SI1- food supply, plant growth and productivity

• Increase in the human population and concern for food security leads to a demand for increased food production.
• Food security is the ability of human populations to access food of sufficient quality and quantity
• Food production must be sustainable and not degrade the natural resources on which agriculture depends
• Intensive agriculture can damage the environment through:

  • global warming
  • pollution (pesticides and fertilisers)
  • deforestation
  • soil erosion
  • reducing soil fertility
  • water abstraction
• Ultimately all food production, plant or animal, depends on the process of photosynthesis
• Agricultural production depends on factors that control photosynthesis and plant growth these include:

  • light intensity
  • CO2 concentration
  • temperature
  • nutrient availability
  • pests and disease
  • competition
• Most human food comes from a small number of plant crops, such as cereals, potato, roots and legumes.
• Despite the increasing demand for food production the area to grow crops is limited.
• Increased food production will depend on controlling factors that affect plant growth:

  • breeding of higher yielding cultivars
  • use of fertiliser to provide essential nutrients
  • protecting crops from pests, diseases and competition through the use of pesticides, herbicides and genetic engineering
• Breeders seek to develop crops:

  • with higher nutritional values
  • resistant to pests and diseases
  • with physical characteristics suited to rearing and harvesting
  • that can thrive in particular environmental conditions- drought, high salinity
• Livestock produce less food per unit area than crop plants due to loss of energy between trophic levels
• Food production is more efficient if the food chain is short
• There are fewer trophic levels at which energy can be lost and therefore a higher percentage of energy is available to humans
• Livestock production is often possible in habitats unsuitable for growing crops
• When light energy from the sun strikes the leaf it is either absorbed, reflected or transmitted
• Photosynthesis is the process by which green plants absorb light energy and use it to make carbohydrates
• Photosynthesis is the process by which green plants absorb light energy and use it to make carbohydrates
• Photosynthesis consists of two stages:

  1. light dependant stage
  2. carbon fixation stage
• Light energy absorbed by photosynthetic pigments located in the chloroplasts
• Plants contain a variety of pigments including chlorophyll a, chlorophyll b and carotenoids
• Each pigment absorbs a different range of wavelengths of light thus increases the level of photosynthesis
• The wavelengths of light absorbed by the different pigments are shown in absorption spectrum
• Chlorophyll a and b are the main photosynthetic pigments, they absorb mainly blue and red light wavelengths
• The carotenoids are accessory pigments that extend the range of wavelengths absorbed
• Carotenoids (and other accessory pigments) pass the energy they absorb to chlorophyll a for photosynthesis
• An action spectrum of a plant shows the rate of photosynthesis occurring at different wavelengths of light
• The action spectrum of a plant has a similar shape to the combined absorption spectra of the photosynthetic pigments present in the plant
• The light energy absorbed is used to generate ATP and for photolysis (splitting water to get hydrogen)
• Absorbed light energy excites electrons in the pigment molecules
• The electrons are transferred through the electron transport chain releasing energy
• This released energy is used for both photophosphorylation and photolysis
• Photophosphorylation is the generation of ATP from ADP and Pi by ATP synthase
• Energy is also used to split water into oxygen and hydrogen ions- a process known as photolysis
• The oxygen is released into the atmosphere (evolved)
• The hydrogen ions are transferred to and become bonded to the coenzyme NADP to form NADPH
• Both the hydrogen ions, in the form of NADPH and the ATP are transferred to the second stage of photosynthesis- carbon fixation
• The carbon fixation stage is a series of enzyme controlled reactions which does not require light.

  It is temperature dependant
• The enzyme RuBisCo fixes carbon dioxide by attaching it to RuBP to produce 3PG
• The 3PG produced is phosphorylated by ATP and combined with hydrogen ions from NADPH to form G3P
• G3P is used to regenerate RuBP allowing continuation of the cycle and for the synthesis of glucose