Unit 3 KA1-KA3

Question 1

Food Security, plant growth and productivity

Answer 1

The human population depends on a sufficient and sustainable supply of food for its survival.
Food security involves humans being able to access food of sufficient quantity and quality (nutritious & varied enough to provide a balanced diet).
As the global human population continues to increase so does the demand for food. This makes it more difficult to ensure food security for all and makes it more likely that some humans will suffer starvation or malnutrition.
Methods of increasing food production must be such that the food production is sustainable and does not degrade the natural resources on which agriculture depends (use of lots of fertilisers, for example, often leads to environmental degradation).

Question 2

Factors Affecting Food Production

Answer 2

The area suited to grow crops is limited, so increased agricultural production will depend on factors that control photosynthesis and plant growth.
Examples of crop plants include cereals, potato, roots and legumes.

Question 3

To increase agricultural production

Answer 3

 Environmental factors which can limit photosynthesis (temperature,
light intensity & CO2 concentration)
 Breeding a higher yielding cultivar (strain) of crop plant
 Breeding crops with higher nutritional values
 Protecting the crops from pests which eat them (using pesticides) and diseases which kill or damage them (using fungicides) will increase food production. Or breeding crops already resistant to pests and diseases.
 Using fertiliser to increase the supply of mineral elements for growth in the soil will increase food production.
 Reducing competition from weeds using herbicides.
 The physical characteristics of the plant which make it suited to rearing and harvesting and also the environmental conditions it thrives in.

Question 4

Photosynthesis

Answer 4

Photosynthesis is the process by where light energy is absorbed by photosynthetic pigments to generate ATP and for photolysis

Most of the light that shines on a leaf is absorbed. The rest is either reflected or transmitted.

Question 5

Photosynthetic Pigments

Answer 5

Green plants have several photosynthetic pigments which are able to absorb light energy and convert it into chemical energy.
The commonly occurring photosynthetic pigments are:-
 Chlorophyll a
 Chlorophyll b
 Carotenoids

Question 6

Absorption Spectrum

Answer 6

Each pigment absorbs a different range of wavelengths of light. Chlorophyll a and b absorbs light mainly in the blue and red regions of the spectrum.
Carotenoids absorb light from a different range of wavelengths and pass the energy onto chlorophyll for photosynthesis.
This means that they extend the range of wavelengths of light which can be absorbed.

Question 7

Action Spectrum

Answer 7

An action spectrum shows the rate of photosynthesis in different wavelengths of light.
X
It can be seen from the action spectrum graph that most photosynthesis occurs in the blue and red regions.
Note that the absorption and action spectra are similar; however, at point X on the action spectrum graph, the rate of photosynthesis remains high even although the absorption by chlorophyll a is low at this point – evidence that there are other pigments involved.

Question 8

Light dependent stage

Answer 8

Stage 1 -
 is the light-dependent stage
 occurs in the granum of a chloroplast
Light energy is absorbed by the photosynthetic pigments.The energy excites electrons in the pigment molecules.
The electrons move through an electron transport chain. As they do so, they release energy.
This energy is used:-
 by the enzyme ATP synthase to generate ATP (from ADP + Pi)
 for photolysis – the splitting of water into oxygen (which is released/evolved) and hydrogen (which is bound onto the coenzyme NADP to form NADPH).
The ATP and NADPH are passed on to the next stage, the Calvin cycle.

Question 9

Calvin cycle

Answer 9

Stage 2 - Calvin cycle
 is the carbon fixation stage and it involves a series of enzyme-controlled
reactions
 It occurs in the stroma of a chloroplast
 It requires ATP and hydrogen produced by the light-dependent stage
The enzyme RuBisCO fixes carbon dioxide by attaching it to ribulose bisphosphate (RuBP) - this results in the formation of 3-phosphoglycerate (3PG).
RuBP is the CO2 acceptor.
The 3PG is phosphorylated by ATP and combined with hydrogen from
NADPH to form glyceraldehyde-3-phosphate (G3P).
Some of the G3P is used to regenerate RuBP; the rest is used for the
synthesis of glucose.
The glucose produced may be used as a respiratory substrate or converted into starch or cellulose. Alternatively, it may pass to other biosynthetic pathways to form metabolites such as DNA, proteins and fats.

