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Recently, scientists from Rothamsted Research Station in Hertfordshire have genetically engineered a new variety of wheat which contains a gene isolated from peppermint. The gene codes for a pheromone which both repels aphids and attracts natural predators of aphids such as ladybirds and parasitic wasps. The pheromone produced naturally by peppermint plants mimics the action of a pheromone which aphids produce themselves when they are under threat of predation, and which warns other aphids to stay away to avoid danger.
Aphids are a major pest of cereal crops, causing an estimated £120m worth of damage to cereal crops per year in the UK alone. The Rothamsted scientists say that the new wheat variety could lead to increased crop yield and reduced use of insecticides, which are costly to apply and environmentally damaging.
Anti-GM campaigners disagree. They say that manipulation of plant genomes, such as introducing genes from other plants, could have unpredictable consequences, and there are safety concerns about eating wheat which produces new phytochemicals. They say that, in the long term, aphids could become habituated to the pheromone, and could learn not to respond to it. They also say the peppermint gene might spread into wild grasses by hybridisation.
Scientists from Rothamstead, however, argue that benefits far outweigh the risks. Further information on the project is at

Pick out two suggested benefits and two suggested risks of introducing the pheromone gene from peppermint into wheat by genetic engineering. (4 marks)

* Suggested benefits:

- Increased crop yield (reduced crop damage).

- Reduced use of insecticides (saving money & reducing environmental damage).
* Suggested risks:

- Unpredictable consequences (safety concerns/habituation of aphids).

- Hybridisation with wild grasses.
Outline the role of Ti plasmid, Agrobacterium tumefaciens and micropropagation in the creation of GM crops such as wheat containing a gene from peppermint. (4 marks)
* Gene coding for pheromone isolated from peppermint.

* Gene introduced into Ti plasmid.

*Ti plasmid placed into Agrobacterium tumefaciens (bacterium).

*This bacterium used to infect wheat plant explant cells.

*Callus cells subdivided and used to grow GM wheat plantlets.

Revision pack 5

Maintaining Biodiversity (AS-2.3.4)

Populations (A2-5.3.2)

Conservation of biological resources can be of economic benefit, both directly and indirectly…there is potential to make a mint out of mint!
Suggest the potential economic benefits of conserving an area where:
(a) A new variety of an existing mint species is discovered. (2 marks)
* There may be varieties of existing species carrying disease resistance alleles

or which produce higher oil yields than plants currently grown.

* These could be interbred with cultivated stock to improve disease resistance / yield.
(b) A new mint species is discovered. (1 mark)
* Undiscovered species could be a valuable source of new mint oils,

food, medicines, insecticides.

Outline two ways in which in-situ conservation could maintain mint biodiversity. (2 marks)

In-situ Conservation
* Legislation – pass laws to stop clearing land for development.

* Conservation Parks – set up national parks, nature reserves or Sites of Special

Scientific Interest (SSSIs).

* Repopulation – Re-create wildlife habitats that have been destroyed.

Outline two ways in which ex-situ conservation could maintain mint biodiversity. (2 marks)
Ex-situ Conservation
* Conserving endangered mint species or varieties by activities that take place outside

its normal environment using botanical gardens and seed banks.

* Grow endangered mint plant species in botanical gardens.

* Collect seeds from the wild and store long-term in seed banks.

Cell Structure (AS-1.1.1)

Cell Membranes (AS-1.1.2)

Figure 4

Diagram of trichome on a mint leaf surface

Figure 5

Electron micrograph of a mint trichome

The essential oils produced by mint plants are secreted by specialised cells and stored in circular structures called trichomes The natural role of mint oils is to inhibit the growth of other competitor plant species (allelopathy), and to protect the mint plants from attack by insects and pathogens such as bacteria and fungi. Many or the constituent ingredients of these oils are known to disrupt cell membranes, this making them more leaky.

Explain which type of electron microscope was used to take the picture in Figure 5. (2 marks)

* Scanning electron microscope.

* 3D image (surface view).
Calculate the actual width of the storage cavity in Figure 5, given that the magnification of the electron micrograph is X2000. Give your answer in µm. (2 marks)
A = I Actual size in µm = Image size X 1000

M Magnification

= 30mm X1000

= 15 µm

State two advantages and two disadvantages of using electron microscopes compared to using light microscopes. (4 marks)
* Advantages: * Disadvantages

- Higher resolution. - Black & white images.

- Higher magnification. - Kills specimens.

- More detail visible. - Very expensive.

- Difficult to use.

Suggest which components of cell membranes could be altered or damaged by mint oils.

(1 mark)
* Proteins / phospholipids / cholesterol.

[probably phospholipids – components of mint oil may dissolve them]

Suggest why the cell surface membranes of plants, insects, bacteria and fungi are all equally susceptible to damage by mint oils. (1 mark)
* All have the same structure (phospholipid bilayer).
Suggest how mint oils could interfere with biochemical reactions such as respiration and photosynthesis. (3 marks)
* Respiration is controlled by mitochondria.

* Photosynthesis is controlled by chloroplasts.

* Both are membrane-bound organelles.

