carbohydrates (unit 3)

Question 1

what are carbohydrates made of?
and the definition

Answer 1

organic polymers composed of the elements carbon, hydrogen and oxygen usually in the ratio Cx (H2O)y. Also known as saccharides or sugars.

Question 2

another name for glucose?

Answer 2

monosaccharides

Question 3

name some monomers

Answer 3

glucose
fructose
galactose

Question 4

name some disaccharides

Answer 4

lactose
sucrose

Question 5

name some polymers

Answer 5

glycogen
cellulose
starch

Question 6

what is glucose?

Answer 6

a hexose monosaccharide
composed of 6 carbon atoms
two variations of a glucose molecule e.g. alpha and beta (OH group on opposite direction of beta in C1)

Question 7

advantage of glucose molecules?

Answer 7

polar and soluble
so can dissolve in cytosol of the cell
hexose sugar

Question 8

how do glucose molecules react?

Answer 8

• react by a condensation reaction - form a glycosidic bond (covalently between the OH groups and C1 and C4 hence 1,4 glycosidic bond)
• two hydrogen atoms and oxygen atoms are removed from the monomers forming a water molecule
  this molecule forms the disaccharide maltose

Question 9

what are other sugars?

Answer 9

fructose - normally occurs in fruit, in combination with glucose forming the disaccharide sucrose
galactose and glucose form lactose used for milk products

Question 10

what are pentose monosaccharide and how are they important?

Answer 10

ribose and deoxyribose used for nucleotides
sugars with five carbon atoms

Question 11

what do many alpha glucose monomers form?

Answer 11

form two different polymers in starch (glucose from photosynthesis is stored as starch)
- amylose
- amylopectin

Question 12

what’s amylose?

Answer 12

formed by 1,4 glycosidic bonds and twists to form a helix, stabilised by h2 bonding making it more compact and less soluble

Question 13

what’s amylopectin?

Answer 13

formed by 1,4 glycosidic bonds with 1,6 glycosidic bonds branched to it from the condensation reaction
there are 1,6 branching points every 25 glucose subunits

Question 14

what’s glycogen?

Answer 14

an equivalent energy storage to starch in animals and fungi - but forms more branches than amylopectin meaning it’s more compact and less space is needed for storage - also allows for many free ends for the addition/removal or glucose molecules which can speed up the process of storing or releasing glucose molecules

Question 15

what’s a hydrolysis reaction?

Answer 15

reactions release glucose for respiration by adding water molecules using a catalyst

Question 16

why can’t beta-glucose molecules react the same way as alpha glucose molecules?

Answer 16

as the alternate b-glucose molecule usually has the hydroxyl group in the opposite direction (flipped over) to the first one, they are too far apart to interact, the it has to be flipped to make the OH molecules closer, however they can never coil or branch, and only form straight chains - these make cellulose

Question 17

what do cellulose molecules do?

Answer 17

they make h2 bonds with each other forming microfibrils and those MICROfibrils join to form MACROfibrils and then combine to produce FIBRES which are strong and insoluble for cell walls

Question 18

what are reducing sugars?

Answer 18

all monosaccharides and some disaccharides are reducing sugars - meaning they lose/donate electrons

Question 19

how does the Benedict’s test work?

Answer 19

reducing sugars from the sample react with Cu2+ ions in the Benedict’s Regent donating the electrons to it, reducing the ions in Benedict’s to Cu+, making them brick red, leaving a red precipitate hence indicating a positive result of a reducing sugar

Question 20

why is the test for reducing sugars qualitative?

Answer 20

more reducing sugar present = more precipitate formed so the colour shown is a mixture of the precipitate and the unchanged Cu2+ ions and so depends on concentration of reducing sugar present as not all coppers are changed - so for a more accurate results (as colours not always determined by the eye - use a colorimeter)

Question 21

using benedicts test for non-reducing sugars

Answer 21

cannot react with the solution - only remains blue after warming indicating a negative result so instead have to add HCl to the sample (e.g sucrose) and boil it to give a positive result which occurs as the sucrose would’ve been hydrolysed by the acid to glucose and fructose which are reducing sugars

Question 22

what is a colorimeter and how is it used?

Answer 22

qualitatively measures the absorbance or transmission of light by a coloured solution - more conc. = more light absorbed and less light transmitted - so able to calculated the conc. of reducing sugar present
- you calibrate a colorimeter using distilled water (will produce a calibration curve later on)
- a filter is used on a colorimeter to select the colour of light which absorbs the most in order to maximise the accuracy of the experiment - usually the opposite of the sample e.g red for blue benedicts Regent

Question 23

monosaccharide def.

Answer 23

a single sugar molecule

Question 24

polysaccharide def.

Answer 24

a polymer made up of many sugar monomers (monosaccharides)

Question 25

glucose def.

Answer 25

a monosaccharide with the chemical formula C6H12O6 - one of the main products of photosynthesis in plants

Question 26

hexose monosaccharide def.

Answer 26

a monosaccharide composed of six carbons

Question 27

glycosidic bond def.

Answer 27

a covalent bond between two monosaccharides

Question 28

condensation reaction def.

Answer 28

a reaction between two molecules resulting in the formation of a larger molecule and the release of a water molecule - the opposite reaction to a hydrolysis reaction

Question 29

maltose def.

Answer 29

two glucose molecules linked by a 1,4 glycosidic bond

Question 30

disaccharide def.

Answer 30

a molecule comprising two monosaccharides, joined together by a glycosidic bond

Question 31

sucrose def.

Answer 31

a disaccharide made up of a fructose and glucose monosaccharides

Question 32

lactose def.

Answer 32

a disaccharide made up of a galactose and glucose monosaccharide

Question 33

pentose monosaccharide def.

Answer 33

a monosaccharide composed of five carbons

Question 34

ribose def.

Answer 34

the pentose monosaccharide present in RNA molecules

Question 35

starch def.

Answer 35

a polysaccharide formed from alpha glucose molecules either joined to form amylose or amylopectin

Question 36

glycogen def.

Answer 36

a branched polysaccharide formed from alpha glucose molecules - a chemical energy store in animal cells

Question 37

hydrolysis reaction def.

Answer 37

the breakdown of a molecule into two smaller molecules requiring the addition of a water molecule - the opposite reaction to a condensation reaction

Question 38

cellulose def.

Answer 38

a polysaccharide formed from beta glucose molecules where alternate beta glucose molecules are turned upside down - it is unable to coil or form branches but makes hydrogen bonds with other cellulose molecules to produce strong and insoluble fibres - major component of plant cell walls

Question 39

reducing sugars def.

Answer 39

saccharides (sugars) that donate electrons resulting in the reduction (gain of electrons) of another molecule

Question 40

benedict’s reagent def.

Answer 40

an alkaline solution of copper (II) sulphate used in the chemical tests for reducing sugars and non-reducing sugars - a brick-red precipitate indicates a positive result

Question 41

iodine test def.

Answer 41

a chemical test for the presence of starch using a potassium iodide solution - a colour change to purple/black indicates a positive result