The key to a healthy balanced diet is not to ban or leave out any foods or food groups but to balance what you eat by consuming a variety of foods from each food group in the right proportions for good health. As shown in the Table 1 below all the nutrients we need are found in lots of different foods.
Table 1: Table of Nutrients and the food they are found in.
- Describe precisely what happens to a meal of fish and chips from being eaten (Ingested) to being used by the body (Assimilated).
Once we have decided what we are going to eat the food has to go through our digestive system in order for us to extract the nutrients from it. Digestion is a long and complicated process with many different structures for the food to pass through. ‘The majority of the food we eat is taken in, in a SOLID INSOLUBLE (Large molecules) form. If the food stayed in this form it would simply pass straight through the digestive system and be of no use to the body. In order to absorb the food it must be changed in to a SOLUBLE (small molecules) form.’ The three main foods which we eat are Carbohydrates, Proteins and Lipids (fats and oils) and they are all digested in a similar way. (Class Notes)
During digestion the chemical bond between the sugar units are split so that the starch which is insoluble ends up as sugar which is soluble. The bonds are split by adding water (H2O) across the bond and this type of reaction is referred to HYDROLYSIS – this means splitting using water. The hydrolysis reactions are controlled by enzymes which are generally known as CARBOHYDRASES. (Class Notes)
The majority of proteins are insoluble but the amino acids which make up the proteins are soluble. During digestion the bonds between the amino acids are broken so all the proteins end up as amino acids. Once again the bands are broken by hydrolysis and the hydrolysis reactions are controlled by PROTEASE enzymes. (Class Notes)
- Lipids (fats and oils):
All lipids are made up of fatty acids and glycerol. During digestion the chemical bonds between the fatty acids and glycerol are broken by hydrolysis so all fats and oils end up as fatty acids and glycerol controlled by LIPASE enzymes. (Class Notes)
The effects of the digestive system can be seen in Tables 1: Summary of Digestive Process and Table 4: A summary of the main human digestive enzymes. (Based on Boyle & Senior, 136)
Table 1: Summary of Digestive Process:
In this instance we have decided to have a meal of fish and chips, this can be a balanced meal depending on the way it is cooked, although it is not necessarily good for you if prepared in the wrong way. To digest this meal the fish and chips have to be broken down in to its component parts and each of these parts are digested in different areas of the digestive system. The following table contains the brake down of fish in batter and chips when fried in Sunflower oil.
Table 3: The Component Parts of a Meal of Fish and Chips
Digestion involves a number of different stages. The first phase is known as the cephalic (head) phase. It starts before food has even entered the mouth. The sight, smell, taste or even the thought of food will activate saliva in the mouth as well as digestive juices, which contain enzymes to break down food.
During the entire journey of the food it will go through a stage called HYDRROLYSIS which is the breaking down of the bonds which keep the molecules in the in the different nutrients together, each nutrient has a different HYDROLITIC ENZYME which is explained in the following procedures. This is also illustrated in Table 4: A summary of the main human digestive enzymes. (Based on Boyle & Senior, 136)
- In the Mouth
Once food is in the mouth, the taste buds begin determining the chemicals within the food via their nerve endings, in order to give the taste sensations of salt, sweet, sour or bitter. As your teeth and jaw muscles chew and grind the food, breaking it down (this is known as Mastication), it's mixed with saliva. This comprises many enzymes including the HYDROLITIC enzyme called Salivary Amylase, which begins to break down the long chains of starch found in the and the flour in the batter. Saliva also contains Mucin, a slimy glycoprotein lubricant (Boyle & Senior, 134) which moistens the food so it can pass easily through the digestive (gastrointestinal) tract. The tongue rolls it in to a ball or bolus which is just the right size to swallow.
- The Oesophagus
After the food has been swallowed, it's carried down the oesophagus (a muscular tube) towards the stomach. The oesophagus can contract and relax in order to propel the food onwards this is called Peristalsis and each mouthful of food takes about six seconds to reach the stomach once swallowed.
- The Stomach
The stomach is a sack made of muscle and, when it's empty, it has a volume of only 50ml but this can expand to hold up to 1.5 litres or more after a meal. The walls of the stomach are made of three different layers of muscle that allow it to churn food around and make sure it's mixed with the stomach's acidic digestive juices which is Hydrochloric Acid. The presence of the hydrochloric acid in the stomach prevents the action of salivary amylase and helps to kill bacteria that might be present. The stomach produces GASTRIC JUICE which produces the HYDROLITIC enzyme Pepsin, which HYDROLISES the proteins found in the in the batter, and breaks them down into Polypeptides. Gastric juice also produces an enzyme called RENIN but this is mainly produced by babies as it hydrolyses the protein in milk, although as adults we do produce a small amount.
Food can stay in the stomach for a few minutes or several hours in the gastric phase where numerous acids and enzymes are released. When the food has been churned into a creamy soup like mixture known as chyme the pyloric sphincter (an opening controlled by muscle) opens and chyme ‘squirts’ into the small intestine.
- The Small Intestine
About 3ml of chyme is squirted into the small intestine at short intervals as the pyloric sphincter opens. This is known as the Intestinal Phase and causes the secretion of many hormones, which all aid the digestive process. The sphincter is designed to open partially so that large particles are kept in the stomach for further mixing and breaking down.
