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Unit notes and practice questions

Grade 10 Biology unit 3: Biochemical Molecules

Everything here is for unit 3 of the Grade 10 Biology textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

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10
Practice questions on unit 3
50–80
The textbook pages for unit 3

Short notes for unit 3

The cards filed under this chapter on the Grade 10 Biology page. Open one at a time.

Target NutrientTest or ReagentPositive Result
Reducing sugarsBenedict's reagent + heatBrick-red precipitate
StarchIodine solutionBlue-black colour
ProteinsBiuret reagentMauve or purple colour
Aromatic amino acidsXanthoproteic test (concentrated HNO3\text{HNO}_3 + NaOH\text{NaOH})Yellow to orange colour

When you use it

Use to identify laboratory procedures and expected colour changes for biological macromolecules.

Watch out

Benedict's test requires heating in a water bath to show the brick-red colour change, while Iodine and Biuret tests occur at room temperature.

Drafted from Grade 10 Biology, pages 50-80, then checked twice before it went up

DisaccharideMonomer CompositionKey Source or Role
MaltoseGlucose+Glucose\text{Glucose} + \text{Glucose}Malt sugar, starch digestion product
LactoseGlucose+Galactose\text{Glucose} + \text{Galactose}Milk sugar
SucroseGlucose+Fructose\text{Glucose} + \text{Fructose}Transport sugar in plants, table sugar

When you use it

Use to recall the specific hexose combinations that form common disaccharides.

Watch out

All three disaccharides contain glucose. The second monomer differs: glucose in maltose, galactose in lactose, fructose in sucrose.

Drafted from Grade 10 Biology, pages 50-80, then checked twice before it went up

FeatureDNARNA
Pentose sugarDeoxyriboseRibose
Nitrogenous basesA, T, C, GA, U, C, G
Number of strandsDouble-strandedSingle-stranded
StabilityHighly stableLess stable

When you use it

Use when distinguishing the structural and functional differences between the two nucleic acids.

Watch out

Thymine is exclusive to DNA, while Uracil is exclusive to RNA. Deoxyribose has one fewer oxygen atom than ribose.

Drafted from Grade 10 Biology, pages 50-80, then checked twice before it went up

BiomoleculeConstituent ElementsMonomer Unit
ProteinsC, H, O, NAmino acids
LipidsC, H, OGlycerol, fatty acids
CarbohydratesC, H, OMonosaccharides
Nucleic acidsC, H, O, N, PNucleotides

When you use it

Use to identify constituent chemical elements and basic building blocks of organic macromolecules.

Watch out

Lipids are formed from glycerol and fatty acids, but they do not form continuous single-monomer chains like true polymers.

Drafted from Grade 10 Biology, pages 50-80, then checked twice before it went up

When you use it

Use when predicting Benedict's test outcomes for different carbohydrate samples.

Watch out

Sucrose is a non-reducing disaccharide and gives a negative (blue) Benedict's test result directly, unlike glucose, maltose, and lactose.

Drafted from Grade 10 Biology, pages 50-80, then checked twice before it went up

Practice questions on unit 3

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

According to the textbook, what percentage of a cell's mass is made up of water?

  1. A.70 to 95 percent
  2. B.20 to 35 percent
  3. C.40 to 50 percent
  4. D.10 to 15 percent
Show the answer and why

Answer: A 70 to 95 percent

Why

Water is the main component of cells, accounting for 70 to 95 percent of a cell's total mass.

Question 2

Which four chemical elements are found in all protein molecules?

  1. A.Carbon, hydrogen, oxygen, and nitrogen
  2. B.Carbon, hydrogen, phosphorus, and sulfur
  3. C.Carbon, oxygen, calcium, and sodium
  4. D.Carbon, nitrogen, potassium, and chlorine
Show the answer and why

Answer: A Carbon, hydrogen, oxygen, and nitrogen

Why

Proteins are polymers of amino acids and are always composed of carbon, hydrogen, oxygen, and nitrogen (C, H, O, and N).

