Calcium entering muscle cells is active transport??

[supertrooper66]

okay, Kaplan is pissing me off. it wanted to know which of the following does NOT require ATP. i chose calcium entering into muscle cell as the answer since i THOUGHT it was passive transport.

kaplan said the urea choice was the correct answer and ruled out the calcium one because it says that process is active transport. i'm a little frustrated so don't mind my irritation...i THOUGHT the AP reaches the the muscle membrane, opens the voltage-gated Ca2+ channels, and then Ca2+ flows in down its gradient. that is passive transport. how is that active transport?

i know ATP is involved in the muscle CONTRACTION with the myosin binding heads, but kaplan specifically says calcium entering muscle cell. umm, what? thanks guys

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[JA Prufrock]

Edit: got mixed up, yeah I would think you're right about voltage-gated Ca++ channels on the sarcoplasmic reticulum. The cytoplasm concentration of Ca++ is low, and there is a high concentration of Ca++ in the sarcoplasmic reticulum, so I don't know why it would be active transport. However, is it possible they are not referring to skeletal muscle? In smooth muscle and cardiac muscle, Ca++ actually enters from the surrounding environment. Is this done via active transport? I'll have to look it up.

 

[minhaj]

I am not sure what the actual question was, but I will try to respond to the best of my ability. When the action potential reaches the muscle cell, the ca+2 in the sarcoplasmic reticulum is released. there is no ca+2 coming in from outside the cell. So I doubt the question dealt with action potential. If it referred to a normal cell, then ca+2 would have to be actively transported through a carrier protein, since calcium ion is too large to diffuse through the plasma membrane.

 

[JA Prufrock]

what minhaj said makes sense, but I assume since OP's post discussed muscle contraction, the answer choice somehow included contraction (and thus, an AP). I think the only active transport of calcium in muscle cells (at least skeletal muscle cells) is the Ca++ ATPase activity following contraction, to sequester calcium in the sarcoplasmic reticulum, and thus maintain cytoplasm calcium concentration at a low level. I think we'd need more info about what the question said.

 

[supertrooper66]

okay, the question says which of the following processes is NOT ATP-dependent. that's it. i chose the muscle one but got it wrong. the muscle choice worded exactly is "movement of Ca2+ into a muscle cell." they eliminate that saying it is active transport. i interpret that answer choice as the calcium entering the sarcoplasm from the sarcoplasmic reticlum upon receiving the AP. that process i would bet $100 on is passive transport.

kaplan's exact wording for ruling out that answer is "movement of calcium ions into muscle cells goes against the conc. gradient; therefore, calcium ions enter cells by active transport and ATP is therefore required."

umm, what are they talking about? what are they defining as "entering the muscle cell?" maybe the muscle cell INCLUDES the sarc. reticulum, so they're saying calcium entering from completely outside the muscle. if that was the case, i guess it'd be active transport. i don't know. i'm lost. what is a muscle cell defined as exactly? one muscle cell has a sarc. reticulum, sarcomere, etc.? one muscle cell is one myofibril? i have no idea. i always thought one muscle cell has a bunch of myofibrils...i guess a myofibril consists of many muscle cells, tho? one actin+myosin system is one muscle cell? i don't know!

also, i hope i didn't violate any rules by quoting that stuff. do me a favor and NO ONE "quote" those lines, that way i can edit and delete those lines later. thanks

 

[TexasTriathlete]

I think you've been hit by a trick question. Calcium moving from the SR to the cytoplasm is passive, but that's not the same as calcium moving into the cell. When its in the SR, its already in the cell, so its just moving from one part of the cell to another. I believe there is another mechanism by which calcium crosses the sarcolemma.

 

[supertrooper66]

I think you've been hit by a trick question. Calcium moving from the SR to the cytoplasm is passive, but that's not the same as calcium moving into the cell. When its in the SR, its already in the cell, so its just moving from one part of the cell to another. I believe there is another mechanism by which calcium crosses the sarcolemma.

yeah, and so that mechanism is what the question is referring to?

SO, ONE muscle cell is comprised of many myofibrils then. the SARCOLEMMA surrounds the entire muscle cell. so there is one sarcolemma, since it is the membrane, around a bunch of myofibrils. we know calcium inside the sarcolemma is high, so completely OUTSIDE the cell it is intuitively low. going from low to high conc. requires ATP. errr...is that it then? that question is so stupid.

i. hate. you. kaplan. these are just the 38 questions following the 200-page bio review in the huge 2007 kaplan review book. it says like "now test what you learned!" there were other just as tricky ones like this, meh. i don't think the real mcat tricks you THAT MUCH, but i guess reviewing with this will teach me to be extra careful on the real one. i already took the test like 2 months ago and i don't remember it being that picky. perhaps that's why i'm having to retake it! haha i didn't review any bio for the 1st test. since i didn't get the score i wanted, i decided to actually review ALL of it. need to work on question details apparently

 

[TexasTriathlete]

Yes, that's my guess. The movement out of the SR is most definitely passive.
I don't know that you'd even be expected to know the mechanism where calcium moves into the cell. I think it might involve vesicles, but I really can't remember. In fact, I'm not sure I ever learned it, and I may be confusing it with something else.

 

[supertrooper66]

why is that on this kaplan question.
kaplan, you are ******ed.

 

[TexasTriathlete]

The MCAT is full of questions like that. Where you don't necessarily know the right answer, but you should be able to eliminate the wrong ones. And sometimes, there will be a trick answer in there like that to throw you off.

 

[Christo1]

I wouldn't have chosen Ca in the case of muscles (cardiac and smooth), simply cause the low intracellular concentrations of the Ca is maintained by the SERCA pumps which do require ATP. The AP which opens the L-Type Ca channels although don't require ATP directly, require the Ca gradient, which does require ATP. This is known as secondary active transport.

And for skeletal, well, the movement of Ca out of he SR due to RyR1 activation may be passive, but only because of, again, secondary active transport from SERCA pumps on the SR.