Biology Form 4

Biology Form 4Chapter 4: Chemical

Composition of the Cell

4.1 Chemical Composition of the CellPractice 4.1

1. What are trace elements?Trace elements are those required in very small amounts which is less than 0.01% of the body mass by an organism.

2. What are the function of the elements hydrogen, carbon, oxygen and nitrogen in the cell?Hydrogen – Component of waterCarbon – Component of organic moleculesOxygen – Component of organic molecules and used in cellular aerobic respirationNitrogen – Synthesis of chlorophyll pigment

3. Complete the following crossword puzzle

Across1. A simple sugar found in grapes.

GLUCOSE2. A fibrous protein found in the skin, tendon, cartilage and in the bone.

COLLAGEN4. They act as a solvent for vitamins A, D, E and K.

LIPIDS5. A complex form of lipids.

STEROIDS7. The major food reserve in plant cells.

STARCHDown8. The major food reserve in animal cells.

GLYCOGEN3. A type of carbohydrate found in plant cell walls.

CELLULOSE6. Protein is required in the synthesis of this compound.

HORMONES8. A type of nucleic acid.

RNA9. Contains genetic information.

DNA

1 G L U 3 C O S E   9 D   6 H

L     E         N   O

Y     L         A   R

2 C O L L A G E N     M

O     U             O

G     4 L I P I D S   N

E     O             E

N     5 S T E 8 R O I D S

      E     N        

        7 S T A R C H  

4.2 CarbohydratesPractice 4.2

1. Name the elements present in carbohydrates.Carbon, hydrogen and oxygen.

2. Name the monosaccharide units that are formed when maltose, sucrose and lactose are hydrolysed.Maltose – 2 glucoseSucrose – Glucose and fructoseLactose – Glucose and galactose

3. How are polysaccharides formed from monosaccharides?Hundreds of monosaccharides combine through condensation to form a long chain of molecules known as polysaccharides.

4. Fructose is one and a half times sweeter than sucrose. Suggest why fructose is better than sucrose as a sugar substitute for diabetic patients.Diabetes is caused by high contents of glucose in the blood system. For that, the doctors have used fructose as a substitute for sucrose in diabetic patients diet. This is simply because, sucrose which is formed from glucose and fructose will not only endanger the condition of the patients but will also cause them fatal. Whereas fructose which is categorised as monosaccharides or simple sugars is a reducing sugar and by far the most suitable substitute for sucrose.

4.3 ProteinsPractice 4.3

1. What are the elements found in proteins?Carbon, hydrogen, oxygen and nitrogen.

2. Explain how a dipeptide can be broken down into amino acids.A dipeptide can be broken down into amino acids means of hydrolysis.

3. Explain how a quarternary structure of protein is formed.A quarternary structure of protein is formed when the combination of two or more tertiary structure polypeptide chains to form one large and complex protein molecule occurs.

4.4 LipidsPractice 4.4

1. How are fats and oils formed?Fats and oils are triglycerides. A triglyceride is an ester that is formed through the condensation of one molecule of glycerol and three molecules of fatty acids.

2. Food labels normally contain nutritional values. What does the term ‘unsaturated fats’ on food labels mean?Unsaturated fats mean fatty acids that have at least one double bond between the carbon atoms.

3. Explain why is it better to cook with unsaturated fats when compared to saturated fats.It is better to cook with unsaturated fats because they exist in liquid form at room temperature whereas saturated fats exist in solid form at room temperature.

4.5 Enzymes

Practice 4.5

1. What is enzyme?Enzymes are biological catalysts that speed up biochemical reactions in the cells.

2. List five characteristics of enzymes.- Alter or speed the rates of chemical reaction.- Have specific sites called the active sites to bind to specific substrates.- Enzyme-catalysed reactions are reversible.- Enzymes remain unchanged at the end of the reactions.- Enzymes are needed in small quantities because they are not used up but released at the end of a reaction.

3. Explain why enzyme-catalysed reactions are specific.Enzymes have specific sites called active sites to bind to specific substrates. This explains why enzymes are highly specific, that is, each enzyme can only catalyse one kind of substrate.

