2.3 Carbohydrates and lipids

Units

- monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers
- General formula: Cx(H2O)y
- Monosaccharides, disaccharides and polysaccharides
- Mon and dis are polar molecules and soluble in water
- Poly are large macromolecules and are insoluble in water

Monosaccharide \#1 - Glucose

- Glucose has the formula C6H12O6
- It forms a hexagonal ring (hexose)
- Glucose is the form of sugar that fuels respiration
- Glucose forms the base unit for many polymers
- 5 of the C atoms form corners of the ring with the 6th by O

Monosaccharide \#2 - Galactose

- Is also a hexose sugar
- It has the same formulas but is less sweet
- Most commonly found in milk, but also found in cereals

Monosaccharide \#3 - Fructose

- Fructose in another pentode sugar
- Commonly found in fruits and honey
- It is the sweetest naturally occurring carbohydrate

Monosaccharide \#4 - Ribose

- Ribose in a pentode sugar (Pentagonal ring)
- It forms the backbone of RNA
- Deoxyribose differs as shown in the diagram

Disaccharides

- Two monosaccharides bonded together by a condensation reaction
- The bond formed between 2 monosaccharides is called a glycosidic bond
- Condensation reactions are anabolic and therefore require more energy
- Most condensation reactions are between the hydroxyl (-OH) group of C1 and C4 on the other monomer
- Disaccharides can be broken apart in monosaccharides by hydrolysis

Disaccharide \#1 - Maltose

- Maltose (C12H22O11)
- Is a dimer of glucose

Disaccharide \#2 - Lactose

- Lactose (C12H22O11) is most commonly found in milk
- The two monosaccharides that make it up are glucose and galactose

Disaccharide \#3 - Sucrose

- Sucrose (C12H22O11) is also known as table sugar
- The two monosaccharides that make it up are glucose and fructose

Some condensation reactions

1. Glucose + Glucose —\> Maltose + Water
2. Glucose + Galactose —\> Lactose + Water
3. Glucose + Fructose —\> Sucrose + Water

Polysaccharides

- E.g. glycogen
- Are polymers with more than two molecules
- They are often very long and may be branched
- Glycosidic bonds can be C1-4 or C1-6

Polysaccharide \#1 - Cellulose

- Cellulose is made by linking together beta-glucose molecules
- Condensation reactions link C1-4 on the next beta glucose (1,4 glycosidic bonds)
- The glucose subunits in the chain are oriented alternately upwards and downwards
- The consequence of this is that the cellulose molecule is a straight chain, rather than curved
- Cellulose molecules are un-branched chains of beta-molecules
- The linked molecules form bundles called cellulose microfibrils
- They have very high tensile strength
- The tensile strength of cellulose (the basis of cell walls) prevent plant cells from bursting, even under high H2O pressure

Polysaccharide #2 - Starch

• Amylose and Amylopectin are both forms of starch and made up from repeating glucose units

• Starch is made by linking together alpha molecules

• Size of molecule is not fixed

• Starch is only made by plant cells

• Molecules of both types of starch are hydrophilic but are too large to be soluble

• Starch does not affect the osmotic balance of cells (i.e. cause too much water to enter)

• It is easy to add or remove extra glucose molecules to starch (good energy store)

• Starch is made as a temporary store in leaf cells when glucose is being made faster by photosynthesis than it can be exported to other parts of the plant

Amylopectin

- The chain is branched so has more globular shape
- Has 1,4 and 1,6 glycosidic bonds
- Due to its branched nature, amylopectin can be much larger consisting of 2,000 - 200,000 units

Amylose

- The chain of alpha glucose molecules in un-branched and forms a helix
- 1,4 glycosidic bonds
- Typically amylose is made up of 300 - 3,000 glucose units, so smaller than amylopectin

Polysaccharide #3 - Glycogen

• Glycogen is not just a simple chain, it branches many times

• Condensation reactions link C1 - 4 on the next alpha glucose

• Branches occur where a condensation reaction links C1-6 on the next

• As a result, the molecule is compact

• Glycogen is a polymer made from repeating glucose subunits

• The molecules varies in size, typically it consists of 30,000 units

• Glycogen is made by animals and also some fungi

• It is stored in the liver and some muscles in humans

• It is used in cells where large stores of dissolved glucose would cause osmotic problems

• Glycogen does not affect the osmotic balance of cells (i.e. cause too much water to enter)

• It is easy to add or remove extra glucose molecule to starch

• Therefore, glycogen is useful in cells for glucose, and consequently, energy storage

What are lipids?

- Non-polar, insoluble in water and hydrophobic
- Soluble in organic solvents (e.g. ethanol)
- Made primarily of CHO

Fatty acids

- General formula
H3C - (CH2)n - C\=O/OH
Chain (or ring) of C & H atoms
Carboxylic group
- When decided whether a fatty acid is saturated or unsaturated, pay attention to double bonds between Cs (C\=C), not C and any other molecule
- Carboxylic acids contain the carboxyl functional group (-COOH)
- Can be saturated or unsaturated
- Saturated fatty acids contains NO double bond
- Unsaturated fatty acids contain one or more double bonds
- Double bonds found in the C chain

Phospholipids

- Where one fatty acid chain is replaced by a phosphate group
- Main component of cell membranes
- Very different structure (4 fused rings instead of straight chains)
- Hydrophobic and insoluble in water so classed as a lipid
- Cholesterol and sex hormones (testosterone, oestrogen...) are examples of steroids

