Glasshouses and fertilisers remove limiting factors
Crop yield is limited by whatever is in shortest supply. A glasshouse or polythene tunnel traps heat (higher temperature faster photosynthesis and growth, up to the optimum) and lets extra carbon dioxide be added, removing as a limiting factor. Fertilisers supply mineral ions: nitrate for amino acids/proteins (growth), phosphate for DNA and membranes, potassium for enzymes. Adding fertiliser to deficient soil raises yield; excess is wasteful and pollutes waterways.
Bread uses yeast; yoghurt uses bacteria
Bread — yeast (a fungus) respires anaerobically: glucose ethanol + carbon dioxide. The carbon dioxide is trapped as bubbles, making the dough rise; baking kills the yeast and evaporates the ethanol. Yoghurt — the bacterium Lactobacillus respires in warm milk, converting the milk sugar lactose into lactic acid. The acid lowers the pH, making the milk proteins coagulate (thicken) into yoghurt. Note: lactose is the sugar (not lactase, an enzyme), and the pH FALLS.
Fermenter conditions and aseptic precautions
A fermenter grows microorganisms under controlled conditions: temperature (a water jacket removes the heat of respiration to hold the optimum), pH, oxygen (sterile air bubbled in), nutrients, and stirring (a paddle distributes them evenly). Aseptic precautions stop unwanted microbes competing or making toxins: equipment and medium are sterilised and the air filtered. Precision trap: too-high a temperature makes the enzymes denature and the microbes die — do not swap those verbs.
Drawn from real examiner reports.
Yoghurt: pH falls; lactose not lactase
In yoghurt the bacteria convert lactose (the milk sugar) into lactic acid. Two marks are routinely lost: writing that the pH RISES or the milk turns alkaline (the acid makes it fall); and writing lactase (an enzyme) for lactose (the sugar). A full answer names lactose lactic acid, states the pH falls, and links it to the milk proteins coagulating.
Enzymes denature; microbes die
Explaining the fermenter's cooling water jacket is a precision trap. Unremoved heat of respiration pushes the temperature above the optimum; then the enzymes denature (their active-site shape changes) AND the microbes die. The marked error is pairing the wrong noun with the wrong verb — "kills the enzymes" or "denatures the bacteria". Enzymes denature; whole organisms die.
Biological control reduces, not exterminates
Biological control has a precise definition: introduce a natural predator (or parasite/pathogen) of the pest to REDUCE the pest population below the damage level — not to wipe it out. Vague answers ("use other animals") miss that the introduced species must specifically attack the pest and that the aim is to control numbers, not exterminate.
Yeast = fungus; Lactobacillus = bacterium
Bread and yoghurt use different microbes. Yeast is a FUNGUS and produces carbon dioxide (making dough rise) by anaerobic respiration. Lactobacillus is a BACTERIUM and produces lactic acid (setting yoghurt). Swapping them — "yeast makes yoghurt" or "bacteria make bread rise" — loses the mark.
Describe the microbe's action, not the recipe
For "how is yoghurt/bread made" answers, describe what the MICROBE does, not the commercial recipe. "Heat the milk, add culture, incubate" scores nothing; the credit is for the bacteria respiring and converting lactose to lactic acid (yoghurt), or the yeast fermenting sugar to carbon dioxide (bread).
Answer as factor -> mechanism -> effect
Build every answer as factor mechanism effect, not a bare list. Glasshouse: adding CO2 removes it as a limiting factor faster photosynthesis and growth higher yield. Always link the named factor to how it changes yield or the microbes.
Compare means evaluate, then conclude
For "compare pesticides with biological control", make a two-column judgement (residues, resistance, speed, specificity) and finish with a conclusion — evaluate, do not just describe. A one-sided list, or a description with no judgement, caps the marks.
Match the command word, be precise
Answer the command word: "describe" wants the pattern, "explain" wants the reason/mechanism — a very common lost mark. Be precise with temperature control too: say "optimum temperature / prevent overheating", not the looser "maintain temperature".
This topic is about using biology to grow more food: controlling the environment of crops (glasshouses, carbon dioxide, temperature, fertiliser), controlling pests, and using microorganisms (yeast and bacteria) to make food and to grow useful products in fermenters.
Two-element, mark-scheme definitions:
Why a glasshouse / polythene tunnel raises yield: it traps heat (higher temperature faster photosynthesis and growth up to the optimum), lets extra carbon dioxide (CO2) be added (removes CO2 as a limiting factor of photosynthesis), and allows light, water and pests to be controlled.
Fertiliser mineral ions: nitrate amino acids/proteins (growth); phosphate DNA (deoxyribonucleic acid) and membranes; potassium enzymes.
Define biological control.
Growers often grow tomato plants in glasshouses rather than outdoors.
State two conditions that can be controlled in a glasshouse to increase the crop yield, and for each one explain how it increases the yield. (4 marks)
