Biology (Sec 2)

Chapter 4: Nutrients

Carbohydrates:

They are made of carbon, hydrogen and oxygen. Hydrogen : oxygen = 2:1

Main groups of carbs:

Monosaccharides (broken down to one simple sugar): glucose, fructose, galactose

Disaccharides: maltose, sucrose, lactose.

Polysaccharides: starch, glycogen, cellulose (consists of glucose molecules but differ in arrangement)

starch-excess glucose in plants
glycogen-excess glucose in animals
cellulose-cell wall for plants

Starch and glycogen are good storage molecules because of:
Large molecules can't be diffused out of cell easily
Insoluble in water, hence will not affect water potential of cell
Compact, hence take up less space than individual glucose molecules
Easily broken down into glucose for tissue/cellular respiration

Carbs are needed for:

• forming supporting structures (like cell wall)
• converted to other organic compounds
• formation of nucleic acids
• synthesize lubricants
• synthesize nectar in some flowers

Definition of condensation: It is a chemical reaction where two simple molecules are joined together to form a disaccharide by removing a water molecule. 

Definition of hydrolysis: Reaction where water molecule is needed to break up complex molecules to simple molecules. 

maltose -> glucose + glucose

sucrose -> glucose + fructose

lactose -> galactose + glucose

How to identify reducing sugars?

-Benedict's test. It contains Cu(2+) ions, and reacts with reducing sugars to be Cu(+) ions. Cu2O is a coloured precipitate

-Equal volume of food solution and Benedict's solution, put in hot water bath for 5 minutes after shaking it.

Application of Benedict's test

• test for presence of glucose in urine. (check for diabetes) 

Iodine test-test for starch (iodine solution turns from yellowish-brown to blue black when coming into contact with starch)

Food sources:

• bread
• noodles
• flour
• fruit

Fats

It consists of hydrogen, carbon, and oxygen. There are much less oxygen atoms than hydrogen atoms. 

One fat molecule -> 3 fatty acids + glycerol

Saturated fats: straight chain, from animals, solid at room temperature. 

Unsaturated fats: bent chain, from plants, liquid at room temperature

Food sources:

• dairy products
• nuts
• oil, margarine
• meat
• fast food

Function of fats:

• source and store of energy (releases most energy but at a slow rate)
• solvent for fat-soluble vitamins
• constituent of protoplasm
• used to form some hormones
• reduce water loss and heat loss from skin surface

Test for fats: Ethanol-Emulsion test

Put a fat droplet and around 3 cubic centimeters of ethanol and shake them vigourously. (For solid sample just put them into ethanol then decant the liquid). Add water and you will see a white emulsion. 

Proteins

It contains carbon, nitrogen, oxygen and hydrogen. 

They are made of polypeptides, and polypeptides are made of amino acids. Amino acids are linked by peptide bonds (strong) while polypeptides are linked by hydrogen bonds (weak)

Food source:

• egg white
• lean meat
• dairy products
• nuts
• soya beans

Functions:

Synthesize:

• New protoplasm for growth and repair of cells
• Enzymes and hormones
• Antibodies to combat diseases

Lack of protein-kwashiorkor 

Symptoms:

• bloated stomach (inflammatory response)
• growth failure, loss of muscle mass
• decreased immunity

Test: Biuret test

1. Equal amount of food solution and NaOH
2. Add drop by drop of CuSO4, shaking after each drop
3. If protein is present, it should turn into a violet colour. If not, just add ten drops of CuSO4 to confirm results. 

Water

Role of water:

• essential in photosynthesis
• used to control body temperature
• essential in blood, lubricants, protoplasm...
• medium where chemical reactions take place
• biological solvent 
• transport medium

Chapter 5: Enzymes

Enzymes are biological catalysts:

• speeds up the rate of chemical reactions
• made of protein (extremely sensitive to pH and temperature changes)

Hence, changes in temperature and pH may break the weak bonds between polypeptides and change its 3D shape.

Enzymes lower the activation rate needed to start a reaction. 

Two types of reactions:

• Anabolic - build up complex substances
• Catabolic - break down complex substances (eg digestion)

Eg: Lactose-intolerant

Some people are lactose-intolerant, they do not produce enough lactase, hence lactase cannot be digested. Food containing lactose should be avoided. Lactose drops should be added to milk 24 hrs before consumption.

Characteristics of enzymes:

• substrate-specific (3D shape)
• remains chemically unchanged, hence can be reused

Factors affecting enzyme action:

• Temperature (@ low temps, enzymes inactive. Too high temps causes denaturation.) 
• extreme changes in pH

As enzyme concentration increases, more active sites to collide with, greater chance of E-S complex, hence rate of reaction increases. 

As substrate concentration increases, greater chance of collision. Yet rate of reaction is constant at a point, because enzymes are saturated. 

Chapter 6: Nutrient in Humans

Why is digestion necessary?

Large nutrients in food must be broken down into small, soluble substances that can be diffused through the partially permeable membrane and enter the bloodstream. 

Two types of digestion:

• Physical digestion (to increase surface area of food so it can be digested efficiently)
• Chemical digestion (breaking down complex substances to simpler substances)

What happens in the mouth?

Chewing breaks up larger food substances into smaller pieces. 

Salivary glands produce salivary amylase to digest starch. 

Tongue rolls food into food bolus. 

Wall of oesophagus made of longitudinal muscles and circular muscles. 

How does these two muscles work?

Peristalsis - rhythmic, wave-like contractions of antagonistic muscles.

1. Wall constricts. Longitudinal muscles relax while circular muscles contract. 
2. Wall dilates for food to enter: Circular muscles relax, longitudinal muscles contract

Stomach wall- contract and relax to:

• break up food into smaller pieces for enzyme to act on
• churns food with gastric juice to produce chyme

Gastric juice contains:

• Hydrochloric acid (active prorennin and pepsinogen, provide acidic medium for enzymes, kills micro-organisms in food)
• Rennin-turns soluble milk protein to insoluble milk protein so it can stay in stomach long enough to be digested.
• Pepsin-digest protein.

Function of the gall bladder-store bile temporarily

Function of the pancreas-secrete pancreatic juice (contains pancreatic amylase)

Function of liver-produce bile for fat emulsification

Intestinal glands produce intestinal juice (contains enzymes)

Bile emulsifies fat by lowering surface tension so it breaks up the large fat globules into smaller fat droplets. Hence, it increases the surface area to volume ratio and speeds up digestion.

All nutrients are digested in the small intestine.

Colon absorbs water and mineral salts from undigested food. Rectum stores faeces temporarily.