The iGCSE specification says that all living organisms share the following basic characteristics and then lists 8 bullet points. This seems unnecessarily unhelpful because every student in the whole word learns MRS GREN for the 7 characteristics of life…
Make sure you understand the exact meaning of each of the following terms:
Not all organisms Move from place to place of course and lots of things move that are not alive. So that doesn’t make me think that this is a good way to start the whole study of Biology. It is true that all living things, without exception, Respire. “Respiration is a series of chemical reactions that happens inside cells in which food molecules are oxidised to release energy for the cell” – good definition that…. Sensitivity means the ability to detect and respond to changes in the environment. Mammals do this through their nervous and hormonal systems, plants through plant growth substances such as auxin. Growth either involves a cell getting larger or in multicellular organisms, the two processes of cell division and cell specialisation. All living things have the potential to Reproduce, to create new individuals of their species. Excretion is the removal of waste molecules (e.g. carbon dioxide, urea) that have been made inside cells. Nutrition means either obtaining food molecules by eating another organism or if you are a plant, and I guess none of you are, by making your own food molecules through photosynthesis.
The people who wrote the specification have added “they control their internal conditions” to the list. This is actually a better characteristic of life than many above as it is a universal feature of all life. The term for this process is Homeostasis – the ability to regulate and control the internal environment.
It is a shame that two of the best ways to decide whether something is alive have been left off the list. All living things on earth are made of cells. Some organisms are unicellular (Paramecium for example) but many are made of many cells. And all living organisms have the molecule DNA as their genetic material. If you get a question on this in the exam, it’s probably better to talk about the 8 characteristics of life the examiner likes… That’s exams for you!
Homeostasis is a term that means maintaining a constant internal environment in spite of changes in the external environment. Many variables in the body are regulated by homeostasis but the two control systems specifically mentioned in your specification for iGCSE are osmoregulation (regulation of water balance) and thermoregulation (regulating of body temperature)
I have looked at osmoregulation in a previous post but in this final post for half term 2015, I will give a few details about thermoregulation.
Thermoregulation means to maintain the core body temperature at a set value. This can be energetically very costly as the animal has to respire at a much higher rate to release the heat needed to warm the body, but it has allowed mammals and birds to colonise habitats that would be inaccessible to all poikilothermic (cold-blooded) animals.
Why regulate body temperature?
All metabolic reactions in the body are catalysed by enzymes. If the body temperature falls too low below the set value, the rate of an enzyme-controlled reaction will drop, and this would be a problem as metabolic reactions would happen too slowly. If the temperate goes much above the optimum temperature, then the enzymes that catalyse all the reactions in cells would denature. This means they will change their shape so that the “lock and key” mechanism of catalysis cannot work at all.
In any homeostatic control system there will be three components:
- Sensors (where the variable is measured)
- Integrating Centre (where the measured value is compared to a set value)
- Effectors (which can bring about a response)
In human thermoregulation, there are two sets of sensors that measure temperature. The skin contains hot and cold receptors which can respond if the skin gets too hot or cold respectively. The temperature of the blood is constantly measured by a second set of thermoreceptors which are found in the hypothalamus in the brain.
The hypothalamus also acts as the integrating centre, collecting information from a variety of sensors and then initiating an appropriate response.
The main effector organ in thermoregulation is the skin.
I have looked at the role of the skin in thermoregulation in an earlier post – click here to be taken to this….
Just check you understand the role of sweating, vasodilation in helping the body lose heat if it gets too hot and vasoconstriction and shivering if it gets to cold. I hope the earlier post will help!
Please add comments/feedback/questions etc using the comment feature at the bottom of this post or tweet me @Paul_Gillam.
The main function of the kidneys is EXCRETION. They remove urea from the blood in a two stage process described in an earlier post, first by filtering the blood under high pressure in the glomerulus and then selectively reabsorbing the useful substances back into the blood as the filtrate passes along the nephron.
But the kidney has an equally important role in HOMEOSTASIS. It actually is the main effector organ for regulating a whole load of variables about the composition of the blood (e.g. blood pH and salt balance) but in this post I want to explain to you how the water balance of the body is regulated and the kidney’s role in this homeostatic system.
Why do you need to regulate the dilution (or water potential) of the blood?
If the blood becomes too dilute, then water will enter all the body cells by osmosis (from a dilute to a more concentrated solution). This net movement into cells would cause them to swell and eventually burst. Bad news all round…
If the blood becomes too concentrated, then water will leave the body cells by osmosis. Cells will shrivel up as they lose water into the blood and this will kill them. Bad news all round….
Remember: a hypertonic solution has a low water potential and is very concentrated. A hypotonic solution has a very high water potential and is very dilute.
The regulation of the water potential of the blood is a very important example of homeostasis in the human. It is often referred to as OSMOREGULATION.
The water potential (dilution) of the blood is measured continuously by a group of neurones in a region of the brain called the hypothalamus.
The hypothalamus is found right next to a very important hormone-secreting gland called the pituitary gland, marked as the red circular structure on the diagram above. When the hypothalamus detects that the blood’s water potential is dropping (i.e. it is getting too concentrated) this causes the posterior lobe of the pituitary gland to start secreting a hormone ADH into the bloodstream.
