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SICM Tuition Biology AS OK, so now we know a little bit about blood cells. Red cells are for transport, white cells are for defence against infection. But come on…that’s not enough! We want more!!! HOW do these things happen? HOW do red blood cells carry oxygen / CO2? HOW do white cells fight infection?! Very valid questions….let’s have a look… Some definitions antigen - foreign protein (on the surface of the membrane of a virus or bacteria), which stimulates the production of a specific antibody antibody - chemical substance produced by a B-lymphocyte in response to exposure to a specific antigen. Phagocytes Phagocytes are white blood cells, which can engulf and digest pathogens (as well as other foreign materials and dead or infected cells). They are non-specific in their actions and so will deal with any type of foreign material, which they come across. They have a distinctive appearance with a lobed nucleus and a grainy cytoplasm. Phagocytes are mainly found in the blood and the lymphatic system, especially in the lymph nodes. They are also capable of squeezing through tiny gaps between cells in the walls of capillaries and entering the tissue fluid, which surrounds every cell. granular cytoplasm lobed nucleus Phagocytosis – phagocytes engulfing/digesting bacteria phagocyte (nucleus not shown) bacterium giving out chemical lysosomes (containing powerful digestive enzymes) vacuole forming bacterium is trapped and some lysosomes move towards the vacuole and fuse with it. the bacterium is broken down and passes into the cytoplasm of the phagocyte Lymphocytes Lymphocytes are white blood cells, which are involved in very specific immune responses against pathogens. They do not engulf and digest pathogens, but use other, more complicated processes to destroy them. Like phagocytes, lymphocytes circulate in the blood and lymph fluid and are also found within the lymph nodes. They have a large rounded nucleus, which almost fills the cell and a small amount of non-grainy cytoplasm. There are two main types of lymphocytes known as B-lymphocytes and T-lymphocytes. Both of these lymphocytes respond to the presence of specific types of foreign material in the body and bring about actions to remove these. Although they actually work in very different ways, the starting point for this is the recognition of the antigen. Page 5 SICM Tuition Biology AS The function of B-lymphocytes B-lymphocytes are involved in the production of antibodies in response to antigens, which is called humoral immunity. On the surface of the membrane of B-lymphocytes are a number of specific antigen receptors. These are sites to which antigens on the surface of pathogens may become attached, leading to a sequence of events in which antibodies are produced to prevent the pathogens from causing harm. There are many thousands of specific types of B-lymphocytes and each is capable of recognising only one specific antigen from a pathogen. (For example, only one type of Blymphocyte will attach to the bacterium causing Tuberculosis, another one for Cholera) This specificity is due to the slight differences in the shape of the antigen receptor on each B-lymphocyte. Sequence of events Pathogens enter the body. This may be via droplets in the air, by a vector (e.g. mosquito), via food, water or body fluids such as saliva, blood or semen. - Antigens on the surface of the pathogen come into contact with their specific antigen receptor on a B-lymphocyte. - The binding of the antigen to the antigen receptor activates the B-lymphocyte and causes it to divide producing a clone of identical B-lymphocytes. This is often known as a clonal explosion. - Most of the B-lymphocytes turn into plasma cells and the rest turn into memory cells. - The plasma cells begin to secrete antibodies against the pathogen concerned. Antibody molecules are secreted at a very high rate: up to 30 000 molecules per second. - The antibodies attach to the antigens on the pathogen and lead to the destruction of the pathogen. - Once the pathogens have been destroyed, the plasma cells eventually die and antibodies stop being secreted. But, the memory cells remain in the lymph nodes and the circulation in case of further infection with the same pathogen. The Immune response B-cell replicated many times: clonal explosion enlargement of B-cells to form plasma cells. These produce antibodies. pathogen with antigens antibodies B-cell is activated due to antigen T-lymphocytes T-lymphocytes recognise foreign antigens and help in antibody production. Others bind to the pathogen and present the antigen to the B-lymphocytes. Page 6 memory B-cells: enable rapid response following subsequent exposure to the same antigen SICM Tuition Biology AS Transport of oxygen and carbon dioxide We already know how blood transports oxygen – we did this not only at GCSE but also about 4 pages