Which of the following tests is used to detect the presence of lipids?
Biuret test
Gram's iodine test
Ninhydrin test
Brown paper test
Benedict's test
The Correct Answer is D
Choice A reason: Biuret test is a test for the presence of proteins or peptides in a solution. It involves adding Biuret reagent, which is a blue solution of copper (II) sulfate and sodium hydroxide, to the solution. If proteins or peptides are present, they form a complex with the copper (II) ions, which changes the color of the solution to violet or pink. ³
Choice B reason: Gram's iodine test is a test for the presence of starch in a solution. It involves adding iodine solution, which is a brown solution of iodine and potassium iodide, to the solution. If starch is present, it forms a complex with the iodine molecules, which changes the color of the solution to blue-black. ⁴
Choice C reason: Ninhydrin test is a test for the presence of amines or amino acids in a solution. It involves adding ninhydrin reagent, which is a purple solution of ninhydrin, to the solution. If amines or amino acids are present, they react with ninhydrin to produce a purple color, often called Ruhemann's purple. ¹
Choice D reason: Brown paper test is a test for the presence of lipids, such as fats or oils, in a solution. The test involves placing a drop of the solution on a piece of brown paper and letting it dry. If lipids are present, they leave a translucent spot on the paper, which can be seen by holding the paper against a light source. The test is based on the fact that lipids are nonpolar and do not dissolve in water, but can dissolve in organic solvents and stain the paper. ²
Choice E reason: Benedict's test is a test for the presence of reducing sugars, such as glucose or maltose, in a solution. The test involves adding Benedict's reagent, which is a blue solution of copper (II) sulfate, sodium carbonate, and sodium citrate, to the solution and heating it in a water bath. If reducing sugars are present, they reduce the copper (II) ions to copper (I) ions, which form a red, orange, or green precipitate of copper (I) oxide. ⁵
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is D
Explanation
Choice A rationale: Proteins contain nitrogen, but this is not the reason why they cannot pass through plasma membranes. Nitrogen is a common element in many organic molecules, including nucleic acids and amino acids, which can cross the membrane under certain conditions.
Choice B rationale: Proteins do not cause emulsification, which is the process of breaking down large fat droplets into smaller ones. Emulsification is facilitated by bile salts, which are amphipathic molecules that have both hydrophilic and hydrophobic regions. Proteins are not amphipathic, and they do not interact with fats in this way.
Choice C rationale: The membrane is made of protein, but this does not prevent proteins from passing through it. The membrane is composed of a phospholipid bilayer with embedded proteins, which can act as channels, carriers, receptors, or enzymes for various substances. Some proteins can cross the membrane by using these transport proteins, or by endocytosis or exocytosis.
Choice D rationale: Proteins are very large molecules, and this is the main reason why they cannot pass through plasma membranes. The size of a molecule determines its permeability across the membrane, and proteins are too big to diffuse through the small gaps between the phospholipids or the pores of the transport proteins. Proteins can only cross the membrane by vesicular transport, which requires energy and specific signals.
Choice E rationale: Proteins do not bind to the phospholipids, which are the main components of the membrane. Phospholipids are also amphipathic molecules, with a hydrophilic head and a hydrophobic tail. Proteins are generally hydrophilic, and they do not associate with the hydrophobic core of the membrane. Proteins can bind to other proteins or carbohydrates on the surface of the membrane, but this does not affect their ability to cross it.
Correct Answer is A
Explanation
Choice A rationale: Diffusion is correct because it is the passive movement of molecules from an area of high concentration to an area of low concentration. Small lipid soluble molecules can easily cross the plasma membrane by diffusing through the hydrophobic core of the phospholipid bilayer.
Choice B rationale: Filtration is incorrect because it is the process of separating solid particles from a fluid by passing it through a porous medium. Filtration does not involve the plasma membrane, and it does not depend on the solubility of the molecules.
Choice C rationale: Osmosis is incorrect because it is the diffusion of water across a selectively permeable membrane. Osmosis does not apply to lipid soluble molecules, which are not water molecules.
Choice D rationale: Active transport is incorrect because it is the movement of molecules across a membrane against their concentration gradient, which requires energy and transport proteins. Active transport does not depend on the solubility of the molecules, and it is not a passive process.
Choice E rationale: Pumping is incorrect because it is a type of active transport that involves the use of specific pumps to move ions or molecules across a membrane. Pumping does not apply to lipid soluble molecules, which are not ions or polar molecules.
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