Human Nutrition,Human alimentary canal, Biology Summarized Notes

These large teeth are at the back of the mouth; they are used for crushing and chewing the food. They have raised points called cusps, and can have up to three roots. The molars at the back of the mouth are sometimes called wisdom teeth.

In the buccal cavity, the food is tasted, chewed,
lubricated and moulded into a bolus before being Swallowed. The tongue is the organ of taste. It has receptor organs, called taste buds, of which there are four types: sNeet, salty, sour and bitter. The pleasant taste of food, reinforced by the appetising smells released from it when it is chewed, causes the salivary glands to secrete even more saliva. The saliva mainly consists ot water and muco-proteins, which lubricate the food. It also contains enzymes, bufters and antibacterial agents. The enzyme is salivary amylase, which hydrolyses starch into maltose.

The structure of a tooth
Although different teeth have specific functions, they all have the same basic structure. Look at the structure of a molar tooth below.

Each tooth consists of:
- a crown - the visible part
- a root that anchors the tooth in its socket in the jawbone.
Teeth consist mainly of a hard substance called dentine. The crown is covered with an additional layer of an even harder substance called enamel. The root is covered with a cement to help anchor the tooth in the jaw. Inside each tooth is a pulp cavity that contains the tooth's blood, nerve and lymph supply.
Tooth decay
These are two of the slogans that appear on tubes of toothpaste.

What is plaque, and is there any scientific reason why baking soda and fluoride should be included in toothpaste? Plaque is a layer of bacteria and undigested foad that builds up on teeth, especially where the gum and teeth meet. It causes tooth decay, known as dental caries. There are always bacteria on the tooth surface, and after a meal some food will always stay behind in between the teeth. If the food residue is not removed regularly, plaque bacteria will feed on or digest sugars that they break down to acids. These acids dissolve the enamel and dentine of teeth, causing cavities/holes, Bacteria also form a sticky covering over the tooth called plaque. Toothache is experienced when the decay reaches the pulp cavity. Baking soda is sodium bicarbonate, a chemical that reacts with and neutralises acids It seems scientific to include baking soda in toothpaste, to neutralise the acids that bacteria produce. Fluoride, together with calcium, are minerals that are essential for healthy teeth and bones. Researchers at Stellenbosch University, in South Africa, have shown that the concentration of fluoride in drinking water, the normal dietary source, is very low in most parts of the country. This fact justifies including fluoride in toothpaste. Another option would be to add fluoride to water supplies. This is known as fluoridation, and is used in several countries where fluoride levels are low Fluoridation of drinking water in parts of southern Africa is being debated at the present time. You should understand the arguments for and against adding fluoride to drinking water.
 Argument for adding fluoride to water:
Research has shown that an increase in the fluoride concentration in water up to 2.0 parts per million will produce a decrease in the percentage of tooth decay. This means less maney being spent on dental care
 Argument against adding fluoride to water:
Some people object to the artificial 'addition' of fluoride to the supplies of drinking water. They claim it is unnatural to put additives into drinking water. Fluoride can produce dark colouration on the teeth In some parts of the world, the amount of fluoride is already ata high level.

 How can you prevent tooth decay?
There are a number of things you can do to care for your teeth and prevent tooth decay:
- brushing and cleaning with dental floss after every meal, to remove plaque and food debris
- not eating sticky and sugary foods
- not drinking truit juices between meals
- using a fluoride toothpaste
- visiting the dentist regularly
- following a diet that has enough vitamin D, calcium and phosphorus.

Oesophagus
Peristalsis moves fod down the oesophagus to the stomach, A special ring of muscle between the stomach and the oesophagus, called the sphincter muscle, stops food passing back up into the oesophagus. There is no digestion in the oesophagus because the food passes through for a few seconds. The wall of the alimentary canal has:
- a lining ot mucous membrane on its inner surface
- a layer of smooth muscle with its fibres arranged in a circle
- a layer of smooth muscle with its fibres arranged longitudinal
The wall of the tube contains muscles, which contract and relax to make food move along. This movement is called peristalsis, as shown in below.

The purpose of peristalsis is to:
- push food along the alimentary canal, and to keep it moving in the same direction
- mix the food with digestive juices
- help with the physical breakdown of food and enlarge the surface area for enzyme reactions, especially in the stomach.
Peristalsis takes place all along your alimentary canal. The muscles work best wnen you have plenty of fibre in your diet. A lack of fibre can cause constipation, which is when the muscles do not work so well and the food stops moving along.

