Digestion. Key Term. Regional specialisations of the human gut

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Vincent Greene
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Key Term Mucus = a slimy glycoprotein, secreted by gland cells in the mucosa, forming a protective layer. It also lubricates food as it passes through the gut.

1 Key Term Mucus = a slimy glycoprotein, secreted by gland cells in the mucosa, forming a protective layer. It also lubricates food as it passes through the gut. Before food is digested it must be broken down mechanically. This takes place by means of the teeth, stomach, and by the peristaltic action of the muscular layers of the gut wall. Structure of the digestive system. liver bile duct gall bladder small intestine caecum appendix 58 mouth duodenum ileum Digestion The absorption of nutrients by the gut epithelial cells is only possible if the large molecules, carbohydrates, fats and proteins are fi rst broken down or digested into smaller products by means of enzymes. Different enzymes are required to carry out digestion of the different food substrates and usually more than one type of enzyme is needed for the complete digestion of a particular food. Carbohydrates (polysaccharides) are fi rst broken down into disaccharides and then into monosaccharides. The enzyme amylase hydrolyses starch to the disaccharide maltose but another enzyme, maltase, is required to break down the maltose to the monosaccharide glucose. Proteins are broken down into polypeptides, then dipeptides, and fi nally into amino acids. The general name given to the protein-digesting enzymes is peptidases. Proteins are extremely large molecules so endopeptidases hydrolyse peptide bonds within the protein molecule and exopeptidases hydrolyse peptide bonds at the ends of these shorter polypeptides. Fats are broken down to fatty acids and glycerol by just one enzyme, lipase. Regional specialisations of the human gut oesophagus stomach colon rectum anus The mouth Mechanical digestion begins in the mouth when food is chewed using the teeth. The food is also mixed with saliva from the salivary glands. Saliva is a watery secretion containing mucus and salivary amylase together with some mineral ions which help to keep the ph in the mouth slightly alkaline, the optimum ph for amylase. Saliva is important for lubricating food before it is swallowed. Amylase breaks down starch to maltose. After chewing, the bolus of food is swallowed and mucus lubricates its passage down the oesophagus. The stomach Food enters the stomach and is kept there by the contraction of two rings of muscles, one at the stomach entrance and one at the junction with the duodenum. Food may stay in the stomach for up to four hours and during this time the muscles of the stomach wall contract rhythmically and mix up the food with gastric juice secreted by glands in the stomach wall. Gastric juice contains acid that gives the stomach contents a ph of 2.0. As well as providing the optimum ph for the enzymes,

2 HB2: Uptake of energy and nutrients the acid kills most bacteria in the food. Gastric juice also contains peptidase enzymes which hydrolyse the protein to polypeptides. Mucus is important in forming a lining to protect the stomach wall from the enzymes and acid as well as assisting in the movement of food within the stomach. The gastric glands in the stomach wall are simple tubular structures containing peptic cells, oxyntic cells and goblet cells. Stomach wall. entrances to gastric pits Different enzymes have different ph optima and therefore function in particular areas of the gut. gastric pit peptic/zymogen/ chief cell sub-mucosa epithelial layer mucus-secreting cells oxyntic cell (secretes acid) Peptic cells (chief cells) secrete the protein-digesting enzyme, pepsin, as an inactive precursor, pepsinogen. This prevents the enzyme from damaging the stomach tissues. Pepsinogen remains inactive until it reaches the lumen of the stomach where it is activated by hydrochloric acid and the resulting pepsin begins the breakdown of proteins to polypeptides. Pepsin is prevented from damaging the stomach wall by the secretion of mucus. Oxyntic cells secrete hydrochloric acid which makes the stomach contents acid; the acid conditions kill off many pathogenic bacteria as well as activating the protein-digesting enzymes. Goblet cells secrete mucus. This forms a protective layer on the stomach wall, thus preventing pepsin and hydrochloric acid from breaking down the gastric mucosa. Mucus also helps the movement of food within the stomach. Rennin is also secreted in an inactive form by peptic cells. It is activated by acid and coagulates the soluble protein in milk. This is important in babies as it allows the semi-solid product to remain in the stomach for a longer period of time enabling digestion to take place more effectively. The small intestine The small intestine is divided into two regions, the duodenum and the ileum. Relaxation of the muscle at the base of the stomach allows the partially digested food into the duodenum a little at a time. The duodenum makes up about the fi rst 20cm of the small intestine and receives secretions from both the liver and the pancreas. Throughout this section highlight the digestion of carbohydrate, fats and proteins in three different colours. This will help you follow the digestion of the separate chemicals through the various parts of the gut. You should be able to explain how the different regions of the gut achieve different ph values, e.g. such as the walls of the duodenum secrete an alkaline juice; oxyntic cells secrete hydrochloric acid. In an exam you would not be expected to write a complete essay on the topic of digestion. Exam essays usually ask for the digestion of either proteins or carbohydrates. 59

