Question 1

Pathway selection: at very low glucose concentrations (below ~0.25 mmol/L), large portions of glucose and glutamine carbon are preferentially shunted through which route in mammalian cells?

Accepted Answer

Introduction / Context: Carbon flux in mammalian cells depends on nutrient availability. When glucose becomes scarce, cells often reduce overflow glycolysis and rely more on oxidative metabolism, with glutamine contributing significantly to the TCA cycle anaplerosis. Given Data / Assumptions: Glucose below ~0.25 mmol/L is limiting. Glutamine can supply carbon to the TCA via glutaminolysis. Oxygen is available (typical culture aeration). Concept / Approach: Under low-glucose conditions, cells increase reliance on oxidative phosphorylation to maximize ATP per substrate molecule, reducing lactate overflow. Glutamine carbon enters as alpha-ketoglutarate, supporting energy and biosynthesis in the TCA cycle. Step-by-Step Solution: Recognize glucose scarcity decreases glycolytic overflow.Map glutamine → glutamate → alpha-ketoglutarate → TCA.Conclude the oxidative route predominates. Verification / Alternative check: Metabolite profiling shows lower lactate accumulation at low glucose and higher oxygen consumption per unit carbon. Why Other Options Are Wrong: B: Anaerobic dominance is typical with abundant glucose, not scarcity. C: Not equal; limitation favors oxidative use. D/E: Metabolism continues; PPP may contribute but is not exclusive. Common Pitfalls: Assuming the Warburg effect persists identically under nutrient limitation; cells adapt to maximize ATP yield. Final Answer: Oxidative pathway (TCA cycle and respiration)

Question 2

Managing nitrogen by-products: which strategy effectively mitigates ammonia (NH3/NH4+) toxicity in mammalian cell cultures?

Accepted Answer

Introduction / Context: Ammonia accumulation from glutamine catabolism can impair growth, productivity, and product quality (e.g., glycosylation patterns) in mammalian bioprocesses. Multiple complementary strategies are used to reduce ammonia stress. Given Data / Assumptions: Ammonia arises primarily from glutamine deamination and from amino acid metabolism. Ammonia is toxic at elevated concentrations and alters culture pH buffering. Medium and feeding strategies can modulate its accumulation. Concept / Approach: Reduce the source (glutamine load), change the nitrogen source (glutamate or dipeptides like Ala-Gln), and/or remove the by-product (adsorbents, dialysis-like systems, perfusion with cell-retentive filtration). Combining approaches often yields the best results. Step-by-Step Solution: Evaluate nitrogen sources: use glutamate/dipeptides to lower free glutamine turnover.Optimize feeding: keep glutamine low to match consumption.Implement removal: perfusion or adsorptive technologies reduce ammonium accumulation. Verification / Alternative check: Process development data commonly demonstrate improved viability and productivity when ammonia is limited via these strategies. Why Other Options Are Wrong: E: Incorrect because proven strategies exist (A–C). Common Pitfalls: Overfeeding glutamine; ignoring ammonium build-up until late in fed-batch; not monitoring pH/osmolality shifts associated with ammonia. Final Answer: All of the above

Question 3

Characteristics of established (continuous) mammalian cell lines: which statement is INCORRECT when describing ECL behavior and properties?

Accepted Answer

Introduction / Context: Established or continuous cell lines have undergone changes enabling indefinite proliferation. They commonly display rapid growth, reduced contact inhibition, and karyotypic abnormalities. However, absolutes (“invariably”) are risky in biology because exceptions can exist. Given Data / Assumptions: ECL are typically faster-growing than primary cultures. Many ECL are aneuploid/heteroploid, but biology admits rare exceptions or near-diploid cases. Loss of spatial orientation and contact inhibition is common in transformed lines. Concept / Approach: Identify the statement that is overly absolute. While aneuploidy is characteristic of many continuous lines, “invariably” implies no exceptions. The safer, correct teaching is “often” or “typically” aneuploid. Therefore, the absolute claim is the incorrect statement in a rigorous sense. Step-by-Step Solution: Evaluate each property against commonly observed line characteristics.Flag the absolute phrasing in the aneuploidy statement.Choose the absolute statement as incorrect. Verification / Alternative check: Cell banks report karyotypes for many lines; while heteroploidy is prevalent, not every continuous line is identically aneuploid at all times, and subclones may vary. Why Other Options Are Wrong: A/C/D/E: These are broadly true descriptors of established lines (rapid growth, high density potential, reduced contact inhibition, indefinite proliferation). Common Pitfalls: Accepting absolute statements in biology; overlooking line-to-line heterogeneity and passage-dependent karyotypic drift. Final Answer: Established cell lines are invariably aneuploid (heteroploid) in every case

Question 4

In animal tissue culture, “organ culture” techniques are commonly divided based on the type of medium employed. Which classification best reflects this division?

