Essential idea: A balanced diet is essential to human health.
D.1 Human nutrition
UNDERSTANDINGS:
UD1.1.1 Essential nutrients cannot be synthesized by the body, therefore they have to be included in the diet.
Essential nutrients:
An essential nutrient is a nutrient required for normal physiological function that cannot be replaced or synthesised by the body (e.g. niacin, choline), and thus must be obtained from a dietary source, e.g. water, minerals, some vitamins, some unsaturated fatty acids and some amino acids (10 out of 20)
Nonessential nutrients:
Nonessential nutrients are those nutrients that can be made by the body; they may often also be absorbed from consumed food however are not necessary in a diet, e.g. carbohydrates, sugars, some vitamins and minerals, some unsaturated fatty acids and some amino acids
UD1.1.2 Dietary minerals are essential chemical elements.
Essential nutrients include:
Minerals are often found in ionic form and are essential for our diet - if minerals are lacking in our diet, it results in a deficiency disease. For example, without the mineral iodine, the thyroid is continuously stimulated to produce hormones resulting in goiter.
UD1.1.3 Vitamins are chemically diverse carbon compounds that cannot be synthesized by the body.
Vitamins have various purposes:
- co-factors for enzymes
- antioxidants
- hormones
UD1.1.4 Some fatty acids and some amino acids are essential.
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism, but must be supplied in its diet. The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.
The term "essential fatty acid" refers to fatty acids required for biological processes but does not include the fats that only act as fuel. Essential fatty acids should not be confused with essential oils, which are "essential" in the sense of being a concentrated essence.
Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).
Some other fatty acids are sometimes classified as "conditionally essential," meaning that they can become essential under some developmental or disease conditions; examples include docosahexaenoic acid (an omega-3 fatty acid) and gamma-linolenic acid (an omega-6 fatty acid).
UD1.1.5 Lack of essential amino acids affects the production of proteins.
Protein cannot be made if there is a shortage of one or more essential amino acids - this is known as protein deficiency malnutrition and can come from a low intake of protein in the diet or an imbalance of the types of protein consumed. PDM can cause a lack of blood plasma proteins which can result in tissue fluid retention.
Kwashiorkor, also known as “edematous malnutrition” because of its association with edema (fluid retention), is a nutritional disorder most often seen in regions experiencing famine. It is a form of malnutrition caused by a lack of protein in the diet. People suffering from kwashiorkor typically have an extremely emaciated appearance in all body parts except their ankles, feet, and belly, which swell with fluid.
Kwashiorkor is rarely found in the United States and other countries with a generally steady food supply. It’s most common in sub-Saharan Africa and other countries where people routinely have a limited supply of food.
The extreme lack of protein causes an osmotic imbalance in the gastrointestinal system causing swelling of the gut diagnosed as an edema or retention of water.
UD1.1.6 Malnutrition may be caused by a deficiency, imbalance or excess of nutrients in the diet.
Malnutrition comes from an unbalanced diet, where the consumption of nutrients is low, lacking or there is an excess comsumption of fats and refined carbohydrates. Starvation and obesity are examples of malnutrition.
UD1.1.7 Appetite is controlled by a centre in the hypothalamus.
Appetite is controlled by hormones (leptin, insulin, CCK) produced in the pancreas, small intestine and adipose tissue. These send messages to the appetite control centre of the brain (hypothalamus), causing a person to feel that they have eaten enough (satiety). The release of hormones can be triggered in a number of ways:
UD1.1.8 Overweight individuals are more likely to suffer hypertension and type II diabetes.
Hypertension is high blood pressure. Obese individuals have more fatty tissue that causes an increase in their vascular resistance and thus increases the 'work' the heart has to do in order to pump blood around the body. Type II diabetes is the body's inability to react to insulin or a resistance to insulin.
UD1.1.9 Starvation can lead to breakdown of body tissue.
