Chapter 3: Biological molecules

• why do we need food?

  The nutrients in food are needed to:

  • provide energy

  • supply raw materials to

  make new protoplasm (new cells!)

  • help us stay healthy (such as vitamins)
• What are nutrients?

  Nutrients are chemical substances in food that

  provide energy and materials needed by the body.

  Nutrients: Water, Carbohydrates, Fats, Proteins
• Functions of water in humans:

  • is the solvent in which chemical reactions

  take place

  • is an essential component of cells, tissue

  fluids, digestive juices and blood.

  • helps control body temperature

  • helps to transport dissolved substances

  around the body
• Functions of water in plants:

  • is essential for photosynthesis

  • helps keep plant cells turgid and plant upright

  • is needed for the transport of mineral salts up

  the xylem

  • is needed for the transport of food substances

  from the leaves to other parts of the plant
• Water is lost from the body when we

  breathe, sweat or urinate. The water

  that is lost needs to be replaced.

  The amount of water depends on:

  • how active the person is

  • how healthy the person is

  • the environmental conditions
• Carbohydrates are organic molecules made up of elements carbon, hydrogen and oxygen. The hydrogen and oxygen atoms are present in the ratio 2:1
• Functions of carbohydrates:

  • to provide energy for cell activities

  • to form supporting structures (e.g. cell wall)

  • for conversion into other organic compounds

  (e.g. amino acids and fats)

  • to form nucleic acids (e.g. DNA)

  • to synthesise lubricants

  • to synthesise nectar in flowers
• Single Sugars:

  Glucose - found in honey, corn syrup etc.

  Galactose - found in dairy products such as yoghurt and cheese.

  Fructose - found in fruits

  Double sugars:

  Maltose — found in bread, sweet potatoes etc.

  Lactose — found in milk.

  Sucrose — found in the form of table sugar.

  Complex sugars:

  Starch, Cellulose, Glycogen

  ALL single and double sugars except sucrose are reducing sugars
• Glucose - found in plants and animals

  Fructose - common in plants but rare in animals\
• A double sugar is formed when two single sugars are joined together. Common double sugars are maltose and sucrose.

  Maltose consists of two glucose molecules bonded together.

  glucose + glucose -> maltose + water

  
• Hydrolysis of double sugars (breaking down)

  • A double sugar can be split into two single sugar

  molecules in the presence of an enzyme.

  
• A sucrose molecule is made up of one glucose molecule and one fructose molecule joined together
• Types of complex carbohydrates:

  Starch:

  • A storage form of carbohydrates in plants
  • Found in storage organs in plants

  Glycogen:

  • A storage form of carbohydrates in mammals
  • Digested to form glucose when needed

  Cellulose:

  • cellulose cell wall protects plant cells
  • serve as dietary fibres to prevent constipation
• Breaking down of starch:

  Starch is broken down into maltose molecules by amylase.

  Maltose molecules are broken down into glucose by maltase.

  
• what are fats:

  •Fats are organic compounds

  made up of:

  • carbon
  • hydrogen
  • oxygen

  •Fats contain much less oxygen in proportion

  to hydrogen.
• Fats are made up of glycerol and fatty acid chains

  
• Functions of fats:

  • a source and store of energy

  • act as an insulating material to

  prevent excessive heat loss

  • are a solvent for fat soluble vitamins

  and some hormones

  • form the main part of cell membranes

  • help reduce water loss from skin surface

  
  

  Examples of fats:

  Butter, cheese, fatty part of meat, nuts
• Hydrolysis of fats (breaking down):

  Fats can be broken down into fatty acids and glycerol.

  
• What are proteins:

  • Proteins are organic compounds

  made up of:

  carbon

  hydrogen

  oxygen

  nitrogen

  sulfur (sometimes)

  • Each protein molecule is made up of smaller basic

  units called amino acids.
• Functions of Proteins:

  Examples of proteins: Meat, Fish, Milk, Yoghurt, bean and eggs

  Proteins are needed for the synthesis of:

  • new protoplasm for growth and repair of worn-out cells
  • enzymes and some hormones
  • antibodies
• What are proteins:

  Amino acids are the building blocks of proteins.

  Peptide bonds link up amino acids to form a polypeptide

  Two or more polypeptide chains fold to form a 3-D molecule, called protein.

  
• Benedict's test for reducing sugar:

  1. Add 2 cm3 of Benedict’s solution to 2 cm3 of food sample in a test tube.
  2. Shake the mixture.
  3. Heat the mixture in a boiling water bath for 2 to 3 minutes.
  4. If a large amount of reducing sugars is present, brick-red precipitate forms. If reducing sugar is absent, Benedict's solution remains blue.

  
• Benedict's Test can be used to find out the amount of reducing sugar:

  Blue solution = no reducing sugars

  Green / yellow solution = trace amount of reducing sugars

  Orange red solution = moderate amount of reducing sugars

  Brick red solution = Large amount of reducing sugars
• Iodine test for starch:

  Add a few drops of iodine solution to some solid food/ food solution on a white tile.

  If starch is present, the solution changes from yellowish-brown to blue-black.

  If starch is absent, the solution remains yellowish- brown.
• Ethanol Emulsion Test for Fats:

  Add 2 cm3 of ethanol to 2 cm3 of food sample in a test tube.

  Shake the contents of the tube vigorously.

  Decant 3 cm3 of water into the test tube and shake the mixture.

  A cloudy white emulsion is formed if fats are present.
• Biuret Test for protein:

  Add 2 cm3 of Biuret solution to 2 cm3 of food solution in a test tube.

  Shake the test tube to mix the contents.

  If proteins are present, the mixture changes from blue to violet.

  If proteins are absent, mixture remains blue.
• Sally wants to lose weight by only eating plain rice for all her meals. Explain why it is not advisable for her to do so for a prolonged period of time?

  Rice is mainly carbohydrates and is a good source of immediate energy. However there are little to no other nutrient classes