.3 Digestion and absorption Flashcards
Digestive system
1) The salivary glands are situated near the mouth. They secretes the enzyme amylase via a duct in the mouth, which breaks down starch to maltose.
2) The oesophagus carries food from the mouth to the stomach. Adapted for transport. Made up of a thick muscular wall.
3) The stomach is a muscular sac that produces enzymes. It stores and digests food, especially proteins. Other glands in the wall produce mucus which prevents it from being digested by its own enzymes.
The pancreas is a large gland situated below the stomach. It secretes pancreatic juice which contains proteases to digest protein, lipase to digest lipids, and amylase to digest starch.
4) The small intestine is a long muscular tube. Food is further digested by enzymes that are produced by its walls. Inner wall is folded into villi, which gives them a large surface area. This is further increased by tiny projections called microvillus on the epithelial cells of each villus. This adapts it for its purpose of absorbing the products of digestion into the blood stream.
5) The large intestine absorbs water. Most of the water that is reabsorbed comes from the secretions of the many digestive glands. Food becomes drier and thicker in consistency and forms faeces.
6) The rectum is the final section of the intestines. The faeces are stored here before being removed via the anus in process called egestion.
7) Anus
What is digestion? Key definition
Digestion is the process in which large insoluble molecules are hydrolysed by enzymes to produce smaller molecules that can be absorbed and assimilated
Two stages of digestion
Physical breakdown
Large food molecules broken down into smaller pieces by teeth
Allows ingestion and a large s.a. for chemical digestion
Food churned by muscles in stomach wall
Chemical digestion
Breaks down large insoluble molecules into smaller soluble ones using enzymes.
All digestive enzymes function by hydrolysis
splitting up of molecules by adding water to the chemical bonds that hold them together).
Chemical digestion
Breaks down large insoluble molecules into smaller soluble ones using enzymes.
All digestive enzymes function by hydrolysis splitting up of molecules by adding water to the chemical bonds that hold them together).
What happens after hydrolysis?
Absorption
Taking soluble molecules from the small intestine into the blood.
Often carried to different parts of the body
Assimilation
Incorporating the absorbed molecules into body tissues
Eg. Amino acids are assimilated to make cellular proteins
Starch digestion
- Food taken in and chewed by the teeth, giving a larger surface area
- Saliva from the salivary glands mixes with the food during chewing
- Salivary amylase starts hydrolysing any starch in the food to maltose. The mineral salts maintain a neutral pH (optimum pH for amylase)
- Swallowed and enters the stomach, where the conditions are acidic. This denatures the amylase and prevents any further hydrolysis of starch
- Passed into small intestine, mixed with pancreatic juice.
- Pancreatic amylase continues the hydrolysis of the remaining starch to maltose. (Alkaline salts are produced to maintain the pH at neutral)
- Muscles push the food along the small intestine, the epithelial lining produces maltase. Maltase is not released into the lumen but is part of the cell-surface membrane. (Membrane-bound disaccharide) This hydrolyses the maltose into α-glucose
Summarise starch digestion
Starch –> Maltose Maltose-> alpha glucose
Amylase Maltase
Mouth (salivary amylase) SI (in CS memb)
SI (pancreatic amylase)
Describe the role of the enzymes of the digestive system in the complete breakdown of starch(5 marks)
(a) Amylase; (Starch) to maltose: Maltase; Maltose to glucose; Hydrolysis; (Of) glycosidic bond;
Q Do not penalise incorrect site for digestion or incorrect site of enzyme production.
Three main enzymes in digestion
Carbohydrases hydrolyse carbohydrates, ultimately to monosaccharides.
• Lipases hydrolyse lipids (fats and oils) in to glycerol and fatty acids.
• Proteases hydrolyse proteins, ultimately to amino acids.
Lipid digestion
Lipases are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatly acids and monoglycerides.
1) Emulsification- Lipids split into tiny droplets called micelles by bile salts in liver- ↑SA- speed up lipases
2) Lipases- hydrolyse micelles’ ester bond in triglycerides → FAs and monoglycerides (glycerol molec w/single FA attached)
Bile salt- hydrophobic (lipophilic) IN fat and hydrophilic (lipophobic) outward- prevents them sticking together- reach ileum in thia form.
Protein digestion
Proteins are large, complex molecules that are hydrolysed by a group of enzymes called peptidases (proteases).
- Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules.
- Exopeptidases hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids.
- Dipeptidases hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum .
SI adaptations for dig
- Long/folded/villi- ↑ SA for eff diff
- Villi have good blood supply- carries away products to maintain conc. grad
- 1 cell thin epithelium (inner layer)- short dist to diffuse- have microvilli ↑SA
- Contain muscle so able to move- maintain diff grad as this mixes w/content of ileum
Co-transport of glucose and aas
1) Na + actively transported out of cell → blood
2) ↓conc. Na+ inside epithelial cell
3) Sets up conc. grad of Na+ btw lumen + cell
4) Na+ diffuses into cell from lumen
5) Co-transport of AAs from lumen→cell w/Na+ (Na takes AA or glucose with it)
6) ↑ conc of aa in cell- now conc. grad
7) AA diffusing into blood
Absorption of triglycerides
1) Micelles come into contact w/ epithelial cells lining villi of the ileum
2) Mics brk↓ by lipases: release mg’s + FAs
3) Diff across CSM→epithelial cells (bcuz they’re non-polar can do so)
4) Mgs + FAs trans to ER + processed (recombined) → TGs
5) In ER + GA associate w/chol + lipoproteins → chylomicrons (CLMs) (baso vesicles) - packaged for release by exocytosis
6) CLMs move out of cell by exocytosis (too big otherwise) + enter lacteals (lymphatic capillaries at centre of each villi)
7) TGs in the CLMs are h’d by an enzyme in bld capillaries