1. Body uses glycogen stores
2. Body begins to break down its own tissue - uses amino acids as energy source
3. Body breaks down its muscle mass as a last resort
APPLICATION:
AD1.1.1 Production of ascorbic acid by some mammals, but not others that need a dietary supply.
- Ascorbic acid is a water-soluble vitamin that is not stored by the human body
- Vitamin C must be obtained by the body in sufficient quantities from the consumption of vitamin C-rich foods, including citrus fruits, cabbage and tomatoes
- Vitamin C is critical for the growth and repair of tissues in all areas of the body
- It assists the body in the production of collagen, an important protein used to form skin, cartilage, tendons, ligaments and blood vessels
- Vitamin C is required for healing wounds as well as repairing and maintaining bones and teeth
- It also allows the body to absorb iron from non-heme sources
- Vitamin C is an antioxidant, along with vitamin E, beta-carotene and many other plant-based nutrients
- Antioxidants protect the body against the destructive effects of free radicals - they neutralise free radicals by donating one of their own electrons, ending the electron-"stealing" reaction
- Free radical damage accumulates with age and can thus contribute to the development of health conditions such as cancer, heart disease and arthritis.
Humans do not produce Vitamin C due to a mutation in the GULO (gulonolactone oxidase) gene, which results in the inability to synthesise the protein. Normal GULO is an enzyme that catalyses the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone.
AD1.1.2 Cause and treatment of phenylketonuria (PKU).
Causes and Consequences of Phenylketonuria:
UNDERSTANDINGS:
UD1.1.1 Essential nutrients cannot be synthesized by the body, therefore they have to be included in the diet.
Essential nutrients:
An essential nutrient is a nutrient required for normal physiological function that cannot be replaced or synthesised by the body (e.g. niacin, choline), and thus must be obtained from a dietary source, e.g. water, minerals, some vitamins, some unsaturated fatty acids and some amino acids (10 out of 20)
Nonessential nutrients:
Nonessential nutrients are those nutrients that can be made by the body; they may often also be absorbed from consumed food however are not necessary in a diet, e.g. carbohydrates, sugars, some vitamins and minerals, some unsaturated fatty acids and some amino acids
UD1.1.2 Dietary minerals are essential chemical elements.
Essential nutrients include:
- Amino acids (e.g. lysine and methionine)
- Fatty acids (omega 3 and omega 6)
- Minerals (iron, sodium, potassium, calcium, phosphorus and iodine)
- Vitamins (vitamins A, C, D and folate)
- Water
Minerals are often found in ionic form and are essential for our diet - if minerals are lacking in our diet, it results in a deficiency disease. For example, without the mineral iodine, the thyroid is continuously stimulated to produce hormones resulting in goiter.
UD1.1.3 Vitamins are chemically diverse carbon compounds that cannot be synthesized by the body.
Vitamins have various purposes:
- co-factors for enzymes
- antioxidants
- hormones
UD1.1.4 Some fatty acids and some amino acids are essential.
An essential amino acid or indispensable amino acid is an amino acid that cannot be synthesized de novo by the organism, but must be supplied in its diet. The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.
The term "essential fatty acid" refers to fatty acids required for biological processes but does not include the fats that only act as fuel. Essential fatty acids should not be confused with essential oils, which are "essential" in the sense of being a concentrated essence.
Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).
Some other fatty acids are sometimes classified as "conditionally essential," meaning that they can become essential under some developmental or disease conditions; examples include docosahexaenoic acid (an omega-3 fatty acid) and gamma-linolenic acid (an omega-6 fatty acid).
UD1.1.5 Lack of essential amino acids affects the production of proteins.
Protein cannot be made if there is a shortage of one or more essential amino acids - this is known as protein deficiency malnutrition and can come from a low intake of protein in the diet or an imbalance of the types of protein consumed. PDM can cause a lack of blood plasma proteins which can result in tissue fluid retention.
