Digestion

Starter

Quiz

Interactive quiz 1

Assessment objectives6.1.1 Explain why digestion of large food molecules is essential.

6.1.2 Explain the need for enzymes in digestion.

3 The need for increasing the rate of digestion at body temperature should be emphasized.

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.

Any human enzymes can be selected. Details of structure or mechanisms of action are not required.

6.1.4 Draw and label a diagram of the digestive system. The diagram should show the mouth, esophagus, stomach, small intestine, large intestine, anus, liver, pancreas and gall bladder. The diagram should clearly show the interconnections between these structures.

6.1.5 Outline the function of the stomach, small intestine and large intestine. 6.1.6 Distinguish between absorption and assimilation. 6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Assessment objectives6.1.1 Explain why digestion of large food molecules is essential.

Food needs to be broken down and reassembled.Large food molecules need to be broken down into smaller ones.

6.1.2 Explain the need for enzymes in digestion.

Enzymes break down large food molecules into smaller ones.Speed up the process of digestion by lowering the activation energy for the reaction.Work at body temperature

The need for increasing the rate of digestion at body temperature should be emphasized.

Assessment objectives6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.

Any human enzymes can be selected. Details of structure or mechanisms of action are not required.

Amylase Lipase Protease

Enzyme

Source

Substrate

Product

Optimum pH

Assessment objectives6.1.4 Draw and label a diagram of the digestive system. The diagram should show the mouth, esophagus, stomach, small intestine, large intestine, anus, liver, pancreas and gall bladder. The diagram should clearly show the interconnections between these structures.

6.1.5 Outline the function of the stomach, small intestine and large intestine (Homework)

Stomach: Secretes HCL which kills bacteria.HCL provides optimum pH for pepsin.Secretes pepsin for protein digestion.Small intestine:Intestinal wall secretes enzymesReceives enzymes from the pancreas.Has villi for absorption of food particles. Large intestine: Moves material that has not been digested along.Absorbes water.Produces faeces.

 

Assessment objectives 6.1.6 Distinguish between absorption and assimilation.

Absorption occurs when the food enters the body as the food molecules pass through a layer of cells and into the bodies tissues. This occurs in the small intestine which has many villi that are specialised for absorption. 

Assimilation occurs when the food molecules becomes part of the bodies tissue. Therefore, absorption is followed by assimilation.  6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Summary:Many villi increase the surface area for absorption.Epithelium is only one cell layer thick and so food is quickly absorbed.Microvilli on the villi increase the surface area for absorption further.Protein channels and pumps are present in the microvilli for rapid absorption.The mitochondria in the epithelium provide ATP needed for active transport. Blood capillaries are very close to the epithelium so diffusion distance is small.The lacteal takes away fats after absorption. 

IA practice

Investigation into the effect of villi on the rate of absorption

Task

•Record any possible limitations/weaknesses in the investigation as you do it and try to suggest improvements. This will help you to complete the CE part of the IA.

•DCP – record quantitative data in a table, including uncertainties and making sure that all data is written to the same precision.•DCP – Processing data. Show calculations for standard deviations and statistical tests.•DCP – Presenting data in a graph.

6.1.4 Draw and label a diagram of the digestive system.

Humans are HETEROTROPHS

• Get their nutrients by breaking down large molecules such as carbohydrates and proteins into smaller ones.

SYNOPTIC• Name three Biological molecules.• Why are some Biological molecules known

as polymers?• What is the monomer for Proteins?• By what process do monomers join to form

larger molecules?• What type of bond is formed between mono-

saccharides?• What are the different types of glucose

molecules?

• Biological molecule made up of amino acid monomers

• Biological molecule made up of glycerol and 3 fatty acids

• Biological molecule made up of alpha-glucose monomers. Composed of amylose and amylopectin.

• Proper scientific term is oesophagus

• A muscular bag with an acidic environment

• Ingestion- taking large pieces of food into the body

• digestion- breaking down the food by mechanical and chemical means

• absorption- taking up the soluble digestion products into the body's cells

• assimilation- using the absorbed materials • egestion- eliminating the undigested

material

• Egestion is NOT the same as EXCRETION.

• Excretion is the removal of waste products formed by biochemical reactions.

What is the function of the digestive system?

• To break down large, insoluble molecules into small, soluble ones so that they can be absorbed into our bloodstream.

