2.3

Question 1

Carbohydrates are made of…

Answer 1

C, H and O (‘carbo’ – contains carbon ; ‘hydrate’ – contains H and O)

Question 2

carbohydrates are composed of

Answer 2

Carbohydrates are composed of recurring monomers called monosaccharides (which typically form ring structures)

Question 3

monosaccharides may be linked together by…

Answer 3

These monosaccharides may be linked together via condensation reactions (water is formed as a by-product)

• Two monosaccharide monomers may be joined via a glycosidic linkage to form a disaccharide
• Many monosaccharide monomers may be joined via glycosidic linkages to form polysaccharides

Question 4

Examples of Carbohydrates:

Answer 4

monosaccharides, disaccharides, polysaccharides

Question 5

Monosaccharides example

Answer 5

Monosaccharides (one sugar unit) are typically sweet-tasting and function as an immediate energy source for cells

Examples of monosaccharides include glucose, galactose and fructose

Question 6

Disaccharides example

Answer 6

Disaccharides (two sugar units) are small enough to be soluble in water and commonly function as a transport form

Examples of disaccharides include lactose, maltose and sucrose

Question 7

Polysaccharides example

Answer 7

Polysaccharides (many sugar units) may be used for energy storage or cell structure, and also play a role in cell recognition

Examples of polysaccharides include cellulose, glycogen and starch

Question 8

Polysaccharides are…

Answer 8

carbohydrate polymers comprised of many (hundreds to thousands) monosaccharide monomers

• The type of polymer formed depends on the monosaccharide subunits involved and the bonding arrangement between them
• Three key polymers can be made from glucose monosaccharides – cellulose, starch (in plants) and glycogen (in animals)

Question 9

Cellulose

Answer 9

Cellulose is a structural polysaccharide that is found in the cell wall of plants

It is a linear molecule composed of β-glucose subunits (bound in a 1-4 arrangement)

Because it is composed of β-glucose, it is indigestible for most animals (lack the enzyme required to break it down)

• Ruminants (e.g. cows) may digest cellulose due to the presence of helpful bacteria in a specialised stomach
• Caecotrophs (e.g. rabbits) will re-ingest specialised faeces that contain digested cellulose (broken down in the caecum)

Question 10

Starch

Answer 10

Starch is an energy storage polysaccharide found in plants

It is composed of α-glucose subunits (bound in a 1-4 arrangement) and exists in one of two forms – amylose or amylopectin

• Amylose is a linear (helical) molecule while amylopectin is branched (contains additional 1-6 linkages)
• Amylose is harder to digest and less soluble, however, as it takes up less space, is the preferred storage form in plants

Question 11

Glycogen

Answer 11

Glycogen is an energy storage polysaccharide formed in the liver in animals

It is composed of α-glucose subunits linked together by both 1-4 linkages and 1-6 linkages (branching)
- It is akin to amylopectin in plants, but is more highly branched (1-6 linkages occur every ~10 subunits as opposed to ~20)

Question 12

Fatty acids can be…

Answer 12

saturated, monounsaturated or polyunsaturated

Question 13

fatty acids are

Answer 13

Fatty acids are long hydrocarbon chains that are found in certain types of lipids (triglycerides & phospholipids)
- Fatty acids may differ in the length of the hydrocarbon chain (typically 4 – 24 carbons) and in the number of double bonds

Question 14

saturated fatty acids

Answer 14

Fatty acids that possess no double bonds are saturated (have maximum number of H atoms)
- Saturated fatty acids are linear in structure, originate from animal sources (i.e. fats) and are typically solid at room temperatures

Question 15

unsaturated fatty acids

Answer 15

Fatty acids with double bonds are unsaturated – either monounsaturated (1 double bond) or polyunsaturated (>1 double bond)
- Unsaturated fatty acids are bent in structure, originate from plant sources (i.e. oils) and are typically liquid at room temperatures

Question 16

unsaturated fatty acids may occur in two distinct structural configurations

Answer 16

cis and trans isomers

Question 17

cis

Answer 17

Cis: The hydrogen atoms attached to the carbon double bond are on the same side

Question 18

trans

Answer 18

Trans: The hydrogen atoms attached to the carbon double bond are on different sides

Question 19

trans fatty acids…

Answer 19

• Trans fatty acids do not commonly occur in nature and are typically produced by an industrial process called - hydrogenation
• Trans fatty acids are generally linear in structure (despite being unsaturated) and are usually solid at room temperature

Question 20

Triglycerides

Answer 20

Triglycerides are the largest class of lipids and function primarily as long-term energy storage molecules
- Animals tend to store triglycerides as fats (solid), while plants tend to store triglycerides as oils (liquid)

Question 21

triglycerides are formed when…

Answer 21

Triglycerides are formed when condensation reactions occur between one glycerol and three fatty acids
- The hydroxyl groups of glycerol combine with the carboxyl groups of the fatty acids to form an ester linkage
This condensation reaction results in the formation of three molecules of water

Question 22

Triglycerides can be either…

Answer 22

saturated or unsaturated, depending on the composition of the fatty acid chains

Question 23

types of fats

Answer 23

Whilst all types of fats consumed as part of dietary intake will cause adverse health effects if taken in excessive amounts, some types of fats are associated with increased health risks

