Topic 3B - More Exchange And Transport Systems Flashcards
Why is food broken down into smaller molecules during digestion?
The large biological molecules like starch and proteins are too big to cross cell membranes. This means they can’t be absorbed from the gut into the blood.
What happens during digestion?
The large molecules are broken down into smaller molecules which can move across cell membranes. This means they can be easily absorbed from the gut into the blood, to be transported around the body for use by the body cells.
What are carbs broken down into?
Into disaccharides then monosaccharides.
What enzymes are used in digestion?
Different digestive enzymes are produced by specialised cells in the digestive system of mammals. These enzymes are then released into the gut to mix with food.
How are carbs broken down?
Amylase is a digestive enzyme that catalyses the conversion of starch into the smaller sugar maltose (a disaccharide). This involves the hydrolysis of glycosidic bonds in starch. Amylase is produced by the salivary glands (which release amylase into the mouth) and by the pancreas (which releases amylase into the small intestine). Membrane-bound disaccharidases are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine). They help to break down disaccharides (like maltose, sucrose and lactose) into monosaccharides (like glucose, fructose and galactose). Monosaccharides can be transported across the cell membranes of the ileum epithelial cells via specific transporter proteins.
How are lipids broken down?
Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are made in the pancreas and work in the small intestine. Bile salts are produced by the liver and emulsify lipids - this means they cause the lipids to form small droplets. Several small lipid droplets have a bigger surface area than a single large droplet. So this greatly increases the surface area of lipid that’s available for lipases to work on. Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles.
How are proteins broken down? What are the types of enzymes?
Proteins are broken down by a combination of different proteases/peptidases. These are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids.
Endopeptidases act to hydrolyse peptide bonds within a protein. This creates more ends/ larger surface area. Trypsin and chymotrypsin are two examples of endopeptidases.
They’re synthesises in the pancreas and secreted into the small intestine. Pepsin is another endopeptidase. It’s released into the stomach by cells in the stomach lining. Pepsin only works in acidic conditions, which are provided by hydrochloric acid in the stomach. Exopeptidases act to hydrolyse peptide bonds at the ends of the protein molecules. They remove single amino acids from proteins. Dipeptidases are exopeptidases that work specifically on dipeptides. They act to separate the two amino acids that make up the dipeptide by hydrolysing the peptide bond between them. Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.
How are different products of digestion absorbed across cell membranes?
They are absorbed across the ileum epithelium into the bloodstream. Monosaccharides: glucose is absorbed by active transport with sodium ions via a Co-transporter protein. Galactose is absorbed in the same way using the same Co-transporter protein. Fructose is absorbed by facilitated diffusion through a different transporter protein.
Monoglycerides and fatty acids: micelles help to move monoglycerides and fatty acids towards the epithelium. Because micelles constantly break up and reform they can release monoglycerides and fatty acids, allowing them to be absorbed - whole micelles are not taken up across the epithelium. Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane. Amino acids: transported via Co-transport similar to glucose and galactose. Sodium ions are actively transported out of the ileum epithelial cells into the blood. This creates a sodium ion concentration gradient. Sodium ions can then diffuse from the lumen of the ileum into the epithelial cells through sodium-dependent transporter proteins, carrying the amino acids with them.
Describe the structure of haemoglobin and where they are found.
Red blood cells contain haemoglobin. It is a large protein with a quaternary structure, made up of 4 polypeptide chains. Each chain has a haem group, which contains an iron ion and gives haemoglobin its red colour.
What are the properties of haemoglobin? What is formed when haemoglobin joins with oxygen?
Haemoglobin has a high affinity for oxygen (tendency to bind with oxygen). Each molecule can carry four oxygen molecules. In the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhaemoglobin. This is a reversible reaction - when oxygen dissociates from oxyhaemoglobin, it turns back to haemoglobin.
How does haemoglobin saturation depend on the partial pressure of oxygen?