 Stomach
The stomach is a highly elastic and muscular organ, which can expand easily to hold a large meal. The stomach wall is highly folded and is dotted with pits leading to tubular gastric glands, which secrete gastric juices. Gastric juices consist mainly of water along with three other secretions
- mucus from goblet cells
- hydrochloric acid
- pepsinogen
Pepsinogen is an inactive form of an enzyme that is converted to pepsin, the active enzyme. Pepsin hydrolyses large polypeptide chains into smaller polypeptides. Continual movements of the muscular stomach wall churn the food, mixing it with gastric juices and helping to break it down mechanically. The combination of chemical and mechanical digestion reduces the food into a uniform creamy paste called acid chyme. Contraction of the stomach muscles mixes the contents very efficiently. Protein digestion starts here, catalysed by pepsin, a protease, which is secreted by gastric glanas in the stomach lining. A sphincter muscle between the stomach and the small intestine controls the exit of the stomach contents, allowing some of the contents into the intestine from time to time. There is some absorption into the bloodstream of water and ingested soluble molecules that require no digestion, like glucose and alcohol.
 Sphincters
The stomach has tvwo sets of sphincter muscles (rings of smooth muscle that can open and close) at its opening. One is at the end of the entry point of the oesophagus, the cardiac sphincter muscle; the other is the pyloric sphincter muscle at the exit point of the stomach. The cardiac sphincter muscle is a circular layer of muscle that controls the passage of food from the mouth through the oesophagus into the stomach, and also prevents the swallowed food from going back into the oesophagus. The pyloric sphincter muscle is a thickening of the middle layer of the stomach around the opening into the first part of the small intestine, the duodenum. It holds the food in the stomach, until it's thoroughly mixed with gastric juice, regulates the entrance of food into the duodenum, and prevents the food going back from the duodenum into the stomach. Occasionally, the cardiac sphincter stays open for a moment after swallowing and allows acid chyme to flow from the stomach into the oesophagus, causing a burning sensation called 'heartburn'.

Cells in the pancreas secrete pancreatic juice into the first part of the small intestine, called the duodenum. The pancreatic juice Contains several enzymes, such as amylase, lipase and protease. These enzymes all need an alkaline pH. The pancreatic juice contains sodium hydrogen carbonate which neutralises the acidic pH in the food from the stomach and changes the pH from acid to alkaline. However, this change makes pepsin from the stomach inactive and enables the enzymes in the pancreatic juice to work at their optimum pH. Each of the enzymes acts on the food and breaks it down further.

Absorption and assimilation
The small intestine
The small intestine is the longest section of the alimentary canal. It is about five to six metres long, of which the duodenum makes up 30 cm. The rest is known as the ileum. It is called the small intestine, because it is narrower than the large intestine, which is much shorter. Digestion is completed in the first part of the small intestine, called the duodenum. Ine enzymes that act in the small intestine are produced by the pancreas and glands in the intestinal wall. The nutrients - monosaccharides, amino acids, fatty acids and glycerol - are now soluble. They are absorbed through the wall of the last part of the m small intestine, called the ileum.
The ileum is equipped for its function of absorption by certain adaptations:
- a large capillary network that absorbs water, amino acIds, salts, Vitamin B and C, and glucose by normal and active diffusion
- a very thin, single layer of columnar epithelium cells that surround the villi so that the difusion distance is minimised to its optimum
- it is in constant movement to move foood along and to keep its suriace area into contact with nutrients
- a surface that is always moist for the easy diffusion of nutrients
- lacteal (ymph vessels) that absorb fatty acids and glycerol, forming fats again that cause the lymph to turn milky.
The leum's extensive surface area for absorption is provided by:
- the many folds of the inner wall of the ileum
- tiny finger-like projections, called villi, on the surface of the folds,
- the microvilli, which are extensions of the surface plasma membranes of the epithelial cells that form the walls of the villi.
Structure of a villus
The structure of a villus speeds up the rate of absorption:
- Villi are finger-like. projections that increase the surface area for absorption.
- Villi are about 1 mm long.
- A villus has a brush border known as microvilli on the epithelial cells to further increase the surface area.
If you look at figure below you can see that one layer of epithelial cells surrounds the illus to minimise the distance for nutrients to travel. figure belowshows that there are blood capillaries and a lacteal inside the villus. There is a single layer of epithelial cells that allows digested food to pass through and reach the blood capillaries and lacteals.

