The Particles That Substances Are Made of

SECTION A: SHORT QUESTIONS

(Total: 30 marks)

A1: BIOLOGICAL TERMS

(Total: 4 marks)

1. The biological molecules composed of carbon, hydrogen, and oxygen in a ratio of 1:2:1, which serve as the primary energy source for living cells.

2. The large biological macromolecules made up of amino acid monomers linked by peptide bonds, which perform diverse functions including catalysis and structural support.

3. The process by which enzymes lose their three-dimensional shape and biological activity due to changes in temperature or pH.

4. The hydrophobic molecules that include fats, oils, and phospholipids, which are important components of cell membranes and energy storage.

A2: TRUE OR FALSE

(Total: 4 marks)

5. Nucleic acids such as DNA and RNA are composed of monomers called amino acids.

6. The lock-and-key model explains how enzymes are highly specific because the passive site has a complementary shape to the substrate.

7. Starch and glycogen are both polysaccharides that function as energy storage molecules in living organisms.

8. All enzymes are proteins, and they function optimally at any temperature and pH.

A3: MATCH THE COLUMN

(Total: 5 marks)

9. Match the biological molecules in Column A with their correct functions or characteristics in Column B. Write only the letter (A-E) next to the question number.

A4: MULTIPLE CHOICE

(Total: 17 marks)

10. Thandi conducted an experiment to investigate the effect of temperature on enzyme activity. She added the enzyme amylase to a starch solution at different temperatures and measured the time taken for the starch to be completely digested. At which temperature would the enzyme most likely denature?

A. 10°C

B. 25°C

C. 37°C

D. 75°C

11. A biology student tested four different food samples using the Benedict's test, iodine test, Biuret test, and ethanol emulsion test. Sample X turned brick-red with Benedict's reagent after heating. What type of biological molecule is present in Sample X?

A. Polysaccharide

B. Reducing sugar

C. Protein

D. Lipid

12. An enzyme isolated from a bacterium living in hot springs has an optimum temperature of 80°C. Which statement best explains why this enzyme differs from human enzymes?

A. The enzyme has a different primary structure that makes its three-dimensional shape more stable at high temperatures.

B. The enzyme does not have an active site.

C. The enzyme works faster at all temperatures.

D. The enzyme does not follow the lock-and-key model.

13. Sipho was investigating the digestion of proteins. He placed egg white (a protein) in a test tube with the enzyme pepsin at pH 2 and in another test tube with pepsin at pH 9. After 30 minutes, the protein was digested only in the test tube at pH 2. What is the most likely explanation for this observation?

A. Pepsin is denatured at pH 9 because the change in pH alters the shape of the active site.

B. Proteins cannot be digested at pH 9.

C. Pepsin works better in basic conditions.

D. The substrate concentration was too high at pH 9.

14. Which of the following correctly describes the relationship between monomers and polymers in biological macromolecules?

A. Glucose monomers link together to form proteins through peptide bonds.

B. Nucleotide monomers link together to form nucleic acids through phosphodiester bonds.

C. Amino acid monomers link together to form lipids through glycosidic bonds.

D. Fatty acid monomers link together to form carbohydrates through ester bonds.

SECTION B: DIAGRAM AND LABELLING QUESTIONS

(Total: 15 marks)

The diagram below represents a simplified model of an enzyme-substrate complex during a biochemical reaction. Study the figure carefully and answer the questions that follow.

15. (a) Identify the correct biological term for each of the labelled structures numbered 1 to 5.

15. (b) Explain the function of the structure labelled 2 in the process of enzyme catalysis.

15. (c) State the function of the structure labelled 4 during the biochemical reaction.

15. (d) Explain what would happen to the rate of product formation if the structure labelled 2 was altered due to high temperature.

SECTION C: STRUCTURED QUESTIONS

(Total: 25 marks)

QUESTION 16: CARBOHYDRATES AND ENERGY STORAGE

Nomsa is a Grade 10 learner who investigated the presence of different carbohydrates in plant and animal tissues. She collected samples from potato tubers, human liver tissue, and celery stalks. She performed the iodine test on all three samples.