Digestion and absorption of fats, protein and carbohydrates occurs in the small intestine. Three important organs are involved:
- Bile Duct: The lipids in the sunflower oil and the fish are broken down by the hydrolytic enzyme lipase; but before lipase can really get to work the lipids first need to be . This is done by bile, a Green Alkaline Fluid secreted in the liver and stored in the gall bladder, which is added to food via the bile duct when food is passing through the duodenum. Bile is not an enzyme. Bile works like detergent - it disperses fats into droplets and so enlarges the surface area so that it can get to work breaking down the fat much more efficiently. Bile salts also neutralise the stomach acid and help to create optimum pH conditions for digestive enzymes in the small intestine
- The Pancreas provides bicarbonate to neutralise the acidic chyme from the stomach, and also produces further digestive enzymes. Which are AMYLASE the hydrolytic enzyme for the remaining starch from the potatoes and flour in the batter, TRYPSIN, the hydrolytic enzyme for protein as found in the fish, and LIPASE the hydrolytic enzyme for fatty acids and glycerol as found in the sunflower oil and the yolk of the egg used to make the batter. The Pancreatic juice reaches the small intestine through the PANCREATIC DUCT. All of these are explained in Table 4: A summary of the main human digestive enzymes. (based on Boyle & Senior, 136)
- The Intestinal Wall contains cells that make up the wall of the small intestine. These cells help to neutralise the acid and also produce the hydrolytic enzymes Erepsin, Maltase, Lactase and Sucrase to digest food, as shown in Table 4: A summary of the main human digestive enzymes. (based on Boyle & Senior, 136)
The functions of all the Hydrolysing Enzymes can be shown in the following table:
Table 4: A summary of the main human digestive enzymes. (Based on Boyle & Senior, 136)
The inner surface of the small intestine is folded into finger-like structures called villi, which greatly increase the surface area available for absorption as they have a single layer of epithelial cells which makes it easier for the end products of digestion monosaccharide’s, amino acids from the fish and chips to pass into the blood system via the capillaries in the villi. The villi also contains a Lacteal Vessel or Lymph which has bigger pores and lets the end product of the digestion of fatty acids and glycerol of the sunflower oil and the fish are taken into the blood system. The mechanism of absorption has been found to be ACTIVE UPTAKE which requires energy – the epithelial cells ‘PUMP’ the end products into the blood and lymph system.
Blood vessels receive the digested food from the villi where it's then transported through the blood stream to the liver via the Hepatic Portal Vein.
The absorption (Assimilation) of the end products of a meal of fish and chips which are; Monosaccharide Sugars, Amino Acids and Fatty Acids and Glycerol are taken to the liver, they are absorbed (assimilated) and used in different ways.
- Monosaccharide Sugars - mainly glucose.
These arrive at the liver via the HPV and they are normally temporarily stored in the liver as GLYCOGEN. The body uses some of the sugars to help maintain body temperature as they may be oxidised to produce heat energy. The blood always has a constant sugar level so the cells of the body have a ready supply of sugar to produce energy (sugar level is normally 0.1g per cm3 of blood). Several organs are involved in maintaining a constant blood sugar level they are:
- The Liver.
- The Pancreas.
- The Blood System.
The ILSETS OF LANGERHAN are areas with in the pancreas which is sensitive to the level of glucose in the blood. INSULIN is a hormone which is produced if the level of sugar in the blood becomes too high it then travels in the blood to the liver where it promotes the change of the excess sugar in the blood into GLYCOGEN and the glycogen is stored in the liver.
If the blood sugar becomes to low the Islets produce a hormone called GLUCAGON which travels in the blood to the liver. Once at the liver glucagon promotes the conversion of any of the stored glycogen into sugar which enters the blood raising the blood sugar level to normal. (Class notes)
If a diabetic cannot produce sufficient insulin either through illness, injury or genetics during their lives they will usually require insulin injections to keep their blood sugar level normal.
- Amino Acids
These arrive at the liver in the HPV. In the blood supply there is a constant pool of amino acid which supplies the cells with the amino acids they require to build proteins for growth and repair. This is maintained by the liver but we always have excess amino acids above our requirements.
Rather than waste the excess the liver oxidises them to produce energy. Unfortunately if the complete amino acid is oxidised poisonous OXIDES OF NITROGEN would be produced, to counteract this, prior to oxidisation the amino acids have their nitrogen removed to prevent the poison from being produced. This is called DEAMINATION and it is carried out by a DEAMINASE enzyme in the liver. The resulting KETO ACID can now be safely oxidised to produce energy. The removed nitrogen forms the waste product UREA which in large amounts is also poisonous so it is filtered from the blood by the KIDNEYS and excreted in URINE. (Class notes)
AMINO ACIDS KETO ACID + UREA
- Fatty Acids and Glycerol
These travel around in the blood and they can be used by the cells for structural purposes such as producing call membranes or they can be oxidised to produce energy. Any excess fat can be stored under the skin as SUB-CUTANIOUS FAT or ADIPOSE TISSUE. This acts as a food store as well as an insulator helping to maintain body temperature. (Class notes)
- The large intestine
The unabsorbed residue of this process finally reaches the end of the small intestine and enters the large intestine. It is named for its wide diameter. The large intestine has six parts: the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.
This is one of the most metabolically active organs in the body. It measures about 1.5 metres and contains over 400 different species of bacteria that break down and utilise the undigested residues of our food, in the first part of the large intestine, the colon, the only things left to be absorbed is mostly water, dietary fibres (roughage) and vitamins and minerals of the meal of fish and chips. As the watery contents move along the large intestine still using the process of peristalsis, water is absorbed and the final product - faeces - is formed, which is stored in the rectum before excretion (defecation) from the body through the sphincter at the anus.
Table 5: ORGANS OF the ALIMENTARY CANAL
Note: The structures are listed in the order in which food passes through them.