earlier…(3 pages to be exact….but that is about 4). So you WILL remember and complete the following: Red blood cells contain haemoglobin, which binds to oxygen so that it can be transported around the body. Red blood cells are adapted to this function in many ways. I can’t be bothered to write them all out, so I will refer back to page 4. But for jokes, the equation of haemoglobin binding is……… Hb + 4O2 HbO8 (oxyhaemoglobin) Carbon Dioxide Carbon dioxide is a waste gas from metabolism. *Sighs*. We would ask you for the equation, but seeing as you may get it wrong (and we don’t want to waste 10 minutes going over the equation), we’ll just ASSUME you know it. The carbon dioxide then needs to be taken to the lungs to be exhaled. It can be transported in different ways. The main way is by converting it into bicarbonate ions: CO2 + H2O → H2CO3 → H+ + HCO3Another way is to just have it dissolved in the plasma. There is one more way…*drum roll*: carbon dioxide can bind to haemoglobin. However, carbon dioxide does NOT bind to the same place as oxygen. But even though this is the case, by binding, it decreases the amount of oxygen that the haemoglobin can take. Looking at oxygen concentration at different concentrations of oxygen a dissociation curve shows the percentage saturation of a sample of haemoglobin in comparison to the partial pressure of oxygen at that point the partial pressure of oxygen (abbreviated to “p(O2)”) shows the amount of oxygen present - Take two points: A and B: 100 ‘A’ shows the partial pressure of oxygen at the lungs 80 percentage saturation 60 40 here, the haemoglobin completely saturated is almost ‘B’ shows the partial pressure at a muscle 20 B - the muscle is respiring so it takes up oxygen - there will obviously be a lower partial pressure of oxygen in a respiring tissue than in the lungs – because a lot of the oxygen has been given up to the tissue A partial pressure of oxygen Page 7 SICM Tuition S-Shape - Biology AS you may also see that in the graph above, the shape of the curve is “S-shaped” this is because each haemoglobin molecule can carry up to four oxygen molecules: o the first molecule of oxygen binds with some difficulty, but as it does, it brings about a change in the shape of haemoglobin o therefore, other oxygen molecules can bind on easier than the first o the last oxygen molecule binds on hundreds of times faster than the first The Bohr Effect the graph we saw above shows what happens then there is very little carbon dioxide present (i.e. low partial pressure of carbon dioxide: low p(CO2)) however, as the p(CO2) increases, the p(O2) decreases so an increase in p(CO2) causes the curve to shift to the right: o this is called the Bohr effect If we once again look at the two points A and B: ‘A’ – as before – shows the partial pressure at the lungs o as the p(CO2) would be very low here (as it is being removed), the p(O2) would be very high: we would be dealing with the top curve 100 80 low partial pressure of CO2 percentage 60 saturation 40 high partial pressure of CO2 20 B A partial pressure of oxygen - ‘B’ shows the partial pressure in a respiring muscle tissue o the muscle would be using the oxygen (so would have a low p(O2)) o but it would also be producing CO2 so would have a high p(CO2) o therefore we would be looking at the lower curve the effect of increasing the p(CO2) results in the haemoglobin giving up more oxygen to the tissues (as is needed in the muscle! Perfect ☺) Different sorts of haemoglobin different animals live in different places with different environments these environments all differ – they even differ in p(O2) therefore, the animals living there must be adapted to this - take the example of a seal o they have lungs, but are still able to stay under water for long periods o they are adapted to be able to do this YAY!! Colouring in!! Page 8 SICM Tuition - Biology AS They have myoglobin o myoglobin is similar to haemoglobin except for a difference in the chemical structure o myoglobin only has one subunit (not four like in haemoglobin) o this results in a different dissociation curve: 100 Myoglobin has two properties making it very useful for its function o it picks up oxygen very readily o it is saturated at very low partial pressures of oxygen o but this means that the oxygen is not given up very readily 80 myoglobin percentage 60 saturation haemoglobin 40 20 partial pressure of oxygen Foetal Haemoglobin A baby (/foetus) does not breathe in the womb. Therefore the amount of oxygen is very limited. The haemoglobin of the foetus is therefore different to the adult’s. - foetal haemoglobin has a higher affinity for oxygen (it picks it up easier) therefore the curve is shifted to the left This haemoglobin is replaced by normal haemoglobin (by the baby’s body) when the baby is born. 100 80 percentage 60 saturation Foetal haemoglobin 40 Adult haemoglobin 20 B A partial pressure of oxygen Page 9