National 5 Biology | Unit 3 | Key Area 5: Food Production

An increasing world population requires what?

An increased food yield.

Fertiliser

A substance that provides chemicals to aid plant growth and increase crop yield.

Pesticide

A chemical that kills unwanted plants and animals that would otherwise reduce crop yield.

Herbicide

A chemical that is used to kill unwanted vegetation, especially weeds.

Name the three elements (most often lacking in soil) which are provided in fertiliser: (3)

- Nitrogen in the form of nitrate

- Phosphorus in the form of phosphate

- Potassium

What is nitrogen used for in plant growth?

Nitrates are used to produce proteins and nucleic acids.

Describe the stages of the nitrogen cycle:

1. Nitrates dissolved in soil water absorbed into plants, used to synthesise amino acids → plant protein

2. Animals consume plants/animals to obtain amino acids for protein synthesis.

3. Protein broken down by decomposer organisms → ammonium compounds from carcasses and excreta

4. Ammonium compounds break down into nitrites

5. Nitrites gains oxygen → nitrates

Eutrophication

The excessive enrichment of a body of water, typically lakes or ponds, frequently due to the leaching of fertilisers and causes a dense growth of plants.

What are the main fertilisers involved in eutrophication?

Nitrates and phosphates

Why are nitrates so easily leached from the ground?

Nitrates are extremely soluble and do not bind to soil particles.

Algal bloom

A rapid increase or accumulation in the population of algae in freshwater.

Describe the effect on biodiversity when fertilisers leach into bodies of water:

- Nitrates from fertilisers leach into bodies of water. Increase in chemicals stimulates growth of algae.

- With excesses of nitrates, algae continues to grow, ie. an algal bloom, and form a “mat” on the surface of the water, reducing light levels.

- Aquatic plants unable to photosynthesise and produce food, therefore they die and decompose. Dead plants and algae become food for aerobic bacteria; bacteria populations increase.

- Bacteria use up the oxygen in the water; no further oxygen produced by photosynthesis. Aerobic organisms suffocate from lack of oxygen.

- Biodiversity decreases.

Explain the effects of pesticides on food chains:

Pesticides sprayed onto crops can accumulate in the bodies of organisms over time, ie bioaccumulation. As they are passed along food chains, toxicity increases and can reach lethal levels in top carnivores.

What are some alternatives to the use of pesticides?

Biological control and genetically modified crops

Biological control

The reduction of a pest population by deliberately introducing a natural enemy of the pest, eg. a predator or a parasite.

How can genetically modified crops reduce the need for pesticides?

GM crops can be altered to be immunised against fungi and disease, have protection against pests, have larger yields, etc.