16. (a) State the biological term for the polysaccharide that is stored in potato tubers.

16. (b) Describe the colour change Nomsa would observe when iodine solution is added to the potato sample if the polysaccharide is present.

16. (c) Explain why the celery stalk sample would not show the same colour change as the potato sample when tested with iodine solution, even though both are plant tissues.

16. (d) The human liver tissue contains a polysaccharide similar to starch but with a different structure. Name this polysaccharide and explain how its structure differs from starch in terms of branching.

16. (e) During vigorous exercise, the polysaccharide stored in the liver and muscles is broken down into glucose monomers. Explain the biological significance of this process for an athlete running a marathon.

QUESTION 17: ENZYME ACTIVITY AND FACTORS AFFECTING IT

Lerato conducted an experiment to investigate the effect of pH on the activity of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. She set up five test tubes, each containing the same concentration of hydrogen peroxide and catalase, but at different pH values: pH 3, pH 5, pH 7, pH 9, and pH 11. She measured the volume of oxygen gas produced in each test tube after 5 minutes. Her results are shown below:

pH 3: 2 cm³ oxygen produced

pH 5: 8 cm³ oxygen produced

pH 7: 15 cm³ oxygen produced

pH 9: 7 cm³ oxygen produced

pH 11: 1 cm³ oxygen produced

17. (a) Identify the optimum pH for catalase activity based on Lerato's results.

17. (b) Explain why very little oxygen was produced at pH 3 and pH 11.

17. (c) Define the term denaturation in the context of enzyme activity.

17. (d) Lerato's teacher asked her to predict what would happen if she repeated the experiment at pH 7 but at a temperature of 80°C instead of room temperature. Predict the outcome and justify your answer using your knowledge of enzyme structure and function.

17. (e) Catalase is found in nearly all living organisms, including the bacterium Escherichia coli and the plant Zea mays. Explain why this enzyme is so important for the survival of these organisms.

SECTION D: SCIENTIFIC ESSAY

(Total: 20 marks)

18. Write a scientific essay on the structure and function of proteins as biological macromolecules. Your essay must include the following:

1. A description of the monomers that make up proteins and how they are linked together to form polypeptide chains
2. An explanation of the four levels of protein structure: primary, secondary, tertiary, and quaternary
3. A discussion of at least three different functions of proteins in living organisms, with specific examples
4. An explanation of how the structure of a protein relates to its specific function, using the enzyme as an example
5. The use of correct binomial nomenclature for at least one organism in your examples

Mark allocation guide:

1. Description of amino acids as monomers and peptide bond formation (4 marks)
2. Explanation of the four levels of protein structure (6 marks)
3. Discussion of three functions with specific examples (6 marks)
4. Explanation of structure-function relationship using enzyme example (3 marks)
5. Correct use of binomial nomenclature (1 mark)

GRAND TOTAL: 90 marks

ANSWER KEY

Well done on completing this worksheet! This answer key will help you understand the correct responses and the biological reasoning behind them. Pay close attention to the terminology used, as using the correct biological terms is essential for achieving full marks in Life Sciences.

SECTION A1 - QUESTION 1

Carbohydrates

Carbohydrates are organic compounds with the general formula C_n(H₂O)_n, meaning they contain carbon, hydrogen, and oxygen in a 1:2:1 ratio. They serve as the primary energy source for cellular respiration.

SECTION A1 - QUESTION 2

Proteins

Proteins are polymers composed of amino acid monomers joined by peptide bonds. They have diverse functions including enzymatic catalysis, structural support, transport, defense, and regulation.

SECTION A1 - QUESTION 3

Denaturation

Denaturation is the irreversible change in the three-dimensional structure of a protein, particularly affecting the secondary and tertiary structures, which results in loss of biological function. This can be caused by extreme temperature, pH changes, or chemical agents.

