2.3 Flashcards
Carbohydrates are made of…
C, H and O (‘carbo’ – contains carbon ; ‘hydrate’ – contains H and O)
carbohydrates are composed of
Carbohydrates are composed of recurring monomers called monosaccharides (which typically form ring structures)
monosaccharides may be linked together by…
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
Examples of Carbohydrates:
monosaccharides, disaccharides, polysaccharides
Monosaccharides example
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
Disaccharides example
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
Polysaccharides example
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
Polysaccharides are…
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)
Cellulose
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)
Starch
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
Glycogen
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)
Fatty acids can be…
saturated, monounsaturated or polyunsaturated
fatty acids are
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
saturated fatty acids
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
unsaturated fatty acids
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
unsaturated fatty acids may occur in two distinct structural configurations
cis and trans isomers
cis
Cis: The hydrogen atoms attached to the carbon double bond are on the same side
trans
Trans: The hydrogen atoms attached to the carbon double bond are on different sides
trans fatty acids…
- 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
Triglycerides
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)
triglycerides are formed when…
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
Triglycerides can be either…
saturated or unsaturated, depending on the composition of the fatty acid chains
types of fats
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
Regulating Blood Cholesterol Levels
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
LDLs vs. HDLs
Hence LDLs raise blood cholesterol levels (‘bad’) while HDLs lower blood cholesterol levels (‘good’)
High intakes of certain types of fats will differentially affect cholesterol levels in the blood
- 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
Health Risks of High Cholesterol
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
Lipid Health Claims
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
how were lipid health claims made?
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
Evidence Supporting Health Claims
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)
Evidence Against Health Claims
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.)
lipids
Lipids are more suitable for long-term energy storage in humans than carbohydrates
how do lipids and carbohydrates differ?
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)
Energy Storage Analogy
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)
Storing energy as carbohydrates (i.e. glycogen) is similar to keeping the cash in a wallet
- 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)
Storing energy as lipids (i.e. triglycerides) is similar to keeping the cash in a safe
- 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)
body mass index (BMI)
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
BMI formula
Body mass index can be calculated according to the following formula:
BMI = kg/m^2
BMI doesn’t apply to
- 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
Nomograms
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
