4.4 Lipids

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  1. Lipids are complex organic compounds that made up of carbon, hydrogen and oxygen. Some lipids contain the elements nitrogen and phosphorus.
  2. The ratio of hydrogen atoms to oxygen atoms in a lipid molecule is higher than the    2 : 1 ratio in carbohydrates.
  3. Lipids are insoluble in water. However, they are soluble in other lipids and organic solvents such as alcohol and acetone.
  4. The basic units of lipids are fatty acids and glycerol. A lipid molecule is made up of one molecule of glycerol and three molecules of fatty acid.

(Formation of molecule of lipid)

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Types of Lipids
  1. Examples of lipids include
    1. fats and oils (triglycerides), 
    2. waxes, 
    3. phospholipids
    4. steroids.
  2. Fats and oils (triglycerides)
    1. A triglyceride is formed from glycerol and three molecules of fatty acids through condensation.
    2. Triglycerides can be broken down into fatty acids and glycerol by hydrolysis.
    3. There are two types of fats.
      1. Saturated fats
        1. Fats containing saturated fatty acids
        2. are solids at room temperature.
      2. Unsaturated fats
        1. Fats containing unsaturated fatty acids
        2. usually Iiquid at room temperature
        3. it is called oil 
  3. Waxes
    1. Waxes are long chain esters.
    2. They are found in the cuticle of leaves.
    3. They are waterproof
    4. They can prevents entry of microorganisms and evaporation of water
  4. Phospholipids
    1. Phospholipids are component of plasma membrane.
    2. Example of phospholipid is lecithin. It is a type of triglyceride, which is the main constituent of the plasma membrane.
  5. Steroids
    1. Steroids include cholesterol and hormones such as testosterone, oestrogen and progesterone.
    2. Steroids have a basic structure which consists of four interconnected rings of carbon atoms. Attached to this basic structure are side chains of different lengths.
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4.3 Protein

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  1. Proteins are complex compounds that made up of carbon, hydrogen and nitrogen. Some proteins contain sulphur and phosphorus.
  2. It is needed by the body for 
    1. growth, 
    2. repair of damaged tissues and 
    3. synthesis of secretions (enzymes, mucus, hormones.)
  3. Proteins are polymer that made up of monomers called amino acids. Each molecule of amino acid has one amino group (-NH2) and one carboxyl group ( -COOH).
  4. Two molecules of amino acids are joined by a peptide bond to form a dipeptide through condensation.
    Amino acid + amino acid → dipeptide + water
  5. Polypeptides (protein) are formed when many amino acids are joined together by condensation.
  6. Proteins can be broken down through hydrolysis into amino acids.
  7. There are 20 types of amino acids in cells.
  8. Amino acid can be divided into essential amino acid and nonessential amino acid.
    1. Essential amino acids are amino acids that cannot be synthesised by the body. They can only be obtained from diets. An example is leucine.
    2. Non-essential amino acids are amino acids that can be synthesised by the body. They are derived from other amino acids. There are 11 non-essential amino acids.
  9. Proteins can be grouped into four levels of organisation according to their structures.
    1. Primary structure (linear sequence of amino acids in a polypeptide.)
    2. Secondary structure (polypeptide is coiled to form an alpha-helix or folded into beta-pleated sheets)
    3. Tertiary structure (helix or beta-pleated sheets are folded in many ways into a three dimensional shape of a polypeptide.)
    4. Examples are hormones, enzymes, antibodies and plasma proteins.
    5. Quarternary structure (Two or more tertiary structure polypeptide chains combine to form a large and complex protein molecule. Example: haemoglobin.)

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3.8.3 Sex Determination and the Occurrence of Twins (Structured Questions)

Question 1:
Diagram 1 shows the schematic diagram of sex determination in human being.

(a) In Diagram 1, complete the sex chromosomes in gametes K and L. [2 marks]

(b) What is gamete L? [1 mark]

(c) State process B. [1 mark]

(d) State the sex of offspring M. [1 mark]

(e) Based on Diagram 1, state the percentage to get a baby boy. [1 mark]