(This topic has strong, synoptic links with BYA1 unit)

Large, insoluble molecule

1. Protein2. Starch (Complex

carbohydrate)3. Lipid

1. __________

2. __________

3. ___________ and ________

Small, soluble molecule

Digestion

• Physical – muscular contractions of the alimentary canal

• Chemical - enzymes

Practical:

1. What is the aim of your investigation? Write a null hypothesis.

2. What is the Independent variable?

3. What is the dependent variable?

4. What variables need to be controlled and why?

5. What control experiment could have been done?

6. Write a conclusion based on the evidence collected.

• It comprises a long tube, the alimentary canal or digestive tract (or simply gut) which extends from the mouth to the anus, together with a number of associated glands. The digestive systems made up of different tissues doing different jobs. The lining wall of the alimentary canal appears different in different parts of the gut, reflecting their different roles, but always has these four basic layers:

• The mucosa, which secretes digestive juices and absorbs digested food. It is often folded to increase its surface area. On the inside, next to the lumen (the space inside the gut) is a thin layer of cells called the epithelium. Mucosa cells are constantly worn away by friction with food moving through the gut, so are constantly being replaced.

• The submucosa, which contains blood vessels, lymph vessels and nerves to control the muscles. It may also contain secretory glands.

• The muscle layer, which is made of smooth muscle, under involuntary control.

• The serosa, which is a tough layer of connective tissue that holds the gut together, and attaches it to the abdomen.

Chemistry of Digestion 

• 1. Digestion of Carbohydrates 

• By far the most abundant carbohydrate = starch found in __________________ but there may also be a lot of sugar (mainly sucrose) and some glycogen (in meat).

• Salivary amylase starts the digestion of starch. Very little digestion actually takes place, since amylase is quickly denatured in the stomach, but is does help to clean the mouth and reduce bacterial infection.

• Pancreatic amylase digests all the remaining starch in the duodenum. Amylase digests starch molecules from the ends of the chains in two-glucose units, forming the disaccharide maltose. Glycogen is also digested here.

• Disaccharidases in the membrane of the ileum epithelial cells complete the digestion of disaccharides to monosaccharides. This includes maltose from starch digestion as well as any sucrose and lactose in the diet. There are three important disaccharidase enzymes:

 

• Active transport requires energy in the form of ATP, but it allows very rapid absorption, even against a concentration gradient.

• The carbohydrates that make up plant fibres (cellulose, hemicellulose, lignin, etc) cannot be digested, so pass through the digestive system as fibre

Digestion of Proteins

•  Pepsin (in gastric juice) digests proteins to peptides, 6-12 amino acids long.

• Pepsin is an endopeptidase, which means it hydrolyses peptide bonds in the middle of a polypeptide chain. It is unusual in that it has an optimum pH of about 2 and stops working at neutral pH.

• Pancreatic endopeptidases continue to digest proteins and peptides to short peptides in the duodenum.

• Different endopeptidase enzymes cut at different places on a peptide chain because they have different target amino acid

• Exopeptidases in the membrane of the ileum epithelial cells complete the digestion of the short peptides to individual amino acids.

• Carboxypeptidases work from the C-terminal end,

• aminopeptidases work from the N-terminal end, and dipeptidases cut dipeptides in half.

•  Protease enzymes are potentially dangerous. SUGGEST why this is.

• Because they can break down other enzymes (including themselves!) and other proteins in cells.

• To prevent this they are synthesised in the RER(BYA1) of their secretory cells as inactive forms.

• The enzymes are only activated in the lumen of the intestine when they are required

• Pepsin is synthesised as inactive pepsinogen, and activated by the acid in the stomach

• The pancreatic exopeptidases are activated by specific enzymes in the duodenum

• The membrane-bound peptidase enzymes are already in their active form – suggest why there is no need for it to be first in an inactive form?

• do not have this problem since they are fixed, so cannot come into contact with cell proteins.

• The lining of mucus between the stomach wall and the food also protects the cells from the protease enzymes once they are activated.

Digestion of Triglycerides

• Fats are emulsified by bile salts to form small oil droplets called micelles, which have a large surface area.

• Pancreatic lipase enzymes digest triglycerides to fatty acids and glycerol in the duodenum.

• Fatty acids and glycerol are lipid soluble and diffuse across the membrane (by lipid diffusion) into the epithelial cells of the villi in the ileum.

• In the epithelial cells of the ileum triglycerides are re-synthesised (!) and combine with proteins to form tiny lipoprotein particles called chylomicrons.

• The chylomicrons diffuse into the lacteal - the lymph vessel inside each villus.

• The chylomicrons are carried through the lymphatic system to enter the bloodstream at the vena cava, and are then carried in the blood to all parts of the body. They are stored as triglycerides in adipose (fat) tissue.

• Fats are not properly broken down until they used for respiration in liver or muscle cells.