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Use the information below, as well as the previous passages and information, to answer this question.

Scientists have long known that cells communicate with each other through the use of membrane receptors. Chemicals and proteins, known as messengers, find a receptor that sits on a target cell, binds to it, and then communicates messages to the cell’s nucleus, where the cellular DNA will make new proteins that in turn affect the overall working of the cell.

Another research group has discovered a different line of proteins that also effect seed germination called the strigolactones. The strigolactones are also present in the smoke and ash of the dying undergrowth. In an experiment very similar to the one run earlier, this group showed that varying concentrations of these chemicals also increase the number of seeds that germinate and shorten the average generation time. Both researchers hypothesize that a combination of the two chemicals will enhance the number of seeds that germinate and shorten even further the time to germinate than either chemical alone. They combine their efforts to run a controlled experiment with the results as shown in the data table below:

Germination Additive Number of Seed Germinated (per 100) Time to First Seed Germinated (days) Average Time of Germination
None 55 2.7 4
20 ug/ml K 88 1.4 2.2
40 ug/ml K 95 1.1 1.8
20 ug/ml S 82 1.3 2.3
40 ug/ml S 91 .8 1.7
20 ug/ml K + 20 ug/ml S 86 1.3 2.2
40 ug/ml K + 40 ug/ml S 94 .9 1.7




Fire is an important way for an ecosystem to rebuild itself. Unchecked undergrowth made up of opportunistic weedy plants robs vital water and soil; nutrients normally available to the large stands of older hardwood trees so important to the wildlife that populates the ecosystem. So while man made forest fires can devastate large parcels of land, causing irreparable harm to the ecosystem, smaller naturally started fires can actually be helpful, ridding the area of unchecked weedy undergrowth, making it more hospitable for the trees and allowing wildlife to flourish.

Scientists are now learning that the dying undergrowth is also helping the ecosystem in another way. The smoke and ash from these natural fires contains chemicals which they believe remain in the soil and aid in the regrowth of the forest. One such family of these chemicals is the karrakins. Karrakins are believed to help the rebirth of a burned forest by aiding in seed germination.

The first experiment involved the karrakins ability to increase the number of seeds that germinate as well as to speed up germination time of various seed plants. The design involved using various members of a typical forest ecosystem, including prickerweed, a pine tree and a beech tree seeds.

Each group contained 50 beech seeds, placed in native soil and watered the same amount on a regular schedule. Treatments are described below:

Group 1-Water only

Group 2-Ash from a burnt prickerweed and water

Group 3-Ash from a pine tree and water

Group 4-Purified karrakin extract at a concentration of 20 ug/ml and water

Group 5-Purifed karrakin extract at a concentration of 40ug/ml and water

Group # # of germinated seeds Time to first seed germinating (days) Ave time for seeds to germinate (days)
1 28 2.3 3.7
2 39 1.6 2.7
3 28 2.7 4.5
4 44 1.2 2.2
5 48 .9 1.9

Based on the passage above and the data assembled, what is the most logical explanation as to why the combined use of both karrakins and strigolactones does not lead to greater germination rates than either chemical alone?


Most cells within the seed lack receptors to either chemical, so they would not affect germination.


The seed cells lack receptors for the karrakins, so adding them has no effect on germination.


The strigolactones and karrakins use the same receptors, so when added together they compete for spots to bind to cells.


The seed cells lack receptors for the strigolactones, so adding them has no effect on germination.

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