BIOLOGICAL MOLECULES: carbohydrates!

Question 1

what do carbohydrates serve as?

Answer 1

• serve as energy fuel and building materials
  > consists of carbon, hydrogen and oxygen
• general formula: Cx(H20)y
  > the ratio of hydrogen to oxygen is always 2:1

Question 2

what are monosaccharides?

Answer 2

• a single unit of carbohydrate that cannot be hydrolysed to simpler carbohydrates
• they are monomers
• general formula: (CH2O)n, where n is 3-7
• a molecule consists of ONE carbonyl group and many hydroxyl groups

Question 3

what are disaccharides?

Answer 3

• disaccharides consists of two monosaccharides/ residues joined together by a glycosidic bond
  >via a condensation reaction, removing a single water molecule in the process

Question 4

describe the formation and breakage of maltose.

Answer 4

maltose consists of two α-glucose monomers

• a α(1,4) glycosidic bond is formed between the hydroxyl group on carbon 1 of an α glucose and a hydroxyl group on carbon 4 of another α glucose
  > via condensation reaction where a single water molecule is lost
• during hydrolysis, the glycosidic bond is broken and a molecule of water is added
  > forming a hydroxyl group on carbon 1 of an α-glucose and a hydroxyl group on carbon 4 of another α-glucose

Question 5

what are polysaccharides?

Answer 5

• polysaccharides are macromolecules that are formed by condensations of numerous monosaccharides joined by glycosidic bonds

STORAGE POLYSACCHARIDES (eg. starch and glycogen)
- serve as energy store and can be hydrolysed to provide simple sugars for cellular respiration

STRUCTURAL POLYSACCHARIDES (eg. cellulose)
- structures that can protect cells or the whole organism

Question 6

STORAGE POLYSACCHARIDES
what is the source, function and structure of starch?

Answer 6

source
- plants store excess glucose produced from photosynthesis as starch which is deposited in the form of starch granules/grains
>starch granules are visible in plant cells, notably in the chloroplasts of leaves and in storage organs such as potato tubers

function:
- starch serves as energy store in plants, storing surplus glucose
- upon hydrolysis by enzymes, large amount of glucose would be released for cellular respiration, releasing energy for cellular metabolism

structure:
- made of amylose and amylopectin
> which both consist of α-glucose units linked together by glycosidic bonds

Question 7

compare amylose and amylopectin.

Answer 7

amylose:
- unbranched polymer
- α-glucose monomers are linked by α(1,4) glycosidic bonds

amylopectin:
- branched polymer
-α-glucose monomers are linked by α(1,4) glycosidic bonds
> branch points are linked by α(1,6) glycosidic bonds

Question 8

STORAGE POLYSACCHARIDES:
what is the source, function and structure of glycogen?

Answer 8

source:
- found in liver and skeletal muscles of vertebrate animals and many fungi
- stored as glycogen granules in the liver and muscle

function:
-glycogen serves as energy store in humans and other vertebrates

structure;
- very similar to amylopectin but shows more extensive branching
- consists of α-glucose monomers linked by α(1,4) glycosidic bonds
> branch points are linked by α(1,6) glycosidic bonds

Question 9

STARCH AND GLYCOGEN:
what is the function of this property?

composed of many (several hundreds to thousands of) α-glucose monomers, linked by glycosidic bonds

Answer 9

function:
-upon hydrolysis, large number of glucose molecules are released for cellular respiration
> releases large amount of energy (ATP) for cellular functions

Question 10

STARCH AND GLYCOGEN:
what is the function of this property?

most of the hydrophilic hydroxyl groups of the glucose residues project into the interior of the helices

Answer 10

function:
- the large molecules are insoluble in water
> can be stored without affecting the water potential of the cells
- can be prevented from diffusing out of the cells

Question 11

STARCH AND GLYCOGEN:
what is the function of this property?

molecules are coiled into helices/ helical shapes

Answer 11

function:
- the molecules are compact
> many molecules can be packed within a fixed volume in the cell

Question 12

STARCH AND GLYCOGEN:
what is the function of this property?

the chains are not cross-linked to form big bundle (like cellulose)

Answer 12

function:
- the molecules can be easily hydrolysed into glucose for cellular respiration when required

Question 13

in amylopectin and glycogen, why are the molecules highly branched with branch points formed by α(1,6) glycosidic bonds?

Answer 13

• more free branch ends for rapid enzyme hydrolysis of amylopectin and glycogen molecules to release glucose for cellular respiration

Question 14

STRUCTURAL POLYSACCHARIDES
what is the source and function of cellulose?

Answer 14

source:
- found in plant cell walls

function:
- serves as building materials for plant cell wall which provides structural support, maintains the shape of plant cells and prevent plant cells from bursting due to excessive uptake of water

Question 15

STRUCTURAL POLYSACCHARIDES
what is the structure of cellulose?

Answer 15

• each cellulose chain is long- made up of a large number (up to 10,000) of β-glucose monomers
• glucose monomers are linked by β(1,4) glycosidic bonds via condensation reactions
  > catalysed by an enzyme called cellulose synthase found on the cell membrane of plant cells
• alternate/ neighbouring β-glucose monomers are inverted/rotated 180 relative to each other
  > for the formation of β(1,4) glycosidic bonds
  > linear and unbranched cellulose chain
• many hydroxyl groups of β-glucoses project out from both sides of each linear chain
• straight chains are lined parallel to one another to form RIGID CROSS-LINKS via NUMEROUS INTER-CHAIN HYDROGEN BONDS
  > formed between hydroxyl groups projected from both sides of each linear chain
  > many cellulose chains are bundles together to form micorfibrils of high tensile strength
• many microfibrils bundle together to form macrofibrils
• many macrofibrils bundle together to form a cellulose fibre

> THUS, cellulose is LONG, INSOLUBLE, LINEAR MACROMOLECULE WITH HIGH TENSILE STRENGTH