VCE Biology Question Thread

[cosine]

When it says signalling molecule, what exactly is meant by it? I know for a response, there must be a stimulus, so could you say the signalling molecules are the cells that pick up the stimulus, and then transfer it to receptor molecules which will further carry out a response? Thanks

[heids]

When it says signalling molecule, what exactly is meant by it? I know for a response, there must be a stimulus, so could you say the signalling molecules are the cells that pick up the stimulus, and then transfer it to receptor molecules which will further carry out a response? Thanks

No, the signalling molecules are the molecules/chemicals that carry the signal, e.g. pheromones, hormones, neurotransmitters, neurohormones.  So rather than picking up the stimulus, they are the things that initiate the stimulus, which a cell then picks up, transducts and the response occurs.  (Note, receptor molecules don't normally carry out a response; they pick up a stimulus and begin the process of transforming it to something the cell can understand and then do.)

[cosine]

No, the signalling molecules are the molecules/chemicals that carry the signal, e.g. pheromones, hormones, neurotransmitters, neurohormones.  So rather than picking up the stimulus, they are the things that initiate the stimulus, which a cell then picks up, transducts and the response occurs.  (Note, receptor molecules don't normally carry out a response; they pick up a stimulus and begin the process of transforming it to something the cell can understand and then do.)

Thanks bangali_lok

Can you give me an example, im having troubles understanding how they initiate the stimulus. Thank you!

[TheAspiringDoc]

Thanks bangali_lok

Can you give me an example, im having troubles understanding how they initiate the stimulus. Thank you!

Example of..?

[cosine]

Example of..?

How signal molecules initiate the stimulus?

[sunshine98]

How signal molecules initiate the stimulus?

I thought signalling molecules initiate the response. Because isn't a stimulus simply a change in either chemical( such as ph, concentration of ions, salts or wastes) or physical factors  (such as temp)  in the internal or external environment? How can a signalling molecule initiate a stimulus? Im confused   

[heids]

Can you give me an example, im having troubles understanding how they initiate the stimulus. Thank you!

I thought signalling molecules initiate the response. Because isn't a stimulus simply a change in either chemical( such as ph, concentration of ions, salts or wastes) or physical factors  (such as temp)  in the internal or external environment? How can a signalling molecule initiate a stimulus? Im confused   

Sorry my fault entirely, don't be confused... I used the wrong word by mistake Yes, they initiate the response - and hence, I think it's right to say that they are a stimulus.

Pretty much, a signalling molecule is just a way that one cell sends another cell a message, like receiving a letter.  The receiving cell's receptors bind to the signalling molecule (get the letter and read it), and then through lots of complex processes (e.g. telling their friends to do XYZ) they do what the letter tells them to do.  If they hadn't got the letter, they wouldn't have known what to do.  You don't have to know the specific biological mechanisms, but when a receptor binds to the signalling molecule (that is, if it's protein based ) it then forms or releases other molecules, called second messengers (the friends), which then go and do other things.

e.g. a neurotransmitter is a signalling molecule.  It's released by another neuron, diffuses across the synapse, and binds with receptors on the next neuron.  To simplify (a lot), the next neuron is then stimulated to transmit an impulse.

Or a hormone like insulin (signalling molecule) binds with a cell receptor, which then triggers various processes and leads to increased uptake of glucose in liver cells.

Edit: I'm bad at explaining

[grannysmith]

^^ Awesome explanation by bangali_lok

To add on, a stimulus is what may trigger the signalling molecule to be produced. If there is an undesirable change in a variable (e.g. decrease in blood glucose levels, which is the stimulus) this will initially be detected (i.e. by the alpha cells of the islets of Langerhans of the pancreas) which then prompts the release of a signalling molecule (i.e. glucagon, a hormone). Now, the purpose of the signalling molecule is to relay this message of counteracting the initial change (stimulus) to the appropriate effectors (in this case, the liver cells in particular, which are stimulated to breakdown glycogen into glucose which is then released into the blood stream, thereby increasing blood glucose levels).

Of course, signal transduction occurs upon the binding of the ligand to the specific receptor. Also, the example of blood glucose is not required knowledge, and was only used as an example to illustrate the point (it's actually negative feedback/homeostasis which is not in the SD).

[cosine]

If hormones are released/secreted by endocrine cells into the bloodstream where they are off to their target cell, how are they produced? Should we know this for VCE, like hormones are the signalling molecules, but for the to be secreted at the stimulus, wouldn't there be other signalling molecules that order the endocrine cells to produce hormones to regulate the internal condition?

