Deforestation means the cutting down of mature forest and woodland for non-forestry purposes. No new trees are planted and so the total area of forest decreases. Humans have cut down forests for many reasons and have built their economies on exploiting natural resources. Forests today are cleared to provide wood for logging, to provide land for building homes, for subsistence farmers as well as for commercial growing of crops and cattle farming.
Deforestation happens all the world. (It is worth noting that the only reason there are no European countries on the list below is because all our forests were cleared very effectively some time ago….)
What are the biological consequences of deforestation?
Wherever deforestation occurs, the biological consequences are the same:
- Atmospheric Gases
- Soil Erosion
- Disturbance to the Water Cycle
- Loss of Biodiversity (inexplicably omitted by the people who wrote the specification
Growing trees have a net uptake of carbon dioxide and a net loss of oxygen due to photosynthesis. Carbon dioxide is a gas that acts as a pollutant in our atmosphere because it is a greenhouse gas. Carbon dioxide concentrations have been rising over the past century and this is leading to permanent and potentially damaging alterations to the earth’s climate system – a process called climate change. Oxygen is the gas that almost all organisms require for their respiration.
In many tropical regions, forests protect the sometimes violent tropical storms from hitting the ground. When forest cover is removed, rainfall hits the soil much harder and this can lead to loss of topsoil in a process called soil erosion. As the water runs through the soil, it will dissolve minerals as it goes, thus leaving the soil that is left denuded of essential minerals for plant growth. This leaching of minerals makes it difficult to use the land cleared for agriculture and so more forest is cleared.
Deforestation also disrupts the water cycle. Trees move large volumes of water a year from the soil into the atmosphere in a process called transpiration. So when trees are lost, less water evaporates from the soil, more water is lost in run-off and so rainfall can be reduced.
The diagram below shows a before and after explanation of how the water cycle is disrupted. (Evapotranspiration is a term for the total water evaporated from a piece of land, combining evaporation directly from the ground and transpiration lost from plants)
There is a final problem with deforestation although the examiners have omitted it from the specification for some inexplicable reason. Forests provide a habitat for a wide variety of animal and plant species. So when forests are lost, species become extinct. This loss of biodiversity is a final terrible consequence of deforestation. 80% of known species live in tropical rainforest so the fact that in the last 50 years, over half of this area has been cleared is a major concern. The rate of loss of rainforest is around 140,000 square kilometres a year although in some parts of the world, the rate of loss is slowing.
Deforestation is a complex issue and a GCSE revision blog like this is not the place to go into the interesting political and cultural details. I would direct you to the WWF site for more information and indeed some ideas as to what we can do to help.
All animals are heterotrophic. This means that they cannot make their own food molecules but need to get them from some external source. Humans get a variety of different food molecules from what they eat. Diet is a term for what an animal eats (and in a biological context has no associations with any attempt to lose weight or change body shape). A balanced diet is a combination of foods that provides the correct proportions of all the various food molecules for any particular individual at any particular stage of their life.
Components of a Balanced Diet
Carbohydrates are a family of molecules that includes sugars, starch and other polysaccharides. They contain C,H and O atoms only and their main function in the diet is to provide molecules that can be respired to release energy for cells. Carbohydrates are thus one of the main respiratory substrates in our diet. All sweet foods will contain sugars of course and starch-rich foods are vegetables like potatoes, pasta and rice. Starch is a polymer of glucose and so needs to be digested to glucose because it is too large a molecule to be absorbed in the small intestine.
Proteins are a family of macromolecules needed to build new cells and thus for growth. Like starch, Proteins are also polymers and thus get digested into their constituent monomers, in this case amino acids in the digestive system. Protein-rich foods include all meat and some pulses and beans. Proteins in the diet are needed to build muscle tissue, to form some components of cell membranes and to make all the enzymes that catalyse all the metabolic reactions in cells.
Lipid is a general term for all fats and oils. Despite the popular misconception that fat is “bad” in our diet, in fact lipids are essential molecules in the diet. We need lipids as a respiratory substrate, for long term energy storage in adipose tissue under the skin and for the electrical insulation of nerve cells. Foods rich in lipids are red meat, many processed foods, and food containing olive oil or other vegetable oils.
Humans need a wide variety of mineral ions in very low concentrations in our diet. The most important mineral in our diet is Calcium which is needed for making healthy teeth and bones. Iron is also needed in relatively high amounts as it is required to make the protein haemoglobin found in red blood cells. Mineral ions come from eating a wide variety of foods, but the main source of calcium is from milk and other dairy products. Iron is found in high concentrations in red meat.
Rather like minerals, vitamins are needed in very small amounts in a diet but are absolutely crucial for the healthy functioning of the body. The diseases associated with a lack of a particular vitamin in the diet are called deficiency diseases. You need to know about three vitamins – A. C and D Vitamin A is a molecule called retinal found in carrots, red peppers and swede. It is needed for healthy growth and a functioning immune system. Vitamin A is also essential for normal vision since it is used to make the pigment found in rod cells in the retina. Vitamin A deficiency in the diet often causes poor vision, especially at night. Vitamin C is needed for the enzyme that produces the protein collagen in the body. It is found in all fruit especially citrus fruits. A lack of vitamin C causes the deficiency disease scurvy. Vitamin D is an unusual vitamin since it can be made in the skin using UV light. Vitamin D is needed in the small intestine to absorb mineral ions such as calcium, magnesium, etc. into the blood. A lack of vitamin D often results in a deficiency disease called rickets in which the bones malform.
Dietary Fibre is actually made up from the molecule cellulose. No mammal including humans possesses a cellulase enzyme and so when plant material passes through the intestines, dietary fibre is never digested. This means it passes into the large intestine where it helps prevent constipation. Foods rich in fibre included wholegrain bread, vegetables and some breakfast cereals.
Water is the final component of a balanced diet. It is needed to replace water lost by sweating and in urine and acts as a solvent of course for all the metabolic reactions that happen in every cell.
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……
I am wary of writing a post about the water cycle as I so rarely teach it. It seems too much like common sense to me to require any elaboration in class, but perhaps writing this post will sooth my guilty conscience for Y10 and Y11 students?
The processes that happen in the water cycle are almost all nothing to do with Biology. Water evaporates from lakes, streams and the sea. Evaporation is when thermal energy from the sun changes water from a liquid to the vapour state. The warmer the day, the more evaporation will occur. The biological component here is that water evaporates from the above ground parts of a plant. This process is called transpiration and mostly happens through the stomata (tiny pores in the lower epidermis of the leaves). Geographers like to combine “transpiration” with the “evaporation” of water direct from the soil to come up with the exciting term “evapotranspiration”. Water vapour condenses in the atmosphere to form clouds and then water falls as a liquid as rain/snow/hail which can be combined together as precipitation.
That’s the water cycle for you: couldn’t be much simpler really, could it?
Just to finish, check your A* understanding of transpiration by answering these questions – if you are feeling really digital, why not add the answers as a comment at the foot of this post?
1) When are stomata open in the leaf and when do they close?
2) What four environmental factors can speed up rates of transpiration?
3) What is the name of the experimental set up that can be used to measure transpiration rates? (Does it actually measure transpiration rate or does it really measure something else entirely?)
4) In what ways would you think of transpiration as a “necessary evil”?