Question 10

Plant & Animal breeding

Answer 10

Breeders of crop plants and livestock are trying to improve characteristics which will help to support sustainable food production.
Improved crops & animals may have:-
 Higher food yields (e.g. cattle with more meat)
 Higher nutritional values (e.g. more protein in soya bean)
 Pest resistance
 Disease resistance
 The ability to thrive in particular environmental conditions

Question 11

Field Trials

Answer 11

Plant field trials are used to investigate the quality of a variety of plant. It could be set up to:-
 Compare the performance of different plant cultivars under the same set of environmental conditions
 Find out the effect of different treatments (e.g. different concentrations of fertiliser) on a new cultivar of a crop plant
 To evaluate the performance of GM crops

Question 12

Selection of treatment

Answer 12

A valid comparison can only be made if there is only one variable factor (e.g. concentration of fertiliser); all other factors must be kept the same.

Question 13

Number of replicates

Answer 13

Several replicates should be set up to take account of uncontrolled variability (e.g. the plots used will not be identical) and reduce the effect of any errors; the more replicates, the more reliable the results will be.

Question 14

Randomisation of treatment

Answer 14

The positioning of the replicated plots should be randomised to eliminate bias created by the environment.

Question 15

Inbreeding

Answer 15

In inbreeding, selected related plants or animals are bred for several generations until the population becomes true breeding (homozygous) for the desired alleles; heterozygotes are eventually eliminated.
A disadvantage of inbreeding though is that deleterious (harmful) recessive alleles also become homozygous. This is known as inbreeding depression and may result in a decline in vigour, size, fertility & yield of the organism. These individuals will do less well at surviving to reproduce.

Question 16

Cross breading

Answer 16

Due to the risk of inbreeding depression, inbreeding is rarely carried out indefinitely. Instead new alleles are introduced to the plant or animal line by cross-breeding it with a strain which has a different desirable genotype.
The new crossbreed produced may have improved characteristics. The two parent breeds can be maintained to produce more crossbreed animals showing the improved characteristic.
In plants F1 hybrids, produced by crossing two different inbred lines, creates a fairly uniform heterozygous crop. F1 hybrids often have increased vigour and yield. This may give the plant increased disease resistance or increased growth rate.
If F1 hybrids are allowed to interbreed, the F2 generation will show a wide variation in genotype.

Question 17

Crop protection

Answer 17

In a natural ecosystem, there is a wide variety of plant species competing with one another.
Weeds compete with crop plants, while other pests and diseases damage crop plants. All of which reduces productivity.

Question 18

Weeds

Answer 18

Weeds compete with crop plants for light, water, soil nutrients and space. Weeds can be annual or perennial plants.

Question 19

Annual weeds

Answer 19

Annual weeds grow from seed and complete their life cycle in 1 year. They have several adaptations (properties) which make them successful competitors:-
 They have rapid growth
 They have a short life cycle
 They produce a large number of seeds
 Their seeds are viable for long periods (even years) in the soil

Question 20

Perennial weeds

Answer 20

Perennial weeds persist from year to year. They are successful because they are already established before the crops are grown. Other properties which make them successful competitors include:-
 The possession of storage organs from the previous year that provide food even when environmental conditions are unfavourable for photosynthesis
 The ability to reproduce vegetatively using structures such as runners and bulbs

Question 21

Pests

Answer 21

Most of the pests of crop plants are invertebrate animals such as:-
 Insects
 Nematode worms
 Molluscs.
All of these pests cause damage to plants and leaves when they feed on them.

