Download OK, so now we know a little bit about blood cells. Red cells are for

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.
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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.
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memory B-cells: enable rapid
response following subsequent
exposure to the same antigen
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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
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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!!
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-
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
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