chapter 3 p2

Question 1

• Carbohydrates

Answer 1

contain the elements carbon, hydrogen, and oxygen.
Carbohydrate literally means hydrated carbon’ (carbon and water).
The elements in carbohydrates usually appear in the ratio Cx (H20)y - This is known as the general formula of carbohydrates.
Carbohydrates are also known as saccharides or sugars.

Question 2

monosaccharides:

Answer 2

• A single sugar unit
  examples include glucose, fructose, and ribose.
  When two monosaccharides link together they form a disaccharide, for example lactose and sucrose.

Question 3

polysaccharide

Answer 3

When two or more (usually many more) monosaccharides are linked they form a polymer
- e.g Glycogen, cellulose, and starch

Question 4

Glucose:

Answer 4

• The basic building blocks, or monomers, of some biologically important large carbohydrates are glucose molecules, which have the chemical formula C6H12O6
• Glucose is a monosaccharide composed of six carbons and therefore is a hexose monosaccharide (hexose sugar)
• In molecular diagrams the cardons are numbered clockwise beginning with the carbon on the right (clockwise) of the oxygen atom within the ring.
• There are two structural variations of the glucose molecule, alpha (a) and beta (3) glucose, in which the OH (hydroxyl) group on carbon 1 is in opposite positions, as shown in Figure 1.

Question 5

properties of glucose molecules

Answer 5

are polar and soluble in water.
This is due to the hydrogen bonds that form between the hydroxyl groups and water molecules.
This solubility in water is important, because it means glucose is dissolved in the cytosol of the cell.

Question 6

Condensation reactions:
p1

Answer 6

• When two alpha glucose molecules are side by side, two hydroxyl groups interact (react).
• When this happens bonds are broken and new bonds reformed in different places producing new molecules.
• As you can see in Figure 2, 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.
• The reaction is called a condensation reaction because a water molecule is formed as one of the products of the reaction.
• Because in this reaction carbon 1 of one glucose molecule 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.
• This is an example of a disaccharide (a molecule made up of two monosaccharides).

Question 7

Other sugars:

Answer 7

• Fructose and galactose are also hexose monosaccharides.
• Fructose naturally occurs in fruit, often in combination with glucose forming the disaccharide sucrose, commonly known as cane sugar or just sugar.
• Galactose and glucose form the disaccharide lactose.
• Lactose is commonly found in milk and milk products.
• Fructose is sweeter than glucose and glucose is sweeter than galactose.
• Pentose monosaccharides are sugars that contain five carbon atoms.
• Two pentose sugars are important components of biological molecules - ribose is the sugar present in RNA nucleotides and deoxyribose is the sugar present in DNA nucleotides.

Question 8

starch.

Answer 8

Many alpha glucose molecules can be joined by glycosidic bonds to form two slightly different polysaccharides known collectively as starch.
Glucose made by photosynthesis in plant cells is stored as starch. It is a chemical energy store.
- the polysaccharides in starch is amylose amylopectin.
Glycogen

Question 9

Amylose

Answer 9

formed by alpha glucose molecules joined together only by 1-4 glycosidic bonds.
The angle of the bond means that 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.

Question 10

amylopectin.

Answer 10

Another type of starch is formed when glycosidic bonds form in condensation reactions between carbon 1 and carbon 6 on two glucose molecules.
also 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 that amylopectin has a branched structure, with the 1-6 branching points occurring approximately once in every 25 glucose subunits.

Question 11

glycogen

Answer 11

The functionally equivalent energy storage molecule to starch in animals and fungi
Glycogen 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.
The coiling or branching of these polysaccharides makes them very 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.

Question 12

key properties of amylopectin and glycogen

Answer 12

insoluble, branched, and compact.
These properties mean they are ideally suited to the storage roles that they carry out.

Question 13

Hydrolysis reactions:

Answer 13

Glucose is stored as starch by plants or glycogen by animals and fungi.
until it is needed for respiration - the process in which biochemical energy in these stored nutrients is converted into a useable energy source for the cell.
To release glucose for respiration, starch or glycogen undergo hydrolysis reactions, requiring the addition of water molecules.
The reactions are catalysed by enzymes.
these are the reverse of the condensation reactions that form the glycosidic bonds.