The greater the concentration of dissolved oxygen in cells, the higher the partial pressure (O2 conc). Haemoglobin’s affinity for oxygen varies depending on the partial pressure of oxygen. Oxygen loads onto haemoglobin to form oxyhaemoglobin where there’s a high pO2. Oxyhaemoglobin unloads it’s oxygen where there’s a lower pO2. Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin. When cells respire, they use up oxygen, which lower the pO2. Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen, and then returns to the lungs.
Describe a dissociation curve for partial pressure of oxygen and haemoglobin saturation.
A dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure.
Where pO2 is high (e.g lungs), haemoglobin has a high affinity for oxygen, so it has a high saturation of oxygen. Where pO2 is low (e.g respiring tissues), haemoglobin has a low affinity for oxygen, which means it releases oxygen rather than combines with it. That’s why it had a low saturation of oxygen. The graph is S shaped because when haemoglobin combines with the first O2 molecule, its shape alters in a way that makes it easier for other molecules to join too. Binding of the first oxygen changes the tertiary structure of haemoglobin, which uncovers another binding site. But as the Hb starts to become saturated, it gets harder for more oxygen molecules to join. So there is a steep bit where it’s really easy for oxygen molecules to join, and shallow parts where it’s harder. When the curve is steep, a small change in pO2 causes a big change in % saturation of haemoglobin.
How does CO2 concentration affect haemoglobin?
Haemoglobin gives up its oxygen more readily at higher partial pressures of CO2. When cells respire they produce carbon dioxide, which raises pCO2. This increases the rate of oxygen unloading, so the dissociation curve shifts right. This is because the blood pH decreases/ becomes more acidic. The saturation of blood with oxygen is lower for a given pO2, meaning that more oxygen is being released. This is the Bohr effect.
How is haemoglobin different in different organisms?
Organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher affinity for oxygen than human haemoglobin, so the dissociation curve is to the left of ours. Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than human haemoglobin, so they unload oxygen more frequently, and the curve is to the right of the human one.
Describe the circulatory system.
It is a mass transport system. Multicellular organisms, like mammals, have a low surface area to volume ratio, so they need a specialised transport system to carry raw materials from specialised exchange organs to their body cells - this is the circulatory system. The circulatory system is made up of the heart and blood vessels. The heart pumps blood through blood vessels. Blood transports respiratory gases, products of digestion, metabolic wastes and hormones round the body. There are two circuits. One circuit takes blood from the lungs from the heart to the lungs, then back to the heart. The other loop takes blood around the rest of the body. The heart has its own blood supply - the left and right coronary arteries.
How are arteries adapted?
Arteries carry blood from the heart to the rest of the body. Their walls are thick and muscular and have elastic tissue to stretch and recoil, which helps to maintain high blood pressure. The inner lining (endothelium) is folded, allowing the artery to stretch, which also helps to maintain high blood pressure. All arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs. Arteries divide into smaller vessels called arterioles. These form a network throughout the body. Blood is directed to different areas of demand in the body by muscles inside the arterioles, which contract to restrict blood flow or relax to allow full blood flow.
How are veins adapted?
Veins take blood back to the heart under low pressure. They have a wider lumen than equivalent arteries, with very little elastic or muscle tissue. Veins contain valves to stop the blood flowing backwards. Blood flow through the veins is helped by the contraction of the body muscles surrounding them. All veins carry deoxygenated blood (because oxygen has been used up by the body cells), except for the pulmonary veins, which carry oxygenated blood to the heart from the lungs.
How are capillaries adapted?
Arterioles branch into capillaries, which are the smallest of the blood vessels. Substances like glucose and oxygen are exchanged between cells and capillaries, so they’re adapted for efficient diffusion. Capillaries are always found very near cells in exchange tissues (e.g alveoli) so there’s a short diffusion pathway. Their walls are only one cell thick, shortening the diffusion pathway. There are a large number of capillaries in capillary beds to increase surface area for exchange.
