Cells needs to bring in molecules to carry out cellular processe. often this required moving the molecules across the cell membrane against the cocentration gradient passive transport by diffusion
active transport using ATP

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.

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