3.4 - Mass transport

Describe the structure of haemoglobin

Globular, water soluble. Consists of four
polypeptide chains, each carrying a
haem group (quaternary structure).

Describe the role of haemoglobin.

Present in red blood cells. Oxygen
molecules bind to the haem groups and
are carried around the body to where
they are needed in respiring tissues.

Name three factors affecting
oxygen-haemoglobin binding.

1. Partial pressure/concentration of oxygen.
2. Partial pressure/concentration of carbon
   dioxide.
3. Saturation of haemoglobin with oxygen.

How does partial pressure of oxygen
affect oxygen-haemoglobin binding?

As partial pressure of oxygen increases, the
affinity of haemoglobin for oxygen also
increases, so oxygen binds tightly to
haemoglobin. When partial pressure is low,
oxygen is released from haemoglobin.

How does partial pressure of carbon
dioxide affect oxygen-haemoglobin
binding?

As partial pressure of carbon dioxide increases, the
conditions become acidic causing haemoglobin to
change shape. The affinity of haemoglobin for
oxygen therefore decreases, so oxygen is released
from haemoglobin. This is known as the Bohr effect.

How does saturation of haemoglobin
with oxygen affect oxygen-haemoglobin
binding?

It is hard for the first oxygen molecule to bind. Once
it does, it changes the shape to make it easier for
the second and third molecules to bind, known as
positive cooperativity. It is then slightly harder for
the fourth oxygen molecule to bind because there is
a low chance of finding a binding site.

Explain why oxygen binds to
haemoglobin in the lungs.

● Partial pressure of oxygen is high.
● Low concentration of carbon dioxide in the lungs,
so affinity is high.
● Positive cooperativity (after the first oxygen
molecule binds, binding of subsequent molecules
is easier).

Explain why oxygen is released from
haemoglobin in respiring tissues.

● Partial pressure of oxygen is low
● High concentration of carbon dioxide
in respiring tissues, so affinity
decreases.

What do oxyhaemoglobin dissociation
curves show?

Saturation of haemoglobin with oxygen
(in %), plotted against partial pressure of
oxygen (in kPa). Curves further to the left
show the haemoglobin has a higher
affinity for oxygen.

How does carbon dioxide affect the
position of an oxyhaemoglobin
dissociation curve?

Curve shifts to the right because
haemoglobin's affinity for oxygen has
decreased.

Name some common features of a
mammalian circulatory system

1. Suitable medium for transport, water-based to
   allow substances to dissolve.
2. Means of moving the medium and maintaining
   pressure throughout the body, such as the heart.
3. Means of controlling flow so it remains
   unidirectional, such as valves

Draw a diagram of the human heart,
including names of chambers, vessels,
and valves.

Relate the structure of the chambers to
their function.

● Atria: thin-walled and elastic, so they can
stretch when filled with blood
● Ventricles: thick muscular walls pump blood
under high pressure. The left ventricle is
thicker than the right because it has to pump
blood all the way around the body.

Relate the structure of the vessels to
their function.

● Arteries have thick walls to handle high pressure
without tearing, and are muscular and elastic to
control blood flow.
● Veins have thin walls due to lower pressure,
therefore requiring valves to ensure blood doesn't
flow backwards. Have less muscular and elastic
tissue as they don't have to control blood flow

Why are two pumps (left and right)
needed instead of one?

To maintain blood pressure around the whole body.
When blood passes through the narrow capillaries of
the lungs, the pressure drops sharply and therefore
would not be flowing strongly enough to continue
around the whole body. Therefore it is returned to the
heart to increase the pressure.

Describe what happens during cardiac
diastole.

The heart is relaxed. Blood enters the atria,
increasing the pressure and pushing open the
atrioventricular valves. This allows blood to
flow into the ventricles. Pressure in the heart
is lower than in the arteries, so semilunar
valves remain closed.

Describe what happens during atrial
systole

The atria contract, pushing any
remaining blood into the ventricles.

Describe what happens during
ventricular systole.

