2.1.2 Biological molecules

what biological molecules are in carbohydrates?

C, H, O

what biological molecules are in lipids?

C, H, O

what biological molecules are in proteins?

C, H, O, N, S

what biological molecules are in nucleic acids?

C, H, O, N, P

what are the main cations used in biological processes?

Ca2+ - used for nerve impulses, muscle contractions

Na+ - used for nerve impulses, kidney function

K+ - used for nerve impulses, stomata

H+ - used for pH determination, catalysts

NH4+ - used for making nitrogen ions

what are the main anions used in biological processes?

NO3- - used for amino acids

HCO3- - used for maintaining blood pH

Cl- - used for balancing sodium and potassium levels in cells

PO₄3- - used for cell membranes, nucleic acids and ATP formation, bone formation

OH- - used for pH determination, catalysts

what is a hydrolysis reaction?

breaks molecules apart

requires/uses water

what is a condensation reaction?

joins molecules together

forms water

what is a hydrogen bond?

covalent bond between hydrogen atoms

what is the structure of water?

oxygen is more electronegative than hydrogen - electrons move around oxygen more

water is therefore polar - a molecule with an unequal share of electrons

what are the properties of water?

cohesive - due to H bonds, creates surface tension, allows flow and transport of substances in organisms, e.g. xylem

solvent - good transport medium, can transport solutes

polar - can dissolve polar, non-polar and ionic molecules, good polar solvent, good reaction medium

metabolite - reactant or product in many important metabolic reactions

high specific heat capacity - due to H bonds, acts as a buffer against rapid temperature change

high latent heat of vaporisation - due to H bonds, lots of heat can be lost without losing lots of water

more dense than ice

what are carbohydrates?

used as energy sources

form hexoses or pentoses

what is a monosaccharide?

a single sugar

e.g. glucose, fructose, ribose, galactose

soluble in water, insoluble in non-polar solvents, sweet reducing sugars

what is a disaccharide?

two monosaccharides joined in a condensation reaction with glycosidic bonds

e.g. maltose = glucose + glucose, sucrose = glucose + fructose, lactose = glucose + galactose

soluble in water, sweet reducing sugars except sucrose

what is a polysaccharide?

many monosaccharides joined in condensation reactions by glycosidic bonds

e.g. glycogen, starch, cellulose

large, insoluble molecules, used as energy stores and storage molecules

what is a glycosidic bond?

covalent bond between carbohydrates

how are disaccharides and polysaccharides formed?

condensation reaction

how are disaccharides and polysaccharides broken down?

hydrolysis reaction

what are pentose and hexose sugars?

pentose sugars have 5 carbons - e.g. ribose, deoxyribose

hexose sugars have 6 carbons - e.g. glucose

what is glucose?

monosaccharide

hexose sugar

major energy source

highly soluble - main way carbohydrates are transported in animals

has 2 structural isomers - alpha glucose and beta glucose

what is alpha glucose?

an isomer of glucose

\-OH group is below the carbon ring

forms polysaccharides e.g. amylose and amylopectin (starch), glycogen

what is beta glucose?

an isomer of glucose

\-OH group is above the carbon ring

forms polysaccharides e.g. cellulose

what is starch?

main carbohydrate storage in plants

made of amylose and amylopectin

produced from glucose made in photosynthesis

broken down during respiration to provide energy

what are amylose and amylopectin?

amylose - has 1-4 glycosidic bonds, helical, insoluble

amylopectin - has 1-4 and 1-6 glycosidic bonds, highly branched, insoluble

both made from alpha glucose

what is glycogen?

storage molecule in animals

has alpha 1-6 glycosidic bonds

branched structure

found as small granules in the muscles and liver

less dense and more soluble than starch - broken down more quickly

what is cellulose?

main component of plant cell walls - chains form microfibrils which are layered to form a network

has beta 1-4 glycosidic bonds

unbranched chains

most abundant organic polymer

what is the iodine test?

test for starch

dissolve the substance in water then add the iodine solution

a positive result is a colour change from red/brown to blue/black

what is a reducing sugar?

a sugar that can reduce (give electrons) to other molecules

what is the test for reducing sugars?

dissolve the substance in water then add benedict’s reagent

heat the solution in a water bath for 2 minutes

a positive result is a colour change from blue to green-brick red depending on the concentration of reducing sugars present

what is the test for non-reducing sugars?

perform the test for reducing sugars first

add hydrochloric acid and heat in a water bath for 2 minutes

neutralise the solution with sodium hydrogencarbonate

add benedict’s reagent and heat the solution in a water bath for 2 minutes

same positive results as reducing sugars test

what are the three types of lipids?

triglycerides (fats and oils)

phospholipids

cholesterol

what is adipose tissue?

fatty connective tissue that surrounds the organs

good thermal insulator

protects the organs from injury

what is a triglyceride?

one glycerol and three fatty acid chains

non-polar molecule

hydrophobic

contain a lot of energy

what are the functions of triglycerides?

major source of energy - provide 2x as much energy as carbohydrates

source of water for metabolic reactions

used to store energy

how are triglycerides formed?

esterification - condensation reaction

ester bonds formed between glycerol and fatty acids

phospholipids are formed the same way

how are triglycerides broken down?

broken down by lipase

hydrolysis reaction

what is glycerol?