Accepted Answer

Introduction / Context: Organ culture is a classical approach in animal cell biotechnology where small pieces of tissue are maintained in vitro so that they preserve their structural organization and differentiated function. A key way textbooks categorize organ culture is by the physical state of the nutrient support: solid (gels or clots) versus liquid (medium bathing or interface methods). Given Data / Assumptions: The question asks how organ culture techniques are divided. Standard laboratory references describe methods using solidified matrices (for example, agar, collagen, fibrin/plasma clots) and liquid-phase systems (for example, raft or interface cultures). We assume routine mammalian organ culture conditions. Concept / Approach: Classification by medium state helps predict oxygenation, diffusion, and mechanical support. Solid matrices stabilize the explant and mimic extracellular scaffolding. Liquid systems favor exchange of nutrients and gases at a liquid–gas interface and are often rotated or rocked to improve diffusion. Step-by-Step Solution: Identify the organizing principle: physical state of the culture support.List categories used in practice: solid (clots/gels) and liquid (overlay/interface).Choose the option that includes both categories. Verification / Alternative check: Protocols for organ culture of trachea, retina, or fetal tissues routinely describe agar/plasma clot (solid) and liquid–gas interface methods, confirming the two-part division. Why Other Options Are Wrong: Solid only or liquid only: incomplete; both are used widely.Semi-solid only: a subset of solid systems, not the entire classification.Gas-phase without medium: not an organ culture technique. Common Pitfalls: Equating “semi-solid” with a separate overarching category; it is simply one form of solid matrix. Final Answer: Both solid and liquid media are used to classify organ culture methods.

Question 5

According to Eagle’s findings in cell culture nutrition, the growth of L-strain and HeLa cell cultures requires the mandatory presence of how many amino acids in the medium?

Accepted Answer

Introduction / Context: Harry Eagle’s pioneering work defined the nutrient requirements of mammalian cells in vitro, leading to Eagle’s Minimum Essential Medium (MEM). A classic question in cell culture is how many amino acids were shown to be essential for growth in certain lines such as L-strain fibroblasts and HeLa cells. Given Data / Assumptions: L-strain and HeLa cells were used to probe essential nutrients. The question focuses on the number of amino acids that must be supplied exogenously for reliable growth. “Mandatory presence” refers to amino acids that the cells cannot synthesize sufficiently under culture conditions. Concept / Approach: Essential components of media are determined empirically by omission experiments. If withdrawal of a specific amino acid halts proliferation or reduces viability, it is deemed essential for that cell line under the test conditions. Step-by-Step Solution: Recall Eagle’s conclusion for these classic lines.Map the conclusion to the nearest option provided.Select the historically cited figure for mandatory amino acids in L-strain/HeLa growth. Verification / Alternative check: Historical exam keys and many training manuals for basic tissue culture list eight amino acids as mandatory for these specific lines, even though later media (for example, MEM) commonly include a broader panel. Why Other Options Are Wrong: 6, 10, 12, 13 amino acids: reflect other formulations or broader medium compositions, not the specific “mandatory” number cited for these lines in the classic Eagle experiments. Common Pitfalls: Confusing the composition of modern MEM (which contains more amino acids) with Eagle’s reported minimum mandatory set for particular cells. Final Answer: 8 amino acids.

Question 6

In standard bicarbonate-buffered media for animal cell culture, what carbon dioxide (CO₂) level is typically maintained in the incubator to support proper pH and metabolism?

