Carbohydrates and lipids

Question 1

What is the general formula for carbohydrates and how can they be classified?

Answer 1

• General formula: C x (H 2 O) y
• Can be either monosaccharides, disaccharides or polysaccharides (these are types of carbohydrates)
• They are constituted of sugars which are polar and soluble in water (apart from polysaccharides)
• Polysaccharides result from condensation reactions of sugars and are insoluble in water

Question 2

What are monosaccharides, where are they joined and what does the joining result in?

Answer 2

Simplest types of carbohydrates. The carbons are joined to a hydroxyl group (-OH) resulting in either ribose, glucose, fructose and galactose
Can have a number of carbon atoms of 3-7

Question 3

How are polysaccharides formed?

Answer 3

Two monosaccharides are linked together by a condensation reaction which forms a glycosidic bond, resulting in a disaccharide and a water molecule is released.
Multiple monosaccharides linked from a polysaccharide

Question 4

What is a condensation reaction?

Answer 4

• When two smaller organic molecules combine to from a larger molecule and a water molecule.
• The opposite of a condensation reaction is hydrolysis
• Hydrolysis is when water is added and breaks apart a polymer

Question 5

What are examples of polysaccharides?

Answer 5

Cellulose, glycogen and starch

Carbohydrates also make up the chitin exoskeleton of insects and crustaceans

Question 6

Give 3 examples of a carbohydrate and its monomers.

Answer 6

Sucrose (Disaccharide) –> glucose and fructose
Maltose (Disaccharide) –> glucose (2 units)
Lactose (Disaccharide) –> glucose and galactose
Starch, Glycogen and Cellulose are composed of only glucose

Question 7

What are the two forms glucose exists in?

Answer 7

D-glucose and L-glucose. They are isomers so have a different structure. There are two types of D-glucose: α-D-glucose and β-D-glucose. These differ by the placement of the -OH

Question 8

What is the structure and function of starch?

Answer 8

• Starch is used to store energy in plants.
• It is composed out of a mixture of two polysaccharides (amylose and amylopectin)
• Glucose is its monomer sub unit
• The structure differs to cellulose as the arrangement of glucose molecules and the glycosidic bonds is different
• The glucose molecules are arranged linear (in the same direction) which is amylose (can have rings) and when branched is called amylopectin
• Hydrogen bonds between glucose sub units stabilize the structure
• Amylopectin has a more complex structure than amylose
• Are branched (depending on which one) and composed of alpha-glucose.

Question 9

What is the structure and function of glycogen?

Answer 9

• Is the carbohydrate used to store energy in animals
• Sub unit is glucose
• Contain hydrogen bonds to stabilize the structure
• Very branched and complex
• Composed of alpha D glucose

Question 10

What is the structure and function of cellulose?

Answer 10

• It is present in the plant cell walls which gives the walls extra strength and protects the cell from over-expanding and bursting
• Is unbranched
• Contains only beta-glucose
• It also stores energy for biofuels
• The glucose monomers that rotate 180 degrees around the backbone chain

Question 11

Give three facts in which starch is used in industry?

Answer 11

• Amylose is an unbranched chain where as amylopectin is a branched chain
• Amylopectin gives starch its characteristic stickiness, which is useful in chemical industries where it is used to make paste, glue or lubricant
• Amylopectin makes up 80% of the starch content in potatoes, useful for adhesive making

Question 12

What are the characteristics of fatty acids?

Answer 12

• They have no affinity (tendency to bind) with water
• They are hydrophobic and water-repellent
• They are non-polar and insoluble in water
• May be soluble in organic solvents
• They have a (-COOH) carboxyl functional group

Question 13

What are triglycerides?

Answer 13

• They are formed by condensation reactions between one glycerol and three fatty acids, creating ester bonds (-COO)
• A water molecule is produced
• It is one of the main groups of lipids
• The types of triglycerides are fats and oils (different state at r.t.p)

Question 14

What are the two types of fatty acids and what are its definitions?

Answer 14

• Saturated: have no double bonds between any of the carbon atoms that make up the hydrocarbon chain
• Unsaturated: contain one or more double bonds

Question 15

Explain unsaturated fatty acids and its different types.

Answer 15

• Monounsaturated means there is only one double bond present, polyunsaturated bonds have two or more
• They can be either cis or trans isomers
• Isomers have the same molecular formula but a different structural formula
• It depends on the position of the two hydrogen atoms around the carbon-carbon double bond (look in notes)
• Trans has a H atom transferred on the other side

Question 16

What are the key features of cis isomers?