4. Explain briefly the effects of temperature and pH on the rate of reactions which is catalysed by enzymes.- pH levelEnzymes are sensitive to the changes of pH of their surroundings. A slight change in pH can have an adverse effect on the rate of an enzyme-catalysed reaction as each enzyme can only function optimally at a particular pH. The optimum pH is the pH at which the rate of reaction is at the maximum. Therefore, a change pH can alter the charges on the active sites of the enzymes and the substrate surfaces. This can reduce the ability of both molecules to bind each other.

- TemperatureAt low temperatures, the rate of enzyme reaction is low. However, as the temperature increases, the rate of enzyme reaction also increases. This is because at high temperatures, the enzyme and substrate molecules moves faster, increasing the force and the rate of collision. Thus, reactions occur more frequently. At low temperature (below 40oc), a rise of 10oc will double the rate of reaction. The optimum temperature for an enzyme is 37oc which is also the body temperature. At high temperature (above 40oc), enzymes, being proteins, becomes easily denatured rapidly due to the changes in the shape of the enzyme molecules. Therefore, the rate of enzyme reaction decreases.

4.6 The Importance of Chemical Composition in Cells

Practice 4.6Complete the table.

Chemical substances Main function Consequence of deficiency

Carbohydrates Important source of energy

- Respiration cannot occur- No food storage- Plant cells are irregular in shape

Proteins To produce new cells Impaired mental and physical development

Lipids Important source of energy

- Plasma membrane is not well formed- No absorption of vitamins A, D, E, K

Enzymes Speed up biochemical reactions in the cells

Biochemical reactions will proceed too slowly

Assessment

Section A

1. Which of these elements is not found in large amounts in the cell?A HydrogenB CarbonC PhosphorusD Oxygen

2. Which compound acts as a medium for chemical reaction in the cell?A Fatty acidsB ProteinsC WaterD Polysaccharides

3. Which of the following carbohydrates is a disaccharide?A GlucoseB FructoseC GalactoseD Sucrose

4. Which type of carbohydrate is found abundantly in liver cells?A StarchB SucroseC GlycogenD Glucose

5. Which of the following is not true about saturated fats and unsaturated fats?A Saturated fats contain less hydrogen atoms compared to unsaturated fats.B Saturated fats do not contain double bonds while unsaturated fats have at least one double bond.C Saturated fats contain more cholesterol compared to unsaturated fats.D Saturated fats are more likely to solidify at room temperature.

6. Proteins can be grouped into four levels of organisation according to their structure. Which structure of protein is formed when a helix polypeptide chain is folded into a three-dimensional shape?A Primary structureB Secondary structureC Tertiary structureD Quarternary structure

7. DNA carries the genetic information of an organism. Which of the following is not found in DNA?A A deoxyribose sugarB A nitrogenous baseC A phosphate groupD A sulphur group

8. Figure 1 shows how an enzyme works.

Figure 1

Which property of the enzyme is not shown in Figure 1?

A An enzyme-catalysed reaction is specific.

B The structure of the enzyme does not change at the end of a reaction.

C The enzyme can be used to catalyse another similar substrate.

D The enzyme is affected by substrate concentration.

9. Which of the following is not a property of an enzyme?A It speeds up the rate of a specific biochemical reaction.B It is not destroyed by the enzyme-catalysed reaction.C The enzyme activity can be slowed down or stopped by a coenzyme.D An enzyme-catalysed reaction is reversible.

10. Which of the following are true about enzymes?I Enzymes convert substrates into products.II Enzymes speed up the rate of biochemical reactions.III Enzymes remain unchanged at the end of the reactions.IV Enzymes have active sites to bind to specific substrates.A I and III onlyB II and IV onlyC I, II and III onlyD I, II, III and IV only

Section B

1. In the human intestine, a triglyceride molecule is converted to glycerol and fatty acids through hydrolysis.

a) Explain what is meant by hydrolysis.Hydrolysis is a reaction that occurs when triglycerides is broken down into fatty acids and glycerol.

b) How many molecules of glycerol and fatty acids are needed to form one molecule of triglyceride?1 glycerol and 3 fatty acids.

c) Name two types of steroid compounds in humans.Cholesterol and hormones.

d) State two differences between saturated fats and unsaturated fats.