Kwashiorkor, also known as “edematous malnutrition” because of its association with edema (fluid retention), is a nutritional disorder most often seen in regions experiencing famine. It is a form of malnutrition caused by a lack of protein in the diet. People suffering from kwashiorkor typically have an extremely emaciated appearance in all body parts except their ankles, feet, and belly, which swell with fluid.
Kwashiorkor is rarely found in the United States and other countries with a generally steady food supply. It’s most common in sub-Saharan Africa and other countries where people routinely have a limited supply of food.
The extreme lack of protein causes an osmotic imbalance in the gastrointestinal system causing swelling of the gut diagnosed as an edema or retention of water.
UD1.1.6 Malnutrition may be caused by a deficiency, imbalance or excess of nutrients in the diet.
Malnutrition comes from an unbalanced diet, where the consumption of nutrients is low, lacking or there is an excess comsumption of fats and refined carbohydrates. Starvation and obesity are examples of malnutrition.
UD1.1.7 Appetite is controlled by a centre in the hypothalamus.
Appetite is controlled by hormones (leptin, insulin, CCK) produced in the pancreas, small intestine and adipose tissue. These send messages to the appetite control centre of the brain (hypothalamus), causing a person to feel that they have eaten enough (satiety). The release of hormones can be triggered in a number of ways:
- Stretch receptors in the stomach lining may become activated when the stomach is full
- Adipose tissue may release hormones (leptin) in response to fat storage
UD1.1.8 Overweight individuals are more likely to suffer hypertension and type II diabetes.
Hypertension is high blood pressure. Obese individuals have more fatty tissue that causes an increase in their vascular resistance and thus increases the 'work' the heart has to do in order to pump blood around the body. Type II diabetes is the body's inability to react to insulin or a resistance to insulin.
UD1.1.9 Starvation can lead to breakdown of body tissue.
1. Body uses glycogen stores
2. Body begins to break down its own tissue - uses amino acids as energy source
3. Body breaks down its muscle mass as a last resort
APPLICATION:
AD1.1.1 Production of ascorbic acid by some mammals, but not others that need a dietary supply.
- Ascorbic acid is a water-soluble vitamin that is not stored by the human body
- Vitamin C must be obtained by the body in sufficient quantities from the consumption of vitamin C-rich foods, including citrus fruits, cabbage and tomatoes
- Vitamin C is critical for the growth and repair of tissues in all areas of the body
- It assists the body in the production of collagen, an important protein used to form skin, cartilage, tendons, ligaments and blood vessels
- Vitamin C is required for healing wounds as well as repairing and maintaining bones and teeth
- It also allows the body to absorb iron from non-heme sources
- Vitamin C is an antioxidant, along with vitamin E, beta-carotene and many other plant-based nutrients
- Antioxidants protect the body against the destructive effects of free radicals - they neutralise free radicals by donating one of their own electrons, ending the electron-"stealing" reaction
- Free radical damage accumulates with age and can thus contribute to the development of health conditions such as cancer, heart disease and arthritis.
Humans do not produce Vitamin C due to a mutation in the GULO (gulonolactone oxidase) gene, which results in the inability to synthesise the protein. Normal GULO is an enzyme that catalyses the reaction of D-glucuronolactone with oxygen to L-xylo-hex-3-gulonolactone.
AD1.1.2 Cause and treatment of phenylketonuria (PKU).
Causes and Consequences of Phenylketonuria:
- A genetic mutation changes the gene coding for tyrosine hydroxylase, which is responsible for metabolising phenylalanine into tyrosine
- Phenylalanine is instead converted into phenylpyruvic acid, resulting in a high level of ketones in the blood and urine
- Over time, this may result in brain damage and mental retardation
Diagnosis and Treatment of Phenylketonuria:
AD1.1.3 Lack of Vitamin D or calcium can affect bone mineralization and cause rickets or osteomalacia.
Calcium and vitamin D work together to protect your bones—calcium helps build and maintain bones, while vitamin D helps your body effectively absorb calcium.