SECTION A1 - QUESTION 4

Lipids

Lipids are hydrophobic biological molecules that are insoluble in water but soluble in non-polar solvents. They include triglycerides, phospholipids, and steroids, and function in energy storage, membrane structure, and signaling.

SECTION A2 - QUESTION 5

FALSE

The corrected word is nucleotides.

Nucleic acids such as DNA and RNA are polymers composed of nucleotide monomers, not amino acids. Each nucleotide consists of a pentose sugar, a phosphate group, and a nitrogenous base.

SECTION A2 - QUESTION 6

FALSE

The corrected phrase is active site.

The lock-and-key model states that the active site of an enzyme has a specific three-dimensional shape that is complementary to the shape of the substrate, ensuring enzyme specificity.

SECTION A2 - QUESTION 7

TRUE

Starch is the energy storage polysaccharide in plants, while glycogen serves the same function in animals. Both are polymers of glucose monomers linked by glycosidic bonds.

SECTION A2 - QUESTION 8

FALSE

The corrected phrase is specific optimum temperature and pH.

Each enzyme has a specific optimum temperature and pH at which it functions most efficiently. Deviations from these optimal conditions can reduce enzyme activity or cause denaturation.

SECTION A3 - QUESTION 9

9.1: Cellulose is a structural polysaccharide that forms the rigid cell wall in plant cells, providing mechanical support and protection.

9.2: Enzymes are biological catalysts that lower the activation energy of biochemical reactions, speeding up the rate of reaction without being consumed.

9.3: Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails, which arrange themselves into a bilayer to form the cell membrane.

9.4: DNA (deoxyribonucleic acid) is the nucleic acid that stores genetic information in the form of genes located on chromosomes in the nucleus.

9.5: Glycogen is a highly branched polysaccharide stored primarily in the liver and skeletal muscles of animals, serving as a readily accessible glucose reserve.

SECTION A4 - QUESTION 10

D. 75°C

At 75°C, the enzyme amylase would denature because this temperature is far above the optimum temperature for most human enzymes (around 37°C). High temperatures disrupt the hydrogen bonds and other weak interactions maintaining the tertiary structure of the enzyme, causing the active site to lose its specific shape.

SECTION A4 - QUESTION 11

B. Reducing sugar

The Benedict's test is specific for detecting reducing sugars such as glucose, fructose, and maltose. When heated with Benedict's reagent, reducing sugars cause a color change from blue to brick-red due to the reduction of copper(II) ions to copper(I) oxide.

SECTION A4 - QUESTION 12

A. The enzyme has a different primary structure that makes its three-dimensional shape more stable at high temperatures.

Thermophilic bacteria have enzymes with different amino acid sequences (primary structure) that result in more stable tertiary structures through additional ionic bonds and disulfide bridges. This allows the enzyme to maintain its functional shape at high temperatures where human enzymes would denature.

SECTION A4 - QUESTION 13

A. Pepsin is denatured at pH 9 because the change in pH alters the shape of the active site.

Pepsin is a proteolytic enzyme found in the stomach that functions optimally at acidic pH (around pH 2). At pH 9, which is alkaline, the ionization of amino acids in the enzyme changes, disrupting the ionic and hydrogen bonds that maintain the active site shape, leading to denaturation and loss of catalytic activity.

SECTION A4 - QUESTION 14

B. Nucleotide monomers link together to form nucleic acids through phosphodiester bonds.

Nucleic acids (DNA and RNA) are polymers of nucleotide monomers. The phosphate group of one nucleotide forms a covalent bond with the pentose sugar of the next nucleotide, creating a phosphodiester bond. This forms the sugar-phosphate backbone of nucleic acids.

SECTION B - QUESTION 15(a)

1. Enzyme

2. Active site

3. Substrate

4. Enzyme-substrate complex

5. Products

Each term must be spelled correctly and be the specific biological term. Accept "active centre" as an alternative to "active site".

SECTION B - QUESTION 15(b)

The active site is the specific region on the enzyme where the substrate binds due to its complementary shape. The active site contains specific amino acids that interact with the substrate, positioning it correctly and lowering the activation energy required for the reaction to occur.