The blood capillaries absorb glucose and amino acids. The concentration of glucose and amino acids is higher inside the blood capillaries, so these substances cannot enter the blood by diffusion. The glucose and amino acids are taken up against their concentration gradients by the process of active transport. They are then transported to the liver, and eventually to the rest of the body. The lacteal is part of the lymphatic system, and absorb fatty acids and glycerol. These molecules recombine to form fats.

The hepatic portal vein
All nutrients that are taken up by capillaries of the villi in the ileum need to go to the liver first, and then to the cells of the body. The ways in which these food materials are then used is called assimilation. In the first stage of assimilation, nutrients are transported from the intestine. Some materials will pass through the liver and to the cells unchanged, some will be stored in the liver, and some will be changed into something else and then released from the liver or stored within it.

You can see in figure below that the hepatic portal vein carries blood from the small intestine to the liver. This vein is unusual, because it links two organs. Blood from an organ, such as the small intestine, returns to the heart and not to another organ. Absorbed food, such as Glucose, amino acids and vitamins, are taken from the intestine to the liver in the hepatic portal vein.

Look carefully at figure above again. You can see the lymphatic vessels that drain the lacteals of the villi. Fats absorbed into the lacteals of the villi are returned through lymphatic ducts to the blood circulation near the heart. Some absorbed nutrients are not used by the liver. These are carried away from the liver in the hepatic vein, which takes blood to the heart. In this way, nutrients are added to the general circulation and are distributed to cells throughout the body, where:
- glucose and fats are oxidised to provide energy for cells
- fats are used to make cell membranes, or are stored in adipose cells
- amino acids are used to make the body's own unique proteins
- nutrients, such as fats and amino acids that are used by cells to synthesise their own structures, are said to be assimilated.

The role of the liver during digestion
Glucose metabolism
Blood glucose levels have to be kept within fairly narrow limits, and the liver plays an important role in this. Immediately after a meal, the liver converts surplus glucose to glycogen. When blood glucose levels drop, the liver converts glycogen back to glucose.
Glucose <---------------------> glycogen
There is a limit to the amount of glycogen the liver can store. If the intake of glucose is excessive, the liver turns it into fat, which is then carried in the.circulation to the fat (adipose) cells, mainly under the skin, and stored there.
 Amino acid metabolism
The body cannot store excess amino acids as amino acids. Any amino acids not assimilated by the cells are broken down by liver cells. Figure below summarises the formation of a waste product, urea, from the breakdown of amino acids.

You can see in figure above that the NH₂ group of amino acids is removed, through the process known as deamination, and turned into ammonia. Most of the ammonia is then turned into urea, which is excreted by the kidneys and removed from the body. The rest of the amino acid molecule is changed by liver cells into fat or carbohydrate molecules that can be used to release energy.

 Large Intestine
Between the small intestine and the large intestine is a sphincter-type valve called the ileocecal valve (ICV). The purpose of this valve is to prevent backflow from the large intestine, once any material leaves the small intestine. Not all the contents entering the digestive tube are going to be absorbed as food. In fact, much of what is ingested and processed continues to flow through the tube for eventual elimination. At the point where the small intestine ends, it sends its watery waste products into the large intestine.

 The leocecal valve:
- remains closed most of the time
- opens briefly to let the contents of the small intestine exit
- closes again quickly to prevent any materials in the large intestine from leaking back.
About nine hours after eating, any undigested food reaches the large intestine, which is shorter than the small intestine, but with a much wider diameter. The undigested food consists mainly of bile salts, fibres, water and mineral salts.
 The main functions of the large intestine are to:
- absorb water, minerals and vitamins, such as vitamin K and some of the Vitamin B-complex vitamins, which are made by bacteria living in the colon
- compact the indigestible food into faeces, and move it to the rectum
- store and release the faeces through the anus defecation is controlled by a sphincter muscle.
Most of the digested food is absorbed in the small intestine, but some substances, such as water, are absorbed further along in the colon. If people get an infection in their digestive system, the colon may stop absorbing water. The water in their digestive system will pass out through the anus with the faeces. This condition is called diarrhoea. A person with diarrhoea quickly becomes dehydrated, and needs to be given a lot of water to drink. if the water has a little glucose and a little salt mixed into it, it will help the colon to absorb the water more efficiently. Babies often get diarrhoea, and need to be given cool, boiled water containing a few teaspoonfuls of sugar and a few grains of salt. Egestion is the elimination of digestive wastes materials that have never left the alimentary canal, and which have never taken part in metabolic reactions inside cells. Egestion is not the same as excretion, which is the elimination of metabolic waste from the body, mainly by the kidneys.

