Macromolecules: A macromolecule is a large biological molecule, composed of polysaccharides, proteins and nucleic acids.
Macromolecules are of 3 types in living things:
Polysaccharides
Protein (polypeptides)
Nucleic acids (polynucleotides)
Monomer: Monomers are relatively simple molecules which are used as building blocks for the synthesis of polymers.
Many monomers are joined together by covalent bonds, usually by condensation reaction to make polymers.
Monomers are:
Monosaccharides
Amino acids
Nucleotides
Condensation reaction: A condensation reaction is a chemical reaction in which two molecules are joined together by the removal of a water molecule.
In a condensation, reaction water is produced
Hydrolysis: Hydrolysis is a chemical reaction in which a chemical bond is broken using water molecules. In this reaction, water is used up. Polymer: Polymers are giant molecules made from many similar repeating subunits joined together in a chain.
Subunits are smaller and simpler molecules called monomers.
Why can RNA be described as a polymer macromolecule?
RNA is a polymer because it is made up of many similar repeating subunits called RNA nucleotides. They are also macromolecules because they are large biological molecules.
Why can a polypeptide be described as a polymer?
A polypeptide is described as a polymer because it is made up of similar repeating subunits called amino acids.
Why are triglycerides and phospholipids not described as polymers?
Triglycerides and phospholipids are not made up of repeating subunits.
Carbohydrates: Carbohydrates are carbon, hydrogen and oxygen (C, H, O) containing compounds with the general equation: (CH2O)n
Carbohydrates can be classified/divided into 3 main groups:
Monosaccharides
They come in different types:
Hexose: six carbon-containing monosaccharides. e.g: Glucose, Fructose, Galactose.
Pentose: five carbon-containing monosaccharides. e.g: deoxyribose (found in DNA) and ribose (found in RNA).
Triose: three carbon-containing monosaccharides. e.g: glyceraldehyde.
Disaccharides (2 monosaccharides)
Disaccharides are complex sugar made up of two monosaccharides. They are:
Soluble in water
Sweet in taste
Crystalline in nature
Examples:
Maltose ---> glucose + glucose
Lactose ---> glucose + galactose
Sucrose ---> glucose + fructose (Note: All disaccharides and monosaccharides are reducing sugars except sucrose.)
Polysaccharides (many monosaccharides)
Polysaccharides are complex carbohydrates made up of many monosaccharides.
They are:
Insoluble in water
Not sweet in taste
It may act as storage for carbohydrates. e.g: Starch found in plants and glycogen found in animals.
May have a structural role. e.g: Cellulose in the plant cell wall.
Joining of α-glucose
Starch: Starch is a mixture of amylose and amylopectin.
Amylose is a polymer of glucose joined by α 1,4 glycosidic bonds.
Amylose forms a helix with 6 glucose molecules per turn and about 300 per helix.
Amylopectin is a polymer of glucose joined by α 1,4 glycosidic bonds with occasional branches of α 1,6 glycosidic bonds, this causes the molecules to be more branched than the helix.
Relationship of structure with function of starch:
The helical shape of starch makes starch compact and insoluble so it acts as a good source of energy.
Starch is branched, so the compound can be easily hydrolyzed to release glucose monomers.
Starch has no osmotic effect so it does not alter the water potential within the cell.
Glycogen: Glycogen is an amylopectin-like molecule.
It is a polymer of α - glucose joined by α - 1,4 glycosidic bonds with more frequent α 1,6 glycosidic.
Glycogen is the main storage polysaccharide in animals. They are found in the liver and muscles.
Relationship of structure with function of glycogen:
Glycogen is highly branched, so:
It can be hydrolyzed easily and
glucose can be attached to it quickly.
Glycogen is compact so takes up less space.
Glycogen is insoluble in water and thus has no osmotic effect.
Comparison between glycogen and amylopectin:
Glycogen and amylopectin are both made up of α glucose.
Glycogen and amylopectin both contain 1,4 and 1,6 glycosidic bonds.
Compared to amylopectin, glycogen contains more than 1,6 glycosidic bonds, glycogen is more frequently branched.
In amylopectin, branching occurs after every 30 molecules of glucose, whereas in glycogen branching occurs every 10 molecules of glucose.
Joining of β - glucose:
Cellulose Cellulose: Cellulose is a polymer of beta glucose. Many beta glucose molecules are joined together by β-1,4 glycosidic bonds (where every successive beta glucose molecule is inverted upside down) to form a straight chain. Many straight chains of β glucose molecules are joined together by hydrogen to form microfibril. Many microfibrils are joined together by hydrogen bonds to form cellulose fibre.
Microfibrils and cellulose fibre are very strong and are structurally important in plant cell walls.
Cellulose fibres provide rigidity to the cell wall and prevent the cell from bursting when it becomes turgid.
