Carbohydrates 🍞🥐🥨🍯

• Glucose has 6 carbon atoms and is a monosaccharide. So it is a hexose sugar.

  
• Monosaccharides are soluble in water because they have a large number of OH groups (hydroxyl groups).
• Monosaccharides are hydrophilic since they can dissolve in water, due to their large number of hydroxyl groups which form hydrogen bonds with water molecules.
• Pentose monosaccharides have 5 carbon atoms, an example is ribose.

  
• In a molecule carbon 1 is the right-most carbon atom in the structure, increasing in number around the structure clockwise.

  
• The 2 isomers of glucose are alpha glucose and beta glucose. The difference is in the carbon-1's bonds. In alpha glucose, carbon-1, the bonded hydroxyl group is below the ring, whereas in beta glucose it is above the ring.
• α-glucose + α-glucose → Maltose + Water

  Condensation reaction

  Maltose is a disaccharide

  
  

  The water molecule comes from a hydrogen atom from one of the monosaccharides and a hydroxyl group from the other.
• A glycosidic bond is between carbon 1 and carbon 4 in maltose, it is a shared oxygen atom between the two original molecules due to the loss of a water molecule.

  
• A glycosidic bond can be broken in a hydrolysis reaction where a water molecule is added. This will revert a disaccharide into the original monosaccharides.
• What is the problem with having glucose as a store of energy?

  1 Glucose is very soluble in water

  2 So if a cell contains lots of dissolved glucose, water will move into the cell in osmosis.
• Plant cell store glucose as starch.
• Starch is found in starch grains.
• What two molecules make up starch?

  1. Amylose
  2. Amylopectin
• Describe the structure of amylose:

  1. It is a polymer of alpha glucose molecules
  2. Joined by 1,4 glycosidic bonds
  3. The bonds are formed in a condensation reaction, producing a molecule of water
  4. The amylose molecule is twisted into a compact helix. (left handed)
  5. There are hydrogen bonds formed between glucose molecules along the chain.

  
• In plants:

  When the cell needs glucose water is needed to break the glycosidic bonds. This is a hydrolysis reaction.
• Amylopectin molecules is similar to amylose, being a polymer of a-glucose molecules joined by 1,4 glycosidic bonds. However they also have a branch every 25-30 a-glucose molecules, a branch is just another chain of alpha glucose, the chain is connected by a 1,6 glycosidic bond.
• How is the structure of starch related to its function?

  Amylose forms a tight helix, making starch compact, so starch can contain a large amount of glucose for its size.

  Starch is also insoluble in water, so starch doesn't cause water to enter the cell by osmosis.

  Also since amylose and amylopectin are polymers they are too large to diffuse through the plasma membrane.
• In plants:

  When the cell need glucose, enzymes are used to break the glycosidic bonds in starch. The enzymes work at the ends of the molecules. Since amylopectin has lots of branches it also has many ends, so enzymes can break down starch quickly.
• Glycogen has the same structure as amylopectin, except glycogen is more branched than amylopectin. This makes glycogen an even more compact molecule, also so it has more free ends. SO enzymes can convert gycogen back to glucose very rapidly.
• Glycogen is insoluble in water so it does not draw in water to cells by osmosis.
• Glycogen is a large molecule so cannot diffuse out of a cell through the plasma membrane.
• Cellulose is a polymer of Beta Glucose so the hydroxyl is above the plane of the ring. To form cellulose, every second beta glucose molecule is flipped to form a beta 1,4 glycosidic bond.
• Since cellulose forms a straight chain without any branches, it allows cellulose molecules to get close together. Hydrogen bonds then form between adjacent chains. Since a huge number of hydrogen bonds are formed, cellulose is made to be very strong.
• Microfibrils are when cellulose chains group together.
• Macrofibrils are just microfibrils grouped together.
• Macrofibrils group together to form a cellulose fibre.
• Cellulose fibres form the cellulose plant cell wall.
• Cellulose's strength allows the cellulose cell wall to carry out it its functions. For example allowing it to withstand the outward pressure due to the cell contents, hence preventing the plant cell from bursting.