Thanks, and sorry for the deep thinking but this question has been haunting me

[grannysmith]

Yes, that's correct - a signalling molecule may just exert an effect on a cell for another signalling molecule to be produced! However, this is going into too much detail and isn't required. If anything, the information would be given to you in an exam and all you'd need is a good understanding of signal transduction and the stimulus-response model.

[cosine]

Yes, that's correct - a signalling molecule may just exert an effect on a cell for another signalling molecule to be produced! However, this is going into too much detail and isn't required. If anything, the information would be given to you in an exam and all you'd need is a good understanding of signal transduction and the stimulus-response model.

Thanks grannysmith, keep up the good work!
Few qs:

1). Why exactly is the endocrine system slower than the nervous system?
2). When a protein-based hormone (hydrophillic) forms a hormone-receptor complex on the cell membrane, do we need to know anything else during signal transduction besides that an enzyme on the lower surface of the membrane is stimulated to convert ATP to cyclic AMP and this AMP is responsible for the further signals and results in the target cell's response? Like do we need to know how cyclic AMP is formed, and the other processes involved?
3). With steroid hormones, when the signal is received by an intracellular receptor molecule, what happens then? Some books say the complex moves into the nucleus, and others say that the complex forms inside the nucleus. Which one is it, or is it both?

Thanks guys, I love how I can build up so many questions, but still  have the relief that someone on AN is kind enough to enlighten me

[grannysmith]

1. Nervous system is electrical (and chemical) whereas endocrine system is only chemical. The former involves neurotransmitters which travel between neurons over exceedingly small distances (think about synaptic gap) to reach their target cell whereas the latter involves hormones generally released into the bloodstream, and hence their target cells would be much further away. Also, nervous signals are more directed than hormones, which are carried by the general flow of blood.

2. You don't even need to know about cyclic AMP or any specific second messengers for that matter. All you need to know for hydrophilic signalling molecules is that the specific receptor is on membrane surface, second messengers are involved which triggers a cascade of intracellular reactions.

3. Either. A steroid-receptor complex may form in the cytoplasm and then move into the nucleus or it may form in the nucleus.

[KingDrogba]

Could someone help me out, what is Venous Return? and how is it affected when the external temperature is increased??

[EspoirTron]

Could someone help me out, what is Venous Return? and how is it affected when the external temperature is increased??

Venous return refer to the blood vessels (veins) that return blood to the heart. The two greater vessels discussed include the superior and inferior vena cava. It is important to realise that not all veins return dexoygenated blood to the heart - so don't add this as a part of your definition of veins! As an example, the pulmonary veins return oxygenated blood from the lungs to the left atrium.

I'm going to postulate here in regards to the latter half of your question. Your arteries to your upper and lower extremities are accompanied by adjacent veins, these veins are referred to as vena comitans, and these veins assist in bringing blood from the extremities (your limbs) back to the heart. Placing these veins near the arteries means that the high pressure of the arterial blood pressure can assist in increasing venous pressure. The comitans being placed so close to the arteries also allow for countercurrent heat exchange between the arteries and the veins. Arterial blood is already quite warm. This means that arterial blood can warm up venous blood by countercurrent heat exchange.  Increasing your core temperature by an increase in external temperature is just going to increase arterial blood temperature, as well as your venous blood temperature. Doing so means that the blood in the vena comitans are further increased in temperature which increases the venous return.

I'm just taking a guess so I may be wrong with that. Hope it helped though!

[Rishi97]

Venous return refer to the blood vessels (veins) that return blood to the heart. The two greater vessels discussed include the superior and inferior vena cava. It is important to realise that not all veins return dexoygenated blood to the heart - so don't add this as a part of your definition of veins! As an example, the pulmonary veins return oxygenated blood from the lungs to the left atrium.

I'm going to postulate here in regards to the latter half of your question. Your arteries to your upper and lower extremities are accompanied by adjacent veins, these veins are referred to as vena comitans, and these veins assist in bringing blood from the extremities (your limbs) back to the heart. Placing these veins near the arteries means that the high pressure of the arterial blood pressure can assist in increasing venous pressure. The comitans being placed so close to the arteries also allow for countercurrent heat exchange between the arteries and the veins. Arterial blood is already quite warm. This means that arterial blood can warm up venous blood by countercurrent heat exchange.  Increasing your core temperature by an increase in external temperature is just going to increase arterial blood temperature, as well as your venous blood temperature. Doing so means that the blood in the vena comitans are further increased in temperature which increases the venous return.

I'm just taking a guess so I may be wrong with that. Hope it helped though!

Do we even need this for yr 12? I don't remember learning it last yr. It was more of a yr 11 bio thing :/