Carbohydrates Flashcards
Carbohydrates,
Monosaccharides/Disaccharides
‘Hydrated carbon’ so for every C there is one O and two H’s. They act as a source of energy (glucose), a store of energy (starch/glycogen), and as structural units (cellulose in plants/chitin in insects).
Glycosidic bond: A bond formed between two monosaccharides by a hydrolysis reaction (across O atom). When glycosidic bonds are formed and hydrolysed in living things, the reactions are catalysed by enzymes.
Monosaccharides
The simplest carbohydrates, important source of energy (well suited to this role as they have many C-H bonds). They are sugars – soluble in water/insoluble in non-polar solvents. Straight chains or ring/cyclic.
Only hexose sugars (e.g. glucose) are the monomers of more complex carbohydrates, they bond together to form polysaccharides and disaccharides.
In solution, triose/tetrose sugars exist as straight chains, pentose/hexose are more likely to be a ring/cyclic. Glucose has two isomers – α and β glucose. Glucose is abundant and the main form in which carbohydrates are transported around the body in animals.
Disaccharides:
Soluble. Common disaccharides are maltose (malt sugar) and lactose (milk sugar) which are reducing sugars. Sucrose is a non-reducing sugar. Disaccharides are made when two monosaccharides join – synthesis.
1. α-glucose + α-glucose -> maltose
2. α-glucose + fructose -> sucrose
3. α-glucose + β-glucose -> lactose
4. β-glucose + β-glucose -> cellobiose
To join the two monosaccharides a condensation reaction occurs to form a glycosidic bond, the two hydroxyl groups line up next to each other and a water molecule is removed, leaving an oxygen atom to link. Disaccharides are broken into monosaccharides by a hydrolysis reaction, addition of water. The water provides a hydroxyl group (-OH) and a hydrogen (-H), which form the two hydroxyl groups on the ends of the monosaccharides once the glycosidic bonds are broken.
Polysaccharides
•Homopolysaccharide: Polysaccharides made solely of one kind of monomer (e.g. starch).
•Hetropolysaccharide: Polysaccharides made of more than one kind of monomer.
Energy sources and stores:
•Glucose is a source of energy, as it is a reactant in respiration (energy released used to make ATP).
•Living things join lots of glucose molecules into polysaccharides to store energy. Plants store energy as starch in chloroplasts and membrane-bound starch grains, humans store glycogen in cells of muscles/liver.
The structure of polysaccharides makes them good energy stores, glycogen in animals and starch (comprising of amylose and amylopectin).
They are good monosaccharide stores because:
• Glycogen/starch are compact due to coiling and folding, they occur in dense granules (starch occurs in amyloplasts in plant cells) within the cell – more space for storage.
• Polysaccharides hold glucose molecules in chains, so they can be easily ‘snipped off’ the end of the chain by hydrolysis reactions (always catalysed by enzymes) when required for respiration.
• Some chains are unbranched (amylose) and some are branched (amylopectin/glycogen). Branched chains tend to be more compact but offer the chance for lots of glucose molecules to be ‘snipped off’ by hydrolysis at the same time, when lots of energy is required quickly. The enzyme amylase is responsible for hydrolysing 1-4 glycosidic linkages, and glucosidase hydrolyses 1-6 glycosidic linkages.
• Polysaccharides are usually less soluble than monosaccharides. If glucose dissolved in the cytoplasm the water potential would reduce, excess water would osmose in, disrupting the cellular function. Polysaccharides are less soluble due to their size and as the regions that can hydrogen bond are hidden inside the molecule. Amylose may form double helix, presenting hydrophobic external surface in contact with the water – making starch insoluble.
Polysaccharides as Structural Units – Cellulose:
Cellulose is a tough, insoluble, straight chain, fibrous substance, it is a homopolysaccharide made up of β-glucose joined by glycosidic bonds. In the condensation reaction between the glucose molecules every other is rotated at 180 degrees so that the OH groups are next to each other. Unlike chains of α-glucose which spiral, cellulose chains are straight and lie side by side.
• Rotation of every other glucose molecule and the β-1-4 glycosidic bonds prevent spiralling.
• Hydrogen bonding between rotated β-glucose molecules in each chain adds strength and prevents
spiralling. Bonds form between OH groups and the O group in the ring of another molecule.
• Hydrogen bonding between rotated β-glucose molecules in different chains give the whole structure
additional strength. The hydroxyl group on carbon 2 sticks out, enabling hydrogen bonding. Bonds form between OH group and the O atom involved in the glycosidic bond.
Microfibrils and Macrofibrils
•Microfibrils – When 60-70 cellulose chains are bound together (10-30nm diameter).
•Macrofibrils – Bundles of microfibrils (approx. 400 microfibrils) which are embedded in pectins (like glue) to form cell walls.
Structure and function of plant cell walls:
•Microfibrils & macrofibrils have high tensile strength, strong glycosidic bonds between monomers
and hydrogen bonds between chains (strong bonds = difficult to digest).
•Macrofibrils run in all directions, criss-crossing for extra strength.
Key features help the plant cell wall do its job:
•As plants do not have a rigid skeleton, each cell needs to have strength to support the whole plant.
•Space between macrofibrils for water/mineral ions to pass, making the cell wall fully permeable.
•The wall has high tensile strength, preventing the cell from bursting when turgid, and supporting the
whole plant. Turgid cells press against each other, which supports the plant’s structure.
•The cell wall protects the delicate cell membrane and cell contents.
Other structural polysaccharides:
•Bacterial cell walls: The whole structure surrounding a bacterial cell is called a peptidoglycan, made from long polysaccharide chains that lie in parallel, cross-linked by short peptide chains.
•Exoskeletons: Insect and crustacean cell walls are made from chitin (like fungi).
