1. The Endocrine System - Introduction

How many signalling pathways are there for chemical intracellular communication?

Which are the 3 types of intracellular signalling pathways?

In the Autocrine Signalling Pathway: A cell [blank_start]synthesises[blank_end] and [blank_start]excretes[blank_end] a chemical into the [blank_start]extracellular fluid[blank_end] which then [blank_start]binds[blank_end] to the [blank_start]receptor of the same cell[blank_end] to generate a response, as seen in the [blank_start]Immune System[blank_end].

In the Paracrine Signalling Pathway: A cell [blank_start]synthesises[blank_end] and [blank_start]excretes[blank_end] a chemical into the [blank_start]extracellular fluid[blank_end], which then [blank_start]travels to nearby cells[blank_end].

In the Endocrine Signalling Pathway: A cell [blank_start]synthesises[blank_end] and [blank_start]excretes[blank_end] a chemical ([blank_start]Hormone[blank_end]) into the [blank_start]blood[blank_end] or [blank_start]lymph[blank_end] system which [blank_start]travels[blank_end] a very [blank_start]long distance[blank_end], [blank_start]circulating[blank_end] throughout [blank_start]the body[blank_end], before affecting the target cell.

Hormones are not as [blank_start]specific[blank_end] as neurotransmitters, they can [blank_start]bind[blank_end] to many [blank_start]different[blank_end] cells to produce an [blank_start]activating[blank_end] or [blank_start]inhibitory influence[blank_end].

Hormones come in many forms which can be broken into 3 categories: [blank_start]Peptide hormones[blank_end] formed from [blank_start]proteins[blank_end]. [blank_start]Steroid hormones[blank_end] formed from [blank_start]Cholesterol[blank_end] or [blank_start]Lipids[blank_end]. [blank_start]Tyrosine derivative hormones[blank_end] formed from the [blank_start]amino acid[blank_end] [blank_start]Tyrosine[blank_end].

Peptide hormones are formed in the [blank_start]RER[blank_end], before proceeding to the [blank_start]Golgi Apparatus[blank_end] in which they may be [blank_start]modified[blank_end]. Once modified it travels into our blood or [blank_start]lymphatic system[blank_end].

Because Blood and Lymph consist mainly of [blank_start]water[blank_end] and [blank_start]Peptide[blank_end] Hormones are made from [blank_start]peptides[blank_end] which are [blank_start]water soluble[blank_end], they can [blank_start]dissolve[blank_end] in blood and lymph and therefore do not require a [blank_start]carrier protein[blank_end].

[blank_start]Peptide[blank_end] Hormones are not [blank_start]lipid[blank_end] soluble so they can’t cross the [blank_start]phospholipid membrane[blank_end] of the target cell via [blank_start]passive diffusion[blank_end].

Because peptide hormones can't [blank_start]diffuse[blank_end] into target cells, they bind to [blank_start]receptor proteins[blank_end] found on the [blank_start]membrane[blank_end] to induce change. For example, opening a channel protein that can change [blank_start]ion concentration[blank_end] or [blank_start]trigger a secondary messenger system[blank_end] to travel into the cell to induce change.

Steroid Hormones are synthesised in the [blank_start]SER[blank_end] or the [blank_start]Mitochondria[blank_end] of cells.

[blank_start]Steroid hormones[blank_end] are [blank_start]lipid soluble[blank_end] opposed to water soluble meaning they cant [blank_start]dissolve[blank_end] in the blood, so they require [blank_start]carrier proteins[blank_end] to reach their target.

[blank_start]Steroid hormones[blank_end] can cross the [blank_start]lipid soluble[blank_end] cell membrane of the target cell via [blank_start]passive diffusion[blank_end].

Steroid hormones bind to receptor proteins found in the [blank_start]cytosol[blank_end] of cells. The [blank_start]receptor protein hormone complex[blank_end] travels into the [blank_start]nucleus[blank_end] to induce [blank_start]transcriptional[blank_end] change, triggering the synthesis of needed [blank_start]proteins[blank_end].

[blank_start]Tyrosine[blank_end] Derivative Hormones are synthesised by the [blank_start]RER[blank_end] or [blank_start]specialised enzymes[blank_end] within the [blank_start]cytosol[blank_end] of cells.

Tyrosine [blank_start]Derivative[blank_end] Hormones can be either water soluble or [blank_start]lipid soluble[blank_end]. This means some don't require [blank_start]carrier proteins[blank_end] and induce change via [blank_start]receptor proteins[blank_end] on the [blank_start]cell membrane[blank_end] which they cannot cross... whilst others do require [blank_start]carrier proteins[blank_end] and cross the membrane to induce [blank_start]transcriptional change[blank_end] within the [blank_start]nucleus[blank_end] of a cell.