3B More exchange systems

• Why do we need digestion?

  Large bilogical moleucles are too big to cross cell membranes so they are digested into smaller molecules that can be easily absorbed and transported around the body.
• How are carbohydrates broken down?

  Amylase is produced by the salivary glands and small intestine and catalyses a hydrolysis reaction that breaks the glycosidic bonds in starch, converting them to disaccharides (maltose).

  
  

  These disaccharides are then broken down further by enzymes attached to the cell membrane of epithelial cells lining the ilium. These enzymes catalyse the hydrolysis of glycosidic bonds between disaccharides to form monosaccharides.
• How are lipids broken down?

  Lipases made in the pancreas are secreted into the small intestine and work by hydrolysing the ester bond in lipids to form monoglycerides.

  
  

  This is aided by bile salts produced in the liver which emulsify the lipids creating a higher surface area to volume ratio.
• How are proteins broken down?

  A combination of different proteases work to hydrolyse the peptide bond and turn proteins to amino acids.

  
  

  Endopeptidases hydrolyse bonds within a protein.

  
  

  e.g. trypsin and chymotrypsin which are synthesised in the pancreas and secreted into the small intestine.

  
  

  E.g. pepsin which is released into the stomach by stomach lining cells, it only works in acidic conditions e.g. HCl in the stomach.

  
  

  Exopeptidases hydrolyse bonds at the end of protein molecules.

  
  

  e.g. dipeptidases which work specifically on dipeptides. They are found on the cell surface membrane of epithelial cells in the small intestine.
• They are absorbed across the ileum epithelium into the blood stream.
• How are monosaccharides absorbed into the blood.

  glucose and galactose are absorbed by active transport with sodium ions via a cotransporter protein.

  
  

  Fructose is absorbed by facilitated diffusion through a transporter protein.
• How are monoglycerides and fatty acids absorbed into the blood?

  Once the lipid has been hydrolysed, the monoglycerides and fatty acids stick with the bile salts to form micelles whichmove these towards the epithelium, they constantly break and reform so release the products at the epithelium. They can diffuse across the epithelial cell membrane because they are lipid soluble.
• How are amino acids absorbed into the blood?


  sodium ions are actively transported out of ileum epithelial cells into the blood causing a concentration gradient. This allows sodium ions to diffuse from the lumen of the ileum into the epithelial cells through sodium-dependant transporter proteins, carrying amino acids with them.
• Where is haemoglobin found?

  red blood cells
• What is the function of haemoglobin?

  transporting oxygen around the body
• What is the structure of haemoglobin?

  Large globular protein with a quaternary structure. Made from four polypeptide chains each containing a haem group with iron.

  
  

  Each molecule can carry 4 oxygen molecules (8 atoms).
• How does haemoglobin move oxygen around the body?

  In the lungs oxygen associates with haemoglobin to form oxyhemoglobin and dissociates when near body cells.
• Write the reaction for oxyhemoglobin.

  Hb + 4O2 =(reversible) HbO8
• What is affinity for oxygen?

  The tendency for a molecule to bind with oxygen.
• What is PO2?

  partial pressure of oxygen ( a measure of oxygen concentration)
• What is the effect of a high PO2 on haemoglobin?

  High PO2 means a high concentration of dissolved oxygen, means a higher affinity of haemoglobin for oxygen.
• How does PO2 affect when oxyhaemoglobin forms?

  Oxygen associates onto haemoglobin when there is a high PO2 and dissociates when it is low.
• How does haemoglobin deliver oxygen to respiring cells?

  Oxygen enters the capillaries at the alveoli and due to a high PO2, it associates onto haemoglobin. Red blood cells then flow through the body and when they reach respiring cells where the PO2 is low, they dissociate the oxygen and return to the alveoli.
• Describe the association of oxygen to hameoglobin as saturation increases.

  Due to the shape of the molecule it is difficult for the first O2 molecule to bind and binding is slow.

  
  

  After the first O2 has binded, the protein changes shape and it is easier and quicker for the next O2 to bind. This is called cooperative binding.

  
  

  The third O2 binds similarly.

  
  

  As saturation increases, it is harder and slower for the last O2 to bind due to the lack of available binding sites.
• What is the Bohr effect?

  PCO2 affects oxygen dissociation from haemoglobin.
• How does PCO2 affect oxygen dissociation in haemoglobin?

  Haemoglobin dissociates oxygen more readily at higher PCO2.
• Why does more oxygen dissociate at respiring cells?

  respiring cells produce CO2 causing a higher PCO2. This results in more oxygen dissociating into the cells and a reduced saturation.
• How does haemogobin in organisms vary?

  Depending on the environment, activity level, and size of the organism.
• How does haemoglobin vary in organisms living in low O2 environments compared to humans?

  Hb has a higher affinity for oxygen so dissociation curve lies left to humans.
• How does haemoglobin vary in organisms living with high activity levels compared to humans?

  Hb has a lower affinity for oxygen as it need to easily dissociate for use, so dissociation curve lies right of humans.
• How does haemoglobin vary in organisms with small size (SA:V) compared to humans?

  Hb has a low affinity for oxygen as it needs to dissociate easily to keep up with the high metabolic rate.
• Describe the flow of blood in the heart.

  Blood comes into the right atrium from the body, moves into the right ventricle and is pushed into the pulmonary arteries in the lungs. After picking up oxygen, the blood travels back to the heart through the pulmonary veins into the left atrium, to the left ventricle and out to the body's tissues through the aorta.
• What are the two circulatory systems in mammals?

  Pulmonary circulation - transports blood between heart and lungs.

  
  

  Systematic circulation - transports blood between the heart and body.
• What does the pulmonary artery do?

  Carries de-oxygenated blood from the heart to the lungs
• What does the pulmonary vein do?

  Carries oxygenated blood from the heart to lungs.
• What does the aorta do?

  Carries oxygenated blood from the heart to the body
• What does the vena cava do?

  Carries deoxygenated blood from the body to the heart
• What does the renal artery do?

  carries oxygenated blood to the kidneys
• What does the renal vein do?

  Carries filtered blood away from the kidney to the vena cava.
• What is transported in the blood?

  Metabolic wastes, hormones, products of digestion, respiratory gases.
• what do arteries do?

  Carry high pressure blood from the heart to the body.

  
  

  All of the blood is oxygenated apart from the blood carried by the pulmonary artery.
• What adaptations do arteries have to aid their function?

  Thick muscular walls with elastic fibres to maintain pressure, and the endothelium is folded to maintain pressure.
• What are arterioles?

  Small vessels divided from arteries that direct blood to areas in high demand.
• How are arterioles adapted to their function?

  contract to restrict the blood flow or relax to allow full blood flow.
• What are capillaries?

  Smaller vessels divided from arterioles that exchange substances with cells.