A diet deficient in vitamin D in conjunction with inadequate sun exposure causes osteomalacia (or rickets when it occurs in children), which is a softening of the bones. In the developed world, this is a rare disease. However, vitamin D deficiency has become a worldwide issue in the elderly and remains common in children and adults. Low blood calcidiol (25-hydroxy-vitamin D) can result from avoiding the sun. Deficiency results in impaired bone mineralization and bone damage which leads to bone-softening diseases.
- Diagnosis can be made by a simple blood test for the levels of phenylalanine – this test is conducted shortly after birth
- As PKU results in the build up of phenylalanine in the bloodstream, it is treated by following a diet that is low in protein
- Diet should include fruits, grains, vegetables and special formula milk and must be undertaken from early in life or else symptoms will appear
- Symptoms may include tremors, seizures, eczema, skin rashes and eventually brain damage (mental retardation)
AD1.1.3 Lack of Vitamin D or calcium can affect bone mineralization and cause rickets or osteomalacia.
Calcium and vitamin D work together to protect your bones—calcium helps build and maintain bones, while vitamin D helps your body effectively absorb calcium.
- UV light causes the skin to synthesise vitamin D, where it is stored in the liver for when UV levels are low (i.e. during the winter seasons)
- Insufficient sun exposure may lead to skeletal deformities (i.e. rickets) due to insufficient vitamin D levels
- However high levels of UV light can damage skin cells and cause skin cancers (malignant melanomas), so exposure must be limited
- Using sun block and covering skin with clothing will prevent UV radiation reaching the skin, inhibiting vitamin D production
- Certain diets (e.g. vegans, vegetarians) are more likely to result in a lack of vitamin D, so that such people need more exposure to sunlight
- Individuals with darker skin pigmentation produce vitamin D more slowly, but are better protected against skin cancers
- Certain ethnicities may be at a greater risk of vitamin D deficiency due to cultural or religious practices (e.g. Muslims wearing a burqa)
- Including vitamin D supplements in the diet can reduce the risk of vitamin D deficiency without requiring sun exposure
A diet deficient in vitamin D in conjunction with inadequate sun exposure causes osteomalacia (or rickets when it occurs in children), which is a softening of the bones. In the developed world, this is a rare disease. However, vitamin D deficiency has become a worldwide issue in the elderly and remains common in children and adults. Low blood calcidiol (25-hydroxy-vitamin D) can result from avoiding the sun. Deficiency results in impaired bone mineralization and bone damage which leads to bone-softening diseases.
AD1.1.4 Breakdown of heart muscle due to anorexia.
Anorexia nervosa is an eating disorder characterised by self-starvation and excessive weight loss. Malnutrition starves body tissues, thus the heart walls can become thin and weak which causes the heart chambers to enlarge . The pumping function of the heart declines and blood pressure falls. Organs that are sensitive to blood pressure e.g. the liver and kidney begin to fail.
AD1.1.5 Cholesterol in blood as an indicator of the risk of coronary heart disease.
Anorexia nervosa is an eating disorder characterised by self-starvation and excessive weight loss. Malnutrition starves body tissues, thus the heart walls can become thin and weak which causes the heart chambers to enlarge . The pumping function of the heart declines and blood pressure falls. Organs that are sensitive to blood pressure e.g. the liver and kidney begin to fail.
AD1.1.5 Cholesterol in blood as an indicator of the risk of coronary heart disease.
- Cholesterol is required for cell membrane structure and steroid synthesis, but very little (~20%) cholesterol comes from the diet
- High cholesterol diets can lead to atherosclerosis, as characterised by the deposition of fatty plaques and subsequent hardening of the arteries
- However the risks are increased when high levels of saturated fats are combined in the diet
- High levels of saturated fats also cause atherosclerosis, but also raise levels of LDL (low density lipoprotein) which retains cholesterol in the body
- Reducing dietary cholesterol levels will likely only be effective in lowering the risk of coronary heart disease if it is combined with lower levels of saturated fats as part of a balanced diet.
