1. Carbohydrates
Only contain carbon, hydrogen and oxygen usually in the ratio Cₓ(H₂O)ₓ
Also known as saccharides or sugars
A single sugar unit is known as a
When two monosaccharides link together, they form a
When two or more monosaccharides link together they form a polymer called a
monosaccharide
disaccharide
Carbohydrates are molecules that only contain carbon, hydrogen and oxygen (literally means hydrated carbon). Elements in carbohydrates usually appear in the ratio Cₓ(H₂O)ₓ.
Carbohydrates are also known as saccharides or sugars. A single sugar unit is known as a monosaccharide e.g. glucose, fructose and ribose. When two monosaccharides link together, they form a disaccharide e.g. lactose and sucrose. When two or more monosaccharides link together they form a polymer called a polysaccharide e.g. glycogen, cellulose and starch.
polysaccharide

2. Examples of Carbohydrates
Monosaccharide
Disaccharide
Polysaccharide
Glucose
Sucrose
Cellulose
Ribose
Lactose
Glycogen
Fructose
Starch
Glucose
Sucrose
Starch
Fructose
Cellulose
Glycogen
Ribose
Lactose

3. Glucose
Glucose molecules, C₆H₁₂O₆, are monomers of some important large carbohydrates
Hexose monosaccharide composed of six carbons
In diagrams, carbons are numbered clockwise beginning with the carbon to the right of the oxygen atom within the ring
Two structural variations; alpha and beta in which the OH (hydroxyl) group on carbon 1 is in opposite positions
Glucose molecules are polar and soluble in water due to exhibiting hydrogen bonds
Basic building blocks/ monomers of some important large carbohydrates are glucose molecules which have the chemical formula C₆H₁₂O₆. Glucose is a hexose monosaccharide composed of six carbons. In molecular structure diagrams, the carbons are numbered clockwise beginning with the carbon to the right of the oxygen atom within the ring. There are two structural variations of the glucose molecule, alpha and beta in which the OH (hydroxyl) group on carbon 1 is in opposite positions. Glucose molecules are polar and soluble in water due to the hydrogen bonds that form between hydroxyl groups and water molecules. This solubility in water is important as it means glucose is dissolved in the cytosol of the cell.

4. Condensation Reactions
When two alpha glucose molecules are side by side, two hydroxyl groups interact and react to produce new molecules
Covalent bond forms between carbons 1 and 4 on the two glucose molecules, known as a 1,4 glycosidic bond
Condensation reaction as a water molecule is formed as a by product
When two alpha glucose molecules are side by side, two hydroxyl groups interact and react – bonds are broken and new bonds reformed in different places producing new molecules.
As seen, two hydrogen atoms and an oxygen atom are removed from the glucose monomers and join to form a water molecule. A bond forms between carbons 1 and 4 on the glucose molecules and the molecules are now joined.
A covalent bond called a glycosidic bond is formed between two glucose molecules. This reaction is termed a condensation reaction because a water molecule is formed as one of the products of the reaction. Because in this reaction, carbon 1 is joined to carbon 4 of the other glucose molecule, the bond is known as a 1,4 glycosidic bond. In this reaction, the new molecule is called maltose- a disaccharide.

5. Additional Sugars Fructose and galactose are hexose monosaccharides
Fructose (found in fruits) in combination with glucose forms sucrose
Galactose and glucose form lactose (found in milk)
Pentose monosaccharides are sugars containing 5 carbon atoms
For example, ribose which is important in RNA nucleotides
I wont go through this but just some other sugars to be aware of
Fructose and galactose are hexose monosaccharides – fructose (fruits) in combination with glucose forms sucrose
Galactose and glucose form lactose (milk)
Pentose monosaccharides are sugars containing 5 carbon atoms e.g. ribose important in RNA nucleotides

6. Starch
Alpha glucose molecules can be joined by glycosidic bonds to form two slightly different polysaccharides (amylose and amylopectin) known collectively as starch
Glucose from photosynthesis is stored as starch in plants
Amylose is formed by alpha glucose molecules joined by 1- 4 glycosidic bonds
Angle of the bond means this long chain of glucose twists to form a helix, stabilised by hydrogen bonding
This makes the polysaccharide more compact and much less soluble than the glucose molecules used to make it
Many alpha glucose molecules can be joined by glycosidic bonds to form two slightly different polysaccharides known collectively as starch. One polysaccharide in starch is amylose, formed by alpha glucose molecules joined together only by 1-4 glycosidic bonds. Angle of the bond means this long chain of glucose twists to form a helix which is further stabilised by hydrogen bonding within the molecule. This makes the polysaccharide more compact and much less soluble than the glucose molecules used to make it.