Leaf damage leads to reduced photosynthesis which results in less sugar being available as an energy supply for the growing plant tissues. Yield, therefore, is greatly reduced.

Question 22

Diseases

Answer 22

Many crop plant diseases are caused by microorganisms such as:-
 Fungi (e.g. potato blight)
 Bacteria (e.g. soft rot in parsley)
 Viruses (e.g. leaf roll in
potato)

Question 23

Cultural methods of control

Answer 23

These are traditional, non-chemical methods developed by trial and error over time.
Such methods tend to be preventative.
 Ploughing involves turning over the top layer of soil. Any weeds tend to be damaged or buried to a depth where they die and decompose.
 Weeding early in the life of the crop allows the crop to become sturdy enough to survive competition by weeds which appear later.

 Crop rotation involves growing different plants in turn on the same piece of ground. Any pests present on a particular crop plant are controlled as they are unlikely to survive for 4 years until the same plant is introduced again.

Question 24

Classical methods of control

Answer 24

G

Question 25

Herbicides

Answer 25

Herbicides are used to kill weeds. There are two types:-  Selective
 Systemic
Weeds are broad-leaved plants and absorb high quantities of weed-killer
through their leaves.
Selective herbicides stimulate the weeds’ rate of growth and metabolism to such an extent that the weeds run out of food and die of starvation.

Narrow-leaved plants such as grass are not affected as they absorb very little weed-killer.
Systemic herbicides are absorbed by the weeds and are transported internally to all parts of the plant through the vascular system where it prevents regrowth.

Question 26

Insecticides

Answer 26

Insecticides are used to kill insect pests.
Systemic insecticides, molluscicides and nematicides spread through the vascular system of the plant and kill the insect when it ingests the chemical whilst feeding on the plant.

Question 27

Fungicides

Answer 27

Fungicides are used to kill fungal parasites that cause diseases of crop plants.
They tend to be used as a protective measure and are sprayed onto plants when it is forecasted that fungal infections are likely. This is often more effective than treating a crop that is already diseased.

Question 28

Problems with chemical methods

Answer 28

DDT is one of the most effective early insecticides.
It is now banned because it is persistent and remains in the environment.
Although harmless at low concentrations, it gradually accumulates and builds up in the bodies of organisms further along the chain. This is called bioaccumulation.

Biomagnification refers to the increase in the concentration of the chemical as you move between trophic levels. This happens because the chemical does not get broken down.
The organisms involved suffer severe poisoning. Therefore it is said to be toxic to these non-target species.
In the case of pesticides, when a pesticide is sprayed on a crop, a few pests may already be resistant to the chemical.
These resistant pests are “naturally selected” and survive to reproduce and produce the next generation, which may also be resistant.
Continued use of the pesticide leads to a population of resistant pests after a period of time.

Question 29

Biological control

Answer 29

This involves the deliberate introduction of one of the pest’s natural enemies. This may be:-
 A predator (e.g. a ladybird feeds on greenfly and may be introduced to control their numbers)
 A parasite (e.g. Encarsia wasp larvae are parasites of whitefly and may be introduced to control
their numbers)
 A pathogen (e.g. the bacterium Bacillus thuringiensis infects
and kills caterpillars and may be introduced to control their numbers)
There are no chemicals involved in biological control and so there are no issues with persistence in the environment, accumulation in food chains or the production of resistant populations.
Biological control works well in a closed system (e.g. a greenhouse) as the organisms cannot escape into the environment and cause problems. However, if the system is not closed and the organisms do escape and have no natural predators or parasites to control their numbers, then they may increase rapidly and threaten other species. This would make them an invasive species.
They may also parasitise, prey on or be a pathogen of other species.

Question 30

Integrated pests management

Answer 30

This involves using a combination of methods (cultural, chemical and biological control) to control pest numbers. It allows a reduction in the use of chemical pesticides.