Organisms and Metabolism
What this domain covers
Organisms and Metabolism tests how cells are built, how substances cross membranes, how enzymes control reactions, and how organisms obtain and release energy through photosynthesis, respiration, digestion and diet. Expect questions that ask you to explain observations and to name the correct enzyme, test or deficiency disease, not just to label a diagram.
Cells and movement across membranes
- Cell membrane: partially (selectively) permeable; controls exchange.
- Diffusion: net movement of particles from high to low concentration; passive; faster with a steeper gradient, higher temperature, shorter distance and larger surface area.
- Osmosis: net movement of water from a region of higher water potential to lower water potential through a partially permeable membrane.
- Active transport: movement against the concentration gradient using energy from ATP, e.g. mineral uptake by root hair cells and glucose reabsorption in the kidney.
- Plant vs animal cells: plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells lack these, though they may have small temporary vacuoles.
Enzymes and metabolism
Enzymes are protein catalysts with a specific active site. Rate rises with temperature up to the optimum (about 37 °C in humans), then falls sharply as the enzyme denatures and the active site loses shape. pH also has an optimum (pepsin acidic, amylase slightly alkaline). Metabolism is anabolism (building, e.g. protein synthesis) plus catabolism (breaking down, e.g. respiration).
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, using light energy trapped by chlorophyll.
- Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP.
- Anaerobic respiration: in muscles, glucose → lactic acid + small ATP; in yeast, glucose → ethanol + CO₂.
- Food tests: Benedict's (reducing sugar, brick red on heating), iodine (starch, blue-black), Biuret (protein, purple), ethanol emulsion (fat, white emulsion).
- Digestive enzymes: amylase converts starch to maltose; protease converts protein to amino acids; lipase converts fat to fatty acids and glycerol.
Worked example
Potato cylinders are placed in 0.0, 0.2, 0.5 and 1.0 mol/dm³ sucrose solutions. Predict and explain the change in length. In 0.0 mol/dm³ (pure water) the potato cells have a lower water potential than the surrounding water, so water enters by osmosis; cells become turgid and the cylinder lengthens. In 1.0 mol/dm³ the solution has a lower water potential than the cells, so water leaves by osmosis; cells become flaccid or plasmolysed and the cylinder shortens. Where length barely changes, the solution is isotonic to the cell sap.
Traps that cost marks
- Defining osmosis without stating that the membrane is partially permeable or mentioning water potential.
- Saying an enzyme is killed by heat; it is denatured.
- Forgetting that plants respire as well as photosynthesise.
- Mixing up Biuret (purple) with Benedict's (brick red).
- Writing that respiration makes energy instead of releases energy as ATP.
- Naming kwashiorkor (protein deficiency) when the case describes marasmus (protein-energy deficiency).