biol+3+membrane+SAC

Here is where you will find relevant info relating to your SACs. You can also use this page to ask questions or me or each other.

Movement across membranes

In case you lose the handout and it contains the criteria you are being assessed on.
 * [|biol 3 membrane sac intro 2008.doc]**


 * Concepts you need to know and understand (no pun intended!)**
 * membrane structure
 * passive movment of substances through membrane
 * active transport of substances through membrane
 * bulk transport (endocytosis and exocytosis)
 * relevant terminology to all the above, including the types of biomolecules


 * How to prepare for the SAC**
 * do a "compare and contrast" and a "terminology analysis" from the making links homework booklet
 * go over the relevant questions on the quizzes and course notes
 * do relevant questions from the chapter review in the textbook
 * do relevant puzzles on membranes from the puzzle booklet
 * read the relvant sections from the course notes and the textbook
 * come up with questions to ask in class or place them below


 * Questions and answers**


 * Q. Vojtech there's a lot of writing space for the aim of the experiment. Are we expected to fill up ALL those lines or should the aim just be short and simple. I'm sure you don't want an aim that's overly long and superfluous :)**

//A. Yes there is a lot of lines and it doesn't mean you need to use it all. The aim should accurately reflect (in a concise manner) what you intend to investigate in this experiment. Use of appropriate terminology is important as is the clarity in what you are saying. A reader should easily be able to get a picture of what you investigated. VM//


 * Q. Is there a reason why there is a table of results for only ONE of the temperatures and then another table of results for all three of the temperatures?**

//A. This is an important question. It relates to how you carry out the experiment and also what the aim of the experiment is. So, yes there is a reason! If you look a the method, I have mentioned there will be groups of three and that each group will carry the experiment out at only one temperature.So the intial table is for your group's results. The second table can then be viewed as the class results incorporating the "same" experiment but at different temperatures. VM//


 * Q. Is it more appropriate to define osmosis as the net movement of free water molecules from an area of low solute concentration to a high solute concentration, or as the net movement of free water molecules from an area where there are more free water molecules to an area where there are less?**

//A. This is an interesting point you raise. My view is that they both mean the same thing. Do you agree or disagree? Lower solute concentration means higher concentration of free water molecules. So they are both correct but just different ways of saying the same thing. Both reflect an accurate definition of osmosis. As to which is better, this is irrelevant, as long as you can explain what they mean (if you are asked to) and this includes annotating a series of diagrams to show your understanding fo the concept. Finally, you should always include "across a semi-permeable membrane" or "across a selectively permeable membrane". VM//


 * Q. How do we determine the change in mass? Is it final - initial? or visa versa? And to determine the % change in mass do we use the formula: change in mass % initial mass X 100?**

//A. Great question and an important one! Yes what you said first. Change in mass is the final mass minus the initial mass. And the % change in mass will be determined using the following formula (final mass-initial mass)/initial mass x100. VM//


 * Q. For part 1 of the SAC (taking place tomorrow) will we be required to answer any questions other than what is on the sheets that we have been given?**

//A. Hmmm... Well, you ask a pertinent question. You want to know if there are some hidden surprises awaiting you all. Well there might be **BUT** tomorrow is about you carrying out the experiment and getting results. It's not about you answering questions that I have set to test your understanding of membranes etc. So you needn't worry about that. Tuesday week is when you will recieve the rest of the SAC which involves written responses to questions. VM//


 * A REMINDER THAT FOR THE WRITTEN PART OF THE SAC YOU CANNOT REFER TO ANY NOTES OR OTHER MATERIALS**

The class results for the membrane prac are shown below.




 * Q. I know you have told us what Concepts we need to know and understand, but does that mean the SAC on tuesday only will be testing us on the prac and other membrane-associated information. (therefore exclude enzymes/synthesising of proteins/photosynthesis and other areas we have studied not to do with the membrane)?**

//A. Yes that is right. I will be testing you on your understanding of the concepts mentioned above. VM//


 * Q. In one of the dot points outlining what we will be tested on you said "relevent terminology to all the above INCLUDING THE TYPES OF BIOMOLECULES". My question is, how much are we required to know and understand (no pun intended) about the types of biomolecules?**

//A. Aha, good question! The relevance of this dot point is that if you are to understand something about how substances move through the plasma membrane, you need to be familiar with what sort of biomolecule the substance is. For example, you need to know that lipid substances are able to "dissolve" other lipid substances and therefore pass with relative ease through the phospholipid bilayer (a lipid itself), whereas proteins and carbohydrates cannot do this, hence the need for using membrane proteins. You do not need to know the intracacies of the different biomolecules and there functions/roles in cells (not for this SAC at least). Although, you should be well familiar with this by now anyway! VM//


 * Q. In order to determine the approximate concentration of sucrose inside the celery pieces would we base our answer on the sucrose solution that has the smallest change in mass? (this indicating that the celery is in a isotonic concentration, therefore water will diffuse in and out of the cell relatively evenly?)**

Your logic seems to be OK, but I will let the others contribute their thoughts to this. VM I assumed that the piece with the smallest change in mass indicates that the concentration of the sucrose solution it was in was quite close to that of the sucrose concentration within the cell, which you mentioned. It might not necessarily be exactly isotonic in this case, but if the change in mass of the celery pieces is small, you can guess it would be pretty close to it. Hope that helps :-) AH


 * Q. Regarding the movement of substances into or out of root hair cells, I know that the water moves by osmosis, but do the root hair cells always uptake minerals using active transport, or also by facilitated diffusion? I realise that the cells won't always necessarily be uptaking minerals against their concentration gradient, but I'm still a bit unclear...**

//A. Minerals are taken up by active transport so that the plant cell ends up having a greater solute concentration compared to the surrounding soil (ie there is less free water molecules in the cell compared to the soil). Subsequently, the water molecules move into the plant cells via osmosis and this is essentially how the roots of a plant take up water. VM//


 * Q. If we are given a question about temperature (im sure we will  J ) and its effects on rate of the reaction for osmosis, do we just say that higher temp causes molecules to have more energy and are moving quicker, therefore osmosis occurs quicker, but with lower temperatures there is less "heat energy" so the molecules are moving slower, therefore osmosis is carried out slower. Even though our graphs seem to contradict this as the coldest temp (fridge) had a larger change in mass (therefore quicker reaction rate) then the room temp. OR.... is there a reason for this???**

//A. Your knowledge of temperature and its affects is logical and accurate for our purposes. But you have raised an important scientific idea and skill to some extent. Doing pracs is about getting you to investigate the theory. Pracs are not perfect, there are limitations and you need to be well aware of this notion. Hence if results don't sit with what theory says then that's a perfect opportunity for you to demonstrate (and me to assess) your understanding of concepts as well experimental procedure. VM//