Accepted Answer

Introduction / Context: Most mammalian cell cultures use bicarbonate-buffered media (for example, DMEM, RPMI) and are maintained in CO₂ incubators. CO₂ equilibrates with bicarbonate and water to form carbonic acid, stabilizing pH near the physiological range. Given Data / Assumptions: Bicarbonate buffer system is present. Typical incubator humidity is high and temperature is 37 °C (for human/rodent cells). We seek the commonly used CO₂ range for stable pH (about 7.2–7.4). Concept / Approach: pH control in bicarbonate media depends on the Henderson–Hasselbalch relation: pH is set by the ratio of bicarbonate to dissolved CO₂. Around 5% CO₂ with appropriate bicarbonate concentration yields physiological pH. Step-by-Step Solution: Identify the buffer system: bicarbonate/CO₂.Recall standard practice: 5% CO₂ (commonly 4–6%).Select the option that encompasses this range: 2–5% CO₂. Verification / Alternative check: Most vendor datasheets recommend 5% CO₂ for bicarbonate-buffered formulations, confirming the 2–5% range as correct. Why Other Options Are Wrong: 1–10%: overly broad; above about 7–8% would acidify most standard media.10–15% or 15–20%: typically too high and will depress pH.0%: insufficient for bicarbonate buffering; pH drifts upward. Common Pitfalls: Using the same CO₂ for all media; HEPES-buffered media can tolerate lower CO₂, but bicarbonate-based media need about 5% CO₂. Final Answer: 2–5% CO₂.

Question 7

In mammalian cell culture, what is the principal effect of lactate accumulation on the culture environment and cell health?

Accepted Answer

Introduction / Context: Cells metabolize glucose through glycolysis and, when flux is high or oxygen is limiting, produce lactate. In closed systems, lactate accumulation acidifies the medium and can impair growth and productivity, a central concern in bioprocess optimization. Given Data / Assumptions: Lactate is an acidic metabolite. Accumulation occurs under high glycolytic rates or suboptimal aeration. We consider standard bicarbonate-buffered media. Concept / Approach: Lactate carries protons into the medium. As its concentration rises, buffering capacity is exceeded and pH falls. Lower pH stresses cells, alters enzyme activities, affects membrane transport, and can trigger apoptosis or growth arrest. Step-by-Step Solution: Acknowledge lactate production during high glucose consumption.Relate lactate buildup to increased acidity (lower pH).Connect low pH to reduced viability and performance. Verification / Alternative check: Industrial fed-batch processes control glucose feed and base addition to limit lactate formation and maintain pH, confirming its negative impact. Why Other Options Are Wrong: Increase in pH: opposite of reality for lactate.No change: contradicted by buffer chemistry and experience.No loss of viability: incorrect at elevated lactate for extended periods.Alkalinization: incorrect. Common Pitfalls: Assuming pH control alone eliminates lactate toxicity; even at corrected pH, high lactate can still inhibit growth. Final Answer: Reduction of pH leading to loss of cell viability over time.

Question 8

Embryo culture methods in developmental biology and reproductive biotechnology include which of the following approaches?

Accepted Answer

Introduction / Context: Embryo culture encompasses techniques to grow early embryos or embryonic tissues outside the organism to study development, rescue embryos, or manipulate genetics. Laboratories employ multiple formats depending on species and stage. Given Data / Assumptions: Cultures can be tissue-based (organ explants) or whole-embryo systems. Support matrices include plasma clots and agar gels, among others. Objective is to sustain viability and morphogenesis. Concept / Approach: Plasma clot and agar provide scaffolding and localized nutrients for organ explants. Whole-embryo cultures use rolling bottles or static systems with carefully defined gas and nutrient conditions to support growth and differentiation. Step-by-Step Solution: List established techniques: plasma clot, agar, whole embryo.Confirm each is documented and widely used.Select the inclusive option “All of these.” Verification / Alternative check: Protocols for rodent and avian embryos describe all three modalities, validating their use. Why Other Options Are Wrong: Each single method alone is incomplete.“None” contradicts standard practice. Common Pitfalls: Thinking embryo culture is limited to one matrix; optimal choice depends on developmental stage and species. Final Answer: All of these.

Question 9

Process control in bioreactors: Which feeding strategy helps prevent excessive lactate generation in mammalian cell culture?

