I hope you had time to relax and recharge your academic batteries during this holiday season. My goal with this post is to get us all focused on learning again. If you are new to the blog be sure to read the previous postings.
To get started I thought I would address a few of Three Question Thursday questions(TQTQ from here on). I plan to use other TQTQ in future posts. So here goes.
TQTQ - 1: Why is there a change in ion concentration after ions flow inside the cell membrane?
Response: When ions such as Na+ move into a cell, for example through a membrane channel, they will move down their concentration gradient. This means that they move from where they are of higher concentration toward lower concentration (thanks to the second law of thermodynamics). This movement will continue until the concentrations become equal. This is to say that the number of Na+ ions per volume is now the same outside and inside the cell. The concentration of Na+ ions inside the cell has changed by the addition of Na+ ions from the exterior to the interior of the cell. Comments?
TQTQ - 2: What is the function of the alpha-helix portion of a trans membrane protein?
Response: Recall that proteins have primary, secondary, tertiary, and quarternary structures. See pages 71-76 of your text for a review. The alpha-helix is a secondary structure of proteins and help anchor the transmembrane protein in the membrane. Also recall that the cell membrane is a phospholipid bilayer with the hydrophillic (water-loving) portions pointing toward the cell exterior and interior and hydrophobic (water-fearing) portions arranged toward the middle of the membrane. This makes the middle of the membrane hydrophobic and because the alpha-helix is typically hydrophobic, it is this part of the membrane protein that is embedded within the membrane, anchoring the protein. See the image below. Comments?
TQTQ - 3: If alternative mechanisms of carbon fixation evolved in arid climates, what would happen to those mechanisms if those plants were to be moved from that climate?
Response: First, a point of clarification. Some CAM plants such as cacti are found in desert (arid) regions. Others such as pineapples (CAM) and corn (C4) live in regions that experience high daytime temperatures. Both climates expose the plants to excessive water loss through their stomates during the day. I know the book uses the term arid but I wanted to make clear that these alternative carbon fixation mechanisms are adaptations to high temperatures not just dry conditions.
Now to the answer. The answer to this question is not related to the content we are currently studying. Rather it is an evolutionary question. The simple answer is we do not know. Because mutation and genetic recombination are the source of new variation and both of these processes are of a random nature, we can not say this or that will happen. We can say that the selective pressures that resulted in these alternative carbon fixation mechanisms would be relaxed if we were to move the plant to a moderate climate such as the Puget lowlands. We could also form some hypotheses as to the direction of CAM plant evolution given a thorough understanding of CAM physiology. Left alone to nature this would take thousands of years if not longer. Alternatively, in a lab we might be able to accelerate CAM evolution but this would be artificial selection - like what we do with dogs, cats, and farm animals. I hope this helps. I love evolution questions, thanks Ana. Comments?
Feel free to comment on this post OR write your own question. I am back to monitoring the blog regularly.
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