18 Questions
Why is sodium hydrogencarbonate added before performing the Benedict’s Test?
To neutralize the test tube
Which color change in Benedict’s Reagent indicates the presence of a non-reducing sugar?
Blue to brick red (orange-brown)
What does a blue/black color change in a solution treated with iodine/potassium iodide indicate?
Presence of starch
What types of lipids are soluble in organic solvents such as alcohols?
Triglycerides and phospholipids
How are triglycerides formed?
By condensation reactions between glycerol and fatty acids
What differentiates saturated from unsaturated lipids?
Unsaturated lipids have no carbon-carbon double bonds
Which polysaccharide is an unbranched chain of glucose molecules joined by 1, 4 glycosidic bonds?
Amylose
What component of plant cells is composed of long, unbranched chains of beta glucose?
Cellulose
What chemical test can be used to test for the presence of reducing sugars?
Benedict's reagent
Which molecule stores a lot of energy due to its coiled structure?
Amylose
What is the function of cellulose in plant cells?
Turgidity maintenance
What type of bonds join the glucose molecules in amylopectin?
Both 1, 4 and 1, 6 glycosidic bonds
What is the purpose of Benedict's Reagent in the Benedict's Test?
To identify reducing sugars.
In the Benedict's Test, what indicates a positive result for the presence of a reducing sugar?
Solution turns brick red
What happens when a non-reducing sugar is tested using the Benedict's Test?
The solution remains blue.
What role does dilute hydrochloric acid play in testing for non-reducing sugars using the Benedict's Test?
To hydrolyze disaccharides and polysaccharides into monosaccharides
Which of the following is true about polysaccharides in relation to the Benedict's Test?
Polysaccharides need to be hydrolyzed before testing with Benedict's Reagent.
What is the main function of Copper(II) Sulfate in the Benedict's Reagent?
To donate an electron to reducing sugars
Study Notes
Reducing Sugars and Benedict's Test
- Monosaccharides and some disaccharides (e.g. maltose) are reducing sugars that can donate an electron to Benedict's Reagent.
- Benedict's Reagent is an alkaline solution of Copper(II) Sulfate.
- When a reducing sugar is added to Benedict's Reagent and heated, it forms an insoluble red precipitate (copper(I) oxide).
- The Benedict's Test involves adding 2cm3 of food sample and 2cm3 of Benedict's Reagent, heating gently in a water bath, and observing a colour change to brick red (orange-brown) if a reducing sugar is present.
Non-Reducing Sugars and Modified Benedict's Test
- Some disaccharides and polysaccharides are non-reducing and require a modified Benedict's Test.
- The modified test involves hydrolysing disaccharides and polysaccharides into monosaccharides using dilute hydrochloric acid.
- Sodium hydrogencarbonate is added to neutralise the solution before retesting with Benedict's Reagent.
Lipids
- Lipids are biological molecules made of carbon, hydrogen, and oxygen, soluble in organic solvents like alcohols.
- Main lipid types are triglycerides and phospholipids.
- Triglycerides are lipids made of one glycerol molecule and three fatty acids joined by ester bonds.
- Fatty acids vary in chain length, presence, and number of double bonds, making them saturated or unsaturated.
Starch and Cellulose
- Starch is a mixture of two polysaccharides, amylose and amylopectin, storing energy in plants.
- Amylose is an unbranched chain of glucose molecules, while amylopectin is branched, allowing for simultaneous breakdown by enzymes.
- Starch is insoluble, compact, and easily hydrolysed to release energy.
- Cellulose is a component of plant cell walls, composed of long, unbranched chains of beta glucose, providing structural rigidity and stopping cell wall bursting under osmotic pressure.
Learn about how the Benedict’s reagent is used to test for reducing sugars in food samples. Understand the process of forming an insoluble red precipitate when a reducing sugar reacts with the reagent.
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