Organisation: Digestion (AQA GCSE Biology)

An organ is a collection of tissues working together to perform one function.

The stomach has three roles:

• To mechanically digest food through the churning of food

• To produce the enzyme protease

• To produce hydrochloric acid (stomach acid)

The stomach is part of the digestive system.

The role of mucus-producing cells in the stomach is to protect the stomach lining from hydrochloric acid (stomach acid).

Two other organs of the digestive system:

• Mouth/tongue/salivary glands

• Oesophagus

• Liver

• Gall bladder

• Pancreas

• Small intestine

• Large intestine

• Rectum

• Anus

The function of the digestive system is to break down large, insoluble food molecules into small, soluble molecules that can be absorbed.

The organs of the alimentary canal through which food passes are:

• Mouth

• Oesophagus

• Stomach

• Small intestine (duodenum and ileum)

• Large intestine

• Rectum

Other organs, e.g. the pancreas, are not in this list because food does not directly pass through them.

True.

Teeth grind food in the mouth; this is mechanical digestion.

Saliva contains amylase enzymes that begin the chemical breakdown of starch into sugars. Saliva also moistens food to aid in swallowing.

The oesophagus is the tube through which food passes between the mouth and the stomach.

True.

Muscular contractions of the stomach wall play a role in mechanical digestion, while protease enzymes start to chemically break down the proteins in food.

The role of the large intestine is to allow absorption of water from solid waste before it is egested; the remaining solid waste makes up the faeces.

False.

Accessory organs, such as the pancreas, are not part of the tube through which food passes. Their role is to produce substances, such as enzymes, needed for digestion.

Three organs of the digestive system that release enzymes include:

• Salivary glands

• Stomach

• Small intestine

• Pancreas

Bile is released from the liver.

An enzyme is a protein that acts as a biological catalyst to speed up the rate of a chemical reaction.

A catalyst speeds up the rate of a chemical reaction without being used up or changed in the reaction.

The enzyme-substrate complex is formed when a substrate attaches to the active site of an enzyme.

The active site is the region on an enzyme where a specific substrate attaches for the reaction to be catalysed.

After the reaction has occurred, the products leave the enzyme's active site as they no longer fit it, and the enzyme is free to take up another substrate.

False.

Enzymes are not changed or used up in the reactions they catalyse; they can be recycled and used in further reactions.

When an enzyme is denatured this is the process of an enzyme losing its specific shape and becoming inactive due to high temperatures or unsuitable pH.

Increasing the temperature towards the optimum increases enzyme activity as the molecules have more kinetic energy, leading to more collisions with substrate molecules and a faster rate of reaction.

The optimum temperature is the temperature at which an enzyme works fastest.

False.

Some enzymes that are produced in acidic conditions have a lower optimum pH, while those produced in alkaline conditions have a higher optimum pH.

The lock is the active site of the enzyme, which is a specific shape to fit the correct substrate, which is the key.

In a practical investigating the effect of pH on rate of reaction in amylase, the iodine solution stops turning blue-black at certain pH levels because the starch is fully broken down by the amylase. This means that there is no longer starch in the solution to turn the iodine blue-black.

In a practical investigating the effect of pH on rate of reaction in amylase, at the optimum pH the iodine solution will stop turning blue-black and remain orange-brown within the shortest amount of time. This is because the amylase is working at its fastest rate to break down the starch.

In a practical investigating the effect of pH on rate of reaction in amylase, at very low pH values the iodine solution will continue turning blue-black throughout the investigation. This is because the enzymes are denatured so the rate of amylase activity has slowed down/stopped.

A buffer solution is a solution that resists changes in pH upon the addition of small amounts of an acid or base.

Buffer solutions are used to maintain a specific pH environment for the amylase enzyme. This keeps the pH the same throughout the investigation for each of the pH conditions.

False.

The starch and amylase solutions should ideally be placed in a water bath at the optimum temperature before being used in the investigation, however it does not need to be the exact optimum as long as the temperature is kept constant throughout.

In a practical that investigates the effect of temperature on rate of reaction in enzymes, the reactants and enzymes must be placed in a water bath at optimum temperature before being used so that they are brought up to the correct temperature before the investigation.

In a practical investigating the effect of pH on rate of reaction in amylase, the variable being changed (independent variable) is the pH of the amylase and starch reaction mixture.

In a practical investigating the effect of pH on rate of reaction in amylase, the variable being measured (dependent variable) is the time taken for the iodine to stop turning black.

In a practical that investigates the effect of pH on rate of reaction the variables that should be kept constant include:

• the concentration and volume of starch solution

• the concentration and volume of enzyme solution

• the temperature

The three main types of digestive enzymes are:

• Carbohydrase

• Protease

• Lipase

Carbohydrase enzymes break down carbohydrates into simple sugars, e.g. glucose.

Amylase is an example of a carbohydrase enzyme which digests starch

The enzyme amylase is produced in the:

• Salivary glands

• Pancreas

• Small intestine

False.

Protease enzymes break down proteins into amino acids. Fatty acids are a product of lipid digestion.

Lipase enzymes carry out lipid digestion.

Lipids (fats) are broken down into glycerol and fatty acids by lipase enzymes.

The pancreas produces the following digestive enzymes:

• Lipase

• Amylase

• Protease

The type of enzyme that digests protein is protease.

The products of protein digestion are amino acids.

The organ that produces the enzyme lipase is the pancreas.

The test for glucose involves adding Benedict's solution to a sample and heating it in a boiling water bath. A positive result is a colour change from blue to brick red.

The test for starch involves adding iodine solution to a food sample. A positive result is a colour change from orange-brown to blue-black.

The protein food test involves adding Biuret solution to a food sample. A positive result is a colour change from blue to violet/purple.

The test for lipids involves mixing a sample with ethanol and then adding the ethanol solution to cold water. A positive result is a cloudy white emulsion forming.

Note that a sample containing solid pieces of food may need to be strained before a positive result can be clearly seen.

False.

A positive test for starch using iodine will turn the solution from brown/orange to blue-black.

The starting colour of Benedict's solution is blue.

The starting colour of Biuret solution is blue.

The Benedict's test for glucose (a reducing sugar) must be heated to observe the results.