Essay - Enzymes

Question 1

Human Digestive System

Answer 1

• Salivary amylase secreted from salivary glands.
• Hydrolyses glycosidic bonds of starch to maltose.
• Food passed from stomach into pancreas, mixes with pancreatic juice containing pancreatic amylase, hydrolysing the rest of starch to maltose.
• Muscles in intestine wall push food along ileum, converts maltose into maltase.
• Maltase hydrolyses maltose into alpha glucose, which bind to a carrier protein and diffuse into the blood.

Question 2

Cholinergic Synapse

Answer 2

• Action potential arrives at presynaptic neurone, calcium channels open.
• Calcium ions enter presynaptic neurone.
• Vesicles fuse to presynaptic membrane, releasing acetylcholine into synaptic cleft.
• Acetylcholine binds to receptor sites on sodium protein channels in postsynaptic neurone, opening sodium channels, sodium ions diffuse in.
• Generates an action potential in postsynaptic neurone.
• Acetylcholinesterase hydrolyses ester bonds in acetylcholine, diffuses back across synoptic cleft into presynaptic membrane, ready to be put back in vesicles for next action potential.
• Enzyme stops acetylcholine from continuously binding to postsynaptic membrane which would prevent transfer of information across synapses.

Question 3

How does transcription work?

Answer 3

1) DNA helicase unwinds section of DNA to expose gene needed for protein synthesis. Breaks hydrogen bonds between bases in double helix.
2) RNA polymerase attaches to template strand and joins complementary RNA nucleotides together.
3) Forms pre-mRNA.
4) Pre-mRNA spliced to form mRNA.
5) mRNA diffuses out nucleus (through nuclear pores).
6) RNA polymerase detaches, DNA rewinds and forms hydrogen bonds between complementary bases.

Question 4

Photolysis and photoionisation of water

Answer 4

1) A chlorophyll molecule absorbs light, boosts a pair of electrons to a higher energy level (Photoionisation).
2) These ‘excited’ electrons leave chlorophyll molecule, taken up by electron carrier. (First sequence of oxidation-reduction reactions).
3) Electrons are passed along a series of electron carriers = electron-transport chain.
4) Energy released as electrons pass down this chain is used to actively transport protons into the thylakoid interior space.
5) Diffuse down gradient and out of thylakoid, into stroma through ATP synthase.

1) Protons are taken up by final electron acceptor - NADP.
2) NADP gains electrons, becomes reduced, becoming reduced NADP, which (along with ATP) is used in the light-independent reaction.
3) In LIR, used to reduce 2 glycerate phosphate molecules into 2 triose phosphate, which then combine to form a glucose molecule - energy storage as starch, stability as cellulose.

Question 5

Link reaction

Answer 5

Pyruvate produced in glycolysis oxidised to acetate.
One of the three carbon atoms forms one molecule of CO2.
Involved reduction of NAD to reduced NAD.
Acetate combines with coenzyme A to form acetylcoenzyme A.
This then enters the Krebs cycle.