C₃H₈O₃

has three hydroxyl groups

what is a saturated fatty acid?

contains only single covalent bonds between carbon atoms

solid at room temperature

fat

what is an unsaturated fatty acid?

monosaturated fatty acid - contains only one double covalent bond between carbon atoms

polysaturated fatty acid - contains many double covalent bonds between carbon atoms

liquid at room temperature

oil

what is the difference between fats and oils?

fats are solid at room temperature and oils are liquid - due to the double bond in the hydrocarbon tail of unsaturated fatty acids in oil, they move further away from each other than saturated fatty acids do, and this weakens intermolecular forces

what is a phospholipid?

contains a phosphate group, glycerol and two fatty acid tails

has a negative polar hydrophilic head (phosphate group and glycerol) and non-polar hydrophobic tails (two fatty acids)

phosphate ester bond forms when phosphoric acid joins to glycerol - condensation reaction

in water they position themselves with the hydrophilic head outwards and the hydrophobic tails cluster together away from the water - forms droplets

what are the functions of phospholipids?

form phospholipid bilayers in cell membranes

what is cholesterol?

part of the lipid family called sterols

the hydroxyl group is hydrophilic and the rest is hydrophobic

can insert into cell membranes - hydrophilic hydroxyl group interacts with the polar head of phospholipids and the rest interacts with the hydrophobic tails

what are the functions of cholesterol?

controls the fluidity of cell membranes

starting point of many hormones - e.g. oestrogen, progesterone

used to make vitamin D

used in the liver to make bile - emulsifies lipids, increases the digestion of lipids

what is the emulsion test?

test for lipids

dissolve the substance in water and add ethanol

a positive result is a white emulsion/precipitate

can be difficult to see if the initial substance is cloudy

what is an amino acid?

monomer of protein

what is the structure of amino acids?

all have the same general structure - central carbon atom bonded to a carboxyl group (-COOH) and an amino group (-NH₄), a hydrogen atom and an R group

an R group is a carbon chain which is different in each amino acid

they differ in size, polarity and charge

how are peptide bonds formed?

condensation reaction

takes place in the ribosomes

how are peptide bonds broken down?

hydrolysis reaction

catalysed by protease

what is a dipeptide?

two amino acids joined by a peptide bond - condensation reaction

what is the difference between polypeptides and proteins?

a protein is a polypeptide folded into a complex 3D shape - once a polypeptide is folded it can carry out its function

many proteins have multiple polypeptide chains

proteins often contain other molecules (prosthetic groups) which help them carry out their functions - e.g. haemoglobin

what is the primary structure of proteins?

the order of amino acids in a polypeptide

determined by the DNA sequence of the gene that encodes the polypeptide

determines the final 3D shape of the protein

what is the secondary structure of proteins?

the shape that the chain of amino acids takes - alpha helix and beta pleated sheets

shape is determined by weak hydrogen bonds

what is the tertiary structure of proteins?

the overall 3D shape of the polypeptide

held together by: hydrogen bonds, ionic bonds, disulphide bonds, and hydrophilic and hydrophobic interactions

what is the quaternary structure of proteins?

only applies to proteins with at least 2 polypeptide chains (subunits)

may also contain a prosthetic (non-protein) group - proteins with these are called conjugated proteins

what are the bonds in the tertiary and quaternary structures of proteins?

form between the R groups of amino acids in polypeptide chains

type of bonding depends on the amino acids present and contributes to the final 3D shape

1) hydrogen bonding - between the slightly negative oxygen atom of a hydroxyl group of one amino acid and the slightly positive hydrogen atom of another, weak bonds, can be broken by high temperatures or pH changes

2) ionic bonding - between amino acids with oppositely charged R groups, can be broken by changes in pH

3) disulphide bonds - between the R groups of cysteines, strong, not broken by temperature or pH changes

4) hydrophilic and hydrophobic interactions - weak bonds

what are the 2 types of proteins?

globular protein

fibrous protein

what are globular proteins?

soluble in water - hydrophilic R groups on their surface

spherical

have more functional roles - e.g. metabolic

e.g. haemoglobin, insulin, lysozyme

what is haemoglobin?

has 4 polypeptide subunits - 2 alpha, 2 beta

found in red blood cells

conjugated protein with the prosthetic group haem - contains Fe2+ ions which bind to the oxygen

each molecule can bind to 4 oxygens - when they bind, the molecule conforms to make it easier for others to bind

what is insulin?

has 2 polypeptide subunits - linked by disulphide bonds

hormone transported in the blood

plays a role in blood glucose regulation

binds to specific receptors on the cell membranes of target cells - shape fits perfectly into the receptor

what is lysozyme?

has a single polypeptide chain

enzyme found in tears and saliva

catalyses the breakdown of a molecule in bacterial cell walls

what are fibrous proteins?

insoluble in water - hydrophobic R groups on its surface

long strands

have more structural roles

e.g. collagen, keratin, elastin

what is collagen?

has 3 polypeptide chains that form a triple helix - many hydrogen bonds join them together

found in tendons (connect muscle to bone) and ligaments (connect bones to each other)

insoluble in water - every third amino acid is glycine (smallest R group), so chains can wrap very tightly

many of the triple helical molecules join to make fibrils and microfibrils - molecules are staggered to avoid weak spots

what is keratin?

found in hair, fingernails, the outer layer of the skin, etc.

insoluble

strong - due to many disulphide bonds

what is elastin?

has 2 polypeptide chains - contain hydrophobic regions, cross linked

found in arteries, skin, lungs, bladder, blood vessels

allows structures to stretch

when they are stretched, the hydrophobic regions on different strands (usually associated) move apart but remain attached at the crosslinks - after stretching, the molecules reassociate and spring back together

what is the biurets test?

test for proteins

dissolve substance in water then add biuret’s solution (or sodium hydroxide and copper sulphate)

colour change is due to a complex forming between nitrogen atoms in peptide chains and Cu2+ ions

a positive result is a colour change from blue to purple