Answer 16

• Occur in nature e.g. oleic acid (found in olive oil
• Two hydrogen atoms are attached on the same side of two carbon atoms
• Lipids/triglycerides formed from cis fatty acids have lower melting points (liquid at r.t.p)
• Healthier and promote ‘good cholesterol’

Question 17

What are the key features of trans-isomers?

Answer 17

• Produced artificially when lipids formed by polyunsaturated fatty acids from plants are ‘partially hydrogenated’ chemically
• They are more solid (like saturated fats)
• The hydrogen atoms are on the opposite side of the two carbon atoms
• Lipids made from trans fatty acids have a higher melting point
• Dangerous for cardiovascular system. Promote ‘bad cholesterol’

Question 18

How are phospholipids produced? What are steroids, what makes them lipids and give two examples?

Answer 18

• When one fatty acid in a triglyceride is replaced by a phosphate group, -PO4
• Steroids are also lipids but have a different structure consisting four fused rings
• They are hydophobic and insoluble in water, which is why they are lipids
• Examples of steroids: cholesterol and sexual hormones

Question 19

Why are lipids better for long-term energy storage than carbohydrates?

Answer 19

• A gram of lipid gives twice the amount of energy as a gram of glycogen
• Glycogen is much lighter than fats (fats are stored in pure form)
• Lipids contribute a sixth as much to the overall body mass as carbohydrates, per unit of energy stored
• The higher energy content also play a role
• This mean animals have a lighter body mass, which is essential for their mobility

The overall ADVANTAGE is that storing lipids long-term means it stores a larger amount of energy for the same body mass

Question 20

What are the two advantages lipids have over carbohydrates and what are their energy contents?

Answer 20

• Lipids have a higher energy content than carbohydrates and have the ability to act as thermal insulators
• The energy storage of them both:
• Carbohydrates 17 kJ/g
• Fats 37 kJ/g
• Fat is useful for heat insulation for many animals and need their fat reserves to also nurse offspring (in the case of some whales)

Question 21

Name 5 properties of lipids.

Answer 21

• The energy content of fats is 37 kJ/g compared to 17kJ/g of carbohydrates
• Fats are less dense than water
• Fats are non-polar and will dissolve other non-polar compounds, but does not affect the movement of water
• Very good heat insulator
• Lipids are water-insoluble and provide a waterproof layer in plants and animals

Question 22

Where can saturated fats and trans fats be found?

Answer 22

• Saturated fatty acids occur naturally in many foods (animal sources) from meat and dairy products.
• Trans fats are artificially synthesized fats, where hydrogen is added to unsaturated fats under high pressure. It extends the shelf life of certain food products

Question 23

What is the scientific evidence for the correlation between consuming trans and saturated fats to the incidence of CHD?

Answer 23

• Trans fats and saturated fats contribute to the formation of atherosclerotic plaques in arteries, which may lead to a heart attack
• This hypothesis is proved by patients who died from CHD that shows that high concentrations of trans fats are present in fatty deposits in diseased arteries
• These is a positive correlation but no casual relationship yet
• In other parts of the world, people consume high amounts of saturated fats (Maasai of Kenya) but have low incidence of CHD
• A relationship cannot be proven yet as other factors e.g. genetics, other food, consumption of alcohol and lifestyle, could intervene and alter the evidence
• Through research must be collected before reaching a conclusion

Question 24

How do you evaluate the evidence for health claims made about lipids?

Answer 24

• Include the strengths and limitations. Some questions to consider:
  1. Is there a correlation between the lipid and the incidence of the disease under investigation?
  2. Has a statistical analysis been carried out?
  3. Is the data collected widely spread? (the more spread, the lower their significance and reliability)
  4. Was the sample used big enough?
  5. Were representatives of the whole human population considered?
  6. Were all important factors effectively controlled?

Question 25

How do you calculate the body mass index (BMI) and what is it?

Answer 25

• A measure and indication of body fat based on height and weight
• It does not provide a diagnosis of the health of an individual
  Body mass index = weight (kg) / height2 (m)
• The height is squared

Question 26

How do you use a nomogram to calculate a BMI?

Answer 26

• Draw a line with a ruler from the body mass to the height of a person. The point where the line intersects the W/H^2 line is person’s BMI
• The value can be used to compare to a health risk classification table to determine the risk of developing health problems