9. on the outside of the plasma membrane, act as signals or markers to cells recognize one another.

The correct answer is carbohydrates.

I"m not explaining why it is the right answer because I almost got banned last time.

thrombocytopenia, which is a deficiency in blood platelets, can cause bleeding and bruising in the dermis similar to senile purpura. other possible causes include: exposure to ultraviolet rays and aging skin are common causes of senile purpura. senile purpura may also be a sign of collagen loss in the skin and bones.