* Mint oils could damage the membranes of these organelles (including the internal

photosynthetic membranes of chloroplasts).

* Organelles could burst / no longer carry our chemiosmosis.

Suggest why mint plants store the oils in specialised trichomes, rather than throughout the mint plant. (2 marks)
* Mint oils may damage organelles and/or the cell plasma membrane.

* Mint oils are stored in trichomes so they can't damage the cells of

the mint plant which produces them.
Diet & Food Production (AS-2.2.1)

Ecosystems (A2-5.3.1) & Populations (A2-5.3.2)

Peppermint, spearmint and cornmint are grown on a vast agricultural scale in North America, China and India. In recent years, farmers such as Sir Michael Colman have re-established peppermint as a commercial crop in the UK. Figure 6 shows one of his peppermint fields.
Figure 6

Peppermint field at Summerdown Farm

in Hampshire, UK

Image courtesy of Sir Michael Colman

eppermint and spearmint thrive in sunny but moist soil preferably in a temperate climate, whereas cornmint prefers a subtropical climate. Mint plants are susceptible to attack by a wide range of pests (e.g. beetles, nematodes & weevils) and pathogens (e.g. mildew), as well as competition from weeds such as thistles and buttercups.

Mint plants occupy the first trophic level in a food chain.
(a) What term is used to describe organisms which occupy this tropic level? (1 mark)

* Producers.

(b) Name the type of nutrition is used by organisms of this trophic level. (1 mark)

* Autotrophic nutrition.

Only a small percentage of the light energy in sunlight which hits the surface of mint plants is used in photosynthesis. State three ways in which light energy is “lost” at this stage. (3 marks)
* Light hits parts of the plant which can’t photosynthesise (e.g. bark).

* Some light energy is used to heat up leaf surface / evaporate water.

* Some light doesn’t hit chloroplasts.

* Some light is the wrong wavelength to get absorbed by photosynthetic pigments /

chlorophyll reflects green light.
Distinguish between gross primary productivity and net primary productivity. (2 marks)
* GPP = rate at which plants convert light energy into chemical energy (fixed biomass)

during photosynthesis.

* NPP = Energy available to plants after some fixed biomass is used in respiration (R).

* NPP = GPP – R.

When a farmer harvests the mint plants, which of these is he or she harvesting? (1 mark)

* NPP.

Explain how each of the following farming practices helps to increase primary productivity:
Herbicides (2 marks)

* Kill weeds.

* Weeds compete for light, water & mineral ions.
Pesticides (2 marks)

* Kill pests.

* Pests graze on mint leaves/roots.
Irrigation (2 marks)

* Provides water.

* Plants can continue to grow if water in short supply (e.g. drought).
Fungicides (2 marks)

* Kill fungal pathogens.

* Fungi infect damage mint plants by causing disease
Crop rotation with legumes (2 marks)

* Improves nitrate ion content of soil

* Nitrogen source to make amino acids/nucleic acids/ ATP/chlorophyll

Many mint species grow in damp, marshy places alongside rivers, streams and ponds. A gardener planted Mentha suaveolens and Mentha aquatica in a wet area of his garden. Initially, both species thrived, but after five years, only one of the two species remained.

What type of competition has occurred here? Explain. (2 marks)

* Interspecific competition – between two species.

What important population concept is illustrated by eventual loss of one species?

Why has it happened? (3 marks)

* Principle of competitive exclusion.

* Two species competing for same resources in same habitat.

* One species out-competes the other, which becomes extinct.
Describe a method that could be used to study the abundance of Mentha suaveolens in the wild, in an area where the plant is uniformly distributed. (4 marks)
* Random sampling.

* Place two long tape measures at right angles.

* Use random numbers as coordinates to place quadrat.

* Use a key to identify Mentha suaveolens plants.

* Count number of /estimate % cover of Mentha suaveolens in the quadrat

* Repeat for reliability (e.g. 10 times).

Plant Responses (A2-5.4.1)

When you took your original mint cuttings from the parent plant, you were instructed to remove the apical bud and to dip the base of the shoot in a rooting powder containing auxins.

With reference to the effects of auxins, suggest why:
(a) the apical bud was removed. (3 marks)
* Remove apical bud to make mint plant bushier.

* Apical bud produces auxins which inhibit growth of lateral buds.

* This is called apical dominance.

* Removal of bud stops auxin production so removes the inhibition

* So allowing lateral buds to grow into lateral shoots.
(b) the shoots were dipped in rooting powder. (1 mark)
* Auxins promote the growth of lateral roots, (so cuttings develop roots).

Over the coming months, you will have to make sure your mint plants get an adequate supply of light.

With reference to auxins and tropisms, explain what would happen to your mint plants if they were illuminated only from one side and why. (3 marks)
* Mint shoots illuminated from one side will bend towards the light.

* Because auxins from the apical bud accumulate on the shaded side.

* This causes cells on the shaded side to elongation.

* So shaded side grows longer than illuminated side, causing the bending.

How does this tropic response help the plant to avoid abiotic stress? (2 marks)
* This helps plants grow towards a light source.

