Food is one of our essential needs. It provides us with energy we need to live and vital nutrients we need to be healthy.
Keywords
Digestion - the breakdown of large insoluble molecules into small soluble molecules
Ingestion - Intake of food (eating)
Absorption - The uptake of soluble nutrients across the small intestine into the bloodstream
Assimilation - Synthesis of larger functional molecules (eg. proteins) from the products of digestion (eg. amino acids)
Egestion - removal of undigested waste
Excretion - Removal of metabolic (cellular) waste
Digestion - the breakdown of large insoluble molecules into small soluble molecules
Ingestion - Intake of food (eating)
Absorption - The uptake of soluble nutrients across the small intestine into the bloodstream
Assimilation - Synthesis of larger functional molecules (eg. proteins) from the products of digestion (eg. amino acids)
Egestion - removal of undigested waste
Excretion - Removal of metabolic (cellular) waste
Different foods have different nutrients which is why we must be aware of what we are eating and should always stick to a balanced diet. A balanced diet is a healthy diet which requires eating a balanced amount of foods containing certain nutrients. For example: someone who ate a lot of burgers and fries for lunch everyday and chocolate cereal for breakfast and then a cheese sandwich and carrots for dinner wouldn't have a balanced diet because it would contain lots of fats (the burgers, fries, cereal and the cheese, to some extent) and only some vitamins (from the carrots).
The nutrients and their functions are given in the table below:
The nutrients and their functions are given in the table below:
The digestive system is a group of organs (for more information on that please do check out our cells page) that helps your food be taken into your body and crushed. Below is a diagram of the digestive system.
Protein, carbohydrate and fat molecules are too big to be absorbed by your body so they need to be broken down by the digestive system. Watch this short video clip to find out more-
The Digestion of Food
Mouth
Naturally, the starting place for the digestive system is the mouth. This is where food enters and begins being broken down. Your teeth mechanically digest the food so it is in smaller parts which are easier to digest and absorb. Your different teeth are each responsible for their own respective method of breaking down certain foods based on their shape. The smaller chunks of food are then subjected to he saliva in your mouth. In your saliva, is an enzyme called amylase (a carbohydrase) which starts the break down of any starch by breaking it up into maltose. Additionally, the saliva (produced and released from the salivary glands) serves as a lubricant aiding in the next part of the food's journey after the tongue pushes it down.
Oesophagus
The oesophagus, is a thin, long tube that carries food from the mouth to the stomach via a process called peristalsis - the movement of food through the oesophagus as a result of the contractions of circular antagonistic muscle pairs around it
Naturally, the starting place for the digestive system is the mouth. This is where food enters and begins being broken down. Your teeth mechanically digest the food so it is in smaller parts which are easier to digest and absorb. Your different teeth are each responsible for their own respective method of breaking down certain foods based on their shape. The smaller chunks of food are then subjected to he saliva in your mouth. In your saliva, is an enzyme called amylase (a carbohydrase) which starts the break down of any starch by breaking it up into maltose. Additionally, the saliva (produced and released from the salivary glands) serves as a lubricant aiding in the next part of the food's journey after the tongue pushes it down.
Oesophagus
The oesophagus, is a thin, long tube that carries food from the mouth to the stomach via a process called peristalsis - the movement of food through the oesophagus as a result of the contractions of circular antagonistic muscle pairs around it
Stomach
Upon, reaching the stomach, the food is mixed with acids such as hydrochloric acid (HCl) and protease enzymes such as pepsin which breaks down proteins into peptides (which are broken down into amino acids by peptidases later). The hydrochloric acid, secreted by the stomach wall, kills bacteria that travels into the stomach with the food, making the stomach acidic (pH2). Pepsin has a lower optimum pH than most enzymes which is why it is able to work in the acidic stomach without denaturing. In the stomach, the smooth muscle in the stomach wall churns up the food via muscle contractions, mixing it with the enzymes and acid. This is all held in the stomach for several hours by the sphincter muscle between the stomach and the small intestines. The mix of food, enzymes and acid is called chyme
Upon, reaching the stomach, the food is mixed with acids such as hydrochloric acid (HCl) and protease enzymes such as pepsin which breaks down proteins into peptides (which are broken down into amino acids by peptidases later). The hydrochloric acid, secreted by the stomach wall, kills bacteria that travels into the stomach with the food, making the stomach acidic (pH2). Pepsin has a lower optimum pH than most enzymes which is why it is able to work in the acidic stomach without denaturing. In the stomach, the smooth muscle in the stomach wall churns up the food via muscle contractions, mixing it with the enzymes and acid. This is all held in the stomach for several hours by the sphincter muscle between the stomach and the small intestines. The mix of food, enzymes and acid is called chyme
Small Intestines
Peristalsis moves the food along the small intestines as well as the oesophagus.