(You might remember that these brain structures appear elsewhere in the iGCSE specification. The hypothalamus also contains the temperature receptors that measure the temperature of the blood in thermoregulation; the pituitary gland plays a role in the menstrual cycle by producing FSH and LH)
Hormones such as ADH exert their effects elsewhere in the body. The main target tissue for ADH is the collecting duct walls in the kidney. ADH binds to receptors on these cells and makes the wall of the collecting duct much more permeable to water. This means as the urine passes down the collecting duct through the salty medulla of the kidney, lots of water can be reabsorbed into the blood by osmosis. This leaves a small volume of very concentrated urine and water loss is minimised.
ADH is secreted whenever the body is dehydrated. It might be because the person is losing plenty of water in sweating in which case it is vital that the kidney produces as small a volume of urine as is possible.
If you drink a litre of water, what effect will this have on the dilution of the blood: of course it makes the blood more dilute. This will be detected in the hypothalamus by osmoreceptors and they will cause the pituitary gland to stop secreting ADH into the bloodstream. If there is no ADH in the blood, the walls of the collecting duct remain totally impermeable to water. As the dilute urine passes down the collecting duct, no water can be reabsorbed into the blood by osmosis and so a large volume of dilute urine will be produced.
This is another beautiful example of negative feedback in homeostasis.
PMG tip: you can avoid getting confused in the exam about the effect of ADH if you can remember what it stands for. ADH is an acronym for anti-diuretic hormone (ADH). A diuretic is a drug that promotes urine production. They are banned drugs from WADA (World Anti-Doping Agency) since they can be taken as a masking drug to help flush out evidence of illegal drug taking. Shane Warne missed the 2003 cricket World Cup and served a ban for failing a drugs test due to diuretics in his sample.
So an anti-diuretic hormone will reduce urine production. This means it will be secreted when the body is dehydrated as the blood gets too concentrated.
Finally remember that it is not the whole nephron that is affected by ADH, just the collecting ducts and part of the distal convoluted tubules. Most water in the glomerular filtrate is absorbed in the nephron but the collecting duct has a role in “fine-tuning” the volume and dilution of urine.
This is a really important topic to master for an A* in your exam. Examiners seem to like asking questions on ADH and osmoregulation and often these questions are amongst the hardest marks to get in the exam, and so serve as a brilliant discriminator between A and A* candidates. Work hard to master this topic and with a little luck from the question-setters an A* grade is within your grasp……
Homeostasis is one of the more difficult topics for students to understand in the iGCSE specification. I have posted already about the skin and its role in thermoregulation so I suggest you read that post again to get the details….
In this post, I am going to try to explain the concept of homeostasis in much more general terms, then in later posts, look at the two examples mentioned in the syllabus. Here goes….
Homeostasis is one of the life characteristics shared by all organisms. Living things all inhabit a world in which the external environment changes from hour to hour, from day to day, from month to month. Even organisms living in the most stable aquatic environments may be subject to changing oxygen concentrations, changing water pH, changing light intensities and so on. This changing external environment poses a challenge for life since how can life processes operate at optimal levels in all these differing conditions. Life has solved this by allowing organisms to keep their internal environments much more constant than the ever-fluctuating external environment.
A definition to learn:
“Homeostasis is the set of processes occurring in an organism to maintain a constant internal environment”
Examples of Homeostasis in Humans
A whole variety of factors are maintained at constant values in the body by homeostasis. For example (there are many more….):
- Blood pH
- Blood temperature
- Blood dilution
- Blood oxygen concentration
- Blood carbon dioxide concentration
- Blood glucose concentration
- Blood pressure
This introduces the first area of common confusion in students’ exam answers. For some reason many students think that homeostasis is a word for the maintenance of body temperature in humans. I hope you can see it is a much more general term than that.
But…. the systems that maintain a constant body temperature in endothermic animals are one example of homeostasis. In fact this example (thermoregulation) is one of the two from the list above that you need to understand for your exam. The other one you might be asked about is osmoregulation (the maintenance of a constant dilution of the blood).
All homeostatic control systems have some common features. The variable that is going to be regulated needs to be measured somewhere in the body. A change in this variable is called a stimulus and is measured by a cell called a receptor. The measured value needs to be compared with a “set value” and this is done by an integrating centre that then controls an effector. The effector is an organ that can bring about a response. But what kind of response do you want in the process of negative feedback?
A common process involved in homeostasis is negative feedback. This is quite tricky to define but in fact it is a really simple idea……. If you want things to stay the same, any change must be corrected. That’s negative feedback in a nutshell.