Question 3

What is the fixed ratio of hydrogen to oxygen atoms in carbohydrates?

  1. A.2:1
  2. B.1:1
  3. C.1:2
  4. D.3:1
Show the answer and why

Answer: A 2:1

Why

In carbohydrates, the ratio of hydrogen atoms to oxygen atoms is always 2:1, just like in water.

Question 4

Which nitrogenous base is present in RNA molecules but absent in DNA?

  1. A.Uracil
  2. B.Thymine
  3. C.Adenine
  4. D.Cytosine
Show the answer and why

Answer: A Uracil

Why

RNA contains uracil instead of thymine. DNA contains adenine, guanine, cytosine, and thymine.

Question 5

Which inorganic ion is needed in the human stomach to produce hydrochloric acid (HCl) and also activates salivary amylase?

  1. A.Chloride ion
  2. B.Potassium ion
  3. C.Calcium ion
  4. D.Phosphate ion
Show the answer and why

Answer: A Chloride ion

Why

Chloride ions are needed to make hydrochloric acid (HCl) in gastric juice and are also responsible for activating the starch-digesting enzyme salivary amylase.

Question 6

Which rare medical condition is caused by abnormal copper deposition in the liver and brain, leading to liver cirrhosis and brain damage?

  1. A.Wilson disease
  2. B.Hemochromatosis
  3. C.Hemosiderosis
  4. D.Goiter
Show the answer and why

Answer: A Wilson disease

Why

Wilson disease is caused by abnormal copper metabolism, leading to copper deposits in the liver, brain, and kidneys.

Question 7

Which common disaccharide is formed by chemically joining one molecule of glucose and one molecule of fructose?

  1. A.Sucrose
  2. B.Maltose
  3. C.Lactose
  4. D.Galactose
Show the answer and why

Answer: A Sucrose

Why

Sucrose is composed of glucose and fructose. Maltose is glucose + glucose, and lactose is glucose + galactose.

Question 8

Why do unsaturated fatty acids typically remain liquid oils at room temperature?

  1. A.They contain one or more double bonds in their carbon chains.
  2. B.They contain the maximum possible number of hydrogen atoms.
  3. C.They contain only single carbon-to-carbon bonds.
  4. D.They have three polar phosphate heads attached.
Show the answer and why

Answer: A They contain one or more double bonds in their carbon chains.

Why

Unsaturated fatty acids contain one or more double bonds (C=C), which make the fatty acids melt more easily, keeping them liquid at room temperature.

Question 9

Which of the following is a tough, water-insoluble fibrous protein that makes up hair, nails, and the outer layers of skin?

  1. A.Keratin
  2. B.Haemoglobin
  3. C.Myoglobin
  4. D.Ovalbumin
Show the answer and why

Answer: A Keratin

Why

Keratin is an insoluble fibrous structural protein that forms hair, nails, and the outer waterproof layers of the skin.

Question 10

A student in Addis Ababa tests a sugar solution using Benedict's reagent. What chemical reaction produces the positive brick-red result?

  1. A.Reducing sugar converts soluble blue copper (II) to insoluble brick-red copper (I) oxide.
  2. B.Non-reducing sugar oxidizes copper (I) into soluble blue copper (II) ions.
  3. C.Iodine combines with peptide bonds in an alkaline solution to form a red complex.
  4. D.Nitric acid reacts with aromatic amino acids to produce a red precipitate.
Show the answer and why

Answer: A Reducing sugar converts soluble blue copper (II) to insoluble brick-red copper (I) oxide.

Why

During the Benedict's test, reducing sugars reduce soluble blue copper (II) ions into an insoluble brick-red precipitate of copper (I) oxide.

The app holds 12 questions on this unit, marked as you go. These are the first 10.

The textbook pages for unit 3

Unit 3 runs from page 50 to page 80 of the Ministry of Education edition.

Ministry of Education edition, English, 177 pages

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All Grade 10 Biology short notes

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