* Important as mint plants are autotrophs and need to absorb light for photosynthesis

Animal Behaviour (A2-5.4.3)
Many of the phytochemicals produced by mint plants have a direct toxic effect on insects and other primary consumers which eat mint plants, and so are classed as natural pesticides. However, peppermint plants (Mentha x piperita) have evolved an additional and more indirect means of protection. They secrete a pheromone which is given off by the peppermint plant leaves, which both repels aphids and attracts natural predators of aphids such as ladybirds and parasitic wasps.
The pheromone mimics the action of a pheromone which aphids produce themselves when they are under threat of predation, and which warns other aphids to stay away to avoid danger. Note that natural predators have evolved to respond to this pheromone, so that although it acts as a warning to other aphids to keep away from predators, it actually attracts predators too!
The pheromone produced by peppermint is released in minute quantities. It deters aphids when there is no real danger, and attracts aphid predators so that any aphids which are present are more likely to get eaten. In short, it protects the plant.
Production of the aphid pheromone by peppermint plants is finely tuned. With reference to habituation, suggest whether or not increased production of the pheromone would lead to increased or decreased damage by aphids. Remember, the pheromone is not actually toxic to aphids. (5 marks)
* Aphids could learn not to respond to the pheromone (habituation) because there is no real

danger, so more aphids are present to damage the crop.

* However, the pheromone could attract more predators, which now have more aphids to

feed on. In this case, those aphids which have become habituated are more likely to get

eaten, so the habituation will only be short term.
* Aphids will be deterred from eating the crop, decreasing damage.
* This could lead to habituation of the predators, which learn not

to respond to the pheromone as they find that there are fewer aphids for them to eat.

* Then, the aphids could learn not to respond to the pheromone (habituation) because there

is no real danger, so more aphids are present to damage the crop. Then more predators are

attracted, and so on……

* A cycle of increased / decreased damage could be established!

Animal Responses (A2-5.4.2)

Peppermint oil is said to relieve a wide range of human ailments. The oil is extracted from Mentha x piperita by steam distillation. It is comprised mainly of L-menthol, a volatile organic compound which has a cooling effect the skin. Peppermint oil helps clear the nasal passages in people suffering from respiratory tract infections; it has antibacterial properties which help to freshen breath and it even helps remove head lice.

There is some evidence that peppermint oil eases stomach and bowel spasms by relaxing muscles in the alimentary (digestive) tract and so may be effective in treating of IBS (irritable bowel syndrome).

If the claimed effects of peppermint oil on the human digestive system are true, suggest which type of muscle peppermint helps relax. (1 mark)

* Smooth (involuntary) muscle – found in digestive system.

Some researchers hypothesise that peppermint helps relax muscles by inhibiting the release of calcium ions in muscle cells. With reference to the sliding filament theory, explain why inhibition of calcium ion release would prevent muscle contraction. (3 marks)

* Release of calcium ions from the sarcoplasmic reticulum in muscle fibres is necessary for

muscle contraction.

* Calcium ions bind to troponin, which changes shape and moves tropomyosin away from actin

– myosin binding sites on the actin filaments.

* This allows myosin heads to attach to the binding sites on the actin filaments, leading to a

power stroke and muscle contraction.

* If peppermint inhibits release of calcium ions from the sarcoplasmic reticulum, then the

actin – myosin binding sites will remain blocked by tropomyosin, and the muscle will stay


Animal Responses (A2-5.4.2)

Carvone is an organic compound which exists in two forms called R-carvone and S-carvone.

As shown in figure 7, the two forms of carvone are mirror images of each other (they are called enantiomers).

Figure 7

Structures of R-carvone and S-carvone

-carvone is produced by spearmint plants (Mentha spicata) whereas S-carvone is produced by caraway plants (Carum carvi).
R-carvone and S-carvone are thought to bind to different olfactory receptors in the nose. The brain interprets R-carvone as a minty smell whereas S-carvone is interpreted as a more spicy smell. Some people cannot differentiate between these two smells!
Suggest why R-carvone cannot bind to an S-carvone olfactory receptor, and why S-carvone cannot bind to an R-carvone olfactory receptor (think about hands fitting into gloves!).

(2 marks)
* R-carvone and S-carvone molecules have different shapes and can only fit into receptors

with the correct complementary shape.

* Therefore, they do not fit into each other’s receptors.

20% of people cannot differentiate between the smells of R-carvone and S-carvone (these people perceive both as smelling like spearmint). Suggest whether these people:

(a) Lack R-carvone receptors

(b) Lack S-carvone receptors

(c) Possess S-carvone receptors with an altered shape which bind to both R-carvone & S-carvone.

(d) Possess R-carvone receptors with an altered shape which bind to both R-carvone & S-carvone.

(e) A combination of (a) and (c)

(f) A combination (b) and (d)

Briefly, explain your choice.(2 marks)
* (f) They must have R-carvone receptors to detect spearmint smell but lack

S-carvone receptors so can’t detect caraway.

* However, as they perceive the smell of caraway to be the same as spearmint,

their R-carvone receptors must be able to bind to S-carvone too.

Science & Plants for Schools: (2013) – Developed by Dr Richard Spencer

Revising A-level biology – answer pack

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