Biology IB HL - 2.3 Carbohydrate and Lipid

Carbohydrates are made of C, H and O (‘carbo’ – contains carbon ; ‘hydrate’ – contains H and O)

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

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

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 (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 (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 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 is an energy storage polysaccharide found in plants

It is composed of α-glucose subunits (bound in a 1-4 arrangement)

exists in one of two forms – amylose or amylopectin

Amylose is a linear (helical) molecule

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 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 are long hydrocarbon chains that are found in certain types of lipids (triglycerides & phospholipids)

YES

Fatty acids may differ in the length of the hydrocarbon chain (typically 4 – 24 carbons) and in the number of double bonds

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

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

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

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 are the largest class of lipids and function primarily as long-term energy storage molecules

Animals tend to store triglycerides as fats (solid)

plants tend to store triglycerides as oils (liquid)

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

all types of fats consumed as part of dietary intake will cause adverse health effects if taken in excessive amounts

The mix of fats in the diet influences the level of cholesterol in the bloodstream

Saturated fats and trans fats raise blood cholesterol levels,

(cis) unsaturated fats lower blood cholesterol levels

NO

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

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

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

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

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

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.)

Storage

Osmolarity

Digestion

ATP yield

Solubility

lipids are more suitable for long-term energy storage

lipids have less of an effect on the osmotic pressure of a cell

carbohydrates are easier to digest and utilise

(lipids store more energy per gram

carbohydrates are easier to transport in the bloodstream

The body mass index (BMI) provides a measure of relative mass based on the weight and height of the individual

BMI = mass in kg/ height in m ^2

BMI ranges from underweight to obese, according to predetermined values based on an average adult population

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

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