Primer — Regulation of Biochemical Pathways

Think of an experiment as a question with one knob. You turn the knob (IV), watch what happens (DV), and clamp everything else (controlled) so the effect you see has only one possible cause.

In the wavelength-of-light photosynthesis experiment, the IV is the colour of cellophane (foil, red, green, blue) because that is what the experimenter deliberately changes. The DV is the colour of the bromothymol blue solution after 24–48 hours, because that is what gets measured. Controlled variables include the number of algae balls per tube, the distance of the lamp, the volume of solution, and the amount of CO₂ dissolved at the start — change any of these between tubes and you can no longer blame the result on the wavelength.

If a marker asks you to identify variables, do it in the language of the experiment — not 'the independent variable is light' but 'the independent variable is the wavelength of light, controlled by different cellophane colours'.

Two independent splits, so every dataset has two labels. The lipase experiment timing a solution going colourless produces *quantitative primary* data — you measured seconds, yourself. The wavelength experiment describing the test-tube colour as 'blue' or 'yellow-green' produces *qualitative primary* data — your own observation, but descriptive.

If your class pooled the three trials from other groups into one table, the data from the other groups is still quantitative, but it becomes *secondary* for you — you didn't collect it.

The exam test here is usually a one-liner: 'State whether the data is qualitative or quantitative, and primary or secondary.' Give both labels. One without the other loses half the mark.

Both test whether the finding is real, but at different scales. If you run the lipase experiment three times yourself and get similar times each time, that's repeatability — the method gives you stable results in your own hands. If a different school group independently follows the same procedure and gets similar results, that's reproducibility — the finding survives being moved to a different lab.

The SAC question about the other school replicating the pea respiration experiment is a straight reproducibility call. Different operator, different equipment, same method, similar result — reproducible. Same operator redoing their own trials is repeatability.

These three look similar but they're measuring different things. Imagine you're shooting arrows at a target. Accurate means your arrows cluster near the bullseye. Precise means your arrows cluster together anywhere — a tight group off to the side is precise but inaccurate. Valid means you're actually shooting at the target you said you'd shoot at.

In the lipase experiment, using a stopwatch that reads to 0.01 s gives high *precision*; calibrating the stopwatch against an atomic clock would improve *accuracy*. The experiment is *valid* only if phenolphthalein colour change really is driven by lipase hydrolysis rather than, say, milk pH drifting on its own.

The trap is using these three words as if they were synonyms. They aren't.

Random errors come from tiny uncontrollable variations — your reaction time on the stopwatch, micro-differences in droplet size. They make repeated trials disagree by small amounts. More trials and averaging reduce their effect.

Systematic errors come from something consistently biased — a miscalibrated pipette that always dispenses 0.2 mL too much, a water bath running 2 °C cool. They push every reading the same way, so averaging does *not* help. You fix systematic errors by fixing the source.

An outlier is a separate beast: a single data point that doesn't fit the rest. In the amylase table, three trials at 40 °C gave 300, 900, 350 seconds — the 900 s at Trial 2, 40 °C is the outlier. It could have come from a personal error (starter tube not swirled, timer started late) or a random fluke. You identify it by eye against its neighbours, then justify your call.

Think of an enzyme as a shaped pocket with a specific job. The pocket is the active site — a small region whose 3D shape and chemistry match only certain molecules. Those matching molecules are the substrates. When a substrate slots into the active site, the enzyme bends or polarises bonds in ways that push the reaction over the hill much faster than it would go alone.

For lipase, the pocket fits triglyceride molecules and lines up water at the right angle to split them into fatty acids and glycerol. That's why the enzyme is called *lipase* — it acts on lipids. Amylase breaks starch; protease breaks protein. The name usually tells you the substrate family and the ending `-ase` tells you it's an enzyme.

Because the active-site fit depends on the enzyme's 3D shape, anything that changes the shape — extreme pH, high temperature, inhibitors — changes how well the enzyme works.

At low substrate concentrations, enzyme molecules spend most of their time waiting between substrates. Adding more substrate means enzymes find a substrate sooner, so the rate goes up roughly in proportion.

As you keep adding substrate, active sites start to be full more of the time. At saturation, every enzyme is occupied the moment it becomes free, and you cannot speed the reaction up any further without adding more enzyme. The rate-vs-substrate curve is the classic shape: steep at first, bending over, then flat.

