Hormones

Transcript

0:00Introduction

Hello everyone! Let us begin with a small puzzle from the kitchen. The table salt sold in shops has a little iodide added to it. Salt is sodium chloride, so why add iodine at all? And how can missing a little iodine make a person tired and overweight? The answer is a hormone, and by the end of this video, you will explain it yourself. Hormones are a part of the biomolecules chapter in JEE chemistry, and these questions are quick marks once the table is clear in your head. So, let's begin.

0:33Hormones: chemical messengers

So, first, the meaning. Hormones are molecules that act as chemical messengers between cells. One set of cells sends the message, and other cells receive it and respond. Okay? They are made in the endocrine glands, which are also called ductless glands, because they have no duct, no tube, to carry what they make out of the gland. So let me ask you this. If there is no duct, how does the hormone travel to the organ where it works? It travels in the blood, that is correct. The gland pours the hormone directly into the blood, and the blood carries it to the site of action. So the gland and its target can be far apart, and the blood is the postman.

1:14Three chemical families

Clear? Now, by their chemical structure, hormones fall into three families. First, the steroids: the estrogens and the androgens, which are the sex hormones, as the brackets say, and also cortisol. Okay? Second, the polypeptides: insulin, glucagon and endorphins. Polypeptide means a chain of amino acids joined by peptide bonds, just as you saw in proteins. Right? And third, the amino acid derivatives: epinephrine, with its other name, adrenaline, in brackets, then norepinephrine, and thyroxine. Each of these is derived from an amino acid, and is not a long chain of them. Now a question for you. Insulin is a polypeptide, so why can it not be given by mouth? Think of digestion, and you have it. In the stomach and intestine, the protein-digesting enzymes would hydrolyse insulin to amino acids, just as they do with the proteins in our food. So the messenger is broken up, and its message is lost. Got it?

2:17The pancreas and the thyroid

Now, which gland makes which hormone, and what each one does. Let's start with the pancreas. It makes insulin, and insulin lowers the blood glucose level. But the body also needs a way to bring a low level back up, and the pancreas has a second hormone for that. So what must that hormone do? It raises the blood glucose level, yes, exactly, and that hormone is glucagon. So insulin lowers it and glucagon raises it, and together they keep blood glucose within a narrow limit. Students often swap these two, so remember it like this: glucagon is called when the glucose is gone. Clear? Next, the thyroid gland. It makes thyroxine, and thyroxine controls metabolism, the chemical reactions going on in the body. With me? And chemically, thyroxine is an iodine-containing derivative of tyrosine, which is an amino acid. That is why it sat in the amino acid family. Remember this iodine, because our salt puzzle and example two both need it.

3:19The adrenal gland: cortex and medulla

Got it? Now the adrenal gland. As the heading says, it has two parts, the cortex and the medulla. The adrenal cortex makes the glucocorticoids, which deal with carbohydrate metabolism, inflammatory reactions, and stress. Okay? The cortex also makes the mineralocorticoids, which control the excretion of water and salt by the kidney. Here is an easy way to remember them: gluco sounds like glucose, so carbohydrates, and mineral sounds like salt. Now, adrenaline also comes from the adrenal gland. So it must come from the cortex as well, right? That is exactly the trap, so be careful here. Epinephrine, which is adrenaline, and norepinephrine come from the other part, the adrenal medulla. They control the body's responses to external stimuli, like a sudden fright or excitement. Right?

4:10The sex hormones

Next, the sex hormones, which are the steroids we met earlier. The testes make testosterone, an androgen, which controls the male sex characteristics. Okay? The ovaries make estradiol, which is an estrogen, and progesterone, and these control the female sex characteristics. Right? And progesterone has one more job. It prepares the uterus for the fertilised egg. So if a question asks which hormone prepares the uterus, the answer is progesterone, and not estradiol. Got it?

4:41When a hormone is missing

Now let us see what happens when a hormone is missing. Low thyroxine is called hypothyroidism, and it causes lethargy, which means tiredness, and obesity. So now you can solve our puzzle. Thyroxine contains iodine. So why is iodide added to table salt? Yes, exactly, the body needs iodine to make thyroxine. Without enough iodine in the diet, we get hypothyroidism, and goitre, which is an enlargement of the thyroid gland. So iodide is added to table salt, and our puzzle is solved: that is how a little iodine is linked to feeling tired. Clear? And one more from the same table. Failure of the adrenal cortex leads to Addison's disease. Link it to the cortex, and not to the medulla. With me?

5:28Example 1: classify six hormones by chemical family

Time for example one, the easier one. Classify these hormones chemically: insulin, testosterone, thyroxine, glucagon, epinephrine and estradiol. Use our three families. Pause the video and try it first. Done? Okay, let's check. Insulin and glucagon, the pancreas pair, are polypeptides. Okay? Testosterone, an androgen, and estradiol, an estrogen, are sex hormones, so they are steroids. Now thyroxine. It is made from an amino acid, so it is a polypeptide too, right? That is exactly the trap, so be careful. Remember, a polypeptide is a chain of amino acids, but thyroxine is a derivative of the amino acid tyrosine. So thyroxine and epinephrine are amino acid derivatives. Clear?

6:23Example 2: percentage of iodine in thyroxine

Now a numerical one, a little harder. Thyroxine contains four iodine atoms per molecule, and has a molar mass of seven hundred and seventy seven. Find the percentage of iodine in it. In the brackets, capital I is the symbol for iodine, and its atomic mass is one hundred and twenty seven. Pause the video and try it first. Done? Okay, let's check. The percentage is the mass of iodine over the molar mass, times a hundred. So, one hundred and twenty seven over seven hundred and seventy seven, times a hundred, right? Wait, that is the trap, so read the question again. Each molecule has four iodine atoms, not just the one. So the iodine is four times one hundred and twenty seven, which equals five hundred and eight. Okay? Now divide by the molar mass. The percentage of iodine is five hundred and eight over seven hundred and seventy seven, times one hundred. Right? And that gives sixty five point four, with the percent sign at the end. So nearly two thirds of thyroxine, by mass, is iodine. With just one iodine atom, the answer would have come out far too small. See how much that word four matters?

7:44Quick revision: three families, insulin, glucagon, thyroxine

So, quick revision. Hormones are chemical messengers from the ductless glands, carried by the blood. They fall into three families: steroids, polypeptides and amino acid derivatives. Insulin lowers blood glucose, glucagon raises it, and thyroxine needs iodine, which is why iodide goes into our salt. Now go and try the practice problems on this page. You are ready for them. All the best!