The cellulose fibres are very difficult to digest because only a few organisms have the enzymes that can break the β 1,4 glycosidic bonds.
How does cellulose have high mechanical strength that makes it suitable for cell walls in plants?
Many hydrogen bonds between parallel β glucose chains form microfibrils.
Microfibrils are held together by more hydrogen bonds to form cellulose fibre.
There are many hydrogen bonds within the molecule.
Why cellulose is a suitable component for cell walls.
Cellulose molecules form fibrils and fibres.
There is hydrogen bonding between cellulose molecules.
Cellulose molecules being a straight chain, allow other cellulose molecules to lie parallel to each other and form hydrogen bonds.
Cellulose fibres give strength to the cell wall to prevent the cell from bursting and withstand turgor pressure.
Cellulose fibres crisscross each other so there are many gaps between cellulose fibres, allowing water molecules to pass through them, and also making cell walls permeable.
Cellulose is insoluble.
Difference between the structure of cellulose and glycogen:
Difference between the structure of cellulose and amylose:
An amylose molecule and a cellulose molecule have very different structures even though they both have glucose as the constituent monomer because:
Amylose is composed of α - glucose monomers and cellulose is composed of β - glucose monomers.
Amylose has α 1,4 glycosidic bonds whereas cellulose has β 1,4 glycosidic bonds.
Adjacent glucose in cellulose is rotated through 180 degrees.
Amylose has an energy storage function whereas cellulose has a structural function.
Sucrose
Structure of sucrose: They're made of glucose and β - fructose joined by 1,2 glycosidic bonds. It's a non-reducing sugar.
β - fructose looks like Pentose, but β - fructose is a hexose sugar.
The structural difference between fructose and sucrose:
Water: Water molecules are charged with oxygen atoms being slightly negative and hydrogen atoms being slightly positive. These opposite charges attract each other forming hydrogen bonds.
Properties of water due to hydrogen bonding:
Solvent: Since water is charged, water is a very good solvent. Water is a polar molecule, so charged or polar molecules, sugars and amino acids dissolve readily in water and so are called hydrophilic (water preferring). Uncharged or non-polar molecules such as lipids do not dissolve well in water and are called hydrophobic (water neglecting).
Specific heat capacity: Water has a high specific heat capacity, which means water does not change temperature very easily. The high specific heat capacity of water minimizes fluctuations in temperature inside cells, this also means sea temperature is remarkably constant.
Latent heat of evaporation: Water requires a lot of energy to change its state from liquid to gas. High latent heat of evaporation of water is used as a cooling mechanism in animals (sweating and panting) and plants (transpiration). As water evaporates, it extracts heat from around it cooling the organism.
Cohesion: Water molecules stick together due to hydrogen bonding between molecules. This explains why long columns can be sucked up by tall trees by transpiration without breaking them. Due to hydrogen bonding between water molecules, water has high surface tension which allows small animals to walk on water.
Test for biological molecules
Carbohydrates:
Starch test with Iodine
Add a few drops of iodine to the sample.
The Colour changes from brown to blue-black, indicating the presence of starch
Benedict's test for reducing sugar
Grind up sample.
Add benedict's solution to the sample and heat up to 90 degrees Celsius.
Colour changes from clear blue to green/yellow/orange/red/brown indicating the presence of reduced sugar.
Benedict's test for non-reducing sugar
HCl is added to the sample (acid hydrolysis)
The sample is then heated at around 40 degrees celsius (to break the bond between glucose and fructose, this is called acid hydrolysis).
NaHCO3 is added to neutralize the acid.
The same volume of Benedict's solution is added.
The mixture is heated in a water bath at 90 degrees Celsius.
If the colour changes from clear blue to green/yellow/orange/red/brown, sucrose is present in the solution.
When benedict's solution is added to sucrose solution and put into a bath, no change is seen, why is that?
Sucrose is a non-reducing sugar.
No hydrochloric acid was used to break down sucrose to reduce sugars.
Sucrose does not have a free aldehyde or ketone group to react with copper ions in benedict's solution.
Fat:
Emulsion test for lipids
Grind up a sample in a dry test tube.
Add ethanol and then water.
The cloudy emulsion will indicate the presence of lipid.
Protein:
Biuret test for protein
The sample is ground up.
Biuret solution is added and then shaken.
The purple colour will indicate the presence of protein.
Describe the advantages of organisms in storing polysaccharides such as glycogen rather than storing glucose.
Glycogen is compact, thus a large amount of glucose can be stored in a relatively small space.
Glycogen is unreactive while glucose is a reactive molecule.
Glycogen is insoluble in water, so it has no osmotic effect, glucose is soluble in water and can make the cytoplasm of the cell concentrate. Water would enter the cell as a result and cell volume would increase which may cause the animal cell to burst.
Lipids
At room temperature, a solid lipid is called fat while a liquid lipid is called oil.
