Carbon and Nitrogen cycle
Why can animals not use nitrogen directly from the atmosphere?
even though it is in abundance.
Atmospheric nitrogen exists as N₂, which has a triple covalent bond that animals cannot break
due to lack of the required enzymes. Nitrogen must first be converted into soluble compounds
like nitrates or ammonium.
Concept tested:
Availability ≠ usability
Why does oxygen diffuse across a cell membrane faster than glucose, even when both are at the same concentration gradient?
...
A drug blocks synaptic transmission but does not affect impulse conduction along neurons. state which step of coordination is disrupted and explainwhy reflexes fail.
Chemical transmission across synapses is disrupted; without neurotransmitter transfer, impulses
cannot pass between neurones, so reflex pathways fail.
Why does a plant cell still require mitochondria even though it has chloroplasts?
Because ATP produced during photosynthesis is used inside the chloroplast and is not exported
efficiently for general cellular processes. Mitochondria supply ATP required for processes such
as active transport, protein synthesis, and cell division throughout the cell.
Core logic:
Photosynthesis ≠ universal ATP supply.
A farmer adds a large amount of ammonium fertiliser to the soil.
After heavy rain, plant growth decreases.
Give one explanation.
Ammonium is converted to nitrates, which are highly soluble and can be leached out of the soil
by rain, reducing nitrogen availability to plants.
Logic focus:
More input does not guarantee more uptake.
Two substances enter a cell at the same rate. One stops entering when ATP is depleted; the other does not.
What does this reveal about their transport mechanisms?
...
Rank the following in decreasing order of ATP demand during normal functioning and justify your ranking:
a) maintaining resting potential in sensory neurones
b) repeated firing in motor neurons during sustained muscles contration
c) hormonal control of blood glucose by insulin
b > a > c
Sustained motor neurone firing has the highest ATP demand, resting potential maintenance is
moderate, and hormonal control has the lowest direct ATP requirement.
A leaf produces glucose at a high rate but shows no increase in growth. Give one metabolic
Explanation based on respiration.
The glucose is being immediately respired, so there is no net accumulation of carbohydrates for
growth or storage. High respiration can offset high photosynthesis.
Concept tested:
Growth depends on net assimilation, not glucose production alone.
In a grassland ecosystem, carbon dioxide levels are high, but plant growth is low.
Use the nitrogen cycle to explain.
Plant growth is limited by nitrogen availability, which restricts protein and enzyme synthesis
required for growth, even when carbon dioxide is abundant.
Key insight:
Carbon supply does not control growth alone
If all membrane proteins were suddenly removed but the phospholipid bilayer remained intact, which cellular function would fail first and why?
.....
A mutation prevents the rapid removal of neurotransmitters from synaptic clefts, but the impulse
Conduction and neurotransmitter release are normal.
Predict and explain the effect on coordination, not muscle function.
Coordination becomes prolonged and imprecise due to continuous stimulation of postsynaptic
Neurones caused by delayed neurotransmitter removal
During intense exercise, muscle cells increase oxygen uptake dramatically. Yet ATP
production per glucose molecule eventually decreases. Why?
Oxygen delivery becomes limiting, forcing cells to rely more on anaerobic respiration, which
produces far fewer ATP molecules per glucose molecule than aerobic respiration.
Logic focus:
ATP yield depends on pathway efficiency, not glucose availability.
If all nitrogen-containing compounds were suddenly removed from an ecosystem, which part for the carbon cycle will fall first.
Photosynthesis would fail first because nitrogen is required to make enzymes such as Rubisco
and chlorophyll-associated proteins. Without these, carbon fixation cannot occur, halting carbon
entry into the ecosystem.
Elite concept:
Nitrogen controls the machinery of carbon fixation.
A cell membrane loses most of its cholesterol. Predict the effect on membrane permeability and justify.
....
Both nervous and hormonal systems use negative feedback.
Explain why nervous control is more precise but less persistent than hormonal control.
Nervous control is precise because signals are targeted and rapidly terminated, while hormonal
Control is less precise but more persistent due to widespread distribution and slower removal.
If all the enzymes of the Krebs cycle suddenly stop functioning,
which process will fail first?:
i)oxygen consumption
ii) ATP synthesis
iii) carbon dioxide production
Give a logical sequence.
Carbon dioxide production fails first (released during Krebs cycle).
ATP synthesis falls next (reduced NADH and FADH₂ supply).
Oxygen consumption decreases last (electron transport slows due to lack of hydrogen carriers).
Key reasoning:
Oxygen is a terminal acceptor, not a direct ATP generator.