6.1 – Digestion
6.1.1 – Explain why digestion of large food molecules is essential
When we eat food, the carbohydrates and other molecules are very large and insoluble. They would not be able to pass through the cell membranes to get into the bloodstream. Digestion breaks them down into smaller molecules that are soluble and can be taken up into the bloodstream. These smaller molecules can then be used to build up the macromolecules necessary for the function of our bodies. For example, large polypeptides are broken up into their amino acids, which are then used to build up new proteins.
6.1.2 – Explain the need for enzymes in digestion
An enzyme is a biological catalyst which lowers the activation energy in order to speed up a reaction. Each enzyme is specific to a particular substrate molecule, so a number of different digestive enzymes are secreted to be able to break down all the different food types. They are secreted for the hydrolysis of large polymers. These enzymes reach optimum functioning at body temperature. The enzymes lower the activation energy to allow the reaction to proceed more readily, whilst still at body temperature.
Without enzymes, we would not be able to digest our food because the temperatures required to break the bonds between the large molecules would be far too high for living organisms. However, without enzymes, the reactions involved in digestion would take place very slowly at body temperature, and we would not be able to survive. With them, the rate of reaction is significantly increased to allow for respiration at the necessary rate.
6.1.3 – State the source, substrate, products and optimum pH condition for one amylase, on protease and one lipase
Salivary Amylase
This enzyme comes from the salivary glands, found in the mouth, and breaks down polysaccharides like amylose, or starch. This is then broken down into disaccharides like maltose and glucose. The optimum pH of this enzyme is roughly neutral: 6.5-7.5.
Pancreatic Lipase
This is sourced from the pancreas and breaks down fats and oils, or triglycerides. The result is fatty acids and glycerol. The triglycerides form tiny droplets called emulsified lipids. Bile salts must be present for the lipids to be emulsified. This process increases the surface area and exposes the head of the glycerol molecule to allow the enzymes to act on it. The optimum pH of lipase is 7.0.
Pepsin
This is a protease found among the gastric juices in the stomach. It works best at pH 2.0. It breaks down large polypeptide chains into smaller peptides. This is done through the hydrolysis of the peptide bonds in the chain.
6.1.4 – Draw and label a diagram of the digestive system
6.1.5 – Outline the function of the stomach, small intestine and large intestine
Stomach
The food enters the stomach in the form of a bolus. The opening between the stomach and the oesophagus is called the cardiac sphincter. In the stomach, the food is churned around. Gastric juices are secreted into the stomach containing HCl and protease enzymes. The main enzyme used to break down proteins in the stomach is pepsin. The pH of the stomach is about 1.5-2.0, which is optimum for protein digestion, as well as killing off many harmful microorganisms.
The mucus in the stomach is secreted from the goblet cells, which are located along the lining of the stomach. This protects the lining from being broken down by all the acids, called autolysis. The end result of the churning is that the food becomes semi-liquid chyme.
Small Intestine
The opening between the stomach and the small intestine is called the pyloric sphincter. In the small intestine, all the soluble products of digestion are absorbed into the bloodstream for use around the body. The first section is the duodenum where the chyme is mixed with bile to lower the pH, then with pancreatic juices containing enzymes for the digestion of lipids, carbohydrates and proteins. This further breaks them down to the smaller monosaccharides, amino acids, small peptides, fatty acids and glycerol. Movement through the small intestine is maintained by peristalsis.
The pancreas secretes both enzymes and a buffer solution made up of a bicarbonate. The buffer helps to maintain a higher pH, resisting the addition of the acidic chyme.
An important feature of the small intestine is that its lining is covered with villi, which in turn are covered in microvilli. These increase the surface area to maximise absorption. The villi absorb nutrients via active transport to the blood.
Bile has an additional function, which is to emulsify the lipids in the duodenum. This breaks them apart, speeding up enzyme action later. 
The small intestine is also coated in mucus along the lining to protect the epithelial cells, which provide the ATP for active transport. The nutrients that are absorbed into the blood then travel to
The nutrients that are absorbed into the blood then travel to all the different parts of the body. They are taken up into the cells through assimilation.
Large Intestine
The large intestine has many folds to increase the surface area for absorption. Most of the products that enter the large intestine are indigestible material such as fibre, dead cells, mucus and other things like minerals and water.
The appendix is located near the opening of the large intestine, however in humans it has no function.
In the first section, the colon, water and minerals are absorbed to leave more solid faeces. These are stored in the rectum until they are excreted through the anus, controlled by sphincter muscles. The movement of food is maintained by peristalsis.
6.1.6 – Distinguish between absorption and assimilation
Absorption
Soluble products of digestion are absorbed into the blood circulation system, or the lymphatic system if they are fats droplets.
Assimilation
Products of digestion are absorbed into the cells from the blood to be stored or used within the tissues.
6.1.7 – Explain how the structure of the villus is related to its role in absorption of transport of the products of digestion
Villi play an important role in absorption of nutrients in the small 
intestine because their structure increases the surface area to maximise the process. In fact, the surface area is increased tenfold due to their presence. Each villus is covered with microvilli to further increase their
Each villus is covered with microvilli to further increase their surface area. Each villus has capillaries and lacteals inside it, which transport the nutrients to the rest of the body. The epithelial cells provide energy for active transport of nutrients.