant municipal baseline; those are extensions, not the canonical default. Final Answer: CSTR (continuous stirred-tank reactor / activated sludge basin)

Question 15

Culture modes: what do we call a culture grown in a closed vessel where no fresh medium is added and no wastes are removed during the run?

Accepted Answer

Introduction / Context: Bioprocess operations are often categorized by how medium and biomass are exchanged during cultivation. This question focuses on the classic closed-system operation with no inflow or outflow during the main growth phase. Given Data / Assumptions: A single charge of medium is inoculated. No feed or bleed occurs during cultivation (aside from sampling). Metabolites accumulate and nutrients deplete over time. Concept / Approach: In a batch culture, the system is closed: biomass, substrate, and product trajectories follow characteristic phases (lag, exponential, stationary, death). This is distinct from continuous (chemostat) and fed-batch (intermittent or continuous feeding without effluent) modes. Step-by-Step Solution: 1) Match definition: “no addition of fresh medium and no removal of wastes” → closed system.2) Closed system in bioprocessing terminology is “batch culture.”3) Therefore select “Batch culture.” Verification / Alternative check: Standard kinetics models (Monod) are often fitted to batch data showing substrate depletion and product accumulation, reinforcing the closed nature of the operation. Why Other Options Are Wrong: Continuous includes balanced inflow/outflow; fed-batch adds medium but removes none; semi-continuous involves periodic harvest/refill; perfusion continuously removes waste/products while retaining cells. Common Pitfalls: Confusing fed-batch with batch because effluent is not removed; the key is that fed-batch still adds fresh medium. Final Answer: Batch culture

Question 16

Exponential (log) phase kinetics: why do cell numbers increase exponentially during the log phase of microbial growth?

Accepted Answer

Introduction / Context: Microbial growth curves show a characteristic exponential (log) phase during which cell concentration rises rapidly. Understanding the kinetic reason for this exponential behavior is central to bioprocess modeling and scale-up. Given Data / Assumptions: Balanced growth with adequate nutrients and constant conditions. Specific growth rate μ is roughly constant during log phase. Cell division proceeds by binary fission. Concept / Approach: In exponential growth, the rate of increase dX/dt is proportional to the current biomass X: dX/dt = μ*X. This is “autocatalytic” because the catalyst (cells) creates more of itself. Constant μ yields X(t) = X0*e^(μt), the hallmark of exponential behavior. Step-by-Step Solution: 1) Define balanced growth: composition and μ remain approximately constant.2) Express growth law: dX/dt = μ*X (rate proportional to amount present).3) Integrate at constant μ to obtain exponential solution X(t) = X0*e^(μt).4) Conclude that exponential rise occurs because the population itself catalyzes its own production. Verification / Alternative check: Batch growth data plotted on a semi-log graph shows a straight line during log phase, confirming exponential kinetics and constant μ. Why Other Options Are Wrong: Producing enzymes (A) is necessary but not sufficient for exponential form; (B) is false biologically; (D) misstates the mechanism—μ is ~constant, not exponentially increasing; (E) violates mass and replication principles. Common Pitfalls: Confusing exponential increase with accelerating μ; it is X that increases, not μ, during balanced growth. Final Answer: Because cells are autocatalysts (growth rate proportional to biomass)

Question 17

Stirred-tank internals: in a standard baffled tank bioreactor, what is the typical width of baffles relative to the tank diameter?

Accepted Answer

Introduction / Context: Baffles suppress swirling and improve mixing and gas dispersion in stirred tanks. Their geometry—especially width relative to vessel diameter—has strong effects on power draw and flow pattern. This item asks for the standard design range. Given Data / Assumptions: Four radial baffles uniformly spaced are assumed. Impeller type and clearance are within common practice. Goal is to avoid vortex formation and increase turbulence. Concept / Approach: Classic design guidelines recommend baffle width around 0.1*D (roughly 1/10 of the tank diameter). Some specifications accept down to about 1/12 depending on viscosity and foaming behavior. Wider baffles (e.g., 1/3*D) overly restrict flow; much narrower baffles permit vortexing. Step-by-Step Solution: 1) Start from the rule-of-thumb: baffle width ≈ 0.1*D.2) Convert to fractional range: approximately 1/10 to 1/12 of D.3) Select the option that captures this accepted range. Verification / Alternative check: Chemical engineering design texts and vendor datasheets list 0.08–0.12*D as common baffle sizing to mitigate vortex and improve mixing. Why Other Options Are Wrong: 1/3–1/4 D is excessively wide; 1/15–1/25 D is too narrow; 1/2 D is unrealistic. Common Pitfalls: Ignoring that viscous or non-Newtonian media may need modified internals; the stated range is the baseline for general service. Final Answer: Between 1/10 and 1/12 of the tank diameter

Question 18

Airlift versus stirred-tank: what key operational advantage(s) do airlift bioreactors typically have over stirred tanks of similar size?

