krit.club logo

Human Physiology - Chemical signalling (HL)

Grade 11IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Chemical signaling involves the release of hormones by endocrine glands into the bloodstream to reach target cells with specific receptors.

•

Hormones are categorized into two main types: Steroid hormones (lipophilic) and Peptide hormones (hydrophilic).

•

Steroid hormones, such as estrogen and testosterone, cross the phospholipid bilayer and bind to intracellular receptors to form a hormone-receptor complex that acts as a transcription factor in the nucleus.

•

Peptide hormones, such as insulin and ADHADH, bind to cell surface receptors (often G-protein coupled receptors) and trigger a signal transduction pathway involving second messengers like cAMPcAMP or Ca2+Ca^{2+}.

•

The hypothalamus acts as the control center, regulating the pituitary gland. It produces 'releasing hormones' for the anterior pituitary and produces hormones like ADHADH and Oxytocin for storage in the posterior pituitary.

•

Negative feedback loops maintain homeostasis, such as the regulation of blood glucose by insulin and glucagon, where normal levels are approximately 4.0−8.0 mmol dm−34.0 - 8.0 \text{ mmol dm}^{-3}.

•

Signal amplification occurs in peptide hormone pathways where one hormone molecule triggers the production of many second messenger molecules, leading to a large cellular response.

📐Formulae

H+R⇌HR→Response\text{H} + \text{R} \rightleftharpoons \text{HR} \rightarrow \text{Response}

t1/2=ln⁡(2)kt_{1/2} = \frac{\ln(2)}{k}

Change=Final Value−Initial Value\text{Change} = \text{Final Value} - \text{Initial Value}

💡Examples

Problem 1:

Explain how the body responds to a decrease in blood glucose levels below 4.0 mmol dm−34.0 \text{ mmol dm}^{-3}.

Solution:

  1. Alpha (α\alpha) cells in the Islets of Langerhans in the pancreas detect low blood glucose.
  2. Glucagon is secreted into the bloodstream.
  3. Glucagon binds to receptors on liver cells (hepatocytes).
  4. This triggers glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis.
  5. Blood glucose levels rise back to the set point.

Explanation:

This is a classic example of negative feedback. The stimulus (low glucose) triggers a response (glucagon release) that opposes the stimulus.

Problem 2:

Contrast the mechanism of action between Epinephrine and Cortisol.

Solution:

Epinephrine (peptide-like/amine) binds to extracellular receptors →\rightarrow activates G-protein →\rightarrow activates Adenylate Cyclase →\rightarrow produces cAMPcAMP (second messenger).

Cortisol (steroid) →\rightarrow diffuses through plasma membrane →\rightarrow binds to cytoplasmic receptor →\rightarrow enters nucleus →\rightarrow alters gene transcription.

Explanation:

Hydrophilic hormones (Epinephrine) use signal transduction at the membrane, while lipophilic hormones (Cortisol) directly influence DNA expression.