Peristalsis moves the food along the small intestines as well as the oesophagus.
Duodenum
The first part of the small intestine is called the duodenum and receives enzymes and bile from the liver and pancreas.
Many digestion enzymes are added to the duodenum to completely break down food into its component parts. These enzymes are the carbohydrases: amylase (from the pancreas) and maltase which breaks down multose into glucose (from the intestine wall); proteases like trypsin (from the pancreas which breaks down protein into peptides) and peptidase (from the intestine wall) which breaks peptides into amino acids; and lipase, produced in the pancreas, which breaks down lipids into glycerol and fatty acids.
Bile is a green alkaline liquid produced by the liver and stored in the gallbladder. It enters the duodenum through the bile duct and mixes with the chyme. As it is alkaline, bile neutralises the acidic chyme to produce the optimum pH (7) for the enzymes to work. Bile also emulsifies lipids to help lipase break down lipids. Emulsion is the suspension of droplets in liquids they can't dissolve into. This breaks up the lipids into smaller droplets which produces a larger surface area for the lipase to work with making it easier to break down the lipids.
The first part of the small intestine is called the duodenum and receives enzymes and bile from the liver and pancreas.
Many digestion enzymes are added to the duodenum to completely break down food into its component parts. These enzymes are the carbohydrases: amylase (from the pancreas) and maltase which breaks down multose into glucose (from the intestine wall); proteases like trypsin (from the pancreas which breaks down protein into peptides) and peptidase (from the intestine wall) which breaks peptides into amino acids; and lipase, produced in the pancreas, which breaks down lipids into glycerol and fatty acids.
Bile is a green alkaline liquid produced by the liver and stored in the gallbladder. It enters the duodenum through the bile duct and mixes with the chyme. As it is alkaline, bile neutralises the acidic chyme to produce the optimum pH (7) for the enzymes to work. Bile also emulsifies lipids to help lipase break down lipids. Emulsion is the suspension of droplets in liquids they can't dissolve into. This breaks up the lipids into smaller droplets which produces a larger surface area for the lipase to work with making it easier to break down the lipids.
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Ileum
As broken down food moves into the Ileum, useful nutrients are absorbed through the villi of the Ileum walls. The Ileum lining has a very large surface area for absorption because it is long and has many folds to help increase surface area, it also contains many tiny projections called villi (sing: villus) which themselves have many tiny projections called microvilli. Each villus (shown on the right) has capillaries and a lacteal into which the useful molecules are absorbed by diffusion and active transport aided by the large surface area of the villi. Most digested food enters the capillaries through which blood circulates and travels through venules and arterioles which are little veins and arteries connected to the capillary. As the blood is circulating, it takes in these nutrients which dissolve into the plasma of the blood (which is mostly water) and is carried round the body. This also keeps the concentration of the nutrients in the villi less than that of the intestines allowing for continual diffusion (until active transport becomes necessary). The villi are also only one layer of cells thick so the molecules have a smaller distance to travel. |
The lacteals are for insoluble molecules and form part of the lymphatic system. As glycerol and fatty acids could react to form lipids which don't dissolve in water and therefore plasma, they travel through the lymphatic system instead so as not to block up the blood vessels by entering the lacteals. |
Large Intestine
The large intestine also consists of multiple parts: the colon and the rectum, and the digestive system ends at the anus.
Indigestible food like fibre, water and bacteria (waste) travels through the large intestine. The first part is called the colon and is where water is absorbed via osmosis (so most water is absorbed but not all) and what is left is called faeces which is stored in the rectum
Anus and Appendix
Finally, he anus ejects faeces out of the body in a process called egestion (pooing - often confused with 'excretion' which is the expulsion of metabolic waste like Carbon Dioxide).
The appendix, which we no longer use, used to be for the digestion of fibre from sources like grass etc..
The large intestine also consists of multiple parts: the colon and the rectum, and the digestive system ends at the anus.
Indigestible food like fibre, water and bacteria (waste) travels through the large intestine. The first part is called the colon and is where water is absorbed via osmosis (so most water is absorbed but not all) and what is left is called faeces which is stored in the rectum
Anus and Appendix
Finally, he anus ejects faeces out of the body in a process called egestion (pooing - often confused with 'excretion' which is the expulsion of metabolic waste like Carbon Dioxide).
The appendix, which we no longer use, used to be for the digestion of fibre from sources like grass etc..
This is a detailed summary music video by the YouTube channel sciencemusicvideos
There are various practicals concerning food and digestion which test, for substances and energy in food.