In the lipase-and-milk experiment, tube A (25 mL milk, most substrate) should go colourless fastest and tube E (5 mL, little substrate) slowest. Tube F with zero milk is the control — no substrate, no colour change, confirming the colour change depends on substrate presence.

A competitive inhibitor looks enough like the substrate to fool the enzyme. It enters the active site, but doesn't get converted to product — it just sits there, taking up the parking space. While it's there, the real substrate can't bind.

Orlistat is a competitive inhibitor of lipase. It locks into the lipase active site and prevents triglycerides from binding, so dietary fat isn't broken down to fatty acids and glycerol in the small intestine. Unabsorbed fat passes out in faeces. Fewer calories absorbed, so (over time) weight loss.

Because it competes with the substrate, you can overcome competitive inhibition by adding more substrate — at high enough substrate concentrations, substrate outnumbers inhibitor and the reaction rate approaches normal. This is the diagnostic difference from non-competitive inhibition.

Instead of competing for the active site, a non-competitive inhibitor binds to a separate pocket — an allosteric site — elsewhere on the enzyme. This binding deforms the whole enzyme slightly, including the active site. The substrate might still arrive, but the pocket no longer has the right shape to hold it or to catalyse the reaction.

The giveaway: non-competitive inhibition cannot be reversed by adding more substrate. More substrate doesn't matter if the active site is the wrong shape.

The Orlistat-style SAC question comparing it to a non-competitive inhibitor wants two contrasts. First: where does the inhibitor bind? Competitive → active site. Non-competitive → allosteric site. Second: can you outcompete it with more substrate? Competitive → yes. Non-competitive → no.

Biological pathways are cascades: enzyme 1 makes molecule A, enzyme 2 turns A into B, enzyme 3 turns B into end product P. Feedback inhibition means P loops back and sticks to enzyme 1 (usually at an allosteric site), stopping the whole pathway at the top.

Why do it this way? Two reasons. First, it prevents waste — if you already have enough P, don't keep making more. Second, it prevents toxic build-up — intermediate molecules don't pile up if the upstream enzyme is off.

For lipase specifically: when fatty acids (the end product of triglyceride digestion) reach a high enough concentration, they bind lipase at an allosteric site and reduce its activity. The cell stops breaking down more fat until fatty acid levels drop. This is why the SAC asks for *two* purposes — efficiency (no wasted enzyme or substrate) and safety (no runaway accumulation of product).

It helps to think of photosynthesis as an energy conversion with two handover points. Stage 1: light energy is absorbed by chlorophyll and used to split water, producing O₂ (released), NADPH (loaded with hydrogen + energy), and ATP. This is the light-dependent stage, and it happens in the thylakoid membranes of the chloroplast.

Stage 2 takes the NADPH and ATP from Stage 1 and uses them to build sugar. CO₂ from the air is fixed into an organic molecule, then rearranged using the energy and hydrogen carried by NADPH and ATP to produce glucose (C₆H₁₂O₆). This is the Calvin cycle (light-independent stage), and it happens in the stroma.

Overall equation: $6\,\text{CO}_2 + 6\,\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$. The oxygen you breathe comes from water, not from the CO₂ — remember this, it's a common exam gotcha.

The light-dependent reactions split water into oxygen, protons (H+), and electrons. Some of those protons and electrons get stored on an empty carrier molecule called NADP+. Once loaded, it is written NADPH. 'Loaded' here means 'carrying hydrogen plus energy' — it's a battery pack.

NADPH then diffuses into the stroma, where the Calvin cycle uses its hydrogen and energy to reduce CO₂-derived intermediates into glucose. Once the hydrogen is handed over, NADPH becomes NADP+ again — the empty form — and cycles back to be reloaded by another round of light reactions.

When the stimulus says 'some of these hydrogen ions are used to form a loaded coenzyme' and asks for its name, the answer is NADPH. Don't confuse it with NADH — that one runs in cellular respiration.

The Calvin cycle is a turning wheel. At one spot on the wheel, CO₂ is clamped onto a 5-carbon acceptor (RuBP) by the enzyme Rubisco — this is *fixation*. The resulting unstable 6-carbon molecule splits into two 3-carbon molecules. These get *reduced* using hydrogen and energy from NADPH and ATP, forming G3P (glyceraldehyde-3-phosphate). Some G3P exits the cycle and is combined into glucose; the rest is used to *regenerate* RuBP so the cycle can spin again.