Question 14

Cellulose: p1

Answer 14

Beta glucose molecules are unable to join together in the same way that alpha glucose molecules can.
As you can see in Figure 5, the hydroxyl groups on carbon 1 and carbon 4 of the two glucose molecules are too far from each other to react.
The only way that beta glucoses molecules can join together and form a polymer is if alternate beta glucose molecules are turned upside down as in Figure 6.
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

Question 15

Cellulose: p2

Answer 15

Cellulose molecules make hydrogen bonds with each other forming microfibrils.
These microfibrils join together forming macrofibrils, which combine to produce fibres (Figure 8).
These fibres are strong and insoluble and are used to make cell walls.
Cellulose is an important part of our diet, it is very hard to break down into its monomers and forms the ‘fibre’ or ‘roughage’ necessary for a healthy digestive system.

Question 16

why Benedict’s test for reducing sugars:

Answer 16

In chemistry reduction is a reaction involving the gain of electrons.
All monosaccharides and some disaccharides (for example maltose and lactose) are reducing sugars.
This means that they can donate electrons, or reduce another molecule or chemical.
In the chemical test for a reducing sugar, this chemical is Benedict’s reagent, an alkaline solution of copper(Il) sulfate.

Question 17

The benedics test is carried out as follows:

Answer 17

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 Benedict’s reagent.
Heat the mixture gently in a boiling water bath for five minutes.

Question 18

what happens within the Benedicts test

Answer 18

Reducing sugars will react with the copper ions in Benedict’s reagent.
This results in the addition of electrons to the blue Cu2+ ions, reducing them to brick red Cu2+ ions.
When a reducing sugar is mixed with Benedict’s 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 (Figure 1).
This makes the test qualitative.

Question 19

Using Benedict’s test for non-reducing sugars:

Answer 19

Non-reducing sugars do not react with Benedict’s 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 Benedict’s solution.
This is because the sucrose has been hydrolysed by the acid to glucose and fructose, both reducing sugars.

Question 20

lodine test for starch:

Answer 20

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.

Question 21

Reagent strips:

Answer 21

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.

Question 22

Quantitative methods to determine concentration:

Answer 22

Colorimetry:

Biosensors:

Question 23

Colorimetry:

Answer 23

In a Benedict’s test, the colour produced is dependent on the concentration of reducing sugar present in the sample.
A colorimeter is a 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.

Question 24

A student was asked to determine the concentration of a solution of glucose.
The procedure was carried out as follows:

Answer 24

A filter was placed in the colorimeter.
The colorimeter was calibrated using distilled water.
Benedict’s test was performed on a range of known concentrations of glucose.
The resulting solutions were filtered to remove the precipitate.
The % transmission of each of the solutions of glucose was measured using the colorimeter.
Using this information a calibration curve was plotted.
Steps 3-6 were repeated using the solution with the unknown concentration of glucose.

Question 25

Describe how you would calculate % absorbance from a % transmission reading

Answer 25

100% – transmission % = absorbance %

Question 26

Explain why it is important to use the correct filter (step 1).

Answer 26

to maximise absorption; complementary colour / red for Benedict’s solution

Question 27

Describe how you calibrate a colorimeter (step 2).

Answer 27

use distilled water; set colorimeter to 100%

Question 28

Describe what you have after the solutions have been filtered (step 4).

Answer 28

unreacted Benedict’s solution (1); supernatant

Question 29

Biosensors:

Answer 29

Biosensors use biological components to determine the presence and concentration of molecules such as glucose.
The basic components of a biosensor are shown in Figure 4.
The analyte is the compound under investigation.

Question 30

3 parts in a biosensor

Answer 30

Molecular recognition
Transduction .
Display

Question 31

Molecular recognition

Answer 31

a protein (enzyme or antibody) or single strand of DNA (ssDNA] is immobilised to a surface, for example a glucose test strip. This will interact with, or bind to, the specific molecule under investigation.

Question 32

Transduction -

Answer 32

this interaction will cause a change in a transducer. A transducer detects changes, for example in pH, and produces a response such as the release of an immobilised dye on a test strip or an electric current in a glucose-testing machine.

Question 33

Display

Answer 33

this then produces a visible, qualitative or quantitative signal such as a particular colour on a test strip or reading on a test machine.