VCE Biology Question Thread

[lzxnl]

Why do chemical reactions occur in a biochemical pathway?

Is it so that the amount of energy released can be controlled, so that not too much energy is released all at once?

There are several reasons.
From a chemistry perspective, chemical reactions, like the complete oxidation of glucose, take several steps. Glucose, as we all know, won't spontaneously combust upon contact with air because although the reaction with oxygen to form CO2 and H2O is thermodynamically favourable, there is an activation energy barrier required, which prevents the reaction from being fast at all. For the combustion of glucose (respiration) to occur at a useful rate, it needs to be catalysed by enzymes, and it so happens that the enzymes we have in our body don't catalyse the complete conversion of glucose to CO2; rather, we have enzymes that catalyse specific steps in the biochemical pathway.
Also, as you've said, if we combusted too much glucose in one step, we'd heat up way too fast. Combusting it step-wise allows our body to take in the energy gradually.

From a biological perspective, the energy released from each step can then individually drive further chemical reactions that may be useful, such as the motion of dynein motors across microtubules. This is in contrast to getting a HUGE spike in the amount of ATP produced.

[DJA]

Please have a look at the question regarding experimental design and see if my answer which is CONTRARY to the VCAA answer is valid
Source: VCAA 2010, Biology Exam 1, Section B, Q2b

It has been suggested that less energy is retained when hard food is the major part of an animal's diet compared with the energy retained when soft food is eaten. The difference in energy retained would be indicated by the weight of an animal.

A pet food company has made two different types of food pellets, one hard and the other soft. Each kind of pellet has the same energy content. The company intends to test the pellets on a group of adult mice. Each mouse is genetically identical and of the same weight.

You are provided with many adult mice. Each mouse is genetically identical and of the same weight
two types of pellets, one hard and one soft. Each kind of pellet has the same energy content.
Outline an experiment that would allow you to determine if the hardness of the food pellets affects the balance between energy intake and energy expenditure.

In your answer you should:
-state the hypothesis that you are testing
-outline the experimental procedure
-describe the results that would support or negate your hypothesis.

My answer:
Mice fed hard pellets will weigh less by the end of the experiment when compared to mice fed soft pellets.

We could set up two groups of many mice and weigh each of them at the beginning of the experiment. Then for a period of time (for say 3 weeks) we could feed one group only soft food pellets and the other group only hard food pellets and control the other variables by supplying both groups with the same amount of water and oxygen as well as environment to live in. After three weeks we could reweigh the mice and record the change in weight. We could repeat this process many times - for example, 20 times.

If the mass gain was consistently smaller for the group of mice taking hard pellets for food, then we could verify the hypothesis and conclude that less energy is retained when hard food is a major part of a mice's diet.

VCAA answer:
Hypothesis: Mice fed hard pellets will weigh more than mice that are fed soft pellets
Experimental procedure: Two groups of mice: one group fed hard pellets and the other soft, and all other variables
controlled
Results: Mice fed hard pellets weighed more than mice fed soft pellets

**In blue above are the phrases I strongly disagree with.
What do you guys think. Could you fault my logic?

[alchemy]

My answer:
Mice fed hard pellets will weigh less by the end of the experiment when compared to mice fed soft pellets.

We could set up two groups of many mice and weigh each of them at the beginning of the experiment. Then for a period of time (for say 3 weeks) we could feed one group only soft food pellets and the other group only hard food pellets and control the other variables by supplying both groups with the same amount of water and oxygen as well as environment to live in. After three weeks we could reweigh the mice and record the change in weight. We could repeat this process many times - for example, 20 times.

If the mass gain was consistently smaller for the group of mice taking hard pellets for food, then we could verify the hypothesis and conclude that less energy is retained when hard food is a major part of a mice's diet.

VCAA answer:
Hypothesis: Mice fed hard pellets will weigh more than mice that are fed soft pellets
Experimental procedure: Two groups of mice: one group fed hard pellets and the other soft, and all other variables
controlled
Results: Mice fed hard pellets weighed more than mice fed soft pellets

**In blue above are the phrases I strongly disagree with.
What do you guys think. Could you fault my logic?

To be honest, it's perfectly fine to agree or disagree with the hypothesis VCAA (or whoever) specifies. The Hypothesis is what YOU think will happen, after all. Since you haven't actually carried out the experiment, the 'results' will be much the same case. However, remember that the aim is something to be careful with. There are usually a few (but only a few) responses that would be given marks for.

[DJA]

To be honest, it's perfectly fine to agree or disagree with the hypothesis VCAA (or whoever) specifies. The Hypothesis is what YOU think will happen, after all. Since you haven't actually carried out the experiment, the 'results' will be much the same case. However, remember that the aim is something to be careful with. There are usually a few (but only a few) responses that would be given marks for.

I'm confused. My answer directly contradicts the VCAA provided answer in that according to their given stem of the question "less energy is retained when hard food is the major part of an animal's diet" my interpretation of this is obviously that hence less weight would be retained as more energy would be used up OR lost when hard food is taken.

So VCAA is right with their faulty logic? - that makes no sense -- at least until someone can prove me otherwise - if you can please do.

[vox nihili]

Please have a look at the question regarding experimental design and see if my answer which is CONTRARY to the VCAA answer is valid
Source: VCAA 2010, Biology Exam 1, Section B, Q2b

It has been suggested that less energy is retained when hard food is the major part of an animal's diet compared with the energy retained when soft food is eaten. The difference in energy retained would be indicated by the weight of an animal.

A pet food company has made two different types of food pellets, one hard and the other soft. Each kind of pellet has the same energy content. The company intends to test the pellets on a group of adult mice. Each mouse is genetically identical and of the same weight.

