Carbohydrates

Question 1

What are carbohydrates?

Answer 1

• Consist of only Carbon, Hydrogen and Oxygen.
• General formula: Cx(H2O)y
• Single sugar unit is knows as Monosaccharide e.g glucose, fructose and ribose.
• When two monosaccharides link they form polymer called polysaccharide e.g glycogen, cellulose and starch.

Question 2

Glucose

Answer 2

• Has six carbon atoms so called Hexose monosaccharide.
• The basic building blocks of some biological important large carbohydrates. C6H12O6.
• Alpha glucose has OH at the bottom.
• Beta has OH at the top.
• Glucose molecules are polar and soluble in water because of the hydrogen bonds that form between the hydroxyl groups and water molecules.
• This allows it to be dissolved in the cytosol of cells.

Question 3

Condensation reaction

Answer 3

• When 2 Alpha glucose molecules are side by side, 2 hydroxyl group interact.
• This produces MALTOSE + WATER.
• A covalent bond called a glycosidic bond forms between carbons 1 and 4. This is called 1,4 glycosidic bond.
• The ‘O’ that is left from the two OH on Alpha molecules is the one that joins the two.

Question 4

Other sugars

Answer 4

• Fructose and galactose are also hexose monosaccharides.
• Fructose naturally occurs in fruit; combines with glucose to form Sucrose.
• Galactose + glucose form Lactose.
• Fructose is sweeter than glucose and glucose is sweeter than galactose.
• Pentose monosaccharides contain 5 carbon atoms.
• Ribose is a pentose sugar and it’s in RNA and Deoxyribose is in DNA.

Question 5

Starch

Answer 5

• Glucose in plant cells is stored as starch, making its the main energy storage material in plants.
• Starch is INSOLUBLE so it doesn’t cause water to enter cells by osmosis (which would make them swell).
• Starch is a mixture of 2 polysaccharides of alpha-glucose: amylose and amylopectin.

Question 6

Amylose

Answer 6

• Long, unbranched chain of alpha-glucose.
• Formed by condensation reactions making glycosidic bonds at carbon 1 and 4.
• Angles of glycosylic bonds give it a coiled structure.
• Making it compact which is good for storage as it can fit in small spaces.

Question 7

Amylopectin

Answer 7

• Long, branched chain of alpha-glucose.
• Also made by 1,4 glycosidic bonds but also some glycol bonds are formed by condensation reactions between carbon 1 and 6, giving branched structure.
• 1-6 branching points occur approx. once ever 25 glucose subunits.
• Side branches allow enzymes that break down the molecule to get at the glycosidic bonds easily.
• This allows glucose to be released quickly.

Question 8

Glycogen

Answer 8

• Main energy storage material in animals.
• Animal cells store excess glucose as glycogen.
• Has more side branches that amylopectin.
• Many branches = glucose can be released quickly.
• Also very compact and insoluble so it’s good for storage.

Question 9

Hydrolysis reactions

Answer 9

• Glucose is stored until its needed for respiration - the process in which biochemical energy in these stored nutrients is converted into useable energy source for the cell.
• To release glucose, starch or glycogen undergo hydrolysis reactions which requires adding water molecules.
• Reactions are catalysed by enzymes.
• These are the reverse of condensation reactions.

Question 10

Cellulose

Answer 10

• Beta-glucose are unable to join in the same way as alpha because the hydroxyl groups on carbon 1 and 4 of the 2 glucose molecules are too far away.
• Only way they can join is if alternate beta-glucose molecules are turned upside down.
• This prevents it from coiling or branching and creates cellulose.
• Cellulose is the major component of cell walls in plants.
• The cellulose chains are linked together by H-bonds to form strong fibres called microfibrils. Microfibrils join together to form macrofibrils, which combine to produce fibres.
• This makes cellulose able to provide structural support for cells.