- Arteries supplying the heart become narrowed and the blood supply to cardiac tissues is reduced
- Heart has to work harder to force blood through narrowed vessels / blood pressure increases
- Angina
- Chest pain due to severe shortage of blood to the heart muscle - cells do not die
- Pain only occurs during activity but not at rest
- Caused by narrowing of coronary arteries (atherosclerosis)
- Heart attack (myocardial infarction)
- When a coronary artery is totally blocked by a thrombus/embolus
- No blood supply to heart muscle and cells die - often fatal
- Heart failure
- Blockage leads to damage of heart muscle and to gradual weakening of muscle
- Less efficient pumping
- Often accumulation of blood on right side → enlargement of heart
- The main cause is atherosclerosis
- Often called hardening of the arteries
- Inner layer of artery wall thickens with deposits of
- Cholesterol
- Fibrous (scar) tissue
- Dead muscle cells
- Blood platelets
- Fats, in the form of lipoproteins, accumulate beneath the endothelium
- Form plaque on the wall of arteries, also called atheroma
- Arteries become less elastic and partially narrowed
- Accelerated by high blood pressure
- Arterial endothelium damage is more likely but also leads to further weakening
- Aneurysm: weak walls may burst leading to severe loss of blood (haemorrhaging)
- Brain aneurysm is called a stroke
- Atheroma increases the risk of blood clotting
- Thrombus (blood clot) may break away and lodge elsewhere in the circulation (e.g. brain, heart)
- Circulating thrombus is called an embolus
Cholesterol:
- Cholesterol has important functions and is needed for
- Vitamin D production in skin
- Sex hormone production in gonads and adrenal glands
- Making cell membranes
- Make bile acid (salts)
- Cholesterol is an alcohol, not a fat but has properties similar to fats - soft, waxy and insoluble (difficult to remove if deposits form)
- Cholesterol is transported in the blood stream from the liver to tissues
- Safe transport is needed due to its insolubility
- This is achieved by lipoproteins, which are soluble fatty proteins
- These are wrapped around the cholesterol
- Usually, only small amounts of free cholesterol escape
- Fatty streaks adhere to wall of arteries - Atheroma/atherosclerosis/plaque forms
- Narrows lumen of artery
- Damages endothelium
- Can lead to formation of thrombus/blood clot
Low density lipo-proteins LDL's:
- Carry cholesterol from the liver to the tissues
- Normal levels: some cholesterol 'leaks' from the lipoprotein and is absorbed to build cell membranes
- High levels:
- Too much cholesterol leaks out
- Cholesterol is deposited on the arterial walls
- White blood cells are trapped within the cholesterol
- Free radicals are released by white blood cells and react with cholesterol
- This damages artery wall which allows further cholesterol deposits (i.e. Atherosclerosis)
- Blood platelets are activated and stick to damaged areas releasing clotting factors (thromboxanes)
- Healthy arteries produce anti-clotting factors (prostaglandins)
- Normally a balance between these two. Healthy vessels do not form clots
High density lipo-protein HDL's:
- Carry cholesterol away from tissues, including artery walls
- Travels to liver, is broken down and removed with bile
Smoking:
- Reduces levels of antitoxidants (vitamins), more damage due to release of free radials by phagocytes
- Nicotine constricts arteries causes platelets to stick together → vasoconstriction → heart must work harder to force blood through → increases blood pressure
- Raises conc. of fibrinogen (in blood) → increased risk of clotting
- Higher blood pressure causes damage to blood vessel lining/endothelium/collagen [EXAM]
- Leads to rise on blood platelets and makes them more sticky/form a plug/adhere to collagen fibres
- Release of thromboplastin/thrombokinase
- Fibrinogen converted to insoluble fibrin
- Platelet plug trapped by fibrin mesh
- Raises blood cholesterol by causing a rise in LDLs in blood
- Carbon monoxide reduces the efficiency of the blood in terms of carrying oxygen
- Haemoglobin combines with CO more readily than with oxygen → forms carboxyheamoglobin
- Associated with plaque formation
- Principle CHD = heart muscle receives inadequate amount of blood or oxygen/(coronary) blood supply reduced
Treatment:
- Lower blood pressure
- Drugs which regulate heart rate/beat - prevent abnormal rhythms (beta blockers)