You are provided with many adult mice. Each mouse is genetically identical and of the same weight
two types of pellets, one hard and one soft. Each kind of pellet has the same energy content.
Outline an experiment that would allow you to determine if the hardness of the food pellets affects the balance between energy intake and energy expenditure.

In your answer you should:
-state the hypothesis that you are testing
-outline the experimental procedure
-describe the results that would support or negate your hypothesis.

My answer:
Mice fed hard pellets will weigh less by the end of the experiment when compared to mice fed soft pellets.

We could set up two groups of many mice and weigh each of them at the beginning of the experiment. Then for a period of time (for say 3 weeks) we could feed one group only soft food pellets and the other group only hard food pellets and control the other variables by supplying both groups with the same amount of water and oxygen as well as environment to live in. After three weeks we could reweigh the mice and record the change in weight. We could repeat this process many times - for example, 20 times.

If the mass gain was consistently smaller for the group of mice taking hard pellets for food, then we could verify the hypothesis and conclude that less energy is retained when hard food is a major part of a mice's diet.

VCAA answer:
Hypothesis: Mice fed hard pellets will weigh more than mice that are fed soft pellets
Experimental procedure: Two groups of mice: one group fed hard pellets and the other soft, and all other variables
controlled
Results: Mice fed hard pellets weighed more than mice fed soft pellets

**In blue above are the phrases I strongly disagree with.
What do you guys think. Could you fault my logic?

Yeah, they've buggered it up

[RazzMeTazz]

For a graph representing the rate of enzyme reaction when there is an increasing concentration of enzyme would the graph be shaped like a bell curve (As the substrate would eventually all be used, so enzyme reaction would cease?)

[alchemy]

For a graph representing the rate of enzyme reaction when there is an increasing concentration of enzyme would the graph be shaped like a bell curve (As the substrate would eventually all be used, so enzyme reaction would cease?)

Depends if there is a limited or unlimited concentration of substrate. If substrate was limited, it would plateau no matter how much the increase in enzyme concentration. If substrate was unlimited, however, the graph would increase linearly (not quite the 'bell curve').

[anon9884]

Since a non competitive inhibitor does not occupy the active site, but attaches to another area of the enzyme and thus changes the shape of the enzyme so that it no longer has affinity for the substrate, how is this process reversible? The bonds in the enzyme have changed, so how can it go back to catalysing the initial reaction?

[DJA]

Since a non competitive inhibitor does not occupy the active site, but attaches to another area of the enzyme and thus changes the shape of the enzyme so that it no longer has affinity for the substrate, how is this process reversible? The bonds in the enzyme have changed, so how can it go back to catalysing the initial reaction?

I always thought that non competitive inhibition is irreversible so yeah...
Correct me if I'm wrong but that's what we got taught at school for VCE level.

[DJA]

A couple of questions regarding enzymes--

1) With enzymes, do extremes of pH cause the denaturing of the enzyme?
2) What bonds in the polypeptide (enzyme) are affected by changing the pH from the optimum? (I thought ionic and H-bonds - am I right or did I miss any? Are disulfide bonds affected?)
    Is this break down of the tertiary structure reversible?

[Tyleralp1]

1. Yes, extremes in pH do cause enzymes do denature. The majority of eznymes will denature with pH 2 or less (except Pepsin), and denature with pH 12 or more. High temperatures may also denature enzymes.
2. The non-neighbouring bonds between the amino acids are broken. Bonds which are broken are part of the 3D tertiary structure. So anything from disulfide bridges, hydrogen bonds, ionic, etc. Just not the peptide bonds! Finally, the denaturation of a protein is irreversible. You are unable to regain functioning of a denatured enzyme. That can only occur to enzymes which are inactive due to low temperatures. 

[katiesaliba]

I always thought that non competitive inhibition is irreversible so yeah...
Correct me if I'm wrong but that's what we got taught at school for VCE level.

Reversible non-competitive inhibitors have weaker bonds, therefore allowing the enzyme to eventually revert back to its original shape. In non-competitive inhibition, it's all dependent on the characteristics of the inhibitor. Heavy metals, for example, usually act permanently on the enzyme due to the strong bonds created and their consequence on the enzyme's conformation. Irreversible non-competitive inhibitors form strong covalent bonds with enzymes, affecting the enzyme's conformational structure. Comparatively, reversible inhibitors would not form bonds as great.

[katiesaliba]

A couple of questions regarding enzymes--

1) With enzymes, do extremes of pH cause the denaturing of the enzyme?
2) What bonds in the polypeptide (enzyme) are affected by changing the pH from the optimum? (I thought ionic and H-bonds - am I right or did I miss any? Are disulfide bonds affected?)
    Is this break down of the tertiary structure reversible?

According to Checkpoints 2014, denaturation by high temperatures break hydrogen bonds and val der Waals forces and denaturation by extremes of pH break ionic bonds.

Denaturation is the breakdown of the quaternary and tertiary structures.

[grannysmith]

Can the products of an enzyme-mediated reaction act as non-competitive inhibitors of the enzyme?

[katiesaliba]

Three questions:
1) If the quaternary structure of an enzyme is destroyed, does an enzyme denature or does denaturation specifically concern the destruction of the tertiary structure?
2) Concerning photosynthesis and cellular respiration, do we ACTUALLY only need to know the outputs and inputs? Teachers always tell us this, but that seems too simple. Do we at least need to know the exact reaction that results in, for example, the oxidation of ATP into ADP? So for glycolysis, the expenditure of ATP occurs during step 1 and step 3. Is that complex enough, or do I need to know the specific names of the enzymes?
3) ATP synthase harnesses the energy from the proton gradient to catalyse the anabolism of ATP from ADP and Pi. HOWEVER, I read somewhere that free e- are also used to help 'fuel' ATP synthase, so to speak. Is this true?

Thank you!!