1 - Glycogen

Structure of glycogen - Function of glycogen - Animals don’t rebuild excess glucose into starch like plants do, instead we make it into glycogen. We store glycogen in muscles as an easily accessible store of glucose when respiration rapidly increases.Liver also stores a lot of glycogen to maintain a stable blood glucose concentration. The energy store in animal cells.

• Consists of alpha glucose with both 1,4 and 1,6 glycosidic bonds. However in glycogen, the 1,6 bonds are much more frequent, so it has way more branches so it can be described as being branched (more than amylopectin).

• The chains are shorter than in starch.

• A polysaccharide of a-glucose.

• Joined by glycosidic bonds.

What are some properties of glycogen that make it useful for its function?

• Has a higher frequency of 1,6 bonds which makes it even more compact than starch as people need more energy compared to plants.

• Much easier to remove monomers as there are more ends by hydrolysis for a rapid source of glucose.

• Provides a short term energy store.

• It’s branched so it has many ends so it can be rapidly hydrolysed to release glucose for respiration to provide energy.

• It’s a large polysaccharide molecule so it can’t leave the cell.

• It’s insoluble in water so the water potential of the cell is not affected (no osmotic effect).

• It is helix/branched so it’s compact.

2 - Starch

• Describe the structure of a starch molecule?

  • Polysaccharide which is formed by the condensation of a-glucose. Mixture of amylose and amylopectin;

  • Amylose (10 - 30% of starch). It has an unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules. The helix shape enables it to be more compact and thus it is more resistant to digestion. Also is coiled.

  • Amylopectin (70 - 90% of starch) has 1,4 glycosidic bonds between α-glucose molecules but also 1,6 glycosidic bonds form between glucose molecules creating a branched molecule. The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage.

• What are the properties of starch that make it useful for its function?

  • Insoluble in water - the water potential of the cell is not affected as there is no osmotic effect.

  • Compact - starch molecules can hold up compactly due to the structure of amylose and amylopectin. This enables them to fit into small storage organelles (like starch grains in potato tuber cells).

  • It’s branched - so glucose is easy released for respiration.

  • It’s coiled - compact for storage as lots can fit in a small space.

  • It’s helical - so it’s compact for storage in the cell.

  • Its a large polysaccharide molecule - so it can’t leave/cross the cell membrane.

  • Can be easily broken down by enzymes due to its branched structure.

  • For plants it can be short term use overnight when photosynthesis cannot occur and long term in organs such as bulbs and tubers to survive through winter.

3 - Cellulose

• Describe the structure of cellulose?

  • Its a polymer consisting of long chains of β-glucose joined together by 1,4 glycosidic bonds.

  • As β-glucose is an isomer of α-glucose to form the 1,4 glycosidic bonds consecutive β-glucose molecules must be rotated 180° to each other

  • So the CH2OH stick out alternatively on opposite sides.

  • Due to the inversion of the β-glucose molecules many weak hydrogen bonds form between the long chains giving cellulose it’s strength.

  • Has bundles of cellulose molecules → microfibrils → macro fibrils → cell wall.

• How is cellulose adapted for its function in plant cells?

  • Every other beta-glucose molecule is inverted in a straight, long unbranched chain due to alternating CH2OH side-chains.

  • Many hydrogen bonds link parallel strands (crosslinks) to form microfibrils (which are strong fibres).

  • Fibres are CRISS-CROSSED in cell walls, making the walls resistant to stretching in any direction.

  • No mammal can make an enzyme to digest cellulose.

  • There are many hydrogen bonds (they are strong in high numbers) which provide strength and rigidity.

  • Provides strength and structural support to plant cell walls.

What are the three structural properties of cellulose and what is the benefit to the plant because of that structure?

• High tensile strength of macro fibrils due to glycosidic bonds and hydrogen bonds →supports the plant and stops plant cells from bursting when turgid

• space between macrofibrils caused by Hydrogen and glycosidic bonds → lets water and minerals pass, the cell is fully permeable.

• glycosidic bonds between the glucose molecules are hard to break → hard for animals to digest cellulose so plants are less likely to be eaten by animals.