This is why CO₂ concentration matters so much. If CO₂ is high, fixation is fast, the cycle turns fast, and lots of glucose gets made. If CO₂ is low, fixation stalls, glucose production drops, and the light reactions back up — NADPH and ATP accumulate because nothing downstream is using them.

That's the link the exam question exploits. CO₂ concentration controls the Calvin cycle, which controls how fast the light reactions can keep running, which is what determines how fast O₂ comes off. More CO₂ → faster Calvin → faster light reactions → more O₂ produced.

Photosynthesis rate depends on how well the chlorophyll can absorb the light hitting it. The absorption spectrum of chlorophyll a has two big peaks — one in the blue, one in the red — and a trough in the middle, where green light sits. Green light bounces off the leaf instead of being absorbed, and that's what reaches your eye.

In the algae-and-cellophane experiment, red and blue cellophane pass through light wavelengths that chlorophyll actually uses — you should see strong photosynthesis (bromothymol blue turns blue as CO₂ is consumed). Green cellophane passes only green light, which chlorophyll barely absorbs — photosynthesis is slow, the bromothymol blue stays yellow-ish. Foil blocks all light, so photosynthesis cannot happen at all and the solution stays yellow-green.

So the ranking you should expect: red ≈ blue > green >> foil. If your results agree, the hypothesis that wavelength affects photosynthesis rate via absorption is supported.

Rubisco, the enzyme that fixes CO₂ in the Calvin cycle, has a bug: it also accepts O₂. In hot, dry conditions, plants close their stomata to conserve water, which traps O₂ (produced by the light reactions) inside the leaf and keeps CO₂ out. The O₂ : CO₂ ratio climbs, Rubisco starts grabbing O₂ instead, and the plant burns energy and releases CO₂ for no gain. That's photorespiration.

CAM plants (Crassulacean Acid Metabolism — pineapples, cacti) dodge this by splitting the two jobs across time. At night, when it's cooler and they can open stomata without losing too much water, they take in CO₂ and lock it up as malic acid. During the day, stomata stay shut (water saved), and the stored CO₂ is released inside the leaf to feed the Calvin cycle. CO₂ is therefore high around Rubisco and O₂ has no chance to compete.

One sentence version for the SAC: *CAM plants fix CO₂ at night and release it internally during the day, so stomata stay closed while the sun is out and Rubisco sees high CO₂ with no O₂ competition.*

Calling cellular respiration a 'biochemical pathway' means two things: it has *multiple steps*, and those steps are *linked* — each intermediate is consumed by the next enzyme.

Glucose doesn't become CO₂ and water in one bang. It's broken down stage by stage: glycolysis converts glucose to two pyruvate in the cytosol; the Krebs (citric acid) cycle processes pyruvate in the mitochondrial matrix; the electron transport chain handles the loaded coenzymes on the inner mitochondrial membrane. Each stage has several enzymes; each enzyme hands its product to the next enzyme as substrate.

When the SAC asks 'why is cellular respiration considered a biochemical pathway' — answer that it's a sequence of enzyme-catalysed reactions, each one taking the previous product as its substrate, converging on final products (CO₂, H₂O, ATP). 'It has multiple steps' alone isn't enough; the linkage through enzymes is the key.

Stage 1 — glycolysis — happens in the cytosol. Glucose (6C) is split into two pyruvate (3C each). Yield per glucose: 2 ATP (net), 2 NADH, 2 pyruvate. Oxygen not required.

Stage 2 — the Krebs cycle — happens in the mitochondrial matrix. Each pyruvate is first converted to acetyl-CoA (releasing 1 CO₂ each), which enters the Krebs cycle. Per glucose (two turns of the cycle): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂ released.

Stage 3 — electron transport chain (ETC) — happens on the inner mitochondrial membrane. NADH and FADH₂ from stages 1–2 drop off electrons. Those electrons cascade down a chain of proteins, powering the pumping of H+ into the intermembrane space. H+ then flows back through ATP synthase, making ATP. Oxygen is the final electron acceptor at the end of the chain — it combines with electrons and H+ to form H₂O. Per glucose: ~26–28 ATP from the ETC.