Functions of lipids:
The energy source for respiration
Energy storage as adipose cell
Formation of cell membrane
Insulation, e.g. blubber in whales
Protection, e.g. cuticle of leaf and hormones.
Lipid acts as a metabolic source of water.
Lipids contain carbon, hydrogen and oxygen.
Lipids are insoluble in water so they are known as hydrophobic.
Carbohydrates are also C, H, and O compounds but carbon to oxygen ratio is less in lipids.
In fats, carbon-hydrogen bonds are present in higher numbers so when fat is broken down it releases more energy as fat contains many C-H bonds.
Lipid and respiration
Hydrolysis of ester bonds and molecular breakdown of lipids release water, Carbon dioxide and energy which is used to make ATP.
The respiration of one gram of lipid gives out twice as much energy as the respiration of a carbohydrate.
Lipids are insoluble, so lipids can be stored in a compact way and don't affect the water potential of surrounding cells.
Respiration of lipids releases more water than carbohydrates, so some organisms use fat as a water supply.
Glycerol and Fatty acids
Fats can be broken down into glycerol and fatty acids with the help of enzyme lipase or by hydrolysis reaction.
Glycerol and fatty acids are found in storage fats, oils and cell membranes.
Glycerol molecules are always the same but fatty acids differ.
Most fatty acids can be made, except for ones called essential fatty acids which must be eaten.
Fatty acids
All fatty acids contain an acid group at one end, like amino acid, the rest of the molecule is a hydrocarbon chain containing only carbon and hydrogen.
Hydrocarbon chains can be 2 to 200 carbons long but most fatty acids contain 18.
Saturated fat compared to unsaturated fat
Fatty acid (or fat) which does not contain any carbon-carbon double bond (C=C) is called saturated fatty acids.
Saturated means full of hydrogen, unsaturated means not full of hydrogen.
Unsaturated fatty acids contain carbon-carbon double bonds (C=C) so fewer hydrogen atoms can be bound to the molecule.
Presence of C=C changes the shape of the hydrocarbon chain
The presence of C=C makes the molecules in lipid push apart, making the lipid more fluid. e.g. oil.
Triglycerides
Triglycerides are made up of 1 glycerol molecule bounded by 3 fatty acid molecules.
1 glycerol molecule is joined with 3 fatty acid molecules by condensation reaction between OH of acid and H of glycerol.
The bond formed is called the ester bond.
Triglyceride is insoluble in water (hydrophobic).
No double bond in glycerol.
Minimum 1 double bond in a fatty acid molecule.
Triglyceride contains a maximum of 1 double bond if unsaturated fatty acid is used triglycerides will contain more than 1 double bond. Fatty acid = minimum 1 double bond Glycerol = 0 (zero) double bond Triglyceride = minimum 1 double bond 3 ester bonds in triglyceride molecule 3 molecules of water
Cholesterol and steroid hormones
Cholesterol is a type of lipid.
Cholesterol is made up of 4 carbon-based rings
Cholesterol is found in all membranes (except prokaryotic bacterial cells)
Cholesterol is small and hydrophobic, this allows cholesterol to sit between phospholipid hydrocarbon fails and help regulate the strength and fluidity of the membranes
Testosterone, estrogen, and vitamin D are made from cholesterol
Cholesterol's lipid nature means it can pass through the phospholipid bilayer to reach its target receptor (site) usually inside the nucleus
Cholesterol can pass through the nuclear envelope
Cholesterol provides mechanical stability by getting in between phospholipid molecules and reducing fluidity.
At low temperatures, phospholipid tails tend to pack together but cholesterol prevents this from happening.
Without cholesterol membranes quickly burst open.
An increase in the proportion of phospholipids with unsaturated fatty acids helps plants survive a decrease in temperature or a cold environment.
Kinks in unsaturated fatty acids prevent the close packing of phospholipids at low temperatures.
Unsaturated fatty acids maintain/increases the fluidity of the membrane.
An increase in unsaturated fatty acids maintains the movement of protein within membranes.
Things that increase the fluidity of cell surface membranes at low temperature
Double bonds between carbon atoms in fatty acid chains
Cholesterol deposition in between fatty acid tails of phospholipids
Short fatty acid tails
A phospholipid
Explain how the structure of phospholipids allows the formation of the phospholipid bilayer of the cell membrane
Phospholipids have a phosphate group which is hydrophilic and fatty acid tails which are hydrophobic
The phosphate head faces the watery environment
Fatty acid tails form hydrophobic core by facing each other because of hydrophobic interactions.
Phospholipids in membrane
Phospholipids may be saturated or unsaturated
Organisms control the fluidity of their membranes by forming either saturated phospholipids or unsaturated phospholipids, depending on the temperature.
Organisms that live in cold climates have more unsaturated fatty acid tails in their phospholipids to ensure that their membranes remain fluid.
The structural difference between phospholipid and triglyceride
Comments