B1.1 Carbs and Lipids

Chemical properties of carbon 

Carbon forms covalent bonds 

• A covalent bond forms when a pair of electrons are shared between two atoms 

•  A single covalent bond is represented by a short straight line between the two atoms 

• Electrons are shaped between atoms to generate strong bonds within compounds 

Carbon in biological molecules 

• Carbon is present in all of the four major categories of biological molecules

• Carbon is present in 

  • Carbohydrates 

  • Lipids 

  • Proteins 

  • Nucleic acids 

• Carbon has four electrons in its outer shell meaning each atom can form four covalent bonds

  • Carbon can be a component of large, stable molecules 

• Carbon forms millions of different covalently-bonded compounds, mainly with H and O 

• Carbon atoms can arrange themselves to form a huge variety of chemical compounds 

  • Bond to other carbon atoms or other atoms such as H, N, O, S 

  • Form molecules with long branches chains such as glycogen 

  • Form long straight chain molecules such as cellulose 

  • Form molecules containing cyclic single rings such as the pyrimidines (thymine, uracil, and cytosine)

  • Form molecules with multiple rings, including starches and the prunes (adenine and guanine)

  • Produce a tetrahedral structure which allow the formation of varied carbon compounds which have different 3-D shapes and different biological properties 

• Carbon atoms can form up to four single covalent bonds or combination of double and single bond 

  • CO2 contains two double bonds 

  • Methane contains four single covalent bonds 

• Double and triple bonds can form with an adjacent carbon atom, allowing unsaturated compounds to form 

• Carbon atoms can also form part of many different functional groups that give organic compounds their individual properties 

  • Hydrozyl groups 

  • Carbozyl groups 

  • Amino groups 

  • Phosphate groups 

NOS: Scientific conventions are based on international agreement

• The professional scientific community is global, meaning that scientists from all over the world may work on the same research, and need to be able to communicate clearly with each other

• Scientific conventions are thereby agreed upon and used internationally

  • SI (which stands for système international) unit prefixes is one example

    • kilo = 10^3

    • Centi = 10^-2

    • Mili = 10^-3

    • Micro = 10^-6

    • Nano = 10^-9 

Formation of macromolecules 

• Carbon compounds can be large molecules made from many small,repeating subunits 

  • Monomers are the smaller units from which larger molecules are made 

  • Polymers are molecules made from a large number of monomers joined together in a chain 

  • The process by which monomers join to form polymers is polymerisation 

• Macromolecules are very large molecules 

• They contain 1000 or more atoms and so have a high molecular mass 

• Polymers can be macromolecules, however, not all macromolecules are polymers; polymers must consist of many repeating subunits 

  • E.g. lipids are not polymers, as they do not consist of repeating monomers

Formation of macromolecules 

• Macromolecules are formed during condensation reactions 

• A condensation reaction occurs when molecules combine together,forming covalent bonds and resulting in polymers (polymerisation) or macromolecules

•  Water is removed as part of the reaction 

Examples of condensation reactions 

• Polysaccharides

  •  Polysaccharides are formed when two hydroxyl(OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond 

• Polypeptides

  •  Polypeptides are formed by condensation reactions Two amino acid monomers interact to form a strong covalent bond called a peptide bond

• Nucleic acids 

  • Separate nucleotides are joined together via condensation reactions to form a phosphodiester bond These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide Itis called a phosphodiester bond because it consists of a phosphate group and two ester bonds

Digestion of Polymers 

• Macromolecules often need to be broken down into their monomers, e.g.this happens in digestion 

• The reaction that allows this to occur is a hydrolysis reaction 

  • Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water) I

• In the hydrolysis of macromolecules, covalent bonds are broken when water is added 

  • The -O and -OH from the water molecule are used to form the functional groups of the products

•  Examples of hydrolysis reactions include: 

  • The hydrolysis of glycosidic bonds in poly- or disaccharides to produce monosaccharides 

  • The hydrolysis of peptide bonds in polypeptides to produce amino acids 

  • Hydrolysis of ester bonds in triglycerides to produce three fatty acids and glycerol 

Monosaccharides 

• The monomers of carbohydrates are monosaccharides

  •  Two monosaccharides can join to form a disaccharide 

  • Many monosaccharides join to form a polysaccharide 

• Monosaccharides can join together via condensation reactions 

  • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond 

• Monosaccharides have the general formula Cn H2n On 

  • Where 'n' is the number of carbon atoms in the molecule 

  • Note that this formula only applies to monosaccharides 

• Monosaccharide properties include:

  •  Colourless crystalline molecules 

  • Soluble in water 

• There are different types of monosaccharide formed from molecules with varying numbers of carbon atoms,for example: 

  • Triose molecules contain 3 carbon atoms, e.g. glyceraldehyde 

  • Pentose molecules contain 5 carbon atoms, e.g. ribose 

  • Hexose molecules contain 6 carbon atoms, e.g. glucose

Glucose 

• The most well-known carbohydrate monomer is glucose

•  Glucose has the molecular formula C6H12 O6

  •  Glucose is the most common monosaccharide and is of central importance to most forms of life

  •  Glucose is the main substrate used in respiration, releasing energy for the production of ATP 

  • Glucose is produced during photosynthesis 

• Glucose exists in two structurally different forms, alpha (α) glucose and beta (β) glucose,these structures are known as the isomers of glucose 

  • This structural variety results in different functions between carbohydrates

  •  This seemingly minor example of isomerism has far-reaching consequences on the functions of the polymers 

  • Different polysaccharides are formed from the two isomers of glucose

  • Starch and glycogen are made from molecules of alpha glucose

  •  Cellulose is made from molecules of beta glucose

Properties of glucose 

• Glucose has several properties that are essential to its function in living organisms 

  • Stable structure due to the presence of covalent bonds which are strong and hard to break

  • Soluble in water due to its polar nature

  •  Easily transportable due to its water solubility

  •  A source of chemical energy when its covalent bonds are broken

Polysaccharides: Energy Storage

 The function of carbohydrates

•  Carbohydrates function as essential energy storage molecules and as structural molecules 

• Starch and glycogen are effective storage polysaccharides because they are:

  •  Compact 

    • Large quantities can be stored in a small space

  •  Insoluble  

    • This is essential because soluble molecules will dissolve in cell cytoplasm, lowering the water potential and causing water to move into cells If too much water enters an animal cell it will burst 

• Cellulose is a structural polysaccharide because itis: 

  • Strong and durable 

  • Insoluble and slightly elastic 

  • Chemically inert;few organisms possess enzymes that can hydrolyse it

Starch 

• Starch is the storage polysaccharide of plants

  •  Starch is stored as granules in chloroplasts 

• It is made of alpha glucose monomers 

• Starch is constructed from two different polysaccharides: 

  • Amylose (10 - 30 % of starch) 

  • Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules 

  • The helix shape enables it to be more compact and thus it is more resistant to digestion

• Amylopectin (70 - 90 % of starch)

  •  Contains 1,4 glycosidic bonds between α-glucose molecules as well as 1, 6 glycosidic bonds, creating a branched molecule 

  • The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration, or added to for storage

Glycogen

•  Glycogen is the storage polysaccharide of animals and fungi 

• The monomer of glycogen is alpha glucose, joined by 1,4- and 1,6 glycosidic bonds 

• Glycogen is more branched than amylopectin, providing more free ends where glucose molecules can be removed by hydrolysis 

  • This means that glycogen can be broken down quickly, supplying the higher metabolic needs of animal cells

•  Liver and muscles cells contain glycogen as visible granules, enabling high rates of cellular respiration

Structure of Cellulose 

• Cellulose is a structural carbohydrate found in the cell walls of plants

•  Molecules of cellulose are straight and unbranched

•  Cellulose is a polymer of β-glucose monomers 

  • β-glucose differs very slightly in structure to α-glucose;the hydroxyl group on carbon 1 sits above the carbon ring in β-glucose, whereas it sits below the ring in α-glucose

  •  It means that in order to form a glycosidic bond with a molecule of β-glucose, every alternate molecule of β-glucose in the chain must invert itself, or flip upside down

• The alternating pattern of the monomers in cellulose allows hydrogen bonding to occur between strands of β-glucose monomers, adding strength to the polymer 

  • Hydrogen bonds link several molecules of cellulose to form microfibrils

Role of Glycoproteins 

• Carbohydrates and polypeptides can combine, via covalent bonds,to make structures called glycoproteins

  •  These are classed as proteins 

• Glycoproteins, along with another group of molecules called glycolipids,form part of the structure of cell surface membranes 

• They act as receptor molecules in processes such as 

  • Cell recognition and identification

  •  Receptors for cell signalling molecules such as hormones and neurotransmitters 

  • Endocytosis 

  • Cell adhesion and stabilisation 

Glycoproteins and ABO blood types 

• Glycoproteins can act as antigens which can identify cells as either"self" or"non-self"

  • Cells that are recognised as non-self will trigger an immune response within the organism 

• A person's blood type is determined by the glycoprotein antigens on the surface of their red blood cells

  •  Blood type A individuals have type A glycoprotein antigens

  •  Blood typeBindividuals have typeBglycoprotein antigens 

  • Blood type AB individuals have both types of glycoprotein antigens 

  • Blood type O individuals have neither 

• The presence of antibodies within an individual can create an interaction with the glycoproteins if blood of the wrong type enters their body

  •  E.g. a person with Type A antigens on their red blood cells will have antibodies in their blood against type B antigens 

• This can cause fatal issues during blood transfusions if the incorrect blood type is given, as the antibodies cause the incorrect antigens (from the transfused blood)to clump together, blocking blood vessels

Lipids : Hydrophobic Properties 

• Examples of lipids in living organisms are 

  • Fats 

  • Oils 

  • Waxes

  • Steroids 

• Lipid macromolecules contain carbon, hydrogen, oxygen atoms 

Lipid solubility 

• The structure of lipids affects their solubility 

• Lipids contain hydrocarbon molecules which contain many non-polar covalent bonds 

• The non-polar nature of lipid molecules means that lipids are insoluble in water or other polar solvents

•  In living organisms, lipid solubility can be improved by combining lipid molecules with other molecules, 

  • e.g. Glycolipids Lipoproteins

Formation of Triglycerides & Phospholipids

•  Formation of triglycerides 

  • Some lipids are categorised as triglycerides 

  • Three fatty acids join to one glycerol molecule to form a triglyceride 

    • Fatty acids contain hydrocarbon chains that can be either saturated or unsaturated 

      • Saturated fatty acids contain only single carbon-carbon bonds 

      • Unsaturated fatty acids contain one or more double bonds

  •  Triglycerides are formed by a process known as esterification

    •  An ester bond forms when the hydroxyl (-OH) group of a glycerol molecule bonds with the carboxyl group (-COOH) of a fatty acid 

    • The formation of an ester bond is a condensation reaction 

      • For each ester bond formed a water molecule is released

      •  Therefore for one triglyceride to form,three water molecules are released

Formation of phospholipids 

• Phospholipids are also formed from glycerol and fatty acids 

• Unlike triglycerides, phospholipids contain only two fatty acids bonded to a glycerol molecule, as the third has been replaced by a phosphate ion (PO )

•  As the phosphate is polar, it is soluble in water, or hydrophilic 

• The fatty acid ‘tails’ are non-polar and therefore insoluble in water, or hydrophobic 

• Phospholipids are said to be amphipathic, meaning that they have both hydrophobic and hydrophilic regions 

• As a result of having hydrophobic and hydrophilic parts, phospholipid molecules can form monolayers or bilayers when placed in water 

Properties of Triglycerides

 Lipids as an energy store

•  The hydrolysis of triglycerides releases glycerol and fatty acids, which can form useful respiratory substrates

•  Lipids are energy-dense in comparison to carbohydrates due to their high number of C-H bonds

  •  They contain 2× more energy per gram than most carbohydrates 

• Lipids are insoluble so are not transported around the body easily and remain in their storage cells 

• When lipids are respired a lot of water is produced compared to the respiration of carbohydrates 

  • This is called metabolic water and can be used as a dietary water source when drinking water is unavailable

    • A camel's hump is not filled with water, but is a lipid-rich storage organ that yields metabolic waterforthe camel in its dry desert habitat 

    • A bird's egg also makes use of lipid-rich yolk to provide energy and metabolic water to the growing chick 

• All these features make lipids ideal for long term energy storage 

Storage of lipids

• In animals, lipids are stored in adipose tissue 

• Subcutaneous fats are stored below the skin 

  • Visceral fats are stored around the major internal organs 

• Fat is stored in adipose cells, which are specialised to contain large globules of fat 

  • Adipose cells shrink when the fat is respired to generate metabolic energy 

• Adipose tissue can be used as a thermal insulator in animals that live in particularly cold environments 

  • Seals and walruses are endotherms and have thick adipose tissue called blubber which helps trap heat generated by respiration 

• In many plants, seeds have evolved to store fats to provide energy for a growing seedling plant 

  • Olives, sunflowers, nuts, coconuts and oilseed rape are good examples of crops whose oils are harvested for edible oil production by humans

Fatty Acids 

• Both triglycerides and phospholipids contain glycerol with molecules known as fatty acids attached 

• These fatty acids have long hydrocarbon ‘tails’ 

  • Hydrocarbons are molecules that contain hydrogen and carbon

•  Fatty acids occur in two forms:

  •  Saturated fatty acids 

  • Unsaturated fatty acids 

    • Unsaturated fatty acids can be monounsaturated or polyunsaturated 

• Saturated fatty acids 

  • In saturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are all single bonds 

  • The fatty acid is said to be ‘saturated’ with hydrogen 

    • This means that each carbon atom in the hydrocarbon tail (exceptforthe final carbon atom) is bonded to two hydrogen atoms

• Saturated fatty acids are straight molecules, meaning that lipid molecules containing them are able to pack tightly together 

• This increases their melting point and causes them to be solid at room temperature 

• Saturated fatty acids are often used as storage molecules in animals for this reason, e.g.the fats in meat and butter 

Unsaturated fatty acids

• In unsaturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds 

  • The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible; each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom instead of two 

• These double bonds can cause the hydrocarbon tail of unsaturated fatty acids to kink, or bend, meaning they are not as straight as saturated fatty acids 

  • Unsaturated fatty acids cannot pack as tightly together as saturated fatty acids, so fats containing unsaturated fatty acids are often liquids at room temperature 

• Unsaturated fatty acids contain at least one carbon-carbon double bond 

  • A fatty acid with one C=C double bond is known as monounsaturated fatty acid

    •  Lipids that contain monounsaturated fatty acids have a lower melting point than saturated fatty acids, meaning that they form liquid oils; some animals and plants store energy in the form of oils 

- In some unsaturated fatty acids,there are many carbon-carbon double bonds;these are known as polyunsaturated fatty acids

•  Lipids containing polyunsaturated fats also have a low melting point, so form oils that are used for energy storage in plants

Formation of Phospholipid Bilayers

•  Phospholipids form the basic structure of the cell membrane

•  Cell membranes are phospholipid bilayers 

• Membranes are formed when a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails 

• Phospholipids have a hydrophobic and a hydrophilic region 

  • The phosphate head of a phospholipid is polar, so is hydrophilic and therefore soluble in water 

  • The fatty acid tail of a phospholipid is nonpolar, so is hydrophobic and therefore insoluble in water 

• Molecules with both polar/hydrophilic and non-polar/hydrophobic regions are said to be amphipathic

• When phospholipids are mixed with water,two-layered structures known as phospholipid bilayers can form;this is the basic structure of the cell membrane 

• The amphipathic nature of phospholipids means that the phospholipid bilayer acts as a barrier to most water-soluble substances

  •  The non-polarfatty acid tails prevent polar molecules or ions from passing between them across the membrane

•  This means that water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get