3 The main cause of air being forced out during normal breathing (at rest) is the recoil of the elastic lungs. Be prepared to label a given diagram of the respiratory system. The human respiratory system The lungs consist of a branching network of tubes called bronchioles arising from a pair of bronchi. The lungs are enclosed within an airtight compartment, the thorax, at the base of which is a dome-shaped sheet of muscle called the diaphragm. The lungs are supported and protected by the rib cage. The ribs can be moved by the intercostals, the muscles between them. This enables the lungs to be ventilated so that air is constantly being replenished. Air is drawn into the lungs via a fl exible airway called the trachea. trachea (with rings of cartilage) Human respiratory system. rib The diffusion pathway is short as the walls of the alveoli are one cell thick and the blood capillaries have a single layer of endothelial cells. The percentage of oxygen in the alveolus is less than in inspired air because the latter mixes with air already in the lungs. This has a lower percentage of oxygen. intercostal muscles alveoli bronchioles pleural membrane Gas exchange in the alveolus The gas exchange surfaces or alveoli provide a very large surface area relative to the volume of the body. They are well suited as a gas exchange surface because the walls are thin, providing a short diffusion path. Each alveolus is covered by an extensive capillary network to maintain diffusion gradients, because blood is always taking oxygen away from the alveolus and returning with carbon dioxide. Deoxygenated blood enters the capillaries surrounding the alveolus. Oxygen diffuses out of the alveolus into the blood in the capillary. Carbon dioxide diffuses out of the capillary into the air in the alveolus. Table showing the % composition of air in the lungs. Inspired air Alveolar air Expired air Oxygen Carbon dioxide Nitrogen Water variable saturated saturated bronchus left lung diaphragm Air fl ows down towards the lungs through the trachea and bronchi. These are lined by cells, called epithelial cells, which remove particles from the air before it reaches the lungs. There are two main types of cells making up the epithelial lining ciliated cells and goblet cells. Cilia are tiny extensions of the cytoplasm that are found on the free surface of the cells. The function of the goblet cells is to secrete mucus. 66

4 HB2: Gas exchange branch of pulmonary artery branch of pulmonary vein from pulmonary artery CO 2 out O 2 in alveoli red cells in blood capillary high CO 2 low O 2 low CO 2 concentration cavity of alveolus high O 2 concentration capillary network epithelial cell of alveolus to pulmonary vein Alveoli Gas exchange in alveolus. When particles in the air, such as dust, bacteria and pollen, are breathed in they are trapped in the mucus. The cilia sweep the mucus upwards to the back of the throat. The mucus is then swallowed. This makes sure that the particles do not reach the lungs. Ventilation of the lungs Mammals ventilate their lungs by negative pressure breathing, forcing air down into the lungs. That is, if air is to enter the lungs then the pressure inside them must be lower than atmospheric pressure. Inspiration or breathing in is an active process since muscle contraction requires energy. The external intercostal muscles contract. The ribs are pulled upwards and outwards. At the same time, the diaphragm muscles contract, causing it to fl atten. Both actions increase the volume of the thorax. This results in a reduction of pressure in the lungs. As the atmospheric air pressure is now greater than the pressure in the lungs, air is forced into the lungs. The essential features of exchange surfaces are lungs which supply a large surface area, increased by alveoli, lined with moisture for the dissolving of gases, thin walls to shorten the diffusion path and an extensive capillary network for rapid diffusion and transport, to maintain diffusion gradients. Expiration or breathing out is a mainly passive process and is essentially the opposite of inspiration. Inspiration Expiration air in air out rib cage moves outwards diaphragm moves downwards rib cage moves inwards position of rib cage during inhalation diaphragm moves upwards Movement of the ribs and diaphragm during ventilation. 67

Key Terms Spirometer = device used to measure the pattern of change in lung volume during human breathing. Tidal volume = volume of air exchanged at rest.

5 Key Terms Spirometer = device used to measure the pattern of change in lung volume during human breathing. Tidal volume = volume of air exchanged at rest. Vital capacity = the total volume of air that can be expired after a maximum inspiration. Residual volume = the minimum volume of air always present in the lungs. A natural surfactant is present in the lungs. The surfactant lowers the surface tension keeping the alveoli open. kymograph rotating drum with chart paper attached Spirometer. You should understand the principles of spirometry and be able to interpret spirometer data in the form of a spirometer trace recorded on a kymograph. However, you are not required to carry out practical work using a spirometer. pen air chamber Surfactant Lining the thorax and surrounding each lung are pleural membranes, between which is a cavity containing pleural fl uid. When breathing, this fl uid acts as a lubricant, allowing friction-free movement against the inner wall of the thorax. To prevent the alveoli from collapsing when breathing out, an anti-sticking chemical, called a surfactant, covers their surfaces and reduces the surface tension. One of the most common and immediate problems facing premature babies is diffi culty in breathing. The most common cause is respiratory distress syndrome (RDS). In RDS the infant s immature lungs do not produce suffi cient surfactant. Immediately after birth and several times afterwards an artifi cial surfactant can be administered. Although, for a while, most premature babies will need to be placed in a ventilator, the use of artifi cial surfactant greatly decreases the amount of time that infants spend on the ventilator. Investigating human breathing by measuring lung capacity The spirometer consists of an air-fi lled chamber of about 6 dm 3 capacity suspended over water. The lid or fl oat of the chamber is arranged so that it rises and falls as the subject breathes in and out through the mouthpiece. A pen connected to the fl oat moves water balanced float which rises and falls during breathing container into which soda lime can be placed to absorb carbon dioxide mouthpiece over a rotating chart, recording movements against time. The movements of the chamber are recorded on a kymograph trace as shown on the next page. The spirometer chamber may be fi lled with atmospheric air that is breathed in (inspired). Soda lime is used to absorb all the carbon dioxide from the expired air before it goes back into the chamber. Human lungs have a volume of about 5 dm 3, but at rest only about 0.45 dm 3 of this will be exchanged. This is called tidal volume. With increasing activity both the frequency and depth of inspiration will increase. Vital capacity is the total volume of air that can be expired after a maximum inspiration. Even after maximum expiration, about 1.5 dm 3 of air remains in the lungs. This is called the residual volume. The rate at which a person breathes is expressed as the ventilation rate. Ventilation rate = tidal volume number of breaths per minute 68