Required for full marks: Must mention "active site", "complementary shape" or "specific shape", and either "lowers activation energy" or "facilitates the reaction".

SECTION B - QUESTION 15(c)

The enzyme-substrate complex is the temporary structure formed when the substrate binds to the active site of the enzyme. This complex holds the substrate in the optimal orientation for the chemical bonds to be broken or formed, facilitating the conversion of substrate into products.

Required for full marks: Must mention "temporary structure", "substrate binds to enzyme/active site", and "facilitates conversion to products" or "allows reaction to occur".

SECTION B - QUESTION 15(d)

If the active site is altered due to high temperature, the enzyme undergoes denaturation. The three-dimensional shape of the active site changes, and it is no longer complementary to the substrate. The substrate cannot bind effectively to the enzyme, so the enzyme-substrate complex cannot form. This results in a dramatic decrease or complete cessation of product formation, as the enzyme loses its catalytic function.

Full marks: Must include denaturation, change in active site shape, substrate cannot bind, and decreased/no product formation.

Partial marks: Mentions denaturation and reduced product formation but lacks explanation of why substrate cannot bind.

SECTION C - QUESTION 16(a)

Starch

Starch is the storage polysaccharide found in plants, particularly in organs such as tubers, roots, and seeds.

SECTION C - QUESTION 16(b)

The iodine solution would change from yellow-brown to blue-black (or dark blue).

This color change indicates the presence of starch, as iodine molecules fit into the helical structure of the starch polymer.

Required for full marks: Both the initial color (yellow-brown or brown) and the final color (blue-black or dark blue) must be mentioned.

SECTION C - QUESTION 16(c)

The celery stalk would not show the same color change because it contains cellulose, not starch. While both are polysaccharides made of glucose monomers, cellulose is a structural polysaccharide with β-glucose units linked by β-1,4-glycosidic bonds, forming straight chains. Iodine does not react with cellulose because it cannot fit into the linear structure the way it fits into the helical structure of starch.

Required for full marks: Must mention "cellulose" (not starch), "structural function" or "cell wall component", and explanation that iodine does not react with cellulose structure.

SECTION C - QUESTION 16(d)

The polysaccharide is glycogen.

Glycogen has a similar structure to starch but is more highly branched than amylopectin (the branched component of starch). Glycogen has branches occurring approximately every 8-12 glucose units, whereas amylopectin has branches every 20-25 glucose units. This extensive branching allows for more rapid mobilization of glucose when energy is needed.

Required for full marks: Must name "glycogen", state "more highly branched" or "more branches", and ideally include functional significance.

SECTION C - QUESTION 16(e)

During vigorous exercise, muscle cells require large amounts of energy in the form of ATP for muscle contraction. The breakdown of glycogen into glucose monomers through the process of glycogenolysis provides a rapid source of glucose for cellular respiration. The glucose is then oxidized through glycolysis and the Krebs cycle to produce ATP. This process is biologically significant because it allows the athlete to maintain high-intensity activity by ensuring a continuous supply of glucose for energy production when blood glucose levels alone are insufficient.

Required for full marks: Must mention glycogen breakdown, glucose release, cellular respiration or ATP production, and the significance for maintaining energy during exercise. Terms "glycogenolysis" and "ATP" should be used.

SECTION C - QUESTION 17(a)

pH 7

This is the pH at which the greatest volume of oxygen (15 cm³) was produced, indicating the highest rate of enzyme activity.

SECTION C - QUESTION 17(b)

Very little oxygen was produced at pH 3 and pH 11 because these pH values are far from the optimum pH of catalase. At these extreme pH values, the enzyme undergoes denaturation. The change in pH alters the ionization of amino acids in the active site, disrupting the ionic and hydrogen bonds that maintain the enzyme's three-dimensional structure. As a result, the shape of the active site changes and is no longer complementary to the substrate (hydrogen peroxide), preventing the formation of the enzyme-substrate complex and drastically reducing the rate of reaction.

