Key Area 3.1: Food Supply, Plant Growth and Productivity

Food security

The ability of human populations to access food of sufficient quality and quantity.

A person is food insecure when:

When lacking regular access to safe and nutritious food for normal growth.

Food production

- An increase in human population and concern for food safety leads to a demand for increased food production.

- More food must be produced for the same area of land, so in order to maintain a sustainable food supply, increased plant productivity and genetic diversity is required.

- Must be sustainable and not degrade the natural resources and ecosystems on which agriculture depends, as all food production ultimately depends on photosynthesis.

Agricultural production depends on factors that control photosynthesis (7):

- Light intensity

- Carbon dioxide concentration

- Water availability in soil

- Nutrient availability

- Temperature

- Competition with other plants

- Chlorophyll concentration

The area to grow plants is limited, so increased food production depends on factors that control plant growth (4):

- Breeding of higher yielding cultivars (cultivated varieties) and planting strains of crops that have higher yields.

- Use of fertiliser to control limiting factors, eg. lack of minerals.

- Protection of crops from pests, disease and competition with weeds by use of pesticides, herbicides and fungicides.

- Developing pest-resistant crops.

Plant crop examples:

- Cereals

- Potato

- Roots

- Legumes

Breeders seek to develop crops with:

- Higher nutritional value

- Resistance to pests and disease

- Physical characteristics suited to rearing and harvesting, and can thrive in particular environmental conditions

Livestock and plant produce

- Livestock produce less food per unit area than crop plants due to the loss of energy between trophic levels.

- Plants produce more energy per area/volume.

- Livestock production often possible in habitats unsuitable for growing crops, eg. steep land and thin soil

Photosynthesis

- The process by which green plants trap light energy and use it to make carbohydrates.

- Carbon dioxide + water (+light, + chlorophyll enzymes) → sugar + oxygen

Describe the three fates of light when it strikes a leaf:

- Absorbed by pigments within chloroplasts (83%)

- Reflected (12%)

- Transmitted through leaves (5%)

Capture of light energy

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

Chlorophyll a and chlorophyll b

- Chlorophyll a is the only pigment to directly participate in light reactions, absorbing red and blue light.

- Chlorophyll b absorbs red and blue light.

Carotenoid

- A naturally occurring accessory pigment which extends the range of wavelengths absorbed by photosynthesis, and passes the energy to chlorophyll for photosynthesis.

- Often yellow or orange pigments in colour, but absorb violet, yellow and green light.

Absorption spectrum

A graph which displays how much light of each wavelength is absorbed by a particular pigment. Having multiple photosynthesis pigments increases the range of wavelengths of light that the plant can absorb, and so increases the rate of photosynthesis.

Action spectrum

A graph which displays the effectiveness of each wavelength of light in causing photosynthesis, ie. it shows the rate of photosynthesis at different wavelengths of light.

When absorbance of light is relatively low, why are there still high levels of photosynthesis?

Carotenoids still extend the range of wavelengths absorbed and pass energy to chlorophyll, as shown in the action spectra.

Light-controlled reactions

Stage 1 of photosynthesis, where light energy is trapped by chloroplasts and converted into chemical energy using ATP.

Describe the process of light-controlled reactions:

- Absorbed light energy excites electrons in the pigment molecule, creating high-energy electrons.

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

- Energy also used for photolysis, in which water is split into evolved oxygen (not kept for this process, ie. given off) and hydrogen, which is transferred to coenzyme NADP.

Coenzyme NADP

- A coenzyme which accepts hydrogen ions from light-controlled reactions and takes them to the carbon fixation stage.

- NADP reduces to NADPH when accepting hydrogen ions, and oxidises back to NADP when offloading hydrogen to carbon fixation.

Carbon fixation

A series of enzyme-controlled reactions which do not require light, and is temperature dependent. Also known as the Calvin cycle.

Describe the process of the Calvin cycle:

- Captured carbon dioxide enters the Calvin cycle.

- Enzyme RuBisCO fixes CO2 by attaching it to RuBP (ribulose phosphate, a carbon acceptor) forming 3PG (3-phosphoglycerate).

- 3PG phosphorylated by ATP from light-controlled reactions (gains a phosphate group).

- Phosphorylated 3PG combines with hydrogen from NADPH, forming G3P (glyceraldehyde-3-phosphate).

- G3P used to regenerate RuBP for continued use in Calvin cycle, or for synthesis of sugars (eg. glucose).

Glucose produced in photosynthesis can be:

- Used as a respiration substrate

- Stored as starch (storage carbohydrate)

- Synthesised to cellulose (structural carbohydrate) for cell walls

- Passed to other biosynthetic pathways to form a variety of metabolites, eg. DNA, protein, fat