Unit 1 · Topic 1.2 end-of-topic test
Over one summer a young oak roughly doubles in mass, adding new wood, leaves and roots.
Where did the carbon atoms in the new wood come from?
Over a year of training, an athlete gains 4 kg of muscle. Apart from its water, muscle is built largely of protein.
Where did the nitrogen atoms in that new protein come from?
Here are the formulas of one molecule from each class of biological molecule. Glucose (a sugar, a carbohydrate): C₆H₁₂O₆. A fatty acid (part of a fat, a lipid): C₁₈H₃₆O₂. One amino acid (a monomer of proteins): C₃H₇NO₂S. One nucleotide (a monomer of nucleic acids): C₁₀H₁₄N₅O₇P.
Which elements appear in every one of the four molecules?
A sample of yeast is dried until all its water is gone, and the kinds of atoms left behind are counted. Most of them are carbon, hydrogen and oxygen; nitrogen, phosphorus and sulfur make up a much smaller share.
What explains this?
Sulfur is one of the six elements that make up almost all of a living cell.
Which class of biological molecule is built using sulfur?
A phosphate group is a phosphorus atom bonded to four oxygens.
Which two kinds of molecule does a cell build using phosphate groups?
Every amino acid, whatever its R group, contains a nitrogen atom in the same place.
Which group carries that nitrogen?
The figure shows the three parts of one nucleotide. A cell has plenty of carbon, hydrogen, oxygen and phosphorus but has run out of nitrogen.
Which part of a new nucleotide is held up by the shortage?
A fat is three fatty acids joined to one glycerol.
Atoms of which elements does a cell need to build a fat?
In a famous experiment, a willow shoot weighing about 2 kilograms was planted in a pot holding 90 kilograms of dried soil. For five years it was given only rainwater. The tree then weighed about 76 kilograms, while the soil, dried again, had lost only about 60 grams.
What do these results show about where the tree's new atoms came from?
Yeast cells are moved into a solution that supplies every element they need but has run out of phosphorus.
Which of these can the yeast still build?
Two cultures of yeast are grown. One culture's supply has run out of nitrogen; the other's has run out of phosphorus. Every other element is plentiful in both.
Which class of molecule stops being built in both cultures?
A farmer adds a fertilizer that supplies nitrogen to a field, and the crop plants grow larger than plants in an untreated field.
Which molecules did the extra nitrogen let the plants build more of?
Yeast cells in a phosphorus-free solution stop dividing, although they still take in sugar.
Why does the lack of phosphorus stop cell division?
A student's summary table reads: nitrogen, used to build nucleic acids only; phosphorus, used to build nucleic acids and phospholipids; sulfur, used to build proteins.
Which row needs correcting, and how?
A cell is short of one element. Its building of new plasma membrane stops, while it goes on building new proteins.
Which element is it short of?
A sealed glass jar containing water, air and a small water plant is weighed. It is left in sunlight for a month, during which the plant doubles in mass, and then weighed again.
How does the total mass of the jar and everything inside it compare with the first weighing?
A cell is given plenty of carbon, hydrogen, oxygen and sulfur, but no nitrogen and no phosphorus.
Which classes of molecule can this cell still build?
(a) Identify the elements that make up most of the atoms in every one of the four classes of biological molecule that the yeast build. (1 point)
A full-credit answer: The most common elements in all four classes are carbon, hydrogen and oxygen: carbohydrates, lipids, proteins and nucleic acids are all built mostly from these three.
Check the box for each point your answer earns
Accept: the symbols C, H and O. A response that also lists nitrogen, phosphorus or sulfur as elements used in particular classes still earns the point, provided carbon, hydrogen and oxygen are named as the most common.
Do not award the point for naming only one or two of the three, or for listing nitrogen, phosphorus or sulfur as the most common elements.
Common slip: Adding nitrogen or phosphorus to the common three. Those are needed by particular classes; carbon, hydrogen and oxygen are the three found in every class.
(b) Explain why the yeast must take these elements in from the solution rather than making them. (1 point)
A full-credit answer: No living thing can make an atom or turn one element into another, so the yeast build every new molecule from atoms they take in from the solution.
