Key Area 4

(b) Hydrophobic signals and control of transcription

Transducing an extracellular message into an intracellular response

How does changes outside a cell trigger changes inside the cell? We have learned in previous Biology courses that increases in blood sugar concentration will trigger the release of insulin into the bloodstream. This binds to receptors on liver cells and glucose is taken into the liver and converted into glycogen. But have you everything wondered HOW?

These receptors are key transducers, converting extracellular messages into intracellular events - how it does this often depends on the location of the receptor.

Sometimes receptors are located on the cell surface (as is shown in the diagram and as we assumed from our N5 Biology lessons on insulin). But, sometimes, receptors may be found inside the cell cytosol.

But what influences whether the receptor is engaged on the cell surface or within the cell?

You guessed it - it's chemical nature.

Task 58

Rob arrived late into class after another seemingly very important Deputy Head Boy task..."Right Rob, just the person. Think you need some "warm your brain up" tasks. Given what you know about hydrophobic and hydrophilic molecules and the cell membrane, how might such molecules transduce their messages across the membrane to give an intracellular response?"

Rob, being has charming self, looked at his friends around the table, "Think, pair, share, Miss?"

"Aye, alright Rob. 2 minutes", replied Dr McRobbie.

On you go, take 2 minutes to think about the answer to this before moving on.

The image opposite shows a hydrophobic signalling molecule, oestrogen, diffusing across the cell membrane and binding to its specific receptor within the cytosol. The oestrogen-receptor complex then moves into the nucleus and interacts directly with DNA.

Hydrophobic signalling molecules can diffuse directly through the phospholipid bilayers of membranes, and so bind to intracellular receptors. They are able to pass through the phospholipid bilayer because the tails of the phospholipids in the plasma membrane are also hydrophobic and allow the molecules to pass across.

Hydrophobic signals

Hydrophobic signalling molecules include steroid hormone, e.g. cortisol, and thyroxine. These molecules can diffuse directly through the phospholipid bilayers of membranes and so bind to intracellular receptors.

The receptors for hydrophobic signalling molecules are transcription factors. Transcription factors are protein molecules that, when bound to DNA, can either stimulate or inhibit the initiation of transcription.

Steroid Hormones - an example of hydrophobic signalling molecules

Steroid hormones are based around the molecule cholesterol - different side chains give the different hormones, e.g. testosterone, oestrogen and progesterone (shown opposite). They are hydrophobic molecules and can pass across the cell membrane of target cells.

Steroid hormones bind to specific receptors in the cytosol or in the nucleus forming a hormone-receptor complex.

The hormone-receptor complex moves to the nucleus where it binds to specific sites on DNA called hormone response elements (HREs). Binding at these sites influences the rate of transcription, with each steroid hormone affecting the gene expression of many different genes.

Mechanism of steroid hormone action

Task 59

"Jist haud on er Miss, this is getting confusing. So testosterone and the like daunder across the membrane like they own the place, hook up with a receptor buddy and then unleash control over gene expression? ", questioned wee Stevie from the side.

"Hmm, well, you're not far wrong there Stevie. Try and complete the diagram in your notes for Task 59 using SQA language this time", replied Dr McRobbie . You should do the same: use the statements in the yellow boxes to help you.

Answers available here.

Task 60

Thyroxine is an example of a hydrophobic signalling molecule that is involved in regulating the metabolic rate. The thyroid gland sits over the windpipe under the larynx and produces thyroid hormones, including thyroxine, under the influence of the pituitary gland. In the absence of thyroxine, the thyroxine receptor protein binds to DNA and inhibits the transcription of the Na/K pump. This ultimately reduces metabolic rate. Conversely, in the presence of thyroxine, thyroxine will bind to the thyroxine receptor protein, preventing it from binding to DNA and, as a consequence, normal transcription of the Na/K pump continues. Metabolic rate increases.

Using the information above, together with the animation linked below, to produce a flowchart and diagram to illustrate the action of thyroxine.