Phospholipid Bilayer?!

[psych93]

okay this is probably going to sound like a really dumb question.
so I was looking through my notes on the phospholipid bilayer and it says that due to the HYPROPHILLIC HEADS only POLAR molecules can pass through (like dissolves like), and they've stated that both alcohol and lipids (eg. steriods) can pass through. But arent lipids largely nonpolar?

2. Also is it correct to say that enzymes arent denatured at low temperatures, but the rate of a reaction is significantly lowered due to fewer collisions between particles?
3. And one last thing, Is the function of cholestorel just to provide fluidity to the membrane? Should I elaborate on this?

[Charmz]

The whole phospholipid bilayed lets only small uncharged molecules through due to the presence of the hydrophobic tails. The lipids are non polar so they get through the heads and then dissolve through. The heads only make up a small portion of the bilayer.

2.  They are inactive due to low temperature. but if incubated to a higher temp they will become active. If you say freeze enzymes, there will be no reaction but if it was a temperature slightly lower than optimum, then it would have a reduced rate of reaction.

3. The cholesterol is there for flexibility and it forms vesicles for phagocytosis and allows the membrane to attach again once they are separated e.g poke a needle or if vesicles are formed. It's always best to elaborate if the answer you are going to give is correct because its best to be safe if aren't sure how much information you need to give. But don't elaborate if there is no need and you have covered everything the question is asking.

[lexitu]

1. Polar molecules can't exactly pass through unaided unless they are exceptionally small. They require protein channels or carriers. Lipophilic substances can pass through simply. This isn't my forte though, so I can't explain it much further other than to say you are right in saying like dissolves like

2. Yep, they are inactivated rather than denatured. Spot on.

3. Pretty much. It maintains membrane structure by helping keeping it from solidifying at lower temperatures and preventing it become overly fluid at higher temperatures. Check the VCAA 2009 question for the way they want you to address that. Basically, think of what the temperature change would do to mess up with the membrane, and cholesterol negates that.

[person]

 Basically, think of what the temperature change would do to mess up with the membrane, and cholesterol negates that.

Could you explain what you mean by the temperature messing up the membrane.

[Ravit]

Well, if the temperature were to increase, the fluidity of the membrane would increase, its dynamic nature would increases as well and this could lead to various parts of the membrane being damaged, the individual phospholipids would have higher kinetic energy as result of temperature increase, this can possibly decrease the attraction between phospholipids and cause disruptions in the phospholipid bilayer, hence leaving the membrane not capable of conducting its role. The presence of cholesterol prevents on this from happening it maintains the integrity of the membrane

Hope this helps  

[psych93]

thanks heaps!!!

also I have a question about autoimune diseases....
Does the body actually produce antibodies against the bodys own tissues? Or does it only involve T-cells attacking self material?

[Ravit]

Yes. It does. Well an autoimmune disease is defined in Vcaa terms as a ' disease resulting from the body's own immune system attacking it's own cells as the body DETECTS IT"S SELF CELLS AS NON SELF', not the body failing to recognize non self cells. Well like most immune responses the, the autoimmune disease involves immune cells such as white blood cells attaching to the body cells and recognizing them as non self. And then through the processes of phagocytosis engulf the cell and display the engulfed cells fragments on its MHC II markers. they travel to lymph nodes and stimulate T helper cells. These will recognize the antigens on the macrophages as foreign and will them stimulate both cytotoxic t cells and B cells. this will lead to the production of cytotoxic cells with appropriate receptors that are able to bind with self cells and undergo the process of cell mediated immunity. B cells would also be activated, and stimulate the production of antibodies by plasma cells which attach to self cells. Hence autoimmune diseease involve both antibody attacks alongside cytotoxic t cells as both are capable of destroying cells

Hope this help            

[person]

With transplant rejection; is it both the humoral and cell-mediated immune responses that are stimulated or..."?


And

Which wavelength of light is stronger (greater rate of photosynthesis), blue or red? 

[Ravit]

Yes, sorry ignore my part upon the humoral response in regards to transplanted organ rejection it only involves cellular mediated immunity, thus the action of T cells.

Yes, well both wavelengths are have the highest absorbing rate compared to other wavelengths, however red is more optimum range for light absorption.
 

[person]

Which is a better source of energy lipid or glucose; and if lipid why isnt it used instead of glucose?

[Russ]

Lipid. It is used (fat cells), glucose is just the primary one for evolutionary reasons

[person]

thanks 

[Ravit]

Lipid is the best source as it stores more neergy per gram.

Well, some books regarding evolution have discussed the following theory. Carbohydrates were the first organic compunds to be constructed, with the following elements C,H,O. Then as time progressed other elements were added on. Such as if you add nitrogen to this you can construct proteins, by adding on phosphate you can construct nucleic acids etc. As glucose is the primary organic substances to be constructed by autotrophs it leads to the possiblity that this was the first organic compound to be catalysed and used a a source of energy. So due to evolution and glucose being more easily broken down than fats, leads it being the primary choice before lipids.

Hope it helps  

[person]

That makes sense