Required for full marks: Must mention "extreme pH", "denaturation", "active site shape changes", and "substrate cannot bind effectively" or "enzyme-substrate complex cannot form".

SECTION C - QUESTION 17(c)

Denaturation is the irreversible change in the three-dimensional structure of an enzyme (or protein), caused by the disruption of hydrogen bonds, ionic bonds, and other weak interactions that maintain the secondary and tertiary structures. This results in the loss of the specific shape of the active site and the loss of the enzyme's biological activity or catalytic function.

Required for full marks: Must define as "change in structure" or "loss of three-dimensional shape", mention "loss of function" or "loss of activity", and ideally reference "active site" or "tertiary structure".

SECTION C - QUESTION 17(d)

At 80°C, very little or no oxygen would be produced, even at the optimum pH of 7.

This is because 80°C is far above the optimum temperature for catalase (around 37°C for human catalase). At this high temperature, the enzyme would undergo thermal denaturation. The increased kinetic energy causes the weak bonds (hydrogen bonds, ionic bonds, and hydrophobic interactions) maintaining the enzyme's tertiary structure to break. The active site loses its specific three-dimensional shape and is no longer complementary to the hydrogen peroxide substrate. Even though the pH is optimal, the denatured enzyme cannot bind to the substrate or catalyze the reaction, so product formation would be severely inhibited or stopped completely.

Required for full marks: Prediction of little/no oxygen production, explanation of thermal denaturation, disruption of tertiary structure/active site, substrate cannot bind, and conclusion that enzyme is non-functional.

SECTION C - QUESTION 17(e)

Catalase is essential for the survival of organisms because it catalyzes the breakdown of hydrogen peroxide (H₂O₂) into water and oxygen. Hydrogen peroxide is a toxic byproduct of various metabolic reactions, particularly those involving cellular respiration and other oxidative processes. If hydrogen peroxide accumulates in cells, it can cause oxidative damage to important cellular components including DNA, proteins, and lipid membranes. By rapidly decomposing hydrogen peroxide, catalase protects cells from oxidative stress and damage, which is critical for maintaining cellular function and organism survival in both Escherichia coli and Zea mays.

Required for full marks: Must state that catalase breaks down hydrogen peroxide, explain that hydrogen peroxide is toxic, mention damage to cellular components (DNA, proteins, or membranes), and conclude with the protective function of catalase.

SECTION D - QUESTION 18: MODEL ESSAY

STRUCTURE AND FUNCTION OF PROTEINS AS BIOLOGICAL MACROMOLECULES

1. Amino acids as monomers and peptide bond formation:

Proteins are large, complex biological macromolecules that are polymers composed of amino acid monomers. Each amino acid has a central carbon atom bonded to four different groups: a hydrogen atom, an amino group (-NH₂), a carboxyl group (-COOH), and a variable R group (side chain) that differs among the 20 different amino acids. Amino acids are linked together through condensation reactions (also called dehydration synthesis), in which the carboxyl group of one amino acid reacts with the amino group of another amino acid, releasing a molecule of water and forming a peptide bond (a covalent bond). When many amino acids are joined in this way, a polypeptide chain is formed, which folds into a functional protein.

Full marks criteria: Must define amino acids as monomers, describe the four groups attached to the central carbon, explain condensation reaction with water release, define peptide bond as the covalent linkage, and state that multiple amino acids form a polypeptide.

Partial marks: Describes amino acids and peptide bonds but omits details of the condensation reaction or the structure of amino acids.

2. The four levels of protein structure:

Proteins have four distinct levels of structural organization. The primary structure is the specific linear sequence of amino acids in the polypeptide chain, determined by the genetic code. This sequence determines all higher levels of structure. The secondary structure refers to the regular, repeating spatial arrangements of the polypeptide backbone, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amino hydrogen of another. The two most common secondary structures are the α-helix (a right-handed coiled structure) and the β-pleated sheet (a folded, sheet-like structure). The tertiary structure is the overall three-dimensional shape of a single polypeptide chain, formed by interactions between the R groups of amino acids. These interactions include hydrogen bonds, ionic bonds, disulfide bridges (covalent bonds between cysteine residues), and hydrophobic interactions. The tertiary structure determines the specific function of the protein, particularly the shape of functional regions such as the active site in enzymes. Finally, quaternary structure occurs when two or more polypeptide chains (subunits) associate to form a functional protein complex. An example is hemoglobin, which consists of four polypeptide subunits that work together to transport oxygen.

