Transport of Respiratory Gases GCE BIOLOGY BY2 Transport of Respiratory Gases

Fluid Mosaic Model of the Plasma Membrane Glycoprotein Carbohydrate chain Intrinsic protein Phospholipids Non-polar hydrophobic fatty acid chain Show/ hide labels

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?

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

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.

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

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.

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

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. Play Animation

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

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-)

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.

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.

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.

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.

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 Show hide CO2 line 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

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

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. Show/ hide titles Show/ hide scale 2 Show/ hide line 1

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?

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

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. 4.8

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: 4.8

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. 4.8

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. 4.8