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#cell membranes

3 public questions tagged with this topic.

Basics of Cell Biology

Which cellular structure is associated with the Freeze-Fracture technique?

Freeze-fracture technique, developed by Branton and refined by Pinto da Silva, revolutionized visualization of membrane interior by physically splitting frozen lipid bilayers. Live cells are rapidly frozen in liquid propane at -180C, fractured with cold knife under high vacuum, fracture plane preferentially passes through hydrophobic interior of membranes where van der Waals forces are weakest, splitting bilayer into exoplasmic and protoplasmic leaflets exposing intramembrane particles representing integral proteins including band 3 anion exchanger, aquaporins, connexin gap junction hexamers, and rhodopsin. Platinum-carbon replica shadowing captures topography, original biological material dissolved with acid, replica examined by transmission EM revealing random or crystalline particle distribution, quantifiable density changes upon hormonal stimulation. While mitochondria cristae, nuclear pores, and even ribosome organization can be studied, classic application was plasma membrane demonstrating proteins embedded within lipid matrix, supporting Singer-Nicolson fluid mosaic, quantifying tight junction strand complexity, and visualizing exocytosis fusion rosettes, establishing direct structural evidence for mosaic protein arrangement.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Freeze-Fracture and Membrane Proteins.

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Properties of Plasma Membrane

Which of the following statements about membrane fluidity is true?

Regulation of fluidity reflects balance between order and disorder of fatty acyl chains governing membrane physical properties and protein function. Saturated acyl chains straight all trans conformation maximize van der Waals contacts small area per lipid about forty eight square angstroms low fluidity high transition temperature. Cis double bond introduces kink thirty degree bend reducing contact area to about sixty square angstroms increasing free volume lowering transition temperature fifty degrees increasing lateral diffusion coefficient and rotational freedom measured by fluorescence anisotropy diphenylhexatriene EPR order parameter. Polyunsaturated arachidonic docosahexaenoic more fluid. Longer chains raise Tm via more contacts decreasing fluidity. Temperature increase disorders chains raising fluidity; decreasing induces gel phase. Cholesterol biphasic modulates ordering. Cells homeoviscously adapt bacteria desaturase induction at low temperature, poikilotherms increased unsaturation winter. Therefore statement that membrane fluidity increases with unsaturated fatty acids accurately describes molecular basis of chain packing disruption enabling maintenance of functional liquid crystalline state required for permeability protein activity vesicular trafficking and signaling platform formation.

Ref: Singer and Nicolson, Fluid Mosaic Model and Fluidity Dependence on Unsaturation, Science 1972.

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Membrane Transportation

Which of the following ions is not actively transported in most cells?

Cellular ion homeostasis requires continuous energy expenditure to maintain steep gradients. Sodium gradient high outside 145 millimolar low inside 12 millimolar and potassium opposite 4 outside 140 inside established by Na+/K+ ATPase hydrolyzing ATP forming phosphoenzyme E1P E2P cycling three sodium out two potassium in per ATP. Calcium gradient extreme low cytosolic 100 nanomolar versus 1 to 2 millimolar extracellular maintained by plasma membrane Ca2+ ATPase PMCA and sarcoplasmic endoplasmic reticulum Ca2+ ATPase SERCA, both P-type pumps transporting two calcium per ATP against ten thousand fold gradient and thapsigargin sensitive. These gradients underlie excitability and signaling. Chloride in contrast distribution of about 110 millimolar extracellular versus 4 to 20 millimolar intracellular in many cells largely approximates passive Donnan equilibrium governed by membrane potential and net impermeable intracellular anions. Transport achieved via KCC potassium chloride cotransporter and NKCC sodium potassium two chloride secondary active, and ClC chloride channels providing conductance, but no mammalian primary ATP dependent pump directly hydrolyzes ATP to pump chloride uphill as main mechanism. Hence chloride considered not primary actively transported in most animal cells.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Ion Distributions - Cl- Passive vs Active Transport.

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