Which cellular organelle is primarily responsible for ATP production?

A. Parathyroid hormone (PTH):PTH is the principal hormone that raises blood calcium. It increases bone resorption (releasing Ca²⁺), enhances renal calcium reabsorption, and stimulates activation of vitamin D (calcitriol) in the kidney, which increases intestinal calcium absorption.

B. Thyroxine:Thyroxine (T4) regulates metabolic rate and growth. It does not play a primary role in acute regulation of serum calcium, although chronic thyroid disease can influence bone turnover indirectly.

C. Cortisol:Cortisol (a glucocorticoid) can affect bone metabolism and calcium balance with long-term excess (promotes bone resorption and can reduce calcium absorption), but it is nota primary hormone for short-term physiologic regulation of serum calcium.

D. Insulin:Insulin primarily regulates glucose uptake and metabolism; it does not directly regulate serum calcium levels.

E. Calcitonin:Secreted by the thyroid C cells, calcitonin lowers blood calcium by inhibiting osteoclast activity and decreasing bone resorption. Its role in humans is less critical than PTH for everyday calcium homeostasis, but it is a classical calcium-lowering hormone.

F. Glucagon:Glucagon primarily raises blood glucose by stimulating glycogenolysis and gluconeogenesis. It does not have a primary role in calcium regulation.

A. Hypodermis:Also known as the subcutaneous layer, the hypodermis acts as an energy reservoir (fat storage) and thermal insulator. While it connects skin to underlying muscle, it is not the primary site for the generation of new skin cells.

B. Dermis:The dermis is the connective tissue layer containing blood vessels, nerves, and collagen. While it plays a role in wound healing (fibroblasts produce collagen here to form scar tissue), the active production of new epithelial cells to resurface a wound occurs in the layer above it.

C. Epidermis:The deepest sub-layer of the epidermis, called the stratum basale(basal layer), contains stem cells that continuously divide (mitosis) to produce new keratinocytes. When skin is injured, these cells proliferate and migrate to cover the wound (re-epithelialization).

D. Stratum corneum:This is the outermost layer of the epidermis, consisting of dead, flattened, keratinized cells. These cells cannot divide or repair tissue; they are constantly shed and replaced by new cells pushing up from the basal layer.