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#temperature effects

2 public questions tagged with this topic.

Properties of Plasma Membrane

Which factor is responsible for phase transitions in phospholipid bilayers?

Lipid phase behavior reflects cooperative melting of hydrocarbon lattice from ordered gel to disordered liquid crystalline phases. Transition temperature Tm defines midpoint where fifty percent of chains contain gauche kinks and lateral expansion occurs, measured by differential scanning calorimetry as sharp endothermic peak representing enthalpy change. Tm rises with longer saturated acyl length because more CH2 groups increase van der Waals energy and with high sphingolipid content forming hydrogen bonding network at amide and hydroxyl groups. Cis unsaturation introduces permanent thirty degree kink disrupting packing lowering Tm by thirty to fifty degrees per double bond. Cholesterol abolishes sharp transition by forming liquid ordered phase intermediate with broadened endotherm. Glycoproteins, integral proteins, ATP levels modulate fluidity via picket fencing and phosphorylation cascades but do not set intrinsic lipid Tm. Cells sense packing defects through Mga2 ubiquitin dependent processing and SREBP pathway regulating desaturases OLE1. Linking Tm to permeability for liposomal drug delivery, cryopreservation, and cold adaptation explains why phase transitions are central to membrane biophysics strongly tested in GATE and NET.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Phase transitions Tm and calorimetry.

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Gene Cloning Process

Insoluble expression of protein X at 37°C in E. coli is due to:

High-level expression of heterologous proteins at 37°C in Escherichia coli under strong promoters like T7 drives rapid polypeptide synthesis exceeding the capacity of endogenous chaperone systems such as GroEL-GroES and DnaK. Hydrophobic stretches improperly exposed interact intermolecularly, forming insoluble aggregates termed inclusion bodies sequestered in cytoplasm. Although protein is abundant, it is misfolded and functionally inactive. This aggregation is temperature and induction rate dependent rather than due to promoter absence, inducer failure, or cytotoxicity, and can be mitigated by lowering culture temperature or using solubility tags.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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