• The mix of fats in the diet influences the level of cholesterol in the bloodstream
• Saturated fats and trans fats raise blood cholesterol levels, while (cis) unsaturated fats lower blood cholesterol levels

Question 24

Regulating Blood Cholesterol Levels

Answer 24

Fats and cholesterol cannot dissolve in blood and are consequently packaged with proteins (to form lipoproteins) for transport

• Low density lipoproteins (LDL) carry cholesterol from the liver to the rest of the body
• High density lipoproteins (HDL) scavenge excess cholesterol and carry it back to the liver for disposal

Question 25

LDLs vs. HDLs

Answer 25

Hence LDLs raise blood cholesterol levels (‘bad’) while HDLs lower blood cholesterol levels (‘good’)

Question 26

High intakes of certain types of fats will differentially affect cholesterol levels in the blood

Answer 26

- Saturated fats increase LDL levels within the body, raising blood cholesterol levels - Trans fats increase LDL levels and decrease HDL levels within the body, significantly raising blood cholesterol levels - Unsaturated (cis) fats increase HDL levels within the body, lowering blood cholesterol levels

Question 27

Health Risks of High Cholesterol

Answer 27

High cholesterol levels in the bloodstream lead to the hardening and narrowing of arteries (atherosclerosis) - When there are high levels of LDL in the bloodstream, the LDL particles will form deposits in the walls of the arteries - The accumulation of fat within the arterial walls lead to the development of plaques which restrict blood flow - If coronary arteries become blocked, coronary heart disease (CHD) will result – this includes heart attacks and strokes

Question 28

Lipid Health Claims

Answer 28

There are two main health claims made about lipids in the diet: - Diets rich in saturated fats and trans fats increase the risk of CHD - Diets rich in monounsaturated and polyunsaturated (cis) fats decrease the risk of CHD

Question 29

how were lipid health claims made?

Answer 29

These health claims are made based on evidence collected in a number of ways: - Epidemiological studies comparing different population groups - Intervention studies that monitor cohorts following dietary modifications - Experimental designs utilising animal models or data based on autopsies

Question 30

Evidence Supporting Health Claims

Answer 30

A positive correlation has been found between the intake of saturated fats and the incidence of CHD in human populations - Counter: Certain populations do not fit this trend (e.g. the Maasai tribe in Africa have a fat-rich diet but very low rates of CHD) Intervention studies have shown that lowering dietary intakes of saturated fats reduces factors associated with the development of CHD (e.g. blood cholesterol levels, blood pressure, etc.) - Counter: Validity of intervention studies is dependent on size and composition of cohort, as well as the duration of the study In patients who died from CHD, fatty deposits in diseased arteries were found to contain high concentrations of trans fats - Counter: Genetic factors may play a role (e.g. blood cholesterol levels only show a weak association to dietary levels)

Question 31

Evidence Against Health Claims

Answer 31

Proportion of saturated and trans fats in Western diets has decreased over the last 50 years, but incidence of CHD has risen - Counter: Increased carbohydrate intake may cause detrimental health effects associated with CHD (e.g. diabetes, obesity) - Counter: Incidence of CHD dependent on a myriad of factors besides dietary intake (e.g. exercise, access to health care, etc.)

Question 32

lipids

Answer 32

Lipids are more suitable for long-term energy storage in humans than carbohydrates

Question 33

how do lipids and carbohydrates differ?

Answer 33

Lipids and carbohydrates both function as energy storage molecules in humans, however differ in several key aspects (SODAS): - Storage (lipids are more suitable for long-term energy storage) - Osmolality (lipids have less of an effect on the osmotic pressure of a cell) - Digestion (carbohydrates are easier to digest and utilise) - ATP Yield (lipids store more energy per gram) - Solubility (carbohydrates are easier to transport in the bloodstream)

Question 34

Energy Storage Analogy

Answer 34

ATP is the energy currency of the cell – in this respect it is akin to cash - Cash is earned when you work (cell respiration) and can be spent in a number of ways (metabolism)

Question 35

Storing energy as carbohydrates (i.e. glycogen) is similar to keeping the cash in a wallet

Answer 35

- It is easier to carry around (monosaccharides and disaccharides are water soluble) - It is readily accessible (carbohydrates are easier to digest) - You cannot carry as much (carbohydrates store less energy per gram)

Question 36

Storing energy as lipids (i.e. triglycerides) is similar to keeping the cash in a safe

Answer 36

- It is not viable to carry around (triglycerides are insoluble in water) - It is harder to access (triglycerides cannot be easily digested) - You can keep more cash in it (triglycerides store more energy per gram)

Question 37

body mass index (BMI)

Answer 37

The body mass index (BMI) provides a measure of relative mass based on the weight and height of the individual - It is commonly used as a screening tool to identify potential weight problems in sedentary adults

Question 38

BMI formula

Answer 38

Body mass index can be calculated according to the following formula: BMI = kg/m^2

Question 39

BMI doesn't apply to

Answer 39

- BMI values are not a valid indicator for pregnant women or professional athletes with atypical muscle / fat ratios - BMI calculations should not be used as a diagnostic tool and should be used in conjunction with other measurements

Question 40

Nomograms

Answer 40

An alternative way of calculating body mass index is by using an alignment chart (nomogram) - Nomograms display height and weight on perpendicular axes and then assign BMI values to colour coded regions