Full marks criteria: Must define all four levels accurately: primary as amino acid sequence, secondary as α-helix and β-pleated sheet with hydrogen bonds, tertiary as 3D shape with multiple bond types listed (hydrogen, ionic, disulfide, hydrophobic), and quaternary as multiple subunits. Must use correct terminology throughout.

Partial marks: Defines three levels correctly but lacks detail on bond types or omits quaternary structure or provides an incomplete explanation of tertiary structure.

3. Functions of proteins with specific examples:

Proteins perform a vast array of functions in living organisms. First, enzymatic catalysis is one of the most important functions. Enzymes are biological catalysts that speed up biochemical reactions by lowering the activation energy. For example, amylase is an enzyme that catalyzes the hydrolysis of starch into maltose during digestion. Second, proteins serve structural functions, providing mechanical support and shape to cells and tissues. Collagen is a fibrous structural protein found in connective tissue, tendons, and bones in animals, while keratin is the structural protein in hair, nails, and the outer layer of skin. Third, proteins function in transport, carrying substances throughout the body. Hemoglobin, found in red blood cells of mammals such as Homo sapiens, transports oxygen from the lungs to body tissues and carbon dioxide back to the lungs. Other functions include defense (antibodies or immunoglobulins that recognize and neutralize pathogens), regulation (hormones such as insulin that regulate blood glucose levels), and movement (contractile proteins such as actin and myosin in muscle cells).

Full marks criteria: Must describe at least three different functions clearly (enzymatic, structural, transport, defense, regulation, or movement), provide specific named examples for each function, and use correct biological terminology. Must include one organism name in binomial nomenclature.

Partial marks: Describes three functions but examples are vague or terminology is imprecise, or only two functions are fully explained with examples.

4. Structure-function relationship using enzyme as example:

The relationship between protein structure and function is best illustrated by enzymes. The specific primary structure (amino acid sequence) of an enzyme determines how the polypeptide chain folds into its unique tertiary structure. This three-dimensional shape creates a highly specific region called the active site, which has a shape complementary to the enzyme's specific substrate. According to the lock-and-key model, only a substrate with the correct shape can fit into the active site, ensuring enzyme specificity. The precise arrangement of amino acids in the active site allows the enzyme to bind the substrate and position it optimally for the reaction to occur, lowering the activation energy. If even one amino acid in the primary structure is changed, the tertiary structure and the shape of the active site may be altered, resulting in reduced or complete loss of enzyme activity. This demonstrates that the specific structure of a protein directly determines its function.

Full marks criteria: Must explain that primary structure determines tertiary structure, describe the active site as a specific 3D region, explain complementary shape to substrate (lock-and-key model), state that this determines specificity and function, and mention that changes in structure affect function.

Partial marks: Explains structure determines function and mentions active site but lacks detail on how amino acid sequence creates specific shape or omits the consequence of structural changes.

5. Correct use of binomial nomenclature:

The essay correctly uses Homo sapiens when discussing hemoglobin in human red blood cells.

Marker's note on common errors: Students often lose marks by: (1) failing to use the correct biological terminology (e.g., saying "enzyme active part" instead of "active site"); (2) not clearly distinguishing between the four levels of protein structure; (3) providing vague or incorrect examples of protein functions; (4) not explaining the cause-and-effect relationship between structure and function; (5) forgetting to use binomial nomenclature or formatting it incorrectly (genus not capitalized or species capitalized); (6) writing in vague, general language instead of precise biological terms. Always use the specific scientific vocabulary you have learned.

MARK ALLOCATION SUMMARY