Iodine Test for Starch
Benedict's Test for Reducing Sugars
(Reducing sugars are so called because the test involves reducing the alkaline solution copper(II) sulphate to copper(I) oxide. All monosaccharides like fructose and glucose are reducing sugars while some disaccharides like lactose are too. Others like sucrose are not.)
*A water bath could be made by half-filling a beaker with water and placing it on a gauze on a tripod to be boiled by a Bunsen burner. Using a water bath is safer than directly heating the test tube with the Bunsen.
Biuret Test for Protein
Emulsion Test for Lipids
- Place the food sample in a spotting tile.
- Add a few drops of iodine solution using a pipette.
- If there is starch, the brown-red iodine reacts with the starch to form a blue-back colour. If there is no starch, the colour remains the same
Benedict's Test for Reducing Sugars
- Place 2 spatulas of the food sample in a test tube and add water of an appropriate depth (1-2cm)
- Shake/mix the solution to allow the reducing sugars (like glucose) to dissolve.
- Add an equal volume of Benedict's solution to the test tube then place it in a water bath*
- If there are reducing sugars, the clear blue Benedict's solution turns brick-red. It might not turn completely brick-red as the more reducing sugars there are, the darker it becomes, it may turn yellow, green or orange as a result.
(Reducing sugars are so called because the test involves reducing the alkaline solution copper(II) sulphate to copper(I) oxide. All monosaccharides like fructose and glucose are reducing sugars while some disaccharides like lactose are too. Others like sucrose are not.)
*A water bath could be made by half-filling a beaker with water and placing it on a gauze on a tripod to be boiled by a Bunsen burner. Using a water bath is safer than directly heating the test tube with the Bunsen.
Biuret Test for Protein
- Place 2 spatulas of the food sample into a test tube and add water of about 1-2cm depth
- Shake the tube to mix the sample with the water
- Add an equal volume of biuret solution* to the test tube and mix it.
- If there is protein in the food, the blue biuret solution turns lilac/purple.
Emulsion Test for Lipids
- Place the sample food in a test tube and add ethanol of about 2cm depth
- Shake the tube to dissolve any lipids in the ethanol (lipids don't dissolve in water)
- Pour the solution into another test tube that is half-filled with water.
- If the water turns cloudy/there is a layer of cloudy white on the water, there are lipids. This is caused by the ethanol dissolving in the water which leaves the lipids behind, suspended in the water it can't dissolve into - an emulsion.
Investigating the Energy Content of Sample Foods
Different foods have different energy contents in J/g (joules per gram), and the energy content of food can be measured by burning samples:
We know the mass of water is 20g as 1g = 1cm cubed = 1ml and the temperature rise is the final temperature - the initial
4.2J is the energy needed to raise the temperature of 1 gram of water by 1 degree.
Calorimeters
The above experiment has some flaws which lead to inaccurate results. One of these flaws is that the energy from the food won't all go to the water and would be lost to the environment. You move the food from the Bunsen to the boiling tube immediately to avoid this as much as possible but energy will still be lost to the environment.
A solution to this problem would be to use a calorimeter as pictured below.
The ignition wires burn the food sample and the heat energy then travels to the water via heat transfer mechanisms (conduction, convection etc.). The water is constantly stirred by the stirrer and the thermometer measures the temperature.
- Using a mass balance, measure the mass of the food sample in grams (g).
- Set up a clamp stand to hold a boiling tube with a thermometer inside and measure the water temperature.
- Measure 20cm cubed of water using a measuring cylinder and pour it into the boiling tube.
- Place the food sample on a mounted needle and burn it with a Bunsen burner until it lights on fire.
- As soon as the sample food lights, transfer the burning food directly underneath the boiling tube.
- Allow the food sample to burn, relighting until it doesn't light anymore.
- Measure the new water temperature. Stir the water with the thermometer so the heat is spread evenly.
- Input the final results into this formula:
We know the mass of water is 20g as 1g = 1cm cubed = 1ml and the temperature rise is the final temperature - the initial
4.2J is the energy needed to raise the temperature of 1 gram of water by 1 degree.
Calorimeters
The above experiment has some flaws which lead to inaccurate results. One of these flaws is that the energy from the food won't all go to the water and would be lost to the environment. You move the food from the Bunsen to the boiling tube immediately to avoid this as much as possible but energy will still be lost to the environment.
A solution to this problem would be to use a calorimeter as pictured below.
The ignition wires burn the food sample and the heat energy then travels to the water via heat transfer mechanisms (conduction, convection etc.). The water is constantly stirred by the stirrer and the thermometer measures the temperature.