6 HB2: Gas exchange Human lung capacity. inspiratory reserve volume vital capacity lung volume / litres dm expiratory reserve volume tidal volume functional residual capacity residual volume time Respiratory disease Lung diseases are a major cause of illness and death in society. Two disorders that affect lung function are asthma and emphysema. Asthma This condition is a reaction, usually allergic, characterised by attacks of wheezing and diffi culty in breathing. It is a common ailment in children and affects about one in twenty of the overall population. Attacks are brought on by spasms of the smooth muscles that lie in the walls of the bronchioles causing the passageways to close partially. The obstruction of these air passages means that more effort is needed to deliver suffi cient air to the lungs. Usually the mucus membranes that line the respiratory passageways become irritated and secrete excessive amounts of mucus that may clog the bronchi and bronchioles and worsen the attack. The possible causes of asthma include air pollution and allergens. The most common allergens are house dust mites, animal fur and feathers, and pollen. Drugs taken in aerosol form are prescribed as an inhalant. They give rapid relief by causing the smooth muscle lining the bronchioles to relax, so widening the airways. Steroids reduce the amount of infl ammation of the bronchioles. volume normal athlete asthma sufferer time Spirometer traces of a healthy person, a healthy athlete, and an asthma sufferer. A common error is to describe a constriction of the trachea rather than the bronchioles. By the end of their teenage years more than half the children with asthma will be relieved of the condition. Be prepared to describe the causes, symptoms and treatment of respiratory diseases. 69

7 & Q A 17 The introduction of an antigen into the blood triggers an immune response. (a) What is meant by the terms antibody and antigen? (2 marks) 10 4 (b) The Rubella virus infects human cells and causes the disease known 10 3 as German measles. The graph shows the effect of immunising 10 2 a child against Rubella. An initial injection is given and this is 10 1 followed by a booster injection four weeks later. 10 (c) (i) Use the information in the graph to describe two differences in the response to the first and second exposure to the antigen. (2 marks) Blood antibody concentration / arbitrary units first injection booster injection Time/days (ii) Explain why there is a difference in the response. (1 mark) Vaccination against smallpox has been effective in eliminating the disease but this method of protection against influenza virus has only limited success. Explain the difference. (2 marks) (WJEC HB2 June 2009) Dylan s answer Examiner commentary Dylan has failed to study the graph thoroughly before attempting the question. Although he has noted the latent period after the first injection is administered, he has not related this to the increase in antibodies produced and the greater production of antibodies in the second response. Dylan has failed to appreciate that memory cells are produced after first contact with an antigen. Dylan has no knowledge of the ability of viruses to mutate and that the smallpox virus has a low mutation rate and so produces few antigenic types. Whereas the influenza virus mutates frequently and new vaccines have to be developed to be effective against new strains of the virus. Summative comment Dylan s knowledge is limited to knowing the difference between antigens and antibodies. He is unable to analyse information supplied in the graph. His weak knowledge limits his ability to access the marks. Dylan achieves 3 out of 7 marks. Cerys s answer (a) An antigen is a non-self molecule that stimulates the production of antibodies. An antibody is a special protein called an immunoglobulin produced in response to an antigen. It is also specifi c to a particular antigen. (b)(i) After the fi rst exposure there is a delay of about 6 days before antibodies are produced but after the booster is given they are produced immediately and in far greater quantity. (ii) Memory cells which remain after the fi rst response cause a rapid response. (c) The virus causing smallpox has a low mutation rate whereas the infl uenza virus mutates frequently so there are many different strains of infl uenza virus. These are called antigenic types. The vaccine is only effective against a particular strain. Examiner commentary Cerys could have also stated that antibodies remain in the blood for longer during the secondary response. Summative comment Cerys has an excellent grasp of this topic. Cerys achieves 7 out of 7 marks. 140

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