Raw Materials of the Cell

The question pairs six materials with six requirements, so an explain answer must say for each one what the cell needs it for — not what it is. "Glucose is a sugar" is a definition and earns nothing here.

6 marks, six materials. One sentence each, every sentence in the form "the cell requires X in order to … , because … ".

The six materials meet three kinds of requirement: energy supply, construction of cell components, and maintenance of the chemical conditions in which reactions occur. Glucose and oxygen serve the first, amino acids the second, and water and ions the third, while carbon dioxide is a special case because it is a waste product in one cell and a raw material in another.

The cell requires glucose as its principal fuel, because the energy stored in its chemical bonds is released during cellular respiration and captured as ATP, the form in which the cell can spend it. It requires oxygen because oxygen is the final electron acceptor in aerobic respiration, allowing glucose to be broken down completely to carbon dioxide and water and so releasing far more energy per molecule than anaerobic breakdown, which stops part-way.

The cell requires amino acids because they are the monomers from which proteins are assembled at the ribosomes, and proteins perform most of the cell’s functional work: enzymes catalyse its reactions, carrier and channel proteins move substances across its membranes, and structural proteins maintain its shape. A cell short of amino acids cannot replace enzymes as they are degraded, so its metabolism slows.

The cell requires water both as the solvent in which all its reactions occur, since substrates and enzymes must be dissolved to meet and collide, and as a reactant in its own right, for instance in the hydrolysis reactions that break large molecules down. It requires ions such as sodium, potassium, magnesium and iron because these maintain osmotic balance and pH, carry the electrical signals of nerve and muscle cells, and form essential parts of molecules, magnesium sitting at the centre of chlorophyll and iron at the centre of haemoglobin.

Carbon dioxide is required differently depending on the cell. In any respiring cell it is a waste product that must be removed, since allowing it to accumulate would lower the cytoplasmic pH and denature enzymes. In a photosynthesising plant cell it is also a raw material, supplying the carbon atoms from which glucose is built. The same molecule is therefore an output of one process and an input to another, which is why a plant cell in light both produces and consumes it.

Predict asks for a likely outcome with the reasoning that makes it likely. Two independent shortages are named, so the answer needs two separate chains — not one general statement about the cell being unhealthy.

5 marks. Roughly three for the oxygen chain, which has more steps, and two for the amino acid chain. Give each a first consequence, a knock-on, and an outcome.

Without oxygen, aerobic respiration cannot proceed past glycolysis, because there is no final electron acceptor for the electron transport chain in the mitochondria. The cell must switch to anaerobic respiration, which yields only about two molecules of ATP per glucose against roughly thirty or more from the aerobic pathway.

The cell’s ATP supply therefore falls sharply while its demand does not, so energy-dependent processes fail first: active transport stops, so ion gradients across the membrane run down and the cell can no longer regulate its internal composition, and protein synthesis and bulk transport slow. In animal cells the lactic acid produced by anaerobic respiration also accumulates, lowering cytoplasmic pH and risking the denaturation of enzymes.

The cell would consume its glucose reserves rapidly to compensate for the poor yield, and if the shortage persisted the failure of ion regulation and falling pH would damage membranes and enzymes irreversibly, so the cell would die. Cells with high continuous energy demands, such as neurones and cardiac muscle, would be affected fastest.

A shortage of amino acids would not affect the cell’s immediate energy supply, since respiration would continue normally, so the onset would be far slower. Proteins are continually degraded and replaced, however, so without a supply of amino acids the cell could not synthesise new enzymes, carrier proteins or structural proteins as existing ones broke down. Enzyme-catalysed reaction rates would progressively fall, membrane transport would become less effective, and the cell could not grow or divide, since division requires a full complement of new proteins. The outcome is a gradual loss of function rather than the rapid collapse caused by oxygen deprivation.