- Drugs which prevent blood clotting making thrombosis less likely (warfarin)
- Heart by-pass
- Vein from the leg is used to by-pass the blocked region of the coronary artery
- Involves open heart surgery
- Angioplasty
- Deflated balloon-like device is passed up to the heart via the aorta
- Guided into damaged coronary artery and inflated to stretch the artery
- Transplant
- Need to find a suitable donor
- Need to prevent rejection → drugs that suppress immune system needed for rest of life
Prevention is more cost effective than treatment
- 1. Screen the population for
- High blood pressure
- High cholesterol
- Stop smoking, healthier diet, more exercises
- Men over 35 are at highest risk
- 2. Monitor the behaviour of the heart during exercise
- Difficult but encouraging the population to adopt a more healthy lifestyle from an early age is important
- Often leads to changes in diet and weight management
- 3. Giving up smoking and reducing alcohol intake
- Reduces blood pressure
- Many people do not heed the advice until it is too late
- Coronary heart disease is a long-term degenerative disease, starts at birth
SKILL:
SD1.1.1 Determination of the energy content of food by combustion.
SD1.1.2 Use of databases of nutritional content of foods and software to calculate intakes of essential nutrients from a daily diet.
SD1.1.1 Determination of the energy content of food by combustion.
SD1.1.2 Use of databases of nutritional content of foods and software to calculate intakes of essential nutrients from a daily diet.
Essential idea: Digestion is controlled by nervous and hormonal mechanisms
D.2 Digestion
UNDERSTANDINGS:
UD2.2.1 Nervous and hormonal mechanisms control the secretion of digestive juices.
UD2.2.3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms.
Both nerves and hormones are involved in controlling the secretion of digestive juices. Gastric juice secretion is an example of this.
SUMMARY:
UD2.2.2 Exocrine glands secrete to the surface of the body or the lumen of the gut.
The passage through which food passes from mouth to anus is called the alimentary canal. Digestive juices are added to food in the alimentary canal at several points. Exocrine glands secrete juices, including salivary glands, the pancreas, gland cells in the stomach wall and in the wall of the small intestine. The composition of the juices secreted by the glands is different, relfecting the processes that occur in each part of the alimentary canal.
Saliva = salivary glands - water, electrolytes, salivary amylase, mucus, lysozyme
Gastric juice = stomach - water, mucus, enzymes (pepsin), rennin and hydrochloric acid
Pancreatic juice = pancreas - water, bicarbonate, enzymes (amylase, lipase, carboxypeptidase, trypsinogen)
Unlike endocrine glands which secrete directly into the bloodstream, exocrine glands secrete into ducts.
UNDERSTANDINGS:
UD2.2.1 Nervous and hormonal mechanisms control the secretion of digestive juices.
UD2.2.3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms.
Both nerves and hormones are involved in controlling the secretion of digestive juices. Gastric juice secretion is an example of this.
- The sight and smell of food triggers the brain to send nerve impulses via the vagus nerve from the medulla
- Gland cells in the stomach wall are stimulated to secrete components of gastric juice
- If chemoreceptors in the stomach wall detect peptides in the stomach contents or if stretch receptors detect distension of the stomach, impulses are sent to the brain
- The brain responds by sending impulses via the vagus nerve to endocrine cells in the wall of the duodenum and the part of the stomach nearest to the duodenum, stimluating them to secrete gastrin
- The hormone gastrin stimulates secretion of acid anf pepsinogen by two types of exocrine glands in the stomach wall
- Two other hormones, secretin and somatostatin, inhibit gastrin secretion if the pH in the stomach falls too low
SUMMARY:
- Food entering the stomach causes distension, which is detected by stretch receptors in the stomach lining
- Impulses are sent to the brain, which triggers the secretion of gastrin from the pits lining the stomach wall
- Gastrin causes the sustained release of gastric juice, particularly its acid component
- When the pH drops too low, gastrin secretion is inhibited by hormones (secretin and somatostatin)
UD2.2.2 Exocrine glands secrete to the surface of the body or the lumen of the gut.