K : X, L : Y





4.2.3 Polysaccharides

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  1. Polysaccharides are formed when more than two monosaccharides (usually more than hendreds) combine through condensation. 
  2. Some characteristics of polysacharides are: 
    1. do not dissolve in water 
    2. do not taste sweet 
    3. do not crystallise
  3. Polysaccharides can be hydrolysed by adding acid, boiling and action of enzymes. 
  4. Examples of polysaccharides 
    1. Starch
      storage of carbohydrate in plants. The iodine test is used for confirming the presence of starch
    2. Glycogen
      storage of carbohydrate in animals. Its polysaccharide chains are branched
    3. Cellulose
      structural polysaccharide in plant cells. They are the constituent of the cell walls of plant cells and chitin of animal cuticles. Gives support to plant cells.
  5. Polysaccharides can be broken down into smaller molecules through hydrolysis using dilute acid or enzymes.
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4.2.2 Disaccharides

  1. Disaccharides are formed when two monosaccharides combine together through a chemical process called the condensation.
  2. Disaccharides are known as complex sugar.
  3. Examples of disaccharides are,
    1. Maltose 
      1. known as malt sugar
      2. It is a reducing sugar
      3. It is formed from condensation of two glucose molecules.
        Glucose + glucose  maltose + water 
      4. Found in germinating cereals
      5. Produced during digestion of starch
    2. Sucrose
      1. It is found in sugar cane, beet root and  sweet fruits
      2. It is a non-reducing sugar
      3. It is  formed from glucose and fructose.
        Glucose + fructose → sucrose + water
    3. Lactose
      1. Present in all mammals
      2. Is a reducing sugar
      3. It is formed from glucose and galactose
        Glucose + galactose  lactose + water
  4. Disaccharides are decomposed to units of monosaccharide through hydrolysis (the addition of one molecule of water)

4.2.1 Monosaccharides

  1. Monosacharides are monomers of carbohydrates. 
  2. Examples of monosacharides are:
    1. Glucose — found in plants and animals (It is the energy source for cellular respiration)
    2. Fructose — found in fruits and honey
    3. Galactose — present in milk
  3. They are reducing sugars, they can remove oxygen from other compounds..
  4. They can be detected by using Fehling's or Benedict's test. 
  5. When monosaccharides are heated with Fehling's or Benedict's solution, they will reduce the blue copper(II) sulphate in the two solutions to a brick red precipitate.

4.2 Carbohydrate

  1. Carbohydrate consist of the elements carbon, hydrogen and oxygen. 
  2. The H : O ratio in all carbohydrate molecules is 2 : 1.
  3. Examples of carbohydrate include starch, sugar, glycogen and cellulose.
  4. Excess carbohydrates are stored in the form of glycogen in the liver. Some are converted to fat and stored in the adipose tissues below the skin and around organs.
  5. There are three types of carbohydrates:
    1. Monosaccharides
    2. Disaccharides
    3. Polysaccharides

3.8.1 Cell Division (Structured Questions)

Question 1:
Diagram 1 shows the stages of cell division.

(a)(i) Name the type of cell division as shown in Diagram 1. [1 mark]
(ii) Give reason to your answer in (a)(i). [1 mark]

(b) Arrange the stages of cell division P, Q, R and S in a correct sequence in the boxes given. [1 mark]

(c)(i) Name structure W. [1 mark]

(ii) How many structures W in each cell at stage P? [1 mark]

(d) What happens to structure W if exposed to radioactive radiation? [1 mark]


The daughter cells contain the same number of chromosomes as the parent cell.




Mutation may occur

Question 2:
Diagram 2 shows a process of cell division. Chromosomes at stage B are not shown.

(a)(i) Name the process shown in Diagram 2. [1 mark]
(ii) State the importance of the process in (a)(i). [1 mark]

(b)(i) During stage C, the chromosomes line up on the equator.
   What happens to the chromosomes during this stage? [1 mark]
(ii) What is the effect of the process in (b)(i) on the offspring? [1 mark]

(c) In Diagram 2, draw the chromosome at stage B. [1 mark]

(d) How many daughter cells are produced at the end of Division II in Diagram 1? [1 mark]


Production of gametes




4 daughter cells are produced

4.1 Chemical Composition of the Cell

  1. The elements found in the cell include
    1. carbon, 
    2. oxygen, 
    3. hydrogen, 
    4. nitrogen, 
    5. sulphur, 
    6. phosphorus, 
    7. calcium,
    8. sodium,
    9. potassium, 
    10. magnesium,
    11. iron 
    12. chlorine, 
  2. The chemical compounds in the cell can be classified into two groups, 
    1. organic: compounds which contain the element carbon and originate from living things. Example: carbohydrates, lipids, proteins and nucleic acids
    2. inorganic: compounds which do not originate from living things. Example: water and mineral salts.
  3. .Organic compounds are synthesised by the cells themselves whereas inorganic compounds are not synthesised by the cells themselves but are obtained from the external environment.