7. Starch
Amylopectin is made by 1-4 glycosidic bonds between alpha glucose molecules
Unlike amylose, amylopectin also has glycosidic bonds formed by condensation reactions between carbon 1 and carbon 6 on two glucose molecules
This means amylopectin has a branched structure, with the 1-6 branching points occurring approximately once in every 25 glucose subunits
The other starch polysaccharide is amylopectin which is made by 1-4 glycosidic bonds between alpha glucose molecules but unlike amylose, in amylopectin there are also some glycosidic bonds formed by condensation reactions between carbon 1 and carbon 6 on two glucose molecules. This means amylopectin has a branched structure, with the 1-6 branching points occurring approximately once in every 25 glucose subunits.
Glucose made by photosynthesis in plant cells is stored as starch- a chemical energy store. Insoluble, branched and compact properties make starch ideally suited for its storage roles.

8. Glycogen
Glycogen is the functionally equivalent energy storage molecule in animals and fungi
Higher branching than amylopectin which means it is more compact – ideal for immobile animals
Branching means there are many free ends where glucose molecules can be added or removed. This speeds up the process of storing or releasing glucose molecules required by the cell
Glycogen is the functionally equivalent energy storage molecule in animals and fungi, the only difference is that it forms more branches than amylopectin which means it is more compact and less space is needed for it to be stored. This is important as animals are mobile unlike plants so the coiling/ branching of these polysaccharides makes them compact which is ideal for storage. The branching also means there are many free ends where glucose molecules can be added or removed. This speeds up the process of storing or releasing glucose molecules required by the cell.
Key properties of amylopectin and glycogen are insoluble, branched and compact – ideally suited for storage roles that they carry out.

9. Hydrolysis Reactions
Glucose is stored as starch by plants or glycogen by animals and fungi until needed for respiration
Hydrolysis reactions involve the addition of water molecules to starch or glycogen in order to release glucose
This reaction is the reverse of condensation reactions that form glycosidic bonds
Require an enzyme to catalyse the reaction
Glucose is stored as starch by plants or glycogen by animals and fungi until needed for respiration – process by which biochemical energy stored in these nutrients is converted to a useable energy source for the cell. To release glucose for respiration, starch or glycogen undergo hydrolysis reactions, requiring the addition of water molecules. These reactions are catalysed by enzymes and are the reverse of the condensation reactions that form the glycosidic bonds.

10. Cellulose
Beta glucose molecules cannot join together in the same way alpha glucose molecules can as OH groups on carbons 1 and 4 are too far apart
In order for beta glucose molecules to join together and form a polymer, alternate beta glucose molecules are turned upside down
Beta glucose molecules are unable to join together in the same way that alpha glucose molecules can. This is because the hydroxyl groups on carbon 1 and 4 of the two beta glucose molecules are too far from each other to react. The only way that beta glucose molecules can join together and form a polymer is if alternate beta glucose molecules are turned upside down.
When a polysaccharide is formed from glucose in this way, it is unable to coil or form branches. A straight chain molecule is formed called cellulose.

11. Cellulose
When a polysaccharide is formed from beta glucose in this way, it is unable to coil or form branches
A straight chain molecule is formed called cellulose
Cellulose molecules make hydrogen bonds with each other forming microfibrils
Microfibrils join together forming macrofibrils, which combine to produce fibres.
Cellulose molecules make hydrogen bonds with each other forming microfibrils. Microfibrils join together forming macrofibrils, which combine to produce fibres. These fibres are strong and insoluble and are used to make cell walls. Cellulose is an important part of our diet, hard to break down and forms the fibre/ roughage necessary for a healthy digestive system.