The passage through which food passes from mouth to anus is called the alimentary canal. Digestive juices are added to food in the alimentary canal at several points. Exocrine glands secrete juices, including salivary glands, the pancreas, gland cells in the stomach wall and in the wall of the small intestine. The composition of the juices secreted by the glands is different, relfecting the processes that occur in each part of the alimentary canal.
Saliva = salivary glands - water, electrolytes, salivary amylase, mucus, lysozyme
Gastric juice = stomach - water, mucus, enzymes (pepsin), rennin and hydrochloric acid
Pancreatic juice = pancreas - water, bicarbonate, enzymes (amylase, lipase, carboxypeptidase, trypsinogen)
Unlike endocrine glands which secrete directly into the bloodstream, exocrine glands secrete into ducts.
- Exocrine glands have ducts through which they secrete their product (these ducts may arise from the convergence of smaller ductules)
- The ducts / ductules arise from a cluster of cells called an acinus (plural = acini), surrounded by a basement membrane
- Acini are lined by a single layer of secretory cells which release the exocrine product into the lumen of the duct via secretory vesicles
- Secretory cells are held together by tight junctions, and possess a highly developed rough ER and golgi network for material secretion
UD2.2.4 Acid conditions in the stomach favour some hydrolysis reactions and help to control pathogens in ingested food.
- Acid is secreted by the parietal cells of the stomach
- The acid disrupts the extracellular matrix that holds cells together in tissues
- It also leads to the denaturing of proteins, exposing the polypeptide chains so that the enzyme pepsin can hydrolyse the bonds within the polypeptides
- Pepsin is released by chief cells as the inactive pepsinogen
- The acid conditions within the stomach convert the inactive pepsinogen to pepsin
- This ensures that the cells that produce pepsinogen are not digested at the same time as the protein in the diet.
UD2.2.5 The structure of cells of the epithelium of the villi is adapted to the absorption of food.
The inner surface of the ileum has numerous folds. Each of the folds is covered in tiny projections called villi. Absorption takes place through the epithelial cells covering each villus.
- Each epithelial cell covering the villus adheres to its neighbours through tight junctions, which ensure that most materials pass into the blood vessels lining the villi through the epithelial cell.
- The cell surface membrane on the intestinal lumen side has a number of extensions called microvilli. The collection of microvilli on the intestinal side of the epithelial cells is termed the brush border. The function of the brush border is to increase the surface area for absorption.
- Relatively high amounts of ATP are required to drive active transport processes. Thus epithelial cells have large numbers of mitochondria.
- Pinocytic vesicles are often present in large numbers due to absorption of some foods by endocytosis.
- The surface facing the lumen of the intestine is referred to as the apical surface and the surface facing the blood vessels is referred to as the basal surface. These surfaces have different types of proteins involved in material transport.
UD2.2.6 The rate of transit of materials through the large intestine is positively correlated with their fibre content.
Dietary fibre is material such as cellulose, lignin and pectin that cannot be readily digested. There are two categories of dietary fibre: soluble and insoluble. A healthy balanced diet contains fibre as it increases the bulk of materials passing through the intestines and helps prevent constipation as it draws water into the intestine. The higher the water content of the intestine, the faster the movement of fecal matter.
There are other possible benefits of fibre in the diet:
- bowel cancer, hemorrhoids and appendicitis risk can be reduced
- the presence of bulky material in the stomach and intestine may increase feelings of satiety, reducing the desire to eat and the risk of obesity
- absorption of sugars may be slowed down, helping to prevent type II diabetes
UD2.2.7 Materials not absorbed are egested.