Accepted Answer

Introduction / Context: Airlift bioreactors use gas-induced circulation rather than mechanical agitation. For shear-sensitive cultures, they are attractive alternatives to stirred tanks. This question focuses on two commonly cited advantages—oxygen transfer and shear environment—and asks whether both apply. Given Data / Assumptions: Similar working volumes and gas flow rates. Well-designed internal or external loop airlifts. Target organisms may be sensitive to impeller-generated shear. Concept / Approach: Airlifts can reach competitive kLa values because circulation increases gas residence time and dispersion, while the absence of impellers yields more uniform, lower peak shear. Although foam control varies with medium and antifoam strategy, shear environment and oxygen transfer together often favor airlifts for delicate cells. Step-by-Step Solution: 1) Compare hydrodynamics: airlifts use buoyancy-driven loops, reducing localized high shear zones.2) With proper geometry, gas holdup and interfacial area improve, sustaining high kLa and OTR.3) Conclude that both higher OTR (in many designs) and more uniform shear are typical advantages. Verification / Alternative check: Literature reports show mammalian and filamentous cultures benefiting from airlift operation due to gentler hydrodynamics, while achieving target oxygenation without high mechanical power input. Why Other Options Are Wrong: (a) or (b) alone understate the combined benefit; foam behavior (d) is medium-dependent; (e) is incorrect—airlifts can outperform for certain applications. Common Pitfalls: Assuming airlifts always have lower OTR; geometry and gas rates matter. The statement is a typical trend, not an absolute law. Final Answer: Both (a) and (b)

Question 19

Lag phase physiology: which statement best describes what cells primarily do during the lag phase before exponential growth begins?

Accepted Answer

Introduction / Context: The lag phase precedes exponential growth in batch culture. Although cell counts may barely change, cells are far from inactive. This question clarifies the dominant activities occurring during lag and why they are necessary for subsequent rapid division. Given Data / Assumptions: Cells are transferred to fresh medium or shifted to new conditions. Nutrients are available but physiological machinery may not yet be optimized. Measurement focuses on viable count or biomass. Concept / Approach: Lag involves physiological adaptation: synthesis of transporters, metabolic enzymes, ribosomes, and membrane components needed to exploit the new environment. Regulatory networks reset gene expression, repair damage from previous conditions, and restore energy reserves. Step-by-Step Solution: 1) Detect environmental shift (carbon source, pH, temperature, osmolarity).2) Upregulate genes for uptake and catabolism appropriate to the new medium.3) Synthesize lipids and membrane proteins to restore membrane integrity and expand surface area.4) Increase ribosome content to boost translational capacity before entering balanced exponential growth. Verification / Alternative check: Proteomic studies show elevated synthesis of transporters and biosynthetic enzymes in lag; ribosomal RNA and energy-charge recovery precede the onset of exponential division. Why Other Options Are Wrong: (A) ignores necessary adaptation; (C) antibiotic production is species- and condition-specific and not the defining feature of lag; (D) physiology differs markedly from log; (E) rapid division is not occurring yet. Common Pitfalls: Interpreting flat growth curves as inactivity; in reality, significant biochemical work is underway. Final Answer: Cells synthesize new proteins, enzymes, and membrane components to adapt

Question 20

In biochemical engineering, a stirred-tank reactor (STR) mixes its contents primarily by a powered impeller system. Which device is chiefly responsible for bulk mixing and suspension of cells/particles in an STR?

Accepted Answer

Introduction / Context: A stirred-tank reactor (STR) is the workhorse of bioprocessing. Its hallmark is an internal mechanical agitation system that provides bulk mixing, solid suspension, and gas–liquid mass transfer when aerated. This question asks which device is chiefly responsible for mixing inside a standard STR. Given Data / Assumptions: Vessel equipped with one or more impellers and baffles. Objective is to mix liquid phases and, when present, disperse gas and suspend cells/solids. Focus on the primary, built-in mixing mechanism of an STR, not on alternative recirculation schemes. Concept / Approach: Mechanical agitation driven by a motor–shaft–impeller assembly generates turbulence and macro-circulation loops. Baffles suppress vortex formation and enhance axial and radial flow, producing uniform composition and temperature. Compressed air may be introduced through a sparger in aerobic cultures, but in an STR it is the impeller that distributes bubbles and closes the mixing loop. Step-by-Step Solution: 1) Identify core STR hardware: tank, impeller(s), baffles, optionally sparger.2) Link function to hardware: impeller creates primary flow fields for mixing.3) Recognize aeration aids oxygen transfer but does not replace impeller mixing.4) External pumps are uncommon and introduce sterility/foaming issues. Verification / Alternative check: Power number correlations (Np), tip speed, and Reynolds number (Re) for impellers are standard tools to predict STR mixing performance—further evidence that agitation is the principal mixing driver. Why Other Options Are Wrong: Compressed air alone: in bubble columns it mixes; in STRs, it is secondary. External pumping: adds complexity and contamination risk. Any of these equally: incorrect; the impeller is primary. Static mixers: not typical inside an STR for bulk mixing. Common Pitfalls: Confusing air-lift/bubble column mixing with STR mixing; assuming aeration alone ensures homogeneity in viscous broths. Final Answer: A mechanical agitator (impeller with baffles)

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