The ventricles contract. The pressure
increases, closing the atrioventricular
valves to prevent backflow, and opening
the semilunar valves. Blood flows into
the arteries.

Name the nodes involved in heart
contraction and where they are situated.

● Sinoatrial node (SAN)= wall of right
atrium.
● Atrioventricular node (AVN)= in
between the two atria.

What does myogenic mean?

The heart's contraction is initiated from
within the muscle itself, rather than by
nerve impulses.

Explain how the heart contracts.

● SAN initiates and spreads impulse across the
atria, so they contract.
● AVN receives, delays, and then conveys the
impulse down the bundle of His.
● Impulse travels into the Purkinje fibres which
branch across the ventricles, so they contract
from the bottom up.

Why does the impulse need to be
delayed?

If the impulse spread straight from the
atria into the ventricles, there would not
be enough time for all the blood to pass
through and for the valves to close.

How is the structure of capillaries suited
to their function?

● Walls are only one cell thick

short diffusion pathway.
● Very narrow, so can permeate tissues and red blood
cells can lie flat against the wall, effectively delivering
oxygen to tissues.
● Numerous and highly branched, providing a large
surface area.

What is tissue fluid

A watery substance containing glucose,
amino acids, oxygen, and other
nutrients. It supplies these to the cells,
while also removing any waste materials.

How is tissue fluid formed?

As blood is pumped through increasingly
small vessels, this creates hydrostatic
pressure which forces fluid out of the
capillaries. It bathes the cells, and then
returns to the capillaries when the hydrostatic
pressure is low enough.

How is water transported in plants?

Through xylem vessels

long, continuous
columns that also provide structural
support to the stem.

Explain the cohesion-tension theory.

Water molecules form hydrogen bonds with
each other, causing them to 'stick' together
(cohesion). The surface tension of the water
also creates this sticking effect. Therefore as
water is lost through transpiration, more can be
drawn up the stem.

What are the three components of
phloem vessels?

● Sieve tube elements= form a tube to transport
sucrose in the dissolved form of sap.
● Companion cells= involved in ATP production for
active loading of sucrose into sieve tubes.
● Plasmodesmata= gaps between cell walls where
the cytoplasm links, allowing substances to flow

Name the process whereby organic
materials are transported around the
plant.

Translocation

How does sucrose in the leaf move into
the phloem?

Sucrose enters companion cells of the phloem
vessels by active loading, which uses ATP and
a diffusion gradient of hydrogen ions. Sucrose
then diffuses from companion cells into the
sieve tube elements through the
plasmodesmata.

How do phloem vessels transport
sucrose around the plant?

As sucrose moves into the tube elements, water potential
inside the phloem is reduced. This causes water to enter
via osmosis from the xylem and increases hydrostatic
pressure. Water moves along the sieve tube towards
areas of lower hydrostatic pressure. Sucrose diffuses into
surrounding cells where it is needed.

Give evidence for the mass flow
hypothesis of translocation.

● Sap is released when a stem is cut, therefore there
must be pressure in the phloem.
● There is a higher sucrose concentration in the
leaves than the roots.
● Increasing sucrose levels in the leaves results in
increased sucroses in the phloem.

Give evidence against the mass flow
hypothesis of translocation.

● The structure of sieve tubes seems to hinder mass flow.
● Not all solutes move at the same speed, as they would in
mass flow.
● Sucrose is delivered at the same rate throughout the
plant, rather than to areas with the lowest sucrose
concentration first.

How can ringing experiments be used to
investigate transport in plants?

The bark and phloem of a tree are removed in a ring,
leaving behind the xylem. Eventually the tissues
above the missing ring swells due to accumulation of
sucrose as the tissue below begins to die. Therefore
sucrose must be transported in the phloem.

How can tracing experiments be used to
investigate transport in plants?

Plants are grown in the presence of radioactive
CO 2 , which will be incorporated into the plant's
sugars. Using autoradiography, we can see
that the areas exposed to radiation correspond
to where the phloem is.