Dietary fibre is the edible parts of plants that are resistant to being digested and are not absorbed from the small intestine. Examples include cellulose and lignin. As a consequence, there is a fraction of ingested food which never leaves the digestive tube. In addition, secretion into the digestive tube occurs. Some of what is added is excretory products such as bilirubin from the breakdown of red blood cells. A large volume of water is added to the tube in the proess of digestion by secretions in the mouth, stomach and small intestine, and has to be reclaimed in the large intestine. The excretory products, the unabsorbed water and undigested dietary fibre are egested as feces.
APPLICATION:
AD2.2.1 The reduction of stomach acid secretion by proton pump inhibitor drugs.
AD2.2.2 Dehydration due to cholera toxin.
AD2.2.3 Helicobacter pylori infection as a cause of stomach ulcers.
SKILLS:
SD2.2.1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.
- Acid is secreted by the parietal cells of the stomach
- The acid disrupts the extracellular matrix that holds cells together in tissues
- It also leads to the denaturing of proteins, exposing the polypeptide chains so that the enzyme pepsin can hydrolyse the bonds within the polypeptides
- Pepsin is released by chief cells as the inactive pepsinogen
- The acid conditions within the stomach convert the inactive pepsinogen to pepsin
- This ensures that the cells that produce pepsinogen are not digested at the same time as the protein in the diet.
UD2.2.5 The structure of cells of the epithelium of the villi is adapted to the absorption of food.
The inner surface of the ileum has numerous folds. Each of the folds is covered in tiny projections called villi. Absorption takes place through the epithelial cells covering each villus.
- Each epithelial cell covering the villus adheres to its neighbours through tight junctions, which ensure that most materials pass into the blood vessels lining the villi through the epithelial cell.
- The cell surface membrane on the intestinal lumen side has a number of extensions called microvilli. The collection of microvilli on the intestinal side of the epithelial cells is termed the brush border. The function of the brush border is to increase the surface area for absorption.
- Relatively high amounts of ATP are required to drive active transport processes. Thus epithelial cells have large numbers of mitochondria.
- Pinocytic vesicles are often present in large numbers due to absorption of some foods by endocytosis.
- The surface facing the lumen of the intestine is referred to as the apical surface and the surface facing the blood vessels is referred to as the basal surface. These surfaces have different types of proteins involved in material transport.
UD2.2.6 The rate of transit of materials through the large intestine is positively correlated with their fibre content.
Dietary fibre is material such as cellulose, lignin and pectin that cannot be readily digested. There are two categories of dietary fibre: soluble and insoluble. A healthy balanced diet contains fibre as it increases the bulk of materials passing through the intestines and helps prevent constipation as it draws water into the intestine. The higher the water content of the intestine, the faster the movement of fecal matter.
There are other possible benefits of fibre in the diet:
- bowel cancer, hemorrhoids and appendicitis risk can be reduced
- the presence of bulky material in the stomach and intestine may increase feelings of satiety, reducing the desire to eat and the risk of obesity
- absorption of sugars may be slowed down, helping to prevent type II diabetes
UD2.2.7 Materials not absorbed are egested.
Dietary fibre is the edible parts of plants that are resistant to being digested and are not absorbed from the small intestine. Examples include cellulose and lignin. As a consequence, there is a fraction of ingested food which never leaves the digestive tube. In addition, secretion into the digestive tube occurs. Some of what is added is excretory products such as bilirubin from the breakdown of red blood cells. A large volume of water is added to the tube in the proess of digestion by secretions in the mouth, stomach and small intestine, and has to be reclaimed in the large intestine. The excretory products, the unabsorbed water and undigested dietary fibre are egested as feces.
APPLICATION:
AD2.2.1 The reduction of stomach acid secretion by proton pump inhibitor drugs.
AD2.2.2 Dehydration due to cholera toxin.
AD2.2.3 Helicobacter pylori infection as a cause of stomach ulcers.
SKILLS:
SD2.2.1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.