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Steroid hormones are lipid soluble, hence, they can pass through the membrane and forming a complex that can activate the transcription of mRNA. One example of steroid hormone is testosterone.
Peptide hormones are hydrophilic and since they are charged, they cannot pass through the plasma membrane, thus they need receptor molecules to combine with and to start the process of phosphorylation cascades.
Peptide hormones cannot enter the nucleus of target cells, hence, the transcription of mRNA is not influenced by peptide hormones. One common example of peptide hormone is represented by insulin.
The steroids hormones include androgens like testosterone and its metabolite androsterone, female hormones like estrogens (e.g. oesterone and progesterone) and the hormones of the adrenal cortex ( glucocorticoids and mineralocorticoids). Steroids are mostly absent from bacteria.
The peptide hormones are oxytocin and vasopressin. Each contain 9 amino acid residues of which 7 are common to both. Both hormones are synthesised in the neurosecretory cells of the hypothalamus and stored in the posterior lobe of the pituitary. Oxytocin causes contraction of the uterus in labour. Vasopressin constricts peripheral blood vessels and thus increases blood pressure.
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