Food Production — Edexcel Linear - OL Revision Notes & Topical Questions

Growing crops outdoors does not allow control of limiting factors.

Polythene tunnels (polytunnels) are large plastic covers that protect crops from wind, rain and extreme temperatures, raise the temperature slightly and keep out pests and diseases.

Explaining costs and benefits

• extra yield from heated, lit glasshouses has to pay for fuel and electricity, so these methods suit high-value crops such as tomatoes.
• Polytunnels are cheaper and protect against frost and rain but give less control.
• Both methods let the farmer extend the growing season, so crops can be harvested earlier or later than outdoors.

With unlimited light, carbon dioxide, water and a warm temperature, the plant itself limits the rate. Usually one factor is in shortest supply, the limiting factor, and the rate cannot rise until it is increased. Three main factors:

Reading graphson a rate against light intensity graph, the line rises then flattens.

Fertilisers raise the amount of key nutrients in the soil so crops grow larger and healthier, increasing yield. Crops remove mineral ions, which must be replaced if crops are grown year after year. Organic fertilisers include manure and compost; chemical fertilisers are applied as granules or sprays.

Learn magnesium for chlorophyll and nitrate for amino acids. Spell nitrate correctly; plants need nitrate ions, not nitrites or nitrogen.

Insects eat crops, weeds compete for space, water and nutrients, and fungi spread disease.

Microorganisms can be used by humans to make foods and other useful substances. Yeast is a single-celled fungus that can respire both aerobically and anaerobically. Bacteria are useful because they can make complex molecules (for example enzymes that turn milk into yoghurt) and because they reproduce rapidly, so the amount of product can increase quickly.

Making bread. Yeast is mixed into the dough.

Aim to find how temperature affects the rate of anaerobic respiration (fermentation) in yeast.

Results and analysis

• bubbles per minute increase with temperature as enzyme activity rises towards the optimum.
• Above the optimum the enzymes denature, so carbon dioxide production slows and stops.
• The best temperature is the one with most bubbles.

CORMS evaluation

• Change the temperature;
  • use the same Organism (yeast species);
  • Repeat for reliability;
  • Measure the number of bubbles (M1) in a set time (M2);
  • Standardise sugar concentration, volume, pH and mass of yeast.

Yoghurt is made using the bacterium Lactobacillus. The stages show why temperature control matters.

The lactic acid lowers the pH, which prevents growth of harmful microorganisms, so yoghurt keeps longer than fresh milk.

Fermenters are containers in which microorganisms (bacteria and fungi) are cultured in large amounts for yoghurt, mycoprotein, antibiotics such as penicillin, and GM microorganisms. Their advantage is careful control of conditions so a large quantity of exactly the right organism is made.

Most fish are still caught from the wild, in the open ocean or in freshwater rivers and lakes. However, overfishing has led to dramatic declines in many fish populations. Fish farms raise large numbers of fish in a small space to provide protein for people. Compared with wild-caught fish, farmed fish offer several advantages:

The disadvantages are that fish farming can be labour intensive and costly.

Because fish farms rely on bought feed and treated water, the farmer must balance running costs against income from the fish sold, and must avoid pollution of surrounding water by waste and uneaten food.

Large numbers of fish are kept in freshwater or seawater enclosures and carefully monitored. A high yield means fast growth of healthy fish. The main methods are summarised below.

Every control method connects to one idea: removing something that would reduce survival or growth.

Evaluate

• benefits include reliable supply, protein for a growing population and less damage to wild stocks;
  • drawbacks include cost, labour, risk of disease outbreaks, and pollution of surrounding water by waste and antibiotics.

Another useful comparison is with other farming: fish convert food into body mass efficiently because they do not use energy to keep warm, so farmed fish can give a high protein yield from a modest amount of feed. Farm design also needs space, clean water flow and monitoring, which is why management and cost matter in evaluations.