#Question id: 11393
#Unit 9. Diversity of Life Forms
Given that the enzymes that catalyze nitrogen fixation are inhibited by oxygen, what are two ʺstrategiesʺ that nitrogen-fixing prokaryotes might use to protect these enzymes from oxygen?
1. couple them with photosystem II (the photosystem that splits water molecules)
2. package them in membranes that are impermeable to all gases
3. be obligate anaerobes
4. be strict aerobes
5. package these enzymes in specialized cells or compartments that inhibit oxygen entry
#Question id: 11584
#Unit 7. System Physiology – Animal
Use the bar graph to answer the following question.
People who suffer from chronic fatigue syndrome (CFS) experience ongoing, debilitating fatigue. In one study, cortisol levels in the saliva during the first hour after waking were measured and compared between CFS patients and people without CFS (controls). The data is summarized in the graph. What can you infer from the data?
#Question id: 31243
#Unit 5. Developmental Biology
#Question id: 26859
#Unit 10. Ecological Principles
#Question id: 1387
#Unit 4. Cell Communication and Cell Signaling
The ability of a population of fibroblasts to migrate along the surface of a tissue culture dish depends on adhesion between the cell surface and the extracellular matrix molecules coating the dish. The dish is coated with laminin, and the only cell-surface protein capable of binding laminin is a cell-adhesion protein called an integrin. Integrins are integral plasma-membrane proteins that function as heterodimers. Under these conditions the rate at which a fibroblast can migrate along the laminin coated culture dish is proportional to the strength of adhesion between the cell and the laminin substrate. The table below lists the rate of cell migration observed for fibroblasts genetically engineered to generate the indicated phenotypes. Microinjection into the cytoplasm of a wild. type cell of a solution of a synthetic peptide possessing the same sequence as the integrin beta subunit cytoplasmic domain would be expected to yield an average fibroblast-cell migration rat
|
Fibroblast
Phenotype |
Level of
Integrin Heterodimer at the Cell Surface (percent of wild type) |
Rate of
Cell Migration (pm/min) |
|
1. Wild type |
100 |
2 |
|
2.
Overexpression of the wild-type integrin alpha subunit |
104 |
2 |
|
3. Overexpression
of an integrin beta subunit lacking the cytoplasmic domain |
96 |
0.6 |
|
4.
Overexpression of the soluble cytoplasmic domain of an integrin beta subunit |
98 |
0.6 |
|
5. Absence of
the integrin alpha subunit |
Less than 1 |
0.05 |