A rave about hormones

Hormones don’t really do much themselves but the effects of hormones on an organism is huge. Hormones are signalling molecules. They tell a cell or group of cells it is time to do something. That’s it. They simply carry a message. Once that message is received though, the effects can be staggering. This is called signal transduction.
Homeostasis and negative feedback

Hormones are one of the mechanisms humans and other animals use to maintain homeostasis, the other being the nervous system. They are also used to initiate change such as growth and reproductive maturity. There is a narrow range that most variables within the body, such as pH of your blood and your body’s temperature, need to be kept within.  Think of enzymes. Remember them? Little worker proteins that catalyse the chemical reactions that occur inside you. What happens to them when they get too hot, or placed in a solution too acidic? They denature, change shape and stop working and you end up feeling pretty shitty at best, dying at worst. The primary mechanism that the body uses to keep things in this narrow range is called the negative feedback system. Positive feedback, while uncommon, is also used but I’m not going to go into that here. Negative feedback is often made to sound incredibly complex, and, while the body’s biochemical pathways can be very complicated, negative feedback is actually a very simple concept. Imaging you are driving, car is flying along the road and you decide to take your hands off the wheel and put them on your head. You continue driving with hands on your head and all of a sudden the car hits the gutter. This is the stimulus, if you don’t do something you’ll crash. You respond by quickly grabbing the wheel and steering the car back into your lane. And then you put your hands back on your head. And after a while the car swerves into the oncoming traffic. And you respond again to avoid crashing. This is how the body drives itself- with its hands on its head! It waits until something goes wrong and then responds. It waits until one of the many variables it is monitoring moves out of a given range, then responds to return it to within those narrow limits. Bad way to drive but seems to work in animals.
Hormones, receptors and signal transduction

You should know by now that, hormones are produced by the endocrine system and, generally, are released into the blood stream. It takes about a minute for your blood to do a lap of your body – pretty quick really when you consider the length of all those arteries, arterioles, capillaries, venuoles and veins. There are two main types of hormones: peptide (protein) and steroid (lipid). This is where you really need to recall hydrophilic and hydrophobic molecules and movements in and out of membranes. Peptide hormones are hydrophilic. Stop here and think: how will this hormone respond when secreted into blood and what will happen when it gets to the cell membrane? While you’re thinking about that, I want you to also recall how important shape is to all the biomolecules you’ve studied so far. Shape is extremely important for hormones and will continue to be important as you look at disease. You may release a hormone from one of your glands and want it to trigger a response in the testes or ovaries. How does the hormone make these cells/organs respond but leave others, say the pancreas, alone? Again it is shape. The cells that the hormone is trying to get a response from are called Target Cells, and these target cells have shape-specific receptors for these particular hormone. And they would most likely have other receptors for other hormones as well. When the right hormone connects with the receptor of the target cell a response is triggered.

Let’s get back to that question I posed a while back. A protein hormone is hydrophilic and is readily carried in the blood around the body until it reaches the target cell. As a hydrophilic molecule it is lipophobic and therefore has issues crossing the cell membrane. So the receptors for protein-based hormones are on the outside of the cell membrane. Steroid hormones, being lipid-based, are hydrophobic and need carrier proteins to help them get around, however, they easily pass through the cell membrane and the signal receptor for them is on the inside of the cell membrane. The triggering of a hormone receptor and the response it causes within a cell is referred to as signal transduction. A signal transduction pathway includes the activation of a receptor by a hormone and the series of responses it causes within the cell. This is often referred to as a cascade effect, like sneezing in the alps and triggering an avalanche. The response seems disproportionate to the stimulus. For the cell to do whatever it needs to do once stimulated by the hormone there are usually a series of steps that are amplified at each stage causing a tsunami of biochemical reactions, each one having a greater impact until the final produce molecule is synthesised or whatever was supposed to happen, happens.

Avalanche? Tsunami?  Time to move on...

Ever wondered how these molecules are removed from the blood? In a nutshell they are broken down, filtered from the blood by the kidneys and peed away. Read your textbook.
Plant hormones and tropisms

Plants also use hormones to regulate themselves and to maintain homeostasis. As they lack a nervous system, they rely on their hormonal system. One would intuitively think that by relying on this system alone, plants would have developed an intricate and complex system of hormones. They have but they only use five of them. (Six if you want to include florigen, the hormone that no one has ever found but some botanists argue must be there). They work by combination and concentration: how much of them there is and what other hormones are with them. They are transported in the phloem and also diffuse through and around cells and are responsible for everything a plant does, from bending toward the light to flowering. I’m going to rant a little about tropisms in plants, then flowering and the importance of environment in the synthesis of hormones in plants. These five hormones and their general impact on a plant would be a good think for a Yr12 student to learn, they are in all textbooks and I’m not getting into them here.

Tropisms in plants are responses in plants to certain stimuli and auxin is a very important hormone in these responses. The three main tropisms that are covered in this course are phototropism (a response to light), gravitropism (a response to gravity) and thigmotropism (a response to touch) and they way they work is relatively simple in concept, the particulars though are quite complicated but unnecessary to discuss at Yr12. Phototropism is a plants response to light. It is usually positive in that a plant will bend and grown toward the light. This occurs by auxin in the growing stem diffusing away from cells in the light and building up in concentration in the cells that are more shaded. Once in these cells they cause them to elongate. These cells grow longer taking up more area and bending the plant toward the light. What actual causes the hormone to move though? The light triggers the response however, what actually causes the auxin to move... I can’t remember. I’ll get back to this later. Gravitropism is similar. If a shoot is on its side gravity causes auxin to move to the underside, causing these cells to elongate and the shoot growing up. Gravitropism also occurs in the roots but works in the opposite way, causing the roots to grow down. I won’t go into thigmotropism but will say that this is a response to touch. It’s what causes pea to wrap those spring-like shoot around branches to give the plant support, or causes a plant to move away when crowded. Don’t worry about this too much.
Environmental stimulation of hormone production

Hormones in plants are often stimulated into production by environmental factors such as light. The example I’m going to give comes from a conversation I had many years ago in Nepal with a Dutch guy. We were sitting in a cafe looking out at a vacant block of land that was covered in wild marijuana plants. This dude started telling me how he grew marijuana hydroponically in Holland. Apparently, the parts of marijuana that drug addicts and malcontents smoke are the flowers. He told me that light prevents these plants from producing flowers and they need at least 10 hours of darkness to allow the build up of the hormone that triggers flowering. If they are exposed to any light during this dark period, the flowering hormone isn’t produced. He was telling me that he had several plants growing in a cupboard and all but on were flowering successfully. He worked out that the one plant that wasn’t flowering was getting a little bit of light through a crack on a couple of leaves. As hormones are transported around the plants even the parts of this plant that were receiving uninterrupted darkness weren’t flowering as the hormones move throughout the plant. Drugs are bad. This is just an example.
Pheromones: external communication hormones

Finally, I’m going to return to animals and pheromones. These are an interesting group of hormones as they work externally and impact primarily behaviour. They are used to communicate and the information is usually species specific. They can communicate territory. This is why dogs pee on poles. They are marking their area with their pheromones. They can communicate a food source. This is why ants will follow a trail. They are following a trail of pheromones laid down by the ants that found the food. And most importantly they can communicate sexual availability. For example a female dog in heat will attract male dogs from all around as they smell her pheromones. Male moths can smell a female moth from kilometres away. That’s how sensitive they are. I’ve got heaps to say about pheromones but I’m going to stop here. That’s about all you need to know. It’s late and I’m tired. I'll type some more about pheromones tomorrow.