Which reagent is used to detect the presence of starch?
Iodine
Biuret
Benedict's
Phenol red
Sudan IV
The Correct Answer is A
Choice A rationale: Iodine is a chemical element that forms a brown solution of iodine and potassium iodide, known as iodine solution. When this solution is added to a sample that contains starch, it forms a complex with the starch molecules, which changes the color of the solution to blue-black. This is based on the fact that starch is a polysaccharide that has a helical structure, which can trap the iodine molecules inside. ²
Choice B rationale: Biuret is a chemical compound that forms a blue solution of copper (II) sulfate and sodium hydroxide, known as biuret reagent. When this reagent is added to a sample that contains proteins or peptides, it forms a complex with the copper (II) ions, which changes the color of the solution to violet or pink. This is based on the fact that proteins and peptides have peptide bonds, which have nitrogen atoms that can coordinate with the copper (II) ions. ³
Choice C rationale: Benedict's is a chemical compound that forms a blue solution of copper (II) sulfate, sodium carbonate, and sodium citrate, known as Benedict's reagent. When this reagent is heated with a sample that contains reducing sugars, such as glucose or maltose, it reduces the copper (II) ions to copper (I) ions, which form a red, orange, or green precipitate of copper (I) oxide. This is based on the fact that reducing sugars have free aldehyde or ketone groups that can donate electrons to the copper (II) ions. ⁴
Choice D rationale: Phenol red is a chemical compound that forms a red solution that is used as a pH indicator. When this solution is added to a sample that has an acidic or neutral pH, it remains red or turns yellow. When this solution is added to a sample that has an alkaline pH, it turns pink or fuchsia. This is based on the fact that phenol red has a sulfonated hydroxyquinone group that can lose or gain protons depending on the pH of the solution. ⁵
Choice E rationale: Sudan IV is a chemical compound that forms a red powder that is used as a stain for lipids. When this powder is dissolved in a solvent and added to a sample that contains lipids, such as fats or oils, it dissolves in the lipids and stains them red. When this solution is added to a sample that does not contain lipids, it remains in the solvent and does not stain the sample. This is based on the fact that Sudan IV is a nonpolar compound that can dissolve in nonpolar substances like lipids, but not in polar substances like water. ⁶.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Diffusion is a passive process that does not require energy input from the cell. It is driven by the random motion of molecules that results in a net movement from regions of higher concentration to regions of lower concentration.
Diffusion is one of the ways that substances can cross the cell membrane, such as oxygen, carbon dioxide, and water. Diffusion is important for cellular functions, such as gas exchange, nutrient uptake, and waste removal. Diffusion also helps maintain the homeostasis of the cell by balancing the concentration gradients of different molecules. Therefore, diffusion is the movement of a molecule from an area of higher concentration to an area of lower concentration.
Correct Answer is E
Explanation
Choice A rationale: The presence of chloroplasts is incorrect because chloroplasts are organelles that perform photosynthesis, not osmosis. Chloroplasts do not affect the water balance of the cell.
Choice B rationale: A plant cell will not burst in a hypotonic solution because water is moving out of the cell is incorrect because water moves into the cell in a hypotonic solution, not out of it. A hypotonic solution has a lower solute concentration than the cell, so water flows from the solution to the cell by osmosis.
Choice C rationale: Chloroplasts that help pump the excess water out of the cell is incorrect because chloroplasts do not have any role in pumping water out of the cell. The cell uses active transport to pump out excess water, which requires energy from ATP, not chloroplasts.
Choice D rationale: The plasma membrane is incorrect because the plasma membrane is permeable to water, so it cannot prevent water from entering the cell. The plasma membrane only regulates the passage of solutes, not water.
Choice E rationale: The cell wall is correct because the cell wall is a rigid structure that surrounds the plasma membrane and provides mechanical support to the cell. The cell wall can withstand the pressure of water entering the cell and prevent the cell from bursting. The cell wall is made of cellulose, a polysaccharide that is resistant to water.
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