Check the box for each point your answer earns
Accept: atoms are neither created nor destroyed, so every atom in a new molecule came in from outside as food, water or air.
Common slip: Saying the yeast make the elements from sugar or from energy. Sugar is itself made of atoms that came in from outside, and energy cannot become an atom.
(c) Predict which kinds of molecule the yeast can build only while phosphorus is supplied. Name each kind. (1 point)
A full-credit answer: The yeast can build new nucleic acids (DNA and RNA) and new phospholipids only while phosphorus is supplied; in the phosphorus-free solution, building of both stops.
Check the box for each point your answer earns
Accept: DNA or RNA for nucleic acids; membrane lipids or 'the lipids of the membrane' for phospholipids. 'Lipids' alone is not enough, because fats and steroids need no phosphorus.
Do not award a response that names proteins or carbohydrates, or that names only one of the two kinds.
Common slip: Answering ‘lipids’ in general, or adding proteins. Fats and steroids need no phosphorus, and proteins need none either; the phosphate group sits in every nucleotide and in every phospholipid head.
(d) Justify your prediction by explaining why the phosphorus-free culture stops dividing while the complete culture keeps dividing. (1 point)
A full-credit answer: To divide, a cell must build a second copy of its DNA and enough new plasma membrane for two cells. Every nucleotide and every phospholipid head contains a phosphate group, so with no phosphorus the cell can build neither and cannot divide, while the culture supplied with phosphorus can build both and keeps dividing.
Check the box for each point your answer earns
Accept: reasoning from either DNA or the membrane alone, provided it names the phosphate group (or phosphorus in the structure) as the reason that class cannot be built.
Common slip: Saying the cells lack energy. The block is a missing atom: without phosphorus there is no phosphate group to build a nucleotide or a phospholipid head from.
(a) Describe where the arrows from the carbon dioxide and water box go, and explain why they go there. (1 point)
A full-credit answer: The arrows from carbon dioxide and water reach all four classes, because carbon, hydrogen and oxygen are the most common elements in every carbohydrate, lipid, protein and nucleic acid, and carbon dioxide and water are where those three elements come from.
Check the box for each point your answer earns
Accept: 'C, H and O go into every class' with all four classes named or 'all four' stated, and the reason given.
Do not award the point for arrows to some classes only, or for 'all four' with no reason from the elements every class is built from.
Common slip: Sending carbon only to carbohydrates. Carbon, hydrogen and oxygen go into lipids, proteins and nucleic acids too, so the arrows reach all four.
(b) Explain why the arrows from nitrogen-containing compounds reach both proteins and nucleic acids. (1 point)
A full-credit answer: Every nitrogenous base in a nucleic acid contains nitrogen, and every amino acid in a protein carries an amine group, –NH₂, so a cell needs nitrogen to build both classes.
Check the box for each point your answer earns
Accept: 'bases contain nitrogen' and 'amino acids contain nitrogen' in plain words; the response must give a reason for each class.
Common slip: Giving a reason for one class only. The point needs nitrogen placed in the base of a nucleotide and in the amine group of an amino acid.
(c) Add the missing arrows from the phosphate box. Write each arrow as 'phosphate → (class)'. (1 point)
A full-credit answer: phosphate → nucleic acids, and phosphate → lipids (the phospholipids).
Check the box for each point your answer earns
Accept: 'phospholipids' or 'membranes' for the lipid arrow. Do not award if an arrow is drawn to proteins or to carbohydrates, or if only one of the two arrows is given.
Common slip: Drawing only the arrow to nucleic acids. The phosphate group also sits in the head of every phospholipid, so the lipid class needs phosphorus too.
(d) Explain how this model relates to the fact that a growing cell, or a growing tree, gains mass. (1 point)
A full-credit answer: A living thing cannot make atoms, so every atom in its new molecules came in from the surroundings as food, water or air. Gaining mass means taking in matter and building it into the four classes; sunlight supplies energy, not atoms.
Check the box for each point your answer earns
Accept: 'the tree's new mass is atoms from carbon dioxide, water and the soil, rearranged into its molecules'; sunlight supplies energy, not atoms.
Common slip: Saying the tree’s new mass comes from sunlight. Light is energy; the atoms come from carbon dioxide, water and the soil.