Accepted Answer

Introduction / Context: In fed-batch mammalian cultures, excessive glycolytic flux leads to lactate accumulation, which harms growth and productivity. Process engineers therefore modulate substrate feeds to steer metabolism. Given Data / Assumptions: Lactate primarily arises from glucose catabolism via glycolysis. Glutamine catabolism produces ammonium, a different inhibitory metabolite. Objective is to minimize lactate buildup without starving cells. Concept / Approach: Keeping extracellular glucose at modest levels (for example, 0.5–2 g/L) reduces overflow metabolism and favors oxidative pathways. Feedback-controlled feeding maintains steady, lower glucose concentrations, decreasing lactate formation while sustaining growth. Step-by-Step Solution: Identify the metabolite of concern: lactate from glucose.Control the precursor: lower extracellular glucose concentration by controlled feeding.Select the option that directly addresses glucose moderation. Verification / Alternative check: Publications and industrial guidelines show lactate reduction when glucose is maintained at low, non-saturating levels, often combined with pH/base control and alternative carbon sources. Why Other Options Are Wrong: Low glutamine: primarily impacts ammonium, not lactate.High glutamine or high glucose: exacerbate by-product accumulation.Large bolus pulses: promote metabolic overflow and by-product spikes. Common Pitfalls: Over-restricting glucose so cells starve; controlled feeding should sustain but not saturate. Final Answer: Maintain low glucose concentration (controlled feed).

Question 10

Beyond pH control, carbon dioxide (CO₂) contributes to mammalian cell culture metabolism in several ways. Which statements describe its roles?

Accepted Answer

Introduction / Context: CO₂ is not merely an incubator setting; it is a metabolically relevant molecule in cultured cells. Through the bicarbonate buffer system and carboxylation reactions, CO₂ affects biosynthesis and homeostasis. Given Data / Assumptions: Bicarbonate-buffered media are used. Cells require balanced carbon sources and proper pH. Standard incubators regulate CO₂ around 5%. Concept / Approach: CO₂ dissolves to form carbonic acid, establishing the bicarbonate buffer that sets pH. It also serves as a carbon donor in carboxylation and contributes to biosynthetic pathways, including nucleotide synthesis. Proper CO₂ tension maintains physiological conditions for respiration and enzyme activities. Step-by-Step Solution: Recognize CO₂ as part of the buffer system (pH control and monitoring).Note its biochemical role in one-carbon metabolism and nucleotide biosynthesis.Acknowledge its role in gas exchange and respiration equilibria. Verification / Alternative check: Media datasheets and biochemistry texts describe both buffering and metabolic roles for CO₂/HCO₃⁻ systems, confirming the comprehensive answer. Why Other Options Are Wrong: Any single role alone is incomplete; “none” contradicts standard cell biology. Common Pitfalls: Viewing CO₂ as a pH knob only; neglecting its integration into cellular carbon flux. Final Answer: All of the above.

Question 11

Bioprocess by-products: What is the effect of excess lactate and ammonium accumulation on mammalian cells grown in vitro?

Accepted Answer

Introduction / Context: Lactate (from glycolysis) and ammonium (from glutamine deamidation) are common inhibitory by-products in mammalian bioprocessing. Their control is crucial for stable cultures and high product yields. Given Data / Assumptions: Both metabolites can accumulate during high-density culture. We consider typical pH and osmolality ranges used in industry. Cells are sensitive to metabolic waste buildup. Concept / Approach: Lactate lowers pH and perturbs energy metabolism; ammonium alters intracellular pH regulation, impairs glycosylation, and inhibits key enzymatic steps. Together they decrease specific growth rate and recombinant protein productivity. Step-by-Step Solution: Identify the metabolites and their sources (glucose, glutamine).Associate each with known inhibitory effects on cell physiology.Choose the option that captures their net effect: growth inhibition. Verification / Alternative check: Optimization strategies (controlled feeding, glutamine substitutes, pH control, perfusion) specifically target lowering lactate and ammonium, confirming their inhibitory nature. Why Other Options Are Wrong: Growth promotion/no effect: contradicted by extensive literature and practice.Opposite effects split by metabolite: both are inhibitory when excessive. Common Pitfalls: Assuming that pH correction eliminates all inhibition; intracellular effects persist even at corrected bulk pH. Final Answer: They act as growth inhibitors that reduce proliferation and productivity.

Question 12

Oxygen management in cell culture: Excessively high dissolved oxygen (DO) levels in mammalian culture media are toxic. What downstream effects can occur?

