FOOD SUPPLY, PLANT GROWTH AND PRODUCTIVITY

FOOD SECURITY

ABSORPTION SPECTRA

Light energy is absorbed by photosynthetic pigments to generate ATP and for photolysis. Each pigment absorbs a different range of wavelengths of light.

Absorption spectra are diagrams which show how much of each wavelength of light has been absorbed.

Chlorophyll a and b absorb light in the red and blue wavelengths of light.

Carotenoids extend the range of wavelengths absorbed and pass the energy to chlorophyll for photosynthesis.

Food security is the ability of human populations to access food of sufficient quality and sufficient 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.

FOOD PRODUCTION

All food production is dependent ultimately upon photosynthesis.

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.

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.

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.

USE OF ABSORBED LIGHT

Absorbed light energy excites electrons in the pigment molecule.

Transfer of these electrons through the electron transport chain releases energy to generate ATP by ATP synthase.

Energy is also used for photolysis, in which water is split into oxygen, which is evolved, and hydrogen ions, which are transferred to the coenzyme NADP.

CARBON FIXATION

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.

FATE OF LIGHT

When light strikes a leaf it is either:

Absorbed

Reflected

Transmitted (passes through the leaf)

A spectroscope, spectrophotometer or a colourimeter could be used to show how much of each wavelength of light is absorbed by a photosynthetic pigment.

FATES OF GLUCOSE

ABSORPTION OF LIGHT

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.

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FOOD SUPPLY, PLANT GROWTH AND PRODUCTIVITY