Saturated Fats

Unsaturated Fats

Differences • Contain saturated fatty acids

• Contain unsaturated fatty acids

• Solid at room temperature

• Liquid at room temperature

e) Explain why daily intake of a diet rich in saturated fats is bad for health.It increases the rate of heart diseases because excess cholesterol will be deposited on the walls of arteries, blocking the arteries and flow of blood.

2. Starch and glycogen are two examples of polysaccharides.

a) State one similarity in starch and glycogen.They are both carbohydrates.

b) What is the main difference between starch and glycogen?Starch is produced by plants as a form of food in storage while glycogen is formed by animals as a means of storing glucose.

c) How is a polysaccharide broken down into smaller units?Polysaccharide can be broken down into smaller units through hydrolysis by adding diluted acid or through enzymatic reaction.

d) Name the monosaccharides obtained after the hydrolysis of(i) sucrose – glucose and fructose(ii) lactose – glucose and galactose

e) Describe briefly how to test for the presence of reducing sugar in an unknown sample of food.The presence of reducing sugar can be tested by using Benedict’s Test.(a) When a simple sugar is boiled with Benedict’s reagent, the colour changes from blue to green, yellow, orange and finally, a brick red precipitate is produced.(b) The simple sugar reduces the copper ions in the Benedict’s reagent to insoluble cuprous oxide.

3. a) Name two types of nucleic acids found in the living cells.DNA and RNA.

Figure 2 shows part of the structure of a nucleic acid.

b) Name the parts labelled P, Q and S.

P – Phosphate group

Q – Pentose sugar

S – Nitrogenous base

c) Draw a circle around a unit of nucleotide in Figure 2.

P

SQ

Figure 2

4. Figure 3 shows two types of polypeptides.

A B

Figure 3

a) What are structures A and B?A – Tertiary structureB – Quarternary structure

b) Explain how structure B is formed from structure A.Quarternary structure is formed from the combination of two or more tertiary structure polypeptide chains to form one large and complex protein molecule.

c) Give one example of protein that has structure B.Haemoglobin

d) Compare and contrast between essential amino acids and non-essential amino acids.

Essential Amino Acids Non-essential Amino Acids

- Cannot be synthesised by the body

- Can be synthesised by the body

- Can only be obtained from the diet

- Derived from other amino acids

Section C

1. a) Explain why enzymes are important to living cells.Because enzymes are biological catalysts that speed up biochemical reactions in the cells.b) Using suitable examples, discuss the uses of enzymes in industrial processes and our daily life.In industrial processes:- Preparation of leather productsIn the leather industry, proteases are used to remove hair from hides. The hides are then treated and used to prepare leather products.- Manufacturing detergentsEnzyme are used in the manufacturing of detergent such as soap powder. The enzymes protease, lipase and amylase are added into the detergent to remove food stains from clothes.- Food processingIn the food processing industry, enzymes are widely used. For example, in the manufacturing of cheese, protein in milk is coagulated by using rennin obtained from calves.

In daily life:- Biological enzymes such as protease,

amylase and lipase which are added to the washing powder to get rid of protein, starch and fat stains have made washing clothes much easier.

- Housewives use papaya juice (papain enzyme) to tenderise meat.

Figure 4 shows the ‘lock and key’ hypothesis in enzyme action.

Figure 4

c) Based on Figure 4, describe(i) which characteristics of the enzyme is explained by this hypothesis.Enzymes have specific sites called active sites to bind to specific substrate. This is why enzymes are highly specific which is each enzyme can only catalyse one kind of substrate.(ii) the enzyme action based on this hypothesis.In the ‘lock and key’ hypothesis, the substrate molecule represents the ‘key’ and the enzyme molecule represents the ‘lock’. The substrate molecule binds to the active site to form an enzyme-substrate complex. The enzyme catalyses the substrate to form products, which then leaves the active site. The enzyme molecule is now free to bind to more substrate molecules.

Thank You!Slides by: Syahirah

Presented by: Sree and AinPictures courtesy of: Googles

Song: Yoku Asobi Yoku ManabePerformed by: NYC