biochemical pathways

Cellular respiration and photosynthesis are both processes that require a variety of different enzymes, however they can differ in that

• cellular respiration requires coenzymes to facilitate the steps, whereas photosynthesis relies on the actions of Rubisco

• cellular respiration is a catabolic pathway that results in the formation of energy-rich glucose, whereas photosynthesis is an anabolic pathway that requires chlorophyll to first absorb energy from the Sun

• they utilise different coenzymes that assist the enzymes involved by acting as electron donors and acceptors.

• photosynthesis results in the assembly of low-energy inorganic carbon dioxide molecules into energy-rich glucose, whereas cellular respiration breaks down glucose into ADP molecules for use in important metabolic reactions

Answer explanation

Cellular respiration uses coenzymes NADH and FADH, whereas photosynthesis uses NADPH. ATP is considered a coenzyme helper for several enzymes that catalyse reactions in cellular respiration

  Coenzymes assist the catalytic actions of enzymes. NAD is a coenzyme that functions to

• accept hydrogen and their associated electrons in its unloaded form

• accept hydrogens and their associated electrons it its reduced state

• donate hydrogens and their associated electrons it its oxidised state

• A.      donate hydrogens and their associated electrons it its unloaded state.

Answer explanation

NAD functions as a hydrogen acceptor when it is unloaded (oxidised). When it is loaded (reduced), it can donate hydrogens and their associated electrons.

  A coenzyme helper that can accept a phosphate group to catalyse an energy-requiring reaction

• ATP

• NADP+

• ADP

• FAD

Above the optimum temperature, enzymes tend to

• move more slowly, and thus colliding less frequently with its substrate, resulting in a slower rate of reaction

• have less energy to interact with its substrate, resulting in a slower rate of reaction 

• collide more frequently with the substrate, resulting in a faster rate of reaction until denaturation occurs

• denature irreversibly at the active site, such that is is unable to bind to its substrate

Bone alkaline phosphatase (BAP) is an enzyme that is attached to the outside of the cell membrane of bone-forming cells called osteoblasts. The BAP enzyme plays a crucial role in the mineralisation of bone, aiding the formation and growth of bones. The pH optimum for the enzyme alkaline phosphatase has been shown to vary under various laboratory conditions but has never been shown to be lower than a pH of 8. This means that

• at pH 10, the BAP enzyme will be most likely have bonds between it's active site and substrate's binding site, allowing for the enzyme to effectively catalyse the reaction

•   at pH levels lower than 8, the bonds within the BAP enzyme can be altered, allowing the enzyme to combine at more sites on the substrate

• at pH levels higher than 8, the ionic bonds within the BAP enzyme denature, reducing the activity of the enzyme.

• at a neutral pH, there are changes in the amount of ionic bonds holding the 3D structure of BAP together, however the changes are negligible such that the reaction rate remains the same.

Answer explanation

we are given a wide pH range for BAP to function effectively — any pH above 8.

Below 8, the pH will be below optimum levels, and would alter bonds that are formed between the enzyme and its substrate.

This leads to a reduced rate of reaction. A neutral pH would be sub-optimal – already the reaction rate would have decreased

When all the active sites of an enzyme are occupied by substrate molecules, the rate of reaction

• increases steadily, producing more products.

• decreases quickly, as less substrates can be reacted with enzymes.

• stays the same, until more substrates are added to the reaction

• stays the same until temperature is increased

Answer explanation

When enzyme sites are full, there can be no significant change in reaction rate. An increase in substrate will not change the rate of reaction as there still will not be enough enzyme active sites to catalyse the reaction. An increase in temperature will, however, increase the frequency of collisions between enzymes and substrates, increasing the rate of reaction

The actions of phosphofructokinase (PFK) enzyme in glycolysis is tightly regulated by ATP. When ATP levels are high, ATP binds to a secondary site on PFK, causing the enzyme’s affinity for its substrate to decrease. In this biochemical pathway, ATP acts as

• cofactor

• competitive inhibitor binding to the enzyme's active site

• a non competitive inhibitor binding to an allosteric site

• an allosteric activator

Which of the following statements about photosynthesis is incorrect? Photosynthesis

•   results in the transformation of radiant energy into chemical energy

• is dependent on the presence of oxygen since it is an anabolic reaction

• requires the use of enzymes to speed up the process

• utilises red and blue wavelengths of light

Which of the following is the most correct sequence of events for the light dependent stage of photosynthesis?

• Light energy absorbed by chlorophyll--> electrons excited--> ATP is produced--> electrons move to NADPH--> NADP is produced

• Light energy absorbed by chlorophyll--> water molecules are split ->electrons are moved between photosystems--> NADPH is oxidised to NADP

• Water molecules are split--> light energy absorbed chlorophyll -->NADPH is produced

• Electrons excited by light energy -->ATP is produced--> NADP⁺ is reduced to NADPH.

which of the following is required for the Calvin Cycle to proceed?

• Rubisco

• NADH

• Water

• Chlorophyll