Condensation reaction

- Condensation reaction between glycerol and fatty acids
- Lipids are glycerol combined with 1,2, or 3 fatty acids, therefore, triglycerides are lipids
- Covalent bonds called ester bonds are formed between the fatty acids and glycerol molecules
- Hydrolysis is the reverse of this process (catalysed by lipase)

Unsaturated fatty acids:

- Monounsaturated fatty acids contain one double bond in the C chain
- Polyunsaturated fatty acids contain two or more double bonds in the C chain

Cis and Trans - unsaturated fatty acids

- Unsaturated fatty acids can be cis or trans depending on the orientation of the H- atoms around the double bond
- If both H atoms are on the same side, it is a cis fatty aicd
- If H atoms are one different sides, it is a trans fatty acid

Isomers

- Identical molecular formula but different structural formulas
- Isomer do not necessarily have the same properties

Cis- isomers

- Very common in nature
- H atoms are on the same side of the C atoms
- The double bond causes a bend in the fatty acid chain
- Therefore, cis-isomers are only loosely packed
- Triglycerides formed from cis-isomers have low m.p, they are usually (l) at r.t

Trans- isomers

- Rare in nature; usually artificially produced to produce solid fats (e.g. margarine)
- The H atoms are on different sides of the 2 C atoms
- The double bond does not cause a bend in the fatty acid chain
- Trans-isomers can be closely packed
- Triglycerides formed from trans-isomers have high m.p; they are usually (s) at r.t

Functions of Lipids

• Structure : phospholipids are a main component of cell membranes

• Hormonal signalling : steroids are involved in hormonal signalling

• Insulation : Fats in animals can serve as heat insulators while sphingolipids in the myelin sheath (of neurons) can serve as electrical insulators

• Protection : Triglycerides may form a tissue layer around many key internal organs and provide protection against physical injury

• Storage of energy : Triglycerides can be used as a long-term energy storage source

• Lipids are normally used for long-term energy storage whereas carbohydrates are used for short-term energy storage ~ The lipids that are usually fats ~ Stored in specialised groups of cells called adipose tissue ~ Located immediately beneath the skin and also around some organs including the kidney

Reasons for using Lipids for long-term Energy storage

- The amount of energy release in cell respiration per grams of lipids is double than for carbohydrates (& proteins)
- Lipid add 1/6 as much to BM as carbohydrates: fats are stored as pure droplets whereas 1g glycogen stored is associated with 2g of H2O
~ Especially critical for active animals as energy stores have to be carried

Why is glycogen needed at all

- Because glycogen can be broken down into glucose rapidly
- Then transported easily (since glucose is soluble) by the blood to where it's needed
- Fats in adipose tissue cannot be mobilised as rapidly
- Glucose can be used either in anaerobic or aerobic cell respiration whereas fats and fatty acids can only be used in aerobic respiration
- Glycogen in the bloodstream is for immediate use and will either be used in respiration to yield ATP or converted to glycogen or fat

Determination of BMI by calculation or use of a nonogram

- BMI is a measure used to assess a person's weight relatively to their height
- It does not give indication of body composition or fat distribution
~ Therfore is not very useful in people who are very muscular as muscle tissue is denser than fat tissue
- However, it can be a useful indicator of health risk in some individuals
- BMI is calculated the same way for adults and children

BMI: (Mass in kg)/ (Height in meters^2)
N.b units for BMI are kg-m^2

Scientific evidence for health risks of trans fats and saturated fatty acids

• A positive correlation has been found between saturated fatty acids and rates of CHD in studies

• Coronary arteries become partially blocked by fatty deposits, leading to blood clot formation and heart attacks

• Correlation does not equal cosation ~ Another factor e.g. dietary fibre could be responsible

• There are populations that do not fit the correlation such as the Maasai of kenya ~ They have a diet that is rich in meat, fat, blood and milk ~ They therfore have high consumption of saturated fats, yet CHD is almost unknown among the Maasai

• Diets rich in olive oil, which contain cis- monounsaturated fatty acids are traditionally eaten in countries around the Mediterranean ~ The population of these countries typically have Low rates of CHD and it has been claimed that this has been due to the intake of cis - monounsaturated fatty acids

• Genetic factors in these populations could be responsible

• Other aspects of the diet could explain the CHD rates

• There is also positive correlation between the amounts of trans-fat consumed and rates of CHD

• Other risk factors have been tested, to see if they account for the correlation, but none did ~ Trans-ft therefore probably do cause CHD

• In patients who had died from CHD, fatty deposits in the diseased arteries have been found to contain high concentration of trans-fats, which give more evidence of a causal link

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: Correlation does not equal cosation
~ Maasai population
~ Dietary fibre could be responsible
- Intervention studies have shown that lowering dietary intakes of saturated fats reduces factors associated with the dev- of CHD (blood cholesterol levels, BP, etc.)
Counter: Validity of intervention studies is dependent on size and composition of cohort, as well as duration of the study
- In patients who have died from CHD, fatty deposits in diseased arteries were found to contain high concentrations of trans fats
Counter: Genetic factors may play a role
~ 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 increased 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. diabetics, obesity)
Counter: Incidence of CHD associated on a myriad of factors besides dietary intake (e.g. exercise, access to healthcare, etc.)