ka1: food supply, plant growth and productivity

• Food security is the ability of human populations to access food of sufficient quality and quantity.
• Increase in human population and concern for food security leads to a demand for increased food production. Food production must be sustainable and not degrade the natural resources on which agriculture depends.
• Agricultural production depends on factors that control photosynthesis and plant growth. The area to grow crops is limited. Increased food production will depend on factors that control plant growth — breeding of higher yielding cultivars, use of fertiliser, protecting crops from pests, diseases and competition.
• All food production is dependent ultimately upon photosynthesis. Plant crop examples include cereals, potato, roots and legumes. Breeders seek to develop crops with higher nutritional values, resistance to pests and diseases, physical characteristics suited to rearing and harvesting as well as those that can thrive in particular environmental conditions.
• The human population is currently growing
• The human population was sitting at 7.7 billion

  November 2018
• Feeding over 7 billion people requires a sufficient and sustainable supply of food
• Food security is a massively important subject for the future of the human race
• Food security


  The ability of human populations to access food of sufficient quality and quantity
• The increase in human population leads to a demand for increased food production
• Food production must be sustainable and not degrade the natural resources on which agriculture depends
• Agricultural production depends on factors that control plant growth
• All food production is ultimately dependent on photosynthesis
• The area of land suitable for growing crops is limited
• Increased food production

  • Adding minerals (fertiliser) or water (irrigation systems) to remove factors which may be limiting plant growth
  • Replacing existing strains of crops with a higher-yielding cultivar (cultured variety)
  • Protecting crops from pests (e.g. insects), diseases (e.g. fungi), and competition (from weeds) by using pesticides, fungicides and herbicides
  • Developing pest-resistant crop plants
• Breeders seek to develop crops with

  • Higher nutritional values
  • Physical characteristics suited to rearing and harvesting
  • Crops which thrive in particular environments
  • Resistance to pests & diseases
• Livestock produce less food per unit area than crop plants due to loss of energy between trophic levels. Livestock production is often possible in habitats unsuitable for growing crops.
• Livestock produce less food per unit area than plants crops due to loss of energy between trophic levels. However, livestock production may be possible in habitats which are unsuitable for growing crops.
• Light energy is absorbed by photosynthetic pigments to generate ATP and for photolysis. Light energy not absorbed is transmitted or reflected.
• Photosynthesis is the process by which green plants trap light energy and use it to make carbohydrates.
• When light strikes a leaf, some is reflected, some is transmitted through the leaf and the rest is absorbed by the pigments within the chloroplasts, this energy is used to generate ATP and for photolysis.
• Pigments are chemicals found within the chloroplasts which absorb light energy and convert it to chemical energy. The principal pigments are chlorophyll a and b.

   

  Plants also contain carotenoids (carotene and xanthophyll) which extend the range of wavelengths absorbed by photosynthesis and pass the energy to chlorophyll. These are often known as accessory pigments.
• Absorption spectra of chlorophyll a and b and carotenoids compared to action spectra for photosynthesis. Carotenoids extend the range of wavelengths absorbed and pass the energy to chlorophyll for photosynthesis. Each pigment absorbs a different range of wavelengths of light.
• The absorption spectra shows the wavelengths of light absorbed by the different pigments in a leaf. The graph shows that most light is absorbed in the blue and red regions of the spectrum and there is limited absorption in the green region of the spectrum with most light reflected or transmitted. This is why most plants are green in colour. Chlorophyll a and b absorb mainly in the blue and red regions of the spectrum. The carotenoids extend the range of wavelengths available for photosynthesis and pass this energy onto chlorophyll a.
• The action spectra shows how effective the different wavelengths of light are at photosynthesis. The graph shows the rate of photosynthesis is highest in the red and blue regions of the spectrum. Having multiple photosynthetic pigments increases the range of wavelengths of light that the plant can absorb and so increases the level of photosynthesis.
• In the carbon fixation stage (Calvin cycle), the enzyme RuBisCO fixes carbon dioxide by attaching it to ribulose bisphosphate (RuBP). The 3-phosphoglycerate (3PG) produced is phosphorylated by ATP and combined with hydrogen ions from NADPH to form glyceraldehyde-3- phosphate (G3P). G3P is used to regenerate RuBP and for the synthesis of glucose.
• In the chloroplast

  1. Light energy is absorbed by the pigments

  2. Electrons in the pigment molecule become excited and get captured by the electron transport chain

  3. Transfer of electrons through the electron transport chain releases energy

  4. Some of this energy is used to generate ATP by the enzyme ATP synthase

  5. The rest of the energy is used for photolysis of water

  6. Photolysis is the process by which water is split into oxygen, which is evolved (released into the atmosphere)

  7. Hydrogen then becomes bonded to the coenzyme NADP to form NADPH and is transferred to the Calvin Cycle along with ATP
• The Carbon Fixation Stage is a series of enzyme controlled reactions which does not require light. It is the temperature-dependent part of photosynthesis.

  Carbon dioxide enters the cycle and becomes attached to ribulose biphosphate (RUBP) by the enzyme RuBisCO.

  The intermediate molecule that is produced, a 3-phosphoglycerate (3PG), is phosphorylated by ATP and combined with hydrogen from NADPH to form glyceraldehyde-3-phosphate (G3P). G3P is used to either regenerate RuBP or can be used to synthesise sugars, for example glucose.
• Glucose may be used as a respiratory substrate, synthesised into starch or cellulose or passed to other biosynthetic pathways. These biosynthetic pathways can lead to the formation of a variety of metabolites such as DNA, protein and fat.
• Glucose may be:

  • used as a respiratory substrate,
  • synthesised into starch (storage) or cellulose (structural)
  • passed to other biosynthetic pathways to form a variety of metabolites such as DNA, proteins and fats.