1. Transport of Respiratory Gases
GCE BIOLOGY BY2
Transport of Respiratory Gases

2. Fluid Mosaic Model of the Plasma Membrane
Glycoprotein
Carbohydrate chain
Intrinsic protein
Phospholipids
Non-polar hydrophobic fatty acid chain
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3. One of the functions of the plasma membrane is to control the movement of substances in and out of cells. Why does this plasma membrane need to be semi-permeable?

4. What’s the connection?
What is the connection between the following structures?
Hint: What do cells need to produce energy?
Alveolus
Red blood cells
Capillary
Small intestine
Lungs

5. Closed Circulatory System
Humans have a closed circulatory system consisting of the heart, arteries, arterioles (narrow, thin walled arteries), capillaries, venules (small veins) and veins. The following diagram shows the relationship between these vessels and illustrates the movement of blood within them.

6. The path of blood from an artery, to a capillary and a vein

7. Blood in the capillaries
Once the blood reaches the capillaries, it is in contact with the endothelium (the lining of the capillary) which is one cell thick. The plasma membrane of these cells are adapted for controlling the exchange of substances across them, as we will see in the following slides.

8. Membrane Permeability
Plasma membranes are semi-permeable – this means that some substances can pass through whilst others cannot.
What determines which substances pass through this membrane?
A substance has to be very soluble in the oily phospholipid bilayer. Steroid hormones, oxygen and carbon dioxide are examples of such molecules.
Plasma membrane
SOLUBLE
steroid hormone
oxygen
carbon dioxide
INSOLUBLE
glucose
protein
lipid
Click on the molecules to start their paths of motion

9. Exchange of gases across a capillary
In the following animation, we zoom into an alveolus and enter the capillary that surrounds it.
Once inside the capillary, we will follow the exchange of gases that occurs in the red blood cell.
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10. This reaction is catalysed by the enzyme carbonic anhydrase.
Formation of hydrogen carbonate and the transport of carbon dioxide in the blood
Tissue cells
First carbon dioxide (CO2) diffuses into the red blood cells where it is converted into carbonic acid (H2CO3).
This reaction is catalysed by the enzyme carbonic anhydrase.
CO2
H2CO3
Diffusion
Endothelium of capillary
Red blood cell

11. Gas exchange CO2 H2CO3 CO2 H+ + HCO3_ H2CO3 H+ + HCO3_
Tissue cells
Endothelium of capillary
H2CO3
CO2
H+ + HCO3_
H2CO3
Red blood cell
Gas exchange
H+ + HCO3_
Carbonic acid dissociates, forming protons (H+) and hydrogencarbonate ions (HCO3-)

12. The hydrogencarbonate ions diffuse out of the cell.
Tissue cells
Endothelium of capillary
H2CO3
Red blood cell
H+ + HCO3_
Diffusion into plasma
HCO3_
The hydrogencarbonate ions diffuse out of the cell.
They are transported in solution in the plasma.

13. This process is called the chloride shift.
Tissue cells
Endothelium of capillary
H2CO3
Red blood cell
Gas exchange
H+ + HCO3_
Chloride shift
Diffusion into plasma
Cl_
HCO3_
Chloride ions (Cl_) diffuse inwards from the plasma to maintain electrical neutrality.
This process is called the chloride shift.

14. Gas exchange Diffusion 4O2 HHb H+ + HCO3_ HbO8
Tissue cells
Diffusion
Endothelium of capillary
4O2
Oxygen unloaded
HHb
Red blood cell
H+ + HCO3_
HbO8
Gas exchange
The proteins left inside the cell are mopped up by haemoglobin to form haemoglobinic acid (HHb). This forces the haemoglobin to release its oxygen load, hence the Bohr shift.

15. Diffusion 4O2 HHb H+ + HCO3_ HbO8
Tissue cells
Diffusion
Endothelium of capillary
4O2
Oxygen unloaded
HHb
Red blood cell
H+ + HCO3_
HbO8
By taking up excess protons haemoglobin is acting as a buffer. This is important in preventing the blood from becoming too acidic.

16. The Effect of CO2 on the Oxygen Dissociation Curve
How much oxygen is transported by a molecule of haemoglobin also depends on partial pressure of carbon dioxide. 2 4 6 8 10 12 20 40 60 80
100
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CO2
Saturation of Haemoglobin / %
From the graph we see that at high partial pressures of carbon dioxide, the oxygen dissociation curve shifts to the right.
This is called Bohr’s shift.
Higher partial pressure of carbon dioxide increases the dissociation of oxyhaemoglobin
Partial Pressure of Oxygen/ kPa

17. O2 O2
Haemoglobin
This diagram shows how a model of haemoglobin reaches saturation with oxygen.
haemoglobin
The molecule is now saturated. O2 O2

18. Oxygen Dissociation Curve
Click on the numbered sections on the graph for an explanation of what happens at each stage
Oxygen Dissociation Curve
Saturation of Haemoglobin / %
Partial Pressure of Oxygen/ kPa 2 4 6 8 10 12 20 40 60 80
100 5 4 3
The red blood cells transport the oxygen to respiring tissues. The partial pressure of oxygen in these tissues is low, as the oxygen is being used for respiration.
Under conditions of a lack of oxygen (low partial pressure of oxygen), the haemoglobin yields its oxygen to the respiring cells – we refer to this as dissociation.
Haemoglobin’s properties allow it to bind with a lot of oxygen at a high partial pressure of oxygen, but at low partial pressure, only a limited amount binds to it.
The partial pressure of oxygen is high and the haemoglobin reaches saturation.
The red blood cells collect oxygen in the capillaries surrounding the lungs.
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19. On the next slide we will see the path that the products of digestion take as they diffuse from the small intestine into the blood capillaries surrounding the gut. The cells must rely on more than diffusion to get all the glucose, amino acids and fatty acid and glycerol into the blood. What form of transport do you think is used?

20. Absorption of digested food from the small intestine
Click on the box to magnify the view

21. Active Transport
This is the movement of substances against a concentration gradient (from a region of low concentration to a region of higher concentration) across a plasma membrane. This process requires energy.
This energy is provided by mitochondria in the form of ATP and cells performing active transport on a large scale contains numerous mitochondria.
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22. How does Active Transport work?
Active transport depends on proteins in the cell membrane to transport specific molecules or ions. These can move. These carriers can move:
i) one substance in one direction (uniport carriers)
ii) two substances in one direction (symport carriers
iii) two substances in opposite directions (antiport carriers)
The exact mechanism of active transport is unclear. Here are two hypotheses:
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23. Cotransport Hypotheses
Sucrose movement in glucose storing cells in a plant.
Show animation
Sucrose
Symport carrier
Proton pump H+
Sucrose
Here the process of pumping protons drives sucrose transport in a plant cell. A pump using ATP as an energy source drives protons out of the cell, as they diffuse back into the cell, sucrose in this case is transported at the same time across a symport carrier.
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24. Another Hypothesis
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ATP
Na+ K+
ATP
Na+ K+
ATP
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ATP
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
Na+ K+
ADP P-
This hypothesis suggests that one protein molecule changes its shape in order to transport solutes across a membrane.
As ATP is hydrolysed to ADP to release energy for the process, ADP binds to the protein and changes its shape.
A sodium-potassium pump is an example of this. These pumps are vital in order to generate impulses in nerve cells.
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