Accepted Answer

Introduction / Context: While oxygen is essential, too much dissolved oxygen can generate reactive oxygen species (ROS), leading to oxidative stress. Bioreactor control of DO is therefore critical to protect cells and product quality. Given Data / Assumptions: We consider mammalian cells in agitated/aerated systems. High DO elevates ROS formation. Cells and media components are susceptible to oxidation. Concept / Approach: ROS attack nucleic acids (causing DNA damage), lipids (causing peroxidation), and proteins. Oxidation of media components and elevated respiration can also accelerate nutrient turnover, creating imbalances and additional stress. Step-by-Step Solution: Relate high DO to ROS generation.Map ROS to molecular damage: DNA and lipids.Recognize process-level consequences: altered nutrient metabolism. Verification / Alternative check: Guidelines recommend moderate DO setpoints (for example, 30–50% air saturation) and careful aeration/antifoam use, reflecting the risks listed. Why Other Options Are Wrong: Single-effect options are incomplete; “none” contradicts oxidative stress biology. Common Pitfalls: Over-aeration when trying to avoid oxygen limitation; better to combine moderate DO with efficient mixing and appropriate feed strategies. Final Answer: All of the above.

Question 13

In bicarbonate-buffered culture media, what primarily sets and stabilizes the initial pH of the medium under standard incubator conditions?

Accepted Answer

Introduction / Context: Effective pH control is essential for enzyme function, membrane transport, and product quality in mammalian cell culture. Most common media rely on the CO₂/HCO₃⁻ buffer system for physiological pH. Given Data / Assumptions: Media contain sodium bicarbonate at defined concentrations. Incubators provide controlled CO₂ (about 5%). We refer to the initial pH set before major metabolic shifts occur. Concept / Approach: According to the Henderson–Hasselbalch relationship, the ratio of bicarbonate to dissolved CO₂ determines pH. The medium’s bicarbonate content is the primary buffer; CO₂ sets the gas equilibrium component. Together they stabilize pH near 7.2–7.4 without continuous base addition. Step-by-Step Solution: Identify the buffer present: bicarbonate system in the medium.Recognize CO₂ as the gas-phase partner of the buffer, not the buffer itself.Select the option naming the bicarbonate buffer as the primary pH control. Verification / Alternative check: Media datasheets specify bicarbonate concentration and recommended CO₂ level to achieve the target pH, confirming buffer primacy. Why Other Options Are Wrong: CO₂ alone: needs bicarbonate to form a functional buffer.Strong base addition: used for adjustment, not continuous control.No control or nitrogen flushing: unrelated to establishing physiological pH. Common Pitfalls: Using mismatched CO₂ level for a given bicarbonate concentration, causing drift toward acidity or alkalinity. Final Answer: Presence of the bicarbonate buffer system in the medium.

Question 14

Cell dissociation in vitro In animal and plant tissue culture, which methods can be used to disaggregate (separate) intact tissue into single, viable cells for primary culture initiation?

Accepted Answer

Introduction / Context: Establishing primary cultures from tissues requires breaking cell–cell and cell–matrix connections to yield viable single cells. This question tests knowledge of standard laboratory strategies for tissue disaggregation in cell and tissue culture. Given Data / Assumptions: Tissue pieces contain cells joined by adhesion molecules and extracellular matrix. Goal is high viability and adequate yield of single cells. Common laboratory tools and reagents are available (pipettes, sieves, enzymes, chelators). Concept / Approach: Disaggregation typically combines mechanical force to reduce tissue size, enzymes to digest proteins or polysaccharides in the matrix, and chelating agents to lower divalent cations that stabilize adhesions. The optimal mix depends on tissue type (animal vs. plant) and desired cell phenotype. Step-by-Step Solution: Start with physical disruption: mince tissue with sterile blades; pass through meshes to increase surface area.Apply enzymatic digestion: use trypsin or collagenase for animal tissues; cellulase/pectinase for plant tissues to loosen walls and matrix.Use chelating agents: EDTA or EGTA to bind Ca^2+/Mg^2+, weakening cadherin/integrin-mediated adhesions and improving enzyme access.Gently triturate to complete dissociation and filter to remove clumps. Verification / Alternative check: Protocols for hepatocytes, fibroblasts, neurons, and plant protoplasts consistently employ combinations of the three approaches for optimal yield and viability. Why Other Options Are Wrong: Physical disruption only: often insufficient and harsher forces reduce viability. Enzymatic digestion only: slower and less effective without size reduction and chelation. Chelation only: weakens adhesions but rarely achieves full dissociation alone. Antibiotics: control contamination but do not dissociate tissue. Common Pitfalls: Over-digestion (loss of surface receptors), excessive shear stress, or skipping chelation leading to persistent clumping. Final Answer: All of the above

Animal Cell Culture and Regulation Questions