digestion and absorption of starch and proteins

what is digestion?

the process in which large insoluble molecules are hydrolysed by enzymes to produce smaller molecules that can be absorbed and assimilated

give and explain 2 examples of mechanical digestion:

• larger food molecules broken down into smaller pieces by teeth - allows ingestion and a large SA:V for chemical digestion

• food churned by muscles in stomach wall

what does chemical digestion involve?

• HCl

• bile

• digestive enzymes !

what is the duodenum?

first part of the SI

what is the ileum?

last part of the SI

describe the process of starch digestion:

• begins in mouth, continues in duodenum, completed in ileum

• glycosidic bonds in starch (polysaccharide) hydrolysed by amylase → maltose (disaccharide)

• glycosidic bonds in maltose hydrolysed by maltase → glucose (monosaccharide)

• glucose is then absorbed into the blood and taken to cells for respiration OR to be stored as glycogen

what does it mean for disaccharidases to be membrane bound?

positioned on membrane surface of epithelial cells in ileum

give 2 examples of disaccharidases and where they are produced:

both produced in ileum:

• sucrase

• lactase

what is sucrase? what does it hydrolyse?

• membrane bound disaccharidase

• hydroluses sucrose → glucose + fructose

what is lactase? what does it hydrolyse?

• membrane bound disaccharidase

• hydrolyses lactose → glucose + galactose

where are carbohydrases produced?

• salivary glands

• pancreas

• SI

describe the process of protein digestion:

• involves exopeptidases, endopeptidases and membrane-bound dipeptidases

• starts in stomach (low pH), continues in duodenum and completed in ileum

• peptide bond hydrolysed

• AAs released to be used in protein synthesis (for fibrous proteins, enzymes, antibodies, hormones etc.)

what are endopeptidases? give 3 examples:

• peptidases that act to hydrolyse peptide bonds w/in a protein, forming multiple peptide fragments

• e.g. trypsin, chymotrypsin, pepsin

what are exopeptidases?

peptidases that act to hydrolyse peptide bonds at the end of protein molecules, removing single AAs from proteins and so forming a single peptide fragment and multiple AAs

what are dipeptidases?

• type of exopeptidase that works specifically on dipeptides

• act to separate the 2 AAs that make up a dipeptide by hydrolysing the peptide bond between them

• membrane bound to the cell surface membrane of epithelial cells

where are peptidases produced?

• (act in stomach) stomach

• (act in duodenum and ileum) pancreas and epithelial cells

describe the absorption of glucose/AAs in the bloodstream:

co transport - how glucose/AAs are absorbed from ileum into bloodstream:

• Na⁺ ions are actively transported out of the epithelial cell into the blood

• this lowers the conc of Na⁺ ions in the epithelial cell, creating a conc gradient of Na⁺ ions

• Na⁺ and glucose bind to a co-transporter protein and enter cell by facilitated diffusion/co-transport (respectively)

• there is now a higher conc of glucose inside the epithelial cell than the blood

• glucose moves from cell to blood by facilitated diffusion

• (there is no glucose buildup in the blood as the blood flows and carries away absorbed glucose)

how is the surface of the ileum adapted to absorb digested substances?

• villi and microvilli - increase SA for absorption

• mitochondria - release ATP for active transport

• carrier proteins - allow active transport/facilitated diffusion into epithelial cells

• network of capillaries and peristalsis of muscles maintain diffusion gradient