12. Cellulose

13. Testing for Carbohydrates
Qualitative Methods Tests which identify whether a substance is present i.e. by a colour change, but not how much of it exists Quantitative Methods Tests which identify the amount of substance that is present
For example; benedict’s test, iodine test for starch, reagent strips, biosensors
For example; colorimeters, biosensors

14. Testing for Carbohydrates
Benedict’s test for reducing sugars
Reduction is a reaction involving the gain of electrons
All monosaccharides and some disaccharides are reducing sugars, meaning they can donate electrons/ reduce another molecule
Reducing sugars react with the Cu²⁺ ions in benedict's reagent resulting in the addition of electrons to blue Cu²⁺ ions to form brick red precipitate, indicating a positive result
Place sample to be test in a boiling tube in liquid form
Add an equal volume of benedict's reagent (alkaline solution of copper (II) sulphate)
Heat the mixture in a boiling water bath for 5 minutes
In chemistry, reduction is a reaction involving the gain of electrons. All monosaccharides and some disaccharides (e.g. maltose and lactose) are reducing sugars which means they can donate electrons or reduce another molecule or chemical.
In the chemical test for a reducing sugar, this chemical is benedicts reagent, an alkaline solution of copper (II) sulfate. The test is carried out as follows;
Place the sample to be tested in a boiling tube. If it is not in liquid form, grind it up or blend it in water
Add an equal volume of benedicts reagent
Heat the mixture gently in a boiling water bath for five minutes
Reducing sugars will react with the copper ions in benedicts reagent. This results in the addition of electrons to the blue Cu²⁺ ions, reducing the to brick red Cu²⁺ ions. When a reducing sugar is mixed with benedicts reagent and warmed, a brick red precipitate is formed indicating a positive result. The more reducing sugar present, the more precipitate formed and the less blue Cu²⁺ ions are left in solution, so the actual colour seen will be a mixture of brick red (precipitate) and blue (unchanged copper ions) and will depend on the concentration of the reducing sugar present. This makes the test qualitative.

15. Testing for Carbohydrates
Benedict’s test for non reducing sugars
Non reducing sugars do not react with benedict's solution and the solution will remain blue after warming, indicating a negative result
If a non reducing sugar, such as sucrose is hydrolysed using hydrochloric acid into reducing sugars before being warmed with benedict's solution, it will then be able to give a positive result
Colour produced is dependent on the concentration of reducing sugar present in the sample
Non reducing sugars do not react with benedicts solution and the solution will remain blue after warming, indicating a negative result. Sucrose is the most common non reducing sugar. If sucrose is first boiled with dilute hydrochloric acid, it will then give a positive result when warmed with benedicts solution. This is because the sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars. Colour produced is dependent on the concentration of reducing sugar present in the sample. Qualitative- deals with descriptions, does not tell you exact amount.
To remember what the colour changes indicate, think of traffic lights; green (low conc of reducing sugar), yellow/amber/orange (medium conc of reducing sugar) and red (high conc of reducing sugar)

16. Testing for Carbohydrates
Iodine test for starch
Sample to be tested is mixed with a few drops of iodine dissolved in potassium iodide solution
If the solution changes colour from yellow/ brown to purple/ black, starch is present in the sample and it is a positive result
If the iodine solution remains yellow/ brown, it is a negative result and starch is not present.
The iodine test is used to detect the presence of starch. To carry out the test, a few drops of iodine dissolved in potassium iodide solution are mixed with a sample. If the solution changes colour from yellow/ brown to purple/ black, starch is present in the sample. If the iodine solution remains yellow/ brown, it is a negative result and starch is not present.

17. Testing for Carbohydrates
Reagent strips
Manufactured reagent test strips can be used to test for the presence of reducing sugars, most commonly glucose
The advantage is that with the use of a colour coded chart the concentration of the sugar can be determined.
Manufactured reagent test strips can be used to test for the presence of reducing sugars, most commonly glucose. The advantage is that with the use of a colour coded chart the concentration of the sugar can be determined.

18. Testing for Carbohydrates
Colorimetry
Equipment used to measure the absorbance or transmission of light by a coloured solution
The more concentrated a solution is, the more light it will absorb and the less light it will transmit
This can be used to calculate the concentration of reducing sugar present.
Piece of equipment used to quantitatively measure the absorbance or transmission of light by a coloured solution. The more concentrated a solution is, the more light it will absorb and the less light it will transmit. This can be used to calculate the concentration of reducing sugar present.

19. Testing for Carbohydrates
Biosensor
Made up of three components
Molecular recognition component that recognises the molecule under investigation and produces a signal
Transducer which detects changes (pH) and produces a response such as immobilised dye on test strip or an electric current in a glucose testing machine
Display reader which produces a visible, qualitative or quantitative signal such as a colour on a test strip or reading on a test machine
Biosensor
Three components- molecular recognition component that recognises the molecule under investigation and produces a signal, transducer which detects changes (pH) and produces a response such as immobilised dye on test strip or an electric current in a glucose testing machine and a display reader which produces a visible, qualitative or quantitative signal such as a colour on a test strip or reading on a test machine.