Module Review & Connections

Amoeba is a single-celled organism approximately 500 μm in diameter. Its very high surface area to volume (SA:V) ratio means that every part of its cytoplasm is within a short diffusion distance of the external environment — typically less than 250 μm from the cell membrane. According to Fick's Law, the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to membrane thickness. With a high SA:V, the entire cell surface is available for gas exchange, the diffusion distance to any organelle is extremely short, and the thin cell membrane offers minimal resistance. O₂ and glucose can diffuse from the surrounding water into the cytoplasm, and CO₂ and wastes can diffuse out, at rates sufficient to meet the metabolic demands of this single cell. No transport system is needed because no cell is far from an exchange surface.

A dog is a large multicellular organism. As size increases, surface area grows as a square of linear dimensions while volume grows as a cube — so the SA:V ratio decreases dramatically with increasing body size. The dog's skin surface area is negligible relative to its total body volume. Most cells are metres away from the body surface — far beyond the effective range of diffusion (which operates efficiently only over micrometres). By Fick's Law, this enormous diffusion distance would reduce the rate of O₂ and glucose delivery to near zero for cells deep in the body. The body surface alone cannot provide sufficient exchange surface area relative to the volume of metabolically active tissue. A dog therefore requires specialised internal exchange surfaces — the alveoli of the lungs provide ~70 m² of thin, moist, highly vascularised surface for gas exchange; the villi and microvilli of the small intestine provide ~250 m² for nutrient absorption. These surfaces maximise SA, minimise membrane thickness, and maintain concentration gradients. However, even these specialised surfaces cannot deliver O₂ and nutrients directly to all cells — the circulatory system (heart, blood, blood vessels) transports O₂ from alveoli and nutrients from villi to every cell in the body, and carries CO₂ and wastes back to the lungs and kidneys for removal. Without the circulatory system, exchange surfaces would serve only the cells immediately adjacent to them — the vast majority of the dog's cells would be unable to meet their requirements.

Investigation design: The independent variable (IV) is pH — the student should test a range of values (e.g. pH 2, 4, 6, 7, 8, 10) using buffer solutions. Buffer solutions are essential because without them, the pH of the reaction mixture may drift during the experiment, making pH a confounding variable. The dependent variable (DV) is the rate of enzyme activity — measured, for example, as the time taken for starch to be fully digested (indicated by iodine no longer turning blue-black) or the rate of O₂ production (for catalase). Rate should be calculated as 1/time, since a shorter time corresponds to a faster rate. Controlled variables must include: temperature (maintained at the enzyme's optimum using a water bath — temperature independently affects enzyme activity); enzyme concentration (same volume from the same stock solution); substrate concentration (same volume and concentration of substrate in each trial); time allowed per measurement interval; and method of measurement (consistent iodine testing protocol). A control should include boiled (denatured) enzyme at the optimum pH to confirm that observed results are due to enzyme activity, not a spontaneous reaction.

Expected results and molecular explanation: The results would show a bell-shaped curve — enzyme activity is low at extreme pH values (both acidic and alkaline), rises to a maximum at the enzyme's optimum pH, and falls again on either side. Molecular explanation: enzymes are proteins whose active sites depend on a precise 3D shape maintained by non-covalent bonds, including ionic interactions between charged amino acid side chains and hydrogen bonds. At the optimum pH, these bonds are intact and the active site is in its ideal configuration for substrate binding. At pH values below the optimum, excess H⁺ ions protonate basic amino acid side chains, disrupting ionic bonds and altering the charge environment of the active site. At pH values above the optimum, excess OH⁻ ions deprotonate acidic side chains, similarly disrupting the ionic and hydrogen bonds. In both cases, the active site geometry changes, reducing the complementarity with the substrate — fewer enzyme-substrate complexes form and the reaction rate falls. At extreme pH values, these disruptions cause irreversible denaturation — the active site is permanently destroyed and activity cannot be recovered by returning to the optimum pH.