AS AQA Biology- Digestion and Absorption Chapter 3.3.3

• What is digestion?

  when large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes
• What hydrolyses carbohydrates?

  amylases, membrane-bound disaccharides
• What hydrolyses lipids?

  Lipases, including action of bile salts
• What hydrolyses proteins?

  Endopeptidases, exopeptidases, membrane-bound dipeptidases
• Where is amylase produced

  salivary glands and pancreas

  
• What does amylase do to polysaccharides?

  hydrolyses polysaccharides into the disaccharide maltose
• Give two examples of membrane-bound disaccharidases.

  Lactase hydrolyses lactose down into glucose and galactose

  Sucrase hydrolyses sucrose into glucose and fructose.
• Why are disaccharidases called disaccharidases?

  They hydrolyse disaccharides
• Where does digestion of carbohydrates start and end in

  Starts in mouth, continues in duodenum, ends in ileum
• Where does digestion of proteins start?

  Starts in stomach, continues in duodenum, ends in ileum
• Where is the duodenum located?

  first part of the small intestine

  
• Where is the ileum located?

  lower right quadrant of small intestine

  
• What are endopeptidases?

  enzymes that hydrolyse peptide bonds between amino acids in the middle of a polypeptide chain

  
• What are exopeptidases?

  enzymes that hydrolyse peptide bonds between amino acids at the end of a polypeptide chain

  
• What is the function of membrane-bound dipeptidases?

  hydrolyse peptide bonds between two amino acids

  
• Where is lipase produced?


  pancreas

  
• What are lipids hydrolysed into?

  Three fatty acids and one glycerol

  
• Where is bile produced?

  Liver

  
• Where is bile stored?

  Gall bladder

  
• What is the role of bile in digestion of lipids?

  Bile can emulsify lipids to form tiny droplets called micelles. Droplets increase the surface area for lipases to act on.

  
• What is the role of micelles?

  deliver fatty acids, glycerol and monoglycerides to the epithelial cells of the ileum for absorption. They can also contain bile salts.

  
• Describe adaptations of the villi.

  . One cell thick so short diffusion distance

  . Large surface area due to folding which maximises absorption

  . Supply of capillaries which maintains a concentration/diffusion gradient

  
• What happens when micelles encounter ileum epithelial cells

  They simply diffuse across due to the non-polar nature. Once in the cell they are modified back into triglycerides

  
• What is a chylomicron?

  Protein + triglyceride

  
• How do pancreatic enzymes secreted in the pancreas reach the duodenum

  Via the pancreatic duct
• Where is pepsin secreted

  Stomach

  
• What does pepsin do?

  Hydrolyses polypeptides into shorter polypeptides

  
• What is the stomach pH?

  Acidic low pH around 2

  
• Where is trypsin secreted and where does it act

  Secreted by the stomach, acts in the duodenum
• What is the pH of the duodenum?

  Around 6
• What are microvilli?

  foldings of membrane which further increase surface area

  
• Describe the co-transport of sodium ions and glucose.

  . Sodium ions are actively transported from columnar epithelial cell into the blood.

  . This creates a concentration gradient that causes more sodium ions to diffuse into the columnar epithelial cell.

  . Na+ ions enter via a co-transporter protein that also transports glucose in at same time.

  . Glucose diffuses through a carrier protein from columnar cell into blood

  
• Why do epithelial cells contain many mitochondria?

  To provide ATP for co-transport and active transport.
• What is the role of the Golgi body in lipid absorption?


  Processes triglycerides and Combines triglycerides with protein to form chylomicrons and puts these into vesicles.
• Describe how a lipid molecule gets from the gut lumen into the bloodstream. [5 marks]

  1) Micelles are formed in the digestion of lipids. Lipids are hydrolysed by lipase into three fatty acids and glycerol.

  2)Fatty acids and glycerol are lipid soluble so diffuse straight through the membrane and into the columnar epithelial cell.

  3) The fatty acids and glycerol are recombined in a condensation reaction to make triglycerides.

  4)The triglycerides are combined with protein to form a chylomicron. This process occurs in the Golgi apparatus.

  5) Chylomicrons are packaged into vesicles.

  6) These vesicles leave the cell via the process of exocytosis and enter the lacteal
• What is the lacteal a part of?

  The lymphatic system
• How do the vesicles containing chylomicrons exit the cell?

  Exocytosis
• Where is lipase produced


  pancreas

  
• Why is combined rate of hydrolysis of protein faster when using both exopeptidases and endopeptidases?

  . Endopeptidases hydrolyse peptide bonds between amino acids in the middle of the polypeptide chain, and increase surface area for exopeptidases to act on
• How do endopeptidases aid exopeptidases?

  Increase surface area for exopeptidases to act on/hydrolyse