Question from Kaplan MCAT45

[thebillsfan]

Osteoblasts secrete large quantities of alkaline phosphatase when they are actively depositing bone matrix. Insufficient dietary calcium would be indicated by:

A. increased conc of alkaline phosphatase in circulation
B. decreased conc of alkaline phosphatase in circulation
C and D were nonsense

The answer is B. I chose A because when the question says "secrete," I thought (since we are talking about osteoBlasts) that the osteoblasts would "secrete" the alkaline phosphatase into the bone matrix. However, Kaplan takes it to mean that osteoblasts secrete alkaline phosphatase into the blood. Thus insufficient dietary calcium --> decreased osteoblast activity --> decreased secretion of alkaline phosphatase (and a decreased concentration in blood).

My question is: how was I supposed to know the secretion was into the blood and not the bone matrix? My reasoning was that since calcium isn't be taken up from the blood to deposit into bone matrix by osteoblasts, then alkaline phosphatase isn't being taken up from the blood to deposit into the bone matrix, either, so that there would be an increased alkaline phosphatase concentration.

Is it common knowledge that when calcium is deposited in bones, the alkaline phosphatase concentration is increased?

---

Members don't see this ad.

 

[loveoforganic]

I'm reading a different way. I'm really weak in biology, but here's my take on it.

Insufficient dietary calcium would lead to bone deterioration. Bone deterioration triggers increased osteoblast activity. Increased osteoblast activity means increased withdrawal of alkaline phosphatase from circulation to deposit into the bone matrix.

 

[thebillsfan]

sorry, but that's def not possible. insufficient calcium means that osteoblasts do not have enough calcium to deposit bone matrix.

 

[loveoforganic]

Is calcium presence necessary for osteoblast activity? The osteoblast wouldn't attempt to function the best it could without calcium present?

 

[thebillsfan]

it needs calcium to deposit bone. that's why calcitonin stimulates osteoblasts

 

[amberisma]

Osteoblasts secrete large quantities of alkaline phosphatase when they are actively depositing bone matrix. Insufficient dietary calcium would be indicated by:

The answer is B. I chose A because when the question says "secrete," I thought (since we are talking about osteoBlasts) that the osteoblasts would "secrete" the alkaline phosphatase into the bone matrix.

Osteoblasts secrete alkaline phosphatase while making bone. If there is not enough calcium in the body to allow osteoblasts to keep depositing bone matrix, the osteoblasts will not be able to secrete alkaline phosphatase because it is not making bone.

You have confused the concept of secretion. Things aren't secreted into cells. Cells secrete things into the blood, nephron, etc.

...then alkaline phosphatase isn't being taken up from the blood to deposit into the bone matrix...

This would be absorbing alkaline phosphatase. The question says nothing about it being absorbed by osteoblasts.

Hope this helps!

 

[thebillsfan]

no i realize that. but i also realize that most bone is not cells...its the extracellular matrix outside of cells that osteoblasts create around them. so osteoblasts SECRETE thing like collagen or whatever into the EC matrix. so why couldnt i have considered them to 'secrete' alkaline phosphatase into the bone matrix as well? it's not inside the cell, it's outside...so it would fit the definition of secretion.

 

[amberisma]

Regardless of how you want to look at it, there would be a decreased secretion of alkaline phosphatase...in the blood, in the matrix, in wherever. The question deliberately states that alkaline phosphatase is ONLY secreted WHILE osteoblasts are depositing bone matrix. You said yourself calcium is needed for osteoblasts to deposit bone. If there isn't enough calcium, the osteoblasts cannot deposit bone. They are not working therefore they are not secreting anything: collagen, alkaline phosphatase, whatever.

You just have to assume from the answers given that the secretion is into the blood because they both say circulation and not matrix.

Remember, they love trying these tricks in order to make you think otherwise. It's not as in depth as an answer as you're making it out to be

 

[thebillsfan]

word.