Digestion and enzymes

Pathogens

• Pathogens are the harmful bacteria and viruses that cause disease.
• Pathogens can be passed from one person to another, so the diseases they cause are called infectious diseases.
• Pathogens are present in and on the food we eat and water we drink.
• They are carried on little droplets of moisture in the air the we breathe out or expel when we sneeze.
• They are also found in the soil and the objects that we touch.
• They find their way into our body via our interfaces.

Our Interfaces

• We have many interfaces with the environment.
• These are the breaks in the skin,
• Our gas exchange system,
• our reproductive and urinary systems and
• the lining of the digestive system.

When a pathogen finds its way into our digestive system it attaches itself to the cells that form the lining. Such pathogens include the bacteria Salmonella and Campylobacter these cause the outbreaks of food poisoning. There are others that cause much more serious diseases such as cholera.

Non-infectious diseases

• Non-infectious diseases are not caused by microorganisms.
• They may result from the genes that we inherit from our parents, or they may be linked with our lifestyle choices and associated factors such as diet, smoking or lack of exercise.

• The digestive system consists of the gut, which forms a tube extending from the mouth at one end, through the body to the anus at the other end.
• Food is ingested - taken in. In the mouth, it is chewed, mixed with saliva and swallowed.
• It must then be digested, the food is mixed with digestive juices secreted by various glands as it is squeezed and pushed along by the muscular walls of the gut.
• The digestive juices contain enzymes.
• The enzymes break down the food to small, soluble molecules which can then be absorbed and transported via the blood to the body's cells.

The enzymes in the digestive juices act upon the large insoluble molecules of protein, starch and fats that are the main components of food.

The enzymes break them down into smaller, soluble molecules:

• Protein is digested to amino acids.
• Starch is digested to glucose.
• Fats are digested to a mixture of fatty acids and glycerol.

Undigestable substances, such as cellulose cannot be digested by the human gut.

Therefore they pass out through the anus, together with cells scraped from the gut lining, enzymes and bacteria which are then egested as faeces.

Large and small molecules

• All food is derived from living organisms.
• It contains substances that once made up organisms, although may be not in the same proportions.
• Three groups of these substances are very important in our diets.
• They are carbohydrates, proteind and lipids, and they all contain carbon.
• These groups are called organic molecules.
• Many of the organic molecules found in living organisms are very large in size and are known as macromolecules.
• Macromolecules are made up of monomers.
• Several monomers join together to from a polymer.

• A carbohydrate molecule contains carbon, hydrogen and oxygen.
• It has twice as many hydrogen atoms as oxygen atoms, this is the same proportion as water.
• Carbohydrates are divided into three main groups:
• Monosaccharides are simple sugars. Different monosaccharides contain a different number of carbon atoms. Mosto of those that are important in our diets are glucose, fructose and galactose which contain six carbon atoms.
• Disaccharides are carbohydrates that contain two monosaccharide residues. Sucrose, maltose and lactose are disaccharides.
• Polysaccharides are very large molecules and conatin many monosaccharide residues. Starch is a polysaccharide.

• Glucose is a monosaccharide, so it is a single sugar.
• Its molecular formula is C6H12O6.
• Galactose and Fructose are also monosaccharides and have exactly the same molecular formula as an alpha glucose.
• This means that, although all three are sugars, they are arranged in different ways. This gives them slightly different proporties.
• Monosaccharide such as glucose are the monomers that join together many carbohydrates.
• Two alpha glucose molecules join by the condensation reaction to form the disaccharide maltose. The bond forms between a carbon1 of one alpha glucose molecule and carbon4 of the other, this is called a glycosidic bond.

• Other disaccharides form in a similar way. Lactose, for example, is the sugar found in milk.
• It is formed in a condensation between a molecule of alpha glucose and a molecule of another monosaccharide, galactose. Sucrose is formed from alpha glucose and fructose.

When sugars such as alpha glucose are boiled with Benedict's solution, an orange precipitate is formed because CU(II) ions in Benedicts's solution are reduced to orange CU(I) ions. This reaction occurs because of the way chemical groups are arranged in such sugars. These sugars are therefore calledreducing sugars. Fructose, maltose and galactose are also reducing sugars.

Sucrose does not give an orange precipitate with benedict's solution: it is a non-reducing sugar. However, when boiled with dilute acid, sucrose is hydrolysed to monosaccharides. The sucrose molecules are split into alpha glucose and fructose, both reducing sugars. Then it will give a postive test with Benedict's.

• Starch is a mixture of two substances, amylose and amylopectin.
• Both substances are made up of alpha glucose.
• To test for starch add a drop of iodine solution. If starch is present the solution will turn blue/black.
• To break down starch you need to use two different enzymes. First you break down starch into maltose using amylase and then you use maltase to break it down into glucose.
• The enzymes do this by hydrolysising the starch and then the maltose.

• The building blocks of proteins are amino acids.
• There are twenty different amino acids, and they can be joined in any order.
• They take on a variety of different roles in the human body, for example an enzyme is a protein, so is a haemoglobin and so are antibodies.
• Amino acids all have the same general structure. There is an amino group (NH2). This the group that gives it its name.
• Then we have a carboxylic acid group (COOH) and a hydrogen atom.
• These groups are all the same in every amino acid.
• Where they differ is in the R-group.
• Amino acids join together through the condensation reaction. The bond formed is a peptide bond.
• Joining two amino acids together is called a dipeptide, when many are joined together it is called a polypeptide.

• A protein that consists of more than one polypeptide chain folds into a 3-d shape.
• The sequence of amino acids in the polypeptide chain(s) is the primary structure.
• The way the chain(s) are folded is the secondary structure, the only ways is either the alpha helix or the beta pleats.
• The way they are futher folded up is called the globular shape and tertiary structure.
• The secondary structure is deeply influenced on the way in which the bonds are formed, where they are formed.
• The tertiary structure influences the function of the protein.
• Different types of bond form between different amino acids and help to maintain the shape
• The types of bonds are: the hydrogen bond - which are formed between the R-groups, these bonds are not very strong, but there are many of them.
• The other is the disulphide bond - which are formed between amino acids that have sulphur in their R-groups, these bonds are stronger than the hydrogen bonds.