For example a school might want students walking round the campus at a sensible speed: not to fast to knock people over, not to slow or people are late for lessons… Imagine a particular group of children who start to run around the place, causing mayhem and injuries to fellow students. Well this will first be detected by the system. There may be a particular teacher who comes out and sees the students running, the school nurse might report an increase in cuts and bruises. However it happens, a change in the system (a stimulus) is detected. There will be an integrating centre in this control system too, probably in the form of a stern deputy head. She will compare the measured speed to her own “set value” of how fast students should move. And she will initiate a response: probably a loud telling off to the entire school in assembly, lots of dire warnings about future conduct and an after school detention for all the rule breakers. The net response of this will be that students will start moving slower around the school…. Eventually of course people will start moving too slowly and will be late for lessons. How do you think the system will react to this new stimulus? This process where the response tends to reduce the stimulus is called negative feedback.
To give you a biological example in thermoregulation, look at the diagram below:
If the body temperature goes up, the system responds by lowering the body temperature.
If the body temperature drops, the system responds by raising body temperature.
Both examples of negative feedback!
The end result of negative feedback is that the value will fluctuate around the set value (see this graph also showing the effect of negative feedback in thermoregulation)
Any questions, problems, queries: please comment using the box below the post or tweet me @Paul_Gillam and I will do my best to help…….
The skin as you all know is the largest organ in the human body. It has a variety of functions including providing a water-tight barrier to minimise evaporation from the cells; it is also a habitat for billions of bacteria that live on the skin, the so-called skin flora and it contains a variety of sensory receptors that provide information about the external world to our central nervous systems. The iGCSE specification requires you to know about the role of skin in thermoregulation.
The skin is made up of an outer layer of dead cells called the epidermis that contains sensory nerve endings. Beneath this is the dermis which is made of living cells and blood vessels, sweat glands, hair follicles and other specialised sensory receptors, e.g. for touch. Underneath the dermis there is a subcutaneous tissue that in humans is packed full of adipose cells that store lipids.
The skin is involved in thermoregulation both as a receptor and more significantly as an effector.
The skin’s role as a receptor in thermoregulation
The brain receives information about temperature from two sets of thermoreceptors. There are receptors in the hypothalamus that measure the temperature of the blood passing through the brain. This provides information about core body temperature. In the skin there are two types of thermoreceptors, called hot and cold receptors, that together monitor the external temperature. Information from both these sets of receptors is used by thermoregulatory centres in the hypothalamus to regulate your body temperature.
The skin’s role as an effector in thermoregulation
The skin is the principle effector organ for thermoregulation. This is because it is found at the boundary between your cells and the external environment and so heat gain and heat loss happen through it. The skin has three ways of altering the heat gain/loss depending on nerve impulses from the CNS.
Humans have sweat glands spread over almost all the surface of the skin. These glands secrete a watery liquid, sweat that contains a solution of salts and a tiny amount of urea dissolved in large volumes of water. Sweat is only produced when the body temperature is too high as the evaporation of sweat from the surface of the skin leads to a cooler skin. How does this process work?
The main idea to understand is that the sweat itself is not in any way cool. Sweat is made in sweat glands from blood plasma so if the blood is getting too hot, the sweat will be hot as well. But it takes energy to evaporate water (to turn it from the liquid to the vapour state) and this energy (called the latent heat of vapourisation) is taken as heat energy from the skin. So as sweat evaporates, it uses thermal energy from the skin to turn the water molecules in sweat into a vapour. This evaporative cooling leaves the skin cooler once the sweat has evaporated than it was at the start.
Hairs on the skin play an important role in thermoregulation in many mammals but not really in our species. If the body temperature drops, the CNS causes hair erector muscles to contract and pull the hair to a more vertical position in the follicle. If an animal’s hairs stand on end, a thicker layer of air is trapped between them and so the body is better insulated against heat loss. Humans are relatively hairless and the only thing that really happens in us when the hair erector muscles contract is that we get “goose bumps”.
3) Shifting patterns of Blood flow in the skin
This is the main effector mechanism in human thermoregulation but it is also the one that tends to catch exam candidates out. Please make sure you understand this process fully and can explain this section of work very well indeed. If the body is getting too cold, the pattern of blood flow switches in the skin so less blood flows in the capillary beds near the surface of the skin and more blood is retained deeper in the skin structure. This is achieved by narrowing the arterioles that supply the capillary beds near the surface (arterioles and arteries have plenty of muscle in their walls that can contract to narrow the lumen of the blood vessel) This narrowing of arterioles is called vasoconstriction.
The converse happens when the body is getting too warm. The muscle in the walls of these arterioles now relaxes to widen the lumen, thus allowing more blood to flow in capillary beds near the surface. This vasodilation allows more heat to be lost from the blood by conduction, convection and radiation and so the blood leaving the skin has lost more heat to the external environment.
You will notice that at no point in these explanations of vasoconstriction and vasodilation do I mention capillaries in the skin moving deeper or nearer the surface. For some reason every year, GCSE candidates think that the reason you look redder when you are hot is because capillaries in the skin move nearer the surface. This cannot be true – blood vessels have a fixed position in the body for a start – but now you should understand that you look redder when you are hot because the capillaries that happen to be near the surface are having a greater volume of blood per minute flowing through them because of vasodilation. If you find yourself in the exam writing about capillaries moving in response to a change in temperature, please stop writing, take a deep breath, count to ten and then cross it all out and start again!