Chemical Coordination and Integration Notes: Class 11 Biology Chapter 19
Class 11 Biology Chapter 19 Chemical Coordination and Integration covers the human endocrine system, the various endocrine glands and their hormones, and the mechanism of hormone action.
Overview of Topics & Subtopics (Click to Jump)
1. The Endocrine System
Short Note:
• Endocrine glands are ductless glands that secrete hormones directly into the bloodstream; hormones act as chemical messengers, regulating and coordinating physiological functions.
2. Major Endocrine Glands and Their Hormones
Short Note:
• Hypothalamus: releasing and inhibiting hormones, regulates the pituitary. Pituitary ('master gland'): secretes GH, TSH, ACTH, FSH, LH, prolactin (anterior); oxytocin, vasopressin (posterior, actually synthesised by hypothalamus).
• Thyroid: secretes thyroxine (T3, T4) — regulates basal metabolic rate; and calcitonin — regulates blood calcium. Parathyroid: secretes PTH — increases blood calcium.
• Adrenal Glands: cortex secretes corticosteroids (glucocorticoids like cortisol, mineralocorticoids like aldosterone); medulla secretes adrenaline and noradrenaline ('fight or flight' hormones).
• Pancreas: islets of Langerhans secrete insulin (lowers blood glucose) and glucagon (raises blood glucose). Gonads: testes secrete testosterone; ovaries secrete oestrogen and progesterone.
3. Mechanism of Hormone Action
Short Note:
• Hormones act via specific receptors, either on the cell membrane (for protein hormones, triggering a second messenger cascade like cAMP) or inside the cell (intracellular receptors, for steroid hormones, directly affecting gene expression).
NCERT Textbook Exercise Questions
Why is the pituitary gland called the master gland?
Answer: The pituitary gland is called the master gland because it secretes a number of trophic hormones (such as TSH, ACTH, FSH, LH) that regulate and control the secretory activity of several other endocrine glands (thyroid, adrenal cortex, gonads) in the body.
Which hormones are involved in the maintenance of blood glucose level?
Answer: Insulin (secreted by beta cells of the islets of Langerhans in the pancreas) lowers blood glucose level by promoting glucose uptake and utilisation by cells, and its storage as glycogen. Glucagon (secreted by alpha cells) raises blood glucose level by promoting the breakdown of glycogen into glucose (glycogenolysis) in the liver, so the two hormones work antagonistically to maintain normal blood glucose levels.
Give the mechanism of action of steroid hormones.
Answer: Steroid hormones, being lipid-soluble, easily diffuse across the plasma membrane and bind to specific intracellular (cytoplasmic or nuclear) receptors, forming a hormone-receptor complex. This complex then interacts with specific regions of DNA in the nucleus, regulating gene expression by stimulating or inhibiting transcription of target genes, leading to specific physiological effects.
Give examples to indicate hormonal interactions in the maintenance of the body.
Answer: For example, calcium homeostasis is maintained by the antagonistic action of PTH (parathyroid hormone, raises blood calcium) and calcitonin (from thyroid, lowers blood calcium). Similarly, blood glucose level is maintained by insulin (lowers glucose) and glucagon (raises glucose) from the pancreas acting antagonistically, demonstrating coordinated hormonal interaction to maintain internal balance (homeostasis).