The lower anal canal and the anal opening are composed of two muscular constrictions that requlate faecal passage. The internal sphincter is part of the inner surface of the canal: it is composed of concentric layers of circular muscle tissue and is not under voluntary control. The external sphincter is a layer of voluntary (striated) muscle encircling the outside wall of the anal canal and anal opening. One can cause it to expand and contract at will, except during the early years of life when it is not yet fully developed. Nerves in the anal canal cause sphincter response and the sensation of pain. The lower part of the canal is very sensitive to heat, cold, cutting, and abrasion. Waste products pass to the anal canal from the rectum. Nerve responses from the rectum cause the internal sphincter to relax while the external one contracts; shortly thereafter the external sphincter also relaxes and allows faecal discharge. The pelvic diaphragm and longitudinal muscles draw the anus and rectum up over the passing faeces, so that they are not extruded (prolapsed) out of the anal opening with the faeces. Numerous blood vessels surround the anal canal and may be subject to enlargement and rupture; this condition, commonly called a haemorrhoid, or pile, may cause pain, bleeding, and projection of the vessels from the anus. Look at Figure below the anal sphincter muscle location.

 Digestion and digestive enzymes
Digestion is the process of breaking down large, complex, insoluble food molecules to small, simple, soluble molecules by using enzymes. The digestive enzymes enable the food molecules to combine chemically with water to form the broken-down product. This type of chemical reaction is called hydrolysis. Large molecules are changed into smaller molecules by hydrolysis.
subunit ---------- subunit + H₂O subunit H + HO --------- subunit (O unit broken).
Most digestive enzymes are produced by salivary glands and the pancreas. Glands in the epithelial lining of the stomach and the small intestine also produce some enzymes. As with all enzymes, digestive enzymes work well at an optimum pH and temperature. Digestive enzymes have an optimum temperature of about 37-40 C. The temperature of the human body is about 37-38 °C. The optimum pH may vary from one enzyme to another.

Digestion of carbohydrates, fats and protein
You need to know the functions of three digestive enzymes: protease, amylase and lipase. Each of these enzymes acts on a particular type of molecule called a substrate, and breaks
down the substrate to form products:
 Amylase
breaks down large carbohydrate molecules, such as starch, into smaller molecules, such as simple sugars; it hydrolyses the glycosidic bonds of polysaccharides, such as starch, which is changed into maltose.
Pepsin
breaks down large molecules of protein into small amino acid molecules; it hydrolyses the peptide bonds in different regions of the polypeptide chain.
 Lipase
breaks down large molecules of fats into small molecules of fatty acids and glycerol; it hydrolyses the fats to fatty acids and glycerol.

 Action of amylase in the mouth
One of the first places where amylase is secreted from, is the salivary glands in the saliva. Amylase starts the digestion of starch in the mouth. The pH of the saliva is slightly alkaline, and provides the optimum pH for amylase. The enzyme works by hydrolysing the glycosidic bonds of the polysaccharides, such as starch, to form maltose. Amylase cannot hydrolyse maltose. Maltose is broken down into smaller units of glucose by another enzyme, called maltase, in the small intestine. The action of amylase stops when the food enters the stomach, because the pH in the stomach is more acidic.

Bile
Bile is not an enzyme, but it helps to break down fats. It is a solution of bile salts and pigments produced by the liver from the breakdown of haemoglobin. Bile is stored in the gall bladder and passes into the duodenum through a duct. it has two main functions:
- to help with the absorption of lipids
- to emulsify fats (to break down fats into smaller droplets).
Emulsification (see figure below) helps to speed up hydrolysis of fats by increasing the surface area for lipase to act on.

Summary of digestion

Part of the alimentary canal
- Mouth pH 7-8
 Source of enzyme
- from salivary glands, in saliva
 Enzyme
- amylase
Function
- starts digestion: starch to maltose

 Part of the alimentary canal
- Stomach pH 2
 Source of enzyme
- from glands in stomach wall, in gastric juices

 Enzyme
• pepsin, a protease
Function
- starts digestion: protein to polypeptides

 Part of the alimentary canal
- Small intestine pH 7- 8
Source of enzyme
- from pancreas, in pancreatic juice and from glands in the wall of the small intestine, in intestinal juice
 Enzyme
1. maltase
2. trypsin, a protease
3. lipase
 Function
1. completes starch digestion: maltose to glucose
2. completes protein digestion: polypeptides to amino acids
3. digests fats to fatty acids and glycerol.

 The end, posted by Miss Fang Xiu.