Total: around 30–32 ATP per glucose, plus 6 CO₂ and 6 H₂O. Without oxygen, the ETC stops, so NADH and FADH₂ can't unload, and glycolysis eventually stalls — that's when anaerobic fermentation kicks in.

The ETC spends its time shuttling electrons down a chain of carrier proteins, with each drop releasing energy that pumps H+ into the intermembrane space. Those electrons need somewhere to go at the end of the chain — otherwise the whole line backs up.

Oxygen is the final electron acceptor. When O₂ reaches the end of the chain, it picks up the electrons and combines with H+ (which also pour in from the intermembrane space) to form H₂O. That's the only reason aerobic respiration needs oxygen: to take the spent electrons off the chain and form water.

For the SAC: water is produced because oxygen, acting as the final electron acceptor in the electron transport chain, combines with H+ and electrons to form H₂O. Two marks means two ideas — (1) oxygen is the final electron acceptor, (2) it combines with H+ to form water.

Here's the problem when oxygen runs out. The ETC stops because there's no final electron acceptor. NADH can't drop off its electrons, so NADH piles up and NAD+ runs out. But glycolysis *needs* NAD+ to keep running. No NAD+ → no glycolysis → no ATP at all → cell dies.

Fermentation is a cheap rescue. In muscle cells, the enzyme lactate dehydrogenase takes pyruvate (from glycolysis) and hydrogens from NADH, and converts them together into lactic acid. Crucially, this regenerates NAD+, which goes back to glycolysis. The cell keeps producing ATP — only 2 per glucose, compared to 30+ from aerobic respiration, but enough to stay alive for short bursts.

Lactic acid is toxic in large amounts (it lowers pH and hurts), but *some* ATP is better than zero. That's the SAC answer: the reaction happens because it regenerates NAD+, letting glycolysis continue and the cell produce ATP during short-term oxygen shortage. Worth the later cost of clearing the lactate.

CRISPR-Cas9 works by combining a guide RNA (programmed to match a specific DNA sequence) with the Cas9 protein (molecular scissors that cut DNA). The guide RNA finds the target; Cas9 cuts it; the cell's own repair machinery then either breaks the gene (knockout) or slots in a new sequence if one is supplied.

In algae, several targets have been proposed. The most exam-friendly answer: edit the Rubisco enzyme so it binds CO₂ more tightly than O₂, which cuts down wasteful photorespiration and lets more CO₂ be fixed for the same amount of light. Another answer: edit pigment genes so the algae absorb a broader range of wavelengths, capturing more incoming light.

For one mark you don't need detail of the mechanism — just a concrete change: '*edit Rubisco to increase its affinity for CO₂ over O₂, reducing photorespiration*' or '*edit chlorophyll synthesis genes to broaden the absorption spectrum*'.

Step 1: grow algae in open ponds or photobioreactors. They photosynthesise and build up carbohydrates (starches, sugars) in their cells.

Step 2: harvest and break open the algal cells to release the sugars (mechanical or enzymatic hydrolysis of starch and cellulose into glucose).

Step 3: add yeast to the sugar solution. Yeast runs anaerobic fermentation — glycolysis of glucose to pyruvate, then pyruvate to ethanol + CO₂ (same shape as the animal lactate pathway but different end product).

Step 4: distill the fermented liquid to concentrate the ethanol. The result is bioethanol, a liquid fuel.

A 2-mark answer needs at least two of: cultivation, cell breakdown / sugar extraction, fermentation by yeast, distillation. Naming the fermentation product (ethanol + CO₂) is a bonus.

Corn and sugarcane are food crops. Turning them into biofuel means either diverting food from humans and livestock, or clearing new farmland to grow extra — often at the cost of forests. Either way, there's a food-versus-fuel tension.

Algae dodge that. They can be grown in saltwater or wastewater, on non-arable land (deserts, rooftops, industrial sites). They produce far more biomass per unit area per year than corn, because they float in 3D in a tank and keep dividing. They don't displace food. Some species can be grown using CO₂ piped in from industrial flue gas, so the cultivation itself can double as carbon capture.

For one mark, pick one angle: *algae don't compete with food crops* OR *algae yield more biomass per hectare per year* OR *algae can grow on non-arable land*. Any single strong reason will do.