An in-depth informative booklet on Mass Transport in animals and plants, covering topics like the circulatory system, formation of tissue fluid, transpiration and translocation. Includes practicals related to this topic, useful summary questions and the specification.
, Introduction
Why do large organisms have a transport system?
All organisms exchange materials between themselves and their environment. Multicellular organisms, like
mammals and plants, have a small surface area to volume ratio so they need a specialised mass
transport system to carry molecules between specialised exchange surfaces and cells. Mass transport
systems ensure the efficient movement of molecules through the organism.
Whether or not there is a specialised transport medium (e.g. blood), and whether it is circulated by a pump
(e.g. a heart), depends on two factors:
• The surface area to volume ratio
• How active the organism is
The smaller the surface area to volume ratio, and the more active the organism is, the greater the need
for a specialised transport system with a pump.
Mass Transport in Animals
Haemoglobin
Haemoglobin is an important part of the circulatory system in humans. Haemoglobin is found in erythrocytes
(red blood cells). Haemoglobin has evolved to make it efficient at loading oxygen under one set of conditions,
and unloading it under a different set of conditions. There are many chemically similar types of haemoglobin
found in many different organisms, all of which carry out the same function. As well as being found in all
vertebrates, haemoglobin is found in earthworms, starfish, some insects, some plants and even in some
bacteria.
Haemoglobin is a protein molecule with quaternary structure, as it contains 4 polypeptide chains, i.e. 4
subunits – 2 alpha subunits and 2 beta subunits. Each subunit contains 1 polypeptide chain and a haem
group which contains a single iron ion, Fe2+. It is this Fe2+ which has a high affinity (attraction) for oxygen.
Because there are 4 subunits, each haemoglobin molecule can bind 4 oxygen molecules when 100%
saturated. In the lungs, oxygen binds to haemoglobin to form oxyhaemoglobin. This is a reversible reaction.
2
,Key terms:
• Association / Loading = The process by which haemoglobin binds with oxygen. In humans this
takes place in the lungs.
• Dissociation / Unloading = The process by which haemoglobin releases oxygen. In humans this
takes place in the respiring tissues.
Dissociation curves
An oxyhaemoglobin dissociation curve shows how saturated the haemoglobin is with oxygen at any given
partial pressure of oxygen (pO2). Partial pressure of oxygen is a measure of oxygen concentration in the
tissues.
3
, The graph is an S shape (sigmoidal curve) because:
1. At low oxygen partial pressure, the haemoglobin does not easily bind oxygen. This is because
the haem groups are in the centre of the haemoglobin which makes it difficult for the oxygen to
bind with it. This results in a low saturation level at low oxygen partial pressures, i.e. low affinity.
2. As the oxygen partial pressure increases, the diffusion gradient into the haemoglobin
increases. This means that eventually an oxygen molecule will associate with one of the haem
groups. This results in a change in the shape of the haemoglobin molecule and makes it easier
for more oxygen molecules to associate with the other haem groups. Therefore the gradient of
the curve increases as the oxygen partial pressure does.
3. However, it is difficult for all the haemoglobin molecules to become 100% saturated even at high
oxygen partial pressures. This is because it is difficult for the last oxygen to diffuse and
associate with the fourth haem group.
Carbon dioxide concentration
Partial pressure of carbon dioxide (pCO2) is a
measure of carbon dioxide concentration in a
cell. pCO2 can affect oxygen unloading.
Haemoglobin unloads its oxygen more readily at a
higher pCO2.
When cells respire they produce carbon dioxide,
which raises pCO2. This increases the rate at which
oxyhaemoglobin dissociates to form haemoglobin
and oxygen. The dissociation curve therefore shifts
to the RIGHT. This is called the Bohr Effect.
The Bohr effect therefore results in more
oxygen being released when more carbon
dioxide is being produced. This means that
when exercising the muscles can be supplied
with more oxygen for continued aerobic
respiration.
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