Unit 4: C1 — Enzymes, Photosynthesis, Chromatography & Experimental Interpretation Summary & Study Notes

🔬 Enzyme activity and temperature

Enzyme activity generally increases with temperature because molecular collisions are more frequent and kinetic energy is higher. This continues until an optimum temperature is reached. Beyond that point the enzyme begins to denature as the active site changes shape, causing activity to fall. The change in activity between points X and Y on a temperature–activity graph is explained by denaturation (the active site losing its complementary shape).

🧩 Enzyme–substrate interaction models

Two commonly referenced models are the lock-and-key and the induced-fit models. The lock-and-key model states the substrate is complementary in shape to the active site. The induced-fit model states the active site changes shape to accommodate the substrate. Permanent alteration of the enzyme (irreversible change) is not implied by the induced-fit model.

⚡ Energy profiles and catalysis

In a reaction energy diagram, an enzyme lowers the activation energy required. The activation energy with an enzyme corresponds to the lower peak. The net energy released (or absorbed) is the vertical difference between reactants and products. Enzymes change the activation energy but do not change the net energy change of the reaction.

🌿 Photosynthesis: which groups produce oxygen?

Oxygenic photosynthesis (producing O_2 as a by-product) is performed by algae and cyanobacteria. Fungi do not perform oxygenic photosynthesis. Therefore the groups that produce oxygen are algae and cyanobacteria.

🌱 FACE (Free-Air Carbon Dioxide Enrichment) experiments

A key advantage of FACE experiments is that they can provide realistic field data on how plants respond to elevated $\mathrm{CO}_2$ under natural conditions; results can more accurately represent future rates of photosynthesis in real ecosystems. FACE does not tightly control variables such as temperature or water the way a lab experiment does.

🧫 Anaerobic respiration in human cells

In human (muscle) cells under anaerobic conditions the main products are lactate (lactic acid) and ATP, and this process occurs in the cytoplasm (glycolysis followed by lactate fermentation). Emphasize: anaerobic respiration in humans yields far less ATP per glucose than aerobic respiration.

🧪 Chromatography and Rf values

The retention factor (Rf) is defined as distance moved by compound / distance moved by solvent front. Rf values reflect relative polarity: less polar pigments typically travel further (higher Rf) on a polar stationary phase (e.g., silica). If a substance Z has an Rf of 0.5, identify other pigments by comparing their Rf values and relative positions: pigments with larger Rf are less polar than Z; those with smaller Rf are more polar.

🧪 Laccase immobilization experiment — interpreting results

• Most effective concentration: the experiment identified the most effective laccase concentration for decolorizing crystal violet as between $2.5,\mathrm{mg},\mathrm{ml}^{-1}$ and $4.5,\mathrm{mg},\mathrm{ml}^{-1}$ (accept $2.5,\mathrm{mg},\mathrm{ml}^{-1}$ or $4.5,\mathrm{mg},\mathrm{ml}^{-1}$ with units).

• Prediction at $5,\mathrm{mg},\mathrm{ml}^{-1}$: extrapolation of the graph gives approximately $94$ relative decolorization (answers between $90$ and $98$ are acceptable).

• Overlapping error bars (3 to $4,\mathrm{mg},\mathrm{ml}^{-1}$): overlapping error bars in that range suggest no significant difference in percent decolorization between those concentrations (i.e., differences are within measurement variability).

• Relationship between concentration and decolorization: there is a positive (direct) relationship at low laccase concentrations (increasing enzyme concentration increases percent decolorization). After about $2.5$–$3,\mathrm{mg},\mathrm{ml}^{-1}$ the curve plateaus (percent decolorization does not increase further), so it is not correct to call this an "optimum" followed by a decline — the data show a plateau.

⚙️ Experimental controls for the laccase study

When discussing controlled variables, explain why each matters (naming alone is insufficient):

• pH — enzymes have an optimum pH; deviations can change enzyme conformation or denature it.
• Temperature — affects kinetic energy and can cause denaturation at high temperatures or reduced activity at low temperatures.
• Size/number/surface area/volume of alginate beads — determines enzyme exposure to substrate and diffusion rates.
• Volume and concentration of crystal violet dye — substrate concentration affects reaction rate; volume affects final concentrations.
• Type/purity of enzyme and dye — substrate specificity and dye chemistry affect reaction outcome.
• Reaction time — amount of substrate converted depends on incubation length; compare samples at the same time point.

Note: do not list laccase concentration as a controlled variable here because it is the independent variable being tested.