Biomolecules and cell structure and function

What the exam asks

Expect a described transport process, a table of concentrations either side of a membrane, or a cell placed in a solution. The items that recur are: decide whether a process is active or passive from evidence rather than from its name, predict the direction of water movement, and work out which organelle a described cell must be rich in. The trap with the highest yield is the isosmotic solution that is not isotonic: if the stem bothers to tell you that a solute crosses the membrane, that detail is the question. The second trap is reading a carrier protein or a saturating curve as proof of active transport, when both are equally at home in facilitated diffusion. Ask what the gradient is doing, not what the mechanism is called.

This topic is the physical chemistry of the cell: what the four classes of macromolecule are built from, why a membrane lets some things through and not others, and which compartment does which job. It sits underneath most of the rest of biology, so the exam uses it as a way of asking chemistry questions with a cell drawn around them.

Almost everything here reduces to one idea. The membrane has a hydrophobic core, and a molecule's fate at that core is decided by its polarity and its size. Nonpolar and small gets through unaided. Charged or large and polar needs a protein, and if it also has to travel against its gradient, it needs energy. Once you can sort a molecule into one of those three boxes you can answer most membrane items without recalling a single transporter by name.

Osmosis is where the marks are lost, because two different quantities are in play. Osmolarity counts every dissolved particle. Tonicity counts only the particles that cannot cross, because only those can hold water on one side. Solutions can match on the first and differ on the second, and the exam knows it.

The organelles are examined at the level of what they are responsible for. You will not be asked to describe the Golgi. You will be given a cell doing something unusual and asked which compartment must be busy, which is a different skill and a much easier one to carry in.

What to hold

• The membrane's core is hydrophobic, so small nonpolar molecules such as oxygen and carbon dioxide cross it unaided while ions and large polar molecules cannot.
• Simple diffusion and facilitated diffusion are both passive: they run down a gradient and consume no energy, and the only difference is whether a protein is involved.
• Active transport is defined by moving a substance against its electrochemical gradient, which is why it, and only it, requires an energy source.
• Facilitated diffusion saturates at high solute concentration because carriers are finite, so saturation is evidence of a protein, not evidence of active transport.
• Osmolarity counts all dissolved particles; tonicity counts only those that cannot cross the membrane, so an isosmotic solution is not necessarily isotonic.
• Water moves across a semipermeable membrane towards the side with the higher concentration of non-penetrating solute.
• The sodium potassium ATPase pumps three sodium ions out for every two potassium ions in per ATP, which makes it electrogenic and sets up the gradient that secondary active transport spends.
• Proteins for secretion are made on the rough ER, modified and sorted in the Golgi, and delivered to the surface in vesicles that fuse with the membrane.
• Lysosomal hydrolases work best near pH 5, which the lysosome maintains by pumping protons in, so the neutral cytosol is a poor environment for them.
• Condensation joins monomers and releases water; hydrolysis uses water to break the bond back apart.
• Denaturation destroys the weak interactions holding the folded shape but leaves the peptide bonds and therefore the primary sequence intact.
• Kinks from unsaturated fatty acids stop the tails packing tightly, so a membrane richer in them stays fluid at lower temperatures.