Particularly with regard to gas exchange, why do we have respiratory and cardiovascular systems, but flatworms, which are only a few cells thick, don't?
What will be an ideal response?
Diffusion is a relatively slow process, and cells that are more than a few layers of cells removed from the environment would not receive enough oxygen. Carbon dioxide would not be removed efficiently either. The cells in flatworms are close enough to the environment to perform gas exchange directly with the environment. The overwhelming majority of our cells is too far away for this, and these cells need a delivery system to bring oxygen and remove carbon dioxide for them. The respiratory system is a way of bringing a huge surface area of air in contact with a huge surface area of blood flow in order to accomplish efficient gas exchange and transport.
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Which of the following statements best describes the difference between a dikaryon and a heterokaryon?
A. There is no difference, the two are identical. B. A dikaryotic cell has two nuclei and a heterokaryon has three or more nuclei. C. A dikaryotic cell is diploid and a heterokaryon is haploid. D. A dikaryotic cell is involved in sexual reproduction and a heterokaryon is involved in asexual reproduction. E. A dikaryotic cell is haploid and a heterokaryon is diploid.
Which of the following has to be a haploid cell?
a. a cell with 6 chromosomes b. a cell with 3 chromosomes c. a cell with 12 chromosomes d. a cell with 24 chromosomes
Human actions have altered the global carbon cycle by ___
a. decreasing the number of photosynthetic organisms b. cutting down large numbers of trees c. inventing the internal combustion engine and burning fossil fuels d. developing and using drilling and mining methods that allow access to previously inaccessible fossil fuel deposits e. all of the above
Based on the graph, the threshold voltage appears to be approximately
A) +60 mV. B) +30 mV. C) 0 mV. D) ?30 mV. E) ?60 mV.