Monosaccharides and Glucose Isomers

• All carbohydrates contain carbon, hydrogen, and oxygen with the formula Cₙ(H₂O)ₙ, where n may be the same or different.
• Carbohydrates include sugars, starch, glycogen, and cellulose.

Monosaccharides

• Monosaccharides are simple sugars with the general formula (CH₂O)ₙ, where n ranges from 3 to 7.
• Monosaccharides are grouped by the number of carbon atoms: trioses (n=3), tetroses (n=4), pentoses (n=5), hexoses (n=6), and heptoses (n=7).
• Glucose is the most abundant hexose, with the chemical formula C₆H₁₂O₆.
• Isomerism is when a molecule can take up a number of different shapes, but has the same chemical formula.
• Important pentoses include ribose and deoxyribose and Important hexoses include fructose and galactose

Glucose Isomers

• Alpha and beta glucose are two common isomers that differ only in the arrangement of hydrogen and hydroxyl groups on carbon atom 1.
• In alpha glucose, the OH group is below carbon atom 1, while in beta glucose, it is above.
• Alpha glucose molecules combine to form starch, while beta glucose molecules form cellulose.
• The detailed positions of atoms in a biological molecule determine both the shape of the molecule and its function.

Disaccharides

• Disaccharides form when two monosaccharides combine through a condensation reaction, releasing a water molecule.
• Maltose consists of two glucose molecules.
• Sucrose consists of glucose and fructose.
• Lactose consists of glucose and galactose.
• Disaccharides can be broken down into monosaccharides by hydrolysis, which involves adding water.

Polysaccharides

• A molecule with between three and ten monosaccharide units is called an oligosaccharide.
• Polysaccharides are formed when many monosaccharide units are added via polymerisation.
• Polymers are large molecules made up of repeating units (monomers).
• Polysaccharides are polymers made up of monosaccharide monomers.
• Monosaccharide monomers may also combine with other types of molecule to form conjugated molecules.

Functions of Simple Sugars

• Glucose is a major energy source for most animals.
• Lactose is the main sugar in milk.
• Maltose is produced by the breakdown of amylose in germinating seeds.
• In plants, carbohydrate is moved as sucrose.

Polysaccharides

• Polysaccharides are polymers of monosaccharide units linked by condensation reactions.
• They have the general formula (C₆H₁₀O₅)ₙ, are relatively insoluble in water, not sweet, and cannot crystallise.
• Monosaccharides can join as straight, helical, coiled, or branched chains.
• Polymer properties depend on the number and type of monomers and how they are joined.
• Each carbon atom in a monosaccharide unit is identified by a number.
• A 1-4 glycosidic bond forms between the hydroxyl groups at carbon 1 of one monosaccharide and carbon 4 of another.
• A 1-6 glycosidic bond forms similarly between carbon 1 and carbon 6.
• Straight chains are formed by monomers linked by 1-4 glycosidic bonds, while branched chains have 1-6 glycosidic bonds.

Starch

• Starch is a polysaccharide of alpha glucose units, consisting of amylose and amylopectin.
• It is formed by the condensation of alpha glucose units.
• In amylose, units are linked by 1-4 glycosidic bonds forming unbranched chains.
• Amylopectin has more 1-6 glycosidic bonds, producing highly branched chains.
• Starch is compact, and ideal for storage.
• In flowering plants, starch granules are inside plastids, which may be amyloplasts in storage structures like potato tubers.

Glycogen

• Glycogen is a storage polysaccharide abundant in liver and muscle cells.
• It has a similar role and structure to starch, and is sometimes called 'animal starch'.
• Glycogen is more branched due to more 1–6 glycosidic bonds.
• Glycogen breaks down more rapidly than starch due to its structure.

Lipids

• Lipids are compounds insoluble in water but soluble in other lipids and organic solvents.
• Triglycerides are lipids made from glycerol and fatty acids (also called triacylglycerols or neutral fats).
• Glycerol is an alcohol with three carbon atoms each linked to a hydroxyl group.
• A fatty acid has a long hydrocarbon chain ending in an acidic carboxyl (-COOH) group.
• The carboxyl group ionizes in water, releasing a hydrogen ion (proton).
• A triglyceride forms when each hydroxyl group on glycerol combines with a carboxyl group on a fatty acid during condensation, forming an ester bond.
• Lipids are grouped as oils (liquid at room temperature) and fats (solid).
• Fatty acids can be saturated or unsaturated.
• A saturated fatty acid has each carbon atom in the hydrocarbon chain linked to a carbon atom on either side and also to two hydrogen atoms (bonded to the maximum number of other atoms).
• Saturated fatty acids have only single bonds, making the chain straight and allowing triglycerides to pack closely (tend to be solid at room temperature).
• An unsaturated fatty acid, has carbon atoms not bonded to the maximum number of other atoms (two or more carbon atoms have double bonds between them)
• Fatty acids with one double bond are called monounsaturated. Fatty acids with two or more double bonds are called polyunsaturated.
• The atoms around a double bond are in either the cis-form or the trans-form.
• Triglycerides with a high proportion of unsaturated cis-fatty acids tend to be oils, because the cis-double bonds twist the hydrocarbon chain, which prevents packing closely.
• Triglycerides with more unsaturated trans-fatty acids tend to be solids.

Functions of Fats and Oils

• Triglycerides have a higher proportion of hydrogen atoms than proteins or carbohydrates.
• Fats and oils have functions including; energy storage, heat insulation, shock absorption, buoyancy.

Phospholipids

• Form a part of cells membranes
• Consist of glycerol attached to two fatty acid chains.
• The third hydroxyl group of glycerol combines with phosphoric acid to form a polar phosphate group.
• Amphipathic, one end hydrophilic and the other end is hydrophobic.

Steroids

• Steroids contain four rings of carbon and hydrogen with various side chains.
• Many animal hormones are steroids, including oestrogen and testosterone, which are made from cholesterol.
• Cholesterol is a raw material for vitamin D and strengthens mammalian cell membranes.

Proteins

• Proteins are large complex biological molecules that play many roles in organisms.
• Proteins make up a large percentage of the structure, and their specific folding enable specific functions.

Types of Protein

• Proteins are grouped into seven major classes based on their functions:
  • Enzymes such as amylase.
  • Structural proteins.
  • Signal proteins.
  • Contractile proteins, myosin.
  • Storage protein, ovalbumin.
  • Defensive proteins.
  • Transport proteins, hemoglobin.
• Although there are many millions of proteins, all are made from the same basic building blocks, namely ~20 kinds of amino acid.

Amino Acids

• All amino acids have an amino group (-NH₂) and a carboxyl group (-COOH).
• The amino group is attached by a covalent bond to the alpha carbon.
• A hydrogen atom, another carbon atom, and a side chain (R group) are also linked to the alpha carbon.
• The R group is different for each of the 20 amino acids.

Amino Acids as buffers

• Amino acids are amphoteric (having both acidic and basic properties).
• Acidic properties are derived from the carboxyl group (can donate a proton).
• Basic properties are derived from the amino group (can take up a proton).
• Zwitterions have positively and negatively charged groups simultaneously.
• The ability to donate or receive protons causes amino acid solutions to behave as buffers.
• Buffer systems play a major role in the human body.

Peptide Bonds

• Two amino acids can combine to form a dipeptide by a condensation reaction, the resulting bond is called a peptide bond.
• Further amino acids connect to form a polypeptide chain.
• Proteins consist of one or more polypeptide chains, and are polymers of amino acid monomers.

Protein Primary Structure

• A protein's primary structure is the sequence of amino acids that make up its polypeptide chain or chains.

Secondary Structure

• When amino acids join up in the polypeptide chain, forces between molecule parts and hydrogen bonding cause coiling into an alpha-helix or folding into a beta-pleated sheet.
• This coiling or folding is the protein's secondary structure.
• The shape is maintained by hydrogen bonds between the -N-H group of one amino acid and -C=O group of another.
• A single polypeptide chain may have some regions coiled into an alpha-helix and others folded into beta-pleated sheets.

Tertiary Structure

• The tertiary structure refers to the overall three-dimensional shape of a polypeptide chain.
• Proteins are either fibrous or globular.
• Fibrous proteins are crossed linked parallel chains.
• Globular proteins have tightly folded polypeptide chains forming a spherical shape.

Quaternary Structure

• Many proteins consist of more than one polypeptide chain chemically bonded to each other
• Proteins are denatured if bonds holding structure are broken.

Nucleotides and Nucleic Acids

• Nucleotides are nitrogen-containing organic substances which play a vital part in every aspect of an organism’s life.
• Nucleotide molecules can be mononucleotides or polynucleotides that consist of three parts; a nitrogen-containing base, a five-carbon sugar and one or more phosphate groups.

ATP

• Adenosine triphosphate (ATP) is a mononucleotide which contains the base adenine, the sugar ribose and three phosphate groups.
• The covalent bond linking the second and third phosphate groups is unstable and easily broken by hydrolysis.
• When the bond is broken a phosphate group is removed and ATP becomes ADP (adenosine diphosphate) and at least 30kJ of energy is released.
• Energy is released in a process called an exergonic reaction.
• ATP can be resynthesised from ADP and inorganic phosphates by a condensation reaction
• This reaction gets the energy needed from respiration.
• The energy of the exergonic breakdown of ATP is coupled to energy-consuming (endergonic) reactions.
• Phosphate